ALT80800 [ALLEGRO]
Automotive-Grade, Constant-Current 2.0 A PWM Dimmable Synchronous Buck LED Driver;
| 型号： | ALT80800 |
| 厂家： | 
ALLEGRO MICROSYSTEMS |
| 描述： | Automotive-Grade, Constant-Current 2.0 A PWM Dimmable Synchronous Buck LED Driver |
| 文件： | 总23页 (文件大小：1105K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
ALT80800
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
DESCRIPTION
FEATURES AND BENEFITS
• AEC-Q100 qualified
• Supply voltage 4.5 to 55 V
The ALT80800 is a synchronous buck switching regulator
that provides constant-current output to drive high-power
LEDs. It integrates both high-side and low-side N-channel
DMOS switches for DC-to-DC step-down conversion. A true
average current is output using a cycle-by-cycle, controlled
on-time method.
• 2.0 A maximum output over operating temperature range
• Integrated high-side and low-side MOSFETs:
200 mΩ/150 mΩTYP
• True average output current control
• Internal control loop compensation
• Integrated 5 V, 14 mA LDO regulator for peripheral circuits
• Dimming via PWM pin or EN pin down to 0.1% at 200 Hz
• Analog dimming (ADIM pin) for brightness calibration
and thermal foldback
Output current is user-selectable by an external current sense
resistor. Output voltage is automatically adjusted to drive
various numbers of LEDs in a single string. This ensures the
optimal system efficiency.
• Low-power shutdown (1 µA typical)
• Cycle-by-cycle current limit
• Active low fault flag output
• LED open fault mask setting for low VIN operation
• Undervoltage lockout (UVLO) and thermal shutdown
protection
• Switching frequency dithering for improved EMC
• Robust protection against:
LED dimming is accomplished by a direct logic input pulse-
width modulation (PWM) signal at the PWM pin while EN is
enabled. Alternatively, applying a PWM signal at the EN pin
while PWM pin is high can enable “chopped battery” PWM
dimming for legacy control modules.
Furthermore, an Analog Dimming input (ADIM pin) can be
used, for example, to calibrate the LED current or implement
thermalfoldbackinconjunctionwithexternalNTCthermistor.
□ Adjacent pin-to-pin short
□ Pin-to-ground short
□ Component open/short faults
The ALT80800 is provided in a 16-pin TSSOP (suffix LP),
with exposed pad for enhanced thermal dissipation.
APPLICATIONS:
PACKAGE:
16-Pin eTSSOP (suffix LP)
Automotive lighting
• Daytime running lights
• Front and rear fog lights
• Turn/stop lights
• Map light
• Dimmable interior lights
Not to scale
VIN
ALT80800
CIN
VIN
EN
SW
CBOOT
L1
RSENSE
LED+
GND
BOOT
VCCIN
TON
CSH
CSL
External PWM
dimming signal
RON
PWM
ADIM
CLED
PWM
ADIM
VCC
External analog
dimming signal
FFn
GND
FFn
VCC
VIN
VCC
CBIAS
SGND
PGND
FDSET
GND
Figure 1: ALT80800 Typical Application Circuit
November 13, 2018
ALT80800-DS, Rev. 1
MCO-0000344
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
SELECTION GUIDE
Part Number
Package
Packing
ALT80800KLPATR
16-pin TSSOP with exposed thermal pad
4000 pieces per 13-inch reel
SPECIFICATIONS
ABSOLUTE MAXIMUM RATINGS
Characteristic
Symbol
VIN, VVCCIN
VBOOT
Notes
Rating
Unit
V
Supply Voltage
–0.3 to 60
Bootstrap Drive Voltage
Switching Voltage
–0.3 to VIN+8
–0.3 to VIN+0.3
–1 to VIN+3
V
Continuous
V
VSW
Pulsed, t < 50 ns
V
EN Voltage
VEN
V
–0.3 to VIN+0.3
Current Sense Voltages
Linear Regulator Terminal
ADIM pin, TON pin
V
CSH, VCSL
VCC
VADIM, VTON
VFDSET
V
V
V
–0.3 to 7
FDSET Voltages
V
FFn and PWM Voltages
Maximum Junction Temperature
Storage Temperature
V
FFn, VPWM
TJ(max)
Tstg
V
150
°C
°C
–55 to 150
THERMAL CHARACTERISTICS*: May require derating at maximum conditions; see application section for optimization
Characteristic
Symbol
Test Conditions*
Value Unit
Package Thermal Resistance
(Junction to Ambient)
RθJA
On 4-layer PCB based on JEDEC standard
34
2
°C/W
°C/W
Package Thermal Resistance
(Junction to Pad)
RθJP
*Additional thermal information available on the Allegro™ website.
Table of Contents
Pinout Diagrams and Terminal List Tables
3
4
5
7
20
Features and Benefits
Description
Applications
1
1
1
1
1
2
2
2
2
Functional Block Diagrams
Electrical Characteristics
Functional Description
Package
Application Circuit Diagrams
Typical Application Circuit
Selection Guide
Specifications
Absolute Maximum Ratings
Thermal Characteristics
2
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
PINOUT DIAGRAM AND TERMINAL LIST TABLE
TSSOP-16 (LP) Pinout Diagram
Terminal List Table
Number
Name
PGND
VIN
Function
1, 2
3
Power ground terminal.
16
15
14
13
12
11
10
9
PGND
PGND
VIN
SW
1
2
3
4
5
6
7
8
Supply input voltage for power stage.
BOOT
VCCIN
FFn
Enable pin for internal LDO regulator and whole IC. EN pin can
also be used as PWM dimming when keeping PWM pin High.
