This paper presents a fully integrated resonant regulating rectifier (IR3) with an on-chip coil used to wirelessly power mm-sized implants. By combining rectification and regulation in a single stage, and controlling this stage via a hybrid pulse-width modulation and pulse-frequency modulation (PFM) feedback scheme, the IR3 avoids efficiencylimiting cascaded losses while enabling tight voltage regulation with low dropout and ripple. The IR3 is implemented in 0.078 mm(2) of active area in 180-nm Silicon oxide insulator (SOI) CMOS, and achieves a 1.87% Delta V-DD/V-DD power supply regulation ratio with a 1-nF decoupling capacitor despite a tenfold load current variation from 8 to 80 mu A. A 0.8-V V-DD is maintained at a 8-k Omega load for 144-MHz RF inputs ranging from 0.98 to 1.5 V. At 1-V regulation, the voltage conversion efficiency is greater than 92% with less than 5.2-mVpp ripple, while the power conversion efficiency is 54%. The measured overall wireless power transfer system efficiency, from the primary coil to VDD output of the IR3, is 2% at 160-mu W load, and reaches 5% at 700 mu W.