A Monolithic Digital Ripple-Based Adaptive-Off- Time DC-DC Converter With a Digital Inductor

Abstract

A ripple-based digital controller requires inductor current ripple as feedback signals and analog RC inductor cur- rent sensors can be used for sensing the ripple. However, the passive RC components are bulky to integrate on-chip and the digital controller cannot use the analog ripple for the control purpose unless extra ADCs are available to quantize the signal. Therefore, a digital inductor current sensor is presented in this paper for obtaining the ripple in the digital domain. As compared to the existing designs, the digital inductor current sensor does not require extra ADCs or knowledge of the inductor DCR. A ripple-based digital controller i s designed to demonstrate how the digital sensor can be utilized. A di gital frequency-lock-loop is also incorporated into the digital controller to alleviate the problem of variable switching-frequency that is commonly found in the ripple-based control scheme. Both the digital sensor and controller are fabricated in UMC 0.13 m digital CMOS process with a small chip area of m m. Measurements results show that a 2 MHz (max. 6.5% in variation) buck converter achieves load-transient responses of about 5 sb yu s i n gt h ed i g i t a l controller. The peak ef ficiency is 91% at a nominal load current of 100 mA.