SI786LRG-T1-E3 [VISHAY]
IC DUAL SWITCHING CONTROLLER, 330 kHz SWITCHING FREQ-MAX, PDSO28, 5.30 MM, LEAD FREE, SSOP-28, Switching Regulator or Controller;
| 型号： | SI786LRG-T1-E3 |
| 厂家： | 
VISHAY |
| 描述： | IC DUAL SWITCHING CONTROLLER, 330 kHz SWITCHING FREQ-MAX, PDSO28, 5.30 MM, LEAD FREE, SSOP-28, Switching Regulator or Controller 信息通信管理 开关 光电二极管 |
| 文件： | 总17页 (文件大小：206K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Si786
Vishay Siliconix
Dual-Output Power-Supply Controller
FEATURES
D Fixed 5-V and 3.3-V Step-down Converters
D Less than 500-mA Quiescent Current per Converter
D 25-mA Shutdown Current
D 5.5-V to 30-V Operating Range
DESCRIPTION
The Si786 Dual Controller for Portable Computer Power
Conversion is pin and functionally compatible with the
MAX786 dual-output power supply controller for notebook
computers. The device is designed as a drop-in replacement
for that circuit.
A complete power conversion and management system can
be implemented with the Si786 Dual Controller for Portable
Computer Power Conversion, an inexpensive linear regulator,
the Si9140 SMP Controller for High Performance Processor
Power Supplies, five Si4410 n-channel TrenchFETR Power
MOSFETs, one Si4435 p-channel TrenchFET Power
MOSFET, and two Si9712 PC Card (PCMCIA) Interface
Switches.
The circuit is a system level integration of two step-down
controllers, micropower 5-V and 3.3-V linear regulators, and
two comparators. The controllers perform high efficiency
conversion of the battery pack energy (typically 12 V) or the
output of an ac to dc wall converter (typically 18-V to 24-V dc)
to 5-V and 3.3-V system supply voltages. The micropower
linear regulator can be used to keep power management and
back-up circuitry alive during the shutdown of the step-down
converters. The comparators can be biased at any voltage
between 2.7 V and the input voltage, simplifying battery
monitoring or providing sufficient voltage to enhance the gate
of a low on-resistance n-channel FET used in switching power
to different zones in the system.
The Si9130 Pin-Programmable Dual Controller for Portable
PCs is another integrated system level devices for portable PC
power systems.
The Si786 is available in both standard and lead (Pb)-free
28-pin SSOP packages and specified to operate over the (0_C
to 70_C), (−10_C to 90_C) and (−40_C to 85_C) temperature
ranges. See Ordering Information for corresponding part
numbers.
FUNCTIONAL BLOCK DIAGRAM
3.3 V
mP
5.5. V
to
30 V
Power
Memory
Section
5 V
Peripherals
Si786
SHUTDOWN
5-V ON/OFF
3.3-V ON/OFF
SYNC
Low-Battery Warning
Power-Good
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
1
Si786
Vishay Siliconix
ABSOLUTE MAXIMUM RATINGS
V+ to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to 36 V
REF, V Short to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Momentary
L
PGND to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "2 V
REF Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 mA
V
to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 7 V
L
V
Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 mA
L
BST , BST to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 36 V
3
5
Continuous Power Dissipation (T = 70_C)a
A
LX to BST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -7 V to 0.3 V
b
3
3
28-Pin SSOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 762 mW
LX to BST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -7 V to 0.3 V
5
5
Operating Temperature Range: (T
to T
)
MIN
MAX
Inputs/Outputs to GND
(D , D , SHDN, ON , REF, SS , CS . FB , SYNC, CS , FB ,
SS , ON ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V, (V + 0.3 V)
Si786CG/CRG/CSG (C-Grade) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 to 70_C
Si786LG/LRG/LSG (L-Grade) . . . . . . . . . . . . . . . . . . . . . . . . . . . −10_ to 90_C
Si786DG/DRG/DSG (D-Grade) . . . . . . . . . . . . . . . . . . . . . . . . . −40_ to 85_C
1
2
5
5
5
5
3
3
3
3)
L
V
H
to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V to 20 V
Lead Temperature (soldering, 10 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . 300_C
Q , Q to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V, (V + 0.3 V)
1
2
H
DL , DL to PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V, (V + 0.3 V)
3
5
L
Notes
DH to LX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V (BST + 0.3 )
3
3
3
a. Device mounted with all leads soldered or welded to PC board.
DH to LX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 V (BST + 0.3 )
b. Derate 9.52 mW/_C above 70_C.
