High Current Stepper Motor Driver Solution with Minimal External Components

Allegro has released the A4989, a dual full-bridge MOSFET controller with integrated microstepping translator suitable for driving a wide range of medium to high-power bipolar two-phase stepper motors. Allegro’s A4989 is based off of the successful design of the Allegro A3986 and adds a change in functionality which improves the ability to more accurately control motor currents over a wider range. Full step to sixteenth step microstepping control reduces system vibration and improves the overall positioning accuracy. Since the A4989 controls external N-channel MOSFETs it is possible to drive stepper motors over a very wide current range with an appropriate choice of external MOSFET’s. Targeted at the industrial and office automation markets, Allegro’s A4989 provides an efficient and easy to implement solution.

Motor power is provided by external N-channel power MOSFETs at supply voltages from 12V to 50V. The A4989 provides all the necessary circuitry to ensure that the gate-source voltage of both high-side and low-side external MOSFETs are above 10V, and the gate capacitance is quickly charged and discharged in order to reduce power dissipation during switching.

This device contains two sinusoidal DACs that generate the reference voltage for two separate fixed-off-time PWM current controllers. These provide current regulation for full bridges of external power MOSFET’s. The PWM regulators automatically switch between slow and fast or mixed decay mode. This patented current decay scheme results in reduced audible noise, increased step accuracy and lower power dissipation.

The two-wire step and direction translator interface is the key to the implementation of the A4989. Simply inputting one pulse on the STEP input drives the motor one step (full, half, quarter or sixteenth depending on the microstep select input). There are no phase-sequence tables, high frequency control lines, or complex interfaces to program. This drastically reduces the load on the application processor or DSP, and the PCB surface area needed.

Using synchronous rectification enhances efficiency and the external power MOSFETs are protected from shoot-through by integrated crossover control and programmable dead time. In addition to crossover current control, internal circuit protection provides thermal shutdown with hysteresis and undervoltage lockout. Special power-up sequencing is not required. Allegro’s A4989 device is supplied in a (Pb) free 38-pin TSSOP package with 100% matte tin leadframe platting, pin-for-pin compatible with the A3986.