A3981KLP-T [ALLEGRO]
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ALLEGRO MICROSYSTEMS 
A3981
Automotive, Programmable Stepper Driver
Description
Features and Benefits
TheA3981isaflexiblemicrosteppingmotordriverwithbuilt-in
• Peak motor current up to ±1.4 A, 28 V
translatorforeasyoperation.Itisasingle-chipsolution,designed
to operate bipolar stepper motors in full-, half-, quarter- and
sixteenth-step modes, at up to 28 V and ±1.4 A. The A3981
can be controlled by simple Step and Direction inputs, or
through the SPI-compatible serial interface that also can be
used to program many of the integrated features and to read
diagnostic information.
• Low RDS(on) outputs, 0.5 Ω source and sink, typical
• Automatic current decay mode detection/selection
• Mixed, Fast, and Slow current decay modes
• Synchronous rectification for low power dissipation
• Internal OVLO, UVLO, and Thermal Shutdown circuitry
• Crossover-current protection
• Short circuit and open load diagnostics
• Hot and cold thermal warning
• Stall detect features
• SPI-compatible or simple Step and Direction motion control
• Highly configurable via SPI-compatible serial interface
The current regulator can be programmed to operate in fixed
off-time or fixed frequency PWM, with several decay modes
to reduce audible motor noise and increase step accuracy.
In addition the phase current tables can be programmed via
the serial interface to create unique microstep current
profiles to further improve motor performance for
specific applications.
Applications
• Automotive stepper motors
• Engine management
The current in each phase of the motor is controlled through a
DMOS full bridge, using synchronous rectification to improve
power dissipation. Internal circuits and timers prevent cross-
conduction and shoot-through, when switching between high-
side and low-side drives.
• Headlamp positioning
Package: 28-pin TSSOP with exposed
thermal pad (suffix LP)
The outputs are protected from short circuits, and features
for low load current and stalled rotor detection are included.
Chip-levelprotectionincludes:hotandcoldthermalwarnings,
overtemperature shutdown, and overvoltage and undervoltage
lockout.
TheA3981 is supplied in a 28-pin TSSOPpower package with
an exposed thermal pad (package type LP). This package is
lead (Pb) free with 100% matte-tin leadframe plating.
Not to scale
Typical Applications
Automotive
12V Power Net
Automotive
12V Power Net
Logic
Supply
Logic
Supply
CP1 CP2
VDD
STEP
DIR
MS0
MS1
ENABLE
RESETn
DIAG
CP1 CP2
VDD
STEP
DIR
MS0
MS1
ENABLE
RESETn
DIAG
VCP VBB
OAP
VCPVBB
OAP
OAM
OAM
Micro-
controller
or
Micro-
controller
or
Stepper
Motor
Stepper
Motor
REF
REF
OBP
OBM
OBP
OBM
SDI
SDI
ECU
ECU
SDO
SCK
STRn
SDO
SCK
STRn
VREG
VREG
SENSA
SENSB
SENSA
SENSB
OSC
OSC
AGND PGND
AGND PGND
Serial Interface Control
Parallel Control
A3981-DS, Rev. 4
A3981
Automotive, Programmable Stepper Driver
Selection Guide
Part Number
A3981KLP-T
Packing*
50 pieces per tube
4.4 mm × 9.7 mm, 1.2 mm nominal height TSSOP
with exposed thermal pad
A3981KLPTR-T
4000 pieces per reel
*Contact Allegro® for additional packing information.
Absolute Maximum Ratings With respect to GND
Characteristic
Symbol
Notes
Rating
–0.3 to 50
–0.3 to 6
Unit
V
Load Supply Voltage
VBBx
Applies to VBBA and VBBB
Logic Supply Voltage
Pin CP1
VDD
V
–0.3 to VBB
–0.3 to VBB+8
–0.3 to 6
V
Pins CP2, VCP
V
Pins STEP, DIR, ENABLE, DIAG
Pin VREG
V
–0.3 to 8.5
–0.3 to 6
V
Pin RESETn
Can be pulled to VBB with 38 kΩ
V
Pin OSC
–0.3 to 6
V
Pins MS0, MS1
–0.3 to 6
V
Pins SDI, SDO, SCK, STRn
Pin REF
–0.3 to 6
V
–0.3 to 6
V
Pins OAP, OAM, OBP, OBM
Pins SENSA, SENSB
–0.3 to VBB
–0.3 to 1
V
V
Ambient Operating Temperature
Range
TA
Range K; limited by power dissipation
–40 to 150
150
°C
°C
Maximum Continuous Junction
Temperature
TJ(max)
Overtemperature event not exceeding 10 s,
lifetime duration not exceeding 10 hours,
guaranteed by design and characterization
Transient Junction Temperature
Storage Temperature Range
TtJ
175
°C
°C
Tstg
–55 to 150
Thermal Characteristics may require derating at maximum conditions
Characteristic
Symbol
Test Conditions*
Value
28
Unit
ºC/W
ºC/W
4-layer PCB based on JEDEC standard
2-layer PCB with 24.52 cm2 of copper area each side
Package Thermal Resistance
(Junction to Ambient)
RθJA
32
Package Thermal Resistance
(Junction to Pad)
RθJP
2
ºC/W
*Additional thermal information available on the Allegro website
Allegro MicroSystems, Inc.
115 Northeast Cutoff
2
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
Functional Block Diagram
OSC
VREG
CP1
CP2
3.3V
VDD
REF
VCP
Oscillator
Regulator
Charge
Pump
REF
DAC
VBAT
DMOS Full Bridge
SENSA
6-bit
DAC
VBBA
+
-
OAP
OAM
STEP
DIR
PWM
Control
MS1
MS0
SENSA
RESETn
ENABLE
System
Control
and
Bridge
Control
Logic
Gate
Drive
Registers
DMOS Full Bridge
VBAT
SDI
SDO
SCK
VBBB
STRn
PWM
Control
OBP
OBM
REF
+
-
6-bit
DAC
SENSB
SENSB
Undervoltage, Overvoltage
Cold Warning, Hot Warning, Overtemperature
Short Detect, Open Load Detect
Stall Detect
DIAG
AGND
PAD
PGND
Allegro MicroSystems, Inc.
115 Northeast Cutoff
3
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
ELECTRICAL CHARACTERISTICS1,2 Valid at TJ = –40°C to 150°C, VBB = 7 to 28 V, VDD = 3 to 5.5 V; unless otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
Supplies
Functional
0
7
–
–
3
–
–
–
–
–
–
–
1
–
–
–
4
–
50
VBBOV
4
V
Load Supply Voltage Range3
VBB
Outputs Driving
ENABLE = 0
Sleep mode
V
mA
μA
V
Load Supply Quiescent Current
Logic Supply Voltage Range
IBBQ
VDD
10
5.5
5
ENABLE = 0
mA
mA
μA
μA
ENABLE=0, VDD > 5 V
Sleep mode, VDD = 3.3 V
Sleep mode, VDD = 5 V
5.5
15
Logic Supply Quiescent Current
IDDQ
25
With repect to VBB, VBB >7.5 V, ENABLE = 0,
RESETn = 1
Charge Pump Voltage
VCP
–
6.7
–
V
Internal Regulator Voltage
Internal Regulator Dropout Voltage
Motor Bridge Output
VREG
ENABLE = 0, RESETn = 1, VBB > 7.5 V
–
–
7.2
–
V
VREGDO ENABLE = 0, RESETn = 1, VBB > 5.6 V
100
200
mV
VBB = 13.5 V, IOUT = –1 A, TJ = 25°C
–
–
–
500
900
625
600
1100
750
mΩ
mΩ
mΩ
High-Side On-Resistance
RONH
VBB = 13.5 V, IOUT = –1 A, TJ = 150°C
VBB = 7 V, IOUT = –1 A, TJ = 25°C
IF = 1 A
High-Side Body Diode Forward
Voltage
VFH
–
–
1.4
V
VBB = 13.5 V, IOUT = 1 A, TJ = 25°C
VBB = 13.5 V, IOUT = 1 A, TJ = 150°C
–
–
–
500
900
625
600
1100
750
mΩ
mΩ
mΩ
Low-Side On-Resistance
RONL
V
BB = 7 V, IOUT = 1 A, TJ = 25°C
Low-Side Body Diode Forward
Voltage
VFL
IF = –1 A
–
–
1.4
V
ENABLE = 0, RESETn = 1, VO = VBB
ENABLE = 0, RESETn = 1, VO = 0 V
ENABLE = 0, RESETn = 0, VO = VBB
ENABLE = 0, RESETn = 0, VO = 0 V
–120
–200
–
–65
–120
<1.0
<1.0
–
–
μA
μA
μA
μA
Output Leakage Current
ILO
20
–
–20
Current Control
OSC = AGND
3.2
3.6
3
4
–
4.8
4.4
5
MHz
MHz
MHz
ns
Internal Oscillator Frequency
fOSC
51 kΩ from OSC to VDD
External Oscillator Frequency Range
Blank Time4
fEXT
tBLANK
tOFF
–
Default Blank-Time
Default Off-Time
–
1500
44
–
Off-Time (In Fixed Off-Time Mode)4
–
–
μs
PWM Frequency (In Fixed Frequency
Mode)4
fPWM
Default PWM Frequency
–
16.7
–
kHz
Continued on the next page…
Allegro MicroSystems, Inc.
115 Northeast Cutoff
4
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
ELECTRICAL CHARACTERISTICS1,2 (continued) Valid at TJ = –40°C to 150°C, VBB= 7 to 28 V, VDD = 3 to 5.5 V; unless
otherwise noted
Characteristics
Current Control (continued)
Fast Decay Time4
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
tFAST
VREF
Default Fast Decay Time
–
8
–
–
2
μs
V
Reference Input Voltage
Internal Reference Voltage
Current Control (continued)
Reference Input Current
Maximum Sense Voltage
Current Trip Point Error5
0.8
1.1
VREFint
REF tied to VDD
1.2
1.3
V
IREF
VSMAX
EITrip
–3
–
0
125
–
3
–
μA
mV
%
VREF = 2 V, MxI0 = MxI1 = 1
–
±5
Logic Input And Output – DC Parameters
–
–
0.3×VDD
V
V
Input Low Voltage
VIL
VIH
VDD > 4.5 V
–
–
0.28×VDD
Input High Voltage
0.7×VDD
–
500
–
–
–
V
Input Hysteresis
VIhys
IIN
250
mV
μA
kΩ
V
Input Current (Except RESETn)
Input Pull-Down Resistor (RESETn)
Output Low Voltage
0 V < VIN < VDD
–1
1
RPD
VOL
VOH
IO
–
–
50
–
IOL = 2 mA
0.2
0.4
–
Output High Voltage
IOL = –2 mA
VDD–0.4
–1
VDD–0.2
–
V
Output Leakage (SDO)
0 V < VO < VDD, STRn = 1
1
μA
Logic Input And Output – Dynamic Parameters
Reset Pulse Width
tRST
tRSD
0.2
10
–
–
–
4.5
–
μs
μs
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
Reset Shutdown Width
Input Pulse Filter Time (STEP, DIR)
Clock High Time
tPIN
35
–
–
tSCKH
tSCKL
tSTLD
tSTLG
tSTRH
tSDOE
tSDOD
tSDOV
tSDOH
tSDIS
tSDIH
A in figure 1
B in figure 1
C in figure 1
D in figure 1
E in figure 1
F in figure 1
G in figure 1
H in figure 1
I in figure 1
J in figure 1
K in figure 1
50
50
30
30
300
–
–
Clock Low Time
–
–
Strobe Lead Time
–
–
Strobe Lag Time
–
–
Strobe High Time
–
–
Data Out Enable Time
–
40
30
40
–
Data Out Disable Time
–
–
Data Out Valid Time from Clock Falling
Data Out Hold Time from Clock Falling
Data In Set-Up Time to Clock Rising
Data In Hold Time From Clock Rising
–
–
5
–
15
10
–
–
–
–
STEP Rising to STRn Rising
Setup Time
tSPS
tSPH
L in figure 1, only when D15 = 1 and D14 = 0
M in figure 1, only when D15 = 1 and D14 = 0
100
300
–
–
–
–
ns
ns
STEP Rising from STRn Rising
Hold Time
Continued on the next page…
Allegro MicroSystems, Inc.
115 Northeast Cutoff
5
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
ELECTRICAL CHARACTERISTICS1,2 (continued) Valid at TJ = –40°C to 150°C, VBB= 7 to 28 V, VDD = 3 to 5.5 V; unless
otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
Logic Input And Output – Dynamic Parameters (continued)
Step Low Time
tSTPH
tSU
1
–
–
–
–
μs
Setup Time Control Input Change
to STEP
MS1, MS2, DIR
MS1, MS2, DIR
200
ns
Hold Time Control Input Change
from STEP
tH
200
–
–
–
–
1
ns
Wake-Up from RESET
tEN
ms
Diagnostics and Protection
VBB Overvoltage Threshold
VBBOV
VBBOVHys
VREGUV
VRGUVHys
VDDUV
VDDUVHys
tWD
VBB rising
VREG falling
VDD falling
Bit 13 = 1
32
2
34
–
36
4
V
V
VBB Overvoltage Hysteresis
VREG Undervoltage Threshold
VREG Undervoltage Hysteresis
VDD Undervoltage Threshold
VDD Undervoltage Hysteresis
OSC Timeout
5.1
–
–
5.4
–
V
1
V
2.6
50
0.5
1.4
3
–
2.9
–
V
100
1
mV
μs
A
1.5
2.65
8
High-Side Overcurrent Threshold
High-Side Current Limit
IOCH
Sampled after tSCT
2.05
5.5
250
2000
–
ILIMH
Active during tSCT
A
Low-Side Overcurrent Sense Voltage
Overcurrent Fault Delay
VOCL
Sampled after tSCT
210
1500
–
290
2700
±10
–
mV
ns
%
tSCT
Default Fault Delay
Open Load Current Threshold Error
Temperature Voltage Output Offset
Temperature Voltage Output Slope
Cold Temperature Warning Threshold
Cold Temperature Warning Hysteresis
Hot Temperature Warning Threshold
Hot Temperature Warning Hysteresis
Overtemperature Shutdown Threshold
Overtemperature Hysteresis
EIOC
VREF = 2 V, Mx0 = Mx1 = 1
Temperature output selected on DIAG pin
VTO
–
1440
–3.92
–10
15
mV
mV/°C
ºC
ºC
ºC
ºC
ºC
ºC
AT
–
–
TJWC
Temperature decreasing
Temperature increasing
–20
–
0
TJWChys
TJWH
TJWHhys
TJF
–
125
–
135
15
145
–
Temperature increasing
Recovery = TJF – TJhys
155
–
170
15
–
TJhys
–
1For input and output current specifications, negative current is defined as coming out of (sourcing) the specified device pin.