4
5
EN
EN
PAD
Logic input for PWM dimming: when PWM = LOW, LED is off; if
PWM = High and at the same time EN is enabled, LED is ON.
PWM
FDSET
ADIM
TON
VCC
SGND
CSL
PWM
FDSET pin to set the LED Open fault mask threshold. Connect
to a voltage divider formed between VIN and PGND. When VIN
is low, resulting in FDSET below the internal reference, LED
Open Fault detection will be masked.
6
FDSET
CSH
Analog dimming control voltage input. If not used for analog
dimming, tie ADIM to 5 V or VCC; if used for analog dimming,
keep ADIM less than 2.5 V.
7
8
ADIM
TON
Regulator on-time setting resistor terminal. Connect a resistor
between TON pin and SGND to set the switching frequency.
9
CSH
CSL
Current Sense (positive end) feedback input for LED current.
Current Sense (negative end) feedback input for LED current.
Signal ground terminal.
10
11
SGND
Internal IC bias regulator output. Connect at least 1 µF MLCC to
PGND. Can be used to supply up to 14 mA for external load.
12
13
14
15
16
–
VCC
FFn
Open-drain output which is pulled low in case of fault. Connect
through an external pull-up resistor to the desired logic level.
It is recommended to connect VCCIN to VIN to bias the internal
LDO regulator.
VCCIN
BOOT
SW
High-side gate driver bootstrap terminal; a 0.47uF capacitor is
recommended between BOOT and SW.
Switched output terminal. The output inductor should be
connected to this pin.
Exposed pad for enhanced thermal dissipation; connect to
ground.
PAD
3
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
VIN
CIN
VIN
BOOT
VOUT
VIN
Boot
Charger
VIN
TON
CBOOT
On-Time
Select
L1
Gate
Driver
SW
RON
On-Time
RSENSE
VOUT
EN
Buck
17 ms
Converter
Duty Cycle
Control
PGND
VCCIN
LED+
VCC
LDO
Internal 5.0 V bias
Up to 14 mA
external load
CSH
CSL
LED
Current
CBIAS
Differential
Amp
VCSREG
RADJ
VCC
UVLO
CLED
PWM
PWM
ADIM
ALT80800
VREF
ADIM
(0 – 200 mV)
i_LED
reference
VIN
Other Faults
FDSET
+
-
VCC
LED Open
Fault
FFn
REF1
FFn
Fault
Handling
LED Short
Fault
VOUT
SGND
Figure 2: Functional Block Diagram
4
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
ELECTRICAL CHARACTERISTICS: Valid at VIN = 12 V, VOUT = 6 V, TJ = –40°C to 125°C, typical values at TJ = 25°C,
unless otherwise noted
Characteristics
Input Supply Voltage
Symbol
Test Conditions
Min.
4.5
–
Typ.
Max.
55
Unit
V
VIN
–
–
–
V
IN Undervoltage Lockout Threshold
IN Undervoltage Lockout Hysteresis
VUVLO(ON) VIN increasing, VIN = VVCCIN, ICC = 0 mA
4.3
V
V
VUVLO
VIN decreasing, VIN = VVCCIN, ICC = 0 mA
100
300
mV
(
HYS)
VCSH – VCSL = 0.5 V, VEN = VIH_EN
VPWM = VIH_PWM, RON = 402 kΩ
,
VIN Pin Supply Current
IIN
–
–
5
1
–
–
mA
µA
V
VIN Pin Shutdown Current
IINSD
VOUT
ISWLIM
VEN = VIL_EN
10
50
Output Current Sense Common Mode
Voltage (measured at CSL pin) [1]
VIN = 55 V, fSW = 500 kHz, iLED = 0.5 A
2.65
Buck Switch Current Limit Threshold
Buck High-Side Switch On-Resistance
Buck Low-Side Switch On-Resistance
BOOT Undervoltage Lockout Threshold
2.5
–
3.25
0.2
0.15
3.4
750
100
65
4.0
0.32
0.24
3.7
–
A
Ω
RDSON(HS) VBOOT = VIN + 4.3 V, TJ = 25°C, ISW = 0.5 A
RDSON(LS) TJ = 25°C, ISW = 0.5 A
–
Ω
VBOOTUV
VBOOT to VSW increasing
3.1
–
V
BOOT Undervoltage Lockout Hysteresis VBOTUVHYS VBOOT to VSW decreasing
mV
ns
ns
ns
ns
–
Switching Minimum Off-Time
Switching Minimum On-Time
Selected On-Time
tOFFmin
tONmin
tON
VCSH – VCSL = 0 V
–
125
90
–
VIN = 12 V, VOUT = 6 V, RON = 42.2 kΩ
200
–
–
300
90
Low-Side Switching Minimum On-Time [2] tLS_ONmin
ON Dithering Range fSW_DITH
Dithering Modulation Frequency fSW_MOD
60
t
RON = 42.2 kΩ
RON = 42.2 kΩ
–
±5%
12.5
–
–
–
kHz
REGULATION COMPARATOR AND ERROR AMPLIFIER
Load Current Sense Regulation
Threshold at 100%[3]
VCSH – VCSL decreasing, SW turns on, ADIM
tied to VCC
VCSREG
194
200
206
mV
CSH Input Sense Current [4]
CSL Input Sense Current
ICSH
ICSL
VCSH – VCSL = 0.2 V
VCSH – VCSL = 0.2 V
–
–250
75
–
µA
µA
50
100
INTERNAL LINEAR REGULATOR
VCC Regulated Output
VCC
0 mA < ICC < 14 mA, VVCCIN > 6 V
4.85
–
5.0
0.3
5.15
0.55
V
V
Measure VVCCIN – VCC: VVCCIN = 4.8 V,
VCC Dropout Voltage
VCC Current Limit
