15.8 Millimeter-Scale Nearly Perpetual Sensor System with Stacked Battery and Solar Cells

Sensors with long lifetimes create new applications in medical, infrastructure and environmental monitoring. Due to volume constraints, sensor systems are often capable of storing only small amounts of energy. Several systems have increased lifetime through VDD scaling [1][2][3]. This necessitates voltage conversion from highervoltage storage elements, such as batteries and fuel cells. Power is reduced by introducing ultra-low-power sleep modes during idle periods. Sensor lifetime can be further extended by harvesting from solar, vibrational and thermal energy. Since the availability of harvested energy is sporadic, it must be detected and stored. Harvesting sources often do not provide suitable voltage levels, so DC-DC up-conversion is required.