CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit, under 35 U.S.C. §119(e)(1), of U.S. Provisional Application No. 60/785,554, filed Mar. 24, 2006, and incorporated herein by this reference. 


   FIELD OF THE INVENTION 
   This invention relates to an integrated circuit protection and more particularly to internal ESD protection an integrated circuit with multiple power supply domains. 
   BACKGROUND OF THE INVENTION 
   Present complementary metal oxide semiconductor (CMOS) and bipolar-CMOS (BiCMOS) circuits employ electrostatic discharge protection (ESD) circuits to protect against electrostatic discharge due to ordinary human and machine handling. This electrostatic discharge occurs when the semiconductor circuit contacts an object that is charged to a substantially different electrostatic potential of typically several thousand volts. The contact produces a short-duration, high-current transient in the semiconductor circuit. This high current transient may damage the semiconductor circuit through joule heating. Furthermore, high voltage developed across internal components of the semiconductor circuit may damage MOS transistor gate oxide. 
   Sensitivity of the semiconductor circuit is determined by various test methods. A human body model test circuit is typically used to determine sensitivity of the semiconductor circuit to human handling. A common human body model test circuit includes a 100 pF capacitor and 1500Ω resistor to emulate a human body resistor-capacitor (RC) time constant of 150 nanoseconds. A stress voltage supply connected in series with a current limiting resistor to charge the 100 pF capacitor to a desired stress voltage. The semiconductor device or device under test (DUT) is connected to the test circuit at a predetermined external terminal for a selected test pin combination. In operation, a switch selects a discharge path through the 1500Ω resistor and the DUT. A post stress current-voltage measurement determines whether the DUT is damaged. Although this test effectively emulates electrostatic discharge from a human body, it fails to comprehend other common forms of electrostatic discharge. Moreover, the relatively high resistance discharge path of the test circuit drops most of the stress voltage during the ESD test, thereby producing a low-voltage test at the DUT. 
   Another common test circuit of the prior art for tests semiconductor circuits under charged device ESD. This circuit is typically used to determine sensitivity of the semiconductor circuit to ESD under automated manufacturing conditions. The test circuit includes a stress voltage supply connected in series with a current limiting resistor. The DUT forms a capacitor above a ground plane that is typically 1-5 pF. The DUT is precharged to a desired test voltage from the stress voltage supply. A small parasitic resistor and parasitic inductor form a discharge path having an RC time constant typically two orders of magnitude less than the tester of  FIG. 5 . In operation, a switch connects an external terminal of the DUT to the discharge path through parasitic resistor and parasitic inductor. This connection produces a high-voltage, high-current discharge at the DUT. 
   All of the foregoing ESD test methods may induce destructive voltage and current levels in internal circuits of a semiconductor. One such internal failure mode was reported by Huh et al., “ESD-induced Internal Core Device Failure: New Failure Modes in System-on-Chip (SOC) Designs,” Proceedings of the Fifth Internal Workshop on System-on-Chip for Real-Time Applications, 47-53 (July 2005) and reproduced in Proceedings of the 9th Internal Database Engineering &amp; Application Symposium (IDEAS &#39;05). The reported internal failure occurred at a power domain crossing circuit (PDCC). Multiple power domains are common for complex integrated and may be separately implemented for analog, digital, input/output, or other circuit functions to reduce noise. The power domain crossing circuit includes transmitter and receiver circuits that transfer signals from circuitry in one power domain to circuitry in another power domain. 
   Referring to  FIG. 1A , there is an exemplary power domain crossing circuit of the prior art. A first power domain includes VDD 1  bus  102  and VSS 1  bus  106 . P-channel transistor  116  and N-channel  118  form an inverter connected between VDD 1  bus  102  and VSS 1  bus  106 . The VSS 1  bus  106  also includes a metal resistance RM. The first power domain also includes a local power supply protection circuit  110  having an N-channel transistor M 1  and a resistor R and capacitor C gate bias circuit. A second power domain includes circuitry powered by VDD 2  bus  104  and VSS 2  bus  108 . P-channel transistor  122  and N-channel  124  form another inverter connected between VDD 2  bus  104  and VSS 2  bus  108 . The second power domain includes a separate local power supply protection circuit  112  having an N-channel transistor M 2  and a resistor R and capacitor C gate bias circuit. Bus VSS 1   106  is connected to bus VSS 2   108  by anti parallel diodes  114  to conduct ESD current during stress testing across the power domains. Alternative power domain designs may omit anti parallel diodes  114  and connect both VSS 1   106  and VSS 2   108  to a common external terminal or use other isolation means. 
