A charge pump for driving CMOS active pixel reset

Abstract

To overcome the limitation of low image signal swing range and long reset time in four transistor CMOS active pixel image sensor, a charge pump circuit is presented to improve the pixel reset performance. The charge pump circuit consists of two stage switch capacitor serial voltage doubler. Cross-coupled MOSFET switch structure with well close and open performance is used in the second stage of the charge pump. The pixel reset transistor with gate voltage driven by output of the pump works in linear region, which can accelerate reset process and complete reset is achieved. The simulation results show that output of the charge pump is enhanced from 1.2 to 4.2 V with voltage ripple lower than 6 mV. The pixel reset time is reduced to 1.14 ns in dark. Image smear due to non-completely reset is eliminated and the image signal swing range is enlarged. The charge pump is successfully embedded in a CMOS image sensor chip with 0.3 × 10⁶ pixels.