Abstract: A design of PFM boost DC-DC converter in CSMC 0.5μm CMOS technology is described. The critical parts such as the reference generation, comparator, PFM controlled circuit and over-current protected circuit are analyzed. The simulation results show that it has advantages of low voltage start, high accuracy of ouput voltage, low power dissipation and over-current protection. According to a standard 0.5μm DPTM double wells CMOS process topological design rule of CSMC,the layout of the proposed circuit is realized.