5
5
5
Exposure to Absolute Maximum rating conditions for extended periods may affect device reliability. Stresses above Absolute Maximum rating may cause permanent
damage. Functional operation at conditions other than the operating conditions specified is not implied. Only one Absolute Maximum rating should be applied at any one
time
SPECIFICATIONS
Limitse
Typb
Specific Test Conditions
V+ = 15 V, I = I
= 0 mA, SHDN = ON = ON = 5 V
VL
REF
3
5
to T
Parameter
Mina
Maxa
Unit
Other Digital Input Levels 0 V or 5 V, T = T
A
MIN
MAX
3.3-V and 5-V Step-Down Controllers
Input Supply Range
5.5
30
0 mV < (CS -FB ) < 70 mV, 6 V < V + < 30 V
5
5
FB Output Voltage
5
4.80
5.08
5.20
(includes load and line regulation)
V
Si786CG/LG/DG
Si786CRG/LRG/DRG
Si786CSG/LSG/DSG
3.17
3.32
3.46
3.35
3.50
3.65
2.5
3.46
3.60
3.75
0 mV < (CS -FB ) < 70 mV
3
3
FB Output Voltage
3
6 V < V + < 30 V
(includes load and line regulation)
Load Regulation
Line Regulation
Either Controller (CS_ to FB_ = 0 to 70 mV)
Either Controller (V+ = 6 V to 30 V)
%
0.03
100
100
4.0
%/V
80
77
2.5
2.3
2
120
120
6.5
6.5
Current-Limit Voltage
CS -FB or CS -FB
5
mV
3
3
5
Si786DG/DRG/DSG
Si786DG/DRG/DSG
SS /SS Source Current
mA
3
5
4.0
SS /SS Fault Sink Current
mA
3
5
Internal Regulator and Reference
ON = ON = 0 V, 5.5 V < V+ < 30 V
5
3
V
V
Output Voltage
4.5
5.5
L
0 mA < I < 25 mA
L
Fault Lockout Voltage
Falling Edge, Hysteresis = 1%
3.6
4.2
4.2
4.7
3.36
3.2
75
L
V
V /FB Switchover Voltage
L
Rising Edge of FB , Hysteresis = 1%
5
5
c
REF Output Voltage
No External Load
3.24
2.4
REF Fault Lockout Voltage
REF Load Regulation
Falling Edge
d
0 mA < I < 5 mA
30
25
mV
L
SHDN = D = D = ON = ON = 0 V
1
2
3
5
V+ Shutdown Current
V+ Standby Current
40
V+ = 30 V
mA
70
70
110
115
8.6
9.0
60
D
1
= D = ON = ON = 0 V, V+ =
2
3
5
30 V
= D = 0 V, FB = CS =
5
Si786DG/DRG/DSG
Si786DG/DRG/DSG
D
1
5.5
5.5
30
2
5
Quiescent Power Consumption
(both PWM controllers on)
5.25 V
FB = CS = 3.5 V
mW
3
3
V+ Off Current
FB = CS = 5.25 V, V Switched Over to FB
5
mA
5
5
L
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
2
Si786
Vishay Siliconix
SPECIFICATIONS
Specific Test Conditions
Limitse
Typb
V+ = 15 V, I = I
= 0 mA, SHDN = ON = ON = 5 V
VL
REF
3
5
to T
Parameter
Mina
Maxa
Unit
Other Digital Input Levels 0 V or 5 V, T = T
A
MIN
MAX
Comparators
1.61
1.60
1.69
1.69
"100
30
D , D Trip Voltage
Falling Edge, Hysteresis = 1%
V
1
2
Si786DG/DRG/DSG
D , D Input Current
D
1
= D = 0 V, 5 V
nA
1
2
2
Q , Q Source Current
12
20
1
2
V
H
= 15 V, V
= 2.5 V
mA
OUT
Q , Q Sink Current
200
500
1000
1
2
Q , Q Output High Voltage
I
= 5 mA, V = 3 V
V - 0.5
H
1
2
SOURCE
H
V
Q , Q Output Low Voltage
I
= 20 mA, V = 3 V
0.4
10
1
2
SINK
H
Quiescent V Current
H
V
H
= 18 V, D = D = 5 V, No External Load
4
mA
1
2
Oscillator and Inputs/Outputs
270
300
300
200
200
330
330
230
230
SYNC = 3.3 V
Si786DG/DRG/DSG
Si786DG/DRG/DSG
260
170
165
200
200
Oscillator Frequency
kHz
ns
SYNC = 0 V, 5 V
SYNC High Pulse Width
SYNC Low Pulse Width
SYNC Rise/Fall Time
Oscillator SYNC Range
Not Tested
200
350
240
89
kHz
%
SYNC = 3.3 V
92
95
Maximum Duty Cycle
Input Low Voltage
SYNC = 0 V, 5 V
92
SHDN, ON , ON SYNC
0.8
3
5
SHDN, ON , ON
3
2.4
V
5
Input High Voltage
Input Current
SYNC
V - 0.5
L
SHDN, ON , ON
V = 0 V, 5 V
IN
"1
mA
3
5
DL /DL Sink/Source Current
V = 2 V
OUT
1
1
3
5
A
DH /DH Sink/Source Current
BST - LX = BST - LX = 4.5 V, V
= 2 V
3
5
3
3
5
5
OUT
DL /DL On-Resistance
High or Low
High or Low
7
7
3
5
W
DH /DH On-Resistance
3
5
BST - LX = BST - LX = 4.5 V
3
3
5
5
Notes
a. The algebraic convention whereby the most negative value is a minimum and the most positive a maximum.
