In phase locked loop, a phase detector detects a phase difference between a first clock signal and a second clock signal and output a first output signal based on the detected difference. A charge pump generates a control voltage in response to the first output signal from the phase detector. A voltage-controlled oscillator generates the second clock signal. A controller controls the control voltage such that the phase difference between the first clock signal and the second clock signal is increased in response to a burn-in test mode signal.