Abstract: A balloon pump for a circulatory assist device, the pump comprising a pumping chamber of variable volume in which a diaphragm assembly is displaceable in first and second pumping directions to inflate and deflate the balloon, the diaphragm assembly comprising a diaphragm sandwiched between a pusher plate and a retainer plate where the pusher plate and retainer plate are affixed to an elongated shaft having an external threaded portion disposed in a planetary roller nut, a rotor assembly is affixed to the roller nut and a stator assembly is disposed outwardly of the rotor assembly to impart bidirectional rotation to the roller nut to cause the shaft and diaphragm assembly to move in the first and second pumping directions, the pump communicating with a pump control unit and microprocessor for controlling pump operation.