b. Typical values are for DESIGN AID ONLY, not guaranteed nor subject to production testing.
c. The main switching outputs track the reference voltage. Loading the reference reduces the main outputs slightly according to the closed-loop gain (AV ) and
CL
the reference voltage load-regulation error. AV for the 3.3-V supply is unity gain. AV for the 5-V supply is 1.54.
CL
CL
d. Since the reference uses V as its supply, its V+ line regulation error is insignificant.
L
e. Limits are for all temperature grades unless otherwise noted.
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
3
Si786
Vishay Siliconix
TYPICAL CHARACTERISTICS (25_C UNLESS NOTED)
Efficiency vs. 5-V Output Current, 200 kHz
Efficiency vs. 5-V Output Current, 300 kHz
100
100
90
80
70
60
50
V+ = 6 V
V+ = 6 V
90
V+ = 15 V
V+ = 15 V
80
V+ = 30 V
V+ = 30 V
70
SYNC = 0 V, 3.3 V Off
3.3 V Off
60
50
0.001
0.01
0.1
1
10
0.001
0.01
0.1
1
10
5-V Output Current (A)
5-V Output Current (A)
Efficiency vs. 3.3-V Output Current, 200 kHz
Efficiency vs. 3.3-V Output Current, 300 kHz
100
90
80
70
60
50
100
90
80
70
60
50
V+ = 6 V
V+ = 6 V
V+ = 15 V
V+ = 15 V
V+ = 30 V
V+ = 30 V
SYNC = 0 V, 5 V On
5 V On
0.001
0.01
0.1
1
10
0.001
0.01
0.1
1
10
3.3-V Output Current (A)
3.3-V Output Current (A)
Quiescent Supply Current vs. Supply Voltage
Standby Supply Current vs. Supply Voltage
0.5
0.4
0.3
0.2
0.1
0.0
30
25
20
15
10
5
ON = ON = High
ON = ON = 0 V
3
5
3
5
0
0
6
12
18
24
30
0
6
12
18
24
30
Supply Voltage (V)
Supply Voltage (V)
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
4
Si786
Vishay Siliconix
TYPICAL CHARACTERISTICS (25_C UNLESS NOTED)
Minimum V to V
Differential
IN
OUT
vs. 5-V Output Current
Shutdown Supply Current vs. Supply Voltage
1.0
0.8
0.6
0.4
0.2
0.0
100
5-V Output
Still Regulating
SHDN = 0 V
75
300 kHz
50
25
0
200 kHz
0
6
12
18
24
30
0.001
0.01
0.1
5-V Output Current (A)
1
10
Supply Voltage (V)
Switching Frequency vs. Load Current
1000.0
100.0
10.0
1.0
SYNC = REF (300 kHz)
ON = ON = 5 V
3
5
5 V, V+ = 30 V
5 V, V+ = 7.5 V
3.3 V, V+ = 7.5 V
0.1
0.1
1
10
Load Current (mA)
100
1000
t
LX 10 V/div
5-V Output
50 mV/div
I
= 100 mA
5-V Output Current = 1 A
Load
V
IN
= 10 V
V
IN
= 16 V
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
5
Si786
Vishay Siliconix
TYPICAL CHARACTERISTICS (25_C UNLESS NOTED)
RRENT
RRENT
t
put
IN
IN
V
= 15 V
V
= 15 V
t
t
16 V
10 V
LOAD
LOAD
I
= 2 A
I
= 2 A
put
ut
16 V
10 V
LOAD
LOAD
I
= 2 A
I
= 2 A
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
6
Si786
Vishay Siliconix
PIN DESCRIPTION AND ORDERING INFORMATION
CS
1
2
28
27
26
25
24
23
22
21
20
19
18
17
16
15
FB
3
3
SS
3
DH
3
ON
D
3
LX
3
3
1
2
BST
4
3
D
5
DL
V+
3
V
H
6
Q
7
V
L
2
1
SSOP-28
Q
8
FB
5
GND
REF
9
PGND
DL
5
10
11
12
13
14
SYNC
SHDN
BST
5
LX
5
ON
DH
5
5
SS
5
CS
5
Top View
PIN DESCRIPTION
Pin
Symbol
Description
1
2
3
4
5
6
CS
Current-sense input for 3.3-V Buck controller—this pins over current threshold is 100 mV with respect to FB .