2All references to “VBB” apply to VBBA and VBBB.
3Function is correct but parameters are not guaranteed above or below the general limits (7 to 28 V). Outputs not operational above VBBOV or
below VREGUVL
.
4Assumes a 4 MHz clock.
5Current Trip Point Error is the difference between actual current trip point and the target current trip point, referred to maximum full scale (100%)
current: EItrip = 100 × [ItripActual – ItripTarget ] / IFullScale (%).
Allegro MicroSystems, Inc.
115 Northeast Cutoff
6
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
No rise when
D15=1 and D14=0
STEP
L
M
STRn
SCK
SDI
C
A
B
D
E
J
K
X
D15
X
D14
X
D0
X
F
I
G
SDO
Z
D15'
D14'
D0'
Z
H
Figure 1. Serial Interface Timing Diagram
Key
A
Characteristic
Clock High Time
Key
Characteristic
H
I
Data Out Valid Time from Clock Falling
Data Out Hold Time from Clock Falling
Data In Set-Up Time to Clock Rising
Data In Hold Time From Clock Rising
STEP Rising to STRn Rising Setup Time
STEP Rising from STRn Rising Hold Time
“Don’t care”
B
Clock Low Time
C
Strobe Lead Time
Strobe Lag Time
Strobe High Time
Data Out Enable Time
J
D
K
L
M
X
Z
E
F
G
Data Out Disable Time
High-impedance (tristate)
tSTPH
tSTPL
STEP
tSU
tH
DIR, MS0, MS1
RESETn
tEN
ENABLE*
* ENABLE(Pin) OR RUN[EN] bit
Figure 2. Control Input Interface Timing Diagram
Allegro MicroSystems, Inc.
115 Northeast Cutoff
7
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
Functional Description
The A3981 is an automotive stepper motor driver suitable for
high temperature applications such as headlamp bending and
leveling, throttle control, and gas recirculation control. It is also
suitable for other low current stepper applications such as air con-
ditioning and venting. It provides a highly flexible microstepping
motor driver that can be configured via the SPI-compatible serial
interface. It can be controlled with simple Step and Direction
inputs, for high speed stepping applications, or directly through
the serial interface by writing a step change value.
AGND Analog reference ground. Quiet return for measurement
and input references. Connect to PGND (see Layout section).
PGND Digital and power ground. Connect to supply ground and
AGND (see Layout section).
OAP, OAM Motor connection for phase A. Positive motor phase
current direction is defined as flowing from OAM to OAP.
OBP, OBM Motor connection for phase B. Positive motor phase
current direction is defined as flowing from OBM to OBP.
The two DMOS full bridges are capable of driving bipolar step-
per motors in full-, half-, quarter-, eighth- and sixteenth-step
modes, at up to 28 V and ±1.4 A. The current in each phase of the
stepper motor is regulated by a peak detect PWM current control
scheme that can be programmed to operate in fixed off-time or
fixed frequency. Several decay modes can be selected to reduce
audible motor noise and increase step accuracy. In addition the
phase current tables, which default to a sinusoidal current profile,
can be programmed via the serial interface to create unique
microstep current profiles to further improve motor performance
for specific applications.
SENSA Phase A current sense. Connect sense resistor between
SENSA and PGND.
SENSB Phase B current sense. Connect sense resistor between
SENSB and PGND.
REF Reference input to set absolute maximum current level for
both phases. Defaults to internal reference when tied to VDD.
STEP Step logic input. Motor advances on rising edge. Filtered
input with hysteresis.
DIR Direction logic input. Direction changes on the next STEP
rising edge. When high, the Phase Angle Number is increased
on the rising edge of STEP. Has no effect when using the serial
interface. Filtered input with hysteresis.
The outputs are protected from short circuits, and features for
open load and stalled rotor detection are included. Chip level pro-
tection includes hot and cold thermal warning, overtemperature
shutdown, and overvoltage and undervoltage lockout.
MS0 Microstep resolution select input.
MS1 Microstep resolution select input.
Pin Functions
VBBA, VBBB Main motor supply and chip supply for internal
regulators and charge pump. VBBA and VBBB should be con-
nected together and each decoupled to ground with a low ESR
electrolytic capacitor and a good ceramic capacitor.
RESETn Resets faults when pulsed low. Forces low-power shut-
down (sleep) when held low for more than the Reset Shutdown
Width, tRSD . Can be pulled to VBB with 30 kΩ resistor.
ENABLE Controls activity of bridge outputs. When held low,
deactivates the outputs, that is, turns off all output bridge FETs.
Internal logic continues to follow input commands.
Note: Any reference to “VBB” in this specification is defined as
applying to both VBBA and VBBB.
CP1, CP2 Pump capacitor connection for charge pump. Connect
SDI Serial data input. 16-bit serial word input MSB first.
a 100 nF (50 V) ceramic capacitor between CP1 and CP2.
SDO Serial data output. High impedance when STRn is high. Out-
puts bit 15 of the diagnostic registers (Fault Register 0 and Fault
Register 1), the Fault Register flag, as soon as STRn goes low.
VCP Above-supply voltage for high-side drive. A 100 nF (16 V)
ceramic capacitor should be connected between VCP and VBB to
provide the pump storage reservoir.
SCK Serial interface clock. Data is latched in from SDI on the
VDD Logic supply. Compatible with 3.3 V and 5 V logic. Should rising edge of the SCK clock signal. There must be 16 rising
be decoupled to ground with a 100 nF (10 V) ceramic capacitor.
edges per write and SCK must be held high when STRn changes.
VREG Regulated supply for bridge gate drive. Should be decou-
STRn Serial data strobe and serial access enable. When STRn
pled to ground with a 470 nF (10 V) ceramic capacitor.
is high any activity on SCK or SDI is ignored, and SDO is high
Allegro MicroSystems, Inc.
115 Northeast Cutoff
8
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
impedance allowing multiple SDI slaves to have common SDI,
SCK, and SDO connections.
partially- or fully-programmed through the serial interface.
Each leg (high-side, low-side pair) of a bridge is protected from
shoot-through by a fixed dead time. This is the time between
switching off one FET and switching on the complementary FET.
Cross-conduction is prevented by lock-out logic in each driver pair.
DIAG Diagnostic output. Function selected via the serial inter-
face, setting Configuration Register 1. Default is Fault output.
OSC With bit 13 in Configuration Register 1 set to 0, either con-
nect this pin to AGND to use the internal oscillator running at the
default frequency of 4 MHz, or connect a resistor to VDD to set
the internal oscillator frequency. (The approximate frequency is
calculated from:
The phase currents and in particular the relative phase currents
are defined in the Phase Current table (table 7). This table defines
the two phase currents at each microstep position. For each of the
two phases, the currents are measured using a sense resistor, RS,
with voltage feedback to the respective SENSx pin. The target
current level is defined by the voltage from the digital-to-analog
converter (DAC) for that phase. The sense voltage is amplified by
a fixed gain and compared to the output of the DAC.
f
OSC = 10 000 / (48 ROSC – 20)
where fOSC is the internal oscillator frequency in MHz, and ROSC
is the value, in kΩ of the resistor between OSC and VDD.)
If bit 13 in Configuration Register 1 is set to 1, then OSC is the
input for an external system clock, which must have a frequency
between 3 and 5 MHz. In this mode a watchdog is provided to
detect loss of the system clock. If the OSC pin remains high or
low for more than the watchdog time, tWD , 1 μs typical, then the
Fault Register flag (bit 15 in the diagnostic registers) is set and
the outputs are disabled until the clock restarts.
There are two types of maximum current: the absolute maximum,
ISMAX, the maximum possible current defined by the sense resis-
tor and the reference input; and the phase maximum, IPMAX, the
maximum current delivered to a motor phase.
The absolute maximum current, ISMAX, is defined as:
ISMAX = VREF / (16 × RS)
where VREF is the voltage at the REF pin, and RS is the sense
resistor value.
Driving a Stepper Motor
A two-phase stepper motor is made to rotate by sequencing
the relative currents in each phase. In its simplest form, each
phase is simply fully energized in turn by applying a voltage to
the winding. For more precise control of the motor torque over
temperature and voltage ranges, current control is required. For
efficiency this is usually accomplished using pulse width modula-
tion (PWM) techniques. In addition current control also allows
the relative current in each phase to be controlled, providing more
precise control over the motor movement and hence improve-
ments in torque ripple and mechanical noise. Further details of
stepper motor control are provided in Appendix A.
The phase maximum, IPMAX , is the 100% reference level for the
phase current table and may be a fraction of the absolute maxi-
mum current, ISMAX , depending on the value of the MXI0 and
MXI1 bits in Configuration Register 0.
For example:
• if RS = 180 mΩ and VREF = 2 V, then ISMAX = 694 mA
• if MXI1= 1 and MXI0 = 0, then IPMAX = 520 mA
The actual current delivered to each phase at each Step Angle
Number is determined by the value of IPMAX and the contents
of the Phase Current table. For each phase, the value in the table
is passed to the DAC, which uses IPMAX as the reference 100%
level (code 63) and reduces the current target depending on the
DAC code. The output from the DAC is used as the input to the
current comparators.
For bipolar stepper motors the current direction is significant,
so the voltage applied to each phase must be reversible. This
requires the use of a full bridge (also known as an H-bridge)
which can switch each phase connection to supply or to ground.
Phase Current Control
In the A3981, current to each phase of the two-phase bipolar
stepper motor is controlled through a low impedance N-channel
DMOS full bridge. This allows efficient and precise control of
the phase current using PWM switching. The full-bridge con-
figuration provides full control over the current direction during
the PWM on-time, and over the current decay mode during the
PWM off-time. Due to the flexibility of the A3981 these control
techniques can be completely transparent to the user or can be
The current comparison is ignored at the start of the PWM
on-time for a duration referred to as the blank time. The blank
time is necessary to prevent any capacitive switching currents
from causing a peak current detection.
The PWM on-time starts at the beginning of each PWM period.
The current rises in the phase winding until the sense voltage
reaches the required current level. At this point the PWM off-time
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A3981
Automotive, Programmable Stepper Driver
starts and the bridge is switched into one of two decay modes,
slow decay or fast decay:
MXI[0..1]) in Configuration Register 0. The phase currents for
each entry in the Phase Current table are expressed as a percent-
age of this maximum phase current.
• Slow decay is most effective when the current is rising from
step to step, and it occurs when the phase winding is effectively
shorted by switching-on either both high-side FETs or both low-
side FETs in the full bridge.
When using the STEP and DIR inputs to control the stepper
motor, the A3981 automatically increases or decreases the Step
Angle Number according to the step sequence associated with
the selected step mode. The default step mode, reset at power-
up or after a power on reset, is full step. Half-, quarter-, and
sixteenth-step sequences are also available when using the STEP
and DIR inputs, and are selected using the logical OR of the MS0
and MS1 inputs and the MS0 and MS1 bits in Configuration Reg-
ister 0. The eighth-step sequence is shown in the Phase Current
table for reference only.
• Fast decay is most effective when the current is falling from
step to step, and it occurs when the voltage on the phase is
reversed.
One disadvantage of fast decay is the increased current ripple in
the phase winding. However, this can be reduced while main-
taining good current control, by using a short time of fast decay
followed by slow decay for the remainder of the PWM off-time.
This technique is commonly referred to as mixed decay
When using the serial interface to control the stepper motor, a
step change value (6-bit) is input through the serial interface to
increase or decrease the Step Angle Number. The step change
value is a two’s complement (2’sC) number, where a positive
value increases the step angle and a negative value decreases
the step angle. A single step change in the Step Angle Number is
equivalent to a single one-sixteenth microstep. Therefore, for cor-
rect motor movement, the step change value should be restricted
to no greater than 16 steps, positive or negative.
The A3981 provides two methods to determine the PWM
frequency: fixed off-time and fixed frequency. At power-up the
default mode is fixed off-time. Fixed frequency can be selected
through the serial interface. Fixed off-time provides a marginal
improvement in current accuracy over a wide range of current
levels. Fixed frequency provides a fixed fundamental frequency
to allow more precise supply filtering for EMC reduction. In both
cases the PWM off-time will not be present if the peak current
limit is not attained during the PWM on-time.
This facility enables full control of the stepper motor at any
microstep resolution up to and including sixteenth-step, plus
the ability to change microstep resolution “on-the-fly” from one
microstep to the next.
Phase Current Table
The relative phase currents are defined by the Phase Current table
(table 7). This table contains 64 lines and is addressed by the Step
Angle Number, where Step Angle Number 0 corresponds to 0° or
360°. The Step Angle Number is generated internally by the step
sequencer, which is controlled either by the STEP and DIR inputs
or by the step change value from the serial input. The Step Angle
Number determines the motor position within the 360° electri-
cal cycle and a sequence of Step Angle Numbers determines the
motor movement. Note that there are four full mechanical steps
per 360° electrical cycle.
