A High-Q Resonant Inductive Link Transmit Driver With Adaptive-Predictive Phase-Continuous Tuning and Deviation Frequency Calibration for Enhanced FSK/PSK Modulation and Power Transfer

Inductively coupled power transfer systems often use high-Q antenna circuits to reduce losses and increase transmission range. However, the narrow bandwidth due to the high Q-factor conventionally constrains the modulation rate on the carrier, creating a tradeoff between power transfer efficiency and data rate. In this article, we show how instantaneous tuning of the antenna can permit frequency and phase modulation data rates beyond the Q-factor limitations, provided that the drive and antenna resonance frequencies are kept the same at all times. It is also necessary to ensure that the modulation symbol boundaries are precisely aligned with the tracking of the resonance tuning to ensure phase continuity and optimum power transfer. With instantaneous tuning, phase modulation can be implemented by defined numbers of cycles at an offset frequency, provided the offset is precisely controlled. We describe how the required instantaneous resonance tuning can be achieved using a switched fractional capacitor technique and present a system architecture that can follow abrupt frequency changes due to the modulation while tracking changes to the resonant condition due to environmental detuning effects at each modulation frequency. An integrated mixed-signal driver/modulator/tuner demonstrator system is presented, operating from 10 kHz-2 MHz using a 0.18-μm 1.8-50-V CMOS/laterally-diffused metal-oxide semiconductor (LDMOS) process. The system maintains phase continuity in the antenna current throughout frequency shift keying (FSK) and phase-shift keying (PSK) modulation and employs self-calibration of the PSK modulation frequencies to achieve precise angles of phase rotation for QPSK. The antenna tuning, modulation, and frequency calibration are verified with experimental results.