Always-On CMOS Image Sensor for Mobile and Wearable Devices

This paper presents a low-power always-on image sensor for mobile and wearable device applications. The sensor continuously captures images for smart sensing, such as face detection, eye tracking, and gesture recognition, and it provides high-resolution images for capturing pictures with a unified sensor. The sensor employs a switchable dual mode: always-on (AO) mode with low power consumption and photo-shooting (PS) mode with high signal-to-noise ratio. For dual-mode operation with high energy efficiency, we implemented dynamic voltage scaling that provides 0.9 V analog–digital supply voltage for the AO mode and 3.3 V analog–1.8 V digital supply voltage for the PS mode. For low-voltage operation in the AO mode, a conventional four-transistor pixel operates as a charge-shared pixel. In order to suppress power consumption and frequency in conventional single-slope analog-to-digital converters (ADC), two neighboring column-parallel ADCs are reconfigured to one successive-approximation ADC for low-voltage and low-frequency operation in the AO mode with negligible area overhead. The fabricated 640 $\,\times\,$ 480 pixel prototype sensor operates at 45.5 $\mu$ W (at 15 fps, 320 $\,\times\,$ 240) in the AO mode, which significantly extends battery life when performing always-on sensing.