A5931 & A5931-3 Three Phase Sensorless Fan Driver Allegro MicroSystems A5931 and A5931-3 Three Phase Sensorless Fan Driver have a sensorless sinusoidal drive to minimize vibration for high-speed server fans. Sensorless control eliminates the requirement for Hall sensors for server fan applications. The device has a flexible integrated closed-loop speed control system. EEPROM provides common functions of the fan speed curve to a specific application.