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A3951

FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3951

FULL-BRIDGE

PWM MOTOR DRIVER

VBB

between the duty cycle on the phase input and the aver-

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

case of ENABLE PWM control (which produces a discon-

tinuous current at low current levels). See also, “DC Motor

Applications” below.

RS

DRIVE CURRENT

RECIRCULATION

Dwg. EP-006-10

Figure 3 — Load-Current Paths

ENABLE Pulse-Width Modulation

Toggling the ENABLE input turns on and off the

selected source and sink drivers; the load inductance

causes the current to flow from ground to the load supply

via the ground clamp and flyback diodes (four-quadrant

operation or fast decay). See figure 3. When the device is

enabled, the internal current-control circuitry will be active

and can be used to limit the load current in the normal

internal PWM slow-decay or two-quadrant mode of opera-

tion.

MISCELLANEOUS INFORMATION

An internally generated dead time prevents crossover

currents that can occur when switching phase or braking.

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 hysteresis of the

thermal shutdown circuit is approximately 15°C.

If the internal current-control circuitry is not used; the

VREF terminal should be connected to VCC, the SENSE

terminal should be connected to ground, and the RC

terminal should be left floating (no connection).

An internal under-voltage lockout circuit prevents

simultaneous conduction of the outputs when the device is

powered up or powered down.

ENABLE

I TRIP

LOAD

CURRENT

Dwg. WP-015-4

Figure 4 — ENABLE PWM Load-Current Waveform

PHASE Pulse-Width Modulation

Toggling the PHASE terminal determines/controls

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

APPLICATIONS NOTES

Current Sensing

The actual peak load current (IOUTP) will be greater

than the calculated value of ITRIP due to delays in the turn

off of the drivers. The amount of overshoot can be ap-

proximated as

IOUTP

≈

(VBB

–

((ITRIP

•

RLOAD) +

LLOAD

VBEMF))

•

tpd(pwm)

where VBB is the load/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

tpd(pwm) is the propagation delay as specified in the electrical

characteristics table.

The reference terminal has an equivalent input resis-

tance of 50 kΩ ±30%. This should be taken into account

when determining the impedance of the external circuit

that sets the reference voltage value.

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