Compact Fast-Waking Light/Heat-Harvesting 0.18- $\mu\text{m}$ CMOS Switched-Inductor Charger

Although microsystems can nowadays consume microwatts, onboard batteries can be so small and leaky that sustaining microwatts for months or years without recharge cycles can be virtually impossible. Tiny photovoltaic cells and thermoelectric generators can help, but only when light or heat is available, and only to the extent that light intensity and thermal gradients allow. This is why energy-harvesting microsystems idle and shut down often and fast-wakeup provisions are important. The 0.18-μm CMOS charger proposed here is 8.31% more efficient during wakeup than the smallest reported and 7.69% more efficient than the next best, but without a 1:60 off-chip transformer and without vibration energy. Although 3.27% less efficient than the most efficient, the system here uses three fewer off-chip inductors. And after waking, the system is 5% to 46% more efficient than the others. The key innovations are low-power management and design. The system in essence charges a 200-pF capacitor with just enough energy to transfer two energy packets per cycle: one to charge a 1.8-μF battery and the other to replenish the 200 pF. This way, and with 100 nF across the source, the system charges a fully depleted 1.8-μF battery to 0.9 V in 45 ms and draws in steady state 98.8%-99.7% of available input power to deliver 76%-86% of the 40-150 μW drawn.