Electrostatic protection circuit

1. An electrostatic protection circuit to prevent damage to a protected circuit due to electrostatic discharge applied to an input/output terminal, comprising:

one or more diodes connected in series and provided between a connection point of the input/output terminal and the protected circuit, and a ground (GND) terminal;

a MOS transistor connected to the diodes in series, and having an operating voltage lower than a signal voltage input from the input/output terminal; and

a current control circuit which is formed at a connection point of the diodes and the MOS transistor and in parallel with the MOS transistor, and controls an electric potential at said connection point of the diodes and the MOS transistor,

wherein the diodes cause a voltage drop.

2. The electrostatic protection circuit according to claim 1 , wherein said MOS transistor is formed between said ground terminal and said diodes.

3. The electrostatic protection circuit according to claim 1 , wherein said current control circuit comprises a resistor.

4. The electrostatic protection circuit according to claim 1 , wherein said current control circuit comprises an N channel MOS transistor having a gate which is connected to a power supply, and a source which is connected to a ground terminal.

5. The electrostatic protection circuit according to claim 1 , wherein said current control circuit comprises a P channel MOS transistor having a gate and drain which are connected to a ground terminal.

6. The electrostatic protection circuit according to claim 1 , wherein said MOS transistor includes a gate which is connected to the ground terminal.

7. The electrostatic protection circuit according to claim 1 , wherein said protected circuit comprises an MOS transistor with a gate oxide film having a thickness which is no greater than a thickness of a gate oxide film of said MOS transistor connected to said diodes in series.

8. The electrostatic protection circuit according to claim 1 , wherein said MOS transistor comprises a plurality of MOS transistors connected in parallel.

9. The electrostatic protection circuit according to claim 1 , wherein said voltage drop is such that a voltage between a source and drain of said MOS transistor is less than said signal voltage.

10. The electrostatic protection circuit according to claim 1 , wherein the diodes cause said voltage drop in normal operation.

11. An electrostatic protection circuit to prevent damage to a protected circuit due to electrostatic discharge applied to an input/output terminal, comprising:

one or more diodes connected in series;

an N channel MOS transistor including a drain which is connected to the cathode of one end of the diodes, and a source which is connected to a ground terminal, and which has an operating voltage less than a signal voltage input from the input/output terminal; and

a current control circuit which is formed at a connection point of the diodes and the MOS transistor and in parallel with the MOS transistor, and controls an electric potential at a connection point of the diodes and the MOS transistor,

wherein the anode of the other end of the diodes, is connected to a connection point of the input/output terminal and the protected circuit, and

wherein the diodes cause a voltage drop.

12. The electrostatic protection circuit according to claim 11 , wherein said current control circuit controls the electric potential of the drain of the N channel MOS transistor.

13. The electrostatic protection circuit according to claim 12 , wherein the current control circuit is provided between the connection point of the diodes and the N channel MOS transistor, and the GND terminal.

14. An electrostatic protection circuit to prevent damage to a protected circuit due to electrostatic discharge applied to an input/output terminal, comprising:

a P channel MOS transistor having a source which is connected to a connection point of the input/output terminal and the protected circuit;

one or more diodes connected in series; and

a current control circuit which is formed at a connection point of the diodes and the MOS transistor and in parallel with the MOS transistor, and controls an electric potential at said connection point of the diodes and the MOS transistor,

wherein the anode of the end of the diodes, is connected to the drain of the P channel MOS transistor, and the cathode of the other end of the diodes is connected to the ground (GND) terminal; and

wherein the diodes cause a voltage drop.

15. The electrostatic protection circuit according to claim 14 , wherein said current control circuit controls the electric potential of the drain of the P channel MOS transistor.

16. The electrostatic protection circuit according to claim 15 , wherein the current control circuit is provided between the connection point of the input/output terminal and the protected circuit, and the connection point of the P channel MOS transistor and the diodes.

17. The electrostatic protection circuit according to claim 14 , wherein said MOS transistor includes a gate which is connected to the connection point of the input/output terminal and the protected circuit.