VLDO
I
CC = 14 mA
iVCCLIM
VCC ≥ 4.35 V
20
–
–
mA
V
VCCUVLO
Rising
3.65
175
3.9
225
4.05
275
VCC Undervoltage Lockout
VCCUVLOHYS Hysteresis
mV
PWM INPUT
Logic High Voltage
Logic Low Voltage
PWM Pin Pull-Up Resistance
EN INPUT
VIH_PWM
VIL_PWM
RPWMPU
VEN increasing
1.8
–
–
–
–
1.2
–
V
V
VEN decreasing
VCC = 5 V
–
100
kΩ
Measured while PWM dimming signal applied at
EN keeping low and exceeding tOFFDelay results
in shutdown
Maximum IC Turn Off Delay
tOFFDelay
10
17
–
ms
Logic High Voltage
Logic Low Voltage
VIH_EN
VIL_EN
EN increasing
EN decreasing
1.8
–
–
–
–
V
V
0.4
Continued on the next page…
5
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
ELECTRICAL CHARACTERISTICS (continued): Valid at VIN = 12 V, VOUT = 6 V, TJ = –40°C to 125°C, typical values at
TJ = 25°C, unless otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
ANALOG DIMMING INPUT
Input Voltage for 100% LED Current
VADIMH
VCSH – VCSL = VCSREG
2.1
–
–
–
–
V
Regulation Threshold at 50% Analog
Dimming
VCSREG50 VADIM = 1.0 V
VCSREG20 VADIM = 0.4 V
100
mV
Regulaton Threshold at 20% Analog
Dimming
38.4
244
40
41.4
284
mV
mV
FAULT
LED Open/Short Detect Condition
ADIM Range
VADIM rising
264
LED Short Fault Output Voltage Low
Threshold
V
OUT falling
1.3
1.5
2.4
1.7
V
V
–
LED Open-Fault Enable Reference
LED Open Fault Current Threshold
VREF1
2.352
2.448
VCSREG = 200 mV start falling (PWM duty =
max), VADIM = VCC, VFDSET = VCC
(20 mV)
10%
(50 mV)
25%
(80 mV)
40%
VCS_OPEN
VCSREG = 200 mV start falling (PWM duty =
max), VADIM = VCC, VFDSET = VCC
(6 mV)
3%
(12 mV)
6%
(18 mV)
9%
LED Open Fault Current Hysteresis [1] VCS_OPEN_HYS
–
Fault Deglitch Timer
Fault Mask Timer
tFDG
35
70
–
50
100
–
65
130
0.4
1
µs
µs
V
tMASK
FFn Pull-Down Voltage
FFn Pin Leakage Current
VFAULT(PD) Fault condition asserted, pull-up current = 1 mA
IFAULT(LKG) Fault condition cleared, pull-up to 5 V
–
–
µA
The transition time FFn pin takes from Low
to High
FFn Rising Time [1]
FFn Falling Time [1]
tRISE
–
–
10
µs
The transition time FFn pin takes from High
to Low
tFALL
–
–
–
1
10
–
µs
Cool Down Timer for Fault Retry
THERMAL SHUTDOWN
tRETRY
ms
Thermal Shutdown Threshold[1]
Thermal Shutdown Hysteresis
TSD
150
–
165
25
180
–
°C
°C
TSDHYS
[1] Determined by design and characterization. Not production tested.
[2] Guaranteed by design, HS and LS switches are interlocked, as illustrated below:
SW
tOFFmin
tdead
≈
(tOFFMIN – tLS_ONmin) / 2
tLS_ONmin
Low Side VGS
tdead
tdead
[3] In test mode, a ramp signal is applied between CSH and CSL pins to determine the VCSH – VCSL regulation threshold voltage. In actual application,
the average VCSH – VCSL voltage is regulated at VCSREG regardless of ripple voltage.
[4] Negative current is defined as coming out of (sourcing) the specified device pin or node.
6
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
FUNCTIONAL DESCRIPTION
The ALT80800 is a synchronous buck regulator designed for
driving a high-current LED string. It uses average current mode
control to maintain constant LED current and consistent bright-
ness. The LED current level is easily programmable by selection
of an external sense resistor, with a value determined as follows:
Synchronous Regulation
The ALT80800 integrates an N-channel DMOS as the low-side
switch to implement synchronous regulation for LED drivers, as
shown in Figure 4.
VIN
iLED = VCSREG / RSENSE
where VCSREG = VCSH – VCSL = 0.2 V typical.
Boot
Charger
BOOT
If necessary, a resistor can be inserted in series with the CSL pin
to fine-tune the LED current, as shown below:
CBOOT
L
i_L
VOUT
Rsc
SW
iCSH
Floating
Gate Driver
SW
iLED
CSH
CLED
GND
+
Integrated
Switch
RSENSE
Radj
VSENSE
VCSREG
CSL
ALT80800
–
–
+
Figure 4: Synchronous Buck LED Driver
iCSL × Radj
iCSL
The Synchronous configuration can effectively pull down SW
to ground by forcing the low-side synchronous switch on even
with small inductor current, as shown in Figure 5. Therefore, the
BOOT capacitor can be charged normally every switch cycle to
ensure the normal operation of buck LED drivers.
