Abstract:
A fail-safe Input/Output buffer bias circuit for digital CMOS chips provides protection for Input/Output buffers which have high voltages applied to the Input/output node and are subjected to power supply failure resulting in a collapsing supply voltage decaying to zero volts while said Input/output circuit has a high voltage remaining applied to its Input/output node. The Input/output buffer bias circuit is comprised of a sensing circuit and a bias generator circuit which acts to drive protection transistors in a manner which optimally minimizes the voltage impressed on input or output devices under all conditions which could persist in the event of V DD  supply voltage failure. Protection circuitry holds all three combinations of voltage stress, gate-to-source, gate-to-drain, and drain-to-source voltages, to acceptable levels.

Description:
This application claims priority under 35 USC §119(e)(1) of Provisional Application No. 60/114,268, filed Dec. 30, 1998. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of this invention is that of integrated circuit input/output buffers and in particular such output buffers that are fail-safe. 
     BACKGROUND OF THE INVENTION 
     Complementary metal oxide semiconductor (CMOS) Input/output buffers which perform the interface between a packaged digital device chip and other such device chips must be able to withstand all anticipated conditions which might occur in normal usage as well as some conditions which could occur only under certain system power supply failure modes. One of the latter conditions, well known to designers, is the condition under which the system supply voltage fails and causes voltage stress, originating from an external load, to be impressed on the input, output, or input/output (I/O) buffer circuitry. This application problem has been made even more difficult in sub-micron CMOS circuitry operating on low voltage power supplies (3.3 volts, for example) but having the requirement that it must drive external circuitry biased with higher voltage supplies (5.0 volts or higher). A number of circuit configurations have been developed to address this problem. 
     One technique widely used to allow low voltage rating transistors to interface to higher voltage is to replace single transistors, which would otherwise have to withstand full voltage stress, with stacked or cascoded multiple transistors across which the stress may be distributed. The major difficulties of prior art solutions have been the effective sensing of the failed-supply condition and the proper biasing of the cascode protection transistors to allow both the required protection in the failed-supply state, and also the correct buffer operation in the normal state. Some chip suppliers have supplied buffers which have been designed using these and other supplementary circuit techniques, yet the buffers are normally not sufficiently robust that the supplier can claim fail-safe operation. Fail-safe operation means unconditional circuit reliability after extended supply voltage failure with high voltage signal levels applied to input/output pins. 
     Sub-micron chips having buffers without fail-safe protection will usually suffer catastrophic failure when the normal chip supply voltage fails for an extended period, if any input, output, or input/output buffer has a positive applied external voltage in the 5.0 volt range. This failure is usually the result of a gate oxide voltage breakdown but can also result from a drain-source impressed voltage beyond normal rated limits. 
     Providing fully fail-safe operation for CMOS input/output buffers is an extension to or generalization of the solution a more basic problem, namely, that of providing high voltage tolerant operation in low-voltage (3.3 volt supply) CMOS. Briefly stated, 5 volt tolerant operation means that a circuit, designed for a 3.3 volt power supply, is able to drive a load or be driven from a source consisting of a resistor connected to a 5 volt power supply. Specifications for 5 volt tolerant circuits define maximum output leakage current flow in the ‘high’ or ‘off’ state, or maximum input leakage in the input ‘high’ state. No appreciable input circuit or output circuit degradation is permitted. Note that a 5 volt tolerant circuit specification does not guarantee protection from the more stringent fail-safe condition, namely that the circuit must not sustain degradation when the normal V DD  supply for the circuit fails, but applied voltage up to 5 volts or higher, at the bond pad from other chips external to the chip in question persists. 
