A 2.03-mW CMOS Image Sensor With an Integrated Four-Stacked Charge-Recycling Driver for Image Signal Transmission

We propose a CMOS image sensor with a dedicated low-power imaging mode and low-power integrated transmitter. The proposed CMOS image sensor selectively operates in dual mode: a high-quality mode with a 1.5 V supply voltage of readout circuits to achieve a high signal-to-noise ratio (SNR), and a low-power mode with a 0.9 V supply voltage to support always-on imaging. To further reduce the power consumption in the low-power mode, a single-slope analog to digital converter (ADC) embeds a power cutoff scheme in the comparator and a two-step conversion with dual reference voltages. To alleviate the SNR degradation in the low-power mode, which inherently occurs from voltage scaling, a correlated multiple sampling technique that consumes negligible power overhead is implemented using the proposed window-counting scheme. To reduce the significant power consumption that occurs during image signal transmission, an integrated transmitter with four-stacked charge-recycling drivers is used so that four symbols are simultaneously transmitted with a shared supply voltage. A prototype CMOS image sensor with 680
$\times $
520 pixels is fabricated using 110-nm CMOS image sensor technology. The fabricated CMOS image sensor consumes only
$301~\mu \text{W}$
(at 15 fps) in the sensor core and 2.03 mW including the transmitter and phase locked loop (PLL) while generating low temporal random noise under 0.27 LSB with correlated multiple sampling.