Abstract:
A circuit that may be configured to provide a first well bias voltage to the output buffer when the output buffer is in a first mode and to provide a second well bias voltage to the output buffer when the output buffer is in a second mode. The first well bias voltage and the second well bias voltage may be used to maintain a reverse bias in diffusion wells used for electrical isolation of transistors.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/204,180, filed May 15, 2000 and U.S. Provisional Application No. 60/204,423, filed May 13, 2000, which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for a high voltage tolerant circuit fabricated with a low voltage process generally and, more particularly, to an n-well bias control within the circuit that may not require high voltage transistors. 
     BACKGROUND OF THE INVENTION 
     The use of low voltage technologies (i.e., 3.3 volts) for CMOS transistor circuits create problems in TTL environments. Some transistors of a low voltage circuit must provide an interface to an output of a TTL capable circuit. The TTL capable circuits may drive the interface to overvoltage (i.e., as high as 5.5 volts) conditions. Consequently, the low voltage technologies must include fabrication steps that result in some or all of the transistors being tolerant of the TTL voltage levels. Such fabrication steps add to the complexity and cost of parts fabricated by using the low voltage technologies. 
     Another problem created by mixing 3.3 volt MOS transistors with 5 volt TTL signals is in the basic operation of p-channel type field effect transistors (PFET). A diode formed between a p-type drain diffusion in an n-type well of the PFET must remain reverse biased for proper operation. When the interface drives a PFET drain to 5 volts, and the n-well is biased to only 3.3 volts, then the drain-to-well diode becomes forward biased. The forward biased diode creates a leakage path from the interface to a power source for the low voltage circuit. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a circuit that may be configured to provide a first well bias voltage to the output buffer when the output buffer is in a first mode and to provide a second well bias voltage to the output buffer when the output buffer is in a second mode. The first well bias voltage and the second well bias voltage may be used to maintain a reverse bias in diffusion wells used for electrical isolation of transistors. 
     The objects, features and advantages of the present invention include providing a well bias voltage control circuit within a high voltage tolerant interface circuit that may (i) maintain low voltages across transistor thin gate oxides, (ii) present signals at the interface at reasonable drive strength levels, and/or (iii) minimize leakage currents between the interface and a supply voltage source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
     FIG. 1 is a block diagram of a circuit implementing the present invention; 
     FIG. 2 is a block diagram of an interface circuit within the circuit; 
     FIG. 3 is a graph of a control signal as a function of a pad voltage; 
     FIG. 4 is a schematic of a switch circuit; 
     FIG. 5 is an alternative embodiment of a high voltage detection circuit; and 
     FIG. 6 is a schematic of an output driver and an output buffer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a block diagram of a circuit  100  is shown in accordance with a preferred embodiment of the present invention. The circuit  100  generally comprises a buffer  102 , an output driver  104 , an output buffer  106 , an input buffer  108 , and an interface circuit  110 . The buffer  102  may be implemented as a data output buffer. 
     The data out buffer  102  may have an input  112  that may receive a signal (e.g., DOUT). The data out buffer  102  may have another input  114  that may receive another signal (e.g., OE). The data out buffer  102  may have an output  116  that may present a signal (e.g., DA). The data out buffer  102  may have another output  118  that may present another signal (e.g., DB). 
     The output driver  104  may have an input  120  that may receive the signal DA from the data out buffer  102 . The output driver  104  may have another input  122  that may receive a signal (e.g., NWB) from the interface circuit  110 . The output driver  104  may have an input  123  that may receive a signal (e.g., CNT) from the interface circuit  110 . The output driver  104  may have an input  124  that may receive a signal (e.g., VPAD). The output driver  104  may have an output  126  that may present a signal (e.g., DC). 
     The output buffer  106  may have an input  128  that may receive the signal DC from the output driver  104 . The output buffer  106  may have another input  130  that may receive the signal DB from the data out buffer  102 . The output buffer  106  may have an input  132  that may receive the signal NWB from the interface circuit  110 . The output buffer  106  may have an output  134  that may present the signal VPAD. 
     The input buffer  108  may have an input  136  that may receive the signal VPAD. The input buffer  108  may have an output  138  that may present a signal (e.g., DIN). The signal DIN may be presented internally and/or externally to the circuit  100 . 