     Circuits which are operated from a 3.3 volt supply often have this requirement so that they can be used in systems along with circuits which are operated from a 5 volt supply. Essentially, protection for the input circuit or output circuit is derived from the placement of a series transistor protection device between the circuit being protected and the bond pad. The biasing of the gate terminals of these series protection transistors has, in prior art, been ineffective to adequately protect all transistors at the input, output or input/output interface from voltage stress exceeding V OX     —     MAX  (the maximum gate oxide voltage) in all failure modes conditions. Also, bias circuits operating in the highly irregular V DD  failed state must be free from other conditions such as latch-up resulting from semiconductor-controlled-rectifier (SCR) action of parasitic transistors present in the device structures. These effects have also limited the application of prior art solutions. 
     SUMMARY OF THE INVENTION 
     The fail-safe CMOS buffer configuration of this invention allows external applied voltages to be applied, which are higher than the maximum gate oxide voltage rating of the transistors provided by the process, even when the integrated circuit V DD  supply has failed, or has been turned off, resulting in zero volts applied at the normal V DD  supply node. This fail-safe protection has as its key element a unique bias circuit which provides two outputs. The first bias, labeled BIAS, drives the gate terminals of the series protection transistors. The second bias, labeled VDF, drives the gate of another protection transistor which acts to pull the gate of the lower output transistor to a low voltage level during the V DD  failed condition. 
     A special connection of stacked transistors detects the V DD  supply voltage collapse. A unique bias voltage supply, derived from the signal pin itself, is developed which is applied to series protection transistors in the input or output buffer circuitry. This bias supply acts to drive the protection transistors in a manner which optimally minimizes the voltage impressed on the input or output devices under all conditions which could persist in the event of V DD  supply voltage failure. Protection circuitry holds all combinations of voltage stress: gate-to-source, gate-to-drain, drain-to-source, gate-to-substrate, and source/drain-to-substrate voltages, to acceptable levels. Parasitic transistor action has also been analyzed to assure that possible destructive latch-up conditions have been eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of this invention are illustrated in the drawings, in which: 
     FIG. 1 illustrates a block diagram of a fail-safe input/output buffer configuration of this invention; 
     FIG. 2 illustrates the bias circuit of this invention for use in fail-safe buffer applications; 
     FIG. 3 illustrates the input buffer circuit which may be used in conjunction with the bias circuit of this invention in fail-safe buffer applications; 
     FIG. 4 illustrates the open-drain output circuit which may be used in conjunction with the bias circuit of this invention in fail-safe buffer applications; and 
     FIG. 5 illustrates the bias circuit of FIG. 2 with critical parasitic PNP transistors shown. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Providing fully fail-safe high voltage operation for low voltage CMOS input/output buffers is an extension to the solution to a more basic problem of providing high voltage tolerant operation. The term ‘high voltage’ is used here to describe a voltage higher than the allowable transistor gate to other terminal voltage for a given CMOS process. The term ‘low voltage’, by contrast, is used to refer to the usual supply voltage V DD  applied to the integrated circuit power terminals. During the initial work on this invention, the predominant high voltage application was 5 volts for use with a CMOS low voltage V DD  supply of 3.3 volts. Briefly stated, 5 volt tolerant operation means that a buffer can function with 5 volts applied to its bond pad terminal while the integrated circuit is powered with 3.3 volts. Specifications for 5 volt tolerant circuits define maximum current flow into the bond pad terminal of a buffer when driven with 5 volts. No appreciable degradation of this specification is allowed over the lifetime of the integrated circuit. A fail-safe input/output buffer must have no appreciable degradation of its specification over the lifetime of the device when 5 volts is applied to its output terminal (bond pad) whether V DD  is active (3.3 volts) or failed (0 volts). 
     A block diagram of a ‘fail-safe’ buffer system is shown in FIG. 1. A single DC supply voltage of 3.3 volts is connected between positive terminal V DD , node  101 , and ground terminal node  107 . Components that are external to the integrated circuit are shown in the dashed box. An external voltage ranging from 0 to 5 volts is applied to the bond pad of the buffer from external high voltage supply, VHV, node  115 , through external resistor  102 . Note that component  102  is not necessarily a resistor but can be any device that limits the current from the 5 volt external supply to the maximum value allowed in the system specifications. 