VCSREG = iLED × RSENSE + iCSL × Radj
Therefore
iLED = (VCSREG – iCSL × Radj) / RSENSE
Figure 3: How To Fine-Tune LED Current Using Radj
For example, with a desired LED current of 1.4 A, the required
RSENSE = 0.2 V / 1.4 A = 0.143 Ω. But the closest power resistor
available is 0.13 Ω. Therefore, the difference is
Radj × iCSL = 0.2 V – 1.4 A × 0.13 Ω = 0.018 V
where iCSL = 75 µA typical
Radj = 0.018 V / 75 µA = 240 Ω
Figure 5: Normal SW waveform with SR configuration
when VIN ≈ VOUT: VIN = 5.4 V, VOUT = 5.14 V
(2 white LEDs)
The LED current is further modulated by the ADIM (Analog
Dimming) pin voltage. This feature can be used for LED bright-
ness calibration, or for thermal foldback protection. See Analog
Dimming section for details.
7
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
Switching Frequency
ENABLE AND DIMMING
The ALT80800 is activated when a logic high signal is applied to
The ALT80800 operates in fixed on-time mode during switching.
The on-time (and hence switching frequency) is programmed
using an external resistor connected between the TON pin and
ground, as given by the following equation:
the EN (enable) pin and VIN = VVCCIN is above UVLO threshold
4.3 V. The buck converter ramps up the LED current to a target
level set by RSENSE when PWM pin = High.
The EN pin is high-voltage tolerant and can be directly connected
to a power supply. However, if VEN is higher than the VIN voltage
at any time, a series resistor (10 kΩ) is required to limit the current
flowing into the EN pin. This resistor is helpful in preventing EN
from damage in case of reverse-battery connection. This series
resistor is not necessary if EN is driven from a logic input.
tON = k × (RON + RINT) × ( VOUT / VIN
fSW = 1 / [ k × (RON + RINT)]
)
where k = 0.0127, with fSW in MHz, tON in µs, and RON and RINT
(internal resistance, 3 kΩ) in kΩ.
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
The PWM pin is a logic input pin and is internally pulled up to
VCC through a resistor.
EN pin and PWM pin function as illustrated below:
EN pin
High
High
Low
PWM pin
Low
VCC
ON
LED
OFF
ON
High/Open
x
ON
Shutdown
When the EN pin is forced from high to low, the LED current is
turned off, but the IC remains in standby mode for up to at least
10 ms. If EN goes high again within this period, the LED current
is turned on immediately if PWM pin is high. If EN pin is low for
more than tOFFDelay, the IC enters shutdown mode to reduce power
consumption. The next high signal on EN will initialize a full
startup sequence, which includes a startup delay of approximately
150 μs. This startup delay is not present during PWM operation.
0
100
200
300
400
500
600
700
800
900
1000
RON (kΩ)
Figure 6: Switching Frequency vs. TON resistance
To minimize the peaks of switching frequency harmonics in EMC
measurement, a dithering feature is implemented. The dithering
range is internally set at ±5%. The actual switching frequency is
swept linearly between 0.95 × fSW and 1.05 × fSW, where fSW is
the programmed switching frequency. The rate of modulation for
fSW is fixed internally at 12.5 kHz.
Active dimming of the LED is achieved with 2 options: by send-
ing a PWM (pulse-width modulation) signal to the EN pin (while
PWM = High), or by sending a dimming PWM signal to the
PWM pin (while EN is enabled) as illustrated in the table above.
The resulting LED brightness is proportional to the duty cycle of
the applied PWM signal. A practical range for PWM dimming
frequency is between 100 Hz (period = 10 ms) and 2 kHz.
If the PWM dimming signal at PWM pin is low when the EN pin
is high, the LED will be off immediately and IC is alive waiting
for next PWM pulse. The internal LDO is still on and can provide
bias to the internal and external circuits.
In PWM dimming operation and when VIN is above 40 V, a
10 kΩ resistor is needed to be in parallel with a 0.047 µF output
capacitor across the LED string to facilitate BOOT charging dur-
ing PWM dimming OFF period.
8
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
It is possible to pull ADIM pin below 0.4 V to achieve lower
PWM DIMMING RATIO
than 20% analog dimming. However, if the average LED cur-
rent determined by ADIM becomes too low and is below half the
inductor current ripple, negative current will flow through the
inductor. To prevent such cases from happening, it is suggested
that ADIM voltage should meet the condition below:
The brightness of the LED string can be changed by adjusting the
PWM duty cycle at the EN pin as follows:
Dimming ratio = PWM on-time / PWM period
For example, by selecting a PWM period of 5 ms (200 Hz PWM fre-
quency) and a PWM on-time of 5 µs, a dimming ratio of 0.1% can
be achieved. This is sometimes referred to as “1000:1 dimming.”
ADIM > RSENSE / 0.2 × (VIN – VOUT) / L × D × T
where D is duty cycle, D ≈ VOUT / VIN, T is switching period,
T = 1 / fSW, L is the inductance.
In an actual application, the minimum dimming ratio is deter-
mined by various system parameters, including: VIN, VOUT
,
inductance, LED current, switching frequency, PWM frequency,
and fault flag usage. The device is easily capable of PWM on-
time as short as 5 µs; however, if fault flag for open/short LED
detection is required, it should be above 130 µs due to the fault
mask timer.
For example, when RSENSE = 0.2 Ω, RON = 178 kΩ, L = 33 µH,
VIN = 12 V, VOUT = 5.2 V, ADIM voltage should be above 0.21 V,
i.e. 11% level, to avoid negative inductor current.