     The interface circuit  110  may have an output  140  that may present the signal CNT to the output driver  104 . The interface circuit  110  may have another output  142  that may present the signal NWB. The interface circuit  110  may have a bidirectional interface  144  that may present and receive the signal VPAD. 
     The circuit  100  may have an input  146  that may receive a supply voltage (e.g., VCC). The circuit  100  may have another input  148  that may receive a ground voltage (e.g., GND). The ground voltage GND may also be implemented as a floating ground (e.g., VSS) or other ground-like voltage to meet the design criteria of a particular implementation. The supply voltage VCC may provide low voltage electrical power to the circuit  100 . In one example, the supply voltage VCC may be 3.3 volts with respect to ground voltage GND. However, other supply voltages VCC may be used to meet the design criterial of a particular application. 
     The signal DOUT may be a data-output signal. The signal DOUT may range from a logical HIGH value (e.g., 3.3 volts) and a logical LOW value (e.g., zero volts or ground) at different times. The signal OE may serve as an output enable signal. While the signal OE is in an enable state, the data out buffer  102  may present the signals DA and DB equal to the signal DOUT. While equal to the signal DOUT, the signals DA and DB may convey information destined for the output buffer  106 . While the signal OE is in a disable state, the data out buffer  102  may present the signal DA in the logical HIGH state and the signal DB in the logical LOW state. While in opposite states, the signals DA and DB may ultimately serve to tri-state the output buffer  106 . 
     The output driver  104  may present the signal DC to be the same as the signal DA while the signal VPAD is below an overvoltage threshold voltage. The output driver  104  may present the signal DC in the logical HIGH state and the logical LOW state at different times, responsive to the signal DA. The output driver  104  may present the signal DC in an overvoltage state (e.g., &gt;3.3 volts) while the signal VPAD is greater than or equal to the overvoltage threshold voltage. While the signal VPAD is greater than or equal to the overvoltage threshold voltage, the signal DC may generally track the signal VPAD in voltage amplitude and polarity (e.g., DC=VPAD). 
     The signal NWB may be an n-well bias voltage. The signal NWB generally has a voltage ranging in amplitude from the supply voltage VCC to the signal VPAD. The signal NWB may be used to bias all n-type wells in the circuit  100  or just selected n-type wells for transistors in the output driver  104 , the output buffer  106 , and the interface circuit  110 . 
     The signal CNT may be a control signal. The signal CNT may be in the logical LOW state while the signal VPAD is less than the overvoltage threshold voltage. The signal CNT may be in the overvoltage state while the signal VPAD is greater than or equal to the overvoltage threshold voltage. The signal CNT generally tracks the voltage amplitude and polarity of the signal VPAD (e.g., CNT=VPAD) while the signal CNT is in the overvoltage state. 
     The signal VPAD may convey input data and output data. The signal VPAD is generally a voltage on an interface pad of the circuit  100 . As output data, the signal VPAD may vary between the logical LOW state and the logical HIGH state at different times. As input data, the signal VPAD may vary between the logical LOW state and the logical HIGH state at different times when driven by another low voltage circuit operating from the supply voltage VCC. The signal VPAD may also vary between the logical LOW state and the overvoltage state (e.g., a TTL logical HIGH state) at different times when driven by a TTL compatible circuit. 
     The signal DIN may be a buffered input signal. The signal DIN may vary between the logical LOW state and the logical HIGH state at different times. The signal DIN generally tracks the signal VPAD in voltage amplitude. However, the signal DIN may be limited in voltage amplitude to the supply voltage VCC. In one example, the input buffer  108  may present the signal DIN with the same logical polarity as the signal VPAD (e.g., the input buffer  108  may be non-inverting). Alternatively, the input buffer  108  may present the signal DIN with an opposite logical polarity as the signal VPAD (e.g., the input buffer  108  may be inverting) to meet the design criterial of a particular implementation. 
     In one example, the transistors of the circuit  100  may be fabricated with a 0.3 micron technology. The 0.3 micron technology may allow for a maximum voltage across a thin gate oxide of 3.6 volts. Since the signals VPAD, CNT and NWB may have voltages greater than 3.6 volts, the design of the circuit  100  is generally arranged so that gate-to-source voltages, gate-to-drain voltages, and gate-to-channel substrate voltages may experience a maximum voltage no greater than the supply voltage VCC (e.g., 3.3 volts). 
