embodiments of the present invention relate to the use of a common discharge or charge path to serve multiple pad - oriented heterogenic current protection structures , even though those protection structures are structured differently or perhaps even serve different voltage domains . accordingly , the common discharge or charge path may provide a discharge path for analog and / or digital pads , high voltage pads and / or normal voltage pads , or other heterogenic voltage domain situations . the integrated circuit may be included within a larger system , such as , for example , a computing system or an automobile to provide significant performance enhancements to that larger system . fig1 , for example , illustrates a system 10 that includes an integrated circuit 100 . the integrated circuit 100 includes a number of pads 101 a through 101 l ( referred to sometimes collectively as “ pads 101 ”). in the illustrated embodiment , the integrated circuit 100 has 12 pads , although there is no significance attached whatsoever to this particular number of pads . the principles of the present invention may just as well apply to integrated circuits with any multiple number of pads . furthermore , although the pads are illustrated as being squares in fig1 taking on perhaps the appearance of a bond pad , the principles of the present invention are not limited to the particular form of the pad . in this description and in the claims , a “ pad ” of a circuit is simply any electrical connection that has the capability to interface with components external to the circuit . an appropriate selection of a pad type is a design choice that will depend on the configuration of the circuit package . each of the pads 101 has an associated current protection structure . with reference to fig1 , each of the pads 101 a through 101 l has a corresponding current protection structure 111 a through 111 l , respectively . the current protection structures 111 a through 111 l will sometimes be referred to herein collectively as “ current protection structures 111 ”. the current protection structures 111 serve to provide appropriate current protection for their corresponding pads . if , at any of the corresponding pads , one or more conditions occur that are indicative of an electrical overstress ( eos ) event , the associated current protection structure activates to discharge current from or charge current to the associated pad . this positive or negative current dissipation protects the core circuitry from excessive currents . the current protection structures 111 may be partially or fully heterogenic . the current protection structures 111 may be “ partially heterogenic ” in that at least some of the current protection structures 111 may have different structures . the current protection structures 111 may even be “ fully heterogenic ” in that the principles of the present invention are still applicable even if all of the current protection structures 111 have different structures . “ different structures ” as the term is used herein when applied to describe the current protection structures 111 is more than mere processing variations that might result from the imperfect manufacture of the same structural design of a current protection structure . processing technology inherently results in some structural differences even if starting from the same circuit design , even when that design is repeated on the same wafer or die . instead , “ different structures ” as the term is used herein are structures that are formulated from different designs . one reason for having different structures for the current protection structures 111 is that each pad 101 may have a distinct voltage domain . proper current protection for a particular voltage domain may require different designs to assign proper current protection given each voltage domain &# 39 ; s anticipated normal operation . for example purposes only , a dashed box 121 a is illustrated as surrounding the pad 101 a , symbolizing that the pad 101 a serves a particular voltage domain ( also referred to herein as “ voltage domain 121 a ”). some pads may be of a common voltage domain . for instance , a dotted box 121 b surrounds the pads 101 b through 101 e , symbolizing that the pads 101 b through 101 e each serve a common voltage domain ( also referred to herein as “ voltage domain 121 b ”). likewise , as symbolically illustrated , pads 101 f and 101 g are associated with a common voltage domain 121 c , pad 101 h is associated with voltage domain 121 d , pad 101 i is associated with voltage domain 121 e , and pads 101 j through 101 l are associated with a common voltage domain 121 f . the voltage domains 121 a through 121 f may be referred to hereinafter collectively as “ voltage domains 121 ”. a “ voltage domain ” is defined by its expected normal voltage operation . as a corollary to that statement , “ different voltage domains ” are domains in which the expected normal operational voltages are different . for instance , pads that correspond to a digital voltage domain are expected to have digital voltages ( but not analog voltages ) applied