A Stacked-Inverter Ring Oscillator for 50 mV Fully-Integrated Cold-Start of Energy Harvesters

Energy harvesting from ambient sources, such as body temperature, is an attractive solution for powering battery-less wearable electronics used for healthcare diagnostics. While feasible from an energy standpoint, initial start-up of an energy harvesting circuit from a millivolt-level thermoelectric generator output poses a particular challenge. One approach to boosting such a low input voltage is to use a low-voltage oscillator to start up a higher voltage DC-DC converter. In this work, we demonstrate a modified ring-oscillator architecture using a stacked three-inverter delay element, which can generate self-sustained oscillation from an input supply voltage as low as 50 mV. Compared to inductor-based on-chip oscillators or those using native transistors, this architecture significantly reduces circuit area and expands process compatibility. The start-up oscillator is implemented in a standard 0.18 μm CMOS process and comprises 21 stages; it generates a clock of frequency 9.5 kHz with a 86% voltage swing from an input supply voltage of 50 mV, while occupying less than 0.003 mm2 and consuming 818 pW.