In both control input method cases, the resulting Step Angle
Number is used to determine the phase current value and current
direction for each phase, based on the Phase Current table. The
decay mode is determined by the position in the Phase Current
table and the intended direction of rotation of the motor.
Diagnostics
The A3981 integrates a number of diagnostic features to protect
the driver and load as far as possible from fault conditions and
extreme operating environments. At the system level the supply
voltages and the chip temperature are monitored. A number of
these features automatically disable the current drive to protect
the outputs and the load. Others only provide an indication of
the likely fault status, as shown in the Fault table (table 1). A
single diagnostic output pin (DIAG) can be programmed through
the serial interface to provide several different internal signals.
At power-up, or after a power-on-reset the DIAG pin outputs a
simple Fault Output flag which will be low if a fault is present.
The Fault Output flag remains low while the fault is present or if
one of the latched faults (for example, a bridge short circuit) has
been detected and the outputs disabled.
Each line of the Phase Current table (table 7) has a 6-bit value per
phase to set the DAC level for that phase, plus an additional bit
per phase to determine the current direction for that phase. The
Step Angle Number sets the electrical angle of the stepper motor
in one-sixteenth microsteps, approximately equivalent to electri-
cal steps of 5.625°.
On first power-up or after a VDD power-on reset, the Phase Cur-
rent table values are reset to define a sinusoidal current profile
and the Step Angle Number is set to 8, equivalent to the electri-
cal cycle 45° position. This position is defined as the “home”
position. The maximum current in each phase, IPMAX , is defined
by the sense resistor and the Maximum Current setting (bits
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A3981
Automotive, Programmable Stepper Driver
Alternative to the Fault Output flag, the DIAG output can be pro- At the system level the supply voltages and chip temperature are
grammed via the serial interface to output: the stall detect signal,
which goes low when a stall is detected; the phase A PWM-on
signal, which is high during the phase A PWM on-time; or an
analog signal indicating the silicon temperature.
monitored.
Supply Voltage Monitors
The logic supply, the motor supply, and the regulator output are
monitored: the motor supply for overvoltage, and the regulator
output and logic supply for undervoltage.
If required, specific fault information can be determined by read-
ing the diagnostic registers (see Serial Interface section).
• If the motor supply voltage, VBBA and VBBB, goes above the
VBB overvoltage threshold, the A3981 will disable the outputs
and indicate the fault. When the motor supply voltage goes be-
low the VBB overvoltage threshold, the outputs will be re-en-
abled and the fault flag removed. The fault bits in the diagnostic
registers remain set until cleared by a diagnostic registers reset.
The first bit (bit 15) in both diagnostic registers contains a com-
mon Fault Register flag which will be high if any of the fault bits
in either register has been set. This allows a fault condition to be
detected using the serial interface, by simply taking STRn low.
As soon as STRn goes low the fist bit in the diagnostic registers
can be read to determine if a fault has been detected at any time
since the last diagnostic registers reset. In all cases the fault bits
in the diagnostic registers are latched and only cleared after a
diagnostic registers reset.
• If the motor supply voltage, VBBA and VBBB, goes below the
VBB undervoltage threshold, the A3981 will indicate the fault
and reduce the VREG undervoltage threshold to the low level.
When the motor supply voltage goes above the VBB under-
voltage threshold, the VREG undervoltage threshold will be
increased to the high level and the fault flag removed. The fault
bits in the diagnostic registers remain set until cleared by a
diagnostic registers reset.
Note that the Fault Register flag in the diagnostic registers, does
not provide the same function as the Fault Output flag on the
DIAG pin. The Fault Output flag on the DIAG pin provides an
indication that either a fault is present or the outputs have been
disabled due to a short circuit fault. The Fault Register flag sim-
ply provides an indication that a fault has occurred since the last
diagnostic registers reset and has been latched.
• If the output of the internal regulator, VREG , goes below the
VREG undervoltage threshold, the A3981 will disable the
outputs and indicate the fault. When the regulator output rises
above the VREG undervoltage threshold, the outputs will be
re-enabled and the fault flag removed. The fault bits in the diag-
nostic registers remain set until cleared by a diagnostic registers
reset.
• If the logic supply voltage, VDD, goes below the VDD under-
voltage threshold, the A3981 will be completely disabled except
to monitor the VDD voltage level. When the logic supply voltage
rises above the VDD undervoltage threshold, a power-on reset
will take place and all registers will be reset to the default state.
Table 1. Fault Table
Diagnostic
Action
Latched
Disable outputs, set
Fault Register flag
VBB Overvoltage
No
Disable outputs, set
Fault Register flag
Note that both the VREG undervoltage monitor and the VBB
undervoltage monitor indicate a fault by using the same fault
bit, UV, in both Fault registers. The state of the UV fault bit is
determined by the logical OR of the fault output from these two
undervoltage monitors.
VREG Undervoltage
No
Power-down,
full reset
VDD Undervoltage
Temperature Warning
Overtemperature
No
No
No
Set Fault Register flag
Disable outputs, set
Fault Register flag
Temperature Monitors
Disable outputs, set
Fault Register flag
Bridge Short
Yes
Three specific temperature thresholds are provided: a hot
warning, a cold warning, and an overtemperature shutdown. In
addition, the analog internal signal used to determine the chip
temperature can be selected in Configuration Register 1 as the
output on the DIAG pin through the serial interface. The analog
scale is TJ ≈ (VDIAG – VTO ) / AT .
Bridge Open
Stall Detect
Set Fault Register flag
Set ST flag
No
No
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A3981
Automotive, Programmable Stepper Driver
FAULT0 and driving the DIAG output low if the Fault Output
flag is selected. The output is switched off and remains off until a
fault reset occurs.
Hot Warning If the chip temperature rises above the Hot Tem-
perature Warning Threshold, TJWH , the Fault flag will go low and
the Hot Warning bits will be set in the diagnostic registers. No
action will be taken by the A3981. When the temperature drops
below the Hot Temperature Warning Threshold, the Fault flag
will go high but the Hot Warning bits remain set in the diagnostic
registers until reset.
Note that the sense resistor cannot distinguish which low-side
FET is in an overcurrent state. So, if more than one low-side FET
is active when the fault is detected, for example during low-side
recirculation with synchronous rectification, then the shorted con-
nection is determined from the internal PWM state.
Cold Warning If the chip temperature falls below the Cold
Temperature Warning Threshold, TJWC , the Fault flag will go low
and the Cold Warning bits will be set in the diagnostic registers.
No action will be taken by the A3981. When the temperature rises
above the Cold Temperature Warning Threshold, the Fault flag
will go high but the Cold Warning bits remain set in the diagnos-
tic registers until reset.
The actual overcurrent that VOCL represents is determined by the
value of the sense resistor and is typically 2 × ISMAX
.
Short to Ground A short from any of the motor connections
to ground is detected by directly monitoring the current through
each of the high-side FETs in each bridge.
When a high-side FET is in the On state the maximum current
is typically always less than 1 A. In this state, an overcurrent is
determined to exist when the current through the active high-side
Overtemperature Shutdown If the chip temperature rises
above the Overtemperature Shutdown Threshold, TJF , the Fault
flag will go low and the Thermal Shutdown bits will be set in the
diagnostic registers. The A3981 will disable the outputs to try to
prevent a further increase in the chip temperature. When the tem-
perature drops below the Overtemperature Shutdown Threshold,
the Fault flag will go high but the Thermal Shutdown bits remain
set in the diagnostic registers until reset.
FET exceeds the High-Side Overcurrent Threshold, IOCH
.
This overcurrent must be present for at least the Overcurrent
Fault Delay, tSCT, before the short fault is confirmed by setting
the relevant bit in FAULT0 and driving the DIAG output low if
the Fault Output flag is selected. The output is switched off and
remains off until a fault reset occurs.
Bridge and Output Diagnostics
Note that when a short to ground is present the current through
the high-side FET is limited to the High-Side Current Limit,
The A3981 includes monitors that can detect a short to supply or
a short to ground at the motor phase connections. These condi-
tions are detected by monitoring the current from the motor
phase connections through the bridge to the motor supply and to
ground.
ILIMH , during the Overcurrent Fault Delay, tSCT . This prevents
large negative transients at the phase output pins when the out-
puts are switched off.
Shorted Load A short across the load is indicated by concurrent
short faults on both high side and low side.
Low current comparators and timers are provided to help detect
possible open load conditions.
Short Fault Blanking All overcurrent conditions are ignored
for the duration of the Overcurrent Fault Delay, tSCT. The short
detection delay timer is started when an overcurrent first occurs.
If the overcurrent is still present at the end of the short detection
delay time then a short fault will be generated and latched. If the
overcurrent goes away before the short detection delay time is
complete, then the timer is reset and no fault is generated.
Short to Supply A short from any of the motor connections to
the motor supply (VBBA or VBBB) is detected by monitoring the
voltage across the low-side current sense resistor in each bridge.
This gives a direct measurement of the current through the low
side of the bridge.
When a low-side FET is in the On state, the voltage across the
sense resistor, under normal operating conditions, should never
be more than the Maximum Sense Voltage, VSMAX. In this state,
an overcurrent is determined to exist when the voltage across the
sense resistor exceeds the Low-Side Overcurrent Sense Voltage,
This prevents false short detection caused by supply and load
transients. It also prevents false short detections resulting from
current transients generated by the motor or wiring capacitance
when a FET is first switched on.
V
OCL, typically 2 × VSMAX. This overcurrent must be continu-
ously present for at least the Overcurrent Fault Delay, tSCT
before the short fault is confirmed by setting the relevant bit in
,
Short Fault Reset and Retry When a short circuit has been
detected all outputs for the faulty phase are disabled until the
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A3981
Automotive, Programmable Stepper Driver
next occurrence of: the next rising edge on the STEP input, the
RESETn input is pulsed low, or until the diagnostic registers
are reset by writing to one of the registers through the serial
interface. At the next STEP command or after a fault reset, the
Fault Register flag is cleared, the outputs are re-enabled, and the
voltage across the FET is resampled. Note that the diagnostic
registers are not cleared by the rising edge of the STEP input.
ary. This can be due to a mechanical blockage such as an end stop
or it can be due to the step sequence exceeding the motor capabil-
ity for the attached load. Reliable stall detection in a simple step-
per driver is only possible by combining the PWM monitor with
a continuous step sequence at a sufficiently high step rate.
When a motor is stopped or moving slowly there is no back EMF
to impede the current in the phase windings. This allows the
current to rise to the limit quickly and the PWM current control
to activate. However, when a motor is running at speed the back
EMF, generated by the speed of the magnetic poles in the motor
passing the phase windings, acts against the supply voltage and
reduces the rise time of the phase current. Therefore the PWM
current control takes longer to activate. Assuming a constant step
rate, this results in fewer PWM cycles for each step of the motor.
While the fault persists the A3981 will continue this cycle,
enabling the outputs for a short period then disabling the out-
puts. This allows the A3981 to handle a continuous short circuit
without damage. If, while stepping rapidly, a short circuit appears
and no action is taken, the repeated short circuit current pulses
will eventually cause the temperature of the A3981 to rise and an
overtemperature fault will occur.
Open Load Detection Open load conditions are detected
by monitoring the phase current when the phase DAC value
is greater than 31. The Open Load Current Threshold, IOL, is
defined by the OL0 and OL1 bits in the Run register as a percent-
age of the maximum (100%) phase current, IPMAX , defined in the
Phase Current table. The 100% level in the Phase Current table is
defined by the sense resistor value and the contents of the MXI0
and MXI1 bits in Configuration Register 0.
The A3981 uses this difference to detect a motor changing from
continuous stepping to stalled. Two PWM counters, one for each
phase, accumulate the number of PWM cycles when the phase
current is stepped from zero to full current. At the end of each
phase current rise, the counter for that phase is compared to the
counter for the previous current rise, in the opposite phase. If the
difference is greater than the number in the PWM compare regis-
ter, then the ST bit in the diagnostic registers is set. In addition, if
the ST signal is selected as the output on the DIAG pin, then the
pin will go low.
For example:
• if RS = 180 mΩ and VREF = 2 V, then ISMAX = 694 mA
• if MXI1 = 1 and MXI0 = 0, then IPMAX = 520 mA
• if OL1=0 and OL0=1, then IOL = 156 mA
This stall detection scheme assumes a number of factors:
• The motor must be stepping fast enough for the back EMF to
reduce the phase current slew rate. Stall detection reliability
improves as the current slew rate reduces.
The open load current monitor is only active after a blank
time from the start of a PWM cycle. An open load can only be
detected if the DAC value for the phase is greater than 31 and the
current has not exceeded the Open Load Current Threshold for
more than 15 PWM cycles.
• The motor is not being stepped in full step mode.
• The phase current table must conform to the 0% and ±100%
conditions at steps 0, 16, 32, and 48.
The A3981 continues to drive the bridge outputs under an open
load condition and clears the Fault Register flag as soon as the
phase current exceeds the Open Load Current Threshold or the
DAC value is less than 32. The diagnostic registers retain the
open load fault bits, OLA and OLB, and will not be cleared until
RESETn is pulsed low or one of the diagnostic registers is written
through the serial interface.
• The phase current profiles must be the same for both phases.
Although stall detection cannot be guaranteed using this detection
method, good stall detection reliability can be achieved by careful
selection of motor speed, count difference, and by conforming to
the above factors.
In addition to using the integrated features of the A3981, it is
also possible to perform stall detection by examining the PWM
on-time for a single phase using an external microcontroller. In
the A3981 the PWM-on signal for phase A can be selected as the
output on the DIAG pin, by using the serial interface.