     Referring to FIG. 2, a schematic of the interface circuit  110  is shown. The interface circuit  110  generally comprises a pad  150 , a circuit  152 , and a circuit  154 . The pad  150  may provide an interface to other circuits or busses internal and/or external to the circuit  100 . The circuit  152  may be implemented as a control generator. The circuit  154  may be implemented as a multiplexer or a switching circuit. 
     The control generator  152  may have an input  155  that may receive the signal VPAD. The control generator  152  may have an input  156  that may receive the signal NWB. The control generator  152  may have an input  157  that may receive the supply voltage VCC. The control generator  152  may have an input  158  that may receive the ground voltage GND. The control generator  152  may have the output  140  that may provide the signal CNT. 
     The switch circuit  154  may have an input  160  that may receive the signal CNT from the control generator  152 . The switch circuit  154  may have an input  162  that may receive the signal VPAD from the pad  150 . The switch circuit  154  may have an input  163  that may receive the supply voltage VCC. The switch circuit  154  may have the output  142  that may present the signal NWB. 
     The control generator  152  generally comprises a circuit  164 , a circuit  166 , and a circuit  168 . The circuit  164  may be implemented as a high voltage detection circuit. The circuit  166  may be implemented as a trim circuit. The circuit  168  may be implemented as an inverter circuit. 
     The high voltage detection circuit  164  may receive the signal VPAD from the pad  150 . The high voltage detection circuit  164  may receive the signal NWB from the switch circuit  154 . The high voltage detection circuit  164  may also receive the supply voltage VCC and the ground voltage GND. The high voltage detection circuit  164  may present a signal (e.g., DET) to the trim circuit  166 . 
     The trim circuit  166  may receive the signal DET from the high voltage detection circuit  164 . The trim circuit  166  may also receive the supply voltage VCC and the ground voltage GND. The trim circuit  166  may present a signal (e.g., TRIM) to the inverter circuit  168 . 
     The inverter circuit  168  may receive the signal TRIM from the trim circuit  166 . The inverter circuit  168  may receive the supply voltage VCC and the ground voltage GND. The inverter circuit  168  may present the signal CNT. 
     The signal DET may be a high voltage detection signal or a high voltage trip signal. The signal DET may range in voltage from the logical LOW state through the overvoltage state. The signal DET may be in the logical LOW state when the signal VPAD is less than the overvoltage threshold. The signal DET may be in the overvoltage state while the signal VPAD is greater than or equal to the overvoltage threshold. 
     The signal TRIM may be another high voltage detection signal or another high voltage trip signal. The signal TRIM may range from the logical LOW state to the logical HIGH state. The signal TRIM may generally be the signal DET arranged to transition between states around a lower voltage. 
     The high voltage circuit  164  may comprise a transistor  170  and a transistor  172 . The transistor  170  may be implemented as a p-type enhancement mode field effect transistor (PFET). The transistor  172  may be implemented as an n-type enhancement node field effect transistor (NFET). However, other types of transistors may be implemented in accordance with the design criteria of a particular implementation. For example, NPN bipolar transistors, PNP biploar transistors, junction field effect transistors, metal-semiconductor field effect transistors, metal-oxide-semiconductor field effect transistors, depletion mode field effect transistors, and the like may be implemented. 
     The transistor  170  may have a source node coupled to the pad  150  that may receive the signal VPAD. The transistor  170  may have a gate node biased by the supply voltage VCC. The transistor  170  may have a drain node coupled to a drain node of the transistor  172 . The transistor  170  may have an n-type well that may receive the signal NWB from the switch circuit  154 . 
     The transistor  172  may have the drain node coupled to the drain node of the transistor  170 . The transistor  172  may have a gate node biased by the supply voltage VCC. The transistor  172  may have a source node biased by the ground voltage GND. The transistor  172  may have a p-type channel substrate biased by the ground voltage GND. 
     The signal DET may be presented from the node connecting the drain node of the transistor  170  to the drain node of the transistor  172 . In general, the transistor  172  may be configured to always conduct or “on”. Consequently, the transistor  172  may bias or pull the signal DET toward the logical LOW state. 
