A compact 120-MHz 1.8V/1.2V dual-output DC-DC converter with digital control

A dual-output cascaded dc-dc buck converter is proposed to meet the challenge of miniaturization for embedded applications. A programmable switching frequency up to 120 MHz, output stage segmentation, and cascoding are used to achieve high power efficiency with small output inductors. A digital constant-off-time controller provides a fast load current step-response and the suppression of cross-regulation among multiple output voltages. Integrated in a 90-nm CMOS technology, an experimental prototype generates 1.8 V and 1.2 V from a 2.5-V input supply. Using only a PCB trace inductance of 4 nH, the 2.5-V to 1.8-V conversion is accomplished with the peak efficiency of 76% at a power density of 2.59 W/mm². The peak efficiency of the dual-output converter is 71.4% at 1.28 W/mm². These efficiency values increase to above 80% if 18-nH air-core inductors are used.