Frequency-Modulated Continuous-Wave (FMCW) radars are widely used in auto-driving scenarios for measuring the distance as well as the speed of multiple targets. The chirp signals, which linearly modulate the output frequency with respect to time, are usually generated by the means of a Phase-Locked Loop (PLL). Due to the limited bandwidth, traditional sawtooth-like chirp signals suffer from the long ramping down time, which degrading the performance of the radar. A fast chirp generation scheme based on a charge pump with segmented current is proposed in this paper. The linearity of the chirp signal was well proved with the optimal charge pump current, i.e., the optimal loop bandwidth. On another hand, much larger current would be employed in the ramping down scene, for the purpose of shorter ramping-down time. The simulation results show that the chirp generator consumes a power of 31.8 mW under 1.2 V supply voltage, when generating a 19.25 GHz to 20.25 GHz chirp. A maximum ramping down slope of 454 MHz/μs is achieved with the proposed technique, which is reduced by 80% compared with constant charge pump current manner.