In this paper, the authors demonstrate an integrated small size buck DC-DC converter for cellular phones. At nominal conditions, the converter produces a 3.7-V output from a 12-V input. Both open loop and closed loop systems are introduced. A simple technique to reduce the ripple is also presented. A converter efficiency of approximately 91 % is achieved. The major advantages and disadvantages are highlighted and the field of application is found. The Circuit will be simulated using MATLAB Simu-link. The converter has less harmonics and its THD value is good.