A Load-Insensitive Hybrid Back Telemetry System for Wirelessly-Powered Implantable Devices

This paper presents a hybrid load-shift-keying (LSK) system for load-insensitive back telemetry to realize near-constant voltage changes on a primary coil against a wide range of load variations. The hybrid-LSK-enabled full-wave rectifier includes built-in open-coil and short-coil functions for hybrid-LSK modulation. When the load changes from 50 Ω to 50
$\mathrm{k}\Omega$
, the variation of voltage change levels on the primary coil can be minimized to 60 mV, achieving 88% reduction compared to the conventional short-coil LSK with 510 mV variation. The 180-nm CMOS hybrid-LSK prototype verified that the normalized variation of voltage changes (VARvc) was reduced by 34.1% compared to short-coil LSK. The primary coil voltage changes can be demodulated with slope detection, enabling robust back telemetry against load variations.