Abstract:
In an integrated circuit, a voltage level shifter transitions an input signal at a first voltage level to an output signal at a second voltage level. The voltage level shifter generally includes switching elements, such as transistors, that control switching the output signal between logical zero and logical one values. The switching elements have a maximum voltage below which they can operate. The maximum voltage is less than the second voltage level. The voltage across the switching elements is limited to less than the maximum voltage.

Description:
FIELD 
     The subject matter herein relates to a voltage level shifter for electronic circuits, such as for translating electrical signals from within an integrated circuit (IC) to outside the IC, where the IC has an internal operating voltage at a different voltage level than an external transfer voltage. 
     BACKGROUND 
     Integrated circuits (IC&#39;s) of today typically operate internally at voltages that are lower than those used in IC&#39;s of just a few years ago. For example, a few years ago, an “internal operating voltage” of about 5.0 volts was common for many IC&#39;s. More recently, an internal operating voltage of about 3.3 volts has become common. Today, internal operating voltages of about 1.8 volts or less have come, or are coming, into use for IC&#39;s. 
     When two IC&#39;s having different internal operating voltages are to be used together, a voltage level shifter is used to shift one of the IC&#39;s output signals from the IC&#39;s internal operating voltage to that of the other IC. Since newer IC&#39;s are typically designed to be compatible with older, “legacy,” IC&#39;s, the voltage level shifter is commonly incorporated into the newer IC&#39;s. 
     A newer IC may have a lower internal operating voltage at which most of the functions of the IC operate and a higher “external transfer voltage” at which output signals are transferred to other IC&#39;s. The voltage level shifter, thus, transitions the output signals from the lower internal operating voltage to the higher external transfer voltage. Additionally, input signals are typically shifted by the IC from the higher external transfer voltage to the lower internal operating voltage. 
     An exemplary prior art voltage level shifter  100 , as shown in FIG. 1, receives an input signal V IN    102 , typically a digital signal operating at a given clock frequency and voltage level, and produces therefrom an output signal V OUT    104  at the same frequency, but a higher voltage level. The input signal V IN    102  is supplied by internal core IC circuitry (not shown), performing the normal functions of the IC (not shown). The output signal V OUT    104  is supplied to output pins (not shown), which connect to other circuitry or IC&#39;s (not shown), possibly through a printed circuit board (not shown). Additionally, a logic one value for the digital input signal V IN    102  has about the same voltage level as a core voltage V CORE    106 , i.e. the internal operating voltage, used to power of the internal core IC circuitry. A logic one value for the digital output signal V OUT    104  has about the same voltage level as an I/O (input/output) voltage V IO    108 , i.e. the external transfer voltage, used to power the I/O functions of the IC. 
     The voltage level shifter  100  includes thick oxide N-MOSFET transistors  110  and  112 , thick oxide P-MOSFET transistors  114  and  116  and a thin oxide inverter  118 . The sources of the transistors  110  and  112  are connected to ground  120 . The drains of the transistors  110  and  112  are connected to the drains of the transistors  114  and  116 , respectively. The sources of the transistors  114  and  116  are connected to the I/O voltage  108 . The drain of the transistor  110  is also connected to the gate of the transistor  116 . The drain of the transistor  112  is connected to the gate of the transistor  114  and also supplies the output signal  104 . The inverter  118  is connected to the core voltage  106  and to the ground  120 . 
     The input signal  102  is supplied to the gate of the transistor  110  and to the input of the inverter  118 . The inverter  118 , powered by the core voltage  106 , inverts the input signal  102 . The inverted input signal  102  is supplied to the gate of the transistor  112 . 
     Therefore, when the input signal  102  is at a logic zero (i.e. approximately zero volts), the logic zero at the gate of the transistor  110  causes the transistor  110  to turn “off,” and the inverted input signal  102  (i.e. logic one) at the gate of the transistor  112  causes the transistor  112  to turn “on.” Since the transistor  112  is “on,” the drain of the transistor  112  (and, thus, the output signal  104  and the gate of the transistor  114 ) is “pulled down” to approximately ground, or zero volts or logic zero. The logic zero on the gate of the transistor  114 , thus, turns “on” the transistor  114 , so the drain of the transistor  114  (and the gate of the transistor  116 ) is “pulled up” to the voltage level of the I/O voltage  108 , i.e. a logic one. The logic one on the gate of the transistor  116 , thus, turns “off” the transistor  116 , so as not to interfere with the logic zero on the drain of the transistor  112  and the output signal  104 . 
