Patent Publication Number: US-8536925-B2

Title: Voltage level translator circuit

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to electrical and electronic circuitry, and more particularly relates to voltage level translation circuits. 
     BACKGROUND OF THE INVENTION 
     A field-effect-transistor (FET) comprises a gate oxide, which is an insulating layer between a gate and a channel region of the transistor. When used in digital logic applications, FETs are often fabricated with what is referred to as a core gate oxide, which, in recent integrated circuit (IC) fabrication technologies, is typically a very thin gate oxide, such as, for example, about 2 nanometers (nm) or less. Core or thin gate oxide transistors are typically capable of supporting, without damage, only relatively low voltages (e.g., core level voltages), for example, about 1.2 volts (V) or less. A transistor comprising a core gate oxide is often referred to as a core transistor and supports core voltage levels. 
     In certain applications, including, for example, some input/output (I/O) buffer and analog applications, transistors capable of supporting, without damage, higher voltages (e.g., I/O level voltages), for example, about 1.98, 3.63 or 5.5 volts, are required. A transistor capable of supporting these relatively higher I/O level voltages is typically fabricated having what is typically referred to as a thick gate oxide which, in recent technologies, may include devices having gate oxide thicknesses of, for example, about 2.3 nm or greater. A transistor comprising a thick gate oxide is often referred to as a thick oxide transistor and supports higher I/O voltage levels. Many IC fabrication processes provide both core transistors and thick oxide transistors. 
     In certain applications, such as, for example, in a hot carrier injection (HCI) application, in order to somewhat increase the voltage that a transistor device can withstand without experiencing long-term damage, a channel length of the device can be increased. However, this can significantly increase the area required by a circuit employing such transistors, which is undesirable. It is also known to use a triple gate oxide process in IC fabrication for providing transistors having even thicker gate oxides, and therefore supporting higher voltage levels without sustaining damage. Such transistors may be used in high-voltage applications, including, for example, electrostatic discharge (ESD) protection. However, in standard IC fabrication processes, such as, for example, 40-nm technology, one is restricted to using only a single thick oxide transistor type, primarily because adding an extra thick gate oxide to the process inherently lowers the yield of the fabricated devices and adds unnecessary cost and complexity. Additionally, as gate oxide increases, gate capacitance increases accordingly, thereby degrading high-frequency performance of the device. This forces a circuit designer to make a decision as to which of the available thick gate oxides will be used in a system-on-a-chip (SoC) design. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide techniques which enable the use of transistors having a thinner gate oxide thickness in applications which require driving voltage levels higher than otherwise supported by the fabrication process. In this manner, transistors having enhanced high-frequency performance can be utilized to drive higher voltage levels without sustaining damage to the transistor. In order to accomplish this, the present invention, in illustrative embodiments thereof, provides a voltage level translator circuit and I/O buffer, employing only core transistors and thick oxide transistors of a single type, that provide translated voltage levels above input voltage levels and above a maximum voltage level otherwise supported by the IC fabrication process. 
     In accordance with one aspect of the invention, a voltage translator circuit includes an input stage adapted for receiving an input signal referenced to a first voltage supply, a latch adapted for connection to a second voltage supply and operative to at least temporarily store a logic state of the input signal, and a voltage clamp coupled between the input stage and the latch. The voltage clamp is operative to set a maximum voltage across the latch to a first prescribed level and to set a maximum voltage across the input stage to a second prescribed level. The voltage translator circuit generates a first output signal at a junction between the latch and the voltage clamp. The voltage translator circuit generates a second output signal at a junction between the voltage clamp and the input stage. 
     In accordance with another embodiment of the invention, a buffer circuit includes at least one voltage translator circuit. The voltage level translator includes an input stage adapted for receiving an input signal referenced to a first voltage supply, a latch adapted for connection to a second voltage supply and operative to at least temporarily store a logic state of the input signal, and a voltage clamp coupled between the input stage and the latch. The voltage clamp is operative to set a maximum voltage across the latch to a first prescribed level and to set a maximum voltage across the input stage to a second prescribed level. The voltage translator circuit generates a first output signal at a junction between the latch and the voltage clamp. The voltage translator circuit generates a second output signal at a junction between the voltage clamp and the input stage. 
     The buffer circuit further includes a first pre-driver circuit coupled to the at least one voltage translator circuit, the first pre-driver circuit being operative to receive the first output signal and to generate a first control signal as a function thereof, and a second pre-driver circuit coupled to the at least one voltage translator circuit, the second pre-driver circuit being operative to receive the second output signal and to generate a second control signal as a function thereof. An output stage is coupled to the first and second pre-driver circuits. The output stage includes at least a first pull-up device adapted for connection between a voltage supply of the buffer circuit and an input/output pad of the buffer circuit, and at least one pull-down device adapted for connection between a voltage return of the buffer circuit and the input/output pad. 
     These and other features, objects and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are presented by way of example only, and without limitation, wherein like reference numerals indicate similar elements throughout the several views of the drawings, and wherein: 
         FIG. 1  illustrates a block diagram of a first output buffer comprising two separate voltage translators, one for driving a pullup transistor and one for driving a pulldown transistor; 
         FIG. 2  is a block diagram depicting an exemplary output buffer circuit  200 , according to an embodiment of the invention; 
         FIG. 3  is an electrical schematic diagram depicting at least a portion of an exemplary composite voltage translator circuit, according to an embodiment of the present invention; 
         FIG. 4  illustrates exemplary voltage levels for certain input, output and internal nodes of the composite voltage translator circuit shown in  FIG. 3 , according to an embodiment of the present invention; 
         FIG. 5  illustrates exemplary waveforms corresponding to the composite voltage translator circuit shown in  FIG. 3 , according to an embodiment of the present invention; 
         FIG. 6  illustrates additional exemplary waveforms corresponding to the composite voltage translator circuit shown in  FIG. 3 , according to an embodiment of the present invention; 
         FIG. 7  is a logical flow diagram depicting an exemplary method for translating signal voltage levels, according to an embodiment of the present invention; 
         FIG. 8  is a partial cut-away view depicting an exemplary packaged IC device comprising a composite voltage translator circuit formed in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described herein in the context of illustrative I/O buffer and voltage level translator circuits. It should be understood, however, that the present invention is not limited to these or any other particular circuit arrangements. Rather, embodiments of the invention are directed broadly to techniques for beneficially translating voltages in a manner which provides a circuit with the capability to drive voltage levels higher than a maximum voltage otherwise supported by transistors used to the circuit without sustaining damage. Furthermore, the techniques presented herein do not require the use of multiple thick gate oxide transistor types, thereby reducing cost and complexity and improving yield. 
