Robust ESD protection circuit, method and design structure for tolerant and failsafe designs

1. A circuit, comprising:

a middle junction control circuit that turns off a top NFET of a stacked NFET electrostatic discharge (ESD) protection circuit during an ESD event, wherein the middle junction control circuit turns off the top NFET by biasing a middle junction between the top NFET and a bottom NFET to substantially a same voltage as a gate of the top NFET.

2. The circuit of claim 1 , wherein the stacked NFET ESD protection circuit comprises the top NFET and the bottom NFET arranged in series between an I/O pad and ground.

3. The circuit of claim 2 , wherein the stacked NFET ESD protection circuit comprises:

a top resistor in series between the I/O pad and the top NFET;

a bottom resistor in series between the bottom NFET and ground; and

the middle junction between the top NFET and the bottom NFET, wherein the middle junction is connected to a source of the top NFET.

4. The circuit of claim 3 , wherein, during operation other than the ESD event, the middle junction control circuit permits the middle junction to float.

5. The circuit of claim 1 , wherein:

the middle junction control circuit comprises an inverter and a switch;

the inverter has a first voltage as an input; and

the switch has an output of the inverter as a switch input, wherein the output of the switch is connected to the middle junction between the top NFET and the bottom NFET of the stacked NFET ESD protection circuit.

6. The circuit of claim 5 , wherein the middle junction is connected to a source of the top NFET.

7. The circuit of claim 6 , wherein:

the switch is closed when the first voltage is in a low state;

the middle junction and the gate of the top NFET receive a second voltage when the switch is closed;

the switch is open when the first voltage is in a high state; and

the middle junction is permitted to float when the switch is open.

8. The circuit of claim 7 , wherein:

the first voltage is Vdd, and

the second voltage is DVdd.

9. The circuit of claim 5 , wherein:

the middle junction control circuit comprises a second switch;

an input of the second switch is connected to the output of the inverter; and

an output of the second switch is connected to a gate of the bottom NFET of the stacked NFET ESD protection circuit.

10. The circuit of claim 9 , wherein the middle junction control circuit turns on the bottom NFET during the ESD event.

11. The circuit of claim 5 , wherein the middle junction control circuit further comprises at least one diode that adjusts DVdd during the ESD event, the at least one diode being located between a DVdd source and the inverter and the switch.

12. A circuit, comprising:

a stacked NFET ESD protection circuit comprising a top NFET and a bottom NFET arranged in series between an I/O pad and ground; and

a middle junction control circuit that turns off the top NFET during an ESD event, wherein the middle junction control circuit comprises an inverter and a switch, wherein the middle junction control circuit at least partially turns on the bottom NFET during the ESD event.

13. The circuit of claim 12 , wherein:

an input of the inverter is tied to a first power supply;

an output of the inverter is tied to a gate of the switch;

an input of the switch is tied to a second power supply;

a gate of the top NFET is tied to the second power supply; and

an output of the switch is tied to a source of the top NFET via a middle junction between the top NFET and the bottom NFET.

14. The circuit of claim 13 , wherein:

the middle junction control circuit comprises a second switch;

a gate of the second switch is tied to the output of the inverter;

an input of the second switch is tied to the second power supply; and

an output of the second switch is tied to a gate of the bottom NFET.

15. The circuit of claim 13 , further comprising at least one diode arranged between the second power supply and the middle junction control circuit.

16. The circuit of claim 13 , wherein:

the first power supply is Vdd, and

the second power supply is DVdd.

17. A method of providing electrostatic discharge protection, comprising:

during an ESD event, turning off a top NFET of a stacked NFET ESD protection circuit by biasing a middle junction between the top NFET and a bottom NFET to substantially a same voltage as a gate of the top NFET.

18. The method of claim 17 , further comprising turning on the bottom NFET during the ESD event.

19. The method of claim 17 , further comprising tuning the voltage using at least one diode.

20. A design structure embodied in a tangible machine readable medium for designing, manufacturing, or testing an integrated circuit, the design structure comprising:

a middle junction control circuit that turns off a top NFET of a stacked NFET electrostatic discharge (ESD) protection circuit during an ESD event, wherein the middle junction control circuit turns off the top NFET by biasing a middle junction between the top NFET and the bottom NFET to substantially a same voltage as a gate of the top NFET.

21. The design structure of claim 20 , wherein the design structure comprises a netlist.

22. The design structure of claim 20 , wherein the design structure resides on storage medium as a data format used for the exchange of layout data of integrated circuits.

23. The design structure of claim 20 , wherein the design structure resides in a programmable gate array.

24. The method of claim 17 , wherein the biasing is performed by a control circuit comprising an inverter and a switch connected between the middle junction and the gate of the top NFET.

25. The circuit of claim 1 , wherein an output of a switch is connected to the middle junction between the top NFET and the bottom NFET of the stacked NFET ESD protection circuit.