     On the other hand, when the input signal  102  is at a logic one (i.e. the internal operating voltage), the logic one at the gate of the transistor  110  causes the transistor  110  to turn “on,” and the inverted input signal  102  (i.e. logic zero) at the gate of the transistor  112  causes the transistor  112  to turn “off.” Since the transistor  110  is “on,” the drain of the transistor  110  (and, thus, the gate of the transistor  116 ) is “pulled down” to approximately ground, or zero volts or logic zero. The logic zero on the gate of the transistor  116 , thus, turns “on” the transistor  116 , so the drain of the transistor  116  (and, thus, the output signal  104  and the gate of the transistor  114 ) is “pulled up” to the voltage level of the I/O voltage,  108  (i.e. a logic one at the external transfer voltage). The logic one on the gate of the transistor  114 , thus, turns “off” the transistor  114 , so as not to interfere with the logic zero on the drain of the transistor  110  and the gate of the transistor  116 . 
     With the lower internal operating voltages and higher clock frequencies coming into use with many IC&#39;s, the thick oxide N-MOSFET transistors  110  and  112  cannot perform adequately. The internal operating voltages, for example, are becoming so low that they are approaching the “threshold voltage” of the transistors  110  and  112 . The threshold voltage of a transistor is the minimum voltage that can be applied to the gate of the transistor to activate the transistor. Therefore, if the internal operating voltage (i.e. the logic one voltage of the input signal  102 ) becomes as low as the threshold voltage of the transistors  110  and  112 , then the transistors  110  and  112  cannot be activated and the voltage level shifter  100  will not operate. Additionally, if the logic on&amp; voltage level of the input signal  102  is relatively larger than the threshold voltage of the transistors  110  and  112 , then the transistors  110  and  112  can be turned on relatively fast. However, if the logic one voltage level of the input signal  102  is relatively close to the threshold voltage of the transistors  110  and  112 , then the transistors  110  and  112  will switch from “off” to “on” relatively slowly. In this case, the transistors  110  and  112  cannot be activated quickly enough for the desired clock frequency of the IC, and the voltage level shifter  100  will not operate. 
     It is with respect to these and other background considerations that the subject matter herein has evolved. 
     SUMMARY 
     The subject matter described herein involves an integrated circuit (IC) having a voltage level shifter capable of operating with the lower internal operating voltages and higher clock frequencies used by current and upcoming IC&#39;s. The transistors (i.e. “switching transistors”) within the voltage level shifter that are activated, or “switched on and off,” by the internal operating voltage of the IC incorporate a thinner oxide than in the prior art. Therefore, the threshold voltage of the switching transistors is lower than the thicker-oxide transistors in the prior art, so the internal operating voltage required to turn “on,” or “activate,” the switching transistors is lower than the voltage required to turn “on” the transistors in the prior art. Additionally, the frequency at which the switching transistors can be switched “on” and “off” at this voltage is higher than that for the transistors in the prior art at this voltage. 
     Also, the switching transistors have a maximum voltage below which they can operate that is less than the external transfer voltage of the IC. Therefore, additional transistors are placed in the current path between the switching transistors and the I/O voltage source in order to limit the voltage drop across the gate oxide (i.e. from the drain to the source) of the switching transistors. The reduced voltage across the gate oxide prevents the switching transistors from “failing” due to the presence of the external transfer voltage on the drain of the switching transistors. 
    
    
     A more complete appreciation of the present disclosure and its scope, and the manner in which it achieves the above noted improvements, can be obtained by reference to the following detailed description of presently preferred embodiments taken in connection with the accompanying drawings, which are briefly summarized below, and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a prior art voltage level shifter. 
     FIG. 2 is a schematic diagram of an improved voltage level shifter. 
     FIG. 3 is a schematic diagram of an alternative improved voltage level shifter. 
    
    
     DETAILED DESCRIPTION 
     A voltage level shifter  200 , as shown in FIG. 2, generally includes thin oxide N-MOSFET transistors  202  and  204 , thick or medium oxide N-MOSFET transistors  206  and  208 , thick oxide P-MOSFET transistors  210  and  212  and a thin oxide inverter  214 . A digital input signal (V IN )  216  having a maximum voltage level about the same as an internal operating voltage (V CORE )  218  is supplied to the voltage level shifter  200 . From the digital input signal  216 , the voltage level shifter  200  generates a digital output signal  220  having a maximum voltage level about the same as an external transfer, or input/output (I/O), voltage (V IO )  222 . 