     Although implementations of the present invention are described herein with specific reference to p-channel metal-oxide-semiconductor (PMOS) and n-channel metal-oxide-semiconductor (NMOS) transistor devices, as may be formed using a complementary metal-oxide-semiconductor (CMOS) fabrication process, it is to be appreciated that the invention is not limited to such transistor devices and/or such a fabrication process, and that other suitable devices, such as, for example, bipolar junction transistors (BJTs), etc., and/or fabrication processes (e.g., bipolar, BiCMOS, etc.), may be similarly employed, as will be understood by those skilled in the art. Moreover, although preferred embodiments of the invention are typically fabricated in a silicon wafer, embodiments of the invention can alternatively be fabricated in wafers comprising other materials, including but not limited to Gallium Arsenide (GaAs), Indium Phosphide (InP), etc. 
     As previously explained, the term “core transistors” may be defined as transistors comprising a relatively thin gate oxide (e.g., less than about 2 nm) and capable of supporting core voltage levels (e.g., about 1.2 volts or less) without damage. For example, an illustrative core transistor device has a gate oxide thickness of about 12 Angstrom (1.2 nm) and can support voltage levels ranging from about 0 volts to about 0.945 volts across any two terminals of the device without sustaining measurable damage. The maximum voltage associated with a core transistor may be defined as the voltage that the core transistor is designed to withstand without sustaining damage over the intended lifetime of the transistor. Damage to a transistor device may be manifested by gate oxide breakdown, substantial increase in gate oxide leakage current, and/or substantial change in a core transistor characteristic, for example, threshold voltage or transconductance. Core transistors are often used in core digital logic circuitry. 
     The term “thick oxide transistors,” on the other hand, may be defined as transistors comprising a relatively thick gate oxide (e.g., greater than about 2.3 nm), in comparison to core transistors, and capable of supporting voltages higher than core voltages (e.g., about 1.98 volts or greater) without damage. Some voltages supported by typical thick oxide transistors are 1.98, 3.63 or 5.5 volts. Generally, the higher the supported voltage, the thicker the gate oxide that is required. For example, an illustrative thick oxide transistor device has a gate oxide thickness of about 2.6 nm and can support voltage levels ranging from about 0 volts to about 1.98 volts. The maximum voltage associated with a thick oxide transistor may be defined as the voltage that the thick oxide transistor is designed to withstand without sustaining damage over the intended lifetime of the transistor. Thick oxide transistors are often used for input, output and I/O buffers and analog applications, and are therefore sometimes referred to as I/O transistors. 
     Many IC fabrication processes offer two types of transistor devices, namely, core transistors and thick oxide transistors. Processes providing only a single thick oxide transistor type require the IC designer to decide which gate oxide thickness to select. This decision is often dependent on the functionality, specifications and/or other performance objectives of the IC. When a high speed interface is needed, such as, for example, double data rate-2 (DDR2) and double data rate-3 (DDR3) synchronous dynamic random-access memory (SDRAM) interfaces, it is advantageous to pick the thinnest of the thick oxide transistors available in the given IC fabrication process that will support the required voltage. The thinnest of the thick oxide transistors available will typically have the highest speed and thus support the required performance of the interface. 
     Some ICs have conflicting requirements pertaining to the selection of gate oxide thickness to meet prescribed speed and voltage handling requirements. For example, an IC may comprise a circuit that requires supporting 1.8-volt signal levels with high-speed (e.g., greater than one gigahertz) performance and thus require, for example, a thinner thick gate oxide transistor. However, the same IC may also comprise circuitry (e.g., I/O buffers) that requires driving a higher voltage, such as, for example, 3.63 volts, which a thinner gate oxide I/O device would not support. However, devices having a thicker gate oxide than typical thick oxide devices are generally not sufficient for high-speed operation. Examples of circuits requiring a higher voltage are circuits designed to accommodate legacy protocols, such as, for instance, advanced technology attachment (ATA) and consumer electronics advanced technology attachment (CE-ATA) which operate at 3.3 volts nominal. 
       FIG. 1  is a block diagram depicting an illustrative output buffer circuit  100 . The output buffer circuit  100  includes a first voltage level translator  110 , which may be a p-type voltage level translator (P-voltage translator), a first pre-driver  120 , which may be a p-type pre-driver (P-pre-driver), coupled to the first voltage level translator, a second voltage level translator  130 , which may an n-type voltage translator (N-voltage translator), and a second pre-driver  140 , which may be an n-type pre-driver (N-pre-driver), coupled to the second voltage level translator. Output buffer  100  may further comprise a PMOS pull-up transistor, MPIO, coupled to the first pre-driver  120 , a first (p-type) over-voltage stress protection circuit  150  coupled to the pull-up transistor device MPIO and to an external pad  170 , which may be an I/O pad (PAD), an NMOS pull-down transistor device, MNIO, coupled to the second pre-driver  140 , and a second (n-type) over-voltage stress protection circuit  160  coupled to the pull-down device MNIO and to the I/O pad. Pull-up transistor MPIO, pull-down transistor MNIO, and first and second over voltage stress protection circuits  150  and  160  form at least a portion of an output stage of the buffer circuit  100 . 
     More particularly, first voltage level translator  110  is preferably operative to receive at least a first input signal  171 , which may be a P-input signal, supplied thereto, and to generate a first level-shifted output signal that is a function of the first input signal. First pre-driver  120  is operative to receive the first level-shifted output signal and to generate a first control signal, VPG, for driving the PMOS pull-up transistor MPIO in the output stage of buffer circuit  100 . A source (S) of PMOS device MPIO is adapted for connection to a first voltage source, which may be an I/O voltage supply, VDD 33  (e.g., about 3.3 volts), a drain (D) of MPIO is connected to a first node of the first over-voltage protection circuit  150  at node PADP, and a gate (G) of MPIO is adapted to receive the first control signal VPG. It is to be appreciated that, because a metal-oxide-semiconductor (MOS) device is symmetrical in nature, and thus bi-directional, the assignment of source and drain designations in the MOS device is essentially arbitrary. Therefore, the source and drain may be referred to herein generally as first and second source/drain, respectively, where “source/drain” in this context denotes a source or a drain. 
