A Fully Integrated x4 Voltage Converter for Monolithic Photodiode Light Harvesting

In this article an integrated switched capacitor charge pump that uses negative photodiodes as the primary energy source is presented. In contrast with conventional bulk CMOS approaches, that use positive photodiodes in the front-end-of-line stack, which have limited photon recollection efficiency, our approach increases the overall energy extraction by a factor of 6 in generating voltages for power management integrated circuits. The proposed solution consists of an interleaved inverting charge pump, that inverts negative input voltage, followed by a single-stage cross-coupled charge pump, that quadruples output voltage by means of an innovative connection of the charge pump. Efficiency loss due to the use of a second charge pump is compensated by the increased efficiency of the negative photodiodes. Charge pumps are clocked by a light-powered ring oscillator. This configuration dynamically adjusts the currents based on light intensity, thereby optimizing the frequency for improved efficiency. This setup is designed to optimize energy harvesting efficiency, striving to achieve maximum power efficiency. The proposed approach, validated on $0.18~\mu m$ standard CMOS technology through experimental measurements, shows that the light harvester generates $8.7~\mu W$ of power ( $75.5~pW/\mu m^{2}$ of power density) from a $240 \times 480~\mu m^{2}$ integrated photodiode area, with a maximum power conversion efficiency of 57%, at $-0.42~V$ input voltage.