PATENT ABSTRACT
Circuitry on integrated circuits usually includes protection against electrostatic discharge (ESD) events. A second ESD current path may be provided in addition to a first ESD current path for shunting ESD current away from circuitry to be protected during an ESD event. In addition to the standard power and ground buses used to provide power and ground voltages to the protected circuitry, one or more extra power and/or ground buses and associated circuitry may be added for improved ESD protection.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is related to U.S. patent application Ser. No. 11/513,638, filed on Aug. 31, 2006, entitled “DISTRIBUTED ELECTROSTATIC DISCHARGE PROTECTION CIRCUIT WITH VARYING CLAMP SIZE,” and assigned to the current assignee hereof. 
     This application is related to U.S. patent application Ser. No. 11/056,617, filed on Feb. 11, 2005, entitled “I/O CELL ESD SYSTEM,” and assigned to the current assignee hereof. 
     This application is related to U.S. patent application Ser. No. 10/914,442, filed on Aug. 9, 2004, entitled “ELECTROSTATIC DISCHARGE PROTECTION FOR AN INTEGRATED CIRCUIT,” and assigned to the current assignee hereof. 
     BACKGROUND 
     1. Field 
     This disclosure relates generally to circuits, and more specifically, to a circuit and method for reducing potential damage to an integrated circuit during an electrostatic discharge event. 
     2. Related Art 
     This disclosure relates generally to circuits, and more specifically, to a circuit and method for reducing potential damage to an integrated circuit during an electrostatic discharge event. An integrated circuit can be damaged when subjected to an overvoltage transient that is higher than the design voltage of the integrated circuit. Electrostatic discharge (“ESD”), originating from such sources as a mechanical chip carrier, a plastic chip storage device, or even a human being can generate a voltage that is many times greater than the design voltage of the integrated circuit. For example, the typical human body can supply an electrostatic discharge of 4 kilovolts or more. For integrated circuits that operate at voltages of less than, for example, 5V (volts), an electrostatic discharge of such proportions can be devastating. In order to protect the internal circuitry on integrated circuits from high voltage, or ESD events, protection circuits are utilized, generally between the internal circuitry and the input/output (“I/O”) terminals (e.g. pads, pins, bumps, etc.) of the integrated circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. 
         FIG. 1  illustrates, in partial schematic diagram form and partial block diagram form, a circuit in accordance with one embodiment of the present invention. 
         FIG. 2  illustrates, in partial schematic diagram form and partial block diagram form, a circuit in accordance with another embodiment of the present invention. 
         FIG. 3  illustrates, in partial schematic diagram form and partial block diagram form, a circuit in accordance with an alternate embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As semiconductor devices are becoming smaller and more fragile, the maximum ESD voltage that protected circuitry can withstand without incurring damage decreases. One way to handle this problem is to provide ESD protection circuitry that can reduce the voltage which must be handled by the increasingly fragile protected circuitry. Thus, it is desired to have ESD protection circuitry that reduces the voltage which must be handled by the increasingly fragile protected circuitry. 
     As used herein, the term “bus” refers to one or more conductors that deliver power (e.g. OVDD, OVSS) or other signals (e.g. TRIGGER) to multiple circuit elements. In some embodiments, one or more buses may be routed to all or a portion of the input/output (I/O) circuitry and to all or a portion of the ESD circuitry. In some embodiments, one or more buses may be routed overlying a plurality of integrated circuit pad cells, wherein the pad cells each comprise I/O circuitry and ESD circuitry associated with an I/O pad. As used herein, the terms “power supply node”, “power bus” and “power supply conductor” may be used interchangeably. 
       FIG. 1  illustrates a circuit  10  in accordance with one embodiment of the present invention. In the illustrated embodiment, circuit  10  comprises circuits  39  and  59  on an integrated circuit that are to be protected from ESD events. Although circuits  39  and  59  are illustrated as being output buffers comprising transistors  90 ,  91 ,  92 , and  93 , alternate embodiments may use any type of one or more circuits within the circuit  39  or  59  which are to be protected from ESD events. For example, in alternate embodiments, circuit  39  or  59  may be an input/output buffer, an input buffer, an analog circuit, or any desired type of circuit or combination of circuits. 
     In the illustrated embodiment, circuit  10  has a BOOST bus  12 , a TRIGGER bus  14 , a first power bus OVDD  16 , a second power bus OVSS  18 , a third power bus OVDD_ 2   20 , a fourth power bus OVSS_ 2   22 , and a fifth power bus VSS  24 . In one embodiment, OVDD  16  and OVSS  18  are used to provide the primary power to circuit  10 . In one embodiment, OVDD  16  provides a first power supply voltage and OVSS  18  provides a second power supply voltage that is less than the first power supply voltage. In some embodiments, the second power supply voltage equals approximately ground. In one embodiment, circuits  39  and  59  are coupled to OVDD  16  and to OVSS  18 . 
     In the illustrated embodiment, circuit  39  comprises a p-channel MOSFET (metal oxide semiconductor field effect) transistor  90  having a first current electrode coupled to OVDD  16 , having a second current electrode, and having a control electrode coupled to receive an input from other circuitry or devices (not shown). In the illustrated embodiment, circuit  39  also comprises an n-channel MOSFET transistor  91  having a first current electrode coupled to the second current electrode of transistor  90 , having a second current electrode coupled to OVSS  18 , and having a control electrode coupled to receive an input from other circuitry or devices (not shown). 
     In addition, circuit  10  has a diode  33  having a first current electrode coupled to OVDD_ 2   20 , and having a second current electrode coupled to the second current electrode of transistor  90 . Circuit  10  also has a diode  35  having a first current electrode coupled to the second current electrode of transistor  90 , and having a second current electrode coupled to OVSS_ 2   22 . A resistive element  40  has a first terminal coupled to the second current electrode of transistor  90 , and has a second terminal coupled to I/O pad  30 . Circuit  10  also has a diode  31  having a first current electrode coupled to OVDD  16 , and having a second current electrode coupled to I/O pad  30 . Circuit  10  also has a diode  32  having a first current electrode coupled to BOOST  12 , and having a second current electrode coupled to I/O pad  30 . Circuit  10  also has a diode  34  having a first current electrode coupled to I/O pad  30 , and having a second current electrode coupled to OVSS  18 . 
     Also, circuit  10  has an n-channel MOSFET transistor  36  having a first current electrode coupled to OVDD  16 , having a second current electrode coupled to OVSS  18 , and having a control electrode coupled to TRIGGER  14 . Circuit  10  also has an n-channel MOSFET transistor  37  having a first current electrode coupled to OVDD_ 2   20 , having a second current electrode coupled to OVSS  18 , and having a control electrode coupled to TRIGGER  14 . Circuit  10  also has an n-channel MOSFET transistor  38  having a first current electrode coupled to OVDD  16 , having a second current electrode coupled to OVSS_ 2   22 , and having a control electrode coupled to TRIGGER  14 . Circuit  10  also has a trigger circuit  41  which is coupled to BOOST  12 , to TRIGGER  14 , and to OVSS  18 . 
     In one embodiment, circuit  10  has an equalizer circuit  45 . Alternate embodiments may use a different equalizer circuit or may not even use an equalizer circuit. In the illustrated embodiment, equalizer  45  has a p-channel MOSFET transistor  42  having a first current electrode coupled to OVDD  16 , having a second current electrode coupled to OVDD_ 2   20 , and having a control electrode coupled to TRIGGER  14 . In the illustrated embodiment, equalizer  45  also has an inverter  44  having an input coupled to TRIGGER  14  and having an output. Equalizer  45  also has an n-channel MOSFET transistor  43  having a first current electrode coupled to OVSS_ 2   22 , having a second current electrode coupled to OVSS  18 , and having a control electrode coupled to the output of inverter  44 . 
     In addition, circuit  10  has a diode  70  having a first current electrode coupled to OVSS  18 , and having a second current electrode coupled to VSS  24 . Also, circuit  10  has a diode  72  having a first current electrode coupled to OVSS  18 , and having a second current electrode coupled to VSS  24 . Alternate embodiments may not use diodes  70  and/or  72 , or may use different circuitry instead of diodes  70  and/or  72 . 
     In the illustrated embodiment, circuit  59  comprises a p-channel MOSFET transistor  92  having a first current electrode coupled to OVDD  16 , having a second current electrode, and having a control electrode coupled to receive an input from other circuitry or devices (not shown) either on the same integrated circuit or from external to the integrated circuit. In the illustrated embodiment, circuit  59  also comprises an n-channel MOSFET transistor  93  having a first current electrode coupled to the second current electrode of transistor  92 , having a second current electrode coupled to OVSS  18 , and having a control electrode coupled to receive an input from other circuitry or devices (not shown) either on the same integrated circuit or from external to the integrated circuit. 
     In addition, circuit  10  has a diode  53  having a first current electrode coupled to OVDD_ 2   20 , and having a second current electrode coupled to the second current electrode of transistor  92 . Circuit  10  also has a diode  55  having a first current electrode coupled to the second current electrode of transistor  92 , and having a second current electrode coupled to OVSS_ 2   22 . A resistive element  60  has a first terminal coupled to the second current electrode of transistor  92 , and has a second terminal coupled to I/O pad  50 . Circuit  10  also has a diode  51  having a first current electrode coupled to OVDD  16 , and having a second current electrode coupled to I/O pad  50 . Circuit  10  also has a diode  52  having a first current electrode coupled to BOOST  12 , and having a second current electrode coupled to I/O pad  50 . Circuit  10  also has a diode  54  having a first current electrode coupled to I/O pad  50 , and having a second current electrode coupled to OVSS  18 . 
     Also, circuit  10  has an n-channel MOSFET transistor  56  having a first current electrode coupled to OVDD  16 , having a second current electrode coupled to OVSS  18 , and having a control electrode coupled to TRIGGER  14 . Circuit  10  also has an n-channel MOSFET transistor  57  having a first current electrode coupled to OVDD_ 2   20 , having a second current electrode coupled to OVSS  18 , and having a control electrode coupled to TRIGGER  14 . Circuit  10  also has an n-channel MOSFET transistor  58  having a first current electrode coupled to OVDD  16 , having a second current electrode coupled to OVSS_ 2   22 , and having a control electrode coupled to TRIGGER  14 . 
