A power-control device for generating and controlling a supply voltage is provided. The power-control device includes a variant delay chain with a delay length, a sampling circuit, a comparison circuit, and a power manager. The variant delay chain receives an initial signal and performs a delay operation on the initial signal according to the delay length to generate a delay signal. The sampling circuit receives the delay signal and performs a sampling operation on the delay signal to generate a sampled signal. The comparison circuit receives the sampled signal and compares the sampled signal with a reference signal to generate a comparison result signal. The power manager receives the comparison result signal and adjusts the supply voltage according to the comparison result signal.