during normal operation . on the other hand , pads that correspond to an analog voltage domain are expected to have analog voltages ( but not digital voltages ) applied during normal operation . also , a pad may correspond to a mixed signal voltage domain if the pad normally operates with both digital and analog signals . the analog voltage domain , the digital voltage domain , and the mixed signal voltage domains are each different voltage domains . a pad may even have multiple voltage domains . for instance , one pad might be a high voltage digital pad , thereby permitting normal operation at high voltages . on the other hand , a normal voltage digital pad might be expected to experience lower voltages during normal operation . a digital voltage domain might also be defined by the expected voltage levels representing binary one and zero signals . a digital voltage domain might also be defined by the number of acceptable digital levels . for instance , a digital pin sometimes might have a high and a low discrete level , as well as one ( or more ) intermediate discrete voltage level defining one ( or more ) intermediate digital values . as another example of multiple voltage domains , an analog pin may have a particular expected voltage range , while another analog pin may have a different voltage range . regardless of whether one voltage range overlaps ( partially or fully ) or not with the other voltage range , these analog pins will have different voltage domains due to their difference in acceptable voltage ranges . as the termed is used herein , pins have “ different voltage domains ” so long as any one of the voltage domains of one pad is different than any of the one or more voltage domains of the other pad . thus , even pads that belong to the same voltage domain ( e . g ., the analog voltage domain ) may have different voltage domains , just as people have different characteristics even though they are in some ways the same . voltage domains may also be defined by whether the corresponding current protection structure has a positive or negative trigger voltage , or both . in this description and in the claims , a “ positive trigger voltage ” is a voltage level that if experienced at the pad will cause significant current to be drawn by the current protection structure from the pad , thereby resulting in a sharp voltage drop at the pad . on the other hand , a “ negative trigger voltage ” is a voltage level that if experienced at the pad will cause significant current to be provided by the current protection structure to the pad , thereby causing a sharp increase in the voltage at the pad towards ground . current protection structures that have both a positive and negative trigger voltage are bi - directional . however , the current protection structure need not have both a positive and a negative trigger voltage to be “ bi - directional ”, as the term is used herein . for example , a bi - directional current protection structure may just have a positive trigger voltage , but when a negative voltage is encountered , the current protection structure may behave more as a diode in the negative direction . on the other hand , a bi - directional current protection structure may just have a negative trigger voltage , but when a positive voltage is encountered , the current protection structure might simply behave as a diode in the positive direction . a voltage domain may also be defined by the level of the positive and / or negative trigger voltage of a current protection structure corresponding to a pad . referring to fig1 , although the circuit 100 is shown as including six voltage domains 121 a through 121 f , the principles of this particular embodiment of the present invention may apply to a circuit that has any configuration of voltage domains and any correlation of such voltage domains to pads . in any case , a current protection structure is activated upon the detection of one or more conditions at an associated pad , or at a node that is close to the pad . the one or more conditions will depend on the voltage domain of the pad . for instance , current protection structure 111 a may be said to be of a first voltage domain 121 a if the current protection structure 111 a serves to discharge current from or charge current to any of its associated pad 101 a when a first set of one or more voltage conditions is present at the pad 101 a . on the other hand , current protection structures 111 b through 111 e may be said to be of a second voltage domain 121 b if the current protection structures 111 b through 111 e each serve to discharge current from or charge current to its associated pads ( in the illustrated case , pads 101 b through 101 e , respectively ) when a second set of one or more voltage conditions is present at the associated pad 101 b through 101 e . similarly , current protection structures 111 f and 111 g may be said to be of a third voltage domain 121 c if the current protection structures 111 f and 111 g each serve to discharge current from or charge current to its associated pad ( in the illustrated case , pads 101 f and 101 g ) when