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A4953ELJTR-T

Full-Bridge DMOS PWM Motor Drivers

Allegro MicroSystems 

A4952 and

A4953

Full-Bridge DMOS PWM Motor Drivers

Synchronous Rectification

When a PWM off-cycle is triggered by an internal fixed off-time

cycle, load current will recirculate. The A4952/A4953 synchro-

nous rectification feature turns-on the appropriate DMOSFETs

during the current decay, and effectively shorts out the body

diodes with the low RDS(on) driver. This significantly lowers

power dissipation. When a zero current level is detected, syn-

chronous rectification is turned off to prevent reversal of the load

current.

Mixed Decay Operation

The bridges operate in Mixed Decay mode. Referring to the

lower panel of the figure below, as the trip point is reached, the

device goes into fast decay mode for 50% of the fixed off-time

period. After this fast decay portion the device switches to slow

decay mode for the remainder of the off-time. During transitions

from fast decay to slow decay, the drivers are forced off for the

Crossover Delay, tCOD . This feature is added to prevent shoot-

through in the bridge. During this “dead time” portion, synchro-

nous rectification is not active, and the device operates in fast

decay and slow decay only.

Mixed Decay Mode Operation

VPHASE

+

IOUT 0

–

See Enlargement A

Enlargement A

Fixed Off-Time, toff = 25 μs

0.50 × toff

0.50 × toff

ITrip

IOUT

tCOD

Fast Decay

tCOD

Slow Decay

tCOD

Allegro MicroSystems, LLC

6

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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