# Patent

## Title
Electrostatic discharge clamp circuit

## Abstract

An electrostatic discharge (ESD) clamp circuit is provided. The ESD clamp circuit includes a first resistor, a second resistor, a first transistor, a second transistor, and a third transistor. A clamp device of the ESD clamp circuit is implemented by the third transistor. A parasitic capacitor of the third transistor forms a detection scheme along with the second resistor to detect the ESD. The first resistor, the second resistor, the first transistor, and the second transistor form a feedback scheme to control the third transistor for discharging the ESD current.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial no. 98116793, filed on May 20, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification.
BACKGROUND
1. Technical Field
The present disclosure generally relates to a protection circuit, in particular, to a protection circuit for electrostatic discharge.
2. Description of Related Art
With the progress of technology, electronic devices are displacing traditional mechanical devices gradually. Electronic devices are often suffered from electrostatic discharge (ESD). The static electricity accumulated in human body (or machine) may be discharged through the electronic devices because of human body (or machine) contact, whether in manufacturing process or in actual use. Because the voltage generated by the ESD event is much higher than the voltage that the electronic devices can endure, the function of the electronic devices may be damaged, or even permanent damage is caused. Moreover, the electronic devices themselves may also accumulate static electricity, and the static electricity can be discharged while the electronic devices are grounded during assembling, thus resulting in unexpected loss.
Therefore, in order to avoid the devices damages caused by the ESD, the corresponding measures are taken to protect the electronic devices. FIG. 1 and FIG. 2 are conventional ESD clamp circuit diagrams. Referring to FIG. 1, an ESD clamp circuit 100 employs a resistor-capacitor (RC) time-delay-triggered architecture. A resistor R1 and a capacitor C1 form a RC circuit used to detect the ESD. A P-channel metal oxide semiconductor (PMOS) transistor MP1 and an N-channel metal oxide semiconductor (NMOS) transistor MN1 form an inverter 101 used to control an NMOS transistor MC1 which serves as a clamp device. A node T3 is an output terminal of the inverter 101, which is coupled to a drain of the transistor MP1 and a drain of the transistor MN1. When the ESD occurs at a power rail VDD, a voltage across the two ends of the resistor R1 is generated, such that the input terminal of the inverter 101 is at a low potential. At this point, the inverter 101 outputs a high potential, turns on the transistor MC1 to form a low-impedance path, and discharge the ESD current to a power rail VSS, so as to protect a core circuit 103 on the back-end. During the discharging of the ESD current, the current flowing through the resistor R1 charges the capacitor C1. At this point, the input terminal of the inverter 101 rises to a high potential gradually, while the output terminal of the inverter 101 drops to a low potential gradually. When the charge for the capacitor C1 is finished, the transistor MC1 is turned off.
Referring to FIG. 2, the ESD clamp circuit 110 employs a capacitor-coupling-triggered architecture. When the ESD occurs at the power rail VDD, the ESD may be coupled to the gate of a transistor MC2 through a capacitor C2, and a voltage across the two ends of a resistor R2 is generated, so as to control an NMOS transistor MC2 which serves as a clamp device. At this point, the transistor MC2 is turned on to form a low-impedance path, so as to discharge the ESD current to the power rail VSS. During the discharging of the ESD current, through the discharge of the resistor R2, the gate voltage of the transistor MC2 is dropped gradually. Finally, the transistor MC2 is turned off because its gate voltage is pulled down to a low potential.
The clamp device can be implemented by a big field effect transistor (BIGFET). Because the BIGFET has a very large channel width, so it can produce a low enough turn-on resistance, thus discharging the ESD current to the power rail VSS rapidly. Referring to FIG. 1 and FIG. 2, in order to discharge the ESD current efficiently, the resistor R1-R2 and the capacitor C1-C2 have to employ a very large resistance value and a capacitance value, so as to enable the transistor MC1-MC2 to keep sufficient channel turn-on time to discharge the ESD current, that is, extending the time constant of the RC circuit. However, the RC circuit having overly large resistance value and capacitance value may result in the problem that the ESD clamp circuits 100 and 110 are liable to be triggered wrongly when they are suffering from large noises. Meanwhile, a quite large layout area is required when the RC circuit having overly large resistance value and capacitance value is applied in circuit layout.
