Design of Monolithic CMOS LDO Regulator With D ² Coupling and Adaptive Transmission Control for Adaptive Wireless Powered Bio-Implants

This paper presents a monolithic CMOS low drop-out regulator for adaptive wireless powered bio-implants. A dual-loop control scheme is proposed to enhance load/DVS transient response, without a filter capacitor. The inner loop employs a direct dynamic coupling technique to improve transient response by directly detecting voltage variations through a capacitive coupling high-pass filter. Meanwhile, the outer loop adopts an adaptive transmission control to improve the regulator's large-signal transient response. The regulator is intrinsically stabilized without frequency compensation. Both small-signal and large-signal analysis are provided as design guidelines. The proposed regulator was fabricated with a 0.35-μm CMOS process, with an active die area of 0.35 μm2. Experimental results demonstrate a 1.6-μs/V down-tracking and 2.2- μs/V up-tracking speed for DVS reference tracking, with undershoot/overshoot voltages controlled below 90 mV. Without a filter capacitor, the output voltage of the regulator recovers within 2.0 μs for 100-mA load change, with less than 90-mV variation. The current/power efficiency stays above 93/78% over the entire interested load current range, with a peak value of 99.96/91.1%, respectively.