Stall Detection A PWM monitor feature is included in the
A3981 to assist in determining the stall condition of the stepper
motor. A stalled motor condition is when the phase currents are
being sequenced to step the motor but the motor remains station-
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A3981
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Serial Interface Description
A three wire synchronous serial interface, compatible with
SPI, can be used to configure and control all the features of the
A3981. A fourth wire can be used to provide diagnostic feedback.
The registers that are accessible through the serial interface are
defined in table 2.
Writing to Configuration and Control Registers
When writing to the serial register, data is received on the SDI
pin and clocked through a shift register on the rising edge of the
clock signal input on the SCK pin. STRn is normally held high,
and is only brought low to initiate a serial transfer. No data is
clocked through the shift register when STRn is high, thus allow-
ing multiple SDI slave units to use common SDI, SCK, and SDO
connections. Each independent slave requires a dedicated STRn
connection.
The A3981 can be operated without using the serial interface,
by using the default configuration and control register settings
and the STEP and DIR logic inputs for motor control. However,
application-specific configurations are only possible by setting
the appropriate register bits through the serial interface. In addi-
tion to setting the configuration bits, the serial interface can also
be used to control the motor directly.
The serial data word has 16 bits, MSB input first. After 16 data
bits have been clocked into the shift register, STRn must be taken
high to latch the data into the selected register. When this occurs,
the internal control circuits act on the new configuration and
control data, and the diagnostic registers are reset.
The serial interface timing requirements are specified in the Elec-
trical Characteristics table, and illustrated in figure 1.
Table 2. Serial Register Definition*
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Configuration and Control Registers (Write)
TOF2 TOF1 TOF0
FRQ2 FRQ1 FRQ0
Configuration
Register 0
(CONFIG0)
SYR
1
MS1
0
MS0
0
MXI1
1
MXI0 PFD2 PFD1 PFD0 TBK1 TBK0
PWM
0
0
0
1
1
0
0
0
1
1
1
0
Configuration
Register 1
(CONFIG1)
OSC
0
TSC1 TSC0
CD3
1
CD2
0
CD1
0
CD0 DIAG1 DIAG0
0
1
1
1
0
1
0
0
0
0
0
1
0
0
0
0
EN
0
OL1
0
OL0
1
HLR SLEW BRK
DCY1 DCY0
SC5
0
SC4
0
SC3
0
SC2
0
SC1
0
SC0
0
Run Register
(RUN)
0
1
0
0
1
Table Load
Register
(TBLLD)
PTP
1
PT5
0
PT4
0
PT3
0
PT2
0
PT1
0
PT0
0
0
0
0
0
0
0
0
Diagnostic Registers (Read)
Fault
Register 0
(FAULT0)
FF
TW1
TW0
OV
OV
UV
UV
ST
ST
OLB
OLB
OLA
OLA
BML
0
BMH
0
BPL
SA5
BPH
SA4
AML
SA3
AMH
SA2
APL
SA1
APH
SA0
Fault
Register 1
(FAULT1)
FF
TW1
TW0
*Power-on reset value shown below each input register bit.
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A3981
Automotive, Programmable Stepper Driver
If there are more than 16 rising edges on SCK, or if STRn goes
high and there are fewer than 16 rising edges on SCK, the write
will be cancelled without writing data to the configuration and
control registers. In addition the diagnostic registers will not be
reset. Instead the FF bit will be set to 1 in the diagnostic registers,
to indicate a data transfer error.
remains high while STRn is low. When STRn goes high the trans-
fer will be terminated and SDO will go into its high impedance
state. Configuration and Run Registers
These registers are used for system configuration and motor con-
trol. Access is described in the section Writing to Configuration
and Control Registers, above.
The first two bits of the serial word are used to select the register
to be written. This provides access to four writable registers:
CONFIG0 sets certain system parameters, and CONFIG1 sets
system and diagnostic output selection parameters. The RUN
register contains motor drive settings used to control the motor
movement and phase current.
• The Configuration registers are used for system configuration:
CONFIG0 for system parameters, and CONFIG1 for system
and diagnostic parameters.
Phase Table Load Register
• The RUN register contains motor drive settings used to control
the motor movement and phase current.
This is one of the configuration and control registers, accessed
when both address bits are 1. It can be used to write a sequence
of values to the phase current table in the A3981. This allows
the current at each Step Angle Number to be tailored to suit
the microstep current profile requirements of a specific motor.
In most cases this feature will not be required and the default
sinusoidal profile will suffice. However for some motor / load
combinations, altering the current profile can improve torque
ripple, resulting in lower mechanical vibration and noise.
• The fourth writable register, TBLLD, is a port that allows se-
quential loading of the 16 distinct phase current table settings.
Reading from Diagnostic Registers
In addition to the writable registers there are two diagnostic
registers. The first eight (most significant) bits of both diagnostic
registers contain the same flags, only the last eight (least signifi-
cant) bits differ, as follows:
Although the phase current table contains 64 entries for each of
two phases, only 16 distinct values are required. These 16 values
correspond to one quadrant of the table for a single phase, and
they are repeated for the other three quadrants and again for the
four quadrants of the other phase. So each of the 16 values writ-
ten to the Phase Table Load register are written to 8 locations in
the phase current table.
• FAULT0 contains the short-circuit fault flags
• FAULT1 contains the present Step Angle Number
Each time a configuration and control register is written, one
of the diagnostic registers can be read, MSB first, on the serial
output pin, SDO (see timing in figure 1). FAULT1 is made the
active register for serial transfer and output on SDO only while
CONFIG1 is being written, that is, only when the first bit of the
input word is 0 and the second bit is 1. FAULT0 is the active
register for serial transfer and output on SDO during writes to any
other configuration or control register.
The 16 values must be entered by sequential writes to the Phase
Table Load register. The first write to the register after writing to
any other register, or after a reset (RESETn pulse low or power-
on), puts that value, PT[5..0], into the first phase table address,
a 6-bit field defined as PT(0). Subsequent writes put values into
successive addresses: PT(1), PT(2), and so forth up to PT(15).
After the sixteenth value has been written, no more values are
accepted and any writes to the Phase Table Load register are
ignored. As each value is received, it is effectively distributed to
all eight required locations in the phase current table.
When STRn goes low to start a serial write, SDO comes out of its
high impedance state and outputs the serial register Fault Register
flag. This allows the main controller to poll the A3981 through
the serial interface to determine if a fault has been detected. If no
faults have been detected then the serial transfer may be termi-
nated without generating a serial read fault by ensuring that SCK
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A3981
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An optional simple odd parity scheme is included to provide
some measure of error checking, if required. Each 6-bit value
can be supplemented with an additional parity bit, PTP, to ensure
an odd number of 1s in the transmission. This is checked by the
A3981 and if a the number of 1s in the value plus parity bit is not
odd, the FF bit will be set and the SDO pin will go high the next
time STRn is taken low, indicating a parity error. That data will
still be written to the next phase table value address; it is incum-
bent upon the external controller to take action, if required.
Diagnostic Registers
The diagnostic registers comprise two read-only fault data regis-
ters. Access is described in the section Reading from Diagnostic
Registers, above.
The diagnostic registers contain fault flags for each fault condi-
tion and are reset to all 0s on the completion of each serial access.
They are also reset to all 0s each time the RESETn input is low
for longer than the Reset Pulse Width, tRST . FAULT0 is set to
all 1s at power-up or after a power-on reset. This indicates to the
external controller that a power-on reset has taken place and all
registers have been reset. Note that a power-on reset only occurs
when the VDD supply rises above its undervoltage threshold.
If the write sequence is broken (by a reset, by writing to another
register, or by a data transfer error) before the sequence has been
completed, then the phase table value address will be reset to
PT(0). If it is required to load the table, then the entire 16-value
sequence must be sent.
Power-on reset function is not affected by the state of the motor
supply or VREG
.
After loading, although the phase current table is volatile, a reset
using a low pulse on the RESETn pin does not corrupt the table.
The table is only reset to default values on a power-on reset.
The first bit in both registers is the Fault Register flag, FF. This
is high if any bits in FAULT0 are set, or if a serial write error or
parity error has occurred.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
16
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
15
0
14
0
13
12
11
10
9
8
7
6
5
4
3
2
1
0
TOF2
TOF1
TOF0
SYR
1
MS1
0
MS0
0
MXI1
1
MXI0
1
PFD2 PFD1 PFD0
TBK1
0
TBK0
1
PWM
0
FRQ2 FRQ1 FRQ0
CONFIG 0
1
0
0
1
1
0
Configuration Register 0
TOF[2..0]
Off time (only valid when PWM bit = 0)
Replaces FRQ bits
Assumes 4-MHz clock
SYR
Synchronous rectification
Synchronous Rectification
Diode recirculation
SYR
Default
0
1
TOF2
TOF1
TOF0 Off Time
Default
Synchronous
D
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
20 μs
24 μs
28 μs
32 μs
36 μs
40 μs
44 μs
48 μs
MS[1..0] Microstep mode for external STEP input control
MS1
MS0
Microstep Mode
Default
0
0
Full Step
D
0
1
Half Step
1
0
Quarter Step
Sixteenth Step
1
1
D
MXI[1..0] Max phase current as a percentage of ISMAX
MXI1
MXI0
Maximum Current
Default
FRQ[2..0]
Frequency (only valid when PWM bit = 1)
Replace TOF bits
Assumes 4-MHz clock
0
0
1
1
0
1
0
1
25%
50%
75%
FRQ2
FRQ1
FRQ0 Period / Frequency
Default
100%
D
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
24 μs / 41.7 kHz
32 μs / 31.3 kHz
40 μs / 25.0 kHz
46 μs / 21.7 kHz
52 μs / 19.2 kHz
56 μs / 17.9 kHz
60 μs / 16.7 kHz
64 μs / 15.6 kHz
PFD[2..0]
Fast decay time for mixed decay
Assumes 4-MHz clock
PFD2
PFD1
PFD0 Fast Decay Time
Default
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
2 μs
3 μs
D
4 μs
6 μs
8 μs
D
PWM
PWM configuration
10 μs
14 μs
20 μs
PWM MODE
Default
0
1
Fixed off-time
D
Fixed frequency
TBK[1..0]
Blank Time
Assumes 4-MHz clock
TBK1
TBK0 Blank Time
Default
0
0
1
1
0
1
0
1
1 μs
1.5 μs
2.5 μs
3.5 μs
D
Allegro MicroSystems, Inc.
115 Northeast Cutoff
17
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
15
0
14
1
13
12
11
10
0
9
0
8
0
7
0
6
0
5
4
3
2
1
0
OSC
0
TSC1 TSC0
CD3
1
CD2
0
CD1
0
CD0
0
DIAG1 DIAG0
CONFIG 1
1
0
0
0
EN
0
OL1
0
OL0
1
HLR
0
SLEW
1
BRK
0
DCY1 DCY0
SC5
0
SC4
0
SC3
0
SC2
0
SC1
0
SC0
0
RUN
1
0
0
1
Configuration Register 1
Run Register
Phase current enable
OR with ENABLE pin
OSC
Selects clock source
Clock Source
Internal
EN
OSC
Default
0
1
D
EN
0
Phase Current Enable
Default
External
Output bridges disabled if ENABLE
pin = 0
D
1
Output bridges enabled
Overcurrent fault delay
TSC[1..0] Assumes 4-MHz clock
Open load current threshold as a percentage of
OL[1..0] maximum current defined by ISMAX and MXI[1..0]
TSC1
TSC0 Detect Delay Time
Default
0
0
1
1
0
1
0
1
0.5 μs
1 μs
OL1
OL0
Open Load Current
Default
0
0
20%
30%
40%
50%
2 μs
D
0
1
D
3 μs
1
0
1
1
PWM count difference for ST detection
CD[3..0] Default to 8
HLR
Selects slow decay and brake recirculation path
HLR
0
Recirculation Path
Default
DIAG[1..0] Selects signal routed to DIAG output
High side
D
DIAG1 DIAG0 Signal on DIAG Pin
Default
1
Low side
0
0
1
1
0
1
0
1
Fault–low true
ST–low true
D
SLEW
Slew rate control
SLEW Slew Rate Control
Default
PWM-on, Phase A
Temperature
0
1
Disable
Enable
D
BRK
Brake enable
Brake
BRK
0
Default
Normal operation
Brake active
D
1
DCY[1..0] Decay mode selection
DCY1
DCY0 Decay Mode
Default
0
0
1
1
0
1
0
1
Slow
Mixed—PFD fixed
Mixed—PFD auto
Fast
D
SC[5..0]
Step change number
2’s complement format
Positive value increases Step Angle Number
Negative value decreases Step Angle Number
Allegro MicroSystems, Inc.
