a4954 Allegro MicroSystems, Inc., a4954 Datasheet - Page 6

• Manufacturer Part Number: a4954
• Description: Dual Full-bridge Dmos Pwm Motor Driver
• Manufacturer: Allegro MicroSystems, Inc.

A4954
Synchronous Rectification
When a PWM off-cycle is triggered by an internal fixed off-time
cycle, load current will recirculate. The A4954 synchronous rec-
tification feature turns-on the appropriate DMOSFETs during the
current decay, and effectively shorts out the body diodes with the
low R
When a zero current level is detected, synchronous rectification is
turned off to prevent reversal of the load current.
DS(on)
driver. This significantly lowers power dissipation.
Mixed Decay Mode Operation
V
PHASE
I
OUT
I
Trip
–
+
0
t
COD
0.50 × t
Fast Decay
Fixed Off-Time, t
off
Enlargement A
I
t
COD
OUT
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, t
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.
off
= 25 μs
Slow Decay
0.50 × t
See Enlargement A
off
t
COD
Dual Full-Bridge DMOS
COD
. This feature is added to prevent shoot-
PWM Motor Driver
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
6