Three-phase brushless DC motor drive technology based on A4936 design

The A4936 is Allegromicro's complete three-phase brushless DC (BLDC) motor pre-driver that delivers up to 28V output and is used directly to drive high current gates for six power N-MOSFETs. The hardship includes three Hall device inputs, a sequencer for commutation control, PWM current control, and synchronous rotor detection. This article describes the main features and benefits of the A4936, functional block diagrams and typical application block diagrams.

The A4936 is a complete 3-phase brushless DC motor pre-driver, high up to 28 V output for direct, high-current gate drive of an all N-channel power MOSFET 3-phase bridge. The 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 PWM, DIR, and BRAKE and STOP inputs can be used to control motor speed, position, and torque. The output current is scaled by the external MOSFETs. Motor speed can be determined using the FG output from an FG coil amplifier and comparator.

The external MOSFETS can be PWMed using an external signal on the PWM input, or using the internal PWM current regulator. In either case, the 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 noise filtering to prevent false commucoming signals. The A4936 provides a regulated 7.5 V supply to power the three Hall elements. Internal circuit protection includes thermal shutdown with hysteresis, undervoltage lockout, and dead time protection Special power-up sequencing is not required. Operating temperature range is –20°C to 105°C.

The device package is a 32-contact, 5 mm × 5 mm, 0.90 mm nominal overall height QFN, with exposed pad for enhanced thermal dissipation. This small-footprint package is lead (Pb) free, with 100% matte tin leadframe plating.

A4936 main features and advantages:

• Drives 6 N-channel MOSFETs

• Synchronous rectification for low power dissipation

• Internal UVLO and thermal shutdown circuitry

• Hall element inputs

• PWM current limiting

• Dead time protection

• FG outputs

• Standby mode

• Lock detect protection

• Overvoltage protection

figure 1. A4936 functional block diagram

figure 2. A4936 Typical Application Block Diagram

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