In this paper, two topics about an on-chip DC-DC buck converter are described. First, the buck converter with an inductor on interposer is investigated for mobile applications. Simulation results indicate that the efficiency of the buck converter is improved by introducing a ferrite film to the inductor. Next, a circuit technique to reduce IR drop in 3D stacked integration using the buck converter is proposed. In this paper, 3D stacked-die system, which consists of stacked dies and silicon interposer, is fabricated and measurement results show that the proposed technique achieves 78% decrease in IR voltage drop compared with the conventional approach.