A Fully Integrated 0.4 mm² Soft-Charging Switched-Capacitor DC-DC Step-Down Converter with 85% Peak Efficiency Minimizing the Circuit Footprint for Sub-mA-Load E-Textile Applications

This work proposes a design approach for a fixed/narrowed-voltage-conversion-ratio (VCR) in a soft-charging switched-capacitor (SC) DC-DC step-down converter, utilizing a continuous scalable- conversion-ratio (CSCR) architecture, specifically designed for sub-mA-load smart e-textile applications where minimizing size is critical. By narrowing the VCR range, the proposed approach reduces area consumption by constraining design parameters such as capacitor size and operating frequency, sacrificing wide VCR flexibility to enhance power density (PD). This makes it particularly suitable for fixed-VCR mA-load smart e-textile applications, where seamless system integration and compact size are essential. Designed for a 1.8V to 1.2V conversion, the system achieves peak efficiencies of 85.4% and 87.8 % in compact footprints of 0.4mm² and 0.6mm², using 65nm CMOS technology. It operates efficiently across a load range of 0.15mA to 2 mA. These simulation-validated results demonstrate the efficiency and PD advantages of the proposed design method over wide-VCR designs in sub-mA-load scenarios. The approach optimizes capacitor sizing and operating frequency, balancing efficiency and area, making it well-suited for space-constrained fixed-VCR-sub-mA smart e-textile applications where maximizing PD in small form factors is crucial.