US Patent 8253502 Spread spectrum clock generator and semiconductor device

A spread spectrum clock generator includes a voltage-controlled oscillator generating an operation clock, a feedback control unit, a modulated pulse generation unit generating a pulse signal obtained by performing a delta-sigma modulation on a component fluctuating a frequency of the operation clock, a level set unit setting an amplitude of the pulse signal, an adder adding a voltage generated by the feedback control unit and the pulse signal whose amplitude is set by the level set unit, and a low pass filter filtering a signal outputted from the adder and generating a control voltage applied to the voltage-controlled oscillator. The feedback control unit compares a phase of the operation clock with a phase of a reference clock, and based on results of the comparison, generates a voltage used as a reference to oscillate the voltage-controlled oscillator.