     External component  104  is a capacitor representing the maximum capacitive system load that an output buffer must discharge to meet its specified timing requirements in the application. Internal signals are generated in the Other Internal Circuitry block  100  which sends logic signals to an output buffer block  176  via line  105  and/or receives logic signals from an input buffer block  156  via line  103 . 
     Consider here only the simple case of an open drain output, and a conventional input circuit without any power reduction functions in the input/output circuit. Addition of three-state output buffers with enable circuitry and power reduction circuitry would not affect the implementation of the fail-safe protection of this invention. The essential difference would be that push-pull output circuits would have not only the cascode N-channel transistor in the pull-down portion of the circuit, but would also have a series or cascode connected P-channel transistor in the pull-up portion of the circuit. Both cascode-connected transistors would be driven at their gate terminals by the bias supply circuit of this invention. 
     In the case of an input/output the bond pad  113  is driven by an output buffer block  176 , and the bond pad also supplies a signal to an input buffer block  156 , thereby functioning bidirectionally. Other buffers external to this integrated circuit may be present in the system. The voltage at the bond pad  113  will be at a level between 0 volts and 5 volts at any time, either due to the operation of output buffer block  176  or an output buffer external to the integrated circuit. The bias generator block  120  contains the circuitry that accomplishes the fail-safe operation. 
     This bias generator block contains a sensing circuit composed of a stacked set of transistors driven by the bond pad, and a switching circuit configuration which detects the failed condition providing proper voltages to BIAS node  111  and VDF node  109  of the internal input and/or output circuits both during normal operation and also when V DD  is failed. The present invention differs from 5 volt tolerant circuits in that the bias generator block  120  is not present in 5 volt tolerant input and output buffers, which instead have the BIAS node at a voltage between V DD  and ground, and the V DD  failed signal VDF does not exist. 
     The Other Internal Circuitry block  100  represents the remainder of the integrated circuit components and is responsible for processing the signals to and from the input/output circuitry at this bonding pad. 
     FIG. 2 contains the schematic of the bias circuit associated with the input/output circuitry at this bonding pad and just sufficient other component details to describe the operation of the fail-safe system function. The function of the bias circuit is to provide proper voltage levels, BIAS  211  and VDF  209 , when the bond pad  213  is at 5 volts. These voltage levels are applied to selected components of the input and/or output buffer to allow them to withstand this voltage. When V DD  has not failed, the correct operation of the input/output buffers must not be affected by the fail-safe bias circuitry. 
     Output node  283  provides the output signal from Other Internal Circuitry  100  (FIG. 1) to drive bond pad  213 . Output circuit transistors  290 ,  294 ,  296 , output circuit resistor  292  and input circuit transistors  260 ,  262 ,  264 , input circuit resistor  258  are enclosed in properly identified dashed boxes to define them as separate from the bias circuit. Resistors  258  and  292  are included here for ESD (electrostatic discharge) protection and may not be required in all applications of this invention. The output load, dashed box components resistor  202 , capacitor  204  and node  215  are external to the integrated circuit. 
     In the output circuit drive section transistor  294  must have its gate at BIAS node  211  maintained at a voltage level such that its drain  285  to gate voltage, and gate to source  287  voltage never exceeds V OX     —     MAX  (the maximum gate oxide voltage) and the drain  287  to ground  207  voltage of transistor  296  is less than V OX     —     MAX  whenever the bond pad  213  is at 5 volts and transistor  296  is ‘off’. Note that the gate to ground voltage, can exceed V OX     —     MAX  because when the transistor is ‘on’ the drain/source channel is inverted and the gate oxide sees the maximum voltage potential from the gate to the source or drain. Additionally the gate of output transistor  294  must be maintained at a voltage level high enough to guarantee that the specified V OL  (voltage output low) from the bond pad  213  to ground  207  can be met when the signal input to the gate of output transistor  296  is high, transistor  296  is ‘on’ and V DD  is not ‘failed’. Additionally a V DD  failed positive voltage is output to VDF node  209  to the gate of output circuit transistor  290  whenever V DD  is failed and the voltage on the bond pad  213  is 5 volts to assure output leakage remains low. This V DD  failed signal and output circuit transistor  290  could be replaced with a resistor from node  283  to ground  207  which would result in additional V DD  current when the output buffer was driving the output to a ‘low’ level. 