     An example ESD event will now be described in order to discuss the functionality of circuit  10  of  FIG. 1 . During an ESD event occurring on I/O pads  30  and  50 , where I/O pad  30  is at a higher potential than I/O pad  50 , the ESD protection circuitry provides a low resistance path between I/O pads  30  and  50  so that circuits  39  and  59  are not damaged by the ESD event. During such an ESD event, the primary ESD current flows from I/O pad  30  through primary diode  31  to OVDD power bus  16 , from OVDD power bus  16  through one or more primary rail clamps (e.g.  36 ,  56 ) to OVSS power bus  18 , and from OVSS power bus  18  through primary diode  54  to I/O pad  50 . Assuming that diodes  33  and  35  are not used and resistor  40  is replaced by a short, note that the ESD stress voltage on device  91  is approximately equal to the voltage across diode  31  and rail clamp  36 . This ESD stress voltage on device  91  may exceed the maximum voltage that the device can withstand without incurring damage. Similarly, assuming that diodes  53  and  55  are not used and resistor  60  is replaced by a short, the ESD stress voltage on device  92  is approximately equal to the voltage across diode  54  and rail clamp  56 . Likewise, this ESD stress voltage on device  92  may exceed the maximum voltage that the device can withstand without incurring damage. 
     In order to better protect devices contained within circuitry  39  and  59 , a secondary ESD protection network or circuitry is added. In one embodiment, one or more additional power busses (e.g. OVDD_ 2   20 , OVSS_ 2   22 ), and/or one or more rail clamps (e.g.  37 ,  38 ,  57 ,  58 ), and/or one or more secondary diodes (e.g.  33 ,  35 ,  53 ,  55 ), and/or one or more resistive elements (e.g.  40 ,  60 ) may be added to form this secondary ESD protection network. In the embodiment illustrated in  FIG. 1 , the primary ESD protection network or circuitry comprises power busses (e.g. OVDD  16 , OVSS  18 ), and/or BOOST bus  12 , and/or TRIGGER bus  14 , and/or one or more rail clamps (e.g.  36 ,  56 ), and/or one or more primary diodes (e.g.  31 ,  34 ,  51 ,  54 ), and/or one or more boost diodes (e.g.  32 ,  52 ) and/or one or more trigger circuits (e.g.  41 ). 
     During the example ESD event described above, in one embodiment, the secondary ESD protection network provides a secondary ESD current path with an ESD current that may be lower than the ESD current of the primary ESD current path as provided by the primary ESD protection network. The secondary ESD protection network further reduces the voltage stress across devices  91  and  92 . During such an ESD event, a first portion of the secondary ESD current used to further protect device  91  flows from I/O pad  30  through resistive element  40  and diode  33  to OVDD_ 2  power bus  20 , from OVDD_ 2  power bus  20  through one or more secondary rail clamps (e.g.  37 ,  57 ) to OVSS power bus  18 , from OVSS power bus  18  through diode  54  to I/O pad  50 . In the illustrated embodiment, due in part to the resistance provided by resistive element  40 , the current flowing through this secondary ESD path is lower than the current flowing through the primary ESD path. As a result, the voltage drop across secondary diode  33  and secondary clamp  37  is lower than the voltage drop across primary diode  31  and primary clamp  36 . Note that for the illustrated embodiment, the voltage drop across secondary diode  33  and secondary clamp  37  is approximately equal to the ESD stress voltage on device  91  with the secondary ESD protection network being used. On the other hand, the voltage drop across primary diode  31  and primary clamp  36  is approximately equal to the ESD stress voltage on device  91  without the secondary ESD protection network being used. Thus in the illustrated embodiment, the voltage drop across resistive element  40  effectively reduces the ESD stress voltage on device  91 . Note that increasing the size of elements  31 ,  36 , and/or  56  may alternately be used to reduce the voltage across protected device  91 ; however, the semiconductor area required to do this may be prohibitively large. Thus, for many embodiments, the secondary ESD protection network provides an advantageous solution for improving ESD protection while using a minimum amount of additional semiconductor area. 
     During the same ESD event, a second portion of the secondary ESD current used to further protect device  92  flows from I/O pad  30  through diode  31  to OVDD power bus  16 , from OVDD power bus  16  through one or more secondary rail clamps (e.g.  38 ,  58 ) to OVSS_ 2  power bus  22 , from OVSS_ 2  power bus  22  through diode  55  and resistive element  60  to I/O pad  50 . Due to the resistance provided by resistive element  60 , the current flowing through this secondary ESD path is lower than the current flowing through the primary ESD path. As a result, the voltage drop across secondary diode  55  and secondary clamp  58  is lower than the voltage drop across primary diode  54  and primary clamp  56 . Note that for the illustrated embodiment, the voltage drop across secondary diode  55  and secondary clamp  58  is approximately equal to the ESD stress voltage on device  92  with the secondary ESD protection network being used. On the other hand, the voltage drop across primary diode  54  and primary clamp  56  is approximately equal to the ESD stress voltage on device  92  without the secondary ESD protection network being used. Thus in the illustrated embodiment, the voltage drop across resistive element  60  effectively reduces the ESD stress voltage on device  92 . Note that increasing the size of elements  54 ,  36 , and/or  56  may alternately be used to reduce the voltage across protected device  92 ; however, the semiconductor area required to do this may be prohibitively large. Thus, for many embodiments, the secondary ESD protection network provides an advantageous solution for improving ESD protection while using a minimum amount of additional semiconductor area. 
     Note that in one embodiment where the protected circuitry  39  and  59  are output buffers, resistive elements  40  and  60  need to be low enough so that the performance (e.g. switching speed, drive strength) of the output buffers  39 ,  59  is not seriously impacted. For one embodiment, resistive elements  40  and  60  each have a resistive value in a range of approximately 1-20 ohms. In alternate embodiments where the protected circuitry  39  and  59  are input buffers, resistive elements  40  and  60  may each have a higher resistive value (e.g. in a range of approximately 10-1000 ohms) without seriously impacting the input buffer performance (e.g. switching speed). Yet other embodiments may use resistive elements  40  and  60  in a broader range of resistive values (e.g. in a range of approximately 1-1000 ohms). Other embodiments may use any desired and appropriate values for the one or more resistive elements. Note that using a higher resistance value for the resistive elements (e.g.  40  and  60 ) allows the secondary ESD protection network to provide a higher level of ESD protection using a given semiconductor area. Therefore, input buffers may particularly benefit from the addition of this secondary ESD protection network. However, the secondary ESD protection network may be helpful regardless of the type of circuit being protected (e.g.  39 ,  59 ) and regardless of the value of resistive elements  40 ,  60 . 
