Bootstrap Capacitor Selection

The bootstrap capacitors, CBOOTx, must be correctly selected

to ensure proper operation of the A4957. If the capacitances are

too high, time will be wasted charging the capacitor, resulting in

a limit on the maximum duty cycle and the PWM frequency. If

the capacitances are too low, there can be a large voltage drop at

the time the charge is transferred from CBOOTx to the MOSFET

gate, due to charge sharing.

To keep this voltage drop small, the charge in the bootstrap

able value, and the following formula can be used to calculate the

=

=

=

The voltage drop across the bootstrap capacitor as the MOSFET

is being turned on,

ΔV, can be approximated by:

∆V ≈

So for a factor of 20,

The maximum voltage across the bootstrap capacitor under

with a good ceramic capacitor the working voltage can be limited

to 16 V.

Bootstrap Charging

It is good practice to ensure the high side bootstrap capacitor is

completely charged before a high side PWM cycle is requested.

approximated by:

100

ΔV is the required voltage of the bootstrap capacitor.

At power-up and when the drives have been disabled for a long

time, the bootstrap capacitor can be completely discharged. In

this case

ΔV can be considered to be the full high-side drive

voltage, 12 V. Otherwise,

ΔV is the amount of voltage dropped

during the charge transfer, which should be 400 mV or less.

The capacitor is charged whenever the Sx pin is pulled low and

current flows from VREG through the internal bootstrap diode

circuit to CBOOT.

Bootstrap Charge Management

The A4957 provides automatic bootstrap capacitor charge

management. The bootstrap capacitor voltage for each phase

is continuously checked to ensure that it is above the bootstrap

voltage drops below this threshold, when the corresponding

high-side is active, the A4957 will turn on the necessary low-side

MOSFET, and continue charging until the bootstrap capacitor

If the bootstrap capacitor voltage is below the threshold, when the

corresponding high-side is commanded to turn on, the A4957 will

not attempt to turn on the high-side MOSFET, but will turn on the

necessary low-side MOSFET to charge the bootstrap capacitor

until it exceeds the undervoltage threshold plus the hysteresis.

The minimum charge time is typically 7 μs, but may be longer

for very large values of bootstrap capacitor (>1000 nF). If the

bootstrap capacitor voltage does not reach the threshold within

approximately 200 μs, an undervoltage fault will be flagged.

VREG Capacitor Selection

gate drive circuits and the charging current for the bootstrap

capacitors. When a low-side MOSFET is turned on, the gate-

drive circuit will provide the high transient current to the gate that

is necessary to turn on the MOSFET quickly. This current, which

can be several hundred milliamperes, cannot be provided directly

by the limited output of the VREG regulator, and must be sup-

plied by an external capacitor connected to the VREG pin.

The turn-on current for the high-side MOSFET is similar in value

to that for the low-side MOSFET, but is mainly supplied by the

bootstrap capacitor. However the bootstrap capacitor must then

be recharged from the VREG regulator output. Unfortunately the

bootstrap recharge can occur a very short time after the low-side

turn-on occurs. This requires that the value of the capacitor con-

nected between VREG and AGND should be high enough to min-

imize the transient voltage drop on VREG for the combination of

a low-side MOSFET turn-on and a bootstrap capacitor recharge.

rated as low as 15 V. This capacitor should be placed as close as

possible to the VREG pin.

Full Bridge MOSFET Driver

A4957

10

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com