A pulse width modulation regulator IC is provided for controlling a duty cycle of at least one switch to convert one input voltage signal into an output voltage. An input pin is provided for receiving an input signal different from the input voltage signal. The input signal has a lasting time substantially the same as the time that input voltage signal situated at a high level, but the waveforms of the two signals are different. The input signal is converted into a square wave signal by a conversion unit, and a PWM signal is generated by a PWM controller according to the square wave signal to control the duty cycle of the switch. Therefore, the input pin can be saved by adjusting an internal or external circuit of the IC for the usage of the different kinds of input signals without increasing the number of input pins of the IC.