     Note that the embodiment illustrated in  FIG. 1  makes use of a boosted and distributed rail clamp network. The rail clamps used in the illustrated embodiment comprise NMOS transistors  36 - 38  and  56 - 58  that shunt ESD current between the power rails using normal NMOS channel conduction. For some embodiment, these NMOS transistors  36 - 38  and  56 - 58  are also known as “active NMOS” rail clamps. In the illustrated embodiment, the NMOS clamp transistors are made conductive during an ESD event by way of a trigger circuit  41 . The trigger circuit  41  detects an ESD event by sensing either a predetermined voltage rise time or voltage threshold on the BOOST bus  12  appropriate to an ESD event, but not appropriate to normal operation of the integrated circuit. Alternate embodiments may detect an ESD event in any desired and appropriate manner. 
     After detection of an ESD event, the trigger circuit  41  then outputs a predetermined voltage (e.g. approximately equal to the voltage on BOOST bus  12 ) on the TRIGGER bus  14 . In this embodiment, the TRIGGER bus  14  drives the control electrode of rail clamp devices  36 - 38  and  56 - 58 , which allows the trigger circuit  41  to be located in any portion of the integrated circuit and does not require the trigger circuit  41  to be co-located with the rail clamps  36 - 38 , and  56 - 58 . In other embodiments, the functionality of the trigger circuit  41  may be combined with the functionality of selected rail clamps (e.g.  36 - 38 ,  56 - 58 ). For example, each I/O pad (e.g.  30 ,  50 ) may have its own trigger circuit  41  associated with the I/O pad. In yet other embodiments, the secondary rail clamps ( 37 ,  38 ,  57 ,  58 ) may have their own trigger circuit or circuits independent of the trigger circuit for the primary rail clamps ( 36 ,  56 ). By having separate trigger circuits for the primary and secondary rail clamps, it may be possible to improve the turn-on speed of the secondary clamps (i.e. the secondary clamps transition from a non-conducting state to a conducting state when an ESD event occurs). In one embodiment, the trigger circuit  41  may be a rise time detector. In alternate embodiments, the trigger circuit  41  may be any circuitry that can detect an ESD event and provide a control signal to turn on the rail clamps ( 36 - 38  and  56 - 58 ). In alternate embodiments, the rail clamps ( 36 - 38  and  56 - 58 ), illustrated in  FIG. 1  as n-channel MOS transistors, may be implemented as p-channel MOS transistors. In yet other embodiments, the rail clamps can be implemented in other ways (e.g. lateral or vertical bipolar transistors or silicon controlled rectifiers, etc.) which may or may not require a trigger circuit to turn “on” during an ESD event (e.g. self-triggered snapback clamps). 
     While the primary rail clamp network is shown as having elements (e.g.  36 ,  56 ) distributed among the various I/O pads, alternate embodiments may use a different approach. For example, alternate embodiments may use one or more rail clamp devices not distributed among the I/O pads, but instead associated with one or more power supply pads or placed in other locations within the I/O region. Yet other embodiments may use one or more rail clamps (e.g.  36 ,  56 ) and locate them anywhere appropriate on the integrated circuit. While the secondary rail clamp network is shown as having elements (e.g.  37 ,  38 ,  57 ,  58 ) distributed among the various I/O pads, alternate embodiments may use a different approach. For example, alternate embodiments may use one or more rail clamp devices not distributed among the I/O pads, but instead associated with one or more power supply pads or placed in other locations within the I/O region. Yet other embodiments may use one or more rail clamps (e.g.  37 ,  38 ,  57 ,  58 ) and locate them anywhere appropriate on the integrated circuit. 
     In the embodiment illustrated in  FIG. 1 , boost circuitry comprises diodes  32  and  52  and the BOOST bus  12 . This boost circuitry is used to provide a higher voltage to the control electrodes of the rail clamp devices ( 36 - 38  and  56 - 58 ) in order to allow the rail clamp devices to conduct as much current as possible. The higher voltage is provided to the trigger circuit  41  via BOOST bus  12 . During the ESD event described above, the voltage on BOOST bus  12  is elevated via diode  32  to a voltage slightly less than the voltage of the “zapped” I/O pad  30 . Note that in one embodiment, diode  32  is smaller than diode  31  as the BOOST bus  12  carries little current compared to the primary ESD current path. Alternate embodiments may not use boost circuitry, or may implement the boost circuitry in a different manner. For example, in an alternate embodiment, BOOST bus  12  may be merged with OVDD bus  16 , and diodes  32 ,  52  may be deleted. 
     Note that although the functionality of circuit  10  has been described in the context of a particular ESD event, the secondary ESD protection circuitry may be useful for any type of ESD event occurring at any I/O pad (e.g.  30 ,  50 ). The secondary ESD protection circuitry may protect the circuitry associated with an I/O pad that experiences ESD current flowing into or out of the I/O pad. For example, an I/O pad may be protected even during a single pad ESD event (e.g. a charged device model (CDM) event). 