a third set of one or more voltage conditions is present at the associated pad 101 f and 101 g . the same might be said for voltage domains 121 d through 121 f as well . a current discharge or charge path 120 serves at least two of the current protection structures 111 , but possibly more than two or even all of the current protection structures 111 . accordingly , if any of the current protection structures 111 connected to the path 120 were to trigger , the trigger current protection structure may then use the path 120 to shunt current . by sharing the current discharge or charge path across multiple current protection structures , the amount of space occupied to discharge or charge current is greatly reduced . furthermore , since the current protection structures 111 are situated between the respective pads 101 and the current charge or discharge path 120 , the voltage on the current charge or discharge path 120 need not be carefully regulated in many applications . thus , there may often be no particular need for distributed voltage clamps on the current charge or discharge path 120 . moreover , when an eos event occurs across any two pads coupled to the common charge or discharge path 120 , the common charge or discharge path 120 serves to provide a low impedance route for the current to shunt through . for instance , suppose that an electrostatic discharge ( esd ) event occurs between pads 101 b and 101 i of circuit 100 of fig1 , each of the current protection structures 111 b and 111 i may trigger thereby providing a low voltage drop across the current protection structures 111 b and 111 i . since the common charge or discharge path 120 may be low impendance since due to it being shared , a low impedance path is created from the pad 101 b , through the current protection structure 111 b , through the common charge or discharge path 120 , through the current protection structure 111 i and to pad 101 i . the dangerous esd current is thus taken safely off the circuit 100 through this shunting operation . the low impedance property of this shunt path means that the current will likely follow the shunt path , rather than flow through undesirable paths in the protected circuitry to other pads thereby causing harm to the protected circuitry . since the esd current has little drive to follow such undesirable paths through protected circuitry , pads of very different voltage domains may be placed closer to each other or even interleaved with greater assurance that the shunt path will be used in case of an esd event . for instance , a high voltage pad may be placed proximate to low voltage pads without particular concern . thus , the principles of the present invention may provide considerable design flexibility in placement of different pad types on an integrated circuit . fig2 illustrates a schematic 200 of the electrical connections of the circuit 100 of fig1 . for each of the pads 101 , the associated current protection structure 111 intervenes to provide current to or draw current from the associated pad 101 . the voltage domains 121 are also identified by bracketing the corresponding pads 101 and current protection structures 111 . referring to pad 101 a , for example , if the current protection structure 111 a is not activated , then signals applied to the pad 101 a are provided to the protected circuit 102 a . on the other hand , if excessive current or voltage is applied to the pad 101 a due to , for example , electrostatic discharge , the current protection structure 111 a activates and draws current from the pad 101 a into the discharge / charge path 120 . conversely , if excessive current is drawn from the pad 101 a due to , for example , negative electrostatic discharge , the current protection structure 111 a may potentially activate and provide current from the current discharge / charge path 120 to the pad 101 a . in any of these cases , the protected circuitry 102 a does not experience physical damage or degradation . fig3 a illustrates a specific layout 300 a of a pad 301 a in conjunction with a current protection structure 303 a and a current discharge / charge path 305 a . as previously noted , the current protection structure 303 a is associated with a single pad 301 a . although the pad 301 a is illustrated as being square - shaped , the pad 301 a may take any form . the pad 301 a may be provided at one terminal of the current protection structure . the region 303 a represents a selectively conductive region of the current protection structure . at the outer perimeter of the current protection structure 303 a lies a conductive material 302 a ( also referred to herein as the “ perimeter terminal 302 a ”) that serves as the other terminal of the current protection structure 303 a . the perimeter terminal 302 a of the current protection structure is electrically connected ( as represented by connection 304 a ) to the current discharge / charge path 305 a that is shared amongst multiple pads and current protection structures . in operation , if the current protection structure is activated , current passes through the selectively conductive