     Likewise, second voltage level translator  130  is preferably operative to receive at least a second input signal  172 , which may be an N-input signal, supplied thereto, and to generate a second level-shifted output signal that is a function of the second input signal. Second pre-driver  140  is operative to receive the second level-shifted output signal and to generate a second control signal, VNG, for driving the NMOS pull-down transistor MNIO in the output stage of buffer circuit  100 . A source of NMOS device MNIO is adapted for connection to a second voltage source, which may be VSS or ground (e.g., about zero volts), a drain of MNIO is connected to a first node of the second over-voltage protection circuit  160  at node PADN, and a gate of MNIO is adapted to receive the second control signal VNG. Second nodes of the first and second over-voltage protection circuits  150  and  160 , respectively, are connected together and adapted for connection to the I/O pad  170 . 
     As apparent from the figure, the first voltage level translator  110 , the first pre-driver circuit  120  and PMOS pull-up device MPIO are all preferably powered from the first voltage source, which in this illustrative embodiment is VDD 33 . Additionally, first voltage level translator  110  and first pre-driver  120  are coupled to a third voltage source, VPBIAS, which serves as a voltage return for the respective circuits. Thus, an output signal generated by either the first voltage level translator  110  or the first pre-driver  120  will have a maximum output voltage swing between VDD 33  and VPBIAS. First voltage source VDD 33  supplies, for example, a nominal voltage of about 3.3 volts. VDD 33  may have a prescribed tolerance associated therewith, typically about ten percent, so that the supplied voltage VDD 33  is between about 2.97 and 3.63 volts. VPBIAS is approximately VDD 33 −1.98 volts and therefore is preferably in a range of about 1.65 to 0.99 volts. 
     Similarly, the second voltage level translator  130 , the second pre-driver circuit  140  and NMOS pull-down device MNIO are all preferably powered from a fourth voltage source, which in this illustrative embodiment is VNBIAS. Additionally, second voltage level translator  130  and second pre-driver  140  are coupled to the second voltage source VSS which serves as a voltage return for the respective circuits. Thus, an output signal generated by either the second voltage level translator  130  or the second pre-driver  140  will have a maximum output voltage swing between VNBIAS and VSS. VSS preferably supplies, for example, a nominal voltage of 0 volts. Since the voltage across the gate-source terminals of NMOS device MNIO should not exceed about 1.98 volts in order to prevent damage to the NMOS device, the maximum voltage supplied by the fourth voltage source VNBIAS is preferably about 1.98 volts. 
     In an illustrative embodiment, the pull-up transistor MPIO is, for example, a 1.8V, 26-angstrom, thick gate oxide PMOS transistor. For this particular IC fabrication process, MPIO, being a thick gate oxide device, can tolerate about 1.98 volts (e.g., 1.8 volts plus ten percent tolerance) across any two of its terminals. The pull-down transistor MNIO is, for example, a 1.8V, 26-angstrom, thick gate oxide NMOS transistor. MNIO, being a thick gate oxide device, can tolerate about 1.98 volts (e.g., 1.8 volts plus ten percent) across any two of its terminals. 
     The voltage on I/O pad  170 , supplied by output buffer  100 , may be as high as 3.63 volts (e.g., nominal 3.3 volts plus ten percent tolerance). Therefore, the pull-down transistor MNIO must be protected from over-voltage stress, that is, any voltage over about 1.98 volts. Second over-voltage protection circuit  160  is coupled between node PADN and I/O pad  170  so that the voltage on node PADN does not exceed 1.98 volts. Similarly, the voltage on I/O pad  170 , supplied by output buffer circuit  100 , may be as low as 0 volts. Therefore, the pull-up transistor MPIO must be protected from over voltage stress, that is, any voltage below about VDD 33  minus 1.98 volts. First over-voltage protection circuit  150  is coupled between node PADP and I/O pad  170  to ensure that the voltage on node PADP does not go below VDD 33  minus 1.98 volts. Over-voltage protection circuits suitable for use in output buffer circuit  100  are known in the art. 
     Because the source of the pull-down transistor MNIO is at 0 volts (VSS), the voltage on either the gate or the drain (at node PADN) of MNIO should not exceed 1.98 volts. Likewise, because the source of the pull-up transistor MPIO is at 1, the voltage on either the gate of the drain (at node PADP) of MPIO should not drop below VDD 33  minus 1.98 volts. Thus, the absolute value of the gate-to-source voltage, |Vgs|, of the pull-down transistor MNIO and of the pull-up transistor MPIO must not exceed 1.98 volts for this illustrative embodiment. It is to be understood that the invention is not limited to this maximum specified voltage, and that, depending upon the particular IC process technology used for fabricating the transistor devices in the output buffer circuit  100 , this maximum specified voltage can be higher or lower than 1.98 volts. 
     Nominal core voltage level signals are, for example, about 0 volts to about 0.9 volts. Because the N-input signal  172  is assumed to be a core voltage level signal, the N-input signal  172  requires translation to a voltage level between 0 volts and VNBIAS for the n-pre-driver  140 . N-voltage translator  130  is preferably operative to translate the core voltage levels, 0 volts and 0.9 volts, to corresponding voltage levels 0 volts and VNBIAS, respectively. Because P-input signal  171  is assumed to be a core voltage level signal, the P-input signal requires translation to a voltage level between VDD 33  and VPBIAS for the p-pre-driver  120 . P-voltage translator  110  is preferably operative to translate the core voltage levels, 0 volts and 0.9 volts, to corresponding voltage levels VPBIAS and VDD 33 , respectively. As shown in  FIG. 1 , the N-voltage translator  130  is a separate and distinct circuit from the P-voltage translator  110 . The N-voltage translator  130  provides different voltage levels than the P-voltage translator  110 , and therefore the N-voltage translator comprises different circuitry than the P-voltage translator. 