115 Northeast Cutoff
18
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
15
1
14
1
13
0
12
0
11
0
10
0
9
0
8
0
7
0
6
5
4
3
2
1
0
PTP
0
PT5
1
PT4
0
PT3
0
PT2
0
PT1
0
PT0
0
TBLLD
Fault 0
Fault 1
FF
FF
TW1
TW1
TW0
TW0
OV
OV
UV
UV
ST
ST
OLB
OLB
OLA
OLA
BML
0
BMH
0
BPL
SA5
BPH
SA4
AML
SA3
AMH
SA2
APL
SA1
APH
SA0
Table Load Register
PTP
PT(0..15)[5..0] Phase Table Value
Fault Register 0
FF
Fault register flag
Parity bit (odd parity)
TW1
TW0
OV
Temperature diagnostic
Temperature diagnostic
Overvoltage on VBB detected
Table Load Register Mapping
Step Angle Number
UV
Undervoltage on VREG or VBB detected
Stall detected
Phase A
Phase B
ST
0%
0
1
2
3
4
5
6
7
8
9
32
16
48
OLB
OLA
BML
BMH
BPL
BPH
AML
AMH
APL
APH
Open load detected on phase B
Open load detected on phase A
PT(0)
PT(1)
PT(2)
PT(3)
PT(4)
PT(5)
PT(6)
PT(7)
PT(8)
PT(9)
PT(10)
PT(11)
PT(12)
PT(13)
PT(14)
PT(15)
31 33 63 15 17 47 49
30 34 62 14 18 46 50
29 35 61 13 19 45 51
28 36 60 12 20 44 52
27 37 59 11 21 43 53
26 38 58 10 22 42 54
Overcurrent detected on BM output low side
Overcurrent detected on BM output high side
Overcurrent detected on BP output low side
Overcurrent detected on BP output high side
Overcurrent detected on AM output low side
Overcurrent detected on AM output high side
Overcurrent detected on AP output low side
Overcurrent detected on AP output high side
25 39 57
24 40 56
23 41 55
9
8
7
6
5
4
3
2
1
0
23 41 55
24 40 56
25 39 57
26 38 58
27 37 59
28 36 60
29 35 61
30 34 62
31 33 63
32
10 22 42 54
11 21 43 53
12 20 44 52
13 19 45 51
14 18 46 50
15 17 47 49
Fault Register 1
16
48
FF
Fault register flag
TW1
TW0
OV
Temperature diagnostic
Temperature diagnostic
Overvoltage on VBB detected
Undervoltage on VREG or VBB detected
Stall detected
UV
ST
OLB
OLA
Open load detected on phase B
Open load detected on phase A
SA[0..5] Step Angle Number read back
OLA Open load detected on phase A
TW[0..1] Temperature diagnostic
TW1
TW0
Thermal Indicator
0
0
No Fault
0
1
Cold Warning
1
0
Hot Warning
1
1
Overtemperature Shutdown
Allegro MicroSystems, Inc.
115 Northeast Cutoff
19
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
Application Information
the phase angle within the full 360° electrical cycle and is called
the Step Angle Number. This is illustrated in figure 3.
Motor Movement Control
The A3981 provides two independent methods to control the
movement of a stepper motor. The simpler is the Step and Direc-
tion method, which only requires two control signals to control
the stepper motor in either direction. The other method is through
the serial interface, which provides more flexible control capa-
bility. Both methods can be used together (although it is not
common), provided the timing restrictions of the STEP input in
relation to the STRn input are preserved.
Figure 5 shows the contents of the phase current table as a phase
diagram. The phase B current, IB, from the phase current table, is
plotted on horizontal axis and the phase A current, IA, is plotted
on the vertical axis. The resultant motor current at each microstep
is shown as numbered radial arrows. The number shown corre-
sponds to the one-sixteenth microstep Step Angle Number in the
phase current table.
Figure 4 shows an example of calculating the resultant motor
current magnitude and angle for step number 28. The target is to
have the magnitude of the resultant motor current be 100% at all
microstep positions. The relative phase currents from the phase
current table are:
Phase Table and Phase Diagram
The key to understanding both of the available control methods
lies in understanding the Phase Current table (table 7). This table
contains the relative phase current magnitude and direction for
each of the two motor phases at each microstep position. The
maximum resolution of the A3981 is one-sixteenth microstep.
That is 16 microsteps per full step. There are 4 full steps per elec-
trical cycle, so the phase current table has 64 microstep entries.
The entries are numbered from 0 to 63. This number represents
IA = 37.50%
IB = –92.19%
Assuming a full scale (100%) current of 1A means that the two
phase currents are:
IA
IA = 0.3750 A
IB = -0.9219 A
17 16 15
18
14
19
13
20
12
21
11
22
10
23
9
24
8
25
7
26
27
28
29
30
6
5
4
IA
24
3
25
26
2
31
1
27
32
33
0
IB
IA28
=37.5%
28
29
30
31
63
62
61
60
59
58
34
28
=
35
36
37
38
39
40
157.9°
IB
32
57
–
92.19%
=
IB28
56
41
55
42
54
43
53
44
52
45
51
46
50
47 48 49
Figure 3. A3981 Phase Current table as a phase diagram; values shown
are referred to as the Step Angle Number
Figure 4. Calculation of resultant motor current
Allegro MicroSystems, Inc.
115 Northeast Cutoff
20
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
The magnitude of the resultant will be the square root of the sum
of the squares of these two currents:
and MS1 can be set to select full step, half step, quarter step, or
sixteenth step microstepping as follows:
| I28 | I A2 IB2 0.1406 0.8499 0.9953 (A)
MS1
MS0
Microstep Mode
0
0
0
1
Full step
Half step
So the resultant current magnitude is 99.53% of full scale. This
is within 0.5% of the target (100%) and is well within the ±5%
accuracy of the A3981.
1
1
0
1
Quarter step
Sixteenth step
The reference angle, zero degrees (0°), within the full electrical
cycle (360°), is defined as the angle where IB is at +100% and IA
is zero. Each full step is represented by 90° in the electrical cycle
so each one-sixteenth microstep is: 90°/16 steps = 5.625°. The
target angle of each microstep position with the electrical cycle
is determined by the product of the Step Angle Number and the
angle for a single microstep. So for the example of figure 5:
MS0 and MS1 can be accessed through the serial interface or
directly on pins 13 and 12 respectively. The values of MS0 and
MS1 are defined as the logical OR of the logic level on the input
pins and the value in Configuration Register 0. The bits in the
register default to 0 so if the serial interface is not used then MS0
and MS1 are defined by the input pins alone. If only the serial
interface is used to set the microstep resolution, then the MS0 and
MS1 logic input pins should be tied low to ensure that the register
retains full control over all resolutions. Note that the microstep
select variables, MS0 and MS1, are only used with the STEP
input; they can be ignored if the motor is fully controlled through
the serial interface.
A28(TARGET ) 285.625 157.5
The actual angle is calculated using basic trigonometry as:
I
A28( ACTUAL) 180 tan1
A28
IB28
180
22.1
157.9
In sixteenth step mode the translator simply increases or
So the angle error is only 0.4°. Equivalent to about 0.1% error in
360° and well within the current accuracy of the A3981.
decreases the Step Angle Number on each rising edge of the
STEP input, depending on the logic state of the DIR input. In the
other three microstep resolution modes the translator outputs spe-
cific Step Angle Numbers as defined in the phase current table.
Note that each phase current in the A3981 is defined by a 6-bit
DAC. This means that the smallest resolution of the DAC is
100 / 64 = 1.56% of the full scale, so the A3981 cannot produce
a resultant motor current of exactly 100% at each microstep. Nor
can it produce an exact microstep angle. However, as can be seen
from the calculations above, the results for both are well within
the specified accuracy of the A3981 current control. The resultant
motor current angle and magnitude are also more than precise
enough for all but the highest precision stepper motors.
Full step uses four of the entries in the phase current table. These
are 8, 24, 40, and 56 as shown in figure 7. Note that the four posi-
tions selected for full step are not the points at which only one
current is active, as would be the case in a simple on-off full step
driver. There are two advantages in using these positions rather
than the single full current positions. With both phases active, the
power dissipation is shared between two drivers. This slightly
improves the ability to dissipate the heat generated and reduces
the stress on each driver.
With the phase current table, control of a stepper motor is simply
a matter of increasing or decreasing the Step Angle Number
to move around the phase diagram of figure 5. This can be in
predefined multiples using the STEP input, or it can be variable
using the serial interface.
The second reason is that the holding torque is slightly improved
because the forces holding the motor are mainly rotational rather
than mainly radial.
Using Step and Direction Control
Half step uses eight of the entries in the phase current table.
These are 0, 8, 16, 24, 32, 40, 48, and 56 as shown in figure 8.
The STEP input moves the motor at the microstep resolution
defined by the two microstep select variables, MS0 and MS1,
logic levels. The DIR input defines the motor direction. These
inputs define the output of a translator which determines the
required Step Angle Number in the phase current table. The MS0
Quarter step uses sixteen of the entries in the phase current table.
These are 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56,
and 60 as shown in figure 9.
Allegro MicroSystems, Inc.
115 Northeast Cutoff
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Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
In half step and in quarter step, the single phase active positions
are used to preserve symmetry. However, if the motor is required
to stop with a significant holding torque for any length of time
it is recommended that the 45° positions be used; those are Step
Angle Numbers 8, 24, 40, and 56, as used with full-step resolu-
tion.
control the output of the A3981:
Mode
Full
Step Angle Numbers used
8, 24, 40, 56
Half
0, 8, 16, 24, 32, 40, 48, 56
Quarter
0, 4, 8, 12, 16, 20, 24, 28, 32,
36, 40, 44, 48, 52, 56, 60
The following table summarizes the Step Angle Numbers used
for the four resolutions available when using the STEP input to
Sixteenth
All
The microstep select inputs can be changed between each rising
edge of the STEP input. The only restriction is that the MSO and
MS1 logic inputs must comply with the set-up and hold timing
constraints. When the microstep resolution changes, the A3981
moves to the next available Step Angle Number on the next rising
edge of the STEP input. For example, if the microstep mode is
sixteenth and the present Step Angle Number is 59, then with the
direction forwards (increasing Step Angle Number), changing
to quarter step mode will cause the phase number to go to 60 on
the next rising edge of the STEP input. If instead the microstep
mode is changed to half step then the phase number will go to 0
on the next rising edge of the STEP input. If the microstep mode
is changed to full step then the phase number will go to 8 on the
next rising edge of the STEP input.
IA
24
8
IB
40
56
Figure 5. Full-step phase diagram using STEP input
IA
16
24
8
Control Through the Serial Interface
The A3981 provides the ability to directly control the motor
movement using only the serial interface. In fact, all features
of the A3981, except sleep mode, can be controlled through the
serial interface thus removing the requirement for individual
control inputs. This can reduce the interface requirement from
multiple I/O signals to a single four wire interface.
32
0
IB
40
56
48
Motor movement is controlled using the serial interface by
increasing or decreasing the Step Angle Number. Note that the
maximum value of the Step Angle Number is 63 and the mini-
mum number is 0.Therefore, any increase or decrease in the
microstep number is performed using modulo 64 arithmetic. This
means that increasing a Step Angle Number of 63 by 1 will pro-
duce a Step Angle Number of 0. Increasing by two from 63 will
produce 1 and so on. Similarly in the reverse direction, decreasing
a Step Angle Number of 0 by 1 will produce a Step Angle Num-
ber of 63. Decreasing by two from 0 will produce 62 and so on.
Figure 6. Half-step phase diagram using STEP input
IA
16
20
12
24
8
28
32
4
0
IB
36
60
The least significant six bits of the Run register, bits 0 to 5, are
the step change number, SC[5..0]. This number is a two’s comple-
ment number that is added to the Step Angle Number causing it
to increase or decrease. Two’s complement is the natural integer
number system for most microcontrollers. This allows standard
40
56
44
52
48
Figure 7. Quarter-step phase diagram using STEP input
Allegro MicroSystems, Inc.
115 Northeast Cutoff
22
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
arithmetic operators to be used, within the microcontroller, to
determine the size of the next step increment. Table 6 shows the
binary equivalent of each decimal number between –16 and +16.
Table 6. Binary Equivalents
2’s
2’s
Decimal
Complement
000000
000001
000010
000011
000100
000101
000110
000111
001000
001001
001010
001011
001100
001101
001110
001111
Decimal
Complement
Each increase in the Step Angle Number represents a forwards
movement of one-sixteenth microstep. Each decrease in the Step
Angle Number represents a reverse movement of one-sixteenth
microstep.
0
1
–1
–2
111111
111110
111101
111100
111011
111010
111001
111000
110111
110110
110101
110100
110011
110010
110001
110000
2
3
–3
To move the motor one full step, the Step Angle Number must be
increased or decreased by 16. To move the motor one half step,
the Step Angle Number must be increased or decreased by 8. For
quarter step the increase or decrease is 4 and for eighth step, 2.
4
–4
5
–5
6
–6
7
–7
So, for example, to continuously move the motor forwards in
quarter-step increments, the number 4 (000100) is repeatedly
written to SC[5..0] through the serial interface Run register (see
figure 10). To move the motor backwards in quarter step incre-
ments, the number -4 (111100) is repeatedly written to SC[5..0]
(see figure 11). The remaining bits in the Run register should be
set for the required configuration and sent with the step change
number each time.
8
–8
9
–9
10
11
12
13
14
15
16
–10
–11
–12
–13
–14
–15
–16
The step rate is controlled by the timing of the serial interface.
It is the inverse of the step time, tSTEP, shown in figure 10. The
motor step only takes place when the STRn goes from low to
010000
+4
1 0 1 0 1 0 1 0 1 0 0 0 0 1 0 0
SDI
SCK
STRn
tSTEP
Figure 8. Serial interface sequence for quarter step in forward direction
-4
1 0 1 0 1 0 1 0 1 0 1 1 1 1 0 0
SDI
SCK
STRn
Figure 9. Serial interface sequence for quarter step in reverse direction
Allegro MicroSystems, Inc.
115 Northeast Cutoff
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Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A3981
Automotive, Programmable Stepper Driver
high when writing to the Run register. The motor step rate is
constant. The current at each Step Angle Number can be set to suit
therefore determined by the timing of the rising edge of the STRn the microstep current profile requirements of a specific motor.
input. The clock rate of the serial interface, defined by the fre-
Note: This is an advanced feature of the A3981, which will not be
quency of the SCK input, has no effect on the step rate.
required for most applications. In general the default sinusoidal
profile will suffice and therefore the phase current table does not
have to be loaded.