region 303 a in an appropriate direction between the pad 301 a and the perimeter terminal 302 a . the perimeter terminal 302 a is electrically coupled to the common current discharge / charge path 305 a through the connection 304 a thereby allowing the path 305 a to serve as a current source or sink for the current protection structure 303 a . the precise nature of the connection between the perimeter terminal 302 a and the current discharge / charge path 305 a is not important to the principles of the present invention . in one embodiment , one of the sides of the perimeter terminal may simply be the common discharge / charge path that is connected to one or more other current protection structures . in another embodiment , the path 305 a may underly or overly much or all of the pad 301 , but at a different metal layer . in the specific example of fig3 a , the selectively conductive region 303 a of the current protection structure is designed to be radially symmetrical around the pad 301 a . in other words , given any radial line extending outwards from the center of the pad 301 a , the ratio of the distance from the center to where the line intersects the outer edge of the pad 301 a to the distance from the center to where the line intersects the perimeter terminal 302 a will be approximately constant as the radial line is rotated in a circle about the center of the pad 301 a . this design is desirable in that breakdown activation of the current protection structure 303 a will occur throughout the entire area of the current protection structure 303 a . however , given the intricate and sometimes imprecise nature of semiconductor processing technology , it is difficult to precisely align one circuit structure with another . for instance , fig3 b illustrates a similar structure as compared to fig3 a , with elements 301 b through 305 b of fig3 b approximately correlating to elements 301 a through 305 a , respectively , of fig3 a . however , one primary difference is that the pad 301 b is not perfectly aligned with respect to the current protection structure 303 b . this might cause breakdown to occur at region 306 , rather than more uniformly across the entire structure . while this misalignment may have been exaggerated , there will always be some finite amount of misalignment between two circuit elements due to mask alignment error . since the current protection structure 303 b is designed to be radially symmetric around the pad 301 b , however , the alignment errors should remain relatively small regardless of the direction of misalignment . without being radial symmetric , the direction of misalignment would be more significant since misalignment in one direction might cause a high activation voltage , whereas the same distance of misalignment in the other direction might cause a lower activation voltage . thus , the radial symmetry in the design of the current protection structure 303 b and the pad 301 a permits for greater control over the activation voltage . accordingly , embodiments have been described in which multiple heterogenic current protection structures may be served by a single common current discharge or charge path . furthermore , an example of a current protection structure that is designed to be radially symmetric with respect to a pad has been described . the precise nature of the current protection structure is not critical to the invention so long as the current protection structure is able to provide a low impedance shunt path to and from a common charge / discharge path . in one example , the current protection structure provides normal operation during a moderate reverse voltage mode , while still providing current dissipation functions if excessive positive or negative currents are experienced . such an example will now be described with respect to fig4 through 7 . although the specific example is described with respect to fig4 through 7 , the principles of the present invention are not limited to any particular usage of a current protection structure . as such , fig4 through 7 should not be construed in any way to restrict the broader principles of the present invention . fig4 illustrates a current protection structure 400 manufactured on a semiconductor substrate that may be used to protect circuitry from eos while permitting operation in a reverse voltage condition . the current protection structure is described in further detail in commonly - owned co - pending u . s . patent application ser . no . 