A control device for controlling a switch unit of a resonant direct current/direct current converter includes a frequency modulation controller and a pulse selector. The frequency modulation controller is adapted to be coupled electrically to the converter for receiving a correcting threshold value and output information of the converter, and for generating a synchronization signal according to the correcting threshold value and the output information received thereby. The pulse selector is adapted to be coupled electrically to the converter and the frequency modulation controller for receiving the correcting threshold value, the output information and the synchronization signal, and for generating a driving signal according to the correcting threshold value, the output information and the synchronization signal received thereby. The driving signal is adapted to drive the switch unit and has a working period. The driving signal switches between high and low signal levels at a frequency that is substantially equal to that of the synchronization signal during the working period.