US 5,512,844 AGranted

Output circuit with high output voltage protection means

AssigneeMatsushita Electric Industrial Co., Ltd.

InventorsYasuhiro Nakakura, Shouichi Yoshizaki

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Abstract

In the case where an external signal line is driven by a circuit other than an output circuit with a voltage higher than an on-chip power-source voltage, if an output transistor is activated in the output state immediately after the driving of the external signal line by the above other circuit was cancelled, a voltage equal to or higher than the breakdown voltage of an oxide film of a transistor composing the output circuit is applied thereto. To prevent this, there is provided a voltage detecting means for detecting a voltage value of the above external signal line being higher than a value in the vicinity of the internal power-source voltage and generating a detection signal, which inhibits the activation of the output transistor. Consequently, in the output circuit of an LSI fabricated using submicron design rules, the application of a voltage equal to or higher than the breakdown voltage of an oxide film of each of the transistors composing the output circuit is prevented, resulting in increased reliability of the output circuit.

Claims20

Dependent claims

An output circuit connected to an internal power source so as to output a signal having a voltage of said internal power source to an external signal line, wherein

a signal having a higher voltage than the voltage of said internal power source is applied to said external signal line by a circuit other than said output circuit,
said output circuit comprises:
an output pad element to which said external signal line is connected;
a signal generating circuit for receiving an external control signal inputted from the outside and generating a pullup control signal based on the external control signal;
an output transistor which is interposed between said internal power source and said output pad element and which is turned ON in response to the pullup control signal from said signal generating circuit so as to supply the voltage of said internal power source as a signal to said output pad element;
and
a voltage detecting means for receiving a signal from said external signal line, detecting a voltage value of the external signal higher than a voltage value in the vicinity of the voltage value of said internal power source, and generating a voltage detection signal,
the generation of the pullup control signal by said signal generating circuit being inhibited by the voltage detection signal from said voltage detecting means, irrespective of the content of said external control signal.

5 more elements

Dependencies are inferred from the claim text.