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This paper introduces a practical analog-to-digital (ADC) converter architecture for the input voltage measurements in low-power digitally controlled switch-mode power supplies (SMPS). The ADC utilizes known reference voltage and a duty ratio of a simple circuit mimicking operation of the converter to obtain the information about the input voltage value. The functionality of the ADC is verified through a discrete implementation, with a 1.5 V/3 W buck converter based experimental prototype. A better than 1.5% accuracy is observed over the entire 1.8 V to 3.3 V input voltage range. A small silicon size of about 0.025mm2 is also estimated, based on the synthesized digital logic and already existing on-chip integrated system functional blocks.