A3985: Digitally Programmable Dual Full-Bridge MOSFET Driver


The A3985 is a flexible dual full-bridge gate driver suitable for driving a wide range of higher power industrial bipolar 2-phase stepper motors or 2-phase brushless DC motors. It can also be used to drive two individual torque motors or solenoid actuators. Motor power is provided by external N-channel power MOSFETs at supply voltages from 12 to 50 V.

Full digital control is provided by two serially-accessible registers that allow programming of off-time, blank-time, dead-time, mixed decay ratios, synchronous rectification, master clock 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 clock such as the system clock of the master controller. 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, and mixed current-decay modes. The desired load-current level and direction is set via the serial port with a direction bit and two 6-bit linear DACs in conjunction with a reference voltage. The seven bits of control allow maximum 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.

The above-supply voltage required for the high-side N-channel MOSFETs is provided by a bootstrap capacitor. Efficiency is enhanced by using synchronous rectification and the power FETs 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. This component is supplied in a 38-pin TSSOP (package LD) with 100% matte tin leadframe plating.

Top Features

  • Serial interface for full digital control
  • Dual full-bridge gate drive for N-channel MOSFETs
  • Dual 6-bit DAC current reference
  • Operation over 12 to 50 V supply voltage range
  • Synchronous rectification
  • Cross-conduction protection
  • Adjustable mixed decay
  • Fixed off-time PWM current control
  • Low-current idle mode

Part Number Specifications and Availability

Part Number Package Type Temperature RoHS
Part Composition /
RoHS Data
Comments Samples Check Stock
A3985SLDTR-T 38-lead TSSOP -20°C to 85°C Yes View Data Contact your
local sales rep
Distributor Stock

Allegro’s products are not to be used in any devices or systems, including but not limited to life support devices or systems, in which a failure of Allegro’s product can reasonably be expected to cause bodily harm.


This component is supplied in a 38-pin TSSOP (package LD) with 100% matte tin leadframe plating.

 LD-TSSOP 38 pin