11 / 532 , 477 entitled “ single well excess current dissipation circuit ” filed on the same day herewith , which application is incorporated herein by reference in its entirety . for clarity , portions of the current protection structure 400 are illustrated in cross - section as they might be processed on a semiconductor substrate , while other portions are illustrated using simple circuit symbols . in addition to providing reverse voltage protection without triggering the current protection structure 400 , the current protection structure 400 may also be processed using a single - well technology in which all wells are manufactured of the same polarity ( i . e ., all n - type or all p - type ). in the illustrated case of fig4 , all of the wells are n - type . in this description and in the claims , an “ n - type ” region or “ n - region ” of a semiconductor material is said to have an n - type polarity and is a region in which there are more n - type dopants than p - type dopants , if there are any p - type dopants at all . on the other hand , a “ p - type ” region or “ p - region ” of a semiconductor material is said to have a p - type polarity and is a region in which there are more p - type dopants than n - type dopants , if there are any n - type dopants . generally , the p - type polarity is considered to be the opposite of the n - type polarity . the current protection structure 400 includes two autonomous n - well regions 411 and 412 within a p - type semiconductor substrate 405 . an “ n - well ” region is a well that is formed as an n - type region within a larger p - type region , as opposed to a “ p - well ” region which is formed as a p - type region within a larger n - type region . techniques for forming n - well and p - well regions in a substrate are well known in the art and thus will not be discussed here . it will be understood that a p - type semiconductor region in contact with an n - type semiconductor region will cause a diode effect , with current being permitted to pass from the p - type region to the n - type region if the voltage at the p - type region is higher than the voltage at the n - type region . however , current is not permitted to flow from the n - type region to the p - type region absent a significantly high voltage at the n - type region with respect to the p - type region . this higher voltage is often referred to as a diode &# 39 ; s “ breakdown ” voltage or “ reverse breakdown ” voltage . occasionally , while describing the operation of the current protection structure 400 of fig4 , reference will be made to the pnpnp stack 600 of fig6 which illustrates the relationship of the p - type and n - type junctions of fig4 . likewise , fig7 illustrates the relationship in the form of interconnected bipolar transistors 700 . since fig6 is used to describe only the principles of operation , the size of the n - type and p - type regions of fig6 are not drawn to scale when compared to the corresponding components of fig4 . in fig6 , the n - region 602 corresponds to the n - well 411 of fig4 , and the n - region 604 corresponds to the n - well 412 of fig4 . the p - region 603 corresponds to the p - type substrate 405 of fig4 . note that in fig4 , there may be an n - channel field 414 surrounding the n - well 411 . the thickness of this n - channel field 414 may be controlled at the time of circuit manufacture to thereby control the breakdown voltage between the diode defined by the n - well 411 and the p - type substrate 405 . mechanisms for forming such an n - channel field of a specific width are known in the art and thus will not be described here . although not shown , an n - tub of higher n - type dopant density than the n - well 411 may be used internal to the n - well 411 to provide a further adjustment to the breakdown voltage . referring to fig6 and 7 , the n - region 602 of fig6 corresponds to the n - type base terminal of the pnp bipolar transistor 701 and the n - type collector terminal of the npn bipolar transistor 702 , which are shown coupled together in fig7 since the terminals are both formed using the same n - type region 602 . also , the n - region 604 of fig6 corresponds to the n - type emitter terminal of the npn bipolar transistor 702 and corresponds to the n - type base terminal of the pnp bipolar transistor 703 . once again , these terminals are coupled together since they are formed of the same n - type region 604 the p - region 603 of fig6 corresponds to the p - type collector terminal of pnp bipolar transistor 701 , the p - type emitter terminal of pnp bipolar transistor 703 , and the p - type base terminal of npn bipolar transistor 702 , which are shown coupled together . referring back to fig4 , the n - well 411 is coupled to a first circuit node 401 through a first parallel combination of a p - type contact region 431 and an n - type contact region 432 . the net dopant density of each of the p - type contact region 431 and the n - type contact region 432 is greater than the net dopant density of the n - well 411 . this higher net dopant density is expressed in fig4 by the p - type contact region 431 being designated as “ p +”, and the n - type contact region 432 being designated as “ n +”. the “ net dopant density ” is the concentration per unit volume of dominant dopant species ( n - type dopants if an n - type region , and p - type dopants if a p - type region ) minus the concentration per unit volume of minority dopant species ( p - type dopants if an n - type region , and n - type dopants if a p - type region ). referring to fig4 and 6 , the p + contact region 431 of fig4 corresponds to the p - region 601 of fig6 . the p - region 601 is coupled to one terminal 621 of the pnpnp stack 600 . the terminal 401 of fig4 corresponds