Dual Full-Bridge PWM Motor Driver with Brake

A3968

6

Allegro MicroSystems, LLC

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

Internal PWM Current Control.

The A3968 dual

internal fixed-frequency PWM control circuit controls the

the load current in each motor. The current-control circuitry

works as follows: when the outputs of the full-bridge are

turned on, current increases in the motor winding. The load

current is sensed by the current-control comparator via

increase until it reaches the predetermined value, set by the

selection of external current-sensing resistors and reference

to the base-drive current of the sink driver transistor.

At the trip point, the comparator resets the source-en-

able latch, turning off the source driver of that full-bridge.

The source turn-off of one full-bridge is independent of

the other full-bridge. Load inductance causes the current to

recirculate through the sink driver and ground-clamp diode.

The current decreases until the internal clock oscillator sets

the source-enable latches of both Full-bridges, turning on

the source drivers of both bridges. Load current increases

again, and the cycle is repeated.

The frequency of the internal clock oscillator is set by

be approximately calculated as:

20 to 100 k

 and 470 to 1000 pF respectively. Nominal

values of 56 k

 and 680 pF result in a clock frequency of

25.4 kHz.

Current-Sense Comparator Blanking.

When the

source driver is turned on, a current spike occurs due to the

reverse-recovery currents of the clamp diodes and switch-

ing transients related to distributed capacitance in the load.

To prevent this current spike from erroneously resetting the

source enable latch, the current-control comparator output

is blanked for a short period of time when the source driver

is turned on. The blanking time is set by the timing compo-

of 1.3 μs.

The current-control comparator is also blanked when

the load current changes polarity (direction or phase

change). This internally generated blank time is approxi-

mately 1.8 μs.

FUNCTIONAL DESCRIPTION

+

–

0

Dwg. WM-003-2

I

OUT

t

d

t

blank

INTERNAL

OSCILLATOR

BRIDGE

ON

SOURCE

OFF

BRIDGE

ON

R C

T T

ALL

OFF

Enlargement A

See Enlargement A

Dwg. EP-006-16

R S

BB

V

BRIDGE ON

SOURCE OFF

ALL OFF