A dual-voltage three-state buffer circuit controls a post driver circuit to operate in a three-state mode and includes a tri-state logic control module operated under a low supply voltage, a level shifter for receiving one or more inputs from the tri-state logic control module and operating with an output control circuit for controlling two differential outputs of the level shifter, and a post driver circuit driven by the two differential outputs of the level shifter, wherein the level shifter, the output control circuit, an the post driver circuit are operated under a high supply voltage, and wherein when the tri-state logic control module generates the inputs for putting the post driver circuit in a high impedance state, the output control circuit operates with the level shifter to turn off the PMOS and NMOS transistors of the post driver circuit while isolating the level shifter from a high supply voltage.