     Since the gate node of the transistor  170  may be biased by the supply voltage VCC, the transistor  170  may be normally configured to be non-conducting or “off” while the signal VPAD is less than the overvoltage threshold voltage. As the signal VPAD increases in voltage above the supply voltage VCC, the source node of the transistor  170  may become more positive than the gate node. When the signal VPAD reaches the supply voltage VCC plus a PFET gate-to-source threshold voltage (e.g., VTP), then the transistor  170  may begin to conduct or switch “on”. The voltage VCC+VTP may be referred to as the overvoltage threshold. By designing the transistor  170  to have a strong conductance and the transistor  172  to have a weak conductance, the transistor  170  may pull the signal DET toward the logical HIGH state. As the signal VPAD increases in voltage, conductance of the transistor  170  may generally increase causing the signal DET to increase. For sufficiently large values of the signal VPAD, the signal DET may generally track the signal VPAD (e.g., DET=VPAD). Consequently, the signal DET may range in voltage from zero volts to 5.5 volts, or greater. The signal DET may be in the overvoltage state while greater than the supply voltage VCC. 
     The trim circuit  166  generally comprises a transistor  174 , a transistor  176 , a transistor  178 , and an inverter  180 . The transistor  174  may be implemented as an NFET. The transistor  176  may be implemented as a PFET. The transistor  178  may be implemented as a PFET. However, other types of transistors may be implemented to meet the design criteria of a particular application. 
     The transistor  174  may have a drain node coupled to the drain node of the transistor  172  that may receive the signal DET. The transistor  174  may have a source node coupled to an input of the inverter  180 . The transistor  174  may have a gate node biased by the supply voltage VCC. The transistor  174  may have a channel substrate couple to the ground voltage GND. 
     The transistor  176  may have a source node coupled to a drain node of the transistor  178 . The transistor  176  may have a gate node coupled to an output of the inverter  180  that may receive the signal TRIM. The transistor  176  may have a drain node coupled to the input of the inverter  180 . The transistor  176  may have a channel substrate biased by the supply voltage VCC. 
     The transistor  178  may have a source node biased by the supply voltage VCC. The transistor  178  may have a gate node biased by the ground voltage GND. The transistor  178  may have the drain node coupled to the source node of the transistor  176 . The transistor  178  may have an n-type well biased by the supply voltage VCC. 
     The inverter  180  may have the input coupled to the source node of the transistor  174  and the drain node of the transistor  176 . The inverter  180  may have the output node coupled to the gate node of the transistor  176  and to the inverter circuit  168 . The inverter  180  may present the signal TRIM. The inverter  180  may receive the supply voltage VCC and the ground voltage GND. 
     The signal DET received at the drain node may be presented as another signal (e.g., DET 2 ) at the source node of the transistor  174 . The transistor  174  may limit an upper voltage that the signal DET 2  may achieve. Since the gate node of the transistor  174  may be biased by the supply voltage VCC, then the signal DET 2  may only have an upper voltage of VCC-VTN, where VTN is an NFET gate-to-source threshold voltage. The signal DET 2  may range from the logical LOW state to the logical HIGH state. Consequently, the transistor  174  generally isolates the rest of the trim circuit  166  from the voltages conveyed by the signal DET greater than the supply voltage VCC. 
     The transistor  176  may be conducting or “on” while the signal TRIM is in the logical LOW state and not conducting or “off” while the signal TRIM is in the logical HIGH state. While the transistor  176  is conducting and the signal DET is in the overvoltage state, the transistor  176  generally biases or pulls the signal DET 2  toward the supply voltage VCC and the top of the logical HIGH state. Here, the signal DET 2  may achieve a sufficiently high voltage to cause a PFET transistor at the input of the inverter  180  to stop conducting or switch “off”. Without the transistor  176 , a small direct current crowbar current may flow through the inverter  180  while the signal DET 2  is at VCC-VTN volts. While the signal DET 2  is in the logical LOW state, the signal TRIM may cause the transistor  176  to stop conducting. With the transistor  176  not conducting, the transistor  172  may bias or pull the signal DET 2  to the bottom of the logical LOW state. 
