Drive DC brushed motors with simple logic from an MPU

13th April 2016
Nat Bowers

Allegro MicroSystems Europe announces a family of full bridge gate-driver ICs designed to drive a wide range of DC brushed motors with simple logic from an MPU. The A4955, A4956 and A5957 suit industrial and consumer applications such as robotic vacuum cleaners, vending machines, cash dispensing machines, coffee vending machines, ticketing machines, sewing machines and automatic gates/turnstiles.

Flexible interface options are offered in three individual products, making the devices easy to fit into customers’ existing designs with little or no firmware changes:

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

Supporting motor supply voltages up to 50V and load currents of up to 20A, unique features of the A4955, A4956 and A5957 include an analogue output function which can be used to monitor current through an external sense resistor (if used). A sample-and-hold circuit is used to capture the peak voltage across the sense resistor and hold it during the ‘off’ time until the next PWM cycle begins. The voltage is amplified by a factor of ten and fed to the analogue output terminal, providing the user with the ability to accurately estimate the current in the load in real time.

Another unique feature is the programmable gate drive. An external resistor sets the magnitude of the current into the gates of the external FET bridge, allowing the user to control the slew rate of the driver to help achieve electromagnetic compliance.

Other key features include internal PWM current control. Peak current is regulated with a fixed ‘off’ time PWM regulator, and the peak current is selected by the voltage on the reference voltage terminal. In addition, an overcurrent flag notifies the user when the device is actively PWM limiting the current in the load.

Protection features include: VDS sensing, where monitoring the drain-source voltage of the external MOSFET provides protection in the case of bridge or motor shorts; undervoltage lockout, which eliminates the need for power sequencing; crossover current protection for driving a wide range of FETS; and thermal shutdown.

The A4955 and A4956 devices are supplied in a low-profile (4x4mm) 20-contact QFN package (suffix “ES”) or a 20-lead eTSSOP (suffix “LP”), both with exposed power tab. The A5957 is only supplied in the low-profile 4x4mm, 20-contact QFN package (suffix “ES) with exposed power pad.

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