DMOS Dual Full-Bridge PWM Motor Driver
A3974
9
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Current Sensing. To minimize inaccuracies in sensing the
I
TRIP
current level caused by ground-trace IR drops, the sense
resistor should have an independent ground return to a ground
terminal of the device. For low-value sense resistors, the IR
drops in the PCB sense traces of the resistor can be signifi cant
and should be taken into account. The use of sockets should be
avoided as they can introduce variation in R
S
due to their contact
resistance.
The maximum value of R
S
is given as R
S
= 0.5/I
TRIPMAX
.
Braking. The braking function is implemented by driving the
device in slow-decay mode via serial port bit D13, enabling
synchronous rectifi cation via bits D11 and D12, and applying
an enable chop command with the combination of D14 and the
ENABLE input terminal. Because it is possible to drive current
in both directions through the DMOS switches, this confi gura-
tion 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 cur-
rent when braking, because the current does not fl ow through the
sense resistor. The maximum brake current can be approximated
by V
BEMF
/R
L
. Care should be taken to ensure that the maximum
ratings of the device are not exceeded in worst-case braking situ-
ations of high speed and high inertial loads.
A. Minimum Data Setup Time .........................................15 ns
B. Minimum Data Hold Time ...........................................10 ns
C. Minimum Setup Strobe to Clock Rising Edge ............50 ns
D. Minimum Clock High Pulse Width .............................50 ns
E. Minimum Clock Low Pulse Width ..............................50 ns
F. Minimum Setup Clock Rising Edge to Strobe .............50 ns
G. Minimum Strobe Pulse Width .....................................50 ns
APPLICATIONS INFORMATION
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 performance, the
driver should be soldered directly onto the board. The ground
side of R
S
should have an individual path to a ground terminal of
the device. This path should be as short as is possible physically
and should not have any other components connected to it. The
load supply terminal, V
BB
, should be decoupled with an electro-
lytic capacitor (>47 μF is recommended) placed as close to the
device as is possible.
Serial Port Write Timing Operation. Data is clocked into
shift register on the rising edge of CLOCK signal. Normally,
STROBE will be held high, and only will be brought low to
initiate a write cycle. Refer to diagram below and specifi cation
table for timing requirements.