Using the Phase Table Load Capability
Torque Ripple Reduction
Loading the Phase Current Table
The performance and audible noise of any motor drive system is
The full phase current table in the A3981 contains one 6-bit value
defined, to a large extent, by the torque ripple generated by both
for each phase, at each microstep position. With 16 microsteps
the motor and the load. In most cases, when using a stepper motor
per mechanical step, 4 mechanical steps per electrical cycle, and
as the mechanical drive, the torque ripple of the load is not related
2 phases this gives a total of 128 values. However, due to symme-
to the mechanical steps of the motor and must be reduced by
try, described below, this reduces to 17 independent values, one
means unrelated to the motor and its drive system. However, for
of which is always zero. The remaining 16 values can be loaded
stepper motors in particular, torque ripple produced by the motor
sequentially through the serial interface using the Phase Table
can be reduced by improvements in the mechanical design of the
Load register. Figure 10 shows the default phase table values
motor and by improvements in the phase current control system.
plotted by Step Angle Number. Similar information is provided in
Torque ripple will naturally be high when driving a stepper motor
in full step mode, due to the nature of stepping. However the
torque ripple can be reduced by using microstepping. Increas-
ing the number of microsteps per mechanical step will result in
reduced torque ripple. This is one of the major reasons for using
microstepping.
table 7.
The diagram in figure 10 is marked with four quadrants, Q1 to
Q4. The set of phase table values is the same in each quadrant in
each phase. Consider phase A (bottom graph), quadrant 1 (Q1).
This contains Step Angle Numbers 0 to 15. The default values
in these 16 positions are selected to produce one quarter of a
sinusoid.
In the majority of cases the standard sinusoidal, microstep current
profile will be sufficient to achieve a good performance with
a good quality motor. In a few cases, further improvements in
torque ripple performance may be achieved by modifying the
microstep current profile to more closely match the motor charac-
teristics. This is usually only necessary for higher quality, higher
power stepper motors.
Now consider the next quadrant (Q2) of phase A. The sequence
of values in this quadrant form a mirror image, by Step Angle
Number, of the values in Q1 so the same values are used but
entered in the reverse sequence.
The following table shows the Step Angle Number in the first
row increasing from 0 to 15, from left to right, and the default
values also increasing from left to right in the second row. These
first two rows are the entries for Q1 of phase A.
When using microstepping, the torque ripple is defined by the
variation in torque at each microstep. In a hybrid stepper motor
this is mostly determined by the mechanical construction of the
motor, particularly the shape of the teeth on the poles of the sta-
tor. The shape of these teeth determine the variation in the torque
constant, the ratio between current and torque, as the motor
rotates. The variation in the torque constant can be seen by mea-
suring the back EMF of the motor when being driven as a genera-
tor, that is when the shaft is driven by external means and the
phase voltage is monitored. The back EMF represents the motor
constant, which is essentially proportional to the torque constant.
Step
0
0
1
5
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Q1
Q2
Value
11 18 23 29 35 40 44 48 52 55 58 60 62 63
Step
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
11 18 23 29 35 40 44 48 52 55 58 60 62 63 63
Value
5
The second two rows are the entries for Q2 of phase A. The Step
Angle Number in the third row increases from 16 to 31, this time
from right to left, but the same default values still increase from
left to right. A single value is therefore placed in more than one
location in the table. Shown outlined above, steps 4 and 28 both
contain the value 23.
If such torque ripple reduction measures are required, the A3981
provides the ability to modify the microstep current profile by
programming the internal phase current table through the serial
interface. The modified profile is then used, in place of the default The same principal can be applied to Q3 and Q4 of phase A. In
sinusoidal profile, to compensate for any variation in motor torque this case the mirror image is in the horizontal axis, about the zero
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A3981
Automotive, Programmable Stepper Driver
63
62
60
58
55
52
48
44
40
35
29
IB
(forwards)
23
18
(DAC value)
11
5
Q4
Q1
Q2
Q3
5
11
18
23
29
35
40
44
IB
(reverse)
(DAC value)
48
52
55
58
60
62
63
Step Angle Number
63
62
60
58
55
52
48
44
40
35
29
IA
(forwards)
23
18
(DAC value)
11
5
Q4
Q1
Q2
Q3
5
11
18
23
29
35
40
44
IA
(reverse)
(DAC value)
48
52
55
58
60
62
63
Figure 10. Default phase table values
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115 Northeast Cutoff
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A3981
Automotive, Programmable Stepper Driver
reference value. Although the current in Q3 and Q4 for phase A is Shown outlined above, steps 4, 28, 36, and 60 all contain the
value 23.
effectively negative, the negation is provided by controlling the
direction of the current. The current control scheme still operates
using positive values.
The other phase, phase B, uses the same values as phase A but
shifted back by 16 Step Angle Numbers. The full distribution of
the value entered in step 4 of phase A is highlighted in figure 12
(and shown in table 7). This single value is used in a total of eight
locations. The same distribution of values applies to all the values
in steps 1 to 15. These values are defined in the A3981 as PT(0)
to PT(14), respectively.
As shown below, the table of values can be extended to include
Q3 and Q4 with the current direction indicated in the last column.
Note that the same value is now applied to four locations in the
full 360-degree electrical cycle.
Step
0
0
1
5
2
3
4
5
6
7
8
9
10 11 12 13 14 15
There are two exceptions to this data distribution principal. These
are the zero value and the maximum value:
Q1 FWD
Q2 FWD
Q3 Rev
Q4 Rev
Value
11 18 23 29 35 40 44 48 52 55 58 60 62 63
Step
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
11 18 23 29 35 40 44 48 52 55 58 60 62 63 63
• The values in phase A steps 0 and 32 and phase B steps 16 and
48 are always set to zero and cannot be programmed.
Value
5
Step 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
• The maximum value, PT(15), is distributed to only two Step
Angle Numbers in each phase. These are the points in the cycle
where the peak current is required, namely phase A steps 16 and
48 and phase B steps 0 and 32.
Value
0
5 11 18 23 29 35 40 44 48 52 55 58 60 62 63
Step
63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48
11 18 23 29 35 40 44 48 52 55 58 60 62 63 63
Value
5
Table 7. Phase Current Table (default, power-on content)
Phase Current
(% of IPMAX
Step
Angle
Phase Current
(% of IPMAX
Step
Angle
Step Angle Number
)
Phase
DAC
Step Angle Number
)
Phase
DAC
Full 1/2 1/4 1/8 1/16
A
B
A
B
A
B
Full 1/2 1/4 1/8 1/16
A
B
A
B
A
B
0
1
2
3
4
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
0.00 100.00
9.38 100.00
0.0
5.4
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
5
63
63
4
5
6
7
0
8
9
16 32
33
17 34
35
18 36
37
19 38
39
0.00 -100.00 180.0
-9.38 -100.00 185.4
-18.75 -98.44 190.8
-29.69 -95.31 197.3
-37.50 -92.19 202.1
-46.88 -87.50 208.2
-56.25 -82.81 214.2
-64.06 -76.56 219.9
-70.31 -70.31 225.0
-76.56 -64.06 230.1
-82.81 -56.25 235.8
-87.50 -46.88 241.8
-92.19 -37.50 247.9
-95.31 -29.69 252.7
-98.44 -18.75 259.2
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
5
63
63
18.75
29.69
37.50
46.88
56.25
64.06
70.31
76.56
82.81
87.50
92.19
95.31
98.44
98.44
95.31
92.19
87.50
82.81
76.56
70.31
64.06
56.25
46.88
37.50
29.69
18.75
9.38
10.8
17.3
22.1
28.2
34.2
39.9
45.0
50.1
55.8
61.8
67.9
72.7
79.2
84.6
90.0
95.4
11 62
18 60
23 58
29 55
35 52
40 48
44 44
48 40
52 35
55 29
58 23
60 18
62 11
11 62
18 60
23 58
29 55
35 52
40 48
44 44
48 40
52 35
55 29
58 23
60 18
62 11
0
2
10 20 40
41
9
10
11
12
13
14
21 42
43
11 22 44
45
23 46
15 100.00
16 100.00
17 100.00
18
19
63
63
63
5
0
5
47 -100.00
-9.38 264.6
0.00 270.0
9.38 275.4
18.75 280.8
29.69 287.3
37.50 292.1
46.88 298.2
56.25 304.2
64.06 309.9
70.31 315.0
76.56 320.1
82.81 325.8
87.50 331.8
92.19 337.9
95.31 342.7
98.44 349.2
63
63
63
5
0
5
0.00
-9.38
12 24 48 -100.00
49 -100.00
98.44 -18.75 100.8
95.31 -29.69 107.3
92.19 -37.50 112.1
87.50 -46.88 118.2
82.81 -56.25 124.2
76.56 -64.06 129.9
70.31 -70.31 135.0
64.06 -76.56 140.1
56.25 -82.81 145.8
46.88 -87.50 151.8
37.50 -92.19 157.9
29.69 -95.31 162.7
18.75 -98.44 169.2
9.38 -100.00 174.6
0.00 -100.00 180.0
62 11
60 18
58 23
55 29
52 35
48 40
44 44
40 48
35 52
29 55
23 58
18 60
11 62
25 50
-98.44
-95.31
-92.19
-87.50
-82.81
-76.56
-70.31
-64.06
-56.25
-46.88
-37.50
-29.69
-18.75
62 11
60 18
58 23
55 29
52 35
48 40
44 44
40 48
35 52
29 55
23 58
18 60
11 62
51
10 20
21
11 22
23
12 24
25
13 26
27
14 28
29
15 30
31
13 26 52
53
27 54
55
1
3
14 28 56
57
29 58
59
15 30 60
61
31 62
63
5
0
63
63
-9.38 100.00 354.6
0.00 100.00 0.0
5
0
63
63
16 32
0
0
0
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A3981
Automotive, Programmable Stepper Driver
Each of the 16 values written to the phase table is a 6-bit num-
ber that determines the current trip point for the associated step.
The highest value, 63, represents the maximum phase current,
Phase Current Table Programming Example
As an example of programming the phase current table, consider
the current profile shown in figure 11. This shows a profile where
the torque from each phase is required to be relatively higher
at the detent points, that is, the points where only one phase is
active. (This current profile does not relate to any specific motor,
it is only shown as an example.)
I
PMAX, defined in the section of the specification on phase cur-
rent control. Other numbers represent a percentage of IPMAX . For
example, the number 23 sets the phase current trip point to 23/63
= 36.51% of IPMAX
.
Figure 13 shows the required current for each phase at each
Step Angle Number as a percentage of the maximum phase
current, IPMAX , defined above. The waveform conforms to the
required symmetry and zero crossing restrictions, so the profile
for phase A for Step Angle Numbers from 0 to 16 (outlined and
shaded) can be used to determine the phase table contents.
There are two restrictions when using the phase table load capa-
bility:
• The required current profile must conform to the symmetry
shown in figure 10. The forward (positive) current part must be
symmetrical about Step Angle Number16 for phase A and about 0
for phase B. The reverse (negative) current part must be sym-
metrical about Step Angle Number 48 for phase A and about 32
for phase B. The forward and reverse profiles for each phase must
be the same.
The first step is to digitize the profile into microsteps and the
percentage values into 6-bit numbers, as shown in figure 12.
At each of the one-sixteenth microsteps, identified by Step Angle
Number, the value of the phase current, as a percentage of the
maximum phase current, IPMAX , is digitized to a 6-bit value from
0 to 63. The value 63 represents 100% of IPMAX , 32 represents
• The phase current must be zero at Step Angle Numbers 0 and 32
for phase A and Step Angle Numbers 16 and 48 for phase B.
Figure 11. Example current profile
Figure 12. Digitizing the example current profile
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A3981
Automotive, Programmable Stepper Driver
32/63=50.8% and so on. The value at each Step Angle Number is
then assigned to its corresponding phase table values as follows:
PT
n 1 DIn
where DIn represents the digitized value of the current at Step
Angle Number n.
A selection of the values and the corresponding phase current
table entries is shown in figure12. The full set of phase current
table values is shown in the table below.
Step
Value 10 20 25 28 29 30 31 32 35 40 50 58 60 62 63 63
PT 10 11 12 13 14 15
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16
0
1
2
3
4
5
6
7
8
9
These 16 values are then loaded sequentially into the phase
current table through the Phase Table Load register of the serial
interface. Each value is then distributed to the appropriate Step
Angle Numbers as described above and as shown in table 4C in
the Phase Table Load Register section.
Figure 13. Resulting example current profile
A representation of the final result is shown in figure 13. This
is the digitized version of the required current profile shown in
figure 13.
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A3981
Automotive, Programmable Stepper Driver
Power Dissipation
chronous rectification is used current will flow most of the time
The A3981 is a power circuit, therefore careful consideration
must be given to power dissipation and the effects of high cur-
rents on interconnect and supply wiring.
through the DMOS transistors that are switched on. When syn-
chronous rectification is not used the current will flow through
the body diode of the DMOS transistors during the decay phase.
A first order approximation of the power dissipation in the A3981 The use of fast or slow decay will also affect the dissipation. All
can be determined by examining the power dissipation in each of
the two bridges during each of the operation modes. When syn-
the above combinations can be calculated from five basic DMOS
output states as shown in figure 14.
