Threshold voltage method and apparatus for ESD protection
An electrostatic discharge protection circuit comprises a comparator coupled between a power supply terminal and ground. The comparator responds to an electrostatic discharge event producing a trigger signal at a comparator output. The comparator comprises a first and second current mirror. The first and second current mirrors each comprise a sense device and a mirror device. The mirror devices are coupled in series between the power supply terminal and ground. The first mirror device produces an incident current and the second mirror device receives an absorption current. With a supply voltage on the power supply terminal equal to or greater than a trigger supply voltage, the absorption current exceeds the incident current and produces a trigger signal at the comparator output. The trigger signal activates a shunt device that shunts current from the power supply terminal to ground.
1. An electrostatic discharge protection circuit, comprising:
a shunt device coupled between a power supply terminal and ground, the shunt device capable of shunting current from the power supply terminal to ground in response to a trigger signal received at a shunt device input;
a latch coupled to the shunt device input, the latch capable of latching the trigger signal received at a latch input;
a buffer coupled to the latch input, the buffer capable of receiving the trigger signal at a buffer input;
a comparator coupled between the power supply terminal, ground, and the buffer input, the comparator capable of responding to an electrostatic discharge event and producing the trigger signal at a comparator output, wherein the comparator comprises:
a first current mirror comprising a first sense device coupled to the power supply terminal and a first mirror device coupled between the comparator output and the power supply terminal;
a plurality of bias devices coupled to the first sense device, at least two of the plurality of bias devices coupled in series at a bias output line;
a transconductance device comprising an input coupled to the power supply terminal and a control input coupled to the bias output line;
a second current mirror comprising a second sense device coupled between the transconductance device and ground and a second mirror device coupled between the comparator output and ground, the second mirror device coupled in series with the first mirror device; and
a compensating device coupled between the plurality of bias devices and ground.
2. The electrostatic discharge protection circuit of claim 1 , wherein the plurality of bias devices is capable of producing a bias voltage on the bias output line proportionate to a first sense current flowing through the first sense device and the plurality of bias devices, the bias voltage determining a second sense current flowing through the transconductance device.
3. The electrostatic discharge protection circuit of claim 1 , wherein the comparator is capable of producing the trigger signal at the comparator output based on a current flowing through the first mirror device and the second mirror device.
4. The electrostatic discharge protection circuit of claim 1 , wherein a resultant current through the series coupled mirror devices activates the trigger signal.
5. The electrostatic discharge protection circuit of claim 1 , wherein the first mirror device is capable of producing an incident current and the second mirror device is capable of accepting an absorption current.
6. The electrostatic discharge protection circuit of claim 1 , wherein the series coupling of mirror devices is capable of producing a resultant current equal to the greater of an incident current flowing through the first mirror device or an absorption current flowing through the second mirror device.
7. The electrostatic discharge protection circuit of claim 6 , wherein the trigger signal is activated when the absorption current is greater than the incident current.
8. The electrostatic discharge protection circuit of claim 1 , wherein a first sense current through the first sense device produces an incident current through the first mirror device and a second sense current through the second sense device produces an absorption current through the second mirror device.
9. The electrostatic discharge protection circuit of claim 1 , wherein the first mirror device is capable of producing an incident current less than an absorption current through the second mirror device with a supply voltage on the power supply terminal equal to or greater than a trigger supply voltage, the absorption current exceeding the incident current producing the trigger signal at the comparator output activating the shunt device.
10. The electrostatic discharge protection circuit of claim 1 , wherein the compensating device is capable of compensating a temperature dependent variation of a threshold voltage of the first sense device, the compensating device providing a voltage drop across the compensating device corresponding to the voltage drop across the first sense device due to temperature variation.