Relevant examples of employing the RC time-delay-triggered architecture may be obtained with reference to “A Compact, Timed-shutoff, MOSFET-based Power Clamp for On-chip ESD Protection” published in EOS/ESD symp. (2004) by Junjun Li et al, pp. 273-279; U.S. Pat. No. 5,946,177 to James Wesley Miller et al; “Design and Characterization of a Multi-RC-triggered MOSFET-based Power Clamp for On-chip ESD Protection” published in EOS/ESD symp. (2006) by Junjun Li et al, pp. 179-185; and “ESD Protection for High-Voltage CMOS Technologies” published in EOS/ESD symp. (2006) by Olivier Quittard et al, pp. 77-86. Relevant examples of employing the capacitor-coupling-triggered architecture may be obtained with reference to U.S. Pat. No. 7,027,275 B2 to Jeremy C. Smith and U.S. Pat. No. 0,285,854 A1 to Thurman John Rodgers et al. It can be known from the above papers or patents that, in order to extend the time of turning on the clamp device, a manner of increasing the RC time constant of the detection circuit may be utilized. Alternatively, a scheme of making the resistors and capacitors charge and discharge through the control circuit can be utilized, so as to extend the control of the turn-on time of the clamp device channel. However, the manner may increase the risk of being wrongly triggered when the circuit is in a fast power-on, and may occupy a large layout area. In addition, utilizing a control circuit having a feedback scheme to implement the clamp circuit still has the risk of being wrongly triggered due to overly large power noise. Moreover, all of the ESD clamp circuits designed in the above-mentioned manners require additional devices to accomplish the design of the detection circuit, which may also occupy a certain layout area.
SUMMARY
The present disclosure is directed to an electrostatic discharge (ESD) clamp circuit, which utilizes a parasitic capacitor of a clamp device to implement a detection circuit, and extends a turn-on time of the clamp device by a control circuit with a feedback scheme, so as to decrease a circuit layout area.
Consistent with the invention, there is provided an ESD clamp circuit, which includes a first resistor, a second resistor, a first transistor, a second transistor, and a third transistor. A first end of the first resistor is coupled to a first rail, and a second end of the second resistor is coupled to a second rail. A control end of the first transistor is coupled to a first end of the second resistor, a first end of the first transistor is coupled to a second end of the first resistor, and a second end of the first transistor is coupled to the second rail. A control end of the second transistor is coupled to the second end of the first resistor, a first end of the second transistor is coupled to the first rail, and a second end of the second transistor is coupled to the first end of the second resistor. A control end of the third transistor is coupled to the first end of the second resistor, a first end of the third transistor is coupled to the first rail, and a second end of the third transistor is coupled to the second rail.
When an ESD occurs, the first transistor is turned on by utilizing a parasitic capacitor of the third transistor and the second resistor. The current generated after the first transistor is turned on flows through the first resistor, so as to generate a voltage drop to turn on the second transistor. The current generated by turning on the second transistor increases the voltage applied to turn on the first transistor and the third transistor through the second resistor. With such a feedback scheme, the voltage for turning on the third transistor is increased, the third transistor is turned on to discharge the ESD current during the ESD, and the channel turn-on time of the third transistor depends on the length of time that the ESD lasts. Meanwhile, such an ESD clamp circuit requires only a few of devices to implement, and can reduce the circuit layout area.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
FIG. 1-FIG. 2 are conventional ESD clamp circuit diagrams.
FIG. 3 is an ESD clamp circuit diagram according to an exemplary embodiment consistent with the present invention.
FIG. 4A is a signal simulation diagram when an ESD occurs to a power rail VDD in FIG. 3 and FIG. 1.
FIG. 4B and FIG. 4C are partial signal simulation diagrams when an ESD occurs to the power rail VDD of the ESD clamp circuit 300 in FIG. 3.
FIG. 4D is a partial signal simulation diagram when an ESD occurs to the power rail VDD of the ESD clamp circuit 100 in FIG. 1.
FIG. 5A is a signal simulation diagram when the power rail VDD in FIG. 3 and FIG. 1 is under fast power-on.
FIG. 5B and FIG. 5C are partial signal simulation diagrams when the ESD clamp circuit 300 in FIG. 3 is under fast power-on.
FIG. 5D is a partial signal simulation diagram when the ESD clamp circuit 100 in FIG. 1 is under fast power-on.
FIG. 6 is an ESD clamp circuit diagram according to an exemplary embodiment consistent with the present invention.
FIG. 7A is a signal simulation diagram when an ESD occurs to the power rail VDD in FIG. 6.
FIG. 7B and FIG. 7C are partial signal simulation diagrams when an ESD occurs to the ESD clamp circuit 400 in FIG. 6.
FIG. 8A is a signal simulation diagram when the power rail VDD in FIG. 6 is under fast power-on.
FIG. 8B and FIG. 8C are partial signal simulation diagrams when the ESD clamp circuit 400 in FIG. 6 is under fast power-on.
FIG. 9 is an ESD clamp circuit diagram according to an exemplary embodiment consistent with the present invention.
FIG. 10A is a signal simulation diagram when the power rail VDD in FIG. 9 is suffering from noise interference.
FIG. 10B and FIG. 10C are partial signal simulation diagrams when the ESD clamp circuit 301 in FIG. 9 is suffering from noise interference.
FIGS. 11-14 are ESD clamp circuit diagrams according to an embodiment of the present invention.
FIG. 15A is a circuit diagram of a first diode unit according to an exemplary embodiment consistent with the present invention.
FIG. 15B is a circuit diagram of a first diode unit according to another exemplary embodiment consistent with the present invention.