     In the embodiment illustrated in  FIG. 1 , the buses or conductors using the letters “VDD” are used during normal operation to provide a first power supply voltage (e.g. approximately a power supply voltage) to one or more circuit elements; and the buses or conductors using the letters “VSS” are used during normal operation to provide a second power supply voltage (e.g. approximately ground) to one or more circuit elements. OVDD power bus  16  and OVSS power bus  18  provide power to the protected circuitry  39  and  59  during normal circuit operation and conduct ESD current during an ESD event. In the illustrated embodiment, the VSS power bus  24  is coupled to the OVSS power bus  18  via diodes  70  and  72  to allow bidirectional ESD current flow between these two power buses. In one embodiment, the VSS power bus  24  may be coupled to the chip substrate and the OVSS power bus  18  may be used for a segment of one or more I/O pads. A second OVSS power bus used for a second segment of one or more I/O pads (not shown) may be electrically isolated from the OVSS power bus  18  to avoid noise coupling between the two segments. During an ESD event in which a potential difference is applied between the two segments, the VSS power bus  24  provides a primary ESD current path between the two segments via diodes  70  and  72 . Likewise, for any given number of I/O pad segments, the VSS power bus  24  provides a common ESD current exchange rail between any of the I/O segments. Alternate embodiments may partition the OVSS power bus  18  in any desired manner. 
     During normal operation of the illustrated embodiment in  FIG. 1 , when OVDD power bus  16  is powered up to its nominal supply voltage with respect to OVSS power bus  18  and when no ESD stress is applied, the secondary power bus OVDD_ 2   20  is actively coupled to the primary power bus OVDD  16  by device  42  so that the potential difference between the two power buses is kept low. This is done to avoid drifting of the voltage on OVDD_ 2  power bus  20 , which may cause parasitic noise coupling between I/O pads (e.g.  30  and  50 ) or variations in I/O leakage current through the secondary diodes (e.g.  33 ,  53 ). In one embodiment, device  42  may be a PMOS transistor with its control terminal coupled to TRIGGER bus  14 . Likewise, the secondary power bus OVSS_ 2   22  is actively coupled to the primary power bus OVSS  18  by device  43  during normal operation so that the potential difference between the two power buses is kept low. This is done to avoid drifting of the voltage on OVSS_ 2  power bus  22 , which may cause parasitic noise coupling between I/O pads (e.g.  30  and  50 ) or variations in I/O leakage current through the secondary diodes (e.g.  35 ,  55 ). In one embodiment, device  43  may be an NMOS transistor with its control terminal coupled to TRIGGER bus  14  via an inverter  44 . In the illustrated embodiment, circuitry  45  thus operates as an equalizer circuit. 
       FIG. 2  illustrates a circuit  110  in accordance with an alternate embodiment of the present invention. In the illustrated embodiment, circuit  110  comprises a circuit  139  that is to be protected from ESD events. Alternate embodiments may use any type of one or more circuits within the circuit  139  which are to be protected from ESD events. For example, in alternate embodiments, circuit  139  may be an input buffer, an output buffer, an input/output buffer, an analog circuit, or any desired type of circuit or combination of circuits. 
     In the illustrated embodiment, circuit  110  has a first power bus OVDD  116 , a second power bus OVSS  118 , and a third power bus OVDD_ 2   120 . In one embodiment, OVDD  116  and OVSS  118  are used to provide the primary power to circuit  110 . In one embodiment, OVDD  116  provides a first power supply voltage and OVSS  118  provides a second power supply voltage that is less than the first power supply voltage. In some embodiments, the second power supply voltage equals approximately ground. In one embodiment, circuit  139  is coupled to OVDD  116  and to OVSS  118  in order to receive power for normal operation. 
     In one embodiment, circuit  110  has a diode  133  having a first current electrode coupled to OVDD_ 2   120 , and having a second current electrode coupled to circuit  139 . A resistive element  140  has a first terminal coupled to the second current electrode of diode  133 , and has a second terminal coupled to I/O pad  130 . I/O pad  130  is coupled to OVDD  116  by way of circuitry  131 , and is coupled to OVSS  118  by way of circuitry  134 . In one embodiment, circuitry  131  may be implemented in the same manner as in  FIG. 1  (e.g. by using one or more diodes such as diode  31 ). In one embodiment, circuitry  134  may be implemented in the same manner as in  FIG. 1  (e.g. by using one or more diodes such as diode  34 ). Alternate embodiments may use any desired and appropriate circuitry to implement circuitry  131  and  134 . 
     Circuit  110  also has circuitry  162  coupled between OVDD  116  and OVSS  118  to function as a primary rail clamp and to be part of a primary ESD current path during an ESD event. Circuit  110  also has circuitry  161  coupled between OVDD_ 2   120  and OVSS  118  to function as a secondary rail clamp and to be part of a secondary ESD current path during an ESD event. This secondary ESD current path through circuitry  161  is used in addition to the primary ESD current path through circuitry  162 . In one embodiment, circuitry  162  may be implemented in the same manner as in  FIG. 1  (e.g. by using one or more clamps such as transistor  36 ). In one embodiment, circuitry  161  may be implemented in the same manner as in  FIG. 1  (e.g. by using one or more clamps such as transistors  37 ,  57 ). Alternate embodiments may use any desired and appropriate circuitry to implement circuitry  161  and  162 . 
