A high-efficiency buck converter circuit was designed in a 0.35 μm BCD process, with 10 V~24 V input voltage, 5 V~12 V output voltage, and 100 mA load current. A simple circuit structure based on the hysteresis control was presented. And the fewer power consumption of the buck converter was achieved by using the sleep mode. In the active mode, the quiescent current was about 110 μA, and in the sleep mode, the quiescent current was about 11 μA. For the phenomenon that the switching loss accounted for a larger proportion under light load conditions, a floating gate voltage circuit for the synchronous power MOSFET MN was designed to reduce the switching loss. Simulation results showed that with 10 V input and 5 V output, the efficiency could reach 67.7% under 0.1 mA load, and the efficiency was higher than 91% above 10 mA load.