3951
FULL-BRIDGE
PWM MOTOR DRIVER
recommended) as close to the device as is physically
practical. To minimize the effect of system ground I•R
drops on the logic and reference input signals, the system
ground should have a low-resistance return to the load
supply voltage.
To minimize current-sensing inaccuracies caused by
ground trace I•R drops, the current-sensing resistor should
have a separate return to the ground terminal of the
device. For low-value sense resistors, the I•R drops in the
PCB can be significant and should be taken into account.
The use of sockets should be avoided as contact resis-
tance can cause variations in the effective value of RS.
See also “Current Sensing” and “Thermal Consider-
ations” above.
Larger values of RS reduce the aforementioned effects
but can result in excessive heating and power loss in the
sense resistor. The selected value of RS must not result in
the SENSE terminal absolute maximum voltage rating
being exceeded. The recommended value of RS is in the
range of
Fixed Off-Time Selection
With increasing values of toff, switching losses de-
crease, low-level load-current regulation improves, EMI is
reduced, the PWM frequency will decrease, and ripple
current will increase. The value of toff can be chosen for
optimization of these parameters. For applications where
audible noise is a concern, typical values of toff are chosen
to be in the range of 15 to 35 µs.
RS = (0.375 to 1.125)/ITRIP
.
Thermal Considerations
For the most reliable operation, it is recommended that
the maximum junction temperature be kept as low as
practical, preferably below 125°C. The junction tempera-
ture can be measured by attaching a thermocouple to the
power tab/batwing of the device and measuring the tab
temperature, TT. The junction temperature then can be
approximated as
DC Motor Applications
In closed-loop systems, the speed of a dc motor can
be controlled by PWM of the PHASE or ENABLE inputs, or
by varying the REF input voltage (VREF). In digital systems
(microprocessor controlled), PWM of the PHASE or
ENABLE input is used typically thus avoiding the need to
generate a variable analog voltage reference. In this case,
a dc voltage on the REF input is used typically to limit the
maximum load current.
TJ ≈ TT + (2 • VF • IOUT • RΘJT
)
where VF is the clamp diode forward voltage and can be
determined from the electrical specification table for the
given level of IOUT. The value for RΘJT is given in the
package thermal resistance table for the appropriate
package.
In dc servo applications that require accurate position-
ing at low or zero speed, PWM of the PHASE input is
selected typically. This simplifies the servo-control loop
because the transfer function between the duty cycle on
the PHASE input and the average voltage applied to the
motor is more linear than in the case of ENABLE PWM
control (which produces a discontinuous current at low-
current levels).
The power dissipation of the batwing package can be
improved by approximately 20% 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 with high load
currents and/or high duty cycles can be improved by
adding external diodes in parallel with the internal diodes.
In internal PWM applications, only the two top-side
(flyback) diodes need be added. For external PHASE or
ENABLE input PWM applications, four external diodes
should be added for maximum junction temperature
reduction.
With bidirectional dc servo motors, the PHASE termi-
nal can be used for mechanical direction control. Similar
to when braking the motor dynamically, abrupt changes in
the direction of a rotating motor produce a current gener-
ated by the back EMF. The current generated will depend
on the mode of operation. If the internal two-quadrant
slow-decay PWM current-control circuitry is used, the
maximum load current generated can be approximated by
I
LOAD = VBEMF/RLOAD where VBEMF is proportional to the
PCB Layout
motor’s speed. If external four-quadrant fast-decay
ENABLE PWM current-control is used, then the maximum
load current generated can be approximated by
The load supply terminal, VBB, should be decoupled
(>47 µF electrolytic and 0.1 µF ceramic capacitors are
ILOAD = (VBEMF + VBB)/RLOAD
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