June 15, 2010
Worcester, MA – Allegro MicroSystems, Inc. has introduced a new three-phase brushless DC motor pre-driver with outputs for direct high current gate drive of an all N-channel power MOSFET three phase bridge with a maximum supply voltage of 38 V. Allegro’s A4936 device has three Hall-element inputs, a sequencer for commutation control, fixed off-time pulse width modulation (PWM) current control, and locked-rotor detection. The key applications for this new device are laser printer and copier drum drives within the office automation market.
Output current is scaled by the capability of the external MOSFETs. Locked rotor detection delay is set by an external capacitor on the CLD terminal. The PWM, DIR, BRAKE and STOP inputs can be used to control motor speed, position, and torque. Motor speed can be determined using motor FG coil signal that is amplified and shaped with the internal differential amplifier and comparator to produce a digital FGS output. The external MOSFETS can be Pulse-Width-Modulated (PWM’d) using an external signal on the PWM input, or using the internal PWM current regulator. In either case, Allegro’s A4936 synchronous rectification feature reduces power dissipation by turning on the appropriate MOSFETs during current decay.
The Hall elements can be inexpensive types when used with internal noise filtering to prevent false commutation signals. The A4936 provides a regulated 7.5 V supply to power the three Hall elements. Internal circuit protection includes thermal shutdown with hysteresis, under voltage lockout, and dead time protection. Special power-up sequencing is not required. The operating temperature range is -20°C to 105°C.
The A4936 is supplied in a 5 mm × 5 mm, 32-terminal QFN package with exposed thermal pad. This small footprint package is lead (Pb) free with 100% matte tin lead frame plating.