# Patent

## Title
Threshold voltage method and apparatus for ESD protection

## Abstract

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.

## Background
RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 11/759,351, filed on Jun. 7, 2007, which is incorporated herein by reference in its entirety.


TECHNICAL FIELD
A present invention generally relates to electrostatic discharge protection devices for integrated circuits. More specifically, the device is activated by threshold voltage for triggering an electrostatic discharge shunting device.
BACKGROUND ART
It is known in the integrated circuit industry that build up of static charge may lead to development of extremely high voltage near input of an integrated circuit. Electrostatic discharge (ESD) refers to the phenomenon of an electrical discharge of high current over a short duration where the current results from a buildup of static charge on or near the integrated circuit package. ESD is a serious problem for semiconductor devices since it has the potential to destroy the entire integrated circuit. It is important to provide a full-chip method of protection against such damage. The principal difficulty for designers is to make a protection device which would be able to protect the integrated circuit without interfering with nominal chip functionality.
A well-known protection method incorporated for protecting devices is a clamp circuit that connects to the power supply rails and distinguishes the ESD event from electronic signals propagating during normal applications. Distinguishing the ESD event from nominal signals is done by a filter with an RC time constant. However, due to this approach to signal differentiation, rapid rise times occurring during integrated circuit ESD event. Undesirable triggering of the protection clamping circuit may occur either due to rapid power-on conditions or during current consumption, such as occurs with simultaneously switching outputs (SSO).
With reference to FIG. 1, a series configuration of a trigger capacitor 115 and a trigger resistor 120 connects between a power supply terminal 105 and ground 110 in a prior art ESD protection circuit 100. An ESD inverter 130 and a trigger latch 140 each connect between the power supply terminal 105 and ground 110. An ESD trigger line 125 connects between a series connection node (between the trigger capacitor 115 and the trigger resistor 120) and an input of the ESD inverter 130. A trigger line 135 connects between an output of the ESD inverter 130 and an input of the trigger latch 140. An ESD shunt device 145 connects between the power supply terminal 105 and ground 110. An ESD shunt trigger line 150 connects between an output of the trigger latch 140 and an input of the ESD shunt device 145.
The ESD protection circuit 100 uses an RC time constant to trigger the ESD shunt device 145, but uses the trigger latch 140 to maintain a triggered state of the ESD shunt device 145. Separating the ESD trigger elements (i.e., the trigger capacitor 115, the trigger resistor 120, and the ESD inverter 130) from an element sustaining the ESD trigger state (i.e., the trigger latch 140), means an RC time constant for triggering can be reduced by a factor of 100. Reduction of an RC time constant helps to eliminate risk of an accidental trigger during a rapid (on the order of hundreds of nanoseconds) power-on of a system. RC time constant reduction does not help in differentiating between ESD events with rise times similar to power-on events. An additional benefit of a reduction in an RC time constant is less risk of false triggering during a switching, which produces noise (on an order of nanoseconds) on the power supply terminal 105 and is caused by simultaneously switching outputs (SSO).
The ESD protection circuit 100 requires additional timeout circuitry (not shown) which produces a release of the trigger latch 140 typically after a few microseconds delay. A timeout circuit is required to release the trigger latch 140 in cases where false triggering has occurred due to RLC noise generation or IR drop during SSO.
What is needed is a triggering mechanism that properly triggers ESD shunting devices yet allows a rapid power-on of a device without a particular requirement for power-on slew rates below a certain minimum. Furthermore, false triggering of the ESD devices should be avoided during SSO events. It is also desirable to have an ESD shunt triggering mechanism that is not a function of RC parasitic components and thus avoids any processing variation in these component values as well as the corresponding expense in required chip area.
SUMMARY
A first exemplary embodiment of an ESD protection circuit comprises 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, and 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.
A second exemplary embodiment is an ESD protection circuit comprising a means for shunting coupled between a power supply terminal and ground, a means for retaining a logic signal level, the retaining means coupled to the shunting means, a means for buffering coupled to the retaining means, a means for triggering coupled to the buffering means, and a means for detecting coupled to the triggering means, the detecting means further comprising a first and second means for mirroring a current and a plurality of means for biasing.
An exemplary method of protecting a device from electrostatic discharge, the method comprising applying an electrostatic discharge to a power supply terminal coupled to a current comparator, sensing a first sense current flowing through a first sense device of a first current mirror, producing a mirror current through a first mirror device of the first current mirror, producing a bias voltage from the first sense current flowing through a plurality of bias devices, sensing a second sense current flowing through a second sense device of a second current mirror, accepting the first sense current through a second mirror device of the second current mirror, and producing a trigger signal at an output of the current comparator.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram of a first prior art shunting circuit for ESD protection.
FIG. 2 is a schematic diagram of an exemplary ESD event shunting circuit according to the present invention.
FIG. 3 is a current diagram of exemplary circuit elements according to FIG. 2.
FIG. 4 is an output voltage diagram of an exemplary current comparator according to FIG. 2.
FIG. 5 is an exemplary process flow diagram of a method for triggering protection from ESD events utilizing the circuit of FIG. 2.



