# Patent

## Title
Electrostatic discharge (ESD) protection circuit and related apparatus and method

## Abstract

An electrostatic discharge (ESD) protection circuit includes a control circuit configured to generate a signal indicating whether an input voltage on an input/output pad is excessive. The protection circuit also includes a voltage divider configured to receive the signal from the control circuit and to divide the input voltage to produce a divided voltage. The protection circuit further includes an inverter chain having multiple inverters, where a first inverter is configured to receive the divided voltage and at least two inverters are configured to generate transistor control signals. In addition, the protection circuit includes a plurality of transistors configured to receive the transistor control signals and, when the input voltage is excessive, to prevent the input voltage from being provided to a protected circuit.

## Background
TECHNICAL FIELD
This disclosure is generally directed to integrated circuits. More specifically, this disclosure is directed to an electrostatic discharge (ESD) protection circuit and related apparatus and method.
BACKGROUND
Electrostatic discharge (ESD) protection circuits are routinely used to protect integrated circuits from damage caused by electrostatic discharge. However, local ESD protection is a major challenge for low-voltage integrated circuits. For example, complimentary metal oxide semiconductor (CMOS) technologies are becoming smaller and smaller, such as when implemented using 0.13 μm or 65 nm CMOS processes. These process technologies are typically associated with corresponding reductions in transistor gate oxide thickness. Gate oxides with reduced thickness are more sensitive to voltage overstress.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of this disclosure and its features, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates an example integrated circuit with electrostatic discharge protection according to this disclosure;
FIG. 2 illustrates an example electrostatic discharge protection circuit according to this disclosure; and
FIG. 3 illustrates an example method for electrostatic discharge protection according to this disclosure.



