Design of a 93% Energy-Efficient Buck-Type Capacitor Charger IC in 250-nm CMOS

This paper presents an energy-efficient buck-type capacitor charger integrated circuit (IC) that can deliver high instantaneous power from a low-capacity battery using a capacitor as an energy buffer. The prototype IC, which was designed and fabricated in 250-nm CMOS, reduces capacitor charging losses by driving the buck converter with gradually increasing pulsewidths and then recycles the remaining energy by operating the buck converter in the reverse direction. The presented digital pulsewidth modulator (DPWM) supports a wide range of duty cycles from 4% to 96% with a fine resolution of 0.36% and a high switching frequency of 1.5 MHz. The optimal switching frequency and transition time that can minimize the charging/discharging losses are analyzed. The prototype charger IC can charge a 10-μF capacitor to 4.2 V with a transition time of 1.25 ms with a peak charging efficiency of 93% while operating at a 1.5-MHz switching frequency.