This paper presents a double pile-up resonance energy harvesting circuit that efficiently and simultaneously extracts energy from a piezoelectric transducer (PZT) and a thermoelectric generator. The proposed harvester operates in a double pile-up mode (DPM) to efficiently extract energy from PZT with the enhanced damping force, resulting in a 1452% improvement in power extraction, which is the best performance among the state-of-the-art works. The harvester also operates in a boost converter mode (BCM) without an additional power switch, achieving 75% conversion efficiency at 450-mu W output power. With a single-shared inductor, a simple control scheme enables the harvester to operate in both DPM and BCM by time-multiplexing method, consuming a low quiescent current of 240 nA. The prototype chip fabricated in a 0.18-mu m BCD occupies an area of 1.5 mm x 1 mm, and it was tested with a 20-nF PZT product (PPA-1001) vibrated by a shaker (Type 4810).