     The internal operating voltage  218  is the voltage that operates most of the components (not shown) of an IC (not shown) of which the voltage level shifter  200  is a part. The digital input signal  216  is an output signal generated by the other components of the IC to be output from the IC. The voltage level shifter  200  uses the digital input signal  216  to generate the digital output signal  220 , which is the actual signal output by the IC. The digital output signal  220  includes logical one values at the voltage level of the external transfer voltage  222  in order to interface with other external circuitry or IC&#39;s (not shown). 
     The digital input signal  216  is applied to the gate of the transistor  202  and to the inverter  214 . The inverter  214  inverts the digital input signal  216  into an inverted digital input signal  224  and supplies the inverted digital input signal  224  to the gate of the transistor  204 . 
     Since the transistors  202  and  204  have a thin gate oxide, the transistors  202  and  204  can be operated (i.e. switched on and off) by the relatively low voltage level of the digital input voltage  216  (and the inverted digital input signal  224 ). For example, voltage levels of about 1.8 volts, 1.0 volts and less may be used by the digital input voltage  216  and the internal operating voltage  218 . Additionally, the thin gate oxide of the transistors  202  and  204  enables the transistors  202  and  204  to be operated, or switched, more rapidly than the thick oxide transistors  110  and  112  (FIG. 1) used in the prior art, so the transistors  202  and  204  can handle a higher desired clock frequency for the digital input voltage  216  than could the prior art. 
     On the other hand, since the transistors  210  and  212  have a thick gate oxide and the transistors  206  and  208  have a thick or medium gate oxide, the transistors  206 - 212  can handle the higher voltage level (e.g. 3.3 volts, etc.) of the digital output voltage  220  and the external transfer voltage  222  that is typically required to interface with other IC&#39;s (not shown), particularly older IC&#39;s. Additionally, the higher voltage level of the external transfer voltage  222  can drive the thick oxide transistors  210  and  212  at the higher desired clock frequency. 
     The drains of the transistors  202  and  204  are connected to the sources of the transistors  206  and  208 , respectively. The sources of the transistors  202  and  204  are connected to a ground  226 . The drains of the transistors  206  and  208  are connected to the drains of the transistors  210  and  212 , respectively. The gates of the transistors  206  and  208  are connected to the internal operating voltage  218 , which also powers the inverter  214 . The sources of the transistors  210  and  212  are connected to the external transfer voltage  222 . The connection between the transistors  206  and  210  is also connected to the gate of the transistor  212 . The connection between the transistors  208  and  212  is not only connected to the gate of the transistor  210 , but also supplies the digital output voltage  220 . 
     When the digital input signal  216  is a logic zero, the inverted digital input signal  224  is a logic one. Therefore, the transistor  202 , having a logic zero at its gate, is turned “off.” The other transistor  204 , having a logic one at its gate, is turned “on.” With the transistor  204  “on,” the connection between the transistors  204  and  208  is pulled down almost to ground, or logic zero. With the internal operating voltage  218  connected to the gate of the transistor  208 , the transistor  208  is always “on,” so the connection between the transistors  208  and  212  is also pulled down almost to ground, or logic zero. The logic zero is thus supplied to the gate of the transistor  210  and as the digital output voltage  220 . Since the transistor  210  is a P-MOSFET device, the logic zero at it&#39;s gate turns the transistor  210  “on.” With the transistor  210  “on,” the connection between the transistors  206  and  210  and the gate of the transistor  212  is pulled up about to the voltage level of the external transfer voltage  222 . This voltage level at the gate of the transistor  212  turns “off” the transistor  212 , so the external transfer voltage  222  does not interfere with the logic zero of the digital output voltage  220  through the transistor  212 . 
     With the internal operating voltage  218  connected to the gate of the transistor  206 , the transistor  206  is also always “on,” but the connection between the transistors  202  and  206  is not pulled up to the external transfer voltage  222 , as is the connection between the transistors  206  and  210 . Instead, since the transistor  202  is turned “off,” the transistor  206  functions so as to limit the voltage level at the connection between the transistors  202  and  206  to a limiting voltage at the gate of the transistor  206  (the internal operating voltage  218 ) minus a conventional gate-source threshold voltage of the transistor  206 . 
     Since the transistor  202  has a relatively thin gate oxide, the maximum drain-source voltage level the transistor  202  can withstand without “breaking down,” or turning “on” when it is supposed to be “off,” is likely to be less than the external transfer voltage  222 . Therefore, the voltage-limiting function of the transistor  206  protects the transistor  202 , so that the transistor  202  can properly perform its switching function in response to the digital input signal  216 . 