       FIG. 2  is a block diagram depicting an exemplary output buffer circuit  200 , according to an embodiment of the invention. The output buffer circuit  200  preferably comprises a single voltage level translator  210 , a first pre-driver  120 , which may be a p-type pre-driver (P-pre-driver), coupled to the voltage level translator, and a second pre-driver  140 , which may be an n-type pre-driver (N-pre-driver), coupled to the voltage level translator. Output buffer  200  may further comprise a PMOS pull-up transistor, MPIO, coupled to the first pre-driver  120 , a first (p-type) over-voltage stress protection circuit  150  coupled to the pull-up transistor device MPIO and to an external pad  170 , which may be an I/O pad (PAD), an NMOS pull-down transistor device, MNIO, coupled to the second pre-driver  140 , and a second (n-type) over-voltage stress protection circuit  160  coupled to the pull-down device MNIO and to the I/O pad. Pull-up transistor MPIO, pull-down transistor MNIO, and first and second over voltage stress protection circuits  150  and  160  form at least a portion of an output stage of the buffer circuit  200 . 
     More particularly, voltage level translator  210  is operative to receive an input signal  270 , which may be a core level input signal (e.g., about 0 to about 0.9 volts nominal), and to generate at least first and second level-shifted output signals, ZP and ZN, respectively, as a function of the input signal. First pre-driver  120  is operative to receive the first level-shifted output signal ZP and to generate a first control signal, VPG, for driving the PMOS pull-up transistor MPIO in the output stage of buffer circuit  200 . A source of PMOS device MPIO is adapted for connection to a first voltage source, which may be an I/O voltage supply, VDD 33  (e.g., about 3.3 volts nominal), a drain of MPIO is connected to a first node of the first over-voltage protection circuit  150  at node PADP, and a gate of MPIO is adapted to receive the first control signal VPG. 
     Likewise, second pre-driver  140  is operative to receive the second level-shifted output signal ZN and to generate a second control signal, VNG, for driving the NMOS pull-down transistor MNIO in the output stage of buffer circuit  200 . A source of NMOS device MNIO is adapted for connection to a second voltage source, which may be VSS or ground (e.g., about zero volts nominal), a drain of MNIO is connected to a first node of the second over-voltage protection circuit  160  at node PADN, and a gate of MNIO is adapted to receive the second control signal VNG. Second nodes of the first and second over-voltage protection circuits  150  and  160 , respectively, are connected together to form an output of buffer circuit  200  and are adapted for connection to the I/O pad  170 . 
     By way of example only, the pull-up transistor MPIO is preferably a 1.8-volt, 26-angstrom, p-channel, thick gate oxide field-effect transistor, although the invention is not limited to this specific device type and/or IC process technology. In this illustrative example, the pull-up transistor MPIO can tolerate 1.98 volts (1.8 volts plus ten percent tolerance) across any pair of its terminals. Similarly, the pull-down transistor MNIO is preferably a 1.8-volt, 26-angstrom, n-channel, thick gate oxide field-effect transistor, although the invention is not limited to this specific device type and/or IC process technology. In this illustrative example, the pull-down transistor MNIO can tolerate 1.98 volts across any pair of its terminals. 
     In this illustration, the voltage on I/O pad  170 , provided by the output stage of output buffer circuit  200 , may be as high as about 3.63 volts (e.g., the sum of the nominal 3.3 volts plus ten percent tolerance). Therefore, the pull-down transistor MNIO will require protection from over-voltage stress, that is, any voltage over about 1.98 volts in this example. Second over-voltage protection circuit  160  is coupled between node PADN and I/O pad  170  so that the voltage on node PADN does not exceed 1.98 volts. Similarly, the voltage on I/O pad  170 , supplied by output buffer circuit  200 , may be as low as 0 volts. Therefore, the pull-up transistor MPIO will require protection from over voltage stress, that is, any voltage below about VDD 33  minus 1.98 volts. First over-voltage protection circuit  150  is coupled between node PADP and I/O pad  170  to ensure that the voltage on node PADP does not go below VDD 33  minus 1.98 volts. 
     Because the source of the pull-down transistor MNIO is at 0 volts (VSS), the voltage on either the gate or the drain (at node PADN) of MNIO should not exceed 1.98 volts. Likewise, because the source of the pull-up transistor MPIO is at VDD 33 , the voltage on either the gate of the drain (at node PADP) of MPIO should not drop below VDD 33  minus 1.98 volts. Thus, the absolute value of the gate-to-source voltage, |Vgs|, of the pull-down transistor MNIO and of the pull-up transistor MPIO must not exceed 1.98 volts for this illustrative embodiment. It is to be understood that the invention is not limited to this maximum specified voltage, and that, depending upon the particular IC process technology used for fabricating the transistor devices in the output buffer circuit  200 , this maximum specified voltage can be higher or lower than 1.98 volts. 
     Voltage level translator  210 , the first pre-driver circuit  120  and PMOS pull-up device MPIO are all preferably powered from the first voltage source, which in this illustrative embodiment is VDD 33 . Additionally, the first pre-driver  120  is coupled to a third voltage source, VPBIAS, which serves as a voltage return for the first pre-driver. Thus, an output signal generated by the first pre-driver  120  will have a maximum output voltage swing between VDD 33  and VPBIAS. First voltage source VDD 33  supplies, for example, a nominal voltage of about 3.3 volts. VDD 33  may have a prescribed tolerance associated therewith, typically about ten percent, so that the supplied voltage VDD 33  is between about 2.97 and 3.63 volts. VPBIAS is approximately VDD 33 −1.98 volts, and therefore is preferably in a range of about 1.65 to 0.99 volts. 
     Similarly, the voltage level translator  210 , the second pre-driver circuit  140  and NMOS pull-down device MNIO are all coupled to the second voltage source VSS which serves as a voltage return for the respective circuits. Additionally, second pre-driver  140  is preferably powered from a fourth voltage source, which in this illustrative embodiment is VNBIAS. Thus, an output signal generated by the second pre-driver  140  will have a maximum output voltage swing between VNBIAS and VSS. VSS preferably supplies, for example, a nominal voltage of 0 volts. Since the voltage across the gate-source terminals of NMOS device MNIO should not exceed about 1.98 volts in order to prevent damage to the NMOS device (in this illustrative embodiment), the maximum voltage supplied by the fourth voltage source VNBIAS is preferably about 1.98 volts. 
