Three-phase brushless DC motor pre-driver saves cost

The device includes three Hall-element inputs, a sequencer for commutation control, fixed off-time pulse width modulation (PWM) current control and locked rotor protection. Output current is scalable with the choice of external Mosfets.

Allegro says that the Hall element inputs used in the A4931 cost less than the Hall switches normally used, plus they benefit from internal noise filtering to prevent false commutation signals. A 7.5V regulated supply output is provided to power the three Hall elements.

'Enable', 'direction' and 'brake' inputs can be used to control the motor speed, position and torque. Output speed can be regulated using the two speed-control outputs, using either one Hall-element or all three Hall transition outputs to provide flexibility for a variety of speed control applications.

The external Mosfets can be PWM-controlled via the 'enable' input or with the internal PWM current regulator. In both cases, Allegro's A4931 synchronous rectification feature will turn on the appropriate Mosfet(s) during current decay to reduce power dissipation.

High side N-channel Mosfets are used because they cost less than P-channel Mosfets of equivalent 'on' resistance. No external components are required to drive the Mosfets.

Internal circuit protection includes thermal shutdown with hysteresis, undervoltage lockout, and locked-rotor and dead-time protection. For the locked-rotor protection, a programmable lock-time detection feature limits the time at stall before the outputs are disabled to prevent output stage or motor overheating.

Special power-up sequencing is not required and the operating temperature range is -20 to +105degC.

The Allegro A4931 is supplied in a compact 5 x 5mm 28-lead QFN package with exposed pad. The exposed pad package has low thermal resistance for improved thermal performance. The package is lead-free with 100 per cent matt tin lead-frame plating.

The A4931 is targeted at the industrial, office automation, telecommunications and consumer markets, for applications such as driving drums on laser printers and copiers or fans in telecommunications or consumer equipment.