     When the digital input signal  216  is a logic one, the inverted digital input signal  224  is a logic zero. Therefore, the transistor  204 , having a logic zero at its gate, is turned “off.” The other transistor  202 , having a logic one at its gate, is turned “on.” With the transistor  202  “on,” the connection between the transistors  202  and  206  is pulled down almost to ground, or logic zero. With the transistor  206  always “on,” the connection between the transistors  206  and  210  is also pulled down almost to ground, or logic zero. The logic zero is thus supplied to the gate of the transistor  212 . Since the transistor  212  is a P-MOSFET device, the logic zero at it&#39;s gate turns the transistor  212  “on.” With the transistor  212  “on,” the connection between the transistors  208  and  212 , the gate of the transistor  210  and the digital output voltage  220  are pulled up about to the voltage level of the external transfer voltage  222 . Therefore, for the digital output voltage  220 , the voltage level of the external transfer voltage  222  is a logic one value. This voltage level at the gate of the transistor  210  turns “off” the transistor  210 , so the external transfer voltage  222  does not passed through the transistor  210  and does not interfere with the logic zero at the gate of the transistor  212 . Likewise, since the transistor  204  is “off,” the ground  226  does not interfere with the voltage level of the digital output signal  220 . 
     Although the transistor  208  is also always “on,” the connection between the transistors  204  and  208  is not pulled up to the external transfer voltage  222 , as is the connection between the transistors  208  and  212  and the digital output voltage  220 . Instead, since the transistor  204  is turned “off,” the transistor  208  functions so as to limit the voltage level at the connection between the transistors  204  and  208  to a limiting voltage at the gate of the transistor  208  (the internal operating voltage  218 ) minus a conventional gate-source threshold voltage of the transistor  208 . 
     Like the transistor  202 , since the transistor  204  has a relatively thin gate oxide, the maximum drain-source voltage level the transistor  204  can withstand without “breaking down,” or turning “on” when it is supposed to be “off,” is likely to be less than the external transfer voltage  222 . Therefore, the voltage-limiting function of the transistor  208  protects the transistor  204 , so that the transistor  204  can properly perform its switching function in response to the inverted digital input signal  224 . 
     Another embodiment for a voltage level shifter  230 , as shown in FIG. 3, generally includes switching N-MOSFET transistors  232  and  234 , thick or medium oxide voltage-limiting N-MOSFET transistors  236  and  238  and P-MOSFET transistors  240  and  242  similar to the transistors  202 - 212  (FIG.  2 ), respectively. The voltage level shifter  230  also includes an inverter  244  similar to the inverter  214  (FIG.  2 ). The voltage level shifter  230  also includes an input digital voltage  246  similar to the input digital voltage.  216  (FIG.  2 ), a digital output voltage  248  similar to the digital output voltage  220  (FIG.  2 ), an internal operating voltage  250  similar to the internal operating voltage  218  (FIG.  2 ), an external transfer voltage  252  similar to the external transfer voltage  222  (FIG. 2) and a ground  254  similar to the ground  226  (FIG.  2 ). 
     The function of the above described components of the voltage level shifter  230  is similar as for the voltage level shifter  200  (FIG.  2 ), except that the internal operating voltage  250  is not applied to the gates of the voltage-limiting transistors  236  and  238 . Instead, a reference voltage (V REF )  256  is applied to the gates of the transistors  236  and  238 . The reference voltage  256  is generated from the internal operating voltage  250 , the external transfer voltage  252  and a “transistor totem pole”  258 . 
     The transistor totem pole  258  may include any appropriate number and type of transistors connected together. The illustrated example includes three transistors  260 ,  262  and  264 . The gate and drain of the transistors  260 - 264  are connected together and to the source of the next transistor in the transistor totem pole  258 . The source of the bottom transistor  260  is connected to the internal operating voltage  250 . The gate and drain of the top transistor  264  are connected to the external transfer voltage  252 . The reference voltage  256  is generated at the connection between the bottom and middle transistors  260  and  262 . 
     The transistor totem pole  258  functions similarly to a voltage divider in which the voltage level of the reference voltage  256  depends on the number and characteristics of the transistors between the reference voltage  256  and the internal operating voltage  250  and the number and characteristics of the transistors between the reference voltage  256  and the external transfer voltage  252 . Generally, the number of transistors in the transistor totem pole  258  depends on the difference between the voltage levels of the external transfer voltage  252  and the internal operating voltage  250  and the desired voltage level for the reference voltage  256 . 
     In the embodiment shown, the reference voltage  256  has a voltage level of the internal operating voltage  250  plus the gate-to-source threshold voltage of the bottom transistor  260 . In a particular embodiment, the bottom transistor  260  and the voltage-limiting transistors  236  and  238  are similar in that they have approximately the same gate-to-source threshold voