3964
DUAL FULL-BRIDGE
PWM MOTOR DRIVER
FUNCTIONAL DESCRIPTION
Internal PWM Current Control.
The A3964SB and
A3964SLB contain a fixed off-time pulse-width modulated
(PWM) current-control circuit that can be used to limit the
load current to a desired value. The peak value of the
current limiting (I
TRIP
) is set by the selection of an external
current-sensing resistor (R
S
) and reference input voltage
(V
REF(IN)
). The internal circuitry compares the voltage
across the external sense resistor to the voltage on the
reference input terminal (V
REF(IN)
) resulting in a
transconductance function approximated by:
I
TRIP
≈
V
REF(IN)
R
S
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.
During internal PWM operation, at the end of the t
OFF
time, the comparator’s output is blanked and C
T
begins to
be charged from approximately 1.1 volts by an internal
current source of approximately 1 mA. The comparator
output remains blanked until the voltage on C
T
reaches
approximately 3 volts.
When a transition of the PHASE input occurs, C
T
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, C
T
is charged by an internal
current source of approximately 1 mA. The comparator
output remains blanked until the voltage on C
T
reaches
approximately 3 volts.
When the device is disabled, via the ENABLE input,
C
T
is discharged to near ground. When the device is
re-enabled, C
T
is charged by an internal current source of
approximately 1 mA. The comparator output remains
blanked until the voltage on C
T
reaches approximately
3 volts.
The minimum recommended value for C
T
is
1000 pF.
This value ensures that the blanking time is
sufficient to avoid false trips of the comparator under
normal operating conditions. For optimal regulation of the
load current, the above value for C
T
is recommended and
the value of R
T
can be sized to determine t
OFF
. For more
information regarding load current regulation, see below.
Load Current Regulation.
Because the device operates
in a slow current-decay mode (2-quadrant PWM mode),
there is a limit to the lowest level that the PWM current
control circuitry can regulate load current. The limitation is
due to the minimum PWM duty cycle, which is a function of
the user-selected value of t
OFF
and the minimum on-time
pulse t
ON(min)
max that occurs each time the PWM latch is
reset. If the motor is not rotating, as in the case of a
stepper motor in hold/detent mode, a brush dc motor when
stalled or at startup, the worst case value of current
regulation can be approximated by:
The reference input voltage is typically set with a
resistor divider from V
REF(OUT)
. To ensure proper operation
of the voltage reference, the resistor divider should have
an impedance of 3 kΩ to 15 kΩ (R
D
= R
A
+R
B
). Within this
range, a low impedance will minimize the effect of the REF
IN input offset current.
The current-control circuitry limits the load current as
follows: when the load current reaches I
TRIP
, the compara-
tor resets a latch that turns off the selected source driver.
The load inductance causes the current to recirculate
through the sink driver and flyback diode.
For each bridge, the user selects an external resistor
(R
T
) and capacitor (C
T
) to determine the time period
(t
OFF
= R
T
C
T
) during which the source driver remains
disabled (see “RC Fixed Off-time” below). The range of
recommended values for C
T
and R
T
are 1000 pF to 1500
pF and 15 kΩ to 100 kΩ respectively. For optimal load
current regulation, C
T
is normally set to 1000 pF (see
“Load Current Regulation” below). At the end of the RC
interval, the source driver is enabled allowing the load
current to increase again. The PWM cycle repeats,
maintaining the peak load current at the desired value.
RC Blanking.
In addition to determining the fixed off-time
of the PWM control circuit, the C
T
component sets the
comparator 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 or
ENABLE inputs). The comparator output is blanked to
I
AVG
≈
[(V
BB
– V
SAT(SOURCE+SINK)
) • t
ON(min)
max] – (1.05 (V
SAT(SINK)
+ V
F
) • t
OFF
)
1.05 (t
ON(min)
max + t
OFF
) • R
LOAD
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