A voltage generator includes an oscillator, a charge pump, a smoothing capacitor, and a driving controller. The oscillator has an output. The charge pump has an input and an output, and the input of the charge pump is coupled to the output of the oscillator. The smoothing capacitor is coupled to the output of the charge pump. The driving controller is coupled to the oscillator, and generates an enable signal to adjust an operation frequency of the oscillator. The voltage generator supplies a driving voltage to a switch for driving the switch via the smoothing capacitor. The driving controller generates the enable signal according to the driving voltage.