0.18-μm Light-Harvesting Battery-Assisted Charger–Supply CMOS System

Wireless microsensors in hospitals, factories, and farms can manage and save lives and resources. Although tiny batteries cannot sustain them for long, harvesters can because ambient energy is abundant. Photovoltaic cells are popular in this regard because they output close to 100× more power from solar light than piezoelectric, electrostatic, and thermoelectric generators can from motion and heat. However, since mm cells can only supply μW's of the mW's that microsystems can draw, and light is not always available, battery assistance is necessary. So to supply functions and sustain operation across extended periods, the system should extract maximum ambient power, draw minimal battery assistance, and deliver as much power as possible. The 0.18-μm CMOS harvester presented here does this, draws 10-100 μW from a 3 × 3 × 1 mm3 cell and assistance from a battery to supply a 1-mW load and recharge the battery with excess cell power. The switched-inductor charger supply regulates 1 V within ±25 mV with 73-86% power-conversion efficiency and keeps the cell within 1% of its maximum power point. This way, the cell outputs 100 μW/mm2 from solar light and 1 μW/mm2 from direct indoor light.