A control circuit for adaptive noise margin control for a constant on time (COT) converter comprises an input reference terminal, amplifier, first switch device, voltage divider, trigger circuit, and output reference terminal. The amplifier has an input terminal coupled to the input reference terminal receiving a reference voltage signal. The first switch device has a control terminal coupled to an output of the amplifier, a first conduction terminal for receiving a voltage source signal, and a second conduction terminal. The voltage divider is coupled to the second conduction terminal and another input terminal of the amplifier. The trigger circuit, coupled to the voltage divider, is for triggering voltage change of a modified reference voltage signal selectively according to a high-side control signal of the COT converter. The output reference terminal coupled to the second conduction terminal outputs the modified reference voltage signal.