Self-Tuning Electrostatic Energy-Harvester IC

Miniature self-powered systems like wireless microsensors that rely only on easily exhaustible tiny in-package batteries suffer from short lifetimes. Harvesters, however, extend life by replenishing consumed energy with energy from the environment. The problem is harvesters generate considerably low power so producing a net gain with which to recharge a battery requires ultra low-energy circuits. This brief presents a 1.5 x1.5 mm² 0.7-μm BiCMOS self-tuning electrostatic energy-harvester integrated circuit (IC) that adapts to changing battery voltages (V_BAT) to produce usable power from vibrations across V_BAT´s entire operating range. The prototype holds C_VAR´s voltage so that kinetic energy in vibrations can generate and steer current into the battery when capacitance decreases. Unlike in [13], the inductor-based precharger that charges C_VAR to V_BAT adapts to a constantly shifting V_BAT target. Collectively, the precharger and its self-tuning reference, system monitors, and other control circuits draw sufficient power to operate yet dissipate low enough energy to yield a net gain. Experimentally, the harvester IC generates 1.93, 2.43, and 3.89 nJ per vibration cycle at battery voltages 2.7, 3.5, and 4.2 V, which at 30 Hz produce 57.89, 73.02, and 116.55 nW. Accordingly, the system charges 1 μF from 2.7 to 4.2 V (a thin-film lithium-ion range) in 69 s and harnesses 47.9% more energy than with a fixed reference in the same time frame.