DETAILED DESCRIPTION
With reference to FIG. 2, a comparator 212 connects between a power supply terminal 205 and ground 210 in an exemplary ESD protection circuit 200. A buffer 230 and a latch 240 each connect between the power supply terminal 205 and ground 210. A comparator output line 225 connects between an output of the comparator 212 and an input of the buffer 230. A trigger line 235 connects between an output of the buffer 230 and an input of the latch 240. A shunt device 245 connects between the power supply terminal 205 and ground 210. A shunt trigger line 250 connects between an output of the latch 240 and an input of the shunt device 245.
The buffer 230 is, for example, a succession of two CMOS inverters in series between the comparator output line 225 and the trigger line 235. The latch 240 is, for example, a cross coupled set of CMOS inverters with a latch input at the trigger line 235 and a latch output coupled to the shut trigger line 250.
In the present exemplary embodiment the comparator 212 comprises a first current mirror 215 coupled between the power supply terminal 205, a first trigger resistor 218, and the comparator output line 225. A second trigger resistor 220 is coupled in series with the first trigger resistor 218 at a bias output line 208. The second trigger resistor 220 is coupled to a compensating device 216.
The first current mirror 215 has a first sense device 207 coupled between the power supply terminal 205 and the first trigger resistor 218. The first current mirror 215 also has a first mirror device 214 coupled between the power supply terminal 205 and the comparator output line 225. The first sense device 207 and the first mirror device 214 may be, for example, PMOS transistors.
The comparator 212 further comprises a second current mirror 217 with a second sense device 209 coupled between a transconductance device 222 and ground 210. Within the second current mirror 217, a second mirror device 211 is coupled between the comparator output line 225 and ground 210. The second sense device 209 and the second mirror device 211 may be, for example, NMOS transistors.
The compensating device 216 serves as a voltage compensating device where variation in voltage across the compensating device 216 tracks, and thus compensates for, corresponding temperature variation in a threshold voltage of the first sense device 207. The compensating device 216 may be, for example, a bipolar transistor. Implemented as a bipolar transistor, the compensating device 216 couples to ground 210 in a diode configuration (i.e., with a transistor base terminal coupled to ground 210).
The first and second trigger resistors 218, 220 are generally incorporated as bias devices. The first trigger resistor 218 may be, for example, 80 kilohms and the second trigger resistors 220 may be, for example, 1.2 megohms. These values are suited to, for example, a 0.18 micrometer (um) technology with a VDD supply voltage level of 3.3 volts (V). Generally, a given magnitude of VDD supply voltage level may affect selection of the values of the first or second trigger resistors 218, 220. One of the first or second trigger resistors 218, 220 being in the megohm range, for example, assures a limited current consumption by the ESD protection circuit during nominal operation (i.e., during non-ESD operation). A plurality of bias devices may be used in various configurations contrived by one skilled in the art to derive a voltage on the bias output line 208 capable of supplying a bias voltage suitable for operating the transconductance device 222.
A skilled artisan may conceive of other solid-state devices, such as field effect transistors, that would suitably equate to the PMOS or NMOS transistors utilized in the first current mirror 215 or the second current mirror 217 respectively. For example, an insulated gate device would be capable of being configured with a gate input connected to a drain node in a first device, for instance a sense device, with an identical device coupled at a gate input forming a mirror device. Other such equivalents may be contemplated by the skilled artisan that would suitably equate to a sense and mirror device.
A transconductance device input is coupled to the power supply terminal 205 and a control input of the transconductance device 222 is coupled to the bias output line 208. An output of the transconductance device 222 couples to the second sense device 209 of the second current mirror 217.
