Method and apparatus to drive a linear resonant actuator at its resonant frequency

An apparatus comprising:

• an input circuit that is configured to receive an input signal;
• a signal generator that is coupled to the input circuit and that is configured to output a drive signal that is based at least in part on the input signal, wherein the drive signal includes a plurality of positive drive intervals, a plurality of negative drive intervals, and a plurality of off intervals;
• and
• a driver that is coupled to the signal generator and the input circuit, that is configured to drive a linear resonant actuator (LRA), and that is configured to receive the drive signal, wherein the input circuit is configured to measure the back electromotive force (back-emf) of the LRA during the plurality of off periods of the drive signal, and wherein the signal generator adjusts the duration of at least one of the positive and negative intervals based at least in part on the measured back-emf so as to drive the LRA substantially at its resonant frequency,
• wherein the apparatus has an operational mode and an initialization mode, wherein each period of drive signal during the operational mode includes at least one positive drive interval or at least one negative drive interval between two consecutive off intervals, and wherein a measurement circuit is configured to determine whether the back-emf has reached a predetermined threshold in each of the two consecutive off intervals, and a controller is configured to measure the duration between the back-emf reaching the predetermined threshold for the two consecutive off intervals,
• wherein the signal generator further comprises:
• an analog-to-digital converter (ADC) that is coupled to the input circuit;
• and
• wherein the controller that is coupled between the ADC and the driver,
• wherein the measurement circuit further comprises:
• a sense amplifier that is coupled to the driver;
• and
• a multiplexer that is coupled to the sense amplifier, the ADC, and the controller, wherein the multiplexer is configured to receive the input signal, and wherein the controller is configured to control the multiplexer,
• wherein the driver is a class D driver having:
• a gate drive circuit that is coupled to the controller;
• and
• an H-bridge that is coupled to the gate drive circuit and the sense amplifier
• wherein the multiplexer receives a supply voltage as an input, and wherein the controller further comprises:
• a logic circuit that is coupled to the ADC;
• and
• a compensator that is coupled between the logic circuit and the gate drive circuit.