3951
FULL-BRIDGE
PWM MOTOR DRIVER
Choosing a small value for RS essentially disables the
current limiting during braking. Therefore, care should be
taken to ensure that the motor’s current does not exceed
the absolute maximum ratings of the device. The braking
current can be measured by using an oscilloscope with a
current probe connected to one of the motor’s leads.
Similarly, when a transition of the PHASE input occurs,
CT is discharged to near ground during the crossover delay
time (the crossover delay time is present to prevent
simultaneous conduction of the source and sink drivers).
After the crossover delay, CT is charged by an internal
current source of approximately 1 mA. The comparator
output remains blanked until the voltage on CT reaches
approximately 3.0 volts.
CAUTION: Because the kinetic energy stored in the
motor and load inertia is being converted into current,
which charges the VBB supply bulk capacitance (power
supply output and decoupling capacitance), care must be
taken to ensure the capacitance is sufficient to absorb the
energy without exceeding the voltage rating of any devices
connected to the motor supply.
Similarly, when the device is disabled via the ENABLE
input, CT is discharged to near ground. When the device is
re-enabled, CT is charged by the internal current source.
The comparator output remains blanked until the voltage
on CT reaches approximately 3.0 V.
RC Fixed Off Time
For most applications, the minimum recommended
value is CT = 820 pF ±5 %. This value ensures that the
blanking time is sufficient to avoid false trips of the com-
parator under normal operating conditions. For optimal
regulation of the load current, the above value for CT is
recommended and the value of RT can be sized to deter-
mine toff. For more information regarding load current
regulation, see below.
The internal PWM current control circuitry uses a one
shot to control the time the driver remains off. The one
shot time, toff (fixed off time), is determined by the selection
of an external resistor (RT) and capacitor (CT) connected in
parallel from the RC terminal to ground. The fixed off time,
over a range of values of CT = 820 pF to 1500 pF and RT =
12 kΩ to 100 kΩ, is approximated by
toff = RT•CT.
LOAD CURRENT REGULATION WITH THE INTERNAL
PWM CURRENT-CONTROL CIRCUITRY
When the PWM latch is reset by the current compara-
tor, the voltage on the RC terminal will begin to decay from
approximately 3 volts. When the voltage on the RC
terminal reaches approximately 1.1 volts, the PWM latch is
set, thereby re-enabling the driver.
During operation, the A3951S– have a lower limit to
the range of PWM current control. This directly relates to
the limitations imposed by the VREF input (2.0 V, minimum).
RC Blanking
Applications requiring a broader or full range (≈0% to
100%) should utilize the A3952S–, which are recom-
mended for the improvements they bring to new designs.
In addition to determining the fixed off time of the
PWM control circuit, the CT component sets the compara-
tor blanking time. This function blanks the output of the
comparator when the outputs are switched by the internal
current control circuitry (or by the PHASE, BRAKE, or
ENABLE inputs). The comparator output is blanked to
prevent false over-current detections due to reverse
recovery currents of the clamp diodes, and/or switching
transients related to distributed capacitance in the load.
LOAD CURRENT REGULATION WITH EXTERNAL
PWM OF THE PHASE OR ENABLE INPUTS
The PHASE or 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 charac-
teristics table. If the normal PWM current control is used,
then the comparator blanking function is active during
phase and enable transitions. This eliminates false
tripping of the over-current comparator caused by switch-
ing transients (see “RC Blanking” above).
During internal PWM operation, at the end of the off
time, the comparator’s output is blanked and CT begins to
be charged from approximately 1.1 V by an internal current
source of approximately 1 mA. The comparator output
remains blanked until the voltage on CT reaches approxi-
mately 3.0 volts.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
6
SANKEN [ SANKEN ELECTRIC ]