ADIM pin can be used in conjunction with PWM dimming to
provide wider LED dimming range over 1000:1. In addition, the IC
can provide thermal foldback protection by using an external NTC
(negative temperature coefficient) thermistor, as shown below:
ANALOG DIMMING
In addition to PWM dimming, the ALT80800 also provides an
analog dimming feature. When VADIM is over 2.0 V, the LED cur-
rent is at 100% level (as defined by the SENSE resistor). When
VADIM is below 2 V, the LED current decreases linearly down to
20% at VADIM = 0.4 V. This is shown in the following figure:
VCC
R1
R
S
ADIM
VCSREG
R
NTC
P
200 mV
±6 mV
(100%)
100 mV
Figure 8: Using an External NTC Thermistor
to Implement Thermal Foldback
ADIM pin
voltage
40 mV
ADIM is tied to 5 V (or VCC) if never used for analog dimming,
or always less than 2.5 V when used for analog dimming. For
long term reliability, or extended period with extreme temperature
condition, it is better to keep ADIM always less than 2.5 V.
0
2 V
0.4 V
1 V
Figure 7: ADIM Pin Voltage Controls SENSE Reference
Voltage (hence LED current)
9
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
During SW on-time:
OUTPUT VOLTAGE AND DUTY CYCLE
The figure below provides simplified equations for approximat-
ing output voltage. The output voltage of a buck converter is
approximately given as:
iRIPPLE = (VIN – VOUT) / L × tON = (VIN – VOUT) / L × t × D
where D = tON / t.
VOUT ≈ VIN × D , D = tON / (tON + tOFF
)
During SW off-time:
where D is the duty cycle.
iRIPPLE = VOUT / L × tOFF = VOUT / L × t × (1 – D)
Simplified equation for output voltage:
VOUT = VIN × D
VIN
MOS
CIN
More precisely:
VOUT = (VIN – iAVG × RDSON(HS)) × D – (1 – D) × RDSON(LS) × iAVG
– (DCR + RSENSE) × iAVG
L
SW
where DCR is the internal resistance of the inductor, RSENSE is
the current sensing resistance, RDSON(HS) is the on-resistance
of high-side switch, RDSON(LS) is the on-resistance of low-side
switch, iAVG is the average current through inductor and equal to
LED current.
iL
RSENSE
VOUT
D
GND
VSW
VIN
t
0
–VD
iL
iRIPPLE
t
tON
tOFF
Period, t
Figure 9: Simplified Waveforms for a Buck Converter
10
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
If the required output voltage is lower than that permitted by the
minimum tON , the controller will automatically extend the tOFF to
maintain the correct duty cycle. This means that the switching
frequency will drop lower when necessary to keep the LED cur-
rent in regulation.
MINIMUM AND MAXIMUM OUTPUT VOLTAGES
For a given input voltage, the maximum output voltage depends
on the switching frequency and minimum tOFF . For example, if
tOFF(min) = 100 ns and fSW = 1 MHz, then the maximum duty
cycle is 90%. So for an 18 V input, the maximum output is
approximately 16.2 V (based on the simplified equation of VOUT
= VIN × D). This means up to 5 LEDs can be operated in series,
assuming Vf = 3.3 V or less for each LED.
If the LED string is completely shorted (VOUT = 0 V), the con-
troller will continue to switch at minimum tON and will not enter
into Hiccup mode.
The minimum output voltage depends on minimum tON and
switching frequency. For example, if the minimum tON = 65 ns
and fSW = 1 MHz, then the minimum duty cycle is 6.5%. That
means with VIN = 18 V, the theoretical minimum VOUT is just
1.2 V. However, the internal current sense amplifier is designed to
guarantee the current accuracy down to VOUT = 2.65 V. When the
output voltage is lower than 2.65 V, the regulator keeps switch-
ing to regulate, but the current accuracy will suffer and not be
guaranteed.
THERMAL BUDGETING
The ALT80800 is capable of supplying a 2 A current through
its high-side switch. However, depending on the duty cycle, the
conduction loss in the high-side switch may cause the package to
overheat. Therefore care must be taken to ensure the total power
loss of package is within budget. For example, if the maximum
temperature rise allowed is ∆T = 60°C at the device case surface,
then the maximum power dissipation of the IC is 1.75 W. Assum-
ing the maximum RDSON(HS) = 0.32 Ω, RDSON(LS) = 0.24 Ω, and
a duty cycle of 70%, then the maximum LED current is limited to
2 A approximately.
To a lesser degree, the output voltage is also affected by other
factors such as LED current, on-resistance of the high-side
switch, and DCR of the inductor.
As a general rule, switching at lower frequencies allows a wider
range of VOUT, and hence more flexible LED configurations.
20
18
16
14
12
10
8
VOUT(max) (V)
VOUT(min) (V)
6
4
2
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Frequency (MHz)
Figure 10: Minimum and Maximum Output Voltage vs.
Switching Freqency
(VIN = 18 V, minimum tON = 90 ns and tOFF = 100 ns)
11
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Automotive-Grade, Constant-Current 2.0 A
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ALT80800
FAULT HANDLING
LED OPEN/OUTPUT SHORT FAULTS
Referring to Fault Function block diagram below, LED Open Fault
The ALT80800 is designed to handle the following faults:
• Pin-to-ground short
• Pin-to-neighboring pin short
• Pin open
• External component open or short
• Output short to ground
is masked when VIN is below the pre-set adjustable threshold at
FDSET pin or ADIM is below 264 mV. When FDSET is below
REF1 or ADIM is below 264 mV with asserting fault flag (FFn =
Low), the fault flag keeps asserted if open LED fault exists. Only
when FDSET is above REF1 and ADIM is above 264 mV, then
the Open fault will be detected by checking current sensing volt-
age VCSREG and duty cycle. LED Open fault will force regulator
into Hiccup mode and assert fault flag, and then fault flag remains
asserted during the remaining hiccup mode periods. Once LED
open fault disappears, fault flag goes high after hiccup mode period
when PWM is high. (refer to Figure 11 and Table 1).
