A3952SW datasheet(5/16 Pages) ALLEGRO

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3952

FULL-BRIDGE

PWM MOTOR DRIVER

FUNCTIONAL DESCRIPTION

INTERNAL PWM CURRENT CONTROL DURING

FORWARD AND REVERSE OPERATION

The A3952S– contains 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 (I

TRIP) is set by the selection of an

external current sensing resistor (R

S) and reference input

voltage (V

REF). 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 approxi-

mated by

TRIP = VREF/(10RS).

In forward or reverse mode the current-control circuitry

limits the load current. When the load current reaches

TRIP, the comparator resets a latch to turn OFF the

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.

Figure 1 — Load-Current Paths

The user selects an external resistor (R

T) and capaci-

tor (C

T) to determine the time period (toff = RTCT) during

which the drivers remain disabled (see “RC Fixed OFF

Time” below). At the end of the R

TCT 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).

ENABLE

MODE

LOAD

CURRENT

TRIP

Dwg. WP-015-1

Dwg. EP-006-2A

R S

DRIVE CURRENT

RECIRCULATION

(SLOW-DECAY MODE)

RECIRCULATION

(FAST-DECAY MODE)

Figure 2 — Fast and Slow Current-Decay Waveforms

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 I

TRIP 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.

As in the case of forward/reverse operation, the drivers

are re-enabled after a time given by t

off = RTCT (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 I

TRIP.

If so, the PWM

cycle will repeat, limiting the load current to the desired

value.

Brake Operation - MODE Input High

During braking, when the MODE input is high, the

current limit can be approximated by

TRIP = VREF/(10RS).

CAUTION: Because the kinetic energy stored in the

motor and load inertia is being converted into current,

which charges the V

BB 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.

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