A reconfigurable micro power solar energy harvester for ultra-low power autonomous microsystems

This paper presents a fully integrated solar energy harvester for ultra-low power autonomous microsystems. This area- and power-efficient circuit harvests solar energy from a micro-power photovoltaic module to charge a rechargeable NiMH microbattery. As the harvested solar energy varies considerably in different lighting condition, the proposed circuit scales the power consumption and performance of the target microsystem, according to the harvested solar energy. Battery voltage is measured when battery is discharged by a high discharge current, to estimate harvested solar energy. Operating frequency and supply voltage of digital circuits and bias currents of analog circuits are reconfigured dynamically, based on measured battery voltage to optimize power-performance of the microsystem. This circuit occupies a core area of only 0.2mm² in a 0.18μm CMOS process and features a low power consumption of 390nW operating at its highest clock frequency.