   In operation, when a sufficient level of positive ESD voltage is applied to VDD 1  bus  102  with respect to VSS 2  bus  108 , the power domain crossing circuit may fail as described with reference to  FIG. 1B . Transistor M 1  of local power supply protection circuit  110  turns on with the aid of the gate bias circuit to conduct ESD stress current in snapback mode. This typically develops about 7 V across local power supply protection circuit  110  during human body model ESD stress. Here and in the following discussion, voltages are presented by way of explanation of circuit operation and not in a limiting manner. Another 2 V may be developed across metal resistance RM and 1 V across diode  114 . A parallel current path between VDD 1  bus  102  and VSS 2  bus  108  is formed by P-channel transistor  116  and N-channel transistor  124 . The gate of P-channel transistor  116  is initially at VSS 1  potential during ESD testing. Thus, P-channel transistor  116  is on, and there is approximately 0 V dropped across the source to drain current path. The entire 10 V, therefore, appears across the gate to source terminals of N-channel transistor  124 . Gate oxide typically fails at 10 MV/cm. Therefore, failure occurs at the gate to source overlap of N-channel transistor  124  if the gate oxide thickness of less than 10 nm. 
   Turning now to  FIG. 2A , there is another power domain crossing circuit of the prior art as reported by Huh et al. Common reference numerals here and throughout the specification indicate the same circuit elements. Huh et al. improved the previously described power domain crossing circuit by adding N-channel transistor  120  across the gate and source terminals of N-channel transistor  124 . Referring to  FIG. 2B , this clamp precludes damage voltage levels between the gate and source terminals of N-channel transistor  124  during positive ESD stress of VDD 1  bus  102  with respect to VSS 2  bus  108 . The gate to source voltage N-channel transistor  124  is clamped to approximately 7 V. The remaining 3 V appears across P-channel transistor  116  as a product of channel current and resistance. However, the N-channel transistor clamp  120  increases the load on P-channel transistor  116  and N-channel transistor  118  during normal circuit operation. Additionally, it may degrade ESD performance for positive ESD stress of VSS 1  bus  106  with respect to VDD 2  bus  104 . In view of these limitations, the present inventors have seen a need for further improvement of power domain crossing circuits for both ESD and normal circuit operation. 
   SUMMARY OF THE INVENTION 
   These problems are resolved by an ESD protection circuit designed on a single integrated circuit. The integrated circuit has a first and a second power supply bus. The circuit includes a first logic gate having a current path coupled to the first power supply bus. A second logic gate has a current path coupled to the second power supply bus. The second logic gate has an input terminal coupled to an output terminal of the first logic gate. A device is coupled in series with the current path of the second logic gate and is always on during normal circuit operation. 
