A self-sustaining regulator harvesting from two energy sources, solar and radio-frequency energy, is designed. A charge pump utilizing charge transfer switch (CTS) with tunable voltages at the bottom of the pumping capacitors is proposed. Through a DC-DC switching converter with a digital pulse-width modulator, the output voltage is regulated at 1V. In the digital DC-DC switching converter, we propose to analyze the error signal by an oversampling analog-to-digital converter (ADC). The system is completely implemented and fully-integrated in a standard 0.18μm CMOS process, and the achieved maximum end-to-end conversion efficiency is about 62%.