DESCRIPTION OF THE EMBODIMENTS
Reference will now be made in detail to these embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
FIG. 3 is an electrostatic discharge (ESD) clamp circuit diagram according to an exemplary embodiment consistent with the present invention. Referring to FIG. 3, an ESD clamp circuit 300 includes a first resistor R3, a second resistor R4, a first transistor MN2, a second transistor MP2, and a third transistor MC3. In this embodiment, the first transistor MN2 is an NMOS transistor, the second transistor MP2 is a PMOS transistor, the third transistor MC3 is an n-channel big field effect transistor (BIGFET), but the present invention is not so limited. A first end of the resistor R3 is coupled to a first rail (for example, a power rail VDD), a second end of the resistor R4 is coupled to a second rail (for example, a power rail VSS). Although the ESD clamp circuit 300 is arranged between the power rail VDD and VSS in this embodiment, those of ordinary skills in the art can derive applications to various rails according to the teachings of this embodiments.
A control end (for example, a gate) of the transistor MN2 is coupled to a first end of the resistor R4, a first end (for example, a drain) of the transistor MN2 is coupled to a second end of the resistor R3, and a second end (for example, a source) of the transistor MN2 is coupled to the power rail VSS. A control end (for example, a gate) of the transistor MP2 is coupled to the second end of the resistor R3, a first end (for example, a source) of the transistor MP2 is coupled to the power rail VDD, and a second end (for example, a drain) of the transistor MP2 is coupled to the first end of the resistor R4. A control end (for example, a gate) of the transistor MC3 is coupled to the first end of the resistor R4, a first end (for example, a drain) of the transistor MC3 is coupled to the power rail VDD, and the second end (for example, a source) of the transistor MC3 is coupled to the power rail VSS.
Because the transistor MC3 which serves as the clamp device has a relatively large channel width, a relatively large parasitic capacitor C3 exists between the gate and drain of the transistor MC3, and therefore, the parasitic capacitor C3 and the resistor R4 form a capacitor-coupling-triggered architecture. FIG. 4A is a signal simulation diagram when an ESD occurs to the power rail VDD in FIG. 3 and FIG. 1. Referring to FIG. 4A, assuming here that the power supply is not obtained, at a time point t1, the ESD occurs to the power rail VDD. At a time point t2, the voltage of the power rail VDD is pulled up to 3 V by the static electricity, with a rise time being 5 nanoseconds. At a time point t3, the ESD ends.
FIG. 4B and FIG. 4C are partial signal simulation diagrams when an ESD occurs to the power rail VDD of the ESD clamp circuit 300 in FIG. 3. Referring simultaneously to FIG. 3 and FIG. 4A-4C, when an ESD occurs to the power rail VDD (at the time point t1), the voltage at a node T2 is also pulled up instantly (as shown in FIG. 4C). When the voltage at the power rail VDD is pulled up quickly, the ESD voltage is coupled to the gate of the transistor MC3 via the parasitic capacitor C3, and at this point, the voltage of a node T1 also rises rapidly (as shown in FIG. 4B). The resistor R4 causes the voltage across the gate and the source of the transistor MN2 to be greater than its threshold voltage, resulting in that the transistor MN2 is turned on. After the transistor MN2 is turned on, the voltage of the node T2 is pulled down to a potential of the power rail VSS (assuming that it is 0 V), as shown in FIG. 4C. The current generated when the transistor MN2 is turned on flows through the resistor R3, thus generating a voltage across the source and the gate of the transistor MP2. When the voltage across the source and the gate of the transistor MP2 is greater than the threshold voltage of the transistor MP2, the transistor MP2 is turned on to pull up the gate voltage of the transistor MC3. When the voltage across the source and the gate of the transistor MC3 is greater than its threshold voltage VTHN, the transistor MC3 forms a low-impedance path, thus discharging the ESD current form the power rail VDD to the power rail VSS.
Likewise, because its gate voltage is pulled up, the transistor MN2 generates an even greater current flowing through the resistor R3, such that the voltage across the source and the gate of the transistor MP2 is much greater, which further pulls up the gate voltage of the transistor MC3, making the ESD current at the power rail VDD be discharged to the power rail VSS more rapidly. In other words, the resistor R3, the resistor R4, the transistor MN2, and the transistor MP2 form a feedback scheme, which turns on the transistor MC3 based on the occurrence of the ESD. That is to say, the length of occurrence time of the ESD determines the turn-on time of the transistor MC3.
When the ESD ends (at the time point t3), the voltage of the node T1 is lowered with the voltage of the power rail VDD (as shown in FIG. 4B). The voltage across the gate and the source of the transistor MN2 drops, such that the voltage across the gate and the source of the transistor MP2 also drops accordingly, and the channel of the transistor MC3 is turned off gradually. Finally, the transistors MN2, MP2 and MC3 cannot be maintained to be turned on, and at this point, the voltage levels of the two power rails VDD and VSS tend to be identical (that is, 0 V).
