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

INPUT VOLTAGE REQUIREMENTS

The Si9976DY operates from a single supply voltage of 20 to

40 V dc. This voltage feeds both the bootstrap and the

low-voltage regulators. The bootstrap voltage regulator

charges the bootstrap capacitor, while the low-voltage

regulator drops the input voltage to a nominal VDD of 16 V for

the low-side logic and the output drive for the low-side

MOSFET.

If the FAULT output is used, a separate voltage (4.5 to 16 V), must

be applied to the VCC pin. This guarantees compatibility with the

logic levels in the motor controller.

OUTPUT DRIVE DETAILS

A unique feature of the Si9976DY is the integral high-side drive

circuitry. This includes logic-signal level shifting, a bootstrap

power supply, a charge pump, an undervoltage lockout, and a

40-mA output driver.

A bootstrap supply and a charge pump comprise the high-side

power supply, and utilize the benefits of each technique. By

itself, bootstrap supply provides sufficient charge for MOSFET

turn-on. However, it has two drawbacks when used alone.

First, a bootstrap capacitor must be recharged after every

MOSFET turn-on. Second, a bootstrap supply cannot sustain

a MOSFET in the on state indefinitely because the gate

leakage current continues to deplete the charge on the

bootstrap capacitor. A charge pump meanwhile, can provide

a continuous source of charge, but in fully integrated form it

cannot provide sufficient charge for MOSFET turn-on at typical

modulation frequencies. Combining the two techniques solves

these problems. The bootstrap supply provides the turn-on

charge while the charge pump provides the leakage current to

allow static operation.

Because a bootstrap supply is used, the bootstrap capacitor

must get charged immediately after power on and then be

recharged after every high-side turn on. Likewise, the low-side

MOSFET must be turned on to complete the charging circuit

for the bootstrap capacitor. Some drive schemes toggle

between the top and bottom MOSFETs, which accomplishes

the required charge and recharge of the bootstrap capacitor

automatically. It is important to understand that the charge

pump operates only when the high-side is turned on.

The bootstrap capacitor provides the charge that turns on the

high-side MOSFET. This capacitor should be sized such that it will

hold 10 times the charge required to turn on a MOSFET fully (i.e.,

VGS = 10 V). A typical capacitor value can be calculated by using

the equation CBOOT = 10 x (Qg/VGS). The value of Qg is taken

from the gate charge curve of the MOSFET being driven at VGS

=10 V. Using this method of capacitor selection, the bootstrap

voltage will drop approximately 1 V when the MOSFET is

turned on. A 0.018-mF capacitor works well for the Si9945DY,

which requires a 15-nC charge to turn on with VGS = 10 V.

A certain minimum recharge time is required for the bootstrap

capacitor after each high-side turn-on. The recharge time is a

function of the amount of charge which has been used to turn

on the high-side MOSFET, the size of the bootstrap capacitor,

and the drain current of the bootstrap transistor in the

Si9976DY. In the case of the Si9976DY, the recharge time

decreases as V+ increases. Part of this decrease is due to the

contribution of the charge pump to the recharging of the

bootstrap capacitor. As V+ increases, the charge pump

contribution increases. In some cases, the charge pump

becomes the only source of charge required to recharge the

bootstrap capacitor.

Bootstrap

Regulator

V+

VDD

Charge

Pump

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

2

Under Voltage

Lockout 1

To High-Side Logic

Figure 2. High-Side Drive

CAP

G1

S1

From High-Side Logic

Document Number: 70582

15-Jun-00

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