     The transistor  176  and the transistor  178  may have a very weak conductance so that while the signal DET is in the logical LOW state, the transistor  172  may bias or pull the signal DET 2  to the logical LOW state. To achieve the very weak conductance, a channel width to length ratio of one or both of the transistor  176  and the transistor  178  may be very small (e.g., W/L may range from 1/200 to 1/50, most preferably W/L=1/100). A large gate node capacitance may result from the large channel length. Driving the large gate node capacitance with the inverter  180  may slow a response time of the signal TRIM. To help reduce the capacitive loading of the output of the inverter  180 , the transistor  176  may have a nominal channel width to length ratio (e.g., W/L may range from 10/1 to 1/10, most preferably W/L=2/1). The transistor  178  may have the large channel width to length ratio (e.g., W/L ranging from 1/200 to 1/50). Consequently, a gate node capacitance of the transistor  176  may be made reasonably small as compared to a gate node capacitance of the transistor  178 . 
     The net effect of the trim circuit  166  may be to present the signal TRIM as a skewed version of the signal VPAD moved downward in voltage and delayed in time. The signal VPAD generally transitions between the logical LOW state and the overvoltage state around the overvoltage threshold (VCC+VTP). The signal TRIM generally transitions between the logical LOW state and the logical HIGH state around a conventional CMOS threshold (e.g., approximately midway between the supply voltage VCC and the ground voltage GND). Therefore, the signal TRIM may cross the CMOS threshold a short time after the signal VPAD crosses the overvoltage threshold. 
     The inverter circuit  168  generally comprises a transistor  182 , a transistor  184 , and a transistor  186 . The transistor  182  may be implemented as an NFET. The transistor  184  may be implemented as an NFET. The transistor  186  may be implemented as a PFET. However, other types of transistors may be implemented to meet the design criteria of a particular application. 
     The transistor  182  may have a drain node coupled to a source node of the transistor  184 . The transistor  182  may have a gate node coupled to the output of the inverter  180  that may receive the signal TRIM. The transistor  182  may have a source node coupled to the ground voltage GND. The transistor  182  may have a channel substrate coupled to the ground voltage GND. 
     The transistor  184  may have a drain node coupled to a drain node of the transistor  186 . The transistor  184  may have a gate node biased by the supply voltage VCC. The transistor  184  may have the source node coupled to the drain node of the transistor  182 . The transistor  184  may have a channel substrate biased by the ground voltage GND. 
     The transistor  186  may have a source node coupled to the signal VPAD. The transistor  186  may have a gate node biased by the supply voltage VCC. The transistor  186  may have the drain node coupled to the drain node of the transistor  184 . The transistor  186  may have a n-type well biased by the signal NWB. 
     The transistor  186  generally has a gate-to-source threshold voltage that matches a gate-to-source threshold voltage of the transistor  170 . Consequently, the transistors  170  and  186  may transition from non-conducting or “off” to conducting or “on” at approximately the same time. Otherwise, the transistor  186  may transition to conducting at a different threshold than the transistor  170 . The transistor  170  becoming conducting may ultimately cause the transistor  182  to become non-conducting. The transistor  184  may protect the transistor  182  from voltages conveyed by the signal CNT greater than the supply voltage VCC. 
     Referring to FIG. 3, a graph of the signal CNT as a function of the signal VPAD is shown. The signal CNT may be presented from the combined drain node of the transistor  186  and the drain node of the transistor  184 . The signal CNT is generally in the logical LOW state (e.g., voltage range 188) while the signal VPAD is less than the overvoltage threshold (e.g., voltage 190). The signal CNT is generally in the overvoltage state (e.g., voltage range 192) while the signal VPAD is greater than or equal to the overvoltage threshold. The signal CNT may track the signal VPAD in voltage amplitude and polarity while in the overvoltage state. 
     Referring to FIG. 4, a schematic of the switch circuit  154  is shown. The switch circuit  154  generally comprises a transistor  194  and a transistor  196 . The transistor  194  may be implemented as a PFET. The transistor  196  may be implemented as a PFET. However, other types of transistors may be implemented to meet the design criteria of a particular application. 
     The transistor  194  may have a source node coupled to a drain node of the transistor  196 . The transistor  194  may have a gate node biased by the supply voltage VCC. The transistor  194  may have a drain node coupled to the pad  150  that may receive the signal VPAD. The transistor  194  may have an n-type well coupled to the drain node. 