     In addition, in various alternate embodiments, circuit  110  may use a BOOST bus  12  as in  FIG. 1 , may use a TRIGGER bus  14  as in  FIG. 1 , may use another ground bus (e.g. VSS  24 ) as in  FIG. 1 , may use yet another ground bus (e.g. OVSS_ 2   22 ) may use a trigger circuit  41  as in  FIG. 1 , may use an equalizer circuit  45  as in  FIG. 1 , may use cross-coupled diodes (e.g.  70 ,  72 ) as in  FIG. 1 , and may use a wide variety of different circuitry that provides a secondary ESD current path to reduce the risk of damaging a protected circuit such as circuit  139 . 
     In one embodiment, the elements illustrated in  FIG. 2  operate in a very similar manner to the elements illustrated in  FIG. 1 . One purpose of  FIG. 2  is to highlight the use of the secondary ESD protection network or circuitry comprised of resistive element  140 , diode  133 , OVDD_ 2  bus  120 , and secondary rail clamp  161 . During an ESD event where current is flowing into I/O pad  130  and to OVSS bus  118 , this secondary ESD circuitry reduces the voltage stress on protected circuitry  139 . Note that, unlike  FIG. 1 , in this embodiment only one secondary power rail OVDD_ 2   120  is used in addition to the primary power rails OVDD  116  and OVSS  118 . The primary ESD current path in the ESD event described above is through elements  131 ,  116 , and  162 . The secondary ESD current is through elements  140 ,  133 ,  120 , and  161 . Note that ESD protection elements  131  and  134  can be implemented in a variety of ways. In one embodiment, elements  131  and  134  may be diodes as illustrated in  FIG. 1 . In an alternate embodiment, elements  131  and/or  134  may be local ESD clamps (e.g. MOS transistors, lateral or vertical bipolar transistors, silicon controlled rectifiers, etc.) that provide direct ESD protection to OVDD power bus  116  and OVSS power bus  118 , respectively. In other embodiments, only one of the two elements  131  and  134  may be present in circuit  110 , providing bidirectional ESD protection to OVDD power bus  116  (in the case only element  131  is present) or OVSS power bus  118  (in the case only element  134  is present), respectively. 
       FIG. 3  illustrates a circuit  210  in accordance with an alternate embodiment of the present invention. In the illustrated embodiment, circuit  210  comprises a circuit  259  that is to be protected from ESD events. Alternate embodiments may use any type of one or more circuits within the circuit  259  which are to be protected from ESD events. For example, in alternate embodiments, circuit  259  may be an input buffer, an output buffer, an input/output buffer, an analog circuit, or any desired type of circuit or combination of circuits. 
     In the illustrated embodiment, circuit  210  has a first power bus OVDD  216 , a second power bus OVSS  218 , and a third power bus OVSS_ 2   222 . In one embodiment, OVDD  216  and OVSS  218  are used to provide the primary power to circuit  210 . In one embodiment, OVDD  216  provides a first power supply voltage and OVSS  218  provides a second power supply voltage that is less than the first power supply voltage. In some embodiments, the second power supply voltage equals approximately ground. In one embodiment, circuit  259  is coupled to OVDD  216  and to OVSS  218  in order to receive power for normal operation. 
     In one embodiment, circuit  210  has a diode  255  having a first current electrode coupled to circuit  259 , and having a second current electrode coupled to OVSS_ 2   222 . A resistive element  260  has a first terminal coupled to the first current electrode of diode  255 , and has a second terminal coupled to I/O pad  250 . I/O pad  250  is coupled to OVDD  216  by way of circuitry  251 , and is coupled to OVSS  218  by way of circuitry  254 . In one embodiment, circuitry  251  may be implemented in the same manner as in  FIG. 1  (e.g. by using one or more diodes such as diode  51 ). In one embodiment, circuitry  254  may be implemented in the same manner as in  FIG. 1  (e.g. by using one or more diodes such as diode  54 ). Alternate embodiments may use any desired and appropriate circuitry to implement circuitry  251  and  254 . 
     Circuit  210  also has circuitry  262  coupled between OVDD  216  and OVSS  218  to function as a primary rail clamp and to be part of a primary ESD current path during an ESD event. Circuit  210  also has circuitry  263  coupled between OVDD  216  and OVSS_ 2   222  to function as a secondary rail clamp and to be part of a secondary ESD current path during an ESD event. This secondary ESD current path through circuitry  263  is used in addition to the primary ESD current path through circuitry  262 . In one embodiment, circuitry  262  may be implemented in the same manner as in  FIG. 1  (e.g. by using one or more clamps such as transistor  56 ). In one embodiment, circuitry  263  may be implemented in the same manner as in  FIG. 1  (e.g. by using one or more clamps such as transistors  38 ,  58 ). Alternate embodiments may use any desired and appropriate circuitry to implement circuitry  263  and  262 . 
     In addition, in various alternate embodiments, circuit  210  may use a BOOST bus  12  as in  FIG. 1 , may use a TRIGGER bus  14  as in  FIG. 1 , may use another ground bus (e.g. VSS  24 ) as in  FIG. 1 , may use another power bus (e.g. OVDD_ 2   20 ), may use a trigger circuit  41  as in  FIG. 1 , may use an equalizer circuit  45  as in  FIG. 1 , may use cross-coupled diodes (e.g.  70 ,  72 ) as in  FIG. 1 , and may use a wide variety of different circuitry that provides a secondary ESD current path to reduce the risk of damaging a protected circuit such as circuit  259 . 
