a first level-shifting unit receiving an input reference signal having a first swing voltage and generating a first level change signal having a second swing voltage and a second level change signal having a third swing voltage, wherein peak-to-peak voltage of the first swing voltage is smaller than peak-to-peak voltage of one of the second and third swing voltages; and

a synchronization unit generating first and second output signals by buffering and level shifting the first and second level change signals, wherein the first and second output signals have substantially the same phase.