Cmos off-chip driver circuit

Abstract

An improved off-chip driver circuit is disclosed which will properly transition from an active mode to a high impedance mode. The circuit includes first and second input nodes for receiving a first and second input signal respectively. An input composite transmission gate including a p-channel transistor in parallel with an n-channel transistor to receive the first input signal is provided in the circuit. A push-pull circuit is also included which includes the pull-up transistor disposed between a voltage supply and an output node and a first pull-down transistor disposed between ground and the output node. The pull-up transistor has a gate electrode for receiving the first input signal provided by the input composite transmission gate. The first pull-down transistor has a gate electrode for receiving the second input signal. A control transistor is included and is coupled between the gate electrode of the pull-up transistor and the output node. The control transistor has a gate electrode connected to a first point of reference potential. An auxiliary transistor is provided coupled between the first input node and a gate electrode of the p-channel transistor for establishing and maintaining a voltage at the gate electrode of the p-channel transistor during a transition of the circuit from an active mode to a high impedance mode sufficient to provide a proper transition from the active mode to the high impedance mode. A biasing transistor is also provided having a gate electrode directly connected to the output node and coupled to the voltage supply for biasing an N-well in which the pull-up transistor, the control transistor, and the auxiliary transistor are situated.