Anti-snapback circuitry for metal oxide semiconductor (MOS) transistor

1. A circuit for protecting a metal oxide semiconductor (MOS) device by pulling down a voltage at a gate of the protected MOS device during an electrostatic discharge (ESD) event, the circuit comprising:

at least one active device connected to the gate of the protected MOS device, configured to actively pull down the voltage at the gate of the protected MOS device when the ESD event occurs,

wherein pulling down the voltage at the gate of the protected MOS device effectively turns off the protected MOS device when the ESD event occurs.

2. The circuit of claim 1 , wherein the at least one active device comprises:

a first anti-snapback MOS device connected to a drain of the protected MOS device to provide a capacitance that behaves like a short circuit when the ESD event occurs; and

a second anti-snapback MOS device connected to the gate of the protected MOS device, the second anti-snapback MOS device being connected to the first anti-snapback MOS device so that the second anti-snapback MOS device is turned on to pull down the voltage at the gate of the protected MOS device when the first anti-snapback MOS device capacitance behaves like a short circuit during the ESD event.

3. The circuit of claim 2 , wherein the capacitance provided by the first anti-snapback MOS device is selected so that the second anti-snapback MOS device is only turned on for high frequency ESD events.

4. The circuit of claim 1 , wherein the at least one active device comprises at least one of:

a diode connected to form a DC current path from the protected MOS device when the ESD event occurs; and

an anti-snapback MOS device connected to form an AC current path from the protected MOS device when the ESD event occurs.

5. The circuit of claim 4 , further comprising:

a current mirror configured to mirror current from the DC current path and/or the AC current path to pull down the voltage of the protected MOS device when the ESD event occurs.

6. A circuit for protecting a metal oxide semiconductor (MOS) device by holding down a voltage at a gate of the protected MOS device during an electrostatic discharge (ESD) event, the circuit comprising:

at least one capacitance-providing element connected to the gate of the protected MOS device, configured to passively hold down the voltage at the gate of the protected MOS device when the ESD event occurs,

wherein pulling down the voltage at the gate of the protected MOS device effectively turns off the MOS device when the ESD event occurs.

7. The circuit of claim 6 , wherein the at least one capacitance-providing element comprises:

an anti-snapback MOS device connected between the gate of the protected MOS device and a fixed voltage reference to form a capacitor divider circuit with a gate-to-drain parasitic capacitance of the protected MOS device, to hold down the voltage at the gate of the protected MOS device when the ESD event occurs.

8. The circuit of claim 7 , wherein the anti-snapback MOS device provides a capacitance that is selected relative to the gate-to-drain parasitic capacitance of the protected MOS device so that the voltage at the gate of the protected MOS device is held down during the ESD event but does not affect normal operation of the protected MOS device.

9. The circuit of claim 6 , wherein the at least one capacitance-providing element comprises:

a first anti-snapback MOS device and a second anti-snapback MOS device connected in series between the gate of the protected MOS device and a fixed voltage reference to form a capacitor divider circuit with a gate-to-drain parasitic capacitance of the protected MOS device, to hold down the voltage at the gate of the protected MOS device when the ESD event occurs.

10. The circuit of claim 9 , wherein the first and second anti-snapback MOS devices together provide a capacitance that is selected relative to the gate-to-drain parasitic capacitance of the protected MOS device so that the voltage at the gate of the protected MOS device is held down during the ESD event but does not affect normal operation of the protected MOS device.

11. A circuit for protecting a metal oxide semiconductor (MOS) device during an electrostatic discharge (ESD) event, the circuit comprising:

anti-snapback means for limiting a voltage at a gate of the protected MOS device when the ESD event occurs,

wherein limiting the voltage at the gate of the protected MOS device effectively turns off the MOS device when the ESD event occurs.

12. The circuit of claim 11 , wherein the anti-snapback means actively pulls down the voltage at the gate of the protected MOS device when the ESD event occurs.

13. The circuit of claim 12 , wherein the anti-snapback means comprises:

a first anti-snapback MOS device connected to a drain of the protected MOS device to provide a capacitance that behaves like a short circuit when the ESD event occurs; and

a second anti-snapback MOS device connected to the gate of the protected MOS device, the second anti-snapback MOS device being connected to the first anti-snapback MOS device so that the second anti-snapback MOS device is turned on to pull down the voltage at the gate of the protected MOS device when the first anti-snapback MOS device capacitance behaves like a short circuit during the ESD event.

14. The circuit of claim 12 , wherein the anti-snapback means comprises at least one of:

a diode connected to form a DC current path from the protected MOS device when the ESD event occurs; and

an anti-snapback MOS device connected to form an AC current path from the protected MOS device when the ESD event occurs.

15. The circuit of claim 11 , wherein the anti-snapback means passively holds down the voltage at the gate of the protected MOS device when the ESD event occurs.

16. The circuit of claim 15 , wherein the anti-snapback means comprises:

at least one capacitance-providing element connected to the gate of the protected MOS device, configured to passively hold down the voltage at the gate of the protected MOS device when the ESD event occurs.

17. The circuit of claim 16 , wherein the at least one capacitance-providing element comprises:

an anti-snapback MOS device connected between the gate of the protected MOS device and a fixed voltage reference to form a capacitor divider circuit with a gate-to-drain parasitic capacitance of the protected MOS device, to hold down the voltage at the gate of the protected MOS device when the ESD event occurs.

18. The circuit of claim 16 , wherein the at least one capacitance-providing element comprises:

a first anti-snapback MOS device and a second anti-snapback MOS device connected in series between the gate of the protected MOS device and a fixed voltage reference to form a capacitor divider circuit with a gate-to-drain parasitic capacitance of the protected MOS device, to hold down the voltage at the gate of the protected MOS device when the ESD event occurs.