This disclosure describes power-efficient ultrasonic techniques for proximity detection and device wake-up from sleep mode. In sleep mode, the host application processor, e.g., the main device processor, is inactive. A low-power embedded controller remains on during sleep mode and generates an ultrasonic waveform that is amplified by a low-power speaker driver that also remains on during sleep mode. In some of the described architectures, a switch is provided that switches between the low-power speaker driver in sleep mode and a high-power speaker amplifier in active mode. A microphone captures echoes of reflected ultrasonic waves. Echoes arising from within a certain distance of the screen of the device are indicative of human presence and trigger the waking up of the device.
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Liu, Fang; Sanders, Nicholas Jordan; and Chang, David F., "Switchable Speaker Driver for Power-efficient Proximity Sensing and Device Wake-up", Technical Disclosure Commons, (April 22, 2020)