An enable circuit includes a first detection controller, a second detection controller, and an enable switch. The first detection controller has a first input terminal coupled to a first input terminal voltage and a first output terminal. The second detection controller has a second input terminal coupled to the first input terminal voltage and a second output terminal. The enable switch has a control terminal coupled to the second output terminal, a third input terminal coupled to a second input terminal voltage, and a third output terminal. When the first input terminal voltage is higher than the first setting voltage but lower than the second setting voltage, the enable circuit is coupled to ground by the second output terminal, thus the enable switch is turned on to output an enable signal from the third output terminal, wherein the enable signal is provided by a second input terminal voltage.