   The present invention provides ESD protection to semiconductor circuit elements. During ESD stress, the series device is off and, therefore, reduces voltage developed across the second logic gate. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention may be gained by reading the subsequent detailed description with reference to the drawings wherein: 
       FIG. 1A  is a schematic diagram of power domain crossing circuit of the prior art; 
       FIG. 1B  is a simplified schematic diagram of the power domain crossing circuit of  FIG. 1A  showing the related components for one polarity of ESD stress; 
       FIG. 2A  is a schematic diagram of a modified power domain crossing circuit of the prior art; 
       FIG. 2B  is a simplified schematic diagram of the power domain crossing circuit of  FIG. 2A  showing the related components for one polarity of ESD stress; 
       FIG. 3A  is a schematic diagram of a first embodiment of the power domain crossing circuit of the present invention; 
       FIG. 3B  is a simplified schematic diagram of the power domain crossing circuit of  FIG. 3A  showing the related components for one polarity of ESD stress; 
       FIG. 3C  is a simplified schematic diagram of the power domain crossing circuit of  FIG. 3A  showing the related components for another polarity of ESD stress; 
       FIG. 4A  is a schematic diagram of a second embodiment of the power domain crossing circuit of the present invention; 
       FIG. 4B  is a schematic diagram of a third embodiment of the power domain crossing circuit of the present invention; 
       FIG. 5  is a schematic diagram of a fourth embodiment of the power domain crossing circuit of the present invention; and 
       FIG. 6  is a schematic diagram of a fifth embodiment of the power domain crossing circuit of the present invention. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 3A , there is a schematic diagram of a power domain crossing circuit of the present invention. The circuit employs previously identified circuit elements having the same reference numerals as well as P-channel transistors  300  and  302  and N-channel transistors  304  and  306 . The gates of P-channel transistors  300  and  302  are connected to local reference terminals VSS 1   106  and VSS 2   108 , respectively. Likewise, the gates of N-channel transistors  304  and  306  are connected to local positive voltage terminals VDD 1   102  and VDD 2   104 , respectively. In normal circuit operation, therefore, P-channel transistors  300  and  302  and N-channel transistors  304  and  306  are always on. Each of the P-channel and N-channel transistors preferably has the same channel width to twice the channel width of the corresponding series connected logic gate transistor and the same channel length. For example, P-channel transistor  300  has a width from that of P-channel transistor  116  to twice that of P-channel transistor  116 . Likewise, N-channel transistor  304  has a width from that of N-channel transistor  118  to twice that of N-channel transistor  118 . Alternative embodiments of the present invention may employ various combinations and sizes of these transistors  300 - 306  as will become apparent in the following discussion. 
   Referring now to  FIG. 3B , operation of the circuit of  FIG. 3A  will be described in detail for positive ESD stress at VDD 1   102  with respect to VSS 2   108 . The positive ESD stress is applied before the semiconductor integrated circuit  100  is installed in a system. All terminals, therefore, are initially at ground or reference potential as determined by the ESD test apparatus. The exemplary ESD stress pulse produces 7 V across local power supply protection circuit  110 , 2 V across metal resistance RM, and 1 V across diode  114  as previously described. A parallel current path between VDD 1   102  and VSS 2   108  includes P-channel transistors  300  and  116  and N-channel transistors  124  and  306 . Control gates of P-channel transistors  300  and  116  are both at VSS 1  potential or about 1 V. Thus, the voltage drop across each of P-channel transistors  300  and  116  is approximately 0 V. The control gate of N-channel transistor  306  is at VDD 2  potential of approximately 0 V. Thus N-channel transistor  306  is off. In this state, N-channel transistor  306  and N-channel transistor  124  act as two series connected capacitors. When N-channel transistor  306  has a channel width approximately equal to the channel width of N-channel transistor  124 , the 10 V between VDD 1   102  and VSS 2   108  is equally divided between them. There is approximately 5 V across the gate to source capacitance of N-channel transistor  124  and 5 V across the drain to gate capacitance of N-channel transistor  306 . This is about 50% of the voltage that would appear across N-channel transistor  124  in the circuit of  FIG. 1B  and about 70% of the voltage that would appear across N-channel transistor  124  in the circuit of  FIG. 2B . This reduction of peak voltage during ESD stress advantageously improves the failure threshold of the semiconductor integrated circuit  100  without significant degradation of normal circuit operation. 