     Input node  253  transmits the input signal sensed from bond pad  213  to Other Internal Circuitry  100  (FIG.  1 ). In the input circuit section transistor  260  must have its gate at BIAS node  211  maintained at a voltage level such that the drain  259  to gate voltage and its gate to source voltage never exceeds the maximum allowed gate oxide voltage (V OX     —     MAX ) and the gate  261  to ground  207  voltage on transistors  262  and  264  is less than V OX     —     MAX  volts whenever the bond pad  213  is at 5 volts. Input transistor  260  performs in a similar manner as output transistor  294 . Additionally the gate of input transistor  260  must be maintained at a voltage level high enough to guarantee that when the bond pad  213  to ground  207  voltage is low (V OL ) and V DD  is not failed, transistor  260  is ‘on’ and node  261  is essentially at V OL . 
     Refer now to the bias switching circuit which has as its function to provide proper BIAS and VDF levels in both the normal and the V DD  failed states. 
     In the V DD  ‘active’ mode (V DD =3.3 volts), transistor  246  is ‘on’ sourcing current through resistor  248  causing a sufficient voltage at the gate  237  of transistor  244  to turn it ‘on’ assuring that the voltage at VDF node  209  is essentially zero volts. In this case transistor  250  is ‘on’ and BIAS node  211  has a low impedance path to V DD  (3.3 volts). Note that transistor  250  is a P-channel transistor with its source tied to BIAS node  211 . This forms an active parasitic substrate PNP transistor which adds to the DC leakage current from the V DD  supply. This connection is required to satisfy circuit operation when V DD  is ‘failed’. In this mode, the remainder of the components in the bias generator have no appreciable effect on VDF node  209  and BIAS node  211  voltages. 
     All P-channel devices have associated parasitic substrate PNP devices. When a P-channel device has its substrate connected to a node where the voltage can be less positive than a source/drain terminal the parasitic substrate PNP can become ‘active’ and thereby sustain current flow during circuit operation. Later in this document, with reference to FIG. 5, the bias generator with all possible ‘active’ parasitic substrate PNP devices will be described. 
     Transistors  234 ,  238  and  240  are source followers and their sources (nodes  233 ,  227  and  235  respectively) are required to have a voltage greater than V DD  (3.3 volts) before they will conduct. In this mode, when the bond pad  213  is at 5 volts, current flow in the voltage divider components is through transistor  238 , limiting the voltage at node  227  to 3.3 volts plus one V THRESHOLD  (approximately 0.8 volt) of transistor  238 . Transistor  234  is non-conducting, limiting the current flow in the divider chain from the bond pad  213 . Any current through the divider chain when 5 volts is applied to bond pad  213  appears to the external circuits as leakage current. 
     When V DD  is in the ‘failed’ mode (0 volts) BIAS node  211  and VDF node  209  must derive their voltage from the bond pad  213  which is assumed biased at 5 volts. The gate  237  of transistor  244  is pulled to ground by resistor  248 , turning it ‘off’, which allows VDF node  209  to achieve a voltage level above ground. Transistor  234  is ‘on’ establishing a ground reference for the voltage divider chain attached to the bond pad  213 . Transistors  238  and  242  will conduct if their sources (nodes  227  and  235 ) have a voltage one V THRESHOLD  (approximately 0.8 volt) higher than ground. 
