This paper presents a high-voltage inverting converter implemented by reconfiguring a buck DC-DC regulator. The negative output voltage is generated from a positive input voltage by exchanging the output and ground on a buck converter. In this way, the complexity of circuit design is greatly reduced. The converter adopts constant on-time control with on-chip ripple compensation and output DC offset elimination techniques to achieve fast transient response and highly accurate output voltage, which also allows the use of output capacitor with low equivalent series resistance (RESR). In addition, an internal bootstrap circuit for high-side driver is proposed in place of conventional LDO structure to improve the efficiency. The converter is fabricated in 0.18-μm BCD process. The input voltage is in the range from 5V to 48V. The measurement results show that the transient response is about 220μs and the overshoot or undershoot voltage is less than 190mV when the load transient between 100mA and 500mA for switching frequency of 300kHz, input voltage of 24V and output voltage of -12V.