     The transistor  196  may have a source node biased by the supply voltage VCC. The transistor  196  may have a gate node coupled to the control generator  152  that may receive the signal CNT. The transistor  196  may have the drain node coupled to the source node of the transistor  194 . The transistor  196  may have an n-type well coupled to a source node of the transistor  196 . 
     The combination of the drain node of the transistor  196  and the source node of the transistor  194  may present the signal NWB. The signal NWB may be controlled by the signals VPAD and the signal CNT. While the signal VPAD is less than the overvoltage threshold  190  the signal CNT may be in the logical LOW state. Consequently, the transistor  194  may be non-conducting and the transistor  196  may be conducting. The signal NWB may be pulled or switched to the supply voltage VCC in response to the transistor  196  conducting and the transistor  194  non-conducting. 
     While the signal VPAD is greater than or equal to the overvoltage threshold  190 , the signal CNT may be in the overvoltage state. Consequently, the transistor  194  may be conducting and the transistor  196  may be non-conducting. The signal NWB may be pulled or switched to the signal VPAD in response to the transistor  194  conducting and the transistor  196  non-conducting. 
     The signal NWB is generally fed back to the high voltage detection circuit  164 , the inverter circuit  168 , the output buffer  106  and the output driver  104 . However, the signal NWB may be presented to other circuits and transistors within the circuit  100 . The signal NWB in the overvoltage state generally prevents diodes formed by p-type drain node diffusions in n-type wells of the PFETs from becoming forward biased during overvoltage conditions. The reverse biased diodes, therefore, generally prevent the signal VPAD from coupling with the supply voltage VCC. 
     Referring to FIG. 5, a schematic of an alternative embodiment of the high voltage detection circuit  164 A is shown. The alternative high voltage detection circuit  164 A may divide the transistor  172  into a transistor  198  and a transistor  200 . The transistor  198  may be implemented as an NFET. The transistor  200  may be implemented as an NFET. However, other types of transistors may be implemented to meet the design criterial of a particular implementation. 
     The transistor  198  may have a drain node coupled to the drain node of the transistor  170 . The transistor  198  may have a gate node biased by the supply voltage VCC. The transistor  198  may have a source node coupled to a drain node of the transistor  200 . The transistor  198  may have a channel substrate biased by the ground voltage GND. 
     The transistor  200  may have the drain node coupled to the source node of the transistor  198 . The transistor  200  may have a gate node biased by the supply voltage VCC. The transistor  200  may have a source node biased by the ground voltage GND. The transistor  200  may have a channel substrate biased by the ground voltage GND. 
     Referring to FIG. 6, a schematic of the output driver  104  and the output buffer  106  is shown. The output driver  106  may comprise a transistor  202 , a transistor  204 , and a transistor  206 . The transistor  202  may be implemented as an NFET. The transistor  204  and the transistor  206  may be implemented as PFETs. 
     The transistor  202  and the transistor  204  may be configured as a pass gate disposed between the input  120  and the output  126 . The transistor  202  may have a drain node that may receive the signal DA from the data out buffer  102 . The transistor  202  may have a gate node biased to the supply voltage VCC. The transistor  202  may have a source node that may present the signal DC to the output buffer  106 . The transistor  202  may have a channel substrate biased to the ground voltage GND. 
     The transistor  204  may have a drain node that may receive the signal DA from the data out buffer  102 . The transistor  204  may have a gate node that may receive the signal CNT from the interface circuit  110 . The transistor  204  may have a source node that may present the signal DC to the output buffer  106 . The transistor  204  may have an n-type well that may receive the signal NWB from the switch circuit  154 . 
     The transistor  206  may have a drain node that may receive the signal VPAD from the pad  150 . The transistor  206  may have a gate node biased by the supply voltage VCC. The transistor  206  may have a source node coupled to the source node of the transistor  202  and the source node of the transistor  204 . The transistor  206  may have an n-type well that may receive the signal NWB from the switch circuit  154 . 
     While the signal OE is in the enable state and the signal CNT is in the logical LOW state, the pass gate formed by the transistors  202  and  204  generally passes the signal DA at the input  120  to the output  126  as the signal DC. While the signal OE is in the disable state and the signals VPAD and CNT are in the overvoltage state, the transistor  206  may be conducting or “on”. With the transistor  206  conducting, the signal VPAD may be presented from the input  124  to the output  126  as the signal DC. 
