A monolithic step-down SC power converter with frequency-programmable subthreshold z-domain DPWM control for ultra-low power microsystems

Abstract

This paper presents a fully integrated switched-capacitor (SC) power converter for ultra-low power microsystems. The design features an efficient step-down charge pump and a frequency-programmable digital feedback controller. The controller is designed in the subthreshold region to reduce power dissipation significantly. It employs a programmable switching frequency digital pulse-width modulation (DPWM) to maintain high efficiency under various loading scenarios, without causing random noise spectrum as in pulse-frequency modulation (PFM). The monolithic implementation effectively suppresses switching noises and glitches introduced by parasitic components from traditional bonding, packaging and PCB wiring. The test prototype was designed and fabricated with a 0.35-μm digital CMOS N-well process. It supplies a variable power output from 0.8 to 1.5V and from 400 μW to 7.5 mW precisely, with a variable switching frequency ranging from 200 KHz to 1 MHz. The converter achieves 66.7% efficiency with a controller power of only 147.5 μW.