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3961

DUAL FULL-BRIDGE PWM MOTOR DRIVER

Allegro MicroSystems 

3961

DUAL FULL-BRIDGE

PWM MOTOR DRIVER

If desired, the reference input voltage can be filtered

by placing a capacitor from REFIN to ground. The ground

return for this capacitor as well as R2 should be indepen-

dent from the high-current power-ground trace to avoid

changes in REFIN due to IR drops.

Thermal Considerations. For reliable operation it is

recommended that the maximum junction temperature be

kept below 110 to 125°C. The junction temperature can

be measured best by attaching a thermocouple to the

power tab/batwing of the device and measuring the tab

temperature, TTAB . The junction temperature can then be

approximated by using the formula:

TJ ≈ TTAB + (ILOAD 2 VF RθJT)

where VF can be chosen from the electrical specification

table for the given level of ILOAD. The value for RθJT is

given in the package thermal resistance table for the

appropriate package.

The power dissipation of the batwing packages can

be improved by 20 to 30% 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 that run at

high load currents and/or high duty cycles can be im-

proved by adding external diodes from each output to

ground in parallel with the internal diodes. Fast recovery

(≤ 200 ns) diodes should be used to minimize switching

losses.

The load supply terminal, VBB, should be decoupled

with an electrolytic capacitor (≥ 47 µF is recommended)

placed as close to the device as is physically practical.

To minimize the effect of system ground IR drops on the

logic and reference input signals the system ground should

have a low-resistance return to the motor supply voltage.

See also “Current Sensing” and “Thermal Consider-

ations” above.

Fixed Off-Time Selection. With increasing values of tOFF,

switching losses will decrease, low-level load current

regulation will improve, EMI will be 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.

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