3
3
SS
3
Soft-start input for 3.3 V. Connect capacitor from SS to GND.
3
ON
ON/OFF logic input disables the 3.3-V Buck controller. Connect directly to V for automatic turn-on.
L
3
D
D
Comparator #1 noninverting input, threshold = 1.650 V. Comparator #1 output = Q1. Connect to GND if unused.
1
Comparator #2 noninverting input (see D ).
1
2
V
H
External bias supply-voltage input for comparators #1 and #2.
Comparator #2 output. Sources 20 mA from V when D is high. Sinks 500 mA to GND when D is low regardless of V input
H
2
2
H
7
Q
2
1
voltage.
8
9
Q
Comparator #1 output (see Q ).
2
GND
REF
Analog ground.
10
3.3-V reference output. Supplies external loads up to 5 mA.
Oscillator control/synchronization input. Connect capacitor to GND, 1-mF/mA output or 0.22 mF minimum. For external clock
synchronization, a rising edge starts a new cycle to start. To use internal 200-kHz oscillator, connect to VL or GND. For
300-kHz oscillator, connect to REF.
11
SYNC
Shutdown logic input, active low. Connect to V for automatic turn-on. The 5-V V supply will not be disabled in shutdown allow-
L
L
12
SHDN
ing connection to SHDN.
13
14
15
16
17
18
19
20
ON
5
ON/OFF logic input disables the 5-V Buck Controller. Connect to V for automatic turn-on.
L
SS
5
Soft-start control input for 5 V Buck controller. Connect capacitor from SS to GND.
5
CS
5
Current-sense input for 5 V Buck controller—this pins over current threshold is 100 mV referenced to FB .
3
DH
Gate-drive output for the 5-V supply high-side n-channel MOSFET.
Inductor connection for the 5-V supply.
5
LX
5
BST
Boost capacitor connection for the 5-V supply.
Gate-drive output for the 5-V supply rectifying n-channel MOSFET.
Power Ground.
5
DL
5
PGND
FB
21
22
23
24
25
26
27
28
Feedback input for the 5-V Buck controller.
5
V
L
5-V logic supply voltage for internal circuitry—able to source 5-mA external loads. V remains on with valid voltage at V+.
L
V+
DL
Supply voltage input.
Gate-drive output for the 3.3-V supply rectifying n-channel MOSFET.
Boost capacitor connection for the 3.3-V supply.
Inductor connection for the 3.3-V supply.
3
BST
3
LX
3
DH
Gate-drive output for the 3.3-V supply high-side n-channel MOSFET.
Feedback input for the 3.3-V Buck controller.
3
FB
3
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
7
Si786
Vishay Siliconix
ORDERING INFORMATION
Lead (Pb)-Free
Part Number
Part Number
Temp Range
VOUT
Si786CG
3.3 V
Si786CG-T1
Si786CRG
Si786CRG-T1
Si786CSG
Si786CG-T1—E3
Si786CRG-T1—E3
Si786CSG-T1—E3
Si786LG-T1—E3
Si786LRG-T1—E3
Si786LSG-T1—E3
Si786DG-T1—E3
Si786DRG-T1—E3
Si786DSG-T1—E3
C-Grade
3.45 V
3.6 V
3.3 V
3.45 V
3.6 V
3.3 V
3.45 V
3.6 V
0 to 70_C
Si786CSG-T1
Si786LG
Si786LG-T1
Si786LRG
L-Grade
−10_ to 90_C
Si786LRG-T1
Si786LSG
Si786LSG-T1
Si786DG
Si786DG-T1
Si786DRG
Si786DRG-T1
Si786DSG
D-Grade
−40_ to 85_C
Si786DSG-T1
Demo Board
Temp Range
Board Type
Si786DB
0 to 70_C
Surface Mount
DESCRIPTION OF OPERATION
The Si786 is a dual step-down converter, which takes a 5.5-V
to 30-V input and supplies power via two PWM controllers (see
Figure 1). These 5-V and 3.3-V supplies run on an optional
300-kHz or 200-kHz internal oscillator, or an external sync
signal. Amount of output current is limited by external
components, but can deliver greater than 6 A on either supply.
As well as these two main Buck controllers, additional loads
can be driven from two micropower linear regulators, one 5 V
(VL) and the other 3.3 V (REF)—see Figure 2. These supplies
are each rated to deliver 5 mA. If the linear regulator circuits fall
out of regulation, both Buck controllers are shut down.
