A Nonresonant Self-Synchronizing Inductively Coupled 0.18- $\mu $ m CMOS Power Receiver and Charger

While the functionality of emerging wireless microsensors, cellular phones, and biomedical implants, to name a few, is on the rise, their dimensions continue to shrink. This is unfortunate because smaller batteries exhaust quicker. Not surprisingly, recharging batteries wirelessly is becoming increasingly popular today. Still, small pickup coils cannot harness much, so induced EMF voltages v
_EMF.S
are low. Modern receivers can resonate these low input voltages to rectifiable levels, but only with a finely tuned capacitor that resonates at megahertz when on-chip and at kilohertz when off-chip. In other words, resonant rectifiers are sensitive to frequency and dissipate considerable switching power when integrated on-chip. Unluckily, excluding the resonant capacitor requires a control signal that synchronizes switching events to the transmitter's operating frequency. The 0.18-μm CMOS prototype presented here derives this synchronizing signal from the coupled v
_EMF.S
by counting the number of pulses of a higher-frequency clock across a half cycle during a calibration phase and using that number to forecast half-cycle crossings. This way, the prototyped IC switches every half cycle to draw up to 557 μW from 46.6 to 585-mV
_PK
signals with 38%-84% efficiency across 1.0-5.0 cm.