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AN709

Designing with the Si9976DY N-Channel Half-Bridge Driver and LITTLE FOOT® Dual MOSFETs

Vishay Semiconductors 

AN709

Vishay Siliconix

A FULLĆBRIDGE APPLICATION

Figure 8 shows a basic implementation of the Si9976DY and

Si9945DY in a full-bridge configuration. Each half-bridge is

made up of one Si9976DY driver IC, one Si9945DY LITTLE

FOOT dual n-channel MOSFET, a bootstrap capacitor, a filter

capacitor for VDD, and decoupling capacitors for each IC. This

configuration yields a full-bridge circuit with a continuous

current rating of 3 A without heatsinking. Use of the

Si9945DY or the Si4946EY yields current ratings of 3.7 A or

4.5 A, respectively.

Any circuit which generates signals with fast rise and fall times

can generate noise. This noise, if not dealt with, can affect the

operation of the circuit. Proper PC board layout techniques

and device decoupling will take care of these problems. The

signal ground trace from the Si9976DY and the trace from the

low-side MOSFET source should be run separately to the

common ground point. This prevents the noise generated by

fast MOSFET transitions from modulating the signal ground of

the Si9976DY. Similarly, the trace to the V+ input of the

Si9976DY and the trace to the drain of the high-side MOSFET

should be connected separately to the supply bypass

capacitor.

In addition to layout considerations, decoupling capacitors are

required to deal with noise. Adding capacitors across the

power supply lines, V+, VDD, and VCC, provides a low

impedance to ground for switching noise and serves as a local

energy reservoir when there is a demand for surge current.

The VDD capacitor provides the surge current required to turn

on the low-side MOSFET.

In addition to basic decoupling, the capacitors added across

the half-bridge itself minimize the surge current in the power

supply traces, and therefore reduce the generated noise.

Although a single capacitor, typically 0.01 mF, works well to

decouple a single pin, it is advisable to apply several decades

of capacitance across the input power, V+ to GND, to handle

the broad spectrum of noise that can be present. The

high-frequency (lower value) capacitors should be located as

close as possible to the device being decoupled, while the

larger capacitors (> 1 mF) can be located farther away and

bypass only the power supply.

Figure 9 shows a typical layout for a Si9976DY with LITTLE

FOOT dual n-channel MOSFETs. The use of surface-mount

packages allows automated assembly of the entire motor drive

circuit, without the need for a separate heatsink and its

associated material and assembly costs.

SUMMARY

The Si9976DY provides both low- and high-side gate drive,

high-side level shifting, a bootstrap/charge pump high-side

power supply, and protection for undervoltage and short circuit

conditions in a single surface-mount IC. The Si4946EY,

Si9945DY, are Si9945DY are surface-mount MOSFETs for

power switching over a broad current range (2 to 5 A) and

require no heatsinking. The use of surface-mount packages

allows automated assembly of the entire drive system while

minimizing use of PC board space. The Si9976DY, when used

with one of the dual n-channel LITTLE FOOT power

MOSFETs, provides a very flexible approach to power

switching in dc motor drives.

C1

GND

IN

EN

VCC

FAULT

C6

GND

IN

EN

VCC

FAULT

C3

U1

U2

A

C4

D1 C2 C8

U3

C5

U4

B

C9

D2

C10

C7

Figure 9 Typical PC Board Layout (Scale 1:1)

Document Number: 70582

15-Jun-00

www.vishay.com S FaxBack 408-970-5600

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