A phase-locked-loop (PLL) circuit is provided. The PLL circuit includes a phase/frequency detector, a digital filter, a digital low pass filter (LPF), a digitally controlled oscillator (DCO), and a frequency divider. The digital LPF performs a low-pass-filtering on least significant bits of first digital data in a digital mode and generates filtered second digital data. The DCO performs a digital-to-analog conversion on the second digital data and most significant bits of the first digital data to generate a first signal, generates an oscillation control signal based on the first signal, and generates an output clock signal oscillating in response to the oscillation control signal.