     Nominal core voltage level signals are, for example, about 0 volts to about 0.9 volts. Because input signal  270  is assumed to be a core voltage level signal, the input signal requires translation to a voltage level between 0 volts and VNBIAS suitable for driving the n-pre-driver  140 . The single voltage translator  210  is preferably operative to translate the core voltage levels, 0 volts and 0.9 volts, to 0 volts and VNBIAS volts, respectively. Voltage translator  210  is further operative to translate the input signal  270  to a voltage level between VDD 33  and VPBIAS suitable for driving the p-pre-driver  120 . The single voltage translator  210  translates the core voltage levels, 0 volts and 0.9 volts, to VPBIAS and VDD 33 , respectively. As shown in  FIG. 2 , the single voltage translator  210  provides the translated voltages to both the p-pre-driver  120  and the n-pre-driver  140 . 
       FIG. 3  is a schematic diagram depicting at least a portion of an exemplary composite voltage translator circuit  300 , according to an embodiment of the invention. Composite voltage translator  300  may be suitable for use, for example, in the single voltage translator  210  shown in the illustrative buffer circuit  200  (see  FIG. 2 ). Voltage level translator circuit  300  can be used to translate an input signal (e.g., signal A) which is referenced to a lower core voltage supply, such as, for example, VDDCORE, to first and second output signals, ZP and ZN, which are referenced to different supply voltages. For example, in this illustrative embodiment, output signal ZP is referenced to voltage supplies VDD 33  and VPBIAS, and output signal ZN is referenced to voltage supplies VNBIAS and VSS. In many applications, the lower core voltage supply VDDCORE is typically about 0.9 volt and the higher voltage supply VDD 33  is typically about 3.3 volts. It is to be understood, however, that the present invention is not limited to these or to any particular voltage levels. 
     Composite voltage translator circuit  300  preferably comprises a voltage level translation circuit  320  including an input stage  322 , a voltage clamp  324  coupled to the input stage, and a latch  326  coupled to the voltage clamp. The input stage  322  preferably comprises a differential input stage operative to receive a first signal, AN, and a second signal, AA. As apparent from the figure, signal AN is a logical complement of input signal A supplied to circuit  300 , such that when signal A is a logic high level, signal AN is a logic low level, and vice versa. Signal AA is preferably a buffered version of input signal A, such that when signal A is a logic high level, signal AA is also a logic high level, and vice versa. Signals AN and AA may be generated, for example, by a buffer circuit  310  which includes a pair of inverters coupled together in series. 
     Specifically, buffer circuit  310  comprises a first PMOS transistor device, MPC 1 , a second PMOS transistor device, MPC 2 , a first NMOS transistor device, MNC 1 , and a second NMOS transistor device, MNC 2 . Sources of devices MPC 1  and MPC 2  are adapted for connection to the lower core voltage supply VDDCORE (e.g., about 0.9 volt nominal), or an alternative voltage supply, and sources of devices MNC 1  and MNC 2  are adapted for connection to voltage return VSS, or an alternative voltage reference source. Gates of MPC 1  and MNC 1  are connected together and form an input of the buffer circuit  310  for receiving the input signal A, and drains of MPC 1  and MNC 1  are connected together and form an output of the first inverter for generating the signal AN. Gates of MPC 2  and MNC 2  are connected together and form an input of the second inverter for receiving signal AN, and drains of MPC 2  and MNC 2  are connected together and form an output of the buffer circuit  310  for generating the signal AA. Although not required, the second inverter, comprising devices MPC 2  and MNC 2 , serves to buffer input signal A and to ensure that rise and fall times of the resulting signal AA are more closely matched to rise and fall times of signal AN. Devices MPC 1 , MPC 2 , MNC 1  and MNC 2  are preferably core transistors, since the highest voltage expected across any two terminals of a given one of the transistors in buffer circuit  310  is about VDDCORE. 
     Input stage  322  preferably comprises first and second NMOS transistor devices, MNIO 1  and MNIO 2 , respectively. Sources of devices MNIO 1  and MNIO 2  are adapted for connection to VSS, a drain of MNIO 1  is connected to a first node of the voltage clamp  324  (node I 5 ), a gate of MNIO 1  is adapted for receiving core signal AA, a drain of MNIO 2  is connected to a second node of the voltage clamp (node I 6 ), and a gate of MNIO 2  is adapted for receiving core signal AN. Devices MNIO 1  and MNIO 2  are preferably thick oxide devices. It is to be understood that alternative input stage configurations are similarly contemplated by the invention. 
     Latch  326  preferably comprises first and second PMOS transistor devices, MPIO 7  and MPIO 8 , respectively, connected in a cross-coupled arrangement. Specifically, sources of MPIO 7  and MPIO 8  are adapted for connection to the higher voltage supply VDD 33  (e.g., about 3.3 volts nominal), a drain of MPIO 7  is connected to a third node of the voltage clamp  324  (node I 1 ), a drain of MPIO 8  is connected to a fourth node of the voltage clamp (node I 2 ), a gate of MPIO 7  is connected to the drain of MPIO 8  at node I 2 , and a gate of MPIO 8  is connected to the drain of MPIO 7  at node I 1 . Latch  326  is operative to at least temporarily store a logic state of the input signal A. Devices MPIO 7  and MPIO 8  are preferably thick oxide devices. It is to be understood that alternative latch arrangements are similarly contemplated by the invention. 
     Voltage clamp  324  is coupled between input stage  322  and latch  326  and is preferably operative to set a maximum voltage across the latch to a first prescribed level and to set a maximum voltage across the input stage to a second prescribed level. More particularly, voltage clamp  324  preferably comprises first and second NMOS transistor devices, MNIO 3  and MNIO 4 , respectively, and first and second PMOS transistor devices, MPIO 5  and MPIO 6 , respectively. A source of MNIO 3  is connected to the input stage  322  at node I 5 , a source of MNIO 4  is connected to the input stage at node I 6 , and the gates of MNIO 3  and MNIO 4  are connected together and adapted to receive a first bias signal, VNBIAS, which may be a supply voltage of N-pre-driver  140  shown in  FIG. 2 . This ensures that the maximum voltage present at nodes I 5  or I 6  is about VNBIAS−V Tn , where V Tn  is a threshold voltage of NMOS devices MNIO 3  or MNIO 4 , respectively. 
