A reconfigurable DC-DC converter that provides both step-down and step-up functions is proposed to ensure a stable output voltage over full voltage range of battery in portable devices. The converter can automatically adjust itself into different modes, so as to enhance system efficiency. Moreover, seamless transition is achieved to reduce output ripple and improve line regulation. Additionally, a novel current-sensing technique is applied to realize lossless and accurate current sensing. The controller IC is designed and fabricated in 1.5-um BCD process. Test results verified the design target.