A device for driving a light source in an image display device includes input terminals to receive a horizontal synchronization signal and a control signal, an oscillator to generate a reference signal having a frequency, a controller to modulate the reference signal in response to the control signal and output a modulated signal, and a phase difference detecting unit to receive the horizontal synchronization signal and the modulated signal and detect a phase difference between the horizontal synchronization signal and the modulated signal to generate an output signal indicating the phase difference. The oscillator adjusts the frequency of the reference signal in response to the output signal of the phase difference detecting unit so that the horizontal synchronization signal and the reference signal are synchronized with each other.