3951
FULL-BRIDGE
PWM MOTOR DRIVER
FUNCTIONAL DESCRIPTION
INTERNAL PWM CURRENT CONTROL DURING
ENABLE
FORWARD AND REVERSE OPERATION
The A3951SB/SW 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 value of
the current limiting (ITRIP) is set by the selection of an
external current sensing resistor (RS) and reference input
voltage (VREF). The internal circuitry compares the
voltage across the external sense resistor to one tenth the
voltage on the REF input terminal, resulting in a function
approximated by
I
TRIP
RC
LOAD
CURRENT
Dwg. WP-015-3
ITRIP = VREF/(10•RS).
Figure 2 — Load-Current Waveform
In forward or reverse mode the current-control cir-
cuitry limits the load current. When the load current
reaches ITRIP, the comparator resets a latch to turn off the
selected sink driver. The load inductance causes the
current to recirculate through the source driver and
flyback diode (two-quadrant operation or slow decay).
See figure 1.
INTERNAL PWM CURRENT CONTROL DURING
BRAKE MODE OPERATION
The brake circuit turns off both source drivers and
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 ITRIP level is reached, the PWM
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.
V
BB
As in the case of forward/reverse operation, the drivers
are re-enabled after a time given by toff = RT•CT (see “RC
Fixed Off Time” below). Depending on the back-EMF
voltage (proportional to the motor’s decreasing speed), the
load current again may increase to ITRIP. If so, the PWM
cycle will repeat, limiting the load current to the desired
value.
DRIVE CURRENT
RECIRCULATION
R
S
Dwg. EP-006-9
Brake Operation
Figure 1 — Load-Current Paths
During braking, the peak current limit defaults inter-
nally to a value approximated by
The user selects an external resistor (RT) and capaci-
tor (CT) to determine the time period (toff = RT•CT) during
which the drivers remain disabled (see “RC Fixed OFF
Time” below). At the end of the RTCT interval, the drivers
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).
ITRIP = 1.5 V/RS.
In this mode, the value of RS determines the ITRIP value
independent of VREF. This is useful in applications with
differing run and brake currents and no practical method of
varying VREF
.
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SANKEN [ SANKEN ELECTRIC ]
