3951

FULL-BRIDGE

PWM MOTOR DRIVER

5

www.allegromicro.com

FUNCTIONAL DESCRIPTION

INTERNAL PWM CURRENT CONTROL DURING

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 (I

external current sensing resistor (R

voltage (V

voltage across the external sense resistor to one tenth the

voltage on the REF input terminal, resulting in a function

approximated by

I

In forward or reverse mode the current-control cir-

cuitry limits the load current. When the load current

reaches I

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.

Figure 1 — Load-Current Paths

The user selects an external resistor (R

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).

Dwg. EP-006-9

R S

BB

V

DRIVE CURRENT

RECIRCULATION

ENABLE

LOAD

CURRENT

RC

I

TRIP

Dwg. WP-015-3

Figure 2 — Load-Current Waveform

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

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

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

If so, the PWM

cycle will repeat, limiting the load current to the desired

value.

Brake Operation

During braking, the peak current limit defaults inter-

nally to a value approximated by

I

In this mode, the value of R

independent of V

differing run and brake currents and no practical method of

varying V