As for the conventional ESD clamp circuit, the channels of its transistors cannot maintain a state of being turned on for a long time during the occurrence of the ESD. FIG. 4D is a partial signal simulation diagram when an ESD occurs to the power ail VDD of the ESD clamp circuit 100 in FIG. 1. Referring simultaneously to FIG. 1, FIG. 4A, and FIG. 4D, when the voltage of the power rail VDD is pulled up to 3 V by the static electricity (at time points t1-t2), the resistor R1 causes that the input terminal of the inverter 101 is maintained at a low potential, and therefore, the voltage of the node T3 is pulled up to a high potential, that is, close to 3 V. After that, as the capacitor C1 starts to be charged, the voltage of the input terminal of the inverter 101 rises gradually, and the voltage of the node T3 drops gradually. When the voltage of the node T3 is lower than the threshold voltage VTHN of the transistor MC1, the channel of the transistor MC1 cannot be maintained a state of being turned on. Therefore, the conventional ESD clamp circuit 100 may turn off the transistor MC1 in advance before the ESD ends, such that the core circuit 103 is faced with the impact of the ESD. In other words, the time constant of the RC circuit in the conventional ESD clamp circuit 100 must be increased in order to discharge the ESD current efficiently.
FIG. 5A is a signal simulation diagram when the power rail VDD in FIG. 3 and FIG. 1 is under fast power-on. Referring to FIG. 5A, at a time-point t4, the power is initiated rapidly, and it is assumed that the voltage of the power rail VDD is pulled up to 1.2 V at a time point t5, with a rise time being 10 nanoseconds. FIG. 5B and FIG. 5C are partial signal simulation diagrams when the ESD clamp circuit 300 in FIG. 3 is under fast power-on. Referring to FIG. 3 and FIG. 5A-5C, when the voltage of the power rail VDD rises fast, with the coupling effect of the parasitic capacitor C3, the voltage of the node T1 also rises but still maintains below the threshold voltage VTHN of the transistor MC3 (as shown in FIG. 5B). Because the voltage of the node T1 is not sufficient to drive the transistor MN2, the transistor MN2 remains in a turn-off state, and the voltage of the node T2 may rise as the voltage of the power rail VDD increases (as shown in FIG. 5C). The voltage of the node T2 remains to be nearly identical to the voltage of the power rail VDD, such that the transistor MP2 remains in the turn-off state. After the power rail VDD rises to a stable voltage of 1.2 V, the voltage of node T1 trends toward the voltage level of the power rail VSS, that is, 0 V.
In comparison, referring to FIG. 1, FIGS. 5A and 5D, when the voltage of the power rail VDD rises fast, the voltage of the node T3 has exceeded the threshold voltage VTHN of the transistor MC1, resulting in that the transistor MC1 is triggered wrongly to be turned on. In other words, the time constant of the RC circuit must be reduced to effectively avoid the ESD clamp circuit 100 from being triggered wrongly. Therefore, in the case of fast power-on, the ESD clamp circuit 300 has the capability of avoiding being triggered wrongly.
FIG. 6 is an ESD clamp circuit diagram according to an exemplary embodiment consistent with the present invention. Referring to FIG. 6, an ESD clamp circuit 400 includes a first resistor R5, a second resistor R6, a first transistor MP3, a second transistor MN3, and a third transistor MC4. In this embodiment, the first transistor MP3 is a PMOS transistor, the second transistor MN3 is an NMOS transistor, the third transistor MC4 is a p-channel BIGFET, but the present invention is not so limited. A first end of the resistor R5 is coupled to a first rail (for example, a power rail VSS), a second end of the resistor R6 is coupled to a second rail (for example, a power rail VDD). A control end (for example, a gate) of the transistor MP3 is coupled to a first end of the resistor R6, a first end (for example, a drain) of the transistor MP3 is coupled to a second end of the resistor R5, and a second end (for example, a source) of the transistor MP3 is coupled to the power rail VDD. A control end (for example, a gate) of the transistor MN3 is coupled to the second end of the resistor R5, a first end (for example, a source) of the transistor MN3 is coupled to the power rail VSS, and a second end (for example, a drain) of the transistor MN3 is coupled to the first end of the resistor R6. A control end (for example, a gate) of the transistor MC4 is coupled to the first end of the resistor R6, a first end (for example, a drain) of the transistor MC4 is coupled to the power rail VSS, and a second end (for example, a source) of the transistor MC4 is coupled to the power rail VDD.
Among these, the transistor MC4 which serves as a clamp device has a relatively large channel width. Therefore, a relatively large parasitic capacitor C4 exists between the gate and the drain of the transistor MC4, and the resistor R6 and the parasitic capacitor C4 form a RC time-delay-triggered architecture. FIG. 7A is a signal simulation diagram when an ESD occurs to the power rail VDD in FIG. 6. Referring to FIG. 7A, assuming here that the power supply is not obtained, at a time point t6, the ESD occurs to the power rail VDD. At a time point t7, the voltage of the power rail VDD is pulled to 3 V by the static electricity, with a rise time being 5 nanoseconds. At a time point t8, the ESD ends.
FIG. 7B and FIG. 7C are partial signal simulation diagrams when an ESD occurs to the ESD clamp circuit 400 in FIG. 6. Referring simultaneously to FIG. 6 and FIG. 7A-7C, when the ESD occurs to the power rail VDD (at the time point t6), due to the delay effect of the RC circuit, the parasitic capacitor C4 makes the voltage of a node T5 at a low potential to turn on the transistor MP3. Therefore, during the time period between time points t6 to t6′, the voltages of both the nodes T4 and T5 may rise with the voltage level of the power rail VDD (as shown in FIG. 7B, FIG. 7C). The current generated by the turned-on transistor MP3 flows through the resistor R5, thus generating a voltage across the gate and the source of the transistor MN3. When the voltage (that is, the voltage of the node T4) exceeds the threshold voltage VTHN of the transistor MN3, that is, at the time point t6′, the transistor MN3 may be turned on, such that the voltage of the node T5 (that is, the gate voltage of the transistor MC4) is pulled down to be close to the voltage level of the power rail VSS (as shown in FIG. 7C). Therefore, the transistor MC4 forms a low-impedance path, thus discharging the ESD current from the power rail VDD to the power rail VSS.