A low-side switching MOSFET connected to DL3 increases
efficiency by reducing the voltage across the rectifier diode. A
low value sense resistor in series with the inductor sets the
maximum current limit, to disallow current overloads at
power-on or in short-circuit situations.
The soft-start feature on the Si786 is capacitor programmable;
pin SS3 functions as a constant current source to the external
capacitor connected to GND. Excess currents at power-on are
avoided, and power-supplies can be sequenced with different
turn-on delay times by selecting the correct capacitor value.
Two voltage comparators with adjustable output voltages are
included in the Si786. They can be used for gate drive in load
switching applications, where n-channel MOSFETs are used.
Logic level voltages can be generated as well, for instance to
serve as mP interfacing (e.g. a Power-good signal).
5-V Switching Supply
The 5-V supply is regulated by a current-mode PWM controller
which is nearly the same as the 3.3-V output. The dropout
voltage across the 5-V supply, as shown in the schematic in
Figure 1, is 400 mV (typ) at 2 A. If the voltage at V+ falls, nearing
5 V, the 5-V supply will lower as well, until the VL linear regulator
output falls below the 4-V undervoltage lockout threshold.
Below this threshold, the 5-V controller is shut off.
3.3-V Switching Supply
The 3.3-V supply is regulated by a current-mode PWM
controller in conjunction with several externals: two n-channel
MOSFETs, a rectifier, an inductor and output capacitors (see
Figure 1). The gate drive supplied by DH3 needs to be greater
than VL , so it is provided by the bootstrap circuit consisting of
a 100-nF capacitor and diode connected to BST3 .
The frequency of both PWM controllers is set at 300 kHz when
the SYNC pin is tied to REF. Connecting SYNC to either GND
or VL sets the frequency at 200 kHz.
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
8
Si786
Vishay Siliconix
3.3-V and 5-V Switching Controllers
full PWM mode. Every cycle from the oscillator asserts the
output latch and drives the gate of the high-side MOSFET for
a period determined by the duty cycle (approximately
VOUT/VIN 100%) and the frequency. The high-side switch
turns off, setting the synchronous rectifier latch and 60ns later,
the rectifier MOSFET turns on. The low-side switch stays on
until the start of the next clock cycle in continuous mode, or
until the inductor current becomes positive again in
discontinuous mode. In over-current situations, where the
inductor current is greater than the 100-mV current-limit
threshold, the high-side latch is reset and the high-side gate
drive is shut off.
Each PWM controller on the Si786 is identical with the
exception of the preset output voltages. The controllers only
share three functional blocks (see Figure 2): the oscillator, the
voltage reference (REF) and the 5-V logic supply (VL). The
3.3-V and 5-V controllers are independently enabled with pins
ON3 and ON5 , respectively. The PWMs are a direct-summing
type, without the typical integrating error amplifier along with
the phase shift which is a side effect of this type of topology.
Feedback compensation is not needed, as long as the output
capacitance and its ESR requirements are met, according to
the Design Considerations section of this data sheet.
During low-current load requirements, the inductor current will
not deliver the 25-mV minimum current threshold. The
Minimum Current comparator signals the PWM to enter
pulse-skipping mode when the threshold has not been
reached. Pulse-skipping mode skips pulses to reduce
switching losses, the losses which decrease efficiency the
most at light load. Entering this mode causes the minimum
current comparator to reset the high-side latch at the beginning
of each oscillator cycle.
The main PWM comparator is an open loop device which is
comprised of three comparators summing four signals: the
feedback voltage error signal, current sense signal,
slope-compensation ramp and voltage reference as shown in
Figure 3. This method of control comes closer to the ideal of
maintaining the output voltage on a cycle-by-cycle basis.
When the load demands high current levels, the controller is in
Soft-Start
To slowly bring up the 3.3-V and 5-V supplies, connect
capacitors from SS3 and SS5 to GND. Asserting ON3 or ON5
starts a 4-mA constant current source to charge these
capacitors to 4 V. As the voltage on these pins ramps up, so
does the current limit comparator threshold, to increase the
duty cycle of the MOSFETs to their maximum level. If ON3 or
ON5 are left low, the respective capacitor is discharged to
GND. Leaving the SS3 or SS5 pins open will cause either
controller to reach the terminal over-current level within 10 ms.
efficiency. The low-side rectifier is shut off when the inductor
current drops to zero.
Shoot-through current is the result when both the high-side
and rectifying MOSFETs are turned on at the same time.
Break-before-make timing internal to the Si786 manages this
potential problem. During the time when neither MOSFET is
on, the Schottky is conducting, so that the body diode in the
low-side MOSFET is not forced to conduct.
