An ultra-low power li-ion battery charger for micro-power solar energy harvesting applications

This paper presents an ultra-low power, fully integrated solar energy harvester circuit for autonomous microsystems. The proposed circuit harvests solar energy from a micro-power photovoltaic module and stores the harvested energy in a miniaturized thin film Li-Ion microbattery, using a highly area- and power-efficient power management circuit. As neither inductor, nor large pumping capacitors have been used in this circuit, it occupies less area comparing to conventional inductive and switched-capacitor DC-DC converters. In addition, thanks to low power design of this circuit, it achieves more than 94% efficiency during battery charging. Even under reduced light intensity, when the harvested energy is only a few tens of microwatts, more than 92% efficiency is achievable. The proposed microsystem has been implemented in a 0.18μm CMOS process and occupies a core area of only 0.12mm². This circuit features a low power consumption of 270nW in average.