     Similarly, a source of MPIO 5  is connected to the latch  326  at node I 1 , a source of MPIO 6  is connected to the latch at node I 2 , and gates of MPIO 5  and MPIO 6  are connected together and adapted to receive a second bias signal, VPBIAS, which may be a supply voltage of P-pre-driver  120  shown in  FIG. 2 . This ensures that the minimum voltage present at nodes I 1  or I 2  is about VPBIAS+V Tp , where V Tp  is a threshold voltage of PMOS devices MPIO 5  or MPIO 6 , respectively. Drains of MNIO 3  and MPIO 5  are connected together at node I 3 , and drains of MNIO 4  and MPIO 6  are connected together at node I 4 . It is to be understood that alternative voltage clamp configurations are similarly contemplated by the invention. Devices MNIO 3 , MNIO 4 , MPIO 5  and MPIO 6 , like the devices in the input stage  322  and latch  326 , are preferably thick oxide devices, such as, for example, thick gate oxide transistors comprising 26 Angstrom gate oxides capable of supporting about 1.98 volts. 
     In accordance with an embodiment of the invention, first and second bias signals VNBIAS and VPBIAS, respectively, may be connected to the same voltage source (e.g., about VDD 33 /2). It is to be appreciated that the invention is not limited to any specific voltage(s) for VNBIAS and VPBIAS. 
     Preferably, composite voltage translator circuit  300  comprises a first output buffer  331 , a second output buffer  332 , a third output buffer  333 , and a fourth output buffer  334 . Although first, second, third and fourth output buffers  331 ,  332 ,  333  and  334 , respectively, are depicted as inverting buffers, one or more of the output buffers may, alternatively, be non-inverting, as will be understood by those skilled in the art (e.g., by adding an inverter to an output node of a given one of the respectively output buffers). Output buffers  331 ,  332 ,  333  and  334  are coupled to nodes I 5 , I 1 , I 6  and I 2 , respectively, of voltage clamp  324  and are operative to buffer the respective output signals generated at these nodes. The output buffers  331 ,  332 ,  333 ,  334  further protect the corresponding nodes of the voltage clamp  324  to which they are connected from undesirable loading effects caused by another circuit or circuits coupled to the voltage translator circuit  300 . In this manner, output buffers  331 ,  332 ,  333 ,  334  advantageously provide symmetry to the voltage level translation circuit  320 , at least in terms of performance and load. It is to be understood that the invention is not limited to the particular buffer circuit arrangements shown. 
     More particularly, first output buffer  331  comprises an NMOS transistor device, MNIO 9 , and a PMOS transistor device, MPIO 10 , connected as a standard inverter. An input of buffer  331  is coupled to node I 5  of the voltage clamp  324  and is operative to generate a first output signal, ZN. Second output buffer  332  comprises an NMOS transistor device, MNIO 11 , and a PMOS transistor device, MPIO 12 , connected as a standard inverter. An input of buffer  332  is coupled to node I 1  of the voltage clamp  324  and is operative to generate a second output signal, ZP. Third output buffer  333  comprises an NMOS transistor device, MNIO 13 , and a PMOS transistor device, MPIO 14 , connected as a standard inverter. An input of buffer  333  is coupled to node I 6  of the voltage clamp  324  and is operative to generate a third output signal, ZNB, which is logical complement of output signal ZN. Fourth output buffer  334  comprises an NMOS transistor device, MNIO 15 , and a PMOS transistor device, MPIO 16 , connected as a standard inverter. An input of buffer  334  is coupled to node I 2  of the voltage clamp  324  and is operative to generate a fourth output signal, ZPB, which is logical complement of output signal ZP. 
     Optionally, voltage translator circuit  300  may comprise a first output latch  350  and a second output latch  360 . Each of the first and second output latches  350  and  360 , respectively, preferably includes a pair of inverters connected in a cross-coupled configuration and is operative to at least temporarily store a logic state of one or more of the output signals generated by the voltage translator circuit  300 . Specifically, first latch  350  preferably comprises first and second inverters,  352  and  354 , respectively. An input of the first inverter  352  is coupled to an output of the second inverter  354  and is adapted to receive the fourth output signal ZPB generate by buffer  334 . An input of the second inverter  354  is coupled to an output of the first inverter  352  and is adapted to receive the second output signal ZP generated by buffer  332 . Likewise, second latch  360  preferably comprises first and second inverters,  362  and  364 , respectively. An input of the first inverter  362  is coupled to an output of the second inverter  364  and is adapted to receive the third output signal ZNB generate by buffer  333 . An input of the second inverter  364  is coupled to an output of the first inverter  362  and is adapted to receive the first output signal ZN generated by buffer  331 . First latch  350  is preferably powered by VDD 33  and VPBIAS, and second latch  360  is powered by VNBIAS and VSS, as shown. In this manner, the first and second latches  350 ,  360  beneficially improve duty cycle distortion in the voltage translator circuit  300 . 
     In terms of operation, voltage level translation circuit  320  comprises first and second conduction paths between VSS and VDD 33 . The first conduction path comprises transistors MNIO 1 , MNIO 3 , MPIO 5  and MPIO 7 . The second conduction path comprises transistors MNIO 2 , MNIO 3 , MPIO 6  and MPIO 8 . When the buffered input signal AA transitions from low to high at substantially the same time that the inverted input signal AN transitions from high to low, the first conduction path is established momentarily until node I 2  is pulled high by MPIO 8 . After node I 2  is pulled high, the first conduction path is terminated by MPIO 7 . When the buffered input signal AA transitions from high to low at substantially the same time that the inverted input signal AN transitions from low to high, the second conduction path is established momentarily until node I 1  is pulled high by MPIO 7 . After node I 1  is pulled high, the second conduction path is terminated by MPIO 8 . When the buffered input signal AA and the inverted input signal AN remain stable, there is essentially no current flowing in either the first or the second conduction paths, except possibly leakage currents. 