The ESD protection circuit 200 incorporates the comparator 212 to trigger the shunt device 245 and uses the latch 240 to maintain a triggered state of the shunt device 245. The ESD protection circuit 200 does not require additional timeout circuitry to release the latch 240 after a few microseconds delay, which is typically the case where passive components set the latch 240. A timeout circuit is required to release a latch in cases of prior art circuits where false triggering has occurred due to RLC noise generation or IR drop during SSO. The exemplary triggering mechanism provided by the comparator 212 avoids such a requirement.
With reference to FIG. 3, a first sense current I1, (FIG. 2) overlays a second sense current I2 in an exemplary current vs. voltage diagram 300. The abscissa represents an input voltage VIN on the power supply terminal 205. For any extent of the input voltage VIN beyond a magnitude of expected power supply voltage VDD, the input voltage VIN may be seen to represent an applied voltage corresponding to an ESD event on the power supply terminal 205. The first sense current increases, for example, essentially linearly from a point where the input voltage VIN is about 0.8 volts (V) through the full range of voltage depicted. The linear characteristic of the first sense current I1 comes from the resistive electrical characteristics of an on-channel resistance within the first current mirror 215, the first trigger resistor 218, and the second trigger resistor 220.
The second sense current I2 increases, for example, in a quadratic fashion from a point where the input voltage VIN is about 0.8 V through the full range of the input voltage VIN depicted. The quadratic characteristic is due to the inherent electrical characteristics of the transconductance device 222. The transconductance device may be, for example, a PMOS transistor operating in a saturation realm. The current characteristic of a PMOS transistor in this realm is a function of the (VGS−VTH)2 type of relationship for the device and so follows the quadratic nature of the current behavior versus the input voltage VIN behavior. In addition, the transconductance device may be, for example, a PMOS transistor operating in a linear conduction realm.
In continuing reference to FIG. 3, for the input voltage VIN ranging from 0.8 V to about 4.3 V, a first sense current I1 exceeds a second sense current I2 for the exemplary comparator 212 (FIG. 2). Since the first mirror device 214 of the first current mirror 215 is coupled in series with the second mirror device 211 of the second current mirror 217, the current through each mirror device is equal. The first mirror device 214 produces an incident current and the second mirror device 211 accepts an absorption current.
The incident current is a measure of the propensity of the first sense current I1 to produce a mirror current in the first mirror device 214. Similarly, the absorption current is a measure of the propensity of the second sense current I2 to induce acceptance of a series current through the second mirror device 211. For a given input voltage VIN applied at the power supply terminal 205, the first sense current I1 is not equal to the second sense current I2 (except at a cross over point, which is explained below).
Since the two mirror currents flowing through the series combination of the respective mirror devices 211, 214 are equal (since they are the same current), the greater of the two sense currents I1, I2 prevails in producing the resultant series current through the mirror devices 211, 214. This comes about since the (relatively) greater of the two internal mirror voltages (not shown) within each current mirror 215, 217 is in effect producing the lower impedance through a corresponding one of the two mirror devices 211, 214.
Consequently, at a given operating condition of input voltage VIN on the power supply terminal 205, the one of the two current sense devices 207, 209 capable of inducing the greater mirror current has a relatively smaller voltage drop across the corresponding one of the two mirror devices 211, 214. The mirror device with the smaller voltage drop across it determines the series current through the two mirror devices 211, 214. The resulting current flowing through the remaining mirror device sustains a greater voltage drop across the device. This scenario provides the behavior of a voltage on the comparator output line VCOUT (FIG. 2).
For a range of input voltage VIN from about 0.8 V to about 4.3 V there is less voltage across the first mirror device 214 than there is across the second mirror device 211. This produces a high output level for the voltage on the comparator output line VCOUT. With a high level comparator output voltage VCOUT, a corresponding high voltage level is produced on the trigger line 235 and a low voltage level is produced on the shunt trigger line 250. Therefore, for VIN ranging from 0.8-4.3 V the shunt device 245 is off leaving voltage on the Power supply terminal 205 at a nominal supply voltage, for example, VDD. As the input voltage VIN ranges from about 0.8 V to about 4.3 V, for example, VCOUT closely approximates VIN in magnitude.
