7

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

3946

Half-Bridge Power MOSFET Controller

Application Information

Bootstrap Capacitor Selection

. C

rectly selected to ensure proper operation of the device. If

too large, time is wasted charging the capacitor, with the

result being a limit on the maximum duty cycle and PWM

frequency. If the capacitor is too small, the voltage drop can

be too large at the time the charge is transferred from the

C

To keep the voltage drop small:

Q

where a factor in the range of 10 to 20 is reasonable. Using

20 as the factor:

Q

and

C

The voltage drop on the BOOT pin, as the MOSFET is being

turned on, can be approximated by:

Delta_v = Q

For example, given a gate charge, Q

typical BOOT pin voltage of 12 V, the value of the Boot

capacitor, C

C

Therefore, a 0.22 μF ceramic (X7R) capacitor can be chosen

for the Boot capacitor.

In that case, the voltage drop on the BOOT pin, when the

high-side MOSFET is turned on, is:

Delta_v = 160 nC / 0.22 μF = 0.73 V

Bootstrap Charging. It is good practice to ensure that the

high-side bootstrap capacitor is completely charged before a

high-side PWM cycle is requested.

The time required to charge the capacitor can be approxi-

mated by:

t

At power-up and when the drivers have been disabled for

a long time, the bootstrap capacitor can be completely

discharged. In this case, Delta_v can be considered to be the

full high-side drive voltage, 12 V. Otherwise, Delta_v is the

amount of voltage dropped during the charge transfer, which

should be 400 mV or less. The capacitor is charged whenever

the S pin is pulled low, via a GL PWM cycle, and current

flows from VREG through the internal bootstrap diode

circuit to C

Power Dissipation. For high ambient temperature

applications, there may be little margin for on-chip power

consumption. Careful attention should be paid to ensure that

the operating conditions allow the A3946 to remain in a safe

range of junction temperature.

The power consumed by the A3946 can be estimated as:

P_total = Pd_bias + Pd_cpump + Pd_switching_loss

where:

Pd_bias = V

and

Pd_cpump = (2V

Pd_cpump = (V

in either case, where

I

and

Pd_switching_loss = Q

where

Ratio = 10 Ω / (R