     The output buffer  106  generally comprises a transistor  208 , a transistor  210 , and a transistor  212 . The transistor  208  and the transistor  210  may be implemented as NFETs. The transistor  212  may be implemented as a PFET. However, other types of transistors may be implemented to meet the design criteria of a particular implementation. 
     The transistor  208  may have a drain node coupled to a source node of the transistor  210 . The transistor  208  may have a gate node that may receive the signal DB from the data out buffer  102 . The transistor  208  may have a source node biased to the ground voltage GND. The transistor  208  may have a channel substrate biased to the ground voltage GND. 
     The transistor  210  may have a drain node coupled to a drain node of the transistor  212 . The transistor  210  may have a gate node biased to the supply voltage VCC. The transistor  210  may have the source node coupled to the drain node of the transistor  208 . The transistor  210  may have a channel substrate biased to the ground voltage GND. 
     The transistor  212  may have a source node biased to the supply voltage VCC. The transistor  212  may have a gate node that may receive the signal DC from the output buffer  104 . The transistor  212  may have the drain node coupled to the drain node of the transistor  210 . The transistor  212  may have an n-type well that may receive the signal NWB from the switch circuit  154 . The combined drain node of the transistor  212  and the drain node of the transistor  210  may present the signal VPAD to the pad  150 . 
     While the signals DB and DC are both in the logical HIGH state or the logical LOW state, the output buffer  106  may be in an active or low voltage mode. The output buffer  106  may drive the signal VPAD to the opposite logical state (e.g., VPAD=/DC) while in the active mode. While the signal DB is in the logical LOW state and the signal DC is in the logical HIGH state, the output buffer  106  may be in a tri-state mode. The transistors  208  and  212  may be non-conducting or “off” while the output buffer  106  is in the tri-state mode. The output buffer  106  thus may present a high impedance to the pad  150  while in the tri-state mode. 
     The signal VPAD may be presented in the overvoltage state to the output buffer  106 . The output buffer  106  may be considered in an overvoltage or high voltage mode. While the output buffer  106  is in the overvoltage mode, the transistor  210  may provide a drain-to-source voltage drop that maintains the gate-to-drain voltage of the transistor  208  to less than the supply voltage VCC. The signal VPAD in the overvoltage state may also cause the transistor  206  of the output driver  104  to conduct or switch “on”. When the transistor  206  conducts, the signal VPAD may be passed to the output  126  of the output driver  104  as the signal DC. In turn, the signal DC in the overvoltage state may insure that the transistor  212  may be non-conducting or “off” since the gate-to-source voltage of transistor  212  may be approximately zero volts. 
     While the signal DC is in the overvoltage state, a source-to-drain voltage drop across the transistor  202  may prevent the signal DA from being driven to the overvoltage state through the transistor  202 . The signal CNT may also enter the overvoltage state thus causing the transistor  204  to be non-conducting or “off”. The transistor  206  may be designed to have a weak conductance so that the signal DC is not driven into the overvoltage state until the signal CNT switches the transistor  204  off. As a result ,the signal DA may be prevented from being driven to the overvoltage state through the transistor  204 . 
     The PFET transistors that may have the signal VPAD applied to a non-well node (e.g., the source node or drain node) may also have the gate node biased to the supply voltage VCC, directly to the signal VPAD, or indirectly to the signal VPAD (e.g., through the signal CNT and the signal DC). Consequently, a voltage drop less than the supply voltage VCC may be maintained across the gate insulators of the PFET transistors. The NFET transistors that may have the signal VPAD applied to the source node or drain node may also have the gate node biased to the supply voltage VCC. As a result, a voltage drop less than the supply voltage VCC may be maintained across the gate insulators of the NFET transistors. 
     The various signals of the present invention are generally “on” (e.g., a logical or digital HIGH, or 1) or “off” (e.g., a logical or digital LOW, or 0). However, the particular polarities of the on (e.g., asserted) and off (e.g., de-asserted) states of the signals may be adjusted (e.g., reversed) accordingly to meet the design criteria of a particular implementation. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.