This paper presents a design of a multi-phase DC-DC buck converter for smart mobile devices. It is designed using more than 2 inductors. Multi-phase structure has some advantage. One is that, it has ripple cancellation effect so, output capacitor size can be decreased. Also, it makes the output voltage regulation more fast. We can use less size inductor, so transient response is more fast. It operates with a switching frequency of 2㎒. The designed DC-DC converter is fabricated with the 0.18㎛ CMOS technology. The overall die size is 3.61㎟, and controller size is 0.7㎟. The converter has the maximum efficiency over 90% for the output voltage of 2V.