LM5117 is a chip which can wide input range synchronous Buck controller. It is qualified for automotive applications. According to the technical requirements of the converter to design, the chip LM5117 is used for this design. The principle of working, the determination of the important parameters and finally the schematic circuit diagram are clearly described. The physical model of the converter is realized. The testing results show that this converter has the advantages of high efficiency, low ripple voltage and good steady-state.