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A3964 Datasheet(PDF) 6 Page - Allegro MicroSystems

Part # A3964
Description  Dual Full-Bridge PWM Motor Driver
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Manufacturer  ALLEGRO [Allegro MicroSystems]
Direct Link  http://www.allegromicro.com
Logo ALLEGRO - Allegro MicroSystems

A3964 Datasheet(HTML) 6 Page - Allegro MicroSystems

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Dual Full-Bridge PWM Motor Driver
A3964
6
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The following procedure can be used to evaluate the worst case
internal PWM load current regulation in the system: set VREF(IN)
to 0 volts. With the load connected and the PWM current control
operating in slow decay mode, use an oscilloscope to measure the
time the output is low (sink on) for the output that is chopping.
This is the typical minimum on time (tON(min)typ) for the device.
The CT then should be increased until the measured value of
tON(min) is equal to tON(min)max as specified in the electrical
characteristics table. When the new value of CT has been set,
the value of RT should be decreased so the value for tOFF =
RTCT (with the artificially increased value of CT) is equal to the
nominal design value. The worst-case load-current regulation
then can be measured in the system under operating conditions.
PWM of the Phase and Enable Inputs.
The PHASE and
ENABLE inputs can be pulse width modulated to regulate
load current. Typical propagation delays from the PHASE and
ENABLE inputs to transitions of the power outputs are specified
in the electrical characteristics table. If the internal PWM current
control is used, the comparator blanking function is active during
phase and enable transitions. This eliminates false tripping of the
over-current comparator caused by switching transients (see the
RC Blanking section, above).
Enable PWM.
Toggling the ENABLE input turns on and off
the selected source and sink drivers. The corresponding pair
of flyback and ground clamp diodes conduct after the drivers
are disabled, resulting in fast current decay. When the device is
enabled the internal current control circuitry will be active and
can be used to limit the load current in a slow current-decay
mode.
Phase PWM.
Toggling the PHASE terminal selects which sink/
source pair is enabled, producing a load current that varies with
the duty cycle and remains continuous at all times. This can have
added benefits in bidirectional brush dc servo motor applications
as 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).
Miscellaneous Information.
An internally generated dead time
prevents crossover currents that can occur when switching phase.
Thermal protection circuitry turns off all drivers should the
junction temperature reach +165°C (typical). This is intended
only to protect the device from failures due to excessive junction
temperatures and should not imply that output short circuits
are permitted. The hysteresis of the thermal shutdown circuit is
approximately 15°C.
APPLICATION NOTES
Current Sensing.
The actual peak load current (IPEAK) will be
above the calculated value of ITRIP due to delays in the turn off of
the drivers. The amount of overshoot can be approximated by:
(VBB – [(ITRIP x RLOAD) + VBEMF]) x tPWM
LLOAD
IOS
where VBB is the motor supply voltage, VBEMF is the back-EMF
voltage of the load, RLOAD and LLOAD are the resistance and
inductance of the load respectively, and t PWM is specified in the
electrical characteristics table.
To minimize current sensing inaccuracies caused by ground trace
IR drops, each current-sensing resistor should have a separate
return to the ground terminal of the device. For low-value sense
resistors, the IR drops in the PCB can be significant and should
be taken into account. The use of sockets should be avoided as
their contact resistance can cause variations in the effective value
of RS.
Generally, 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 should not cause the absolute
maximum voltage rating of 1.0 V, for the SENSE terminal, to be
exceeded. The recommended value of RS is in the range of:
0.5
ITRIPmax
RS
± 50%
If desired, the reference input voltage can be filtered by placing
a capacitor from REFIN to ground. The ground return for this
capacitor as well as RB should be independent from the high-
current power-ground trace to avoid changes in REFIN 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, TTAB .
The junction temperature can then be approximated by using the
formula:
TJ ≈ TTAB + (2 ILOAD VF RθJT)
where VF can be chosen from the electrical specification table
for the given level of ILOAD. The value for RθJT is approximately
6°C/W.
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.


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