A Switched Capacitor DC-DC Buck Converter for a Wide Input Voltage Range

Abstract

In this paper, a power-efficient multiphase Recursive Switched Capacitor (RSC) converter is presented. Conventionally, RSC converters are used to obtain many different output voltages from a fixed input voltage. Here, the converter provides a fixed output voltage of 1 V at 1 mA from an input voltage ranging from 1.4 V to 4.5 V. It has one programmable stage (2:1 or 3:2) followed by four 2:1 stages. Contrary to most conventional topologies, depending on the input voltage, not all the stages are always deployed. This allows to increase the power efficiency of the whole architecture. The flying capacitance of the non-activated stages is transferred to the activated ones. Hence, for any given input voltage, 100 % of the on-chip capacitance is always used for the conversion. For a general 2:1 topology, an analytical analysis of the power losses is carried out and the impact of the overdrive voltage of the switches on the power efficiency is quantified.
A novel gate-driver technique for the switches involved in the conversion is proposed. It ensures an optimal overdrive voltage of the transistor, irrespective of its source and drain potentials. The 16-phase interleaved converter employs a charge recycling technique and uses a total on-chip capacitance of 3 nF.
The RSC converter is designed to be implemented in a standard 40 nm CMOS process which offers a capacitor density of approximately 2 nF/mm^2. Circuit simulations over the whole input voltage range show a power efficiency never lower than 54 % with a peak value of 92.7 %.