A DC-DC converter, which tracks minimum power loss under various conditions, is presented in this paper. With this DC-DC converter, a new approach for optimum efficiency, which implements theoretical equations intactly at the circuit level, was adopted. This approach has high reliability against process variation. The concept of lossless soft-switching was also adopted in discontinuous-conduction-mode (DCM) operation. The proposed DC-DC converter has a maximum power efficiency of 91% at the switching frequency of 1 MHz. The proposed converter is able to maintain a maximum power efficiency of around 90% over the whole range of input voltage from 2.9 to 4.5 V and output voltage from -3.8 to -5.9 V. This chip is implemented in a 0.35 pm BCD process and occupies an area of 2.1x 1.4 mm(2).