VIN
TON Resistor Open/Short,
SENSE Open/Short,
Inductor Open/Short,
Overcurrent
FDSET
R
+
-
REF1
1 ms
Hiccup
Mode
ADIM
+
-
LED Open
VCC
0.264 V
VCSREG
-
+
FFn
25% i
Duty
LED
FFn
+
-
SGND
MaxDuty
LED Short
VOUT
-
+
1.5 V
Figure 11a: Simplified Faults Block Diagram
FDSET
(or ADIM
@264 mV)
REF1
PWM
VCSREG
25% × i
LED
LED OPEN FAULT
LED OPEN FAULT
FFn Flag
Figure 11b: LED Open Fault Timing Diagram
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
Table 1: LED Open Fault Truth Table
LED Open Fault Event?
VCSREG < 25% × iLED Max Duty
No Open Fault
Yes, Open Fault
x
FDSET
ADIM
FFnn
PWM
FFnn+1
High
High
Low
x
High
High
x
x
x
1
1
0
0
0
0
1
0
1
1
0
0
1
1
x
x
x
x
Low
x
x
Low
x
Yes, Open Fault
Yes, Open Fault
No Open Fault
No Open Fault
Low
x
Low
x
Low
FDSET High means FDSET > REF1; FDSET Low means FDSET < REF1;
ADIM High means ADIM > 264 mV; ADIM Low means ADIM < 264 mV
When output Short fault (such as LED shorted to ground or out-
put capacitor shorted to ground) occurs, FFn will be flagged as
flagged if the fault remains active after a deglitch period (tFDG).
A mask timer (tMASK) is also introduced whenever PWM signal
goes from Low to High. During this mask time, faults will not be
detected, so the fault will not be detected when the PWM pulse
width is less than this mask time. When PWM goes low, fault
flag is latched. Fault flag will keep prior state when PWM is
Low.
V
OUT drops below 1.5 V and ADIM voltage is above 264 mV; but
regulator will not enter into Hiccup mode and will work continu-
ously. When short is removed, ALT80800 will return to normal
operation.
When an LED Open/Short fault occurs, the Fault pin will be
13
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
The Fault deglitch time is fixed; and the Fault mask time is also
fixed (refer to Electrical Characteristics table). The LED Open/
Short Fault timing diagrams are illustrated below:
Short
Removed
Short
Removed
Short
Fault
Short
Fault
Short
Fault
tMASK
PWM
FFn
tFDG
tMASK
Figure 12a: LED Short Fault Timing Diagram Overview
Open
Removed
Open
Fault
Open
Fault
Open
Fault
Open
Removed
tMASK
PWM
tFDG
tMASK
~1ms Hiccup period
SW
~1 ms Hiccup
~1ms Hiccup
~1 ms Hiccup
FFn
Current to
regulation timer
Current to
regulation timer
Figure 12b: LED Open Fault Timing Diagram Overview
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
The basic timing configurations are detailed below for LED Open/Short faults:
Case 1: LED Open/Short Event is outside
Mask Timer at PWM = H
Case 2: LED Open/Short Event is within
Mask Timer at PWM = H
PWM
PWM
Mask Timer
Mask Timer
Fault Event
Fault Event
Fault
Fault
No LED Fault
Event
Event
LED Open/Short
LED Open/Short
Fault Flag
Fault Flag
Deglitch
Timer
Mask Timer
Case 3: LED Open/Short Event is close to
Case 4: LED Open/Short Event is at PWM = L
↓
PWM at PWM = H
Mask Timer
PWM
PWM
Mask Timer
Mask Timer
Fault Event
Fault Event
Fault
Event
Fault
Event
No LED Fault
LED Open/Short
LED Open/Short
Fault Flag
Fault Flag
Mask Timer
Mask Timer
Case 5: LED Open/Short Removed at PWM = L
a) LED Short Removed at PWM = L
b) LED Open Removed at PWM = L
PWM
PWM
Mask Timer
Mask Timer
Fault Removed
Fault Removed
No LED Short
No LED Open
Open
Event
Short
Event
Current to
regulation timer: *
Mask Timer
Fault Flag
Fault Flag
Case 6: LED Open/Short Removed outside Mask Timer at PWM = H
a) LED Short Removed outside
Mask Timer at PWM = H
b) LED Open Removed outside
Mask Timer at PWM = H
PWM
PWM
Open
Mask Timer
Mask Timer
Fault Removed
No LED Short
Fault Removed
No LED Open
Short
Event
LED Open
Event
LED Short
Hiccup period:
~1 ms
Current to
Fault Flag
Fault Flag
regulation timer *
Case 7: LED Open/Short Removed within Mask Timer at PWM = H
a) LED Short Removed within
Mask Timer at PWM = H
b) LED Open Removed within
Mask Timer at PWM=H
PWM
PWM
Mask Timer
Mask Timer
Fault Removed
Fault Removed
No LED Short
No LED Open
Short
Event
Open
Event
LED Open
LED Short
Current to
regulation Timer *
Fault Flag
Fault Flag
* Current to regulation timer is 256 switching cycles.