Synchronous Fast Decay
Non-Synchronous Fast Decay
• Diagonally opposite DMOS output
transistors are on
• Diagonally opposite body diodes
conducting
• Current flows from ground through
load to positive supply
• Current flows from ground through
load to positive supply
Dissipation is I2R losses in the
DMOS transistors:
Dissipation is IV losses in the diodes:
PD(NF) = I × (VFH + VFL
)
PD(SF) = I2 × (RDS(on)H+RDS(on)L
)
Synchronous Slow Decay
Non-Synchronous Slow Decay
• Both low-side DMOS output
transistors are on
• Current circulates through both
transistors and the load
• One low-side DMOS output transistor
and one body diode conducting
• Current circulates through the diode,
the transistor and the load
Dissipation is I2R losses in the DMOS
transistors:
Dissipation is I2R losses in the DMOS
transistors plus IV loss in the diode:
PD(SS) = I2 × (2 × RDS(on)L
)
PD(NS) = (I2 × RDS(on)L) ⁄ (I × VF)
Drive Current Ramp-up
(Used in all combinations)
• Diagonally opposite DMOS output
transistors are on
• Current flows from positive supply
through load to ground
Dissipation is I2R losses in the
DMOS transistors:
PD = I2 × (RDS(on)H + RDS(on)L
)
Figure 14. Basic output states
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A3981
Automotive, Programmable Stepper Driver
The total power dissipation for each of the four decay modes,
PD(TOT) XX, is the average power for the drive current ramp por-
tion, PD , and the drive current decay portion, PD(XX) of the PWM
cycle. For slow decay the current will be rising for approximately
20% of the cycle and decaying for approximately 80%. For fast
decay the ratio will be approximately 50%. Note that these are
approximate figures and will vary slightly depending on the
motor characteristics and the use of synchronous rectification.
The following formulas may be used to estimate total power dis-
sipation:
• Synchronous fast decay mode
PD(TOT)SF = 0.5×PD + 0.5×PD(SF)
PD(TOT)SF = I2 (RDS(on)H +RDS(on)L
)
• Non-synchronous fast decay mode
PD(TOT)NF = 0.5×PD + 0.5×PD(NF)
PD(TOT)NF = 0.5(I2 [RDS(on)H+RDS(on)L]) + 0.5(I×[VFH +VFL])
An approximation of the total dissipation can be calculated by
summing the total power dissipated in both bridges and adding
the control circuit power due to VBB ×IBB and VDD ×IDD
.
• Synchronous slow decay mode
P
D(TOT)SS = 0.2×PD + 0.8×PD(SS)
The total power at the required ambient temperature can then be
compared to the allowable power dissipation shown in figure 15.
For critical applications, where the first order power estimate is
close to the allowable dissipation, the power calculation should
take several other parameters into account including: motor
parameters, dead time, and switching losses in the controller.
PD(TOT)SS = 0.2(I2 [RDS(on)H+RDS(on)L])+0.8(I2 ×2×RDS(on)L
)
• Non-synchronous slow decay mode
P
D(TOT)NS = 0.2×PD + 0.8×PD(NS)
PD(TOT)NS = 0.2(I2[RDS(on)H+RDS(on)L])+0.8(I2×RDS(on)L+I×VF)
5
4
R
= 28 °C/W
θJA
3
2
(on 4-layer PCB)
R
= 32 °C/W
θJA
1
0
(on 2-layer PCB)
25
50
75
100
125
150
Ambient Temperature (°C)
Figure 15. Allowable power dissipation, on typical PCBs
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A3981
Automotive, Programmable Stepper Driver
together externally. The copper ground plane located under the
exposed thermal pad is typically used as the star ground point.
Layout
Traces
Current Sense Resistor
PCB The printed circuit board (PCB, or printed wiring board)
should use a higher weight copper thickness than a standard small
signal or digital circuit board. This helps to reduce the impedance
of the printed traces when conducting high currents. PCB traces
carrying switching currents should be as wide and short as pos-
sible to reduce the inductance of the trace. This will help reduce
any voltage transients caused by current switching during PWM
current control.
In sensing the output current level, to minimize inaccuracies
caused by ground-trace IR drops, the current sense resistor (RS)
should have an independent ground return to the star ground
point. This path should be as short as possible. For low-value
sense resistors, the IR drop in the PCB trace to the sense resis-
tor can be significant and should be taken into account. Surface
mount chip resistors are recommended to minimize contact
resistance and parasitic inductance. The value, RS , of the sense
resistor is given by:
For optimum thermal performance, the exposed thermal pad on
the underside of the A3981 should be soldered directly onto the
board. A solid ground plane should be added to the opposite side
of the board, and multiple vias through the board to the ground
plane should be placed in the area under the thermal pad.
V
REF
RS
16 ISMAX
There is no restriction on the value of RS or VREF , other than the
range of VREF over which the output current precision is guaran-
teed. However, it is recommended that the value of VREF be kept
as high as possible to improve the current accuracy. The table
below provides increasing values of ISMAX for suggested values
of VREF and standard E96 values of RS.
Decoupling
All supplies should be decoupled with an electrolytic capacitor in
parallel with a ceramic capacitor. The ceramic capacitor should
have a value of 100 nF and should be placed as close as pos-
sible to the associated supply and ground pins of the A3981. The
electrolytic capacitor connected to VBB should be rated at least
1.5 times the maximum circuit voltage, and selected to support
the maximum ripple current provided to the motor. The value of
the capacitor is unimportant but should be the lowest value with
the necessary ripple current capability.
Suggested Values
ISMAX
(mA)
RS
(mΩ)
VREF
(V)
100
200
300
405
501
610
702
812
912
1008
499
499
417
309
249
205
178
154
137
124
0.8
1.6
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
The pump capacitor between CP1 and CP2, the pump storage
capacitor between VCP and VBB, and the compensation capaci-
tor between VREG and ground should be connected as close as
possible to the respective pins of the A3981.
Grounding
A star ground system, with the common star point located close to
the A3981, is recommended. The reference ground, AGND (pin
7), and the power ground, PGND (pin 21), must be connected
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A3981
Automotive, Programmable Stepper Driver
Pin-out Diagram
1
2
28
27
26
25
24
23
22
21
20
19
18
17
16
15
SENSA
STRn
DIR
VBBA
RESETn
ENABLE
OAM
3
4
OAP
OSC
SDI
5
CP2
6
CP1
7
AGND
REF
VCP
Ref
8
PGND
VREG
STEP
OBM
9
Reg
SCK
VDD
10
11
12
13
14
VDD
OBP
MS1
SDO
MS0
DIAG
VBBB
SENSB
Terminal List Table
Name
AGND
CP1
Number Description
Name
PGND
REF
Number Description
7
Analog reference ground
21
8
Power Ground
24
23
16
3
Charge pump capacitor terminal
Charge pump capacitor terminal
Diagnostic output
Reference input voltage
Chip reset
CP2
RESETn
SCK
27
9
DIAG
DIR
Serial data clock
Direction select input
SDI
6
Serial data input
ENABLE
MS0
26
13
12
25
4
Bridge enable input
SDO
17
1
Serial data output
Current sense node – bridge A
Current sense node – bridge B
Step input
Microstep select input
Microstep select input
Bridge A negative output
Bridge A positive output
Bridge B negative output
Bridge B positive output
Oscillator input
SENSA
SENSB
STEP
STRn
VBBA
VBBB
VCP
MS1
14
19
2
OAM
OAP
Serial data strobe
Motor supply – bridge A
Motor supply – bridge B
Above supply voltage
Logic Supply
OBM
OBP
18
11
5
28
15
22
10
20
OSC
PAD
–
Connect exposed tab to ground
VDD
VREG
Regulated voltage
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A3981
Automotive, Programmable Stepper Driver
Package LP, 28-Pin TSSOP
with Exposed Thermal Pad
0.45
0.65
9.70±0.10
28
8º
0º
28
1.65
0.20
0.09
B
6.10
3.00
3 NOM 4.40±0.10 6.40±0.20
0.60 ±0.15
1.00 REF
A
2
1
5.08 NOM
1
2
0.25 BSC
Branded Face
5.00
SEATING PLANE
GAUGE PLANE
C
SEATING
PLANE
28X
0.10
C
C
PCB Layout Reference View
0.30
0.19
0.65 BSC
For Reference Only; not for tooling use (reference MO-153 AET)
Dimensions in millimeters
1.20 MAX
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
0.15
0.00
Terminal #1 mark area
A
B
C
Exposed thermal pad (bottom surface); dimensions may vary with device
Reference land pattern layout (reference IPC7351
SOP65P640X120-29CM);
All pads a minimum of 0.20 mm from all adjacent pads; adjust as
necessary to meet application process requirements and PCB layout
tolerances; when mounting on a multilayer PCB, thermal vias at the
exposed thermal pad land can improve thermal dissipation (reference
EIA/JEDEC Standard JESD51-5)
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A3981
Automotive, Programmable Stepper Driver
Appendix A. Driving a Stepper Motor
A stepper motor is a particular form of brushless DC motor. As
for any electric motor, motion is created by magnetic interaction
between the stationary part of the motor, known as the stator, and
the moving part of the motor, known as the rotor. The information
presented here concentrates on a specific type of motor known as
a hybrid stepper motor. This is the most common type of small
stepper motor. It uses permanent magnets in the rotor to produce
one set of constant magnetic fields and electromagnets in the
stator to produce another set of varying magnetic fields. The term
hybrid relates to the use of both electromagnets and permanent
magnets.
remainder of the information presented here relates specifically to
2-phase bipolar motors.
Moving a 2-Phase Bipolar Stepper Motor
Figure A1 shows the four possible current combinations in two
phase windings, A and B, and the effect on a simplified repre-
sentation of part of a stepper motor. In each case the stator with
the electromagnets is shown at the top of the diagram and the
rotor with the permanent magnets is shown at the bottom of the
diagram.
In figure A1 the stator consists of alternate phase A and phase
B electromagnets. The winding direction of the electromagnet
changes for each sequential electromagnet in each phase as indi-
cated by the overbar above the phase letter and identified below
as A-bar and B-bar. The result is that the magnetic poles will
alternate for each sequential electromagnet of each phase. That
means, for example, when the A electromagnet produces a north
(N) magnetic pole at the end nearest to the rotor, then the A-bar
electromagnet will produce a south (S) magnetic pole at the end
nearest to the rotor.
Comparing Bipolar and Unipolar Motors
There are two options in small hybrid stepper motor construction.
In the first, known as a unipolar stepper motor, there are indepen-
dent electromagnets to generate each magnetic polarity, so two
electromagnets are required per phase. Each of these is energized
with current in only one direction, producing a single magnetic
field direction (unipolar). Because the current in each electromag-
net only flows in a single fixed direction, the control circuit can
be very simple. The drawback is that only one electromagnet per
phase can be energized at any time so, at most, only half of the
motor volume is ever used to create torque on the rotor.
The windings for all the A and A-bar electromagnets are con-
nected in series and driven by a single full bridge. Similarly the
windings for all the B and B-bar electromagnets are connected
in series and driven by another single full bridge. So a 2-phase
bipolar stepper motor requires two full bridges for full control.
A bipolar motor, in contrast, uses each electromagnet to pro-
duce two opposing fields (bipolar) at different times, by allow-
ing the current to flow in both directions. This means that the
motor volume required for a bipolar motor is half of the volume
required for a unipolar motor for the same torque output. The
minor drawback is that a bipolar motor requires a more complex
drive circuit in order to reverse the forcing voltage across the coil
of the electromagnet. However, if the drive circuit is integrated
into a single IC then the drive becomes cost effective. This, along
with the improvement in torque output makes the bipolar motor
a better solution for applications where the volume available is
restricted. For this reason the following information will relate
only to bipolar motors.
The rotor is much simpler than the stator, and consists of a solid
base holding permanent magnets with alternating pole directions.
There are no windings on the rotor, so there is no requirement to
conduct current to the moving part of the motor. In addition the
lack of current and windings means that there is no heat generated
in the rotor, making cooling of the moving parts much simpler.
The diagrams in figure A1 provide a representation of a small
section of the mechanics of the motor. In practice the motor struc-
ture is a little different from this, but the principle of operation is
the same.
In order to create continuous motion in one direction it is neces-
sary to have two or more sets of electromagnets, that is, two or
more phases. The simplest and most cost effective configuration
for a stepper motor is to have two phases. For some applications
Starting at the top, panel (a) in figure A1, the current is flowing
down through the phase A winding from top to bottom and there
is no current in phase B. The result is an N magnetic pole on the
A electromagnets and an S pole on the A-bar electromagnets. The
that require an extremely low torque ripple, 3 phase, 5 phase, and rotor position is such that that the poles of the permanent magnets
even 9 phase stepper motors are sometimes used. However, the
align with the poles of the electromagnets, N to S.
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Stator
_
_
_
_
A
B
A
B
A
B
A
B
(a)
• Phase A energized in positive direction
• Phase B not energized
A
B
N
S
S
N
N
S
S
N
Permanent magnet poles on the rotor aligned with
electromagnet poles on the stator
N
S
N
S
Rotor
Stator
_
_
_
_
A
B
A
B
A
B
A
B
(b)
• Phase A not energized
• Phase B energized in positive direction
A
B
N
S
S
N
N
S
S
N
Rotor moves to the right to realign permanent
magnet poles on the rotor to the electromagnet
poles on the stator.
N
S
N
S
Rotor
Stator
_
_
_
_
A
B
A
B
A
B
A
B
(c)
• Phase A energized in negative direction.
• Phase B not energized
A
B
S
N
N
S
S
N
N
S
Rotor moves to the right to realign permanent
magnet poles on the rotor to the electromagnet
poles on the stator.
S
N
S
N
Rotor
Stator
_
_
A
_
A
_
B
(d)
A
B
B
A
B
• Phase A not energized.
• Phase B energized in negative direction
A
B
S
N
N
S
S
N
N
S
Rotor moves to the right to realign permanent
magnet poles on the rotor to the electromagnet
poles on the stator.
S
N
S
N
Rotor
Figure A1. Basic principle of bipolar stepper motor operation
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In the next panel, panel (b), the current is flowing down through
the phase B winding from top to bottom and there is no current
in phase A. The result is an N pole on the B electromagnets and
an S pole on the B-bar electromagnets. These magnetic poles will
attract and repel the permanent magnets on the rotor producing a
force that moves the rotor from left to right in the diagram until
the poles of the permanent magnets again align with the poles of
the electromagnets.