DETAILED DESCRIPTION
FIGS. 1 through 3, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the invention may be implemented in any type of suitably arranged device or system.
FIG. 1 illustrates an example integrated circuit 100 with electrostatic discharge (ESD) protection according to this disclosure. The embodiment of the integrated circuit 100 shown in FIG. 1 is for illustration only. Other embodiments of the integrated circuit 100 could be used without departing from the scope of this disclosure.
As shown in FIG. 1, the integrated circuit 100 includes protected circuitry 102, multiple ESD protection circuits 104a-104n, and multiple input/output (I/O) pads 106a-106n. The protected circuitry 102 generally represents any integrated circuit chip or other circuitry having one or more I/O pins or other structures requiring ESD protection. For example, the protected circuitry 102 could represent integrated circuitry produced using a low-voltage complimentary metal oxide semiconductor (CMOS) process, such as a 0.13 μm or 65 nm CMOS process.
The ESD protection circuits 104a-104n are coupled to I/O pins or other structures of the protected circuitry 102. Each of the ESD protection circuits 104a-104n provides ESD protection for the protected circuitry 102, such as by limiting or preventing excessive voltages from reaching the protected circuitry 102. As described in more detail below, each of the ESD protection circuits 104a-104n could include a dual-base triggered silicon controlled rectifier (SCR) structure. This structure could simultaneously provide a low triggering voltage and a high-breakdown and low-leakage ESD clamp in a highly compact area. One example embodiment of the ESD protection circuits 104a-104n is shown in FIG. 2.
The I/O pads 106a-106n are coupled to the ESD protection circuits 104a-104n. The I/O pads 106a-106n represent square pads or other structures on which wires or other electrical connections can be placed. The I/O pads 106a-106n provide electrical connection to the protected circuitry 102, allowing external components to send electrical signals to or receive electrical signals from the protected circuitry 102. Each of the I/O pads 106a-106n represents any suitable structure configured to be coupled to a wire or other electrical connection.
Although FIG. 1 illustrates an example integrated circuit 100 with ESD protection, various changes may be made to FIG. 1. For example, the integrated circuit 100 could include any number of protected circuits, ESD protection circuits, and I/O pads.
FIG. 2 illustrates an example ESD circuit 104a-104n according to this disclosure. The embodiment of the ESD circuit 104a-104n shown in FIG. 2 is for illustration only. Other embodiments of the ESD circuit 104a-104n could be used without departing from the scope of this disclosure.
As shown in FIG. 2, the ESD circuit 104a-104n includes four general components: a control circuit 202, a voltage divider 204, an inverter chain 206, and a transistor section 208. These components are coupled to and between two rails 210-212.
In this example embodiment, the control circuit 202 includes four transistors 214-220. The transistors 214 and 218 represent p-channel MOS (PMOS) transistors, and the transistors 216 and 220 represent n-channel MOS (NMOS) transistors. A source of the transistor 214 is coupled to the rail 210, and a source of the transistor 216 is coupled to the rail 212. Drains of the transistors 214-216 are coupled together and to gates of the transistors 218-220. A source of the transistor 218 is coupled to the rail 210, and a source of the transistor 220 is coupled to the rail 212. Drains of the transistors 218-220 are coupled together and to a gate of the transistor 214. A gate of the transistor 216 can receive a supply voltage VDD when a protected circuit is powered on, or the gate of the transistor 216 could float when the protected circuit is powered off.
The voltage divider 204 in this example embodiment is implemented using a transistor 222 and a resistor 224. The transistor 222 represents an NMOS transistor having a drain coupled to the rail 210, a source coupled to the resistor 224, and a gate. The gate is configured to receive a voltage formed between the transistors 214-216 in the control circuit 202. The resistor 224 is coupled between the transistor 222 and the rail 212 and could have any suitable resistance.
The inverter chain 206 in this example embodiment includes three inverters. The first inverter is formed by transistors 226-228, the second inverter is formed by transistors 230-232, and the third inverter is formed by transistors 234-236. The transistors 226, 230, and 234 represent PMOS transistors, and the transistors 228, 232, and 236 represent NMOS transistors. The transistors 226-228 have gates configured to receive a voltage produced between the transistor 222 and the resistor 224 in the voltage divider 204. The transistors 230-232 have gates configured to receive a voltage produced between the transistors 226-228. The transistors 234-236 have gates configured to receive a voltage produced between the transistors 230-232.
In this example embodiment, the transistor section 208 includes two bipolar junction transistors 238-240 arranged to form a silicon controlled rectifier (SCR). The transistor 238 represents a PNP transistor, and the transistor 240 represents an NPN transistor. The transistor 238 has an emitter coupled to the rail 210 and an I/O pad, a base coupled to an output of the third inverter in the inverter chain 206, and a collector coupled to an output of the second inverter in the inverter chain 206. The transistor 240 has a collector coupled to an output of the third inverter in the inverter chain 206 and to the base of the transistor 238. The transistor 240 also has a base coupled to an output of the second inverter in the inverter chain 206 and to the collector of the transistor 238. The transistor 240 further has an emitter coupled to the rail 212.
During normal operation with no ESD event, an integrated circuit chip or other circuitry being protected (such as circuitry 102) is powered, and the voltage VDD is high. The high VDD voltage causes the transistor 216 to pull its drain to VSS, and the voltage produced between the transistors 214-216 is low. This causes the voltage produced between the transistor 222 and the resistor 224 to be low. The second inverter therefore outputs a low voltage that pulls the p-well of the transistor 240 to VSS, while the third inverter outputs a high voltage that pulls the n-well of the transistor 238 to the voltage on the I/O pad. In this condition, both base-emitter junctions of the transistors 238-240 can have zero or near-zero bias, so leakage current can be very small. The voltage on the I/O pad is therefore supplied to the protected circuit.
During an ESD condition when an integrated circuit chip or other circuitry being protected is not powered, the voltage VDD is floating. In this case, the transistors 214 and 218 pull up their drains, but the transistor 214 can be sized larger so that it pulls up its drain faster and turns on the transistor 220 (forming a positive feedback loop). The voltage at the gate of the transistor 222 is then pulled up to the voltage on the I/O pad. Once the voltage at the interior of the voltage divider 204 reaches the switching threshold of the first inverter, the first inverter outputs a low voltage. The second inverter therefore outputs a high voltage that pulls up the p-well of the transistor 240, while the third inverter outputs a low voltage that pulls down the n-well of the transistor 238. In this condition, current is injected into both bases of the transistors 238-240 simultaneously, triggering silicon controlled rectification. As a result, the ESD protection circuit prevents most or all of the excessive voltage on the I/O pad from reaching the protected circuit.
