Lithium-ion batteries are generally used in mobile devices, but the voltage range of the battery varies from 2.7 to 4.2V. To provide a mid-3V-range output from the battery, a converter capable of step-up/down-conversion is necessary. For this purpose, non-inverting buck-boost topologies with multimode control [1-3] have been widely used. However, they have limited efficiency slightly higher than 90%, which comes from the fact that a main current path always encompasses two switches. To increase the efficiency in the buck mode where the converter operates for most of the usage time, a flying capacitor buck-boost (FCBB) was proposed in . Despite its high power efficiency, it requires large-size LDMOS to endure a large voltage range up to 8V at switching node, resulting in cost inefficiency. Since all these topologies have a common controller that covers both buck and boost modes of operation, compensator design is challenging. Moreover, a non-minimum-phase system of boost operation makes it hard to achieve a fast loop response. In this paper, we propose a step-up/down DC-DC converter based on buck operation only over the whole input voltage range, which greatly simplifies the controller design and consequently gives fast response. Furthermore, it achieves high efficiency because of the reduced effective resistance on the main current path.