A 20nW-to-140mW input power range, 94% peak efficiency energy-harvesting battery charger with frequency-sweeping input voltage monitor and optimal on-time generator

This paper presents a boost-type energy-harvesting battery charger IC that can maintain high efficiencies over a wide range of input power. In other words, the IC has a fast response to achieve 94% peak efficiency at high input power conditions, yet dissipates only 14.7nW quiescent power to achieve a net positive charging at low input power conditions. The key to this fast, low-power operation is the use of frequency-sweeping input voltage monitor, i.e., a clocked comparator of which sampling clock frequency is exponentially swept from 800-kHz to 100-Hz. In addition, an optimal on-time pulse generator improves the power efficiency by scaling the switching pulse-width proportional to the battery-to-input voltage ratio. The prototype IC fabricated in 0.25-μm CMOS can charge a 3.0V battery from the input source ranging 0.3-2V and 20nW-140mW, which is the widest input power range reported to date.