Power Electronics

Bridge Driver Supports Multiple Motor Types

Allegro MicroSystems A3985 is a digitally programmable dual full-bridge gate driver suitable for driving a wide range of higher power industrial bipolar two-phase stepper motors, two-phase brushless DC motors or two brush DC motors. Motor power is provided by external N-channel power MOSFETs at supply voltages from 12 V to 50 V. The A3985 provides all the necessary circuitry to ensure that the gate-source voltage of both high-side and low-side external MOSFETs are above 10 V, and the gate capacitance is quickly charged and discharged in order to reduce switching losses.

Full digital control is provided by two serially accessible registers that permit programming of off-time, blank-time, dead-time, mixed decay ratios, synchronous rectification, system oscillator source selection and division ratio and idle mode. All internal timings are derived from a master clock that can be generated on-chip or provided by an external system clock. A programmable divider allows for a wide range of external system clock frequencies.

The internal fixed off-time PWM current-control timing is programmed via the serial interface to operate in slow, fast, or mixed current-decay modes. The desired load current level and direction is set via the serial port with a direction bit and two six-bit linear DACs in conjunction with a reference voltage.

The seven bits of control allow flexibility in torque control for a variety of step methods, from microstepping to full-step drive. Load current in the external power MOSFET full-bridges is set in 1.56% increments of the maximum value.

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 under-voltage lockout. Special power up sequencing is not required.

The A3985 is supplied in a 38-pin TSSOP package with 100% matte tin leadframe platting. The A3985SLD-T is priced at $3.87 in quantities of 1000.

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