     More particularly, by way of example only and without loss of generality,  FIG. 4  shows exemplary voltage levels for illustrative input signals, output signals and internal nodes of the composite voltage translator circuit  300  shown in  FIG. 3 , according to an aspect of the present invention. A first table  410  presents input signal voltage levels and corresponding node voltages and output signal voltage levels for the voltage translator circuit  300  in terms of general voltage supply levels (e.g., VDD 33 , VPBIAS, VNBIAS, VSS) and threshold voltages (e.g., V Tn , V Tp ). A second table  430  presents input signal voltage levels and corresponding node voltages and output signal voltage levels in terms of actual voltage values. In generating the voltage values shown in table  430 , exemplary voltages of the various voltage supplies and transistor thresholds are assumed to be as indicated in table  420 . 
     As apparent from  FIG. 4 , the nodes within the voltage translator circuit  300  reside at voltages corresponding to two logic levels. The logic levels are referred to herein as a low logic level and a high logic level. These logic levels correspond to voltages; for example, the low logic level corresponds to VSS, which may be approximately ground or 0 volt, and the high logic level corresponds to VDD 33 , which may be about 3.3 volts nominal (3.63 volts maximum). The input signal A to the voltage translator circuit  300 , as well as inverted input signal AN and buffered input signal AA supplied to the voltage level translation circuit  320  ( FIG. 3 ), may be referenced to different voltage supplies. Thus, in the present example, a logic low level (LOW) input signal corresponds to about 0 volt nominal and a logic high level (HIGH) input signal corresponds to about 0.9 volt nominal. 
     With reference again to  FIG. 3 , consider the case when the input signal A is high, and therefore the buffered input signal AA is high and the inverted input signal AN is low. Signal AA being high turns on NMOS device MNIO 1  and node I 5  is pulled low (e.g., to VSS). Since NMOS device MNIO 3  will be turned on (assuming VNBIAS is greater than about an NMOS transistor threshold voltage V Tn ), node I 3  will be pulled low. Since PMOS device MPIO 5  is turned on by VPBIAS, the voltage at node I 1  will be a PMOS transistor threshold voltage (V Tp ) above VPBIAS. This, in turn, will turn on PMOS device MPIO 8 , thereby pulling node I 2  high (e.g., to VDD 33 ). Node I 2  being high turns off PMOS device MPIO 7 , thereby allowing node I 1  to be controlled by MPIO 5  essentially without interference. 
     Since PMOS device MPIO 6  is turned on by VPBIAS, node I 4  will be pulled high to about VDD 33 . Signal AN being low turns off MNIO 2 , thereby allowing node I 6  to be controlled by NMOS device MNIO 4  essentially without interference. With MNIO 4  turned on as a result of signal VNBIAS supplied to the gate thereof, node I 6  would otherwise be pulled to VDD 33 . However, the gate voltage VNBIAS on MNIO 4  prevents node I 6  from exceeding a threshold voltage below VNBIAS (i.e., VNBIAS−V Tn ), thereby protecting NMOS device MNIO 2  from sustaining damage. Thus, under the condition when input signal A is high, the maximum voltage across any two terminals of devices MNIO 1  or MNIO 2  will be about VNBIAS−V Tn , and the maximum voltage across any two terminals of devices MNIO 3  or MNIO 4  will be about VDD 33 −(VNBIAS+V Tp ), which will be less than about 2.0 volts for the illustrative case shown in  FIG. 4 . Similarly, the maximum voltage across any two terminals of devices MPIO 5  or MPIO 6  will be about VDD 33 −(VPBIAS+V Tp ), which will be less than about 2.0 volts, and the maximum voltage across any two terminals of devices MPIO 7  or MPIO 8  will be about VPBIAS+V Tp , which will be about 2.0 volts. 
     Likewise, consider the case when the input signal A is low, and therefore the buffered input signal AA is low and the inverted input signal AN is high. Signal AN being high turns on MNIO 2  and node I 6  is pulled low (e.g., to VSS). MNIO 4  will be turned on (assuming VNBIAS is greater than about an NMOS threshold voltage V Tn ), and therefore node I 4  will be pulled low. Since MPIO 6  is turned on by VPBIAS, the voltage at node I 2  will be a PMOS threshold voltage (V Tp ) above VPBIAS. This, in turn, will turn on MPIO 7 , thereby pulling node I 1  high (e.g., to VDD 33 ). Node I 1  being high turns off MPIO 8 , thereby allowing node I 2  to be controlled by MPIO 6  essentially without interference. 
     Since MPIO 5  is turned on by VPBIAS, node I 3  will be pulled high (e.g., to about VDD 33 ). Signal AA being low turns off MNIO 1 , thereby allowing node I 5  to be controlled by MNIO 3  essentially without interference. With MNIO 3  turned on as a result of VNBIAS supplied to the gate thereof, node I 5  would otherwise be pulled to VDD 33 . However, the gate voltage VNBIAS on MNIO 3  prevents node I 5  from exceeding a threshold voltage below VNBIAS (i.e., VNBIAS−V Tn ), thereby protecting MNIO 1  from sustaining damage. Thus, when input signal A is low, like the condition when signal A is high, the maximum voltage across any two terminals of devices MNIO 1  or MNIO 2  will be about VNBIAS−V Tn , and the maximum voltage across any two terminals of devices MNIO 3  or MNIO 4  will be about VDD 33 −(VNBIAS+V Tp ), which would be less than about 2.0 volts for the illustrative case shown in  FIG. 4 . 