Figure A2 shows the basic principle of microstepping. Panels (a)
and (c) of figure A2 correspond to panels (a) and (b) of figure
A1. Panel (b) shows each phase energized such that there are now
two adjacent N poles and two adjacent S poles. In this example
the currents in both phases is the same, and so the S and N poles
of the rotor now move to half way between the positions in
diagrams (a) and (c). Figure A2 only shows a single mechanical
step in total, which is one quarter of a full electrical cycle. This
sequence is the lowest resolution form of microstepping, known
as half step, and is the simplest method of driving a stepper motor
in half-step mode.
In panel (c), the current is flowing up through the phase A wind-
ing from bottom to top and there is no current in phase B. This
reverses the pole orientation from the top panel, such that there
is an S pole on the A electromagnets and an N pole on the A-bar
electromagnets. As before, these magnetic poles will attract and
repel the permanent magnets on the rotor producing a force that
moves the rotor from left to right in the diagram, until poles of
the permanent magnets again align with the poles of the electro-
magnets.
The currents are switched-on in the correct direction in sequence
and no current control is required. The current is simply defined,
Stator
_
_
A
B
A
B
A
The bottom panel, panel (d), shows the final combination with
current flowing up through the phase B winding from bottom to
top and there is no current in phase A. This produces an N pole
on the B electromagnets and a S pole on the B-bar electromag-
nets. As before, these magnetic poles will attract and repel the
permanent magnets on the rotor producing a force that moves the
rotor from left to right until poles of the permanent magnets again
align with the poles of the electromagnets.
(a) Same as
figure A1(a)
A
B
N
S
S
N
N
S
N
S
N
Rotor
Stator
_
_
A
N
B
N
A
B
A
N
Each of the four steps in figure A1 represents a single full
mechanical step of the stepper motor. The four steps together
represent a single electrical cycle.
(b) Half-step
position
A
B
S
N
S
S
N
The step resolution depends entirely on the mechanical construc-
tion of the motor and typically there will be 200 or more full
steps per mechanical revolution of the motor. A 200-step motor
will provide a resolution of 360/200 = 1.8° of rotation per step.
S
Rotor
Stepping in the opposite direction to that described above is sim-
ply a case of changing the step sequence or inverting one of the
phase current directions.
Stator
_
A
_
B
A
B
A
(c) Same as
figure A1(b)
A
B
N
S
S
N
Microstepping
N
In many applications it is necessary to improve the resolution of
the stepper motor, for more precise positioning control, or simply
to increase the number of steps per revolution to reduce the
torque ripple and therefore the vibration and noise of the motor.
Fortunately this can be achieved by driving both phases at the
same time in order to move the rotor to a position between two
electromagnets. This is known generically as microstepping.
N
S
Rotor
Figure A2. Half step operation
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in the first instance, by the resistance of the winding and the
applied voltage.
line and the phase B current by the horizontal line. The half-step
numbers correspond to the numbers in figure A3. For example,
at step 1 in figure A3, the phase A current and the phase B cur-
rent are both positive and with the same magnitude. These two
currents are shown in figure A4 as the two solid arrows. Adding
these two current vectors together gives the resultant motor cur-
rent vector indicated. The resultant is the hypotenuse of a right-
angled triangle with the two other sides equal. If the other two
sides are assumed to be 1 then the magnitude of the hypotenuse
will be:
From figure A2(b) it is also apparent that varying the relative
current in each phase will make it possible to move the rotor to
any intermediate position between the four positions of figure A1,
which occur when only a single phase is energized. When there is
one intermediate position this is known as half step. When there
are three intermediate positions this is known as quarter step
and so on. Higher resolution microstepping is described in more
detail below.
12 12 2 1.41
Phase Current-Sequence Diagrams
Figure A3 shows the full sequence of the two phase currents illus-
trated in figure A2. This shows two electrical cycles, equivalent to
4 full mechanical steps (8 half steps). The full-step positions are
marked F and the half-step positions are marked H. Each half step
in the electrical cycle is numbered, from 0 to 7, for reference later.
So the resultant current vector will be 141% of the value of the
current in phase A or B, positioned at 45°.
Torque Ripple
Now, the torque output of any electrical motor is directly propor-
tional to the magnitude of the motor current, and the motor cur-
rent is the resultant phase current. It is clear from figure A4 that
the resultant phase current at the half-step position is higher than
the current at the full-step position. This means that the motor
torque will be changing as the motor rotates, resulting in what is
known as torque ripple. Torque ripple in any rotating system will
cause mechanical vibration and will result in increased audible
noise and possible wear on other mechanical components. Torque
ripple can be reduced by ensuring that the resultant current at the
half-step point has the same magnitude as the full current in the
single phase at the full-step positions.
This figure shows that, when discussing stepper motor control, it
is necessary to know the relative magnitude and direction of the
current in each phase. So, rather than use physical representations
of the motor, such as in figure A1 and A2, or simple time-based
current waveforms, such as figure A3, it is simpler to use a phase
diagram. For a 2-pole bipolar motor this diagram is created by
plotting the current in the two phases as orthogonal vectors, that
is, as vectors at 90° to each other as shown in figure A4.
Phase Current-Phase Diagrams
Figure A4 shows the currents of figure A3 plotted on a phase
diagram where the phase A current is represented by the vertical
F
F
F
F
F
F
F
F
F
H
H
H
H
H
H
H
H
Phase A
Current
Phase
B
3
2
F
1
H
H
Current
Resultant
0
F
4
Phase B
Current
F
Phase
A
Current
H
H
5
6
F
7
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
Figure A3. Phase current sequence for simple half step
Figure A4. Phase diagram for simple half step
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The current vectors at half-step position 1 are shown specifically
to illustrate that the magnitude of the resultant sits on the 100%
circle.
Improved Half Step
Figure A5 shows a circle superimposed on the phase diagram.
This circle represents the required locus of the resultant phase cur-
rent vectors to maintain 100% current magnitude. At the full-step
positions, 0, 2, 4, and 6, only one phase is active and the magni-
tude of the phase current is at 100%. At the half-step positions, 1,
3, 5, and 7, both phases are active. To ensure that the magnitude
of the resultant current is 100%, the magnitude of each phase cur-
rent must be 70.7%. Calculating the value of the resultant current
as before gives a resultant current of 100%.
For a standard stepper motor to operate with minimum torque
ripple, the resultant current must always lie on the constant torque
circle irrespective of the number of microsteps. For higher resolu-
tion microstepping this then defines the relative phase currents at
each microstep position.
Quarter Step
For example consider the next resolution in microstepping; quar-
ter step. The locus of the required phase currents are shown in
figure A6. The required current level in each phase can be calcu-
lated using simple trigonometry. For example, consider microstep
position 7 in figure A6 as detailed in figure A7.
0.7072 + 0.7072
=
0.5 + 0.5
=
1
=
1
Phase A
Current
2 F
There are 4 quarter steps for each full step. A full step on the
phase diagram is represented by 90°. So each quarter step incre-
ments the phase angle by 90°/4 = 22.5°.
3
1
H
H
In figure A7 the resultant motor current at quarter-step position 8
is one quarter step from the horizontal, so it is at 22.5°. The mag-
Resultant
0
F
nitude of the current in phase A at quarter-step position 7, IA7
is therefore sin 22.5°, which is equal to 0.383 or 38.3% of the
maximum current.
,
4
F
Phase B
Current
Similarly, the magnitude of the current in phase B at quarter-step
position 7, IB7 , is therefore cos 22.5°, which is equal to 0.924 or
92.4% of the maximum current.
H
5
H
7
6
F
At the 45° positions, 2, 6, 10 and 14, the magnitude of the current
in phase A and phase B will be cos 45° = 0.707 or 70.7%, which
is the same magnitude as in the half-step case shown in figure A5.
Due to symmetry, the phase A current is the same at quarter-step
positions 7 and 1. The phase A current at quarter-step positions
9 and 15 also has the same magnitude, but the current is in the
Figure A5. Phase diagram for improved half step
IA
4
5
3
6
2
IA
6
7
1
0
8
IB
IA7
7
=sin22.5°
9
15
10
14
22.5°
11
13
IB
12
8
I
B7=-cos22.5°
Figure A6. Phase diagram for quarter step
Figure A7. Calculating phase current magnitudes
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opposite direction. In addition the phase B current at quarter-
step positions 3, 5, 11, and 13 also have the same magnitude as
that of phase A at quarter-step position 7, with a positive current
direction for steps 3 and 13 and a negative direction for steps 5
and 11. Similar symmetry can be applied to the phase B current at
quarter-step position 7, calculated above.
Higher Microstep Resolution
The principles described above can easily be extended to higher
microstep resolutions. As the microstep resolution increases, it
becomes more apparent that the phase current sequences approxi-
mate ever closer to a sin and cosine function. Figure A9 shows
the measured phase current sequence of the A3981 running in
sixteenth-step mode. The phase current sequences for eighth-step
and sixteenth-step resolutions are shown in figures A10 and A11.
This means that only five discrete current magnitudes are
required, including 0% and 100%, in order to drive the stepper
motor to all 16 quarter-step positions. Using the same nomencla-
ture as figure A7, that is, IPn , where P is the phase, A or B, and n
is the quarter-step number from figure A6, table A1 shows where
each of the five magnitude values are used.
Most applications using small motors are limited to sixteenth-step
mode due to the mechanical precision of the motor. Larger, high-
precision stepper motors are sometimes driven at 32, 64, or even
up to 256 microsteps in some extreme cases.
Figure A8 shows these values plotted as a current sequence
diagram. This figure is therefore the time-based equivalent of the
phase diagram in figure A6.
Practical Implementation
A system to drive a stepper motor with microstep capability
requires sequencers, current reference generators, and current
controllers. Developing such a system from discrete components,
or even using a fast microcontroller, is a complex task. The
A3981 is one of several fully integrated stepper drivers that are
available with microstep resolutions, from simple half step to
sixteenth step and higher, using programmable current tables. All
aspects of the stepper control system are included in these single
chip solutions and many of them can be controlled by a simple
Step and Direction interface.
Table A1. Quarter-Step Phase Current Magnitudes
Magnitude
Phase B
Phase A
(%)
0.
IA0
IA1
IA2
IA3
–
–
IA8
IA9
IA10
IA11
–
–
–
IB4
IB5
IB6
IB7
–
–
IB12
IB13
IB14
IB15
–
38.3
70.7
92.4
IA7
IA6
IA5
IA4
IA15
IA14
IA13
IA12
IB3
IB2
IB1
IB0
IB11
IB10
IB9
100.
IB8
IB
100%
92%
70%
38%
0
-38%
-70%
-92%
-100%
100%IA
92%
70%
38%
0
-38%
-70%
-92%
-100%
12 13 14 15
0
1
2
3
4
5
6
7
8
9
10 11 12
Figure A8. Phase current sequence for quarter step
Figure A9. Measured sixteenth-step phase current sequence
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voltage the resulting voltage difference will be insufficient to
Practical limitations
drive the phase current required to produce the necessary output
torque. When this occurs the motor will stall and slip out of syn-
chronization with the driving circuit.
The information presented here assumes ideal stepper motors
being stepped slowly, with accurate, efficient current control
circuits. In practice the stepper motor phase windings are repre-
sented by two non-ideal inductors and the motor may be driven at
a high stepping rate.
The mechanical precision of the motor will also have an effect
on the overall performance of the system. If the effect of the
motor windings on the rotor are non-linear then the relationship
between current and torque may not be linear. The magnitude of
the currents at each microstep may then require a relationship
other than sinusoidal. The A3981 and a few other integrated driv-
ers are able to accommodate this by allowing the phase current
values for each microstep position to be reprogrammed. In most
systems this effect will be very small and can be ignored but in
some cases some improvement in torque ripple and audible noise
can be achieved.
A high stepping rate will produce a back EMF, like any other
motor, that will act against any current control circuits. The
current control circuits must also be able to work with inductive
loads. In general the current control circuit will be a PWM cur-
rent control scheme to make the driver as efficient as possible and
reduce the dissipation in the driver.
Like any other motor, the back EMF will also limit the maximum
stepping rate of the motor. As the motor speed increases the back
EMF will increase. When it reaches a value close to the supply
100%
98%
92%
83%
70%
56%
38%
19%
IB
19%
38%
56%
70%
83%
92%
98%
100%
24 25 26 27 28 29 30 31 0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
100%
98%
92%
83%
70%
56%
38%
19%
IA
19%
38%
56%
70%
83%
92%
98%
100%
Figure A10. Phase current sequence for eighth step
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100%
98%
99%
96%
92%
88%
83%
77%
70%
63%
56%
47%
38%
29%
19%
10%
IB
10%
19%
29%
38%
47%
56%
63%
70%
77%
83%
88%
92%
96%
99%
98%
100%
100%
98%
99%
96%
92%
88%
83%
77%
70%
63%
56%
47%
38%
29%
19%
10%
IA
10%
19%
29%
38%
47%
56%
63%
70%
77%
83%
88%
92%
96%
99%
98%
100%
Figure A11. Phase current sequence for sixteenth step
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Revision History
Revision
Revision Date
Description of Revision
Rev. 4
June 21, 2012
Update Electrical Characteristics table
Copyright ©2010-2012, Allegro MicroSystems, Inc.
Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to per-
mit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the
information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use;
nor for any infringement of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
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相关型号：
A3981KLPTR-T
Stepper Motor Controller, PDSO28, 9.70 X 4.40 MM, 1.20 MM HEIGHT, LEAD FREE, MO-153AET, TSSOP-28
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