The transistor MP3 generates an even greater current because its gate is at a low potential, such that the voltage across the gate and the source of the transistor MN3 is even greater, which further pulls down the gate of the transistor MC4, making the ESD current be discharged to the power rail VSS more rapidly. In other words, the resistor R5, the resistor R6, the transistor MN3, and the transistor MP3 form a feedback scheme, which turns on the transistor MC4 based on the occurrence of the ESD. That is to say, the length of occurrence time of the ESD determines the turn-on time of the transistor MC4.
When the ESD ends (at the time point t8), the voltage of the node T4 drops with the voltage of the power rail VDD (as shown in FIG. 7B). The voltage across the gate and the source of the transistor MN3 reduces, such that the voltage across the gate and the source of the transistor MP3 also reduces accordingly, thus turning off the channel of the transistor MC4 gradually. Finally, the transistors MP3, MN3 and MC4 cannot be maintained to be turned on, and at this point, the voltage levels of the two power rails VDD and VSS tend to be identical (that is, 0 V).
FIG. 8A is a signal simulation diagram when the power rail VDD in FIG. 6 is under fast power-on. Referring to FIG. 8A, assuming that the power is initiated fast at a time point t9, the voltage of the power rail VDD is pulled up to 1.2 V at a time point t10, with a rise time being 10 nanoseconds. FIG. 8B and FIG. 8C are partial signal simulation diagrams when the ESD clamp circuit 400 in FIG. 6 is under fast power-on. Referring simultaneously to FIG. 6 and FIG. 8A-8C, when the voltage of the power rail VDD rises fast (at time points t9-t10), because the voltage of the node T5 can meet up with the voltage of the power rail VDD (as shown in FIG. 8C), the channels of the transistor MP3 and the transistor MC4 are turned off. In the state of the transistor MP3 remaining to be turned off, the voltage of the node T4 may remain lower than the threshold voltage VTHN, thereby making the transistor MN3 remain to be turned off. Therefore, in the case of fast power-on, the ESD clamp circuit 400 has the capability of avoiding to be triggered wrongly.
In normal application, that is, in the case that the power rail VDD obtains the power supply, both the above ESD clamp circuits 300 and 400 have a considerable degree of noise interference resistance, and may not trigger the transistors MC3 and MC4 wrongly to cause short circuits of the two power rails VDD and VSS. However, when the ESD clamp circuit is triggered when suffering from very large noise interference (or ESD), after the noise interference disappears, a latch-on phenomenon may occur to the ESD clamp circuit. Referring to FIG. 6, assuming that the power rail VDD obtains the power supply, when the ESD clamp circuit 400 encounters noises, a voltage across the two ends of the resistor R6 is generated to turn on the transistor MP3, and at this point, the transistor MN3 is turned on according to the feedback scheme of the ESD clamp circuit 400, and the transistor MC4 is driven to be turned on. When the noises disappear, the voltage of the power rail VDD can support the continuous operation of the feedback scheme, thus resulting in that the transistor MC4 continues to be in a turn-on state, which is the latch-on phenomenon.
FIG. 9 is an ESD clamp circuit diagram according to an exemplary embodiment consistent with the present invention. Referring to FIG. 3 and FIG. 9, the ESD clamp circuit 301 is similar to the ESD clamp circuit 300, so only the differences between them are illustrated below. Referring to FIG. 9, the ESD clamp circuit 301 further includes a first diode unit 311. In this embodiment, the diode unit 311 is a diode string of two diodes D1 and D2 connected in series, but the present invention is not so limited. An anode end of the diode D1 is coupled to the second end of the resistor R3, a cathode end of the diode D1 is coupled to an anode end of the diode D2, and a cathode end of the diode D2 is coupled to the drain of the transistor MN2. Those of ordinary skills in the art can modify the number of the diodes correspondingly according to the specification of this embodiment in actual application. For example, assuming that the turn-on voltage for the diodes D1 and D2 is 0.6 V, then the turn-on voltage for the diode unit 311 is 1.2 (0.6×2) V. When a forward bias greater than 1.2 V is applied to the two ends of the diode unit 311, the diode unit 311 may be turned on. In this embodiment, the turn-on voltage for the diode unit 311 is greater than or equal to the working voltage of the power rail VDD. How to utilize such a physical characteristic to avoid latch-on phenomenon is illustrated below.
FIG. 10A is a signal simulation diagram when the power rail VDD in FIG. 9 is suffering from noise interference. Assuming here that the power rail VDD is suffering from noise interference at a time point t11, the voltage of the power rail VDD is pulled up from 1.2 V to 3 V (at a time point t12), with a voltage rise time being 3 nanoseconds. FIGS. 10B and 10C are partial signal simulation diagrams when the ESD clamp circuit 301 in FIG. 9 is suffering from noise interference. When the ESD clamp circuit 301 is triggered to operate when suffering from noise interference, the feedback scheme of the ESD clamp circuit 301 is initiated due to the coupling effect of the capacitor C3. During the time points t11-t12, when the voltage of a node T7 is greater than the turn-on voltage for the diode unit 311 (as shown in FIG. 10C), the voltage of the node T6 also rises accordingly (as shown in FIG. 10B) to drive the transistor MC3. During the time points t12-t13, by utilizing the forward bias physical characteristic of the diode unit 311, the voltage of the node T7 may be maintained approximately around 1.5 V.
