The present invention provides a constant on time controller packaged in one single package for controlling a buck converting circuit to convert an input voltage into an output voltage. The constant on time controller includes an input voltage detecting circuit, an on-time determining circuit and a driving circuit. The input voltage detecting circuit receives a bootstrap voltage to generate an input voltage detecting signal indicative of the input voltage. The on-time determining circuit receives the input voltage detecting signal and generates an on-time signal in response to the input voltage detecting signal and the output voltage. The driving circuit controls the buck converting circuit according to the n-time signal.