A 4.2-to-0.5-V, 0.8- \mu A-0.8-mA, Power-Efficient Three-Level SIMO Buck Converter for a Quad-Voltage RISC-V Microprocessor

This article presents a Li-ion battery-compatible single-inductor-multiple-output (SIMO) buck converter that fulfills the power management need of an integrated sub-mW RISC-V microprocessor. The proposed converter can directly take a 4.2-V battery voltage and produce four power rails ranging from 1.8 V for I/O to 0.5 V for the processor core. The three-level input stage is chosen to reduce the inductor ripple size and switching loss, thus increasing power conversion efficiency (PCE). In addition, the fully digital implementation using novel domino flash analog-digital converters (ADCs) enables low static current. Also, pulse frequency modulation (PFM) results in a wide dynamic range. The proposed three-level SIMO converter has been prototyped in a 65-nm CMOS technology with the 32-bit RISC-V processor. Measurement results show that the converter achieves a 1000 \times load current range (0.8 \mu A-0.8 mA) to support the active or sleep modes of the processor. The converter marks the PCE of 56.2%-72.8%. Compared to the ideal buck-low-dropout voltage regulator (LDO) architecture (LDO-only), it improves the PCE by 23.8% (46.4%).