15
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
SYSTEM FAILURE DETECTION AND PROTECTION DEMONSTRATION
VIN
C1
C1,C2 = open or short
L1 =open
or short
C2
ALT80800
C4 open
or short
RSENSE
SENSE open
or short
VIN
SW
BOOT
CSH
GND
R1
L1
R
R1 =open
or short
C4
TON
EN
EN
LED+
C3
PWM
PWM
VCC
CSL
LED string open
or short to GND
C3 open
or short
SGND
PGND
C5 =open
or short
GND
C5
GND
System-Level Failure Modes
Protected against open/short fault for
all external components, including:
- LED string
- Sense resistor
- Inductor
IC-Level Failure Modes
Protected against
- Any pin open
- Any pin shorted to ground
- Adjacent pin-to-pin short
- Input/output caps, etc.
Figure 13: Demonstration of various possible fault cases in an application circuit
Table 2: System Failure Mode Table (partial)
FAULT flag
asserted?
Failure Mode
Symptom Observed
ALT80800 Response
Inductor open
Dim light from LED
Yes [1]
When VIN is below preset FDSET setting, regulator switches at
maximum duty cycle; when VIN is above FDSET setting, enters into
Hiccup mode with 1 ms retry period.
Inductor shorted
Sense resistor open
Sense resistor shorted
LED string open [1]
Dim light from LED
Dim light from LED
Dim light from LED
Yes
Yes
Yes
Current spike trips SW OCP and turns off switching, entering into
Hiccup mode with about 1 ms retry period.
High differential sense voltage causes IC to shut off switching,
entering into Hiccup mode with about 1 ms retry period.
Triggers SW OCP fault, entering into Hiccup mode with about 1 ms
retry period.
No light from LED
Dim light from LED
Yes [1]
Yes
Enter into Hiccup mode with about 1 ms retry period.
LED Strings shorted [2]
(Either LED shorted to GND or
Output cap shorted to GND) < 1.5 V
Continues switching at minimum TON; regulator will not enter into
Hiccup mode.
Output cap open
Boot capacitor open
Boot capacitor shorted
Normal light from LED
Dim light from LED
No light from LED
No
Yes
No
Normal operation (since IC only monitors inductor current)
IC attempts to switch but can’t fully turn on SW.
IC detects undervoltage fault across BOOT capacitor and will not
start switching.
TON resistor open
Dim light from LED
Dim light from LED
Yes
Yes
Enter into Hiccup mode with about 1 ms retry period.
Enter into Hiccup mode with about 1 ms retry period.
TON resistor shorted
[1] For LED Open Fault, fault flag will not be asserted when VIN is below preset mask threshold, ADIM is below 0.264 V or PWM dimming pulse width is
below fault mask timer.
[2] For LED Short Fault, fault flag will not be asserted when ADIM is below 0.264 V or PWM dimming pulse width is below fault mask timer.
16
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Automotive-Grade, Constant-Current 2.0 A
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ALT80800
CLAMPDIODESFORLEDOPEN/SHORTPROTECTION
Refer to Figure 14. It is recommended to add clamp diode D1 to
provide LED short protection when VIN is above 40 V; if VIN is
below 40 V, D1 is not needed. Diode D2 is needed to clamp the
overshoot from L-C resonance due to LED Open fault when VIN
is above 45 V; when VIN is below 45 V, D2 is not required.
VIN
D2
L1
RSENSE
VOUT
SW
CSH
CLED
D1
Figure 14: Clamp Diode D1 for LED Short Protection when VIN is above 40 V.
Clamp Diode D2 for LED Open Protection when VIN is above 45 V.
17
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
the inductor and LEDs can handle the peak current (average cur-
COMPONENT SELECTIONS
rent × 1.2 in this case). However, higher ripple current percentage
affects the accuracy of LED current, and limits the minimum
current that can be regulated when using ADIM.
The inductor is often the most critical component in a buck con-
verter. Follow the procedure below to derive the correct param-
eters for the inductor:
• In general, allowing a higher ripple current percentage enables
lower-inductance inductors to be used, which results in smaller
size and lower cost.
1. Determine the saturation current of the inductor. This can be
done by simply adding 20% to the average LED current:
iSAT ≥ iLED × 1.2.
• If lower ripple current is required for the LED string, one
solution is to add a small capacitor (such as 1 to 2.2 μF) across
the LED string from LED+ to ground. In this case, the induc-
tor ripple current remains high while the LED ripple current is
greatly reduced.
2. Determinetheripplecurrentamplitude(peak-to-peakvalue).As
a general rule, ripple current should be kept between 10% and
30% of the average LED current:
0.1 < iRIPPLE(pk-pk) / iLED < 0.3.
3. Calculate the inductance based on the following equations:
L = (VIN – VOUT) × D × t / iRIPPLE , and
• The effectiveness of this filter capacitor depends on many fac-
tors, such as: switching frequency, inductors used, PCB layout,
LED voltage and current, and so forth.
D = VOUT / VIN
,
where
D is the duty cycle, and
t is the period 1/fSW
OUTPUT FILTER CAPACITOR
The ALT80800 is designed to operate in current regulation mode.
Therefore it does not require a large output capacitor to stabilize
the output voltage. This results in lower cost and smaller PCB
area. In fact, having a large output capacitor is not recommended.
• The addition of this capacitor introduces a longer delay in LED
current during PWM dimming operation. Therefore the accuracy
of average LED current is reduced at short PWM on-time.
.
INDUCTOR SELECTION CHART
The chart in the figure below summarizes the relationship
between LED current, switching frequency, and inductor value.
