A post driver comprises a source follower and a first sub-unit. The source follower includes an input to receive a first voltage from a pad, and an output to provide a second voltage. The first sub-unit includes a first transistor and a second transistor. The first transistor is coupled between the pad and a first power rail, and is configured to operate in a sub-threshold region in response to the second voltage and a first range of the first voltage. The second transistor is coupled in parallel with the first transistor between the pad and the first power rail, and is configured to electrically connect the pad to the first power rail in response to a second range of the first voltage.