In contrast, for VIN above a magnitude, for example, of 4.3 V, the above characteristics take on complementary relationships since the second mirror device 211 is capable of conducting more current than the first mirror device 214 in this realm. For an increasing magnitude of input voltage VIN, a cross over point or a trigger supply voltage (considering that the input voltage VIN is applied to the power supply terminal 205) is defined where the second mirror device 211 begins to be capable of conducting more current than the first mirror device 214.
Therefore, above the trigger supply voltage, the voltage across the second mirror device 211 is less than the voltage across the first mirror device 214. A condition follows that the comparator output voltage VCOUT is at a low level and the shunt device 245 is turned on. With the shunt device 245 turned on, high levels of VIN for example above 4.3 V, the power supply terminal 205 is shunted to ground through the shunt device 245. High levels of VIN that produce the protective shunting effect, produced by the shunt device 245, correspond with elevated voltages like those produced by an ESD event on the power supply terminal 205. That is, the ESD protection circuit 200 produces protection against elevated voltages such as those of an ESD event. The protection is derived from a threshold voltage detection provided by a current comparator.
The characteristics of the first and second sense currents I1, I2 verses input voltage VIN may be complementary to the two instances given. The first sense current I1 may increase, for example, in a quadratic fashion and the second sense current I2 may increase, for example, in an essentially linear fashion. One skilled in the art may readily envision any of a number of types of functional variation of a sense current provided by current sensing devices where a cross over point is present in the combined sense current characteristics.
With reference to FIG. 4, a comparator output voltage VCOUT attains a peak value prior to an input voltage VIN attaining a magnitude of 5 V in an exemplary comparator output voltage versus input voltage diagram 400. For VIN ranging from about, for example, 0.8-4.3 V, the shunt device 245 is off leaving voltage on the power supply terminal 205 at a nominal supply voltage, for example, VDD. Therefore, as the input voltage VIN ranges from about 0.8 V to about 4.3 V, for example, VCOUT closely approximates VIN in magnitude.
Subsequent to the input voltage VIN attaining a magnitude of about, for example, 4.3 V, the comparator output voltage VCOUT reduces to about 0.4 V for the input voltage VIN exceeding about 8 V. Above about the 4.3 V level for VIN, the comparator output voltage VCOUT is a trigger signal or ESD trigger voltage capable of causing the ESD protection circuit 200 (FIG. 2) to activate the shunt device 245. For VIN below about the 4.3 V level, and since VCOUT closely approximates VIN for VIN ranging from about 0.8 V to about 4.3 V, VCOUT does not vary from VIN enough to cause a trigger signal on the shunt trigger line 250.
With reference to FIG. 5, applying an electrostatic discharge to a power supply terminal 505 is a first step in an exemplary method of protecting a device from electrostatic discharge 500. A next step is sensing a first sense current flowing through a first sense device 510 followed by a step of producing a mirror current through a first mirror device 515. A continuing step is producing a bias voltage from the first sense current 520 followed by a step of sensing a second sense current flowing through a second sense device 525. A subsequent step is accepting the first sense current through a second mirror device 530 and next producing a trigger signal at an output of the current comparator 535. A next step is taken of activating a shunt device coupled between the power supply terminal and ground 540 followed by shunting current from the electrostatic discharge to ground 545.
It would be clear to one of skill in the art that alternate embodiments of the above detailed description may exist. Therefore, the above description is illustrative and not restrictive. The scope of the invention should therefore be determined by reference to the appended claims and not by the above description.