to the terminal 621 of fig6 . the resistor 403 of fig4 corresponds to the resistor 611 of fig6 having resistance r . referring to fig4 and 7 , the p + contact region 431 of fig4 corresponds to the p - type emitter terminal of the pnp bipolar transistor 701 . the terminal 401 of fig4 corresponds to terminal 721 of fig7 . the resistor 403 of fig4 corresponds to the resistor 711 a of fig7 . referring back to the illustrated embodiment of fig4 , the n + contact region 432 is coupled to the first circuit node 401 through a resistor circuit element 403 . in this description and in the claims , a “ resistor circuit element ” is a resistor that is specifically formed as a desired portion of a circuit pattern . the p + contact region 431 is coupled to the first circuit node 401 without an intervening resistor circuit element in the illustrated embodiment . a second n - well 412 is coupled to the second circuit node 402 through a parallel combination of a p + contact region 421 and an n + contact region 422 . in the illustrated embodiment , the third and fourth contact regions 421 and 422 are coupled to the second circuit node 402 without an intervening resistor element . in one embodiment , the first circuit node 401 is an i / o pad in which input and / or output signals may be applied . the second circuit node 402 may be a substantially fixed voltage supply such as , for example , ground . the substrate 405 may also be connected to ground . the remaining circuit elements 423 serve to reduce the breakdown voltage of the diode defined by the interface between the n - well 411 to p - type substrate 405 . referring to fig4 and 6 , the p + contact region 421 of fig4 corresponds to the p - region 605 of fig6 . the second circuit node 402 of fig4 corresponds to the circuit node 622 of fig6 . since the n - well 412 is connected through the n + region 422 to the circuit node 402 with some resistance , fig6 shows a small resistor 612 having resistance r 1 coupled between the n - region 604 and the second circuit node 622 . furthermore , since p - type substrate 405 may well be grounded , and the second circuit node 402 is grounded , the p - region 603 is shown coupled to the second circuit node 622 through resistor 613 having resistance r 2 . the resistors r 1 and r 2 may be parasitic , as opposed to an expressed resistor circuit element in the design . however , the resistors may also be expressed design elements . referring to fig4 and 7 , the p + contact region 421 of fig4 corresponds to the p - type collector terminal of pnp bipolar transistor 703 of fig7 . the second circuit node 402 of fig4 corresponds to the circuit node 722 of fig7 . since the n - well 412 is connected through the n + region 422 to the circuit node 402 with some resistance , fig7 shows a small resistor 712 having resistance r 1 coupled between the n - type base terminal of pnp bipolar transistor 703 and the second circuit node 722 . furthermore , since p - type substrate 405 may well be grounded , and the second circuit node 402 may well be grounded , the p - type base terminal of npn bipolar transistor 702 is shown coupled to the second circuit node 722 through resistor 713 having resistance r 2 . as will be apparent to those of ordinary skill in the art , the polarities of each of the regions of fig4 , 6 and 7 , may be reversed . in other words , p - type regions may be replaced by n - type regions , and vice verse . fig5 illustrates a dual reference mode form of the current protection structure 400 of fig4 . while the current protection structure 400 of fig4 uses a single reference node 402 as a current source or sink , the current protection structure 500 of fig5 includes two references nodes 502 and 504 to source current to or sink current from the circuit node 501 . the operation of the components 501 , 502 , 503 , 505 , 511 , 512 , 521 , 522 , 523 , 531 and 532 of fig5 will operate much as described above for the components 411 , 412 , 421 , 422 , 423 , 431 and 432 described with respect to fig4 in sourcing or sinking current to or sinking current from circuit node 501 using reference node 502 . however , the reference node 504 will operate using regions 541 , 542 and 543 within n - well 513 much as described above for the reference node 402 operating using regions 421 , 422 and 423 within n - well 412 . accordingly , dual reference node current dissipation is achieved . therefore , a current protection structure is further described that permits for proper and adjustable current dissipation while permitted normal reverse voltage operation . furthermore , this is achieved by using single well technology thereby simplifying the fabrication of the current protection structure . the present invention may be embodied in other specific forms without departing from its spirit or essential characteristics . the described embodiments are to be considered in all respects only as illustrative and not restrictive . the scope of the invention is , therefore , indicated by the appended claims rather than by the foregoing description . all changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope .