Electrostatic protection circuit

Abstract

An electrostatic protection circuit including a first terminal for outputting a signal, a second terminal for receiving power or a ground potential, a first MOS (metal-oxide semiconductor) transistor including a drain and a source coupled to the first and second terminals, respectively, a first electrostatic protection element connecting the first and second terminals, and a second electrostatic protection element connecting a gate of the first MOS transistor and either of the first terminal and the drain of the first MOS transistor. The first terminal includes an output terminal, the second terminal includes a power source terminal or ground terminal, and the electrostatic protection circuit further includes an output driver circuit including the output terminal as its output, the first MOS transistor being included in the output driver circuit.

What is claimed is: 
     
         1 . An electrostatic protection circuit comprising:
 a first terminal for outputting a signal;   a second terminal for receiving power or a ground potential;   a first MOS (metal-oxide semiconductor) transistor including a drain and a source coupled to said first and second terminals, respectively;   a first electrostatic protection element connecting said first and second terminals; and   a second electrostatic protection element connecting a gate of said first MOS transistor and either of said first terminal and the drain of said first MOS transistor,   wherein said first terminal comprises an output terminal, said second terminal comprises a power source terminal or ground terminal, and said electrostatic protection circuit further comprises an output driver circuit having said output terminal as its output, said first MOS transistor being included in said output driver circuit.   
     
     
         2 . The electrostatic protection circuit according to  claim 1 , wherein an internal circuit is connected to the gate of said first MOS transistor, and
 wherein gate-to-drain voltage of said first MOS transistor is limited to a value below a prescribed value by a current that flows into said second electrostatic protection element owing to application of static electricity to the first terminal, and a resistance of the internal circuit as seen from the gate of said first MOS transistor.   
     
     
         3 . The electrostatic protection circuit according to  claim 2 , wherein the internal circuit includes a second MOS transistor of the same conductivity type as that of said first MOS transistor, said second MOS transistor having a drain connected to the gate of said first MOS transistor and a source connected to a ground terminal, and
 wherein the resistance of the internal circuit includes a drain-to-source resistance component of said second MOS transistor.   
     
     
         4 . A semiconductor device having the electrostatic protection circuit set forth in  claim 1 . 
     
     
         5 . The electrostatic protection circuit according to  claim 1 , further comprising an output driver circuit comprising the first terminal,
 wherein said first terminal comprises an output terminal providing an output external to the output driver circuit and the electrostatic protection circuit.   
     
     
         6 . The electrostatic protection circuit according to  claim 1 , wherein the second electrostatic protection element is connected from the gate of said first MOS transistor and either of said first terminal and the drain of said first MOS transistor. 
     
     
         7 . An electrostatic protection circuit comprising:
 a first terminal for outputting a signal;   a second terminal for receiving a power or ground;   a first MOS (metal-oxide semiconductor) transistor including a drain and a source connected between said first and second terminals;   a first element connecting said first and second terminals; and   a second element connecting a gate of said first MOS transistor to one of said first and second terminals,   wherein said one of said first and second terminals is connected to the drain of said first MOS transistor,   wherein said first and second elements have impedances that fall when a voltage exceeding a prescribed voltage is applied, and   wherein said first terminal comprises an output terminal, said second terminal comprises a power source terminal or ground terminal, and said electrostatic protection circuit further comprises an output driver circuit having said output terminal as its output, said first MOS transistor being included in said output driver circuit.   
     
     
         8 . A semiconductor device having the electrostatic protection circuit set forth in  claim 7 . 
     
     
         9 . The electrostatic protection circuit according to  claim 7 , wherein the first terminal outputs a signal external to the electrostatic protection circuit. 
     
     
         10 . A semiconductor device comprising:
 a first terminal of an output driver circuit for outputting a signal external from the output driver circuit;   a second terminal for receiving power or a ground potential;   a first MOS (metal-oxide semiconductor) transistor including a drain and a source coupled to said first and second terminals, respectively;   a first electrostatic protection element connecting said first and second terminals; and   a second electrostatic protection element connecting a gate of said first MOS transistor and either of said first terminal and the drain of said first MOS transistor,   wherein said first terminal comprises an output terminal, said second terminal comprises a power source terminal or ground terminal, and said electrostatic protection circuit further comprises an output driver circuit having said output terminal as its output, said first MOS transistor being included in said output driver circuit.   
     
     
         11 . The semiconductor device according to  claim 10 , wherein an internal circuit is connected to the gate of said first MOS transistor, and
 wherein gate-to-drain voltage of said first MOS transistor is limited to a value below a prescribed value by a current that flows into said second electrostatic protection element owing to application of static electricity to the first terminal, and a resistance of the internal circuit as seen from the gate of said first MOS transistor.   
     
     
         12 . The semiconductor device according to  claim 11 , wherein the internal circuit includes a second MOS transistor of the same conductivity type as that of said first MOS transistor, said second MOS transistor having a drain connected to the gate of said first MOS transistor and a source connected to a ground terminal, and
 wherein the resistance of the internal circuit includes a drain-to-source resistance component of said second MOS transistor.   
     
     
         13 . The semiconductor device according to  claim 11 , wherein said first terminal comprises an output terminal providing an output external to the output driver circuit and the electrostatic protection circuit. 
     
     
         14 . The semiconductor device according to  claim 11 , wherein said output driver circuit includes an amplifier circuit for output purposes, and
 wherein said first MOS transistor comprises one transistor in said differential amplifier circuit.