## Claims

1. A circuit comprising:
a current mirror based comparator for coupling to a power supply terminal, wherein currents differ from each other responsive to an electrostatic discharge event, wherein the current mirror based comparator includes:
a first current mirror configured provide a first mirror current related to a voltage applied to the power supply terminal;
a second current mirror configured to provide a second mirror current non-linearly related to the voltage applied to the power supply terminal;
a bias device configured to receive a first sense current from the first mirror device and to provide a bias voltage, and

a transconductance device configured to receive the bias voltage at a control input and to provide a second sense current to the second mirror device using the voltage applied to the power supply terminal;
a latch coupled to the current mirror based comparator for latching a comparison signal generated by the comparator as a function of the currents generated by the electrostatic discharge event; and
a shunt device coupled to the latch, and responsive to shunt current from the power supply terminal to a ground during an electrostatic discharge event.

2. The circuit of claim 1, wherein the first mirror device and the second mirror device are coupled to a common conductor, wherein the common conductor is configured to provide the comparison signal.

3. The circuit of claim 1, wherein the current mirror based comparator includes a compensation circuit to compensate the comparator for a variable operating condition.

4. The circuit of claim 3, wherein the variable operating condition includes temperature.

5. An electrostatic discharge protection method, comprising:
receiving an electrostatic discharge at a power supply terminal;
generating a first current and a second current using the electrostatic discharge, wherein generating the first current includes generating a bias voltage, and wherein generating the second current includes receiving the bias voltage at a transconductance device;
comparing the first and second current to generate an output value indicative of the electrostatic discharge;
latching the output value; and
activating a shunt device, based on the latched output value, to shunt current from the power supply terminal to a ground.

6. The method of claim 5, including compensating for temperature variation using at least one of the first current or the second current.

7. The method of claim 5, wherein
generating the first current includes generating a first mirror current using a first current mirror; and
generating a second current includes generating a second minor current using a second current mirror,
wherein the first mirror current and the second mirror current are equal.

8. The method of claim 5, wherein activating a shunt device includes activating shunt device sized to compensate for different power supply voltages.

9. A method comprising:
receiving a voltage at a power supply terminal coupled to a current comparator;
generating a first sense current using a bias device coupled to the power supply terminal through a first current mirror, the first sense current configured to linearly track a value of the voltage;
generating a bias voltage at a terminal of the bias device using the first sense current;
generating a second sense current at an output of a transconductance device using the bias voltage, the bias voltage coupled to a control input of the transconductance device, wherein the second sense current is configured to track the value of the voltage in a nonlinear fashion, and wherein the second sense current is less than the first sense current when the voltage is below a trigger voltage and is greater than the first sense current when the voltage is above the trigger voltage; and
receiving the second sense current at a second current mirror; and
generating a trigger output using a mirror current coupled between the first minor device and the second mirror device,
wherein generating a trigger output includes generating a first trigger output when the voltage is below the trigger voltage is near the trigger voltage, and
wherein generating a trigger output includes generating a second trigger output when the voltage is higher than about the trigger voltage.

10. The method of claim 9, including receiving the second trigger output at a control input of the shunt and shunting the voltage to a ground potential.

11. The method of claim 10, wherein receiving the trigger output includes buffering the trigger output at a buffer coupled to the shunt.

12. The method of claim 9, including adjusting the bias voltage to compensate for a variable operating condition.

13. The method of claim 9, wherein the variable operating condition includes temperature.

14. The method of claim 9, wherein generating a trigger output includes:
generating a first mirror current using the first mirror device and the first sense current;
generating a second mirror current using the second mirror device and the second sense current; and
flowing the first mirror current and the second mirror current through a conductor common to the first mirror device and the second mirror device.