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3961

DUAL FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3961

DUAL FULL-BRIDGE

PWM MOTOR DRIVER

where tOFF = RTCT, RLOAD is the series resistance of the

load, VBB is the motor supply voltage and t ON(min)max is

specified in the electrical characteristics table. When the

motor is rotating, the back EMF generated will influence

the above relationship. For brush dc motor applications,

the current regulation is improved. For stepper motor

applications when the motor is rotating, the effect is

dependent on the polarity and magnitude of the motor’s

back EMF.

The following procedure can be used to evaluate the

worst-case internal PWM load current regulation in the

system:

Set VREF to 0 volts. With the load connected and

the PWM current control operating in slow decay mode,

use an oscilloscope to measure the time the output is

low (sink ON) for the output that is chopping. This is the

typical minimum on time (tON(min)typ) for the device. The

CT then should be increased until the measured value of

tON(min) is equal to tON(min)max as specified in the electrical

characteristics table. When the new value of CT has been

set, the value of RT should be decreased so the value for

tOFF = RTCT (with the artificially increased value of CT)

is equal to the nominal design value. The worst-case load-

current regulation then can be measured in the system

under operating conditions.

PWM of the Phase and Enable Inputs. The PHASE and

ENABLE inputs can be pulse width modulated to regulate

load current. Typical propagation delays from the PHASE

and ENABLE inputs to transitions of the power outputs are

specified in the electrical characteristics table. If the inter-

nal PWM current control is used, the comparator blanking

function is active during phase and enable transitions. This

eliminates false tripping of the over-current comparator

caused by switching transients (see “RC Blanking” above).

Enable PWM. Toggling the ENABLE input turns ON and

OFF the selected source and sink drivers. The corre-

sponding pair of flyback and ground clamp diodes conduct

after the drivers are disabled, resulting in fast current de-

cay. When the device is enabled the internal current-con-

trol circuitry will be active and can be used to limit the load

current in a slow decay mode.

Phase PWM. Toggling the PHASE terminal selects which

sink/source pair is enabled, producing a load current that

varies with the duty cycle and remains continuous at all

times. This can have added benefits in bidirectional brush

dc servo motor applications as the transfer function be-

tween the duty cycle on the PHASE input and the average

voltage applied to the motor is more linear than in the case

of ENABLE PWM control (which produces a discontinuous

current at low current levels).

Miscellaneous Information. An internally generated dead

time prevents crossover currents that can occur when

switching phase.

Thermal protection circuitry turns OFF all drivers

should the junction temperature reach 165°C (typical).

This is intended only to protect the device from failures

due to excessive junction temperatures and should not

imply that output short circuits are permitted. The hyster-

esis of the thermal shutdown circuit is approximately 15°C.

APPLICATION NOTES

Current Sensing. The actual peak load current (IPEAK)

will be above the calculated value of ITRIP due to delays in

the turn off of the drivers. The amount of overshoot can be

approximated by:

IOS ≈

(VBB – [(ITRIP RLOAD) + VBEMF]) tPWM

LLOAD

where VBB is the motor supply voltage, VBEMF is the

back-EMF voltage of the load, RLOAD and LLOAD are the

resistance and inductance of the load respectively, and

t PWM is specified in the electrical characteristics table.

To minimize current sensing inaccuracies caused by

ground trace IR drops, each current-sensing resistor

should have a separate return to the ground terminal of the

device. For low-value sense resistors, the IR drops in the

PCB can be significant and should be taken into account.

The use of sockets should be avoided as their contact

resistance can cause variations in the effective value of

RS.

Generally, larger values of RS reduce the aforemen-

tioned effects but can result in excessive heating and

power loss in the sense resistor. The selected value of RS

should not cause the absolute maximum voltage rating of

1.0 V, for the SENSE terminal, to be exceeded. The

recommended value of RS is in the range of:

RS ≈

0.5

± 50%

ITRIPmax

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

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