After the noise disappears (at time point t13), the voltage of the power rail VDD returns to 1.2 V, and at this point, with the characteristic of the diode unit 311, the voltage of the node T7 may be approximately equal to the voltage of the power rail VDD. Because the voltage across the source and the gate of the transistor MP2 after the voltage of the power rail VDD returns to 1.2 V is lower than its threshold voltage, the transistor MP2 may be turned off, thereby pulling down the voltage of the node T6 (as shown in FIG. 10B). Therefore, after the noise disappears, the channel of the transistor MC3 is also turned off accordingly.
According to the above illustration, the ESD clamp circuit described in the present invention can be implemented in other ways. FIG. 11 is an ESD clamp circuit diagram according to an embodiment of the present invention. Referring to FIG. 3 and FIG. 11, the operation of the ESD clamp circuit 303 is similar to that of the ESD clamp circuit 300. Referring to FIG. 11, the ESD clamp circuit 303 further includes a second diode unit 313. The diode unit 313 is a diode string of two diodes D3 and D4 connected in series, but the present invention is not so limited. An anode end of the diode D3 is coupled to the drain of the transistor MP2, a cathode end of the diode D3 is coupled to an anode end of the diode D4, and a cathode end of the diode D4 is coupled to the first end of the resistor R4. When the ESD clamp circuit 303 is triggered to operate due to suffering from noise interference, the transistor MC3 is driven to be turned on. When the noise disappears, the gate voltage of the transistor MN2 drops to be lower than its threshold voltage due to the voltage drop of the diode unit 313. The transistor MN2 is turned off because its gate voltage is lower than the threshold voltage, such that the feedback scheme of the ESD clamp circuit 303 cannot maintain its operation, thus turning off the transistor MC3. Therefore, the ESD clamp circuit 303 can return to a state before being triggered.
FIG. 12 is an ESD clamp circuit diagram according to an embodiment of the present invention. Referring to FIG. 3 and FIG. 12, the operation of the ESD clamp circuit 305 is similar to that of the ESD clamp circuit 300. Referring to FIG. 12, the ESD clamp circuit 305 further includes a first diode unit 311 and a second diode unit 313. When the ESD clamp circuit 305 is triggered to operate due to suffering from noise interference, the transistor MC3 is driven to be turned on. When the noise disappears, with the characteristics of the diode units 311 and 313, the gate voltage of the transistor MP2 may be close to the voltage of the power rail VDD, and the gate voltage of the transistor MN2 may close to the voltage of the power rail VSS. The transistors MN2 and MP2 are turned off because the voltages across their gates and sources are lower than the threshold voltage, so the feedback scheme of the ESD clamp circuit 305 cannot maintain its operation, and the channel of the transistor MC3 is also turned off accordingly.
FIG. 13 is an ESD clamp circuit diagram according to an exemplary embodiment consistent with the present invention. Referring to FIG. 6 and FIG. 13, the operation of an ESD clamp circuit 401 is similar to that of the ESD clamp circuit 400. Referring to FIG. 13, the ESD clamp circuit 401 further includes a first diode unit 411 and a second diode unit 413. The diode unit 411 is a diode string of two diodes D5 and D6 connected in series, and the diode unit 413 is a diode string of two diodes D7 and D8 connected in series, but the present invention is not so limited. An anode end of the diode D5 is coupled to the drain of the transistor MP3, a cathode end of the diode D5 is coupled to an anode end of the diode D6, and a cathode end of the diode D6 is coupled to the second end of the resistor R5. An anode end of the diode D7 is coupled to the first end of the resistor R6, a cathode end of the diode D7 is coupled to an anode end of the diode D8, and a cathode end of the diode D8 is coupled to the drain of the transistor MN3.
After the ESD clamp circuit 401 is triggered when suffering from noise interference, the transistor MC4 is turned on to release the noise. When the noise disappears, with the characteristics of the diode units 411 and 413, the gate voltage of the transistor MP3 can be close to the voltage of the power rail VDD, and the gate voltage of the transistor MN3 can be close to the voltage of the power rail VSS. The transistors MN3 and MP3 are turned off because the voltages across their gates and sources are lower than the threshold voltage, so the feedback scheme of the ESD clamp circuit 401 cannot maintain its operation, and the channel of the transistor MC4 is also turned off accordingly.
In other embodiments, one of the diode units 411 and 413 in the ESD clamp circuit 401 can be omitted. The number of the diodes in the diode units 411 and 413 can be determined based on design requirements.
