This paper proposes a novel DC-DC converter with only one switch on the main current path of the power stage. Since the number of switches is reduced on the current path, the efficiency is improved. The wide input/output voltage range is also realized by having three types of operation modes of step-up mode, step-up/step-down mode and step-down mode. The advantages of the proposed circuit have been confirmed by theoretical analysis and simulations. Post layout simulation results show that the proposed circuit has an efficiency of more than 87% in the 0.5V to 2.5V input range with 1.8-V output voltage and 10- mA output current.