Synchronous rectification is always active when the Si786 is
powered-up, regardless of the operational mode.
Soft start helps prevent current spikes at turn-on and allows
separate supplies to be delayed using external
programmability.
Gate-Driver Boost
Synchronous Rectifiers
The high-side n-channel drive is supplied by a flying-capacitor
boost circuit (see Figure 4). The capacitor takes a charge from
VL and then is connected from gate to source of the high-side
MOSFET to provide gate enhancement. At power-up, the
low-side MOSFET pulls LX_ down to GND and charges the
BST_ capacitor connected to 5 V. During the second half of the
oscillator cycle, the controller drives the gate of the high-side
MOSFET by internally connecting node BST_ to DH_. This
supplies a voltage 5 V higher than the battery voltage to the
gate of the high-side MOSFET.
Synchronous rectification replaces the Schottky rectifier with
a MOSFET, which can be controlled to increase the efficiency
of the circuit.
When the high-side MOSFET is switched off, the inductor will
try to maintain its current flow, inverting the inductor’s polarity.
The path of current then becomes the circuit made of the
Schottky diode, inductor and load, which will charge the output
capacitor. The diode has a 0.5-V forward voltage drop, which
contributes a significant amount of power loss, decreasing
efficiency. A low-side switch is placed in parallel with the
Schottky diode and is turned on just after the diode begins to
conduct. Because the rDS(ON) of the MOSFET is low, the I*R
voltage drop will not be as large as the diode, which increases
Oscillations on the gates of the high-side MOSFET in
discontinuous mode are a natural occurrence caused by the
LC network formed by the inductor and stray capacitance at
the LX_ pins. The negative side of the BST_ capacitor is
connected to the LX_ node, so ringing at the inductor is
translated through to the gate drive.
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
9
Si786
Vishay Siliconix
SCHEMATIC DRAWINGS
INPUT
5.5 V to 30 V
100 W
C1
22 mF
C10
22 mF
D2A
1N4148
0.1 mF
D2B
1N4148
5 V at 5 mA
Si786
10 mF
C5
0.1 mF
C4
0.1 mF
23
22
18
16
17
V
V+
L
25
27
26
BST
BST
3
5
N1
N3
N2
N3
DH
DH
LX
3
5
R1
25 mW
L1
L2
R2
25 mW
10 mH
10 mH
LX
3
5
5 V at
3 A
3.3 V
at 3 A
D1
D1FS4
D1
D1FS4
24
19
C7
150 mF
C6
330 mF
DL
DL
3
5
1
28
2
15
21
14
CS
CS
FB
SS
3
3
3
5
5
5
C12
150 mF
(Note 1)
(Note 1)
FB
SS
C9
0.01 mF
C8
0.01 mF
3
6
4
ON
ON
3.3 V ON/OFF
5 V ON/OFF
SHUTDOWN
V
COMPARATOR SUPPLY INPUT
3
H
13
12
11
9
IN
D
5
1
1
COMPARATOR 1
8
OUT
SHDN
SYNC
GND
Q
5
D
IN
OSC SYNC
2
2
COMPARATOR 2
7
Q
OUT
10
20
REF
PGND
Note 1: Use short, Kelvin-connected
PC board traces placed very
close to one another.
3.3 V at 5 mA
C3
1 mF
FIGURE 1. Si786 Application Circuit
5-V LDO
FB
CS
BST
3
V+
Linear
3
Regulator
3.3-V
PWM
Controller
(See Figure 3)
3
V
L
DH
LX
3
3.3-V
Reference
3
REF
DL
3
ON
ON
SS
3
4.5 V
SHDN
PGND
4 V
ON
3
FB
5
CS
5
2.8 V
5-V
PWM
Controller
(See Figure 3)
BST
5
DH
5
STANDBY
300 kHz/200 kHz
Oscillator
SYNC
ON
LX
5
DL
5
ON
SS
5
ON
5
V
H
D
1
Q
1
1.65 V
D
2
Q
2
1.65 V
FIGURE 2. Si786 Block Diagram
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
10
Si786
Vishay Siliconix
CS
_
1X
60 kHz
LPF
FB_
REF, 3.3 V
(or Internal 5-V
Reference)
Summing
Comparator
BST_
R
S
Q
Level
DH
Shift
_
LX_
OSC
Slope
Comp
Minimum Current
(Pulse-Skipping)
25 mV
V
L
Current
Limit
4 mA
Shoot-
Through
Control
0 mV to
100 mV
SS_
ON_
30R
1R
3.3 V
Synchronous
Rectifier Control
V
L
R
S
Q
Level
Shift
DL_
PGND
FIGURE 3. Si786 Controller Block Diagram
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
11
Si786
Vishay Siliconix
BATTERY
INPUT
V
L
V
L
BST_
DH_
Level
Translator
PWM
LX_
DL_
V
L
FIGURE 4. Boost Supply for Gate Drivers
OPERATIONAL MODES
PWM Mode
Pulse-Skipping Mode
When the load requires less than 25% of its maximum, the
Si786 enters a mode which drives the gate for one clock cycle
and skips the majority of the remaining cycles. Pulse-skipping
mode cuts down on the switching losses, the dominant power
consumer at low current levels.