FIG. 14 is an ESD clamp circuit diagram according to an exemplary embodiment consistent with the present invention. Referring to FIG. 12 and FIG. 14, the operation of an ESD clamp circuit 500 is similar to that of the ESD clamp circuit 305. Referring to FIG. 14, the ESD clamp circuit 500 further includes a fourth transistor MC5. In this embodiment, the fourth transistor MC5 is a p-channel BIGFET, but the present invention is not so limited. A first end (for example, a source) of the transistor MC5 is coupled to the power rail VDD, a second end (for example, a drain) of the transistor MC5 is coupled to the power rail VSS, and a control end (for example, a gate) of the transistor MC5 is coupled to the second end of the resistor R3. Among these, the transistor MC5 which serves as a clamp device has a relatively large channel width, and therefore, a relatively large parasitic capacitor C5 exists between the gate and the drain of the transistor MC5. The resistor R3 and the parasitic capacitor C5 form a RC time-delay-triggered architecture, which can enhance the detection of the occurrence of the ESD, and discharge the ESD current to the power rail VSS. For example, when an event of ESD occurs, the ESD clamp circuit 500 can be triggered to operate, the feedback scheme formed by the resistor R3, the resistor R4, the transistor MN2, and the transistor MP2 begins to operate, such that the transistor MC3 and the transistor MC5 can be turned on to discharge the ESD current.
After the ESD clamp circuit 500 is triggered wrongly due to suffering from excessive noise interference, the feedback scheme formed by the resistor R3, the resistor R4, the transistor MN2, and the transistor MP2 begins to operate, such that the transistors MC3 and the transistor MC5 are driven to be turned on. When the noise disappears, with the characteristics of the diode unit 311 and the diode unit 313, the gate voltage of the transistor MP2 may be close to the voltage of the power rail VDD, and the gate voltage of the transistor MN2 may be close to the voltage of the power rail VSS. The transistors MN2 and MP2 are turned off because the voltages across their gates and sources are lower than the threshold voltage, so the feedback scheme of the ESD clamp circuit 500 can be stopped, and the channels of the transistor MC3 and the transistor MC5 are turned off accordingly.
In other embodiments, one of the diode units 311 and 313 in the ESD clamp circuit 500 can be omitted. Alternatively, both the diode units 311 and 313 can be omitted.
Moreover, in certain embodiments, the diode units shown in FIGS. 9, 11, 12, 13, and 14 can be implemented in other ways. For example, a diode connection structure formed by connecting transistors is used to implement the diodes shown in FIG. 9, 11, 12, 13, or 14. The diode unit 311 in FIG. 12 is taken as an example to be illustrated, and the diode unit 313 or the diode units shown in other drawings can be implemented with reference to the illustration of the diode unit 311.
FIG. 15A is a circuit diagram illustrating the diode unit 311 in FIG. 12 according to an exemplary embodiment consistent with the present invention. Referring to FIG. 15A, the diode unit 311 includes the diodes D1 and D2. In this embodiment, a diode connection structure is formed by connecting NMOS transistors Q1 and Q2 to implement the diodes D1 and D2 respectively. That is to say, a first end (for example, a drain) of the transistor Q1 serves as a first end (for example, an anode) of the diode D1, a second end (for example, a source) of the transistor Q1 serves as a second end (for example, a cathode) of the diode D1, and a control end (for example, a gate) of the transistor Q1 is coupled to the drain of the transistor Q1. Similarly, a first end (for example, a drain) of the transistor Q2 serves as a first end (for example, an anode) of the diode D2, a second end (for example, a source) of the transistor Q2 serves as a second end (cathode) of the diode D2, and a control end (for example, a gate) of the transistor Q2 is coupled to the drain of the transistor Q2.
FIG. 15B is a circuit diagram of the diode unit 311 according to another exemplary embodiment consistent with the present invention. Referring to FIG. 15B, the diode unit 311 includes the diodes D3 and D4. In this embodiment, a diode connection structure is formed by connecting PMOS transistors Q3 and Q4 to implement the diodes D1 and D2 respectively. That is to say, a first end (for example, a drain) of the transistor Q3 serves as the first end (for example, the cathode) of the diode D1, a second end (for example, a source) of the transistor Q3 serves as the second end (for example, the anode) of the diode D1, and a control end (for example, a gate) of the transistor Q3 is coupled to the drain of the transistor Q3. Moreover, a first end (for example, a drain) of the transistor Q4 serves as the first end (for example, the cathode) of the diode D2, a second end (for example, a source) of the transistor Q4 serves as the second end (anode) of the diode D2, and a control end (for example, a gate) of the transistor Q4 is coupled to the drain of the transistor Q4.
Therefore, those of ordinary skills in the art can modify the first diode unit 311, the second diode unit 313, the first diode unit 411, and the second diode unit 413 in FIG. 9 and FIG. 11-FIG. 14 correspondingly based on the illustration in FIG. 15A or FIG. 15B.
In view of the above, taking the parasitic capacitor of the third transistor and the second resistor as a detection scheme, when it is detected that an ESD occurs to a first rail, the feedback scheme formed by the first resistor, the second resistor, the first transistor, and the second transistor is triggered to turn on the third transistor which serves as the clamp device, so as to discharge the ESD current from the first rail to a second rail, thus avoiding the damage to the core circuit by the ESD current. Meanwhile, such a feedback scheme has a capability of resisting being triggered wrongly when under fast power-on. The parasitic capacitor of the fourth transistor and the first resistor can also serve as a detection scheme, and can accelerate the discharging of the ESD current. When the ESD clamp circuit is suffering from noise interference, the latch-on phenomenon can be avoided through the first diode unit and the second diode unit. Moreover, the ESD clamp circuit in this embodiment only needs a small number of devices to implement, thus reducing the area of circuit layout.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.

