A3952SW ALLEGRO [Allegro MicroSystems], A3952SW Datasheet - Page 5

• Manufacturer Part Number: A3952SW
• Description: FULL-BRIDGE PWM MOTOR DRIVER
• Manufacturer: ALLEGRO [Allegro MicroSystems]

INTERNAL PWM CURRENT CONTROL DURING
FORWARD AND REVERSE OPERATION
modulated (PWM) current-control circuit that can be used
to limit the load current to a desired value. The value of
the current limiting (I
external current sensing resistor (R
voltage (V
across the external sense resistor to one tenth the voltage
on the REF input terminal, resulting in a function approxi-
mated by
limits the load current. When the load current reaches
I
selected sink driver (in the slow-decay mode) or selected
sink and source driver pair (in the fast-decay mode). In
slow-decay mode, the selected sink driver is disabled; the
load inductance causes the current to recirculate through
the source driver and flyback diode (see figure 1). In fast-
decay mode, the selected sink and source driver pair are
disabled; the load inductance causes the current to flow
from ground to the load supply via the ground clamp and
flyback diodes.
tor (C
which the drivers remain disabled (see “RC Fixed OFF
Time” below). At the end of the R
are re-enabled allowing the load current to increase again.
The PWM cycle repeats, maintaining the load current at
the desired value (see figure 2).
TRIP
3952
FULL-BRIDGE
PWM MOTOR DRIVER
The A3952S– contains a fixed OFF-time pulse-width
In forward or reverse mode the current-control circuitry
, the comparator resets a latch to turn OFF the
The user selects an external resistor (R
T
) to determine the time period (t
REF
). The internal circuitry compares the voltage
Figure 1 — Load-Current Paths
R S
I
TRIP
TRIP
V
BB
) is set by the selection of an
= V
REF
/(10R
T
C
S
) and reference input
DRIVE CURRENT
RECIRCULATION
(SLOW-DECAY MODE)
RECIRCULATION
(FAST-DECAY MODE)
T
S
).
interval, the drivers
off
= R
T
T
) and capaci-
C
T
Dwg. EP-006-2A
) during
INTERNAL PWM CURRENT CONTROL DURING
BRAKE MODE OPERATION
turns ON both sink drivers. For dc motor applications, this
has the effect of shorting the motor’s back-EMF voltage,
resulting in current flow that brakes the motor dynamically.
However, if the back-EMF voltage is large, and there is no
PWM current limiting, then the load current can increase to
a value that approaches a locked rotor condition. To limit
the current, when the I
circuit disables the conducting sink driver. The energy
stored in the motor’s inductance is then discharged into
the load supply causing the motor current to decay.
are re-enabled after a time given by t
Fixed OFF Time” below). Depending on the back-EMF
voltage (proportional to the motor’s decreasing speed), the
load current again may increase to I
cycle will repeat, limiting the load current to the desired
value.
Brake Operation - MODE Input High
current limit can be approximated by
motor and load inertia is being converted into current,
which charges the V
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.
CURRENT
ENABLE
Figure 2 — Fast and Slow Current-Decay Waveforms
MODE
The brake circuit turns OFF both source drivers and
As in the case of forward/reverse operation, the drivers
During braking, when the MODE input is high, the
CAUTION: Because the kinetic energy stored in the
LOAD
I
TRIP
I
BB
TRIP
TRIP
supply bulk capacitance (power
RC
= V
level is reached, the PWM
REF
/(10R
TRIP
S
off
).
= R
. If so, the PWM
T
RC
C
T
(see “RC
Dwg. WP-015-1