The 3.3-V and 5-V Buck controllers operate in
continuous-current PWM mode when the load demands more
than approximately 25% of the maximum current (see typical
curves). The duty cycle can be approximated as Duty_Cycle
In the region between pulse-skipping mode and PWM mode,
the controller may transition between the two modes,
delivering spurts of pulses. This may cause the current
waveform to look irregular, but will not overly affect the ripple
voltage. Even in this transitional mode efficiency will stay high.
= VOUT/VIN
.
In this mode, the inductor current is continuous; in the first half
of the cycle, the current slopes up when the high-side
MOSFET conducts and then, in the second half, slopes back
down when the inductor is providing energy to the output
capacitor and load. As current enters the inductor in the first
half-cycle, it is also continuing through to the load; hence, the
load is receiving continuous current from the inductor. By using
this method, output ripple is minimized and smaller form-factor
inductors can be used. The output capacitor’s ESR has the
largest effect on output ripple. It is typically under 50 mV; the
worst case condition is under light load with higher input
battery voltage.
Current Limit
The current through an external resistor, is constantly
monitored to protect against over-current. A low value resistor
is placed in series with the inductor. The voltage across it is
measured by connecting it between CS_ and FB_. If this
voltage is larger than 100 mV, the high-side MOSFET drive is
shut down. Eliminating over-currents protects the MOSFET,
the load and the power source. Typical values for the sense
resistors with a 3-A load will be 25 mW.
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
12
Si786
Vishay Siliconix
Oscillator and SYNC
DESIGN CONSIDERATIONS
Inductor Design
There are two ways to set the Si786 oscillator frequency: by
using an external SYNC signal, or using the internal oscillator.
The SYNC pin can be driven with an external CMOS level
signal with frequency from 240 kHz and 350 kHz to
synchronize to the internal oscillator. Tying SYNC to either VL
or GND sets the frequency to 200 kHz and to REF sets the
frequency to 300 kHz.
Three specifications are required for inductor design:
inductance (L), peak inductor current (ILPEAK), and coil
resistance (RL). The equation for computing inductance is:
ǒV ǓǒV
OUTǓ
IN(MAX)–V
OUT
L + ǒ
Ǔ( )ǒ Ǔ(
VIN(MAX) f IOUT LIR
)
Operation at 300 kHz is typically used to minimize output
passive component sizes. Slower switching speeds of
200 kHz may be needed for lower input voltages.
Where:
V
= Output voltage (3.3 V or 5 V);
OUT
V
= Maximum input voltage (V);
IN(MAX)
f = Switching frequency, normally
300 kHz;
I
= Maximum dc load current (A);
OUT
Internal VL and REF
LIR = Ratio of inductor peak-to-peak ac current to
average dc load current, typically 0.3.
A 5-V linear regulator supplies power to the internal logic
circuitry. The regulator is available for external use from pin VL ,
able to source 5 mA. A 10-mF capacitor should be connected
between VL and GND. To increase efficiency, when the 5 V
switching supply has voltage greater than 4.5 V, VL is internally
switched over to the output of the 5-V switching supply and the
linear regulator is turned off.
When LIR is higher, smaller inductance values are acceptable,
at the expense of increased ripple and higher losses.
The peak inductor current (ILPEAK) is equal to the steady-state
load current (IOUT) plus one half of the peak-to-peak ac current
(ILPP). Typically, a designer will select the ac inductor current
to be 30% of the steady-state current, which gives ILPEAK equal
to 1.15 times IOUT
.
The 5-V linear regulator provides power to the internal 3.3-V
bandgap reference (REF). The 3.3-V reference can supply 5
mA to an external load, connected to pin REF. Between REF
and GND connect a capacitor, 0.22 mF plus 1 mF per mA of load
current. The switching outputs will vary with the reference;
therefore, placing a load on the REF pin will cause the main
outputs to decrease slightly, within the specified regulation
tolerance.
The equation for computing peak inductor current is:
ǒV ǓǒV
OUTǓ
Ǔ
IN(MAX)–V
OUT
ILPEAK + IOUT
)
ǒ
(2)(f)(L) VIN(MAX)
Output Capacitors
VL and REF supplies stay on as long as V+ is greater than 4.5
V, even if the switching supplies are not enabled. This feature
is necessary when using the micropower regulators to keep
memory alive during shutdown.
