A low-voltage power supply circuit is developed for micro-power CMOS LSI applications, especially for microwatt smart-sensor LSIs. The circuit consists of a switched-capacitor voltage converter and a series regulator. The switched converter lowers battery voltage 1.5-3V to a low voltage of 0.5-0.7V to drive the series regulator, and the series regulator provides LSI logic gates with a power voltage of 0.4-0.6V such that the logic gates operate in the subthreshold region. In a sample circuit designed to produce a supply voltage of 0.6V, the switched converter lowered a 1.5-V battery voltage to 0.68V, and the series regulator lowered the 0.68V to 0.6V. The power conversion efficiency of the switched converter was 83%, and the total efficiency was 73%, with a 13-µA output current.