An output buffer

摘要

An output buffer includes a pair of P-channel transistors and two cascode pull-down N-channel transistors to drive an output node. The output pull-up transistor (Q6) has the gate thereof connected through a P-channel control transistor (Q5) to an input driving signal. The control signal is isolated from the output node by a shunt P-channel transistor (Q4) which only conducts during overvoltage conditions. During normal operation, the control transistor (Q5) is maintained in a conductive state to allow the gate of the output pull-up transistor (Q6) to be pulled high and low. During an overvoltage condition, the shunt P-channel transistor (Q4) connected between the output node and the control transistor is turned on to effectively turn off the control transistor. The output P-channel transistor (Q6) is protected from excess voltages across the gate oxide when initially coming out of the hiZ state with a logic high signal on the input. This is facilitated by pulling the gate thereof low to a voltage above ground and then pulling the gate fully to ground when the voltage on the output node falls to below the supply voltage. This is provided by a NAND gate structure (120,122,124,128) that disposes a limiting P-channel transistor (128) in series with the gate of the P-channel pull-up transistor (Q6) to prevent it from going to a full logic low level. When the voltage on the output is pulled down by the shunt P-channel transistor (Q4), this will then bypass the limiting P-channel transistor (128) in the NAND gate and pull the gate of the P-channel pull-up transistor (Q6) down to the full low logic level.