A circuit, comprising: an output buffer comprising at least one P-type transistor (MP1, MP2) and a plurality of N-type transistors (MN1, MN2, MN3) connected in series, wherein the at least one P-type transistor is coupled between a supply voltage (2×VDD) and a pad (IO_PAD), and the plurality of N-type transistors comprises: a first N-type transistor, wherein a drain electrode of the first N-type transistor is coupled to the pad; a second N-type transistor, wherein a drain electrode of the second N-type transistor is coupled to a source electrode of the first N-type transistor; and a third N-type transistor, wherein a drain electrode of the third N-type transistor is coupled to a source electrode of the second N-type transistor, and a source electrode of the third N-type transistor is coupled to the ground voltage; a tracking circuit, coupled to the pad, for generating a tracking signal control signal (PTRK) according to a voltage level at the pad; and a pre-driver coupled to the tracking circuit the pre-driver is configured to refer to the tracking signal to generate a control signal to one of the plurality of N-type transistors according to the tracking signal; wherein the circuit selectively operates in an input mode or an output mode, when the circuit operates in the input mode, the circuit receives an input signal (Vin) from the pad, the output buffer is disabled, and the pre-driver refers to the tracking signal to generate the control signal to control the one of the plurality of N-type transistors, and a gate electrode of the second N-type transistor is biased by a first bias voltage lower than the supply voltage, a gate electrode of the third N-type transistor is biased by a second bias voltage lower than the first bias voltage; and if a voltage level of the input signal is greater than the supply voltage of the output buffer, the pre-driver generates the control signal whose voltage level is equal to the supply voltage to control the gate electrode of the first N-type transistor; if the voltage level of the input signal is between the supply voltage and the first bias voltage, the pre-driver generates the control signal whose voltage level is proportional to the voltage level of the input signal to control the gate electrode of the first N-type transistor; and if the voltage level of the input signal is lower than the first bias voltage, the pre-driver generates the control signal whose voltage level is equal to the first bias voltage to control the gate electrode of the first N-type transistor. a first N-type transistor, wherein a drain electrode of the first N-type transistor is coupled to the pad; a second N-type transistor, wherein a drain electrode of the second N-type transistor is coupled to a source electrode of the first N-type transistor; and a third N-type transistor, wherein a drain electrode of the third N-type transistor is coupled to a source electrode of the second N-type transistor, and a source electrode of the third N-type transistor is coupled to the ground voltage; a tracking circuit, coupled to the pad, for generating a tracking signal control signal (PTRK) according to a voltage level at the pad; and a pre-driver coupled to the tracking circuit the pre-driver is configured to refer to the tracking signal to generate a control signal to one of the plurality of N-type transistors according to the tracking signal; wherein the circuit selectively operates in an input mode or an output mode, when the circuit operates in the input mode, the circuit receives an input signal (Vin) from the pad, the output buffer is disabled, and the pre-driver refers to the tracking signal to generate the control signal to control the one of the plurality of N-type transistors, and a gate electrode of the second N-type transistor is biased by a first bias voltage lower than the supply voltage, a gate electrode of the third N-type transistor is biased by a second bias voltage lower than the first bias voltage; and if a voltage level of the input signal is greater than the supply voltage of the output buffer, the pre-driver generates the control signal whose voltage level is equal to the supply voltage to control the gate electrode of the first N-type transistor; if the voltage level of the input signal is between the supply voltage and the first bias voltage, the pre-driver generates the control signal whose voltage level is proportional to the voltage level of the input signal to control the gate electrode of the first N-type transistor; and if the voltage level of the input signal is lower than the first bias voltage, the pre-driver generates the control signal whose voltage level is equal to the first bias voltage to control the gate electrode of the first N-type transistor.