Input/output buffer for protecting a circuit from signals received from external devices and method of use

• US 6,924,673 B2
• Filed: 02/20/2003
• Issued: 08/02/2005
• Est. Priority Date: 05/31/2002
• Status: Expired due to Term

• Current Assignee: Socionext Inc.
• Original Assignee: Fujitsu Limited
• Inventors: Tanishima, Hideaki
• Primary Examiner(s): Wells, Kenneth B.
• Application Number:
  US10/368,409
• Publication Number:
  US 20030222684A1
• Time in Patent Office: 894 Days
• Field of Search: 327108-112, 327/309, 327/314, 327/315, 327318-320, 327/325, 327/544
• US Class Current: 327/108
• CPC Class Codes: H03K 19/00315 in field-effect transistor circuits

Abstract

An input/output buffer that protects a circuit from voltage signals provided from an external device. The input/output buffer includes a reference power generation circuit connected to a high voltage power supply and a low voltage power supply to convert the voltage of an external voltage signal and generate reference power. The reference power generation circuit has a protection circuit including a plurality of MOS transistors for decreasing the voltage of the external voltage signal to a predetermined voltage when the input/output buffer is not supplied with the voltage of the high voltage power supply. Each of the MOS transistors has a back gate connected to a predetermined node at which the voltage is less than the voltage of the high voltage power supply and greater than the voltage of the low voltage power supply.

Claims

1. An input/output buffer for use with a high voltage power supply that is variably provided and a low voltage power supply and receiving an external voltage signal, the input/output buffer comprising:

a reference power generation circuit connected to the high voltage power supply and the low voltage power supply for converting the external voltage signal to a reference power, the reference power generation circuit having a protection circuit including a plurality of MOS transistors for decreasing the external voltage signal to a predetermined voltage when the input/output buffer receives the external voltage signal and is not supplied with the voltage of the high voltage power supply, each of the MOS transistors having a back gate connected to a predetermined node, each predetermined node having a voltage less than the voltage of the high voltage power supply and greater than the voltage of the low voltage power supply.

2. The input/output buffer according to claim 1, wherein the plurality of MOS transistors are connected in series and each MOS transistor is diode-connected, and wherein at least one of the MOS transistors is diode-connected so as to undergo reverse bias due to the reference power.

3. The input/output buffer according to claim 2, wherein the back gate of each MOS transistor is connected to a node between two adjacent MOS transistors.

4. The input/output buffer according to claim 1, wherein each of the MOS transistors is an n-channel MOS transistor.

5. The input/output buffer according to claim 4, wherein each n-channel MOS transistor includes a gate, a source terminal, and a drain terminal, and the gate is connected to the source terminal and drain terminal of the n-channel MOS transistor at which the voltage is greater when generating the reference power.

6. The input/output buffer according to claim 4, wherein the back gate of each n-channel MOS transistor is supplied with voltage generated by resistor-dividing the voltage difference between the voltage of the low voltage power supply and the greater one of the voltages at the source terminal and drain terminal of the n-channel MOS transistor when generating the reference power.

7. The input/output buffer according to claim 1, wherein each of the MOS transistors is a p-channel MOS transistor.

8. The input/output buffer according to claim 7, wherein each p-channel MOS transistor includes a gate, a source terminal, and a drain terminal, and the gate is connected to the source terminal and drain terminal of the p-channel MOS transistor at which the voltage is less when generating the reference power.

9. The input/output buffer according to claim 7, wherein each p-channel MOS transistor includes a back gate, a source terminal, and a drain, and the back gate is supplied with voltage generated by resistor-dividing the voltage difference between the voltage of the low voltage power supply and the lesser one of the voltages at the source terminal and drain terminal of the p-channel MOS transistor when generating the reference power.

10. The input/output buffer according to claim 1, wherein the reference power generation circuit includes a voltage-maintaining circuit having at least two MOS transistors connected in series between the reference power and the low voltage power supply, wherein each MOS transistor includes a gate, a source terminal, and a drain terminal, wherein the gate of the one of the at least two MOS transistors is connected to the reference power, and wherein the gate of each other one of the MOS transistors is connected via its drain terminal to the source terminal of the adjacent MOS transistor.