     In one embodiment, the elements illustrated in  FIG. 3  operate in a very similar manner to the elements illustrated in  FIG. 1 . One purpose of  FIG. 3  is to highlight the use of the secondary ESD protection network or circuitry comprised of resistive element  260 , diode  255 , OVSS_ 2  bus  222 , and secondary rail clamp  263 . During an ESD event where current is flowing out of I/O pad  250  coming from OVDD bus  216 , this secondary ESD circuitry reduces the voltage stress on protected circuitry  259 . Note that, unlike  FIG. 1 , in this embodiment only one secondary power rail OVSS_ 2   222  is used in addition to the primary power rails OVDD  216  and OVSS  218 . The primary ESD current path in the ESD event described above is through elements  254 ,  218 , and  262 . The secondary ESD current is through elements  260 ,  255 ,  222 , and  263 . Note that ESD protection elements  251  and  254  can be implemented in a variety of ways. In one embodiment, elements  251  and  254  may be diodes as illustrated in  FIG. 1 . In an alternate embodiment, elements  251  and/or  254  may be local ESD clamps (e.g. MOS transistors, lateral or vertical bipolar transistors, silicon controlled rectifiers, etc.) that provide direct ESD protection to OVDD power bus  216  and OVSS power bus  218 , respectively. In other embodiments, only one of the two elements  251  and  254  may be present in circuit  210 , providing bidirectional ESD protection to OVDD power bus  216  (in the case only element  251  is present) or OVSS power bus  218  (in the case only element  254  is present), respectively. 
     Referring to  FIG. 1 , note that any number of transistors  36 ,  56  may be used to form a primary rail clamp. In one embodiment, each I/O pad (e.g.  30 ,  50 ) may have one rail clamp associated with it. In alternate embodiments, each I/O pad (e.g.  30 ,  50 ) may have any number of rail clamps associated with it. Although the embodiments illustrated in  FIGS. 1-3  shown both a primary ESD current path and a secondary ESD current path between power and ground, alternate embodiments may have any number of ESD current paths between power and ground. Note that  FIG. 1  illustrates an embodiment that uses both a secondary VDD power bus or rail (OVDD_ 2   20 ) as well as a secondary VSS power bus or rail (OVSS_ 2   22 ). In this embodiment, the primary VDD power bus or rail is OVDD  16  and the primary VSS or ground power bus or rail is OVSS  18 .  FIG. 2 , on the other hand, illustrates an embodiment that uses a secondary VDD power bus or rail (OVDD_ 2   120 ) and no secondary VSS power bus or rail. In the embodiment of  FIG. 2 , the primary VDD power bus or rail is OVDD  116  and the primary VSS or ground bus or rail is OVSS  118 .  FIG. 3  illustrates an embodiment that uses a secondary VSS power bus or rail (OVSS_ 2   222 ) and no secondary VDD power bus or rail. In the embodiment of  FIG. 3 , the primary VDD power bus or rail is OVDD  216  and the primary VSS or ground bus or rail is OVSS  218 . 
     By now it should be appreciated that there has been provided circuitry that can provide a plurality of ESD current paths in order to better protect circuitry on an integrated circuit from potentially damaging ESD events 
     Because the apparatus implementing the present invention is, for the most part, composed of electronic components and circuits known to those skilled in the art, circuit details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 
     Although the invention has been described with respect to specific conductivity types or polarity of potentials, skilled artisans appreciated that conductivity types and polarities of potentials may be reversed. 
     Furthermore, those skilled in the art will recognize that boundaries between the functionality of the above described operations merely illustrative. The functionality of multiple operations may be combined into a single operation, and/or the functionality of a single operation may be distributed in additional operations. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments. In addition, one or more of circuits  10 ,  110 ,  210  or other embodiments of circuitry used to provide ESD protection may be used on one or more integrated circuits. These integrated circuits may be incorporated into a wide variety of apparatus, such as, for example, electronic equipment (e.g. cell phones, computers, etc.), products using electronic control (e.g. vehicles, appliances, etc.), or any apparatus at all that makes use of an integrated circuit. 
     Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, I/O pads  30 ,  130 , and  250  do not have to be implemented as pads, but may be any portion of an integrated circuit that is susceptible to receiving the stress of an ESD event. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims. 
     The term “coupled,” as used herein, is not intended to be limited to a direct coupling or a mechanical coupling. 
     Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. 
     Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. 
     Additional Text
     1. An apparatus comprising an integrated circuit, the integrated circuit comprising:
       a pad for example: ( 250 ,  130 ,  50  or  30 ) for communicating external to the integrated circuit;   a first ESD protection element for example: ( 254 ,  131 ,  54  or  31 ) which couples the pad to a first power supply node for example: ( 218 ,  116 ,  18  or  16 ) in response to an ESD event;   a second ESD protection element for example: ( 262 ,  162 ,  56  or  36 ) which couples the first power supply node to a second power supply node for example: ( 216 ,  118 ,  16  or  18 ) in response to the ESD event;   a resistive element for example: ( 260 ,  140 ,  60  or  40 ) having a first terminal coupled to the pad and having a second terminal;   a first diode for example: ( 255 ,  133 ,  55  or  33 ) having a first terminal coupled to the second terminal of the resistive element and having a second terminal coupled to a third power supply node for example: ( 222 ,  120 ,  22  or  20 ); and   a third ESD protection element for example: ( 263 ,  161 ,  58  or  37 ) which couples the third power supply node to the second power supply node in response to the ESD event,   wherein the first power supply node, the second power supply node, and the third power supply node are different nodes.   
