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This paper presents an input-powered energy-harvesting interface circuit that eliminates standby power consumption by automatically shutting down when the ac input voltage amplitude is too low for successful energy reclamation. This feature eliminates the need for precharging the load and allows for indefinitely long intervals between energy harvesting cycles. The interface comprises two subcircuits: an input-powered ac/dc converter and an input-powered dc/dc boost converter with regulated output. The two subcircuits are separately fabricated in the ON Semi 3M-2P 0.5 μm CMOS process. The entire interface circuit starts up when the ac input amplitude exceeds 1 V and supplies a regulated dc output up to 3 V. When the input amplitude drops below 600 mV, the interface automatically enters standby mode and consumes no power. The system achieves a maximum efficiency of 60% with 1.5-V ac input amplitude and 3 V regulated dc output, delivering 3.9 mW of output power. The interface also functions properly in tests with an electrodynamic (magnetic) vibrational energy harvester.