Buck circuit having fast transient response mechanism and operating method thereof

The invention provides a buck circuit having fast transient response mechanism comprising a high side metal oxide semiconductor transistor, a low side metal oxide semiconductor transistor, a transient control module and an operation control module. The low side metal oxide semiconductor transistor and the high side metal oxide semiconductor transistor are connected by a connection point, so that an output voltage is generated according to an input voltage received by the high side metal oxide semiconductor transistor at the connection point and then is output to an external load. When the external load is in a heavy load state and thus the transient control module is disabled, the operation control module controls operation of the high side metal oxide semiconductor transistor and the low side metal oxide semiconductor transistor according to the output voltage. And when the heavy load state of the external load is transferred to a light load state and thus the output voltage is raised and exceeds a preset le