25-nA Modified Hybrid Ladder Converter with Efficient Output-Capacitor Charge Recycling and 90% Battery Lifetime Extension

Systems-on-Chip (SoCs) in wearable devices require a high-efficiency buck converter to convert battery voltage (V<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BAT</inf>, 2.8-4.2V for Li-ion battery) to Vo (supply voltage of SoC, < 1V). Integrating the buck converter with the SoC is cost-effective. Given that the fabrication process for SoC typically has <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{low}-V_{\text{DS}}$</tex> MOSFETs, stacking these MOSFETs as power switches in the buck converter to sustain up to 4.2V is a practical solution [1].