Si786

Vishay Siliconix

www.vishay.com

8

Document Number: 70189

S-40807—Rev. J, 26-Apr-04

ORDERING INFORMATION

Part Number

Lead (Pb)-Free

Part Number

Temp Range

Si786CG

33 V

Si786CG-T1

Si786CG-T1—E3

3.3 V

Si786CRG

C-Grade

3 45 V

Si786CRG-T1

Si786CRG-T1—E3

C-Grade

0 to 70_C

3.45 V

Si786CSG

36 V

Si786CSG-T1

Si786CSG-T1—E3

3.6 V

Si786LG

33 V

Si786LG-T1

Si786LG-T1—E3

3.3 V

Si786LRG

L-Grade

3 45 V

Si786LRG-T1

Si786LRG-T1—E3

L-Grade

−10_ to 90_C

3.45 V

Si786LSG

36 V

Si786LSG-T1

Si786LSG-T1—E3

3.6 V

Si786DG

33 V

Si786DG-T1

Si786DG-T1—E3

3.3 V

Si786DRG

D-Grade

3 45 V

Si786DRG-T1

Si786DRG-T1—E3

D-Grade

−40_ to 85_C

3.45 V

Si786DSG

36 V

Si786DSG-T1

Si786DSG-T1—E3

3.6 V

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

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 mP interfacing (e.g. a Power-good signal).

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

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;

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 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

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