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A3951SW

FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3951

FULL-BRIDGE

PWM MOTOR DRIVER

To minimize current-sensing inaccuracies caused by

ground trace I•R drops, the current-sensing resistor should

have a separate return to the ground terminal of the

device. For low-value sense resistors, the I•R drops in the

PCB can be significant and should be taken into account.

The use of sockets should be avoided as contact resis-

tance can cause variations in the effective value of RS.

Larger values of RS reduce the aforementioned effects

but can result in excessive heating and power loss in the

sense resistor. The selected value of RS must not result in

the SENSE terminal absolute maximum voltage rating

being exceeded. The recommended value of RS is in the

range of

RS = (0.375 to 1.125)/ITRIP.

Thermal Considerations

For the most reliable operation, it is recommended that

the maximum junction temperature be kept as low as

practical, preferably below 125°C. The junction tempera-

ture can be measured by attaching a thermocouple to the

power tab/batwing of the device and measuring the tab

temperature, TT. The junction temperature then can be

approximated as

TJ ≈ TT + (2 • VF • IOUT • RΘJT)

where VF is the clamp diode forward voltage and can be

determined from the electrical specification table for the

given level of IOUT. The value for RΘJT is given in the

package thermal resistance table for the appropriate

package.

The power dissipation of the batwing package can be

improved by approximately 20% by adding a section of

printed circuit board copper (typically 6 to 18 square

centimeters) connected to the batwing terminals of the

device.

The thermal performance in applications with high load

currents and/or high duty cycles can be improved by

adding external diodes in parallel with the internal diodes.

In internal PWM applications, only the two top-side

(flyback) diodes need be added. For external PHASE or

ENABLE input PWM applications, four external diodes

should be added for maximum junction temperature

reduction.

PCB Layout

The load supply terminal, VBB, should be decoupled

(>47 µF electrolytic and 0.1 µF ceramic capacitors are

recommended) as close to the device as is physically

practical. To minimize the effect of system ground I•R

drops on the logic and reference input signals, the system

ground should have a low-resistance return to the load

supply voltage.

See also “Current Sensing” and “Thermal Consider-

ations” above.

Fixed Off-Time Selection

With increasing values of toff, switching losses de-

crease, low-level load-current regulation improves, EMI is

reduced, the PWM frequency will decrease, and ripple

current will increase. The value of toff can be chosen for

optimization of these parameters. For applications where

audible noise is a concern, typical values of toff are chosen

to be in the range of 15 to 35 µs.

DC Motor Applications

In closed-loop systems, the speed of a dc motor can

be controlled by PWM of the PHASE or ENABLE inputs, or

by varying the REF input voltage (V ). In digital systems

REF

(microprocessor controlled), PWM of the PHASE or

ENABLE input is used typically thus avoiding the need to

generate a variable analog voltage reference. In this case,

a dc voltage on the REF input is used typically to limit the

maximum load current.

In dc servo applications that require accurate position-

ing at low or zero speed, PWM of the PHASE input is

selected typically. This simplifies the servo-control loop

because the transfer function between 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).

With bidirectional dc servo motors, the PHASE termi-

nal can be used for mechanical direction control. Similar

to when braking the motor dynamically, abrupt changes in

the direction of a rotating motor produce a current gener-

ated by the back EMF. The current generated will depend

on the mode of operation. If the internal two-quadrant

slow-decay PWM current-control circuitry is used, the

maximum load current generated can be approximated by

ILOAD = VBEMF/RLOAD where VBEMF is proportional to the

motor’s speed. If external four-quadrant fast-decay

ENABLE PWM current-control is used, then the maximum

load current generated can be approximated by

ILOAD = (VBEMF + VBB)/RLOAD

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