3964
DUAL FULL-BRIDGE
PWM MOTOR DRIVER
If desired, the reference input voltage can be filtered
by placing a capacitor from REF
IN
to ground. The ground
return for this capacitor as well as R
B
should be indepen-
dent from the high-current power-ground trace to avoid
changes in REF
IN
due to IR drops.
Thermal Considerations.
For reliable operation, it is
recommended that the maximum junction temperature be
kept below 110°C to 125°C. The junction temperature can
be measured best by attaching a thermocouple to the
power tab/batwing of the device and measuring the tab
temperature, T
TAB
. The junction temperature can then be
approximated by using the formula:
T
J
≈
T
TAB
+ (2 I
LOAD
V
F
R
θJT
)
where V
F
can be chosen from the electrical specification
table for the given level of I
LOAD
. The value for R
θJT
is
approximately 6°C/W for both package styles.
The power dissipation of the batwing packages can be
improved by 20 to 30% by adding a section of printed
circuit board copper (typically 6 to 18 square centimeters)
connected to the batwing terminals of the device.
The thermal performance in applications that run at
high load currents and/or high duty cycles can be im-
proved by adding external diodes from each output to
ground in parallel with the internal diodes. Fast-recovery
(≤200 ns) diodes should be used to minimize switching
losses.
The load supply terminal, V
BB
, should be decoupled
with an electrolytic capacitor (≥47
µF
is recommended)
placed as close to the device as is physically practical.
To minimize the effect of system ground IR drops on the
logic and reference input signals the system ground should
have a low-resistance return to the load supply voltage.
See also “Current Sensing” and “Thermal Consider-
ations” above.
Fixed Off-Time Selection.
With increasing values of t
OFF
,
switching losses will decrease, low-level load current
regulation will improve, EMI will be reduced, the PWM
frequency will decrease, and ripple current will increase.
The value of t
OFF
can be chosen for optimization of these
parameters. For applications where audible noise is a
concern, typical values of t
OFF
are chosen to be in the
range of 15 to 35
µs.
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
