High-Efficiency Ultrasound Energy Harvesting Interface With Auto-Calibrated Timing Control From −25 °C to 85 °C

This work presents a high-efficiency ultrasound energy harvesting interface with auto-calibrated timing control, featuring: 1) the proposed $C_{\text {P}}$ auto-calibration, consisting of the half bias-flip time ( $t_{\text {half}}$ ) detection and adaptive closed-loop time calibration (ACTC) to improve the system’s robustness against piezoelectric transducer (PZT) materials and environmental variations; 2) the proposed charge recycling (CR) bootstrapping driver to reduce conduction loss and improve the $C_{\text {P}}$ auto-calibration accuracy as well as the peak voltage flipping efficiency ( $\eta _{\text {flip}}$ ); and 3) the proposed coarse detection and fine calibration technique to eliminate the inherent timing offset and increase the acceptable input excitation frequency range. The fabricated chip prototype in 0.18- $\mu $ m silicon on insulator (SOI) CMOS process can adapt to both PZT5A (nominal ${C_{\text {P}}}~{\sim }~114$ pF) and PZT5H (nominal ${C_{\text {P}}}~{\sim }~190$ pF) and is capable of operating over a wide temperature range from ${-} 25~{^{\circ }}$ C to $85~{^{\circ }}$ C. The proposed $C_{\text {P}}$ auto-calibration and CR bootstrapping driver can improve the $\eta _{\text {flip}}$ to as high as 93.6% at an output power of $496.6~{\mu }$ W. With the proposed coarse detection and fine calibration technique, this work demonstrates a high measured peak power conversion efficiency (PCE) of 94.5%, corresponding to a ~23% improvement when compared with the prior ultrasound energy harvesting interface while achieving a favorable figure of merit (FoM) of $8.13{\times }$ .