New three-phase, sensorless, brushless DC motor driver IC

Paul Boughton
Allegro Microsystems Europe is introducing the A4941 three-phase, sensorless, brushless DC motor driver integrated circuit (IC) that is designed for fan and blower applications in the white goods and office automation sectors. Key features of the new device, which is housed in a small eTSSOP package, include sensorless commutation, a wide (5-16V) supply voltage range, 1.25A peak output current, -40 to +105 degrees C operating temperature range, lock detection with auto restart, and soft switching for reduced audible noise.
The motor drive system in the A4941 consists of three half-bridge NMOS outputs, back-EMF sensing circuitry, adaptive commutation control and a state sequencer. The sequencer determines which output devices are active, while the back EMF sensing circuits and adaptive commutation circuits determine when the state sequencer advances to the next state. The three half-bridge outputs are controlled by a state machine with six possible states.
Motor back-EMF is sensed at the tri-stated output for each state; back-EMF sensing motor commutation relies on the accurate comparison of the voltage on the tri-stated output to the voltage at the centre tap of the motor. The back-EMF zero crossing - the point where the tri-stated motor winding voltage crosses the centre tap voltage - is used as a positional reference. The zero crossing occurs roughly halfway through one commutation cycle.
At start-up, commutation is provided by an onboard oscillator in order to step the motor to generate back-EMF. When legitimate back-EMF zero crossings are detected, normal back-EMF sensing commutation is achieved.
Adaptive commutation circuitry and programmable timers automatically determine the optimal commutation points with only a minimal number of external components required. Output speed can be monitored with the output signal that occurs at the FG terminal when a back-EMF zero crossing is detected.
Duty cycle control of the pulse-width modulated (PWM) input is applied directly to the motor windings to control speed or torque. The PWM input is also used to disable motor operation and put the IC into a low-power sleep mode.
Protection features include thermal shutdown with hysteresis, under-voltage lockout (UVLO), crossover current and locked rotor protection with auto restart.
The A4941GLP-T is supplied in a small (5 × 4.4mm) 16-lead TSSOP package with exposed power tab. The package is lead (Pb) free, with 100 per cent matt tin lead-frame plating.
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