3958
DMOS FULL-BRIDGE
PWM MOTOR DRIVER
APPLICATIONS INFORMATION
Current Sensing. To minimize inaccuracies in sensing
the ITRIP current level, which may be caused by ground
trace IR drops, the sense resistor should have an
independent ground return to the ground terminal of the
device. For low-value sense resistors the IR drops in the
PCB sense resistor’s traces can be significant and should
be taken into account. The use of sockets should be
avoided as they can introduce variation in RS due to their
contact resistance.
The maximum value of RS is given as RS ≤ 0.5/ITRIP
.
Braking. The braking function is implemented by
driving the device in slow-decay mode via serial port bit
D13, enabling synchronous rectification via bit D12, and
chopping with the combination of D14 and the ENABLE
input terminal. Because it is possible to drive current in
either direction through the DMOS drivers, this
configuration effectively shorts out the motor-generated
BEMF as long as the ENABLE chop mode is asserted. It
is important to note that the internal PWM current-control
circuit will not limit the current when braking, because the
current does not flow through the sense resistor. The
maximum brake current can be approximated by VBEMF
RL. Care should be taken to ensure that the maximum
ratings of the device are not exceeded in worst-case
braking situations of high speed and high inertial loads.
/
Thermal Protection. Circuitry turns off all drivers
when the junction temperature reaches 165°C typically. It
is intended only to protect the device from failures due to
excessive junction temperatures and should not imply that
output short circuits are permitted. Thermal shutdown has
a hysteresis of approximately 15°C.
Layout. The printed wiring board should use a heavy
ground plane. For optimum electrical and thermal perfor-
mance, the driver should be soldered directly onto the
board. The ground side of RS should have an individual
path to the ground terminals of the device. This path
should be as short as is possible physically and should not
have any other components connected to it. It is recom-
mended that a 0.1 µF capacitor be placed between SENSE
and ground as close to the device as possible; the load
supply terminal, VBB, should be decoupled with an
electrolytic capacitor (> 47 µF is recommended) placed as
close to the device as is possible.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
8
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
