Design of 93% energy efficiency buck-type capacitor charger IC in 250nm CMOS

This paper presents an energy-efficient bucktype capacitor charger IC for delivering large instantaneous power using a low-capacity battery and a super-capacitor. The prototype IC designed and fabricated in 250nm CMOS reduces the capacitor charging loss by driving a buck converter with gradually increasing pulse-widths and recycles the remaining energy by operating the buck converter in the reverse direction. The presented digital pulse-width modulator (DPWM) supports both high switching frequency of 3.3-MHz and fine duty-cycle resolution of 0.39%. The analysis is carried out on the optimal switching frequency and transition time that can minimize the charging/discharging losses. In case of charging a 0.01-mF capacitor to 4.2V, the peak charging efficiency of 93% is achieved with a switching frequency of 1.5-MHz and transition time of 1.25-ms.