Full bridge driver links MCU to brushed motor

Allegro MicroSystems has introduced a family of full bridge gate-driver ICs that interface brushed dc motors to microcontrollers.

Allegro A5957

The devices can support motors up to 50V and 20A, and are aimed at industrial and consumer applikcations, for example in robotic vacuum cleaners, vending machines, cash dispensers, ticketing machines, automatic gates, automatic turnstiles, and sewing machines.

Uniquely, according to Allegro, the chips include an analogue peak hold circuit that captures the maximum voltage across an optional motor current sense resistor. Via a 10x amplifier, this voltage is made available throughout the PWM cycle to allow the motor current to be estimated at all times.

There are three products, varying in input signals:

  • A4955 offers parallel control inputs which drive the bridge in forward, reverse, brake, and standby modes
  • A4956 offers phase, enable and mode inputs which drive the bridge in forward, reverse, brake and fast decay modes
  • A5957 (pictured) offers phase, enable, and ‘sleep’ inputs which drive the bridge in forward or reverse modes with slow decay synchronous rectification.

In all of them, gate drive current to the external FETs is programmed by an external resistor to control slew rate for electromagnetic compliance – another unique feature according to the firm.

Other feature is the regulation of peak load current with a fixed off-time PWM regulator. Peak current is selected by the voltage on the reference voltage terminal. An over-current flag notifies the user when the device is actively PWM limiting load current.

Protection features include: monitoring mosfet drain-source voltage to detect bridge or motor shorts, under-voltage lockout, crossover current protection for driving a wide range of FETs, and thermal shut-down.

A4955 and A4956 come in a 4 x 4mm 20-contact QFN or 20 lead eTSSOP, both with exposed power tab.

A5957 only comes in the QFN.

 


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