Based on this chart: assuming LED current = 1 A and L = 22 μH,
then minimum fSW = 0.68 MHz in order to keep the ripple current
at 20% or lower. If the switching frequency is lower, then a larger
inductance must be used to meet the same ripple current require-
ment.
In most applications, however, it is beneficial to add a small filter
capacitor (around 0.1 μF) across the LED string. This capacitor
serves as a filter to eliminate switching spikes seen by the LED
string. This is very important in reducing EMI noises, and may
also help in ESD testing.
In PWM dimming operation and when VIN is above 40 V, it is
suggested to use a 0.047 µF output capacitor, as described in
Enable and Dimming section.
ADDITIONAL NOTES ON RIPPLE CURRENT
• For consistent switching frequency, it is recommended to
choose the inductor and switching frequency to ensure the induc-
tor ripple current percentage is at least 10% over normal operat-
ing voltage range (ripple current is lowest at lowest VIN).
• If ripple current is less than 10%, the switching frequency may
jitter due to insufficient ripple voltage across CSH and CSL pins.
However, the average LED current is still regulated.
• For best accuracy in LED current regulation, a low current
ripple of less than 20% is required.
Figure 15: Minimum switching frequency vs. LED current,
given different inductance used
(VIN = 12 V, VOUT = 6 V, ripple current = 20%)
• There is no hard limit on the highest ripple current percentage
allowed. A 40% ripple current is still acceptable, as long as both
18
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
Effects of Output Capacitor on LED Ripple Current
VIN
VIN
L1
L1
iRIPPLE
iRIPPLE
RSENSE
RSENSE
LED+
GND
LED+
GND
iRIPPLE
Without output capacitor:
With a small capacitor across LED string:
The same inductor ripple current flows through
sense resistor and LED string.
Ripple current through LED string is reducted, while
ripple voltage across RSENSE remains high.
Figure 16: Using an Output Filter Capacitor to Reduce
Ripple Current in LED String
19
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
APPLICATION CIRCUIT DIAGRAMS
VIN
ALT80800
4.7 µF
RON
178 kΩ
33 µF +
VIN
SW
RSENSE
L1 33 µH
CBOOT
0.2 Ω
TON
PWM
ADIM
VCC
BOOT
0.47 µF
LED+
PWM
CSH
CSL
ADIM
CLED
0.1 µF
External PWM
dimming signal
VCC
VCC
10 kΩ
2.2 µF
VIN
VCCIN
EN
FFn
FFn
187 kΩ
100 kΩ
VIN
0.1 µF
GND SGND
PGND
FDSET
187 kΩ
Figure 17: Application Circuit Example for ALT80800
(LED current = 1 A, 500 kHz)
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Automotive-Grade, Constant-Current 2.0 A
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ALT80800
APPLICATION CIRCUIT DIAGRAMS (continued)
Input
Voltage
VIN
5 V
BOOT
SW
ADIM
Voltage
CBOOT
L1
i_L1
Rsc1
SW
ADIM
CSL1
CSH1
CLED1
GND
FFn
VOUT
ALT80800
VIN
BOOT
SW
CBOOT
L2
i_L2
Rsc2
ADIM
SW
CSH2 CSL2
CLED2
GND
FFn
ALT80800
Figure 18: Using 2 (or more) ALT80800 in parallel to drive the same LED string.
Total LED current is the sum of currents from each LED driver.
(Note: each LED driver shares the same VIN and ADIM as illustrated).
21
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
PACKAGE OUTLINE DRAWINGS
For Reference Only – Not for Tooling Use
(Reference MO-153 ABT)
Dimensions in millimeters. NOT TO SCALE
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
0.45
0.65
8º
0º
16
5.00 0.10
16
1.70
0.20
0.09
B
3.00
4.40 0.10
6.10
3 NOM
6.40 0.20
A
0.60 0.15
1.00 REF
1
2
3 NOM
1
2
0.25 BSC
Branded Face
SEATING PLANE
GAUGE PLANE
3.00
C
16X
SEATING
PLANE
0.10
C
C
PCB Layout Reference View
0.30
0.19
1.20 MAX
0.65 BSC
NNNNNNN
YYWW
LLLL
0.15
0.00
A
B
C
Terminal #1 mark area
Exposed thermal pad (bottom surface); dimensions may vary with device
1
D
Standard Branding Reference View
Reference land pattern layout (reference IPC7351 SOP65P640X110-17M);
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)
N = Device part number
= Supplier emblem
Y = Last two digits of year of manufacture
W = Week of manufacture
L
= Characters 5-8 of lot number
D
Branding scale and appearance at supplier discretion
Package LP, 16-Pin TSSOP with Exposed Thermal Pad
22
Allegro MicroSystems, LLC
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Automotive-Grade, Constant-Current 2.0 A
PWM Dimmable Synchronous Buck LED Driver
ALT80800
Revision History
Number
Date
Description
–
1
December 7, 2017
November 13, 2018
Initial release
Updated Enable and Dimming section (page 8) and Output Filter Capacitor section (page 18).
Copyright ©2018, Allegro MicroSystems, LLC
Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to
permit 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 any devices or systems, including but not limited to life support devices or systems, in which a failure of
Allegro’s product can reasonably be expected to cause bodily harm.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its
use; nor for any infringement of patents or other rights of third parties which may result from its use.
Copies of this document are considered uncontrolled documents.
For the latest version of this document, visit our website:
www.allegromicro.com
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相关型号：
ALT80800KLPATR
Automotive-Grade, Constant-Current 2.0 A PWM Dimmable Synchronous Buck LED Driver
ALLEGRO
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