A digitally controlled dc-dc power converter is useful to dc power supply systems for telecommunications and data communications systems that require high reliability and high performance. This paper presents a design criterion and output characteristics of 1 MHz high-speed digitally controlled dc-dc converter using a voltage controlled oscillator (VCO) as an analog to digital signal converter in the digital control circuit. As a result, it is revealed theoretically and experimentally that the proposed 1 MHz high-speed digitally controlled dc-dc converter has excellent static and dynamic characteristics. The adjustability of output voltage is less than 10 mV and there is no steady-state error within the integral controlled regulation range. Also, the overshoot is suppressed enough within 0.1 V and the transient time is very short.