A novel bootstrap circuit was proposed for high-voltage buck converter. The bootstrap capacitor was charged directly by the chip’s external power. The circuit featured fast response and high driving current, meeting the driving requirements of the N-type power switch with large size. In this bootstrap circuit, even though the high-side and low-side switches did not work, the bootstrap capacitor also could be charged well. It was especially suitable for light-load and high-efficiency buck converters. The problem of the traditional bootstrap circuit can not adjust the bootstrap voltage stably while the low-side switch did not work was solved. Based on 0.25 μm UMC process, simulation results showed that as long as the input voltage was sufficient, even if the low-side switch did not work, and the load had a high voltage, the circuit could keep the bootstrap voltage above 3.6 V and improve the converter’s reliability.