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Document Number: 70189
S-40807-Rev. J, 26-Apr-04
Vishay Siliconix
Si786
Product is End of Life 3/2014
DESCRIPTION OF OPERATION
The Si786 is a dual step-down converter, which takes a 5.5V
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 sup-
ply. As well as these two main Buck controllers, additional
loads can be driven from two micropower linear regulators,
one 5 V (V
L
) 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.
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 P interfacing (e.g. a Power-good signal).
3.3 V Switching Supply
The 3.3 V supply is regulated by a current-mode PWM con-
troller in conjunction with several externals: two N-Channel
MOSFETs, a rectifier, an inductor and output capacitors (see
Figure 1). The gate drive supplied by DH
3
 needs to be
greater than V
L
 , so it is provided by the bootstrap circuit con-
sisting of a 100 nF capacitor and diode connected to BST
3
 .
A low-side switching MOSFET connected to DL
3
 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 programma-
ble; pin SS
3
 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.
5 V Switching Supply
The 5 V supply is regulated by a current-mode PWM control-
ler 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 V
L
 lin-
ear regulator output falls below the 4 V undervoltage lockout
threshold. Below this threshold, the 5 V controller is shut off.
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 V
L
 sets the frequency at 200 kHz.
ORDERING INFORMATION
Part Number
Lead (Pb)-free
Part Number
Temp Range
V
OUT
Si786CG
C-Grade
0 to 70 癈
3.3 V
Si786CG-T1
Si786CG-T1-E3
Si786CRG
3.45 V
Si786CRG-T1
Si786CRG-T1-E3
Si786CSG
3.6 V
Si786CSG-T1
Si786CSG-T1-E3
Si786LG
L-Grade
- 10?to 90 癈
3.3 V
Si786LG-T1
Si786LG-T1-E3
Si786LRG
3.45 V
Si786LRG-T1
Si786LRG-T1-E3
Si786LSG
3.6 V
Si786LSG-T1
Si786LSG-T1-E3
Si786DG
D-Grade
- 40?to 85 癈
3.3 V
Si786DG-T1
Si786DG-T1-E3
Si786DRG
3.45 V
Si786DRG-T1
Si786DRG-T1-E3
Si786DSG
3.6 V
Si786DSG-T1
Si786DSG-T1-E3
Demo Board
Temp Range
Board Type
Si786DB
0 to 70 癈
Surface Mount
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