The output capacitors determine loop stability and ripple
voltage at the output. In order to maintain stability, minimum
capacitance and maximum ESR requirements must be met
according to the following equations:
VREF
CF
u ǒ
Both linear regulators can be connected to their respective
switching supply outputs. For example, REF would be tied to
the output of the 3.3 V and VL to 5 V. This will keep the main
supplies up in standby mode, provided that each load current
in shutdown is not larger than 5 mA.
ǒ
Ǔ
Ǔ
VOUT RCS (2)(p)(GBWP)
and,
ǒ
Ǔ
Ǔ
ǒ
VOUT RCS
ESRCF t
VREF
Fault Protection
Where:
C
= Output filter capacitance (F)
F
V
V
= Reference voltage, 3.3 V;
= Output voltage, 3.3 V or 5 V;
= Sense resistor (W);
REF
OUT
The 3.3 V and 5 V switching controllers as well as the
comparators are shut down when one of the linear
regulators drops below 85% of its nominal value; that is, shut
down will occur when VL < 4.0 V or REF < 2.8 V.
R
CS
GBWP = Gain-bandwidth product, 60 kHz;
ESR = Output filter capacitor ESR (W).
CF
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
13
Si786
Vishay Siliconix
Both minimum capacitance and maximum ESR requirements
must be met. In order to get the low ESR, a capacitance value
of two to three times greater than the required minimum may
be necessary.
Lower Voltage Input
The application circuit shown here can be easily modified to
work with 5.5-V to 12-V input voltages. Oscillation frequency
should be set at 200 kHz and increase the output capacitance
to 660 mF on the 5-V output to maintain stable performance up
to 2 A of load current. Operation on the 3.3-V supply will not be
affected by this reduced input voltage.
The equation for output ripple in continuous current mode is:
1
VOUT(RPL) + ILPP(MAX)
ESR
)
CF
ǒ
Ǔ
ǒ
Ǔ
2 P x f CF
The equations for capacitive and resistive components of the
ripple in pulse-skipping mode are:
–4
ǒ
Ǔ
(4) 10 (L)
1
1
VOUT(RPL)(C) +
ǒ
Ǔ
)
2
VOUT VIN–VOUT
ǒ
Ǔǒ
Ǔ
RCS CF
ǒ
Ǔ
(0.02) ESRCF
RCS
VOUT(RPL)(R) +
The total ripple, V
as follows:
, can be approximated
OUT(RPL)
if
then
otherwise,
V
V
V
V
(R) < 0.5 V
(C),
OUT(RPL)
OUT(RPL)
OUT(RPL)
OUT(RPL)
OUT(RPL)
= V
(C),
OUT(RPL)
= 0.5 V
(C) +
OUT(RPL)
(R).
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
www.vishay.com
14
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Vishay
Notice
Specifications of the products displayed herein are subject to change without notice. Vishay Intertechnology, Inc.,
or anyone on its behalf, assumes no responsibility or liability for any errors or inaccuracies.
Information contained herein is intended to provide a product description only. No license, express or implied, by
estoppel or otherwise, to any intellectual property rights is granted by this document. Except as provided in Vishay's
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Document Number: 91000
Revision: 08-Apr-05
www.vishay.com
1
Package Information
Vishay Siliconix
SSOP: 28-LEAD (5.3 MM) (POWER IC ONLY)
28
15
−B−
E
1
E
1
14
−A−
D
e
GAUGE PLANE
R
c
A
2
A
1
A
−C−
L
SEATING PLANE
ꢀ
SEATING PLANE
0.076
C
L
1
b
S
M
0.12
A
B
C
MILLIMETERS
Dim
A
A1
A2
b
c
D
E
E1
e
Min
Nom
1.88
Max
1.99
0.21
1.78
0.38
0.20
10.33
8.00
5.40
1.73
0.05
1.68
0.25
0.09
10.07
7.60
5.20
0.13
1.75
0.30
0.15
10.20
7.80
5.30
0.65 BSC
0.75
0.63
0.95
L
1.25 BSC
0.15
L1
R
0.09
− − −
0_
4_
8_
ꢀ
ECN: S-40080—Rev. A, 02-Feb-04
DWG: 5915
Document Number: 72810
28-Jan-04
www.vishay.com
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Vishay
Disclaimer
ALL PRODUCT, PRODUCT SPECIFICATIONS AND DATA ARE SUBJECT TO CHANGE WITHOUT NOTICE TO IMPROVE
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Revision: 12-Mar-12
Document Number: 91000
1
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