     The illustrative voltage values shown in table  430  of  FIG. 4  are based on the assumption that VSS is about 0 volt, VDD 33  is about 3.63 volts maximum (e.g., 3.3 volts plus ten percent), VNBIAS is about 1.98 volts maximum (e.g., 1.8 volts plus ten percent), VPBIAS is about 1.65 volts minimum (e.g., 1.8 volts minus ten percent), and NMOS and PMOS threshold voltages, V Tn  and V Tp , respectively, are about 0.2 volt for each of the thick oxide transistors. Thus, for the case when the input signal A is low (e.g., about 0 volt), signal AA will be about 0 volt, signal AN will be about 0.9 volt, node I 1  will be about 3.63 volts, node I 2  will be about 1.85 volts, node I 3  will be about 3.63 volts, node I 4  will be about 0 volt, node I 5  will be about 1.78 volts, node I 6  will be about 0 volt, output signal ZN will be about 0 volt, and output signal ZP will be about 1.65 volts. Similarly, for the case when signal A is high (e.g., about 0.9 volt), signal AA will be about 0.9 volt, signal AN will be about 0 volt, node I 1  will be about 1.85 volts, node I 2  will be about 3.63 volts, node I 3  will be about 0 volt, node I 4  will be about 3.63 volts, node I 5  will be about 0 volt, node I 6  will be about 1.78 volts, output signal ZN will be about 1.98 volts, and output signal ZP will be about 3.63 volts. It is to be understood that these voltage values are merely illustrative, and that the invention is not limited to any particular voltage values. 
       FIG. 5  depicts exemplary waveforms  500  corresponding to the composite voltage translator circuit  300  shown in  FIG. 3  under worst-case fast conditions. The waveforms  500  correspond to the illustrative voltages shown in tables  420  and  430  of  FIG. 4 . Waveform  510  represents the input signal A toggling between low (e.g., 0 volt) and high (e.g., 0.9 volt) logic states. The remaining waveforms correspond to internal nodes of the voltage level translation circuit  320  ( FIG. 3 ). Specifically, waveform  520  represents node I 6 , waveform  530  represents node I 5 , waveform  540  represents node I 4 , waveform  550  represents node I 3 , waveform  560  represents node I 2 , and waveform  570  represents node I 1  of voltage level translation circuit  320 . The voltages in each waveform (y-axis) are labeled in units of volts, and each of the waveforms is referenced to some arbitrary unit of time (x-axis). 
       FIG. 6  shows additional exemplary waveforms  600  corresponding to the composite voltage translator circuit  300  shown in  FIG. 3  under worst-case fast conditions. The waveforms  600  correspond to the illustrative voltages shown in tables  420  and  430  of  FIG. 4 . The first waveform  510  represents the input signal A toggling between low (e.g., 0 volt) and high (e.g., 0.9 volt) logic states. The remaining waveforms correspond to output signals generated by the composite voltage translator circuit  300 . Specifically, waveform  620  represents output signal ZN generated by first output buffer  331 , waveform  630  represents output signal ZP generated by second output buffer  332 , waveform  640  represents output signal ZNB generated by third output buffer  333 , and waveform  650  represents output signal ZPB generated by fourth output buffer  334  of the composite voltage translator circuit  300  ( FIG. 3 ). As apparent from  FIG. 6 , output signals ZN and ZNB toggle between VSS and VNBIAS as a function of the input signal A, and output signals ZP and ZPB toggle between VDD 33  and VPBIAS as a function of the input signal A. The voltages in each waveform (y-axis) are labeled in units of volts, and each of the waveforms is referenced to some arbitrary unit of time (x-axis). 
       FIG. 7  is a logical flow diagram depicting an exemplary method  700  for translating signal voltage levels, according to an embodiment of the present invention. In step  710 , an input signal referenced with respect to a first voltage level, which may be ground or VSS, and a second voltage level, which may be a core voltage supply, VDDCORE, is provided to a voltage translator circuit (e.g., circuit  300  shown in  FIG. 3 ). The input signal may toggle, for example, between about 0 volts and 0.9 volts, although the invention is not limited to any specific voltage levels. 
     In step  720 , the voltage translator circuit preferably generates, as a function of the input signal, at least a first output signal referenced with respect to a third voltage level, which may be a P-bias voltage, VPBIAS, and a fourth voltage level, which may be an I/O voltage supply, VDD 33 . For example, the first output signal may toggle between the third voltage level of about 1.65 volts and the fourth voltage level of about 3.63 volts, although the invention is not limited to any specific voltage levels. 
     In step  730 , the voltage translator circuit generates, as a function of the input signal, at least a second output signal referenced with respect to a fifth voltage level, which may be an N-bias voltage, VNBIAS, and a sixth voltage level, which may be ground or VSS. For example, the second output signal may toggle between the fifth voltage level of about 0 volts and the sixth voltage level of about 1.98 volts, although the invention is not limited to any specific voltage levels. 
     At least a portion of the techniques of the present invention may be implemented in one or more ICs. In forming ICs, die are typically fabricated in a repeated pattern on a surface of a semiconductor wafer. Each of the die includes a device described herein, and may include other structures or circuits. Individual die are cut or diced from the wafer, then packaged as integrated circuits. One skilled in the art would know how to dice wafers and package die to produce integrated circuits. Integrated circuits so manufactured are considered part of this invention. 
       FIG. 8  is a partial cut-away view depicting an exemplary packaged IC device  800  comprising a voltage translator circuit according to an embodiment of the present invention. The packaged IC device  800  comprises a leadframe  802 , a die  804  attached to the leadframe and a plastic encapsulation mold  808  surrounding the die and leadframe. Although the figure depicts only one type of IC package, the invention is not so limited; rather, the invention may comprise an IC die enclosed in any package type (e.g., ceramic, ball grid array (BGA), quad flat pack (QFP), etc.). 
     The die  804  comprises at least one voltage translator circuit according to an embodiment of the invention, such as, for example, one or more of the embodiments depicted in  FIGS. 2 and 3 . For example, in one embodiment, the die  804  comprises a single voltage translator (e.g.  210  in  FIG. 2 ). In another embodiment, the die  804  preferably comprises the composite voltage translator circuit  300  shown in  FIG. 3 . In yet another embodiment, the die  804  comprises an output buffer (e.g.,  200  in  FIG. 2 ) comprising the composite voltage translator circuit. 
     An IC in accordance with the present invention can be employed in essentially any application and/or electronic system. Suitable systems for implementing aspects of the invention may include, but are not limited to, personal computers, communication networks, portable communications devices (e.g., cell phones), solid-state media storage devices, etc. Systems incorporating such integrated circuits are considered part of this invention. Given the teachings of the invention provided herein, one of ordinary skill in the art will be able to contemplate other implementations and applications of the techniques of the invention. 
     Although illustrative embodiments of the invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made therein by one skilled in the art without departing from the scope of the appended claims.