11. The input/output buffer according to claim 10, wherein the at least two MOS transistors are each an n-channel MOS transistor.

12. The input/output buffer according to claim 1, wherein the plurality of MOS transistors includes at least five MOS transistors.

13. The input/output buffer according to claim 1, further comprising:

an internal circuit;

an input circuit for generating a voltage signal having a voltage that is less than the external voltage signal, the input circuit providing in the voltage signal to the internal circuit;

an output circuit for providing a data signal from the internal circuit to an external device; and

an input/output circuit connected to the input circuit and the output circuit for invalidating an output of the output circuit and providing the external voltage signal to the input circuit when the internal circuit receives the data signal from the external device;

wherein the input circuit, the output circuit, and the input/output circuit are each supplied with the reference power as operational power.

14. The input/output buffer according to claim 13, further comprising a tolerant circuit having a voltage that is equal to the voltage of the high voltage power supply when the external voltage signal is less than or equel to the voltage of the high voltage power supply, wherein the tolerant circuit generates the voltage signal having a voltage derived from the external voltage signal when the voltage of the external voltage signal is greater than the voltage of the voltage signal;

wherein the input/output circuit, the input circuit, and the reference power generation circuit each receive the voltage signal from the tolerant circuit.

15. A method for protecting an input/output buffer from a voltage signal that is provided from an external device, wherein the input/output buffer is connected to a high voltage power supply that is variably provided and a low voltage power supply and includes an input/output circuit for transferring data with the external device, the method comprising the steps of:

decreasing the voltage of the voltage signal to a predetermined voltage with a plurality of MOS transistors, which are connected in series between the high voltage power supply and the low voltage power supply, each MOS transistor having a back gate to generate reference power when the input/output buffer is not supplied with the voltage of the high voltage power supply;

supplying the input/output circuit with the reference power; and

supplying the back gate of each MOS transistor with voltage that is less than the voltage of the high voltage power supply and greater than the voltage of the low voltage power supply.

16. The method according to claim 15, wherein the step of supply the back gate of each MOS transistor with voltage includes supplying the back gate with the voltage of a node between adjacent MOS transistors.

17. An input buffer for use with a high voltage power supply that is variably provided and a low voltage power supply and for receiving an external voltage signal, the input buffer comprising:

a reference power generation circuit connected to the high voltage power supply and the low voltage power supply for converting the external voltage signal to a reference power, the reference power generation circuit having a protection circuit including a plurality of MOS transistors for decreasing the external voltage signal to a predetermined voltage when the external voltage signal is received and the voltage of the high voltage power supply is not supplied, each of the MOS transistors having a back gate connected to a predetermined node, each predetermined node having a voltage less than the voltage of the high voltage power supply and greater than the voltage of the low voltage power supply.

18. The input buffer according to claim 17, further comprising:

an input circuit including an n-channel MOS transistor and a comparator connected to the n-channel MOS transistor, the n-channel MOS transistor including;

a source for receiving the external voltage signal via a resistor;

a gate connected to the source; and

a drain for receiving the reference power, the voltage of which is divided, by a divisional resistor;

wherein the comparator compares the external, voltage signal with a reference voltage signal to determine whether the voltage of the external voltage signal is greater than a predetermined threshold voltage from the comparison.

19. An output buffer for use with a high voltage power supply that is variably provided and a low voltage power supply and receiving an external voltage signal, the output buffer comprising:

a reference power generation circuit connected to the high voltage power supply and the low voltage power supply for converting the external voltage signal to a reference power, the reference power generation circuit having a protection circuit including a plurality of MOS transistors for decreasing the external voltage signal to a predetermined voltage when the output buffer receives the external voltage signal and is not supplied with the voltage of the high voltage power supply, each of the MOS transistors having a back gate connected to a predetermined node, each predetermined node having a voltage less than the voltage of the high voltage power supply and greater than the voltage of the low voltage power supply.