AN709 データシートの表示（PDF） - Vishay Semiconductors

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AN709

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

Vishay Semiconductors 

AN709

Vishay Siliconix

Si9976

Si9976

Dual

Dual

DIR

IN

LITTLE

LITTLE

IN

FOOT

FOOT

MOSFET

MOSFET

EN

EN

PWM

DIR

Figure 7a. Sign-Magnitude Control

Si9976

IN

EN

Dual

LITTLE

FOOT

MOSFET

Dual

LITTLE

FOOT

MOSFET

Si9976

IN

EN

PWM

Figure 7b. Sign-Magnitude Control for Low-Side MOSFET PWM

There are a couple of things to be aware of in this mode of

operation. Application of the PWM signal to the EN input when

the IN input is held low will create an erroneous Fault signal

which is the inverse of the PWM signal. This can be eliminated

by applying the inverse of the PWM signal to the IN input as

shown in Figure 7b. Secondly, care must be taken to ensure

that the bootstrap capacitor has been charged prior to a

high-side turn on. As low-side on-times decrease, this

becomes of greater concern. Minimum low-side on-times must

be observed to ensure that the high-side will turn on.

Remember that this minimum time can be reduced by adding

an external bootstrap diode (see Figure 8). When this is done,

it increases the load on VDD and therefore on the decoupling

capacitor. The value of the VDD decoupling capacitor should

be doubled to prevent an undervoltage condition from

occurring.

CURRENT SENSING

If current sensing is required, a fractional W resistor can be

inserted in between the low-side MOSFET source connection

and ground. External op amps or comparators can then be

used to implement current limit or some other current control.

A Schottky diode must be connected from the half-bridge

output to ground to protect output from negative voltage

spikes. In addition to causing potential damage to the Si9976,

negative spikes can cause an erroneous latching FAULT. The

sensing resistor provides a small amount of isolation of the

MOSFET decoupling capacitors from ground. Make sure that

decoupling capacitors on MOSFETs are connected directly

across the MOSFET pair, high-side drain to low-side source to

maximize their effectiveness at reducing noise (see Figure 7).

CBOOT

BRAKING

Braking is accomplished by turning on both upper or both lower

MOSFETs in the H-bridge so the motor windings are shorted

together. If the upper MOSFETs are used for this function, be

certain that the bootstrap capacitors are charged prior to

turning them on.

IN4148

or

Equivalent

2 x CDD

CAP

S1

Si9976

VDD

Document Number: 70582

15-Jun-00

Figure 6. External Bootstrap Diode

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