       2. An apparatus as in statement 1, wherein the first ESD protection element comprises a diode.   3. An apparatus as in statement 1, wherein the second ESD protection element comprises a transistor.   4. An apparatus as in statement 1, wherein the third ESD protection element comprises a transistor.   5. An apparatus as in statement 1, wherein the third power supply node and the first power supply node are both at approximately a first voltage during normal operation of the integrated circuit.   6. An apparatus as in statement 1, wherein the third power supply node and the first power supply node are both at approximately VSS (ground) during normal operation of the integrated circuit.   7. An apparatus as in statement 1, wherein the third power supply node and the first power supply node are both at approximately VDD (power) during normal operation of the integrated circuit.   8. An apparatus as in statement 1, wherein the integrated circuit further comprises protected circuitry for example: ( 259 ,  139 ,  59  or  39 ) coupled to the second terminal of the resistive element.   9. An apparatus as in statement 8, wherein the protected circuitry comprises a buffer coupled to the first power supply node and coupled to the second power supply node.   10. An apparatus as in statement 9, wherein the buffer comprises at least one of an input buffer and an output buffer.   11. An apparatus as in statement 1, further comprising:
       a fourth ESD protection element for example: ( 51  or  34 ) which couples the pad to the second power supply node in response to the ESD event.   
       12. An apparatus as in statement 11, further comprising:
       a second diode for example: ( 53  or  35 ) having a first terminal coupled to the second terminal of the resistive element and having a second terminal coupled to a fourth power supply node for example: ( 20  or  22 ).   
       13. An apparatus as in statement 12, further comprising:
       a fifth ESD protection element for example: ( 57  or  38 ) which couples the fourth power supply node to the first power supply node in response to the ESD event.   
       14. An apparatus as in statement 12, wherein the third power supply node and the first power supply node are both at approximately a first voltage during normal operation of the integrated circuit, wherein the fourth power supply node and the second power supply node are both at approximately a second voltage during normal operation of the integrated circuit, and wherein the second voltage is substantially different than the first voltage.   15. An apparatus as in statement 1, further comprising:
       a trigger circuit for example: ( 41 ), coupled to detect the ESD event and to trigger both the first ESD protection element and the second ESD protection element in response to detecting the ESD event.   
       16. An apparatus as in statement 15, further comprising:
       a boost bus for example: ( 12 ), coupled to the trigger circuit, wherein the trigger circuit detects the ESD event by monitoring voltage on the boost bus.   
       17. An apparatus comprising an integrated circuit, the integrated circuit comprising:
       a pad for example: ( 250 ,  130 ,  50  or  30 ) for communicating external to the integrated circuit;   a first diode for example: ( 254 ,  131 ,  54  or  31 ) which couples the pad to a first power supply node for example: ( 218 ,  116 ,  18  or  16 ) in response to an ESD event;   a first transistor for example: ( 262 ,  162 ,  56  or  36 ) which couples the first power supply node to a second power supply node for example: ( 216 ,  118 ,  16  or  18 ) in response to the ESD event;   a resistive element for example: ( 260 ,  140 ,  60  or  40 ) having a first terminal coupled to the pad and having a second terminal;   a second diode for example: ( 255 ,  133 ,  55  or  33 ) having a first terminal coupled to the second terminal of the resistive element and having a second terminal coupled to a third power supply node for example: ( 222 ,  120 ,  22  or  20 ); and   a second transistor for example: ( 263 ,  161 ,  58  or  37 ) which couples the third power supply node to the second power supply node in response to the ESD event,   wherein the first power supply node, the second power supply node, and the third power supply node are different nodes.   
       18. A method, comprising:
       providing protected circuitry for example: ( 259 ,  139 ,  59  or  39 ) on an integrated circuit, wherein the protected circuitry is to be protected from an ESD event;   providing a first power conductor for example: ( 218 ,  116 ,  18  or  16 ) for providing a first power voltage to the protected circuitry;   providing a second power conductor for example: ( 216 ,  118 ,  16  or  18 ) for providing a second power voltage to the protected circuitry;   wherein the first power voltage and the second power voltage are substantially different during normal operation of the protected circuitry;   using the first power conductor and the second power conductor in a first ESD current path for example: ( 250 / 254 / 218 / 262 / 216 ,  130 / 131 / 116 / 162 / 118 ,  50 / 54 / 18 / 56 / 16  or  30 / 31 / 16 / 36 / 18 ) for conducting a first portion of ESD current due to the ESD event;   providing a third power conductor for example: ( 222 ,  120 ,  22  or  20 ) which provides no power to the protected circuitry; and   using the third power conductor in a second ESD current path for example: ( 250 / 260 / 255 / 222 / 263 / 216 ,  130 / 140 / 133 / 120 / 161 / 118 ,  50 / 60 / 55 / 22 / 58 / 16  or  30 / 40 / 33 / 20 / 37 / 18 ) for conducting a second portion of the ESD current due to the ESD event,   wherein the first ESD current path is not identical to the second ESD current path, and   wherein the third power conductor is held to approximately a same voltage as one of the first power conductor or the second power conductor during normal operation of the protected circuitry.   
       19. A method as in statement 18, further comprising:
       providing a trigger circuit for example: ( 41 ) to enable the first and second ESD current paths to be conductive during the ESD event and to enable the first and second ESD current paths to be non-conductive during normal operation of the protected circuitry.   
       20. A method as in statement 18, further comprising:
       providing an equalizer circuit for example: ( 45 ) for decoupling the first power conductor and the third power conductor in response to the ESD event, and for coupling the first power conductor and the third power conductor during normal operation of the protected circuitry.