Fully Integrated Capacitive DC–DC Converter With All-Digital Ripple

Abstract

This paper presents an adaptive all-digital ripple mit- igation technique for fully integrated capacitive dc–dc converters. Ripple control is achieved using a two-pronged approach where coarse ripple control is achieved by varying the size of the bucket capacitance, and fine control is achieved by charge/discharge time modulation of the bucket capacit ors used to transfer the charge between the input and output, both of which are completely digital techniques. A dual-loop control was used to achieve regulation and ripple control. The primary s ingle-bound hysteretic control loop achieves voltage regulation and the secondary loop is re- sponsible for ripple control. The d ual-loop control modulates the charge/discharge pulse width in a h ysteretic variable-frequency environment using a simple digital pulse width modulator. The fully integrated converter was implemented in IBM’s 130-nm C M O Sp r o c e s s .R i p p l er e d u c e sf r o m9 8t o3 0m V ,w h e nr i p p l e control secondary loop is enabled for a load of 0.3 V and 4 mA without signi ficantly impacting the converter’s core ef ficiency. Measurement results show constant ripple, independent of output voltage. The converter achieves a maximum ef ficiency of 70% for 1.3 V and 0.5 V and a maximum power density of 24.5 mW/mm , including the areas for the decoupling capacitor. The maximum power densit y increases to 68 mW/mm if the decoupling capacitor is assumed to be already present as part of the digital design.