   Turning now to  FIG. 3C , operation of the circuit of  FIG. 3A  will be described in detail for positive ESD stress at VSS 2   108  with respect to VDD 1   102 . This is similar to the previously discussed ESD stress of  FIG. 3B  but with opposite polarity. All terminals are initially at ground or reference potential of VDD 1   102  for this polarity. The exemplary ESD stress pulse produces 1 V across diode  114 , 2 V across metal resistance, and 1 V across local power supply protection circuit  110 . The reduced voltage (1 V) across local power supply protection circuit  110  is due to forward bias conduction of the parasitic lateral NPN transistor associated with transistor M 1  rather than snapback operation as discussed for the polarity of  FIG. 3B . A parallel current path between VSS 2   108  and VDD 1   102  includes P-channel transistors  300  and  116  and N-channel transistors  124  and  306 . Here, however, control gates of P-channel transistors  300  and  116  are both at VSS 1  potential or about 3 V with respect to VDD 1   102 . Thus, the gate to drain voltage drop (VSS 1 -VDD 1 ) across P-channel transistor  300  is 3 V. The control gate of N-channel transistor  306  is at VDD 2  potential of approximately 0 V. Thus N-channel transistor  306  is off, and the drain to gate voltage drop (VSS 2 -VDD 2 ) for N-channel transistor  306  is 4 V. Finally, when P-channel transistor  116  has a channel width approximately equal to the channel width of N-channel transistors  124  and  306 , the 3 V between VSS 1   106  and VDD 2   104  is equally divided between them by charge sharing. 
   Referring now to  FIG. 4A , there is a schematic diagram of a second embodiment of the power domain crossing circuit of the present invention. The circuit is the same as previously described with respect to  FIG. 3A  with the addition of CMOS pass gate  400 . The CMOS pass gate includes a P-channel transistor having a control gate connected to VSS 1   106  and an N-channel transistor having a control gate connected to VDD  1102 . The CMOS pass gate, therefore, is always on during normal circuit operation. During positive ESD stress of VDD 2   104  with respect to VSS 1   106 , the N-channel transistor of the CMOS pass gate  400  is off. The P-channel transistor of the CMOS pass gate  400  is on. This condition facilitates charge sharing between P-channel transistors  302  and  122 , N-channel transistor  118 , and both transistors of the CMOS pass gate  400 . During positive ESD stress of VSS 1   106  with respect to VDD 2   104 , both the N-channel transistor and the P-channel transistor of the CMOS pass gate  400  are off. Peak voltage for this polarity is distributed between P-channel transistors  302  and  122  and the CMOS pass gate  400 . As previously described, this charge sharing process advantageously reduces peak voltage across gate oxide of power domain crossing circuits, thereby increasing the failure threshold of the semiconductor integrated circuit. 
   Turning now to  FIG. 4B , there is a schematic diagram of a third embodiment of the power domain crossing circuit of the present invention. This embodiment is similar to the embodiment of  FIG. 4A  except that the positions of the logic gate transistors and their corresponding series connected transistors are reversed with respect to the previous embodiment. For example, P-channel transistors  116  and  300  are reversed as well as N-channel transistors  118  and  304 . The power domain circuit operates substantially in the same manner as previously described, except the body effect for each of the logic gate transistors  116 ,  118 ,  122  and  124  is reduced by moving the series connected transistors from the source side to the drain side. This advantageously improves normal circuit operation without compromising ESD performance. 
   The power domain crossing circuit of  FIG. 5  is similar to the previously discussed embodiment of  FIG. 4A . Logic gate transistors  116 ,  118 ,  122  and  124 , however, are replaced with NAND gates  500  and  502 , respectively. The embodiment of  FIG. 6  is also similar to  FIG. 4A , but the logic gate transistors  116 ,  118 ,  122  and  124  are replaced with NOR gates  600  and  602 , respectively. In each embodiment, ESD performance is substantially improved through charge sharing of the logic gate transistors with their corresponding series connected transistors. Moreover, the present invention may be readily adapted to any complex logic gates where analog or digital signals must cross power domains. 
   It is to be further understood that numerous changes in the details of the embodiments of the invention will be apparent to persons of ordinary skill in the art having reference to this description. It is contemplated that such changes and additional embodiments are within the spirit and true scope of the invention as claimed below.
An ESD protection circuit is designed on an integrated circuit ( 100) having a first power supply bus ( 106) and a second power supply bus ( 108). The circuit includes a first logic gate ( 116, 118) having a current path coupled to the first power supply bus. The first logic gate includes an output terminal. A second logic gate ( 122, 124) has a current path coupled to the second power supply bus. The second logic gate has an input terminal coupled to the output terminal of the first logic gate. A first device ( 306) is coupled in series with the current path of the second logic gate and is always on during normal circuit operation.