Date: 21/06/2012

3-phase brushless DC motor driver from Allegro with closed loop speed control

Allegro MicroSystems, Inc has launched a new three phase brushless DC motor driver chip with BEMF sensing for commutation control. Allegro’s A4942 device has a maximum operating voltage of 16 V at up to 1.45 A. This new fan driver is aimed at the industrial and office automation markets and was designed for fan and blower applications.

The motor drive system consists of three P-Channel high side N-Channel low side DMOS half bridges, closed loop speed control with minimum and maximum speed settings, soft switching, soft start, internal PWM current limit control, BEMF sensing circuits, adaptive commutation control, and state sequencer.

Features:
1. 3 phase angle settings from 6 degrees to 24 degrees
2. BLDC motor efficiency can be increased at high speed by adjusting lead angle (phase advance) of motor commutation.
3. Output current is controlled by an internal fixed off time PWM current regulator
4. Motor start-up current (Soft Start) is controlled with an external capacitor
5. Protection features such as thermal shutdown with hysteresis, under voltage lockout (UVLO), crossover current and locked rotor protection with auto restart
6. Lead (Pb) free package with 100% matte tin leadframe plating
7. A4942GES-T: 20 lead 4 × 4 mm QFN package with exposed power tab

The sequencer determines which output devices are active. The BEMF sensing circuits and adaptive commutation circuits determine when the state sequencer advances to the next state. The three half-bridge outputs are controlled by the state sequencer with six possible states. Motor BEMF is sensed at the tri-stated output for each state. BEMF sensed motor commutation relies on the accurate comparison of the voltage on the tri-stated output to the voltage at the center tap of the motor. The BEMF zero crossing is used as a positional reference and it is the point where the tri-stated motor winding voltage crosses the center tap voltage.

The A4942 soft switching feature reduces the rate of change of the motor output current resulting in reduced motor speed variations and lower fan audible noise.