In particular embodiments, the components shown in FIG. 2 are produced using a low-voltage CMOS process, such as the OPTIMOS, OPTIMOS2, or CBC8 process of NATIONAL SEMICONDUCTOR CORPORATION or any suitable CMOS or BiCMOS process. A particular design of the protection circuit could be implemented in 0.13 μm BiCMOS technology with a core VDD of 1.2V, an I/O voltage of 2.5V, and a width of 50 μm intended to protect a 2.5V I/O pad from triggering voltages above 2.5V when not powered.
Depending on the implementation, the ESD protection circuit 104a-104n shown in FIG. 2 could offer the following benefits. The ESD protection circuit 104a-104n could have a variable triggering voltage, such as a 2-3V triggering voltage when VDD is floating and a higher triggering voltage when VDD is high. This means that the VDD line can be used as a control signal to increase the triggering voltage when the protected circuit is powered, which can help to prevent inadvertent triggering of the transistors 238-240. The ESD protection circuit 104a-104n could also have very low leakage when in the off (non-triggered) state. For example, the protection circuit could have a relatively flat leakage over its normal operating temperature range, such as a leakage of 0.2 nA/μm at 2.5V. In fact, since the leakage of the protection circuit can be decoupled from its triggering voltage, it is possible for the protection circuit to have both low leakage and a low triggering voltage (unlike conventional diode-triggered silicon controlled rectifiers). The ESD protection circuit 104a-104n could further provide a very fast response to an ESD event. For instance, the time required for the components 202-204 to produce a high enough voltage for the inverter section 206 to trigger rectification could be 10-50 ns. In addition, the protection circuit could have low parasitic features while simultaneously having a low triggering voltage, making the protection circuit useful in low-voltage CMOS applications.
Among the various design considerations of the protection circuit, the width/resistance of the transistor 222 and the resistance of the resistor 224 can affect the triggering voltage, but in particular embodiments the exact relationship may be somewhat complicated by several conflicting effects within the protection circuit. As a result, circuit simulations can be used to design the voltage divider 204 for a target triggering voltage (such as to select a width of the transistor 222 and/or a resistance of the resistor 224).
As another design consideration, the triggering voltage may be independent of rise time during an ESD condition, but a dV/dt effect can exist when the protected circuit is powered. The magnitude of the dV/dt effect may depend on the transistor sizing ratios (PMOS/NMOS) of the second and third inverters in the inverter chain 206, where large inverter sizing ratios can lead to an increased dV/dt effect. The dV/dt effect can therefore be mitigated with a proper design of the inverter sizing ratios in the inverter chain 206.
Although FIG. 2 illustrates an example ESD circuit 104a-104n, various changes may be made to FIG. 2. For example, each of the components 202-208 in FIG. 2 could be implemented using any suitable sub-components. As particular examples, the inverter stage 206 could be implemented using any suitable number of inverters, and the transistor stage 208 could include any suitable transistors.
FIG. 3 illustrates an example method 300 for electrostatic discharge protection according to this disclosure. The embodiment of the method 300 shown in FIG. 3 is for illustration only. Other embodiments of the method 300 could be used without departing from the scope of this disclosure. Also, for ease of explanation, the method 300 is described with respect to the ESD protection circuit 104a-104n of FIG. 2 operating in the circuit 100 of FIG. 1. The method 300 could be used by or in any other suitable device or system.
A control signal is received at step 302. This could include, for example, the ESD protection circuit 104a-104n receiving a VDD signal that is either high or floating. An ESD event may or may not occur at step 304. If one does not occur, a normal voltage can be received at an I/O pad at step 306. This could include, for example, receiving a voltage between 0-2.5V on an I/O pad 106a-106n. A low output is generated by a control circuit at step 308. This could include, for example, the control circuit 202 outputting a low voltage in response to the normal voltage on the I/O pad. The low output of the control circuit causes a voltage divider to generate a low output at step 310. A first set of transistor control signals is generated by an inverter chain using the output of the voltage divider at step 312. This could include, for example, the second inverter in the inverter chain 206 outputting a low voltage and the third inverter in the inverter chain 206 outputting a high voltage. These control signals are used to provide zero (or substantially zero) bias to transistor bases at step 314. In this way, the transistors 238-240 have very low leakage and do not interfere with the protected circuit (such as circuitry 102) receiving the normal voltage on the I/O pad.
If an ESD event does occur at step 304, the ESD protection circuit receives an excessive voltage on an I/O pad at step 316. This could include, for example, the I/O pad 106a-106n receiving a voltage greater than 2.5V when VDD is floating. This creates a positive feedback loop in the control circuit at step 318. For instance, this may include the transistors 214 and 218 pulling up their drains, where the transistor 214 pulls its drain up faster and turns on the transistor 220 to form a positive feedback loop. A high output is generated by the control circuit at step 320, and a high output is generated by the voltage divider at step 322. Eventually, the high output of the voltage divider passes the threshold of the inverters in the inverter chain, and a second set of transistor control signals is generated at step 324. This could include, for example, the second inverter in the inverter chain 206 outputting a high voltage and the third inverter in the inverter chain 206 outputting a low voltage. These control signals are used to inject current onto the transistor bases at step 326. In this way, the transistors 238-240 protect the protected circuit against the excessive voltage associated with the ESD event. This may continue until the voltage on the I/O pad returns to normal or is no longer representative of an ESD event.
Although FIG. 3 illustrates an example method 300 for electrostatic discharge protection, various changes may be made to FIG. 3. For example, while shown as a series of steps, various steps in FIG. 3 could overlap, occur in parallel, occur multiple times, or occur in a different order.
It may be advantageous to set forth definitions of certain words and phrases that have been used within this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more components, whether or not those components are in physical contact with one another. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like.
While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this invention. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this invention as defined by the following claims.

