Si9913

Vishay Siliconix

Document Number: 71343

S-40133—Rev. B, 16-Feb-04

www.vishay.com

7

Bootstrap Supply Operation

(see Functional Block Diagram)

The power to drive the high-side MOSFET (Q2) gate comes

through D1 during the time when the low-side MOSFET is on

sized to be greater than ten times the high-side MOSFET gate

capacitance, and large enough to supply the bootstrap current

droop.

Synchronous Enable

The synchronous enable pin serves to enable and disable the

drive to the low-side MOSFET gate. With SYN high, the

low-side MOSFET is driven on and off in antiphase with the

improves efficiency at high load currents because the flyback

drop. With SYN low, the low-side MOSFET is held off all the

under light load or pulse skipping mode, where there is a long

off time, because it prevents current flowing back from the

output to ground during the off time.

Layout Considerations

There are a few critical layout considerations for these parts.

Firstly, the IC must be decoupled as closely as possible to the

power pins. Secondly the IC should be placed physically close

to the high- and low-side MOSFETs it is driving. The major

consideration is that the MOSFET gates must be charged or

discharged in a few nanoseconds, and the peak current to do

this is of the order of 1 A. This current must flow from the

decoupling and bootstrap capacitors to the IC, and from the

output driver pin to the MOSFET gate, returning from the

MOSFET source to the IC. The aim of the layout is to reduce

the parasitic inductance of these current paths as much as

possible. This is accomplished by making these traces as

short as possible, and also running trace and its current return

path adjacent to each other.

APPLICATIONS

FIGURE 1.

Typical Applications Schematic Circuit Used to Obtain Typical Rising and Falling Switching Waveforms

8

6

7

5

1

3

2

4

GND

IN

SYN

BOOT

U1

Si9913

+5 V

C2

0.1 mF

PWM IN

GND

1 mF

C5

C1

0.1 mF

Enable

4

4

Q1

Si4412

Q2

Si4412

L1

15 mH

0.1 mF

C3

15 mF

C4

GND

+

GND