## Claims

1. An electrostatic discharge (ESD) clamp circuit, comprising:
a first resistor, having a first end coupled to a first rail;
a second resistor, having a second end coupled to a second rail;
a first transistor, wherein a control end of the first transistor is coupled to a first end of the second resistor, and a second end of the first transistor is coupled to the second rail;
a first diode unit, having two ends coupled to a second end of the first resistor and a first end of the first transistor respectively;
a second transistor, wherein a control end of the second transistor is coupled to the second end of the first resistor, a first end of the second transistor is coupled to the first rail, and a second end of the second transistor is coupled to the first end of the second resistor; and
a third transistor, wherein a control end of the third transistor is coupled to the first end of the second resistor, a first end of the third transistor is coupled to the first rail, and a second end of the third transistor is coupled to the second rail.

2. The ESD clamp circuit according to claim 1, wherein the first transistor is an N-channel metal oxide semiconductor (NMOS) transistor, the second transistor is a P-channel metal oxide semiconductor (PMOS) transistor, and the third transistor is an NMOS transistor.

3. The ESD clamp circuit according to claim 1, wherein the first transistor is a PMOS transistor, the second transistor is an NMOS transistor, and the third transistor is a PMOS transistor.

4. The ESD clamp circuit according to claim 1, wherein the third transistor is a big field effect transistor (BIGFET).

5. The ESD clamp circuit according to claim 1, further comprising:
a fourth transistor, wherein a control end of the fourth transistor is coupled to the second end of the first resistor, a first end of the fourth transistor is coupled to the first rail, and a second end of the fourth transistor is coupled to the second rail.

6. The ESD clamp circuit according to claim 5, wherein the first transistor is an NMOS transistor, the second transistor is a PMOS transistor, the third transistor is an NMOS transistor, and the fourth transistor is a PMOS transistor.

7. The ESD clamp circuit according to claim 5, wherein the third transistor and the fourth transistor are BIGFETs.

8. The ESD clamp circuit according to claim 5, further comprising:
a second diode unit, coupled between the second end of the second transistor and the first end of the second resistor.

9. The ESD clamp circuit according to claim 8, wherein the second diode unit comprises a second diode string formed by a plurality of diodes connected in series, and two ends of the second diode string are coupled to the second end of the second transistor and the first end of the second resistor respectively.

10. The ESD clamp circuit according to claim 9, wherein the diodes are implemented by transistors.

11. The ESD clamp circuit according to claim 1, wherein the first diode unit comprises a first diode string formed by a plurality of diodes connected in series, and two ends of the first diode string are coupled to the first end of the first transistor and the second end of the first resistor respectively.

12. The ESD clamp circuit according to claim 11, wherein the diodes are implemented by transistors.

13. The ESD clamp circuit according to claim 1, further comprising:
a second diode unit, coupled between the second end of the second transistor and the first end of the second resistor.

14. The ESD clamp circuit according to claim 13, wherein the second diode unit comprises a second diode string formed by a plurality of diodes connected in series, and two ends of the second diode string are coupled to the second end of the second transistor and the first end of the second resistor respectively.

15. The ESD clamp circuit according to claim 14, wherein the diodes are implemented by transistors.

16. The ESD clamp circuit according to claim 1, wherein the first rail and the second rail are power rails.

17. An electrostatic discharge (ESD) clamp circuit, comprising:
a first resistor, having a first end coupled to a first rail;
a second resistor, having a second end coupled to a second rail;
a first transistor, wherein a control end of the first transistor is coupled to a first end of the second resistor, a first end of the first transistor is coupled to a second end of the first resistor, and a second end of the first transistor is coupled to the second rail;
a second transistor, wherein a control end of the second transistor is coupled to the second end of the first resistor, and a first end of the second transistor is coupled to the first rail; and
a second diode unit, having two ends coupled to the first end of the second resistor and a second end of the second transistor respectively;
a third transistor, wherein a control end of the third transistor is coupled to the first end of the second resistor, a first end of the third transistor is coupled to the first rail, and a second end of the third transistor is coupled to the second rail.

18. The ESD clamp circuit according to claim 17, wherein the first transistor is an N-channel metal oxide semiconductor (NMOS) transistor, the second transistor is a P-channel metal oxide semiconductor (PMOS) transistor, and the third transistor is an NMOS transistor.

19. The ESD clamp circuit according to claim 17, wherein the first transistor is a PMOS transistor, the second transistor is an NMOS transistor, and the third transistor is a PMOS transistor.

20. The ESD clamp circuit according to claim 17, wherein the third transistor is a big field effect transistor (BIGFET).

21. The ESD clamp circuit according to claim 17, further comprising:
a fourth transistor, wherein a control end of the fourth transistor is coupled to the second end of the first resistor, a first end of the fourth transistor is coupled to the first rail, and a second end of the fourth transistor is coupled to the second rail.

22. The ESD clamp circuit according to claim 21, wherein the first transistor is an NMOS transistor, the second transistor is a PMOS transistor, the third transistor is an NMOS transistor, and the fourth transistor is a PMOS transistor.

23. The ESD clamp circuit according to claim 21, wherein the third transistor and the fourth transistor are BIGFETs.

24. The ESD clamp circuit according to claim 21, further comprising:
a first diode unit, having two ends coupled to the second end of the first resistor and the first end of the first transistor respectively.

25. The ESD clamp circuit according to claim 24, wherein the first diode unit comprises a first diode string formed by a plurality of diodes connected in series, and two ends of the first diode string are coupled to the first end of the first transistor and the second end of the first resistor respectively.

26. The ESD clamp circuit according to claim 25, wherein the diodes are implemented by transistors.

27. The ESD clamp circuit according to claim 17, further comprising:
a first diode unit, coupled between the second end of the first resistor and the first end of the first transistor.

28. The ESD clamp circuit according to claim 27, wherein the first diode unit comprises a first diode string formed by a plurality of diodes connected in series, and two ends of the first diode string are coupled to the first end of the first transistor and the second end of the first resistor respectively.

29. The ESD clamp circuit according to claim 28, wherein the diodes are implemented by transistors.

30. The ESD clamp circuit according to claim 17, wherein the second diode unit comprises a second diode string formed by a plurality of diodes connected in series, and two ends of the second diode string are coupled to the second end of the second transistor and the first end of the second resistor respectively.

31. The ESD clamp circuit according to claim 30, wherein the diodes are implemented by transistors.

32. The ESD clamp circuit according to claim 17, wherein the first rail and the second rail are power rails.