Patent ID: 7449928

Claim:
A semiconductor device comprising: a phase locked loop circuit having a phase frequency detector which receives a reference signal and a frequency-divided signal, and outputs a phase difference detection signal between the reference signal and the frequency-divided signal, a charge pump which receives the phase difference detection signal and outputs a charge pump signal by converting a voltage change into a current change, a loop filter which receives the charge pump signal, and outputs a control voltage by passing frequencies not more than a predetermined frequency, a voltage controlled oscillator which outputs a frequency signal having a frequency based on the control voltage, and a frequency divider which receives the frequency signal, and outputs the frequency-divided signal by dividing the frequency; a mask signal generator which generates a mask signal masking a timing at which said phase frequency detector compares phases of the frequency-divided signal and the reference signal; a lock detector which detects a locked state of said phase locked loop circuit and outputs a lock detection signal; and an isolating circuit which receives the mask signal and the lock detection signal, and electrically isolates the output of said loop filter and the input of said voltage controlled oscillator from each other upon phase comparison in the locked state.