     Note that all the P-channel transistors in the 5 volt divider chain ( 222 ,  224 ,  226 ,  234 ,  238 ,  240 ,  242 ) are in isolated N-wells and have specific source/drain to N-well short orientations. Transistors  222 ,  224  and  226  must be P-channel transistors to limit the voltage across their gate oxides when the externally applied bond pad  213  voltage transitions from 5 volts to ground. Their orientation eliminates parasitic substrate PNP current flow when the bond pad  213  is at 5 volts and discharges node  227  to within 3 V BE  (V BE =base-emitter ‘on’ voltage of bipolar transistor) of ground when the bond pad is at 0 volts. Transistor  234  has the orientation of a source follower with a threshold one V THRESHOLD  (approximately 0.8 volt) above V DD . Transistor  240  guarantees that the voltage at BIAS node  211  is at least one V THRESHOLD  (approximately 0.8 volt) lower than the voltage at VDF node  209  which guarantees that transistor  250  is ‘off’ presenting a high impedance path from BIAS node  211  to V DD . Transistor  240  is oriented to prevent substrate PNP action when BIAS node  211  has a positive voltage less than one V THRESHOLD  (approximately 0.8 volt) above the voltage at VDF. Transistors  238 ,  240  and  242  are oriented to make their associated parasitic substrate PNP transistors ‘inactive’ when the bond pad  213  is at 5 volts. 
     Operation of the bias generator in the V DD  ‘failed’ mode is as follows. Assume the bond pad  213  is at 0 volts. Since there is no voltage stress applied to the integrated circuit transistors and no functionality is required, VDF node  209  and BIAS node  211  can be near zero volts. When the applied bond pad  213  voltage rises to 5 volts, current flows in the divider chain (including resister  236  and transistors  222 ,  224 ,  226 ,  228 ,  230 ,  232  and  234 ) establishing a voltage less than V OX     —     MAX  (maximum allowed gate oxide voltage) at node  227 . The voltage at VDF node  209 , the gate of transistor  250 , rises faster than the voltage at BIAS node  211  ensuring that transistor  250  remains ‘off’ and turning ‘on’ output transistor  294  and/or input transistor  260 . As transistors  294  and/or  260  turn ‘on’, capacitance is established from their drain/source nodes to their gate and the changing voltage across this capacitance causes BIAS node  211  voltage to increase to one V THRESHOLD  (approximately 0.8 volt) above VDF node  209  at which point transistor  250  conducts to V DD , limiting the positive excursion of the voltage on BIAS node  211 . Parasitic substrate PNP action at this node also limits this voltage. The positive voltage at VDF node  209  is also applied to the gate of transistor  290  assuring that transistor  296  is held ‘off’. 
     The divider components are designed to conduct as little as possible and still assure proper circuit operation to minimize the current drawn from the external source when it is at 5 volts. They act as forward biased diode connected transistors with very high ‘on’ resistance. When the externally applied voltage at the bond pad  213  transitions to zero volts, the parasitic substrate PNP transistors associated with the divider chain transistors become ‘active’ and reduce the voltage on the internal nodes of the bias generator. 
     FIG. 3 illustrates the schematic of the 5 volt ‘fail-safe’ input buffer. Elements the same as those illustrated in FIG. 2 will have the same reference number and will not be described in detail. The 3.3 volt supply is applied between V DD    201  and ground  201 . The 5 volt supply is applied to VHV  215  external to the integrated circuit and ground  207 , which can cause 5 volts to appear on the bond pad  213 , and through resistor  358  on the drain terminal  359  of transistor  360 . In this design, resistor  358  is used for ESD (electro-static discharge) protection and might not be required in all possible configurations. The gate of transistor  360  has a voltage applied from the bias generator (discussed previously) that is approximately 3.3 volts when V DD  is not ‘failed’ or sufficient when V DD  is ‘failed’ (zero volts) and the bond pad  213  is 5 volts to guarantee the transistor is ‘on’ and its drain to gate voltage is less than V OX     —     MAX  (the maximum allowed gate oxide voltage). Transistor  360  acts as a source follower whenever the voltage at the bond pad  213  is 5 volts, limiting the voltage at node  361  to a value less than the voltage on BIAS node  211  minus V THRESHOLD  (approximately 0.8 volts), protecting transistors  362  and  364  from the external 5 volt signal levels. When V DD  is ‘active’ and the bond pad  213  is zero volts, transistor  360  is ‘on’ and the voltage at node  361  is approximately zero volts. The action of transistor  360  driven by the bias circuit of FIG. 2 is the heart of the input circuit protection of this invention. 
     The conventional input circuit (not a part of the invention) is now described for clarity. Transistors  362  and  364  form an inverter circuit stage, transistors  366 ,  368  and  370  provide hysteresis for noise reduction and transistors  372  and  374  form a second inverter stage. The full input circuit shown is a non-inverting input buffer with output at SIG_IN to Other Internal Circuitry  100  (FIG.  1 ). 
     FIG. 4 illustrates the schematic of the 5 volt ‘fail-safe’ output buffer. Elements the same as those illustrated in FIG. 2 will have the same reference number and will not be described in detail. The open drain output circuit comprises transistor  496  driven at its gate by internal logic and the fail-safe protection related transistor  490 . Transistor  496  is protected by the action of transistor  494  driven at its gate by the bias circuit of this invention. 
     The 3.3 volt V DD  supply is applied between V DD    201  and ground  207 . The 5 volt supply is applied to VHV  215  external to the integrated circuit and ground  207 , which can cause 5 volts to appear on the bond pad  213 , and through resistor  492  on the drain terminal of transistor  494 . In this design resistor  492  is used for ESD protection and might not be required in all possible configurations. The gate of transistor  494  has a voltage applied from the bias generator (discussed previously) that is approximately 3.3 volts when V DD  is not ‘failed’ or sufficient when V DD  is ‘failed’ (zero volts) and the bond pad  213  is 5 volts to guarantee the transistor  260  is ‘on’ and its drain  259  to gate  211  voltage is less than V OX     —     MAX  (the maximum allowed gate oxide voltage). When V DD  is ‘failed’ (zero volts) and the bond pad  213  is at 5 volts transistor  496  ‘off’ and no current flows through transistor  494 , which is in the ‘on’ condition, to ground. When V DD  is not ‘failed’ (V DD =3.3 volts) transistor  490  is ‘off’ and transistor  496  will be ‘on’ if the voltage at its gate is 3.3 volts, resulting in its drain voltage approaching zero volts and the bond pad  213  voltage being below V OL  (the maximum specified voltage output low). The output signal is in phase with SIG_OUT  405  in polarity but varies from 0 volts to 5 volts, whereas SIG_OUT varies from 0 volts to 3.3 volts. The action of transistor  494  driven at its gate by the BIAS output of the bias circuit of FIG. 2 of this invention and the action of transistor  490  driven at its gate by the VDF output of the bias circuit of FIG. 2 of this invention is the heart of the output circuit protection of this invention. 
     The three stage inverter driver comprised by transistors  478 ,  480 ,  482 ,  484 ,  486 , and  488  with output at node  483  are conventional circuitry for driving an open drain output buffer and are shown for illustration only and are not part of the invention. 
     FIG. 5 illustrates the schematic of the bias generator of this invention showing all P-channel transistors and associated parasitic substrate PNP transistors that can become ‘active’ during circuit operation. The operation of the circuit was described above but now the parasitic substrate PNP transistor action will be further illustrated in the modes of circuit operation where it could occur. All P-channel devices have associated parasitic substrate PNP devices. When a P-channel device has its substrate connected to a node where the voltage can be less positive than a source/drain terminal the parasitic substrate PNP can become ‘active’ and thereby sustain current flow during circuit operation. 