## Claims

1. An electrostatic discharge (ESD) protection circuit comprising:
a control circuit configured to generate a signal indicating whether an input voltage on an input/output pad is excessive;
a voltage divider configured to receive the signal from the control circuit and to divide the input voltage to generate a divided voltage;
an inverter chain comprising multiple inverters, a first inverter configured to receive the divided voltage, at least two inverters configured to generate transistor control signals; and
a plurality of transistors configured to receive the transistor control signals and, when the input voltage is excessive, to protect a protected circuit from the excessive input voltage;
wherein the control circuit comprises:
first and second transistors coupled in series; and
third and fourth transistors coupled in series;
wherein a gate of the first transistor is coupled to receive a voltage generated between the third and fourth transistors;
wherein gates of the third and fourth transistors are coupled to receive a voltage generated between the first and second transistors; and
wherein a gate of the second transistor is configured to receive a supply voltage.

2. The electrostatic discharge (ESD) protection circuit of claim 1, further comprising:
the control circuit further configured to receive an input voltage on an input/output pad and generate a signal indicating whether the input voltage on the input/output pad is excessive.

3. The ESD protection circuit of claim 2, wherein the voltage divider comprises a transistor configured to receive the input voltage and a resistor coupled in series with the transistor, a gate of the transistor configured to receive the signal from the control circuit.

4. The ESD protection circuit of claim 3, wherein the transistor comprises an NMOS transistor.

5. The ESD protection circuit of claim 2, wherein the inverter chain comprises the first inverter, a second inverter, and a third inverter coupled in series, the second and third inverters configured to generate the transistor control signals.

6. The ESD protection circuit of claim 5, wherein the plurality of transistors comprises:
a first bipolar junction transistor having an emitter configured to receive the input voltage, a base configured to receive the transistor control signal from the third inverter, and a collector coupled to an output of the second inverter; and
a second bipolar junction transistor having a collector coupled to an output of the third inverter and to the base of the first bipolar junction transistor, a base configured to receive the transistor control signal from the second inverter and coupled to the collector of the first bipolar junction transistor, and an emitter.

7. The ESD protection circuit of claim 6, wherein the first bipolar junction transistor comprises a PNP transistor, and the second bipolar junction transistor comprises an NPN transistor.

8. The ESD protection circuit of claim 1, wherein the first and third transistors comprise PMOS transistors, and the second and fourth transistors comprise NMOS transistors.

9. An apparatus comprising:
integrated circuitry to be protected;
an input/output pad in electrical connection with the integrated circuitry; and
an electrostatic discharge (ESD) protection circuit coupled to the integrated circuitry and the input/output pad, wherein the ESD protection circuit comprises:
a control circuit configured to generate a signal indicating whether an input voltage on the input/output pad is excessive;
a voltage divider configured to receive the signal from the control circuit and to divide the input voltage to generate a divided voltage;

an inverter chain comprising multiple inverters, a first inverter configured to receive the divided voltage, at least two inverters configured to generate transistor control signals; and
a plurality of transistors configured to receive the transistor control signals and, when the input voltage is excessive, to protect the integrated circuitry from the excessive input voltage; and

wherein the control circuit comprises:
first and second transistors coupled in series; and
third and fourth transistors coupled in series;
wherein a gate of the first transistor is coupled to receive a voltage generated between the third and fourth transistors;
wherein gates of the third and fourth transistors are coupled to receive a voltage generated between the first and second transistors; and
wherein a gate of the second transistor is configured to receive a supply voltage.

10. The apparatus of claim 9, wherein the inverter chain comprises the first inverter, a second inverter, and a third inverter coupled in series, the second and third inverters configured to generate the transistor control signals.

11. The apparatus of claim 10, wherein the plurality of transistors comprises:
a first bipolar junction transistor having an emitter configured to receive the input voltage, a base configured to receive the transistor control signal from the third inverter, and a collector coupled to an output of the second inverter; and
a second bipolar junction transistor having a collector coupled to an output of the third inverter and to the base of the first bipolar junction transistor, a base configured to receive the transistor control signal from the second inverter and coupled to the collector of the first bipolar junction transistor, and an emitter.

12. The apparatus of claim 9, wherein:
the input/output pad comprises one of multiple input/output pads; and
the ESD protection circuit comprises one of multiple ESD protection circuits, each ESD protection circuit coupled to one of the input/output pads.