     In the V DD  ‘active’ mode (V DD =3.3 volts), transistor  546  is ‘on’ sourcing current through resistor  548  causing a sufficient voltage at the gate  537  of transistor  544  to turn it ‘on’ assuring that the voltage at VDF  509  is essentially zero volts. In this case transistor  550  is ‘on’ and BIAS  511  has a low impedance path to V DD  (3.3 volts). Note that transistor  550  is a P-channel transistor with its source tied to BIAS  511  which forms an active parasitic substrate PNP transistor  560  which adds to the DC leakage current from the V DD  supply. This connection is required to satisfy circuit operation when V DD  is ‘failed’. In this mode, the remainder of the components in the bias generator have no appreciable effect on VDF  509  and BIAS  511  voltages. 
     Note that all the P-channel transistors in the 5 volt divider chain ( 522 ,  524 ,  526 ,  534 ,  538 ,  540 ,  542 ) are in isolated N-wells and have specific source/drain to N-well short orientations. Transistors  522 ,  524  and  526  must be P-channel transistors to limit the voltage across their gate oxides when the externally applied bond pad  213  voltage transitions from 5 volts to ground. Their orientation eliminates parasitic substrate PNP current flow when the bond pad  213  is at 5 volts and discharges node  527  to within 3 V BE  (V BE =base−emitter ‘on’ voltage of bipolar transistor) of ground when the bond pad is at 0 volts. Transistor  540  is oriented to prevent substrate PNP action when BIAS node  211  has a positive voltage less than one V THRESHOLD  (approximately 0.8 volt) above the voltage at VDF node  209 . Transistors  538 ,  540  and  542  are oriented to make their parasitic substrate PNP transistors ‘inactive’ when the bond pad  213  is at 5 volts. 
     Operation of the bias generator in the V DD  ‘failed’ mode is as follows. Assume the bond pad  213  is at 0 volts. Since there is no voltage stress applied to the integrated circuit transistors and no functionality is required VDF node  209  and BIAS node  211  can be near zero volts. When the applied bond pad  213  voltage rises to 5 volts, current flows in the divider chain (including resistor  536  and transistors  522 ,  524 ,  526 ,  528 ,  530 ,  532  and  534 ) establishing a voltage less than V OX     —     MAX  (maximum allowed gate oxide voltage) at node  527 . The voltage at VDF node  209 , the gate of transistor  550  rises faster than the voltage at BIAS node  211  ensuring that transistor  550  remains ‘off’ and turning ‘on’ output transistor  294  (not illustrated in FIG. 5, see FIG. 2) and/or input transistor  260  (FIG.  2 ). As transistors  294  and/or  260  turn ‘on’, capacitance is established from their drain/source nodes to their gate and the changing voltage across this capacitance causes BIAS node  211  voltage to increase to one V THRESHOLD  (approximately 0.8 volt) above VDF node  209  at which point transistor  550  conducts to V DD , limiting the positive excursion of the voltage on BIAS node  211 . Parasitic substrate PNP action at this node also limits this voltage. The positive voltage at VDF node  209  is also applied to the gate of transistor  290  assuring that transistor  296  is held ‘off’. The divider components (including resistor  536  and transistors  522 ,  524 ,  526 ,  528 ,  530 ,  532  and  534 ) are designed to conduct as little as possible and still assure proper circuit operation to minimize the current drawn from the external source when it is at 5 volts. They act as forward biased diode connected transistors with very high ‘on’ resistance. When the externally applied voltage at the bond pad  212  transitions to zero volts, the parasitic substrate PNP transistors associated with the divider chain transistors become ‘active’ and reduce the voltage on the internal nodes of the bias generator. 
     Because of the irregular conditions which circuit nodes are exposed to under the onset of the V DD  failed condition, analysis of the full fail-safe bias circuit operation with parasitic transistor action included in the simulation models was an essential part of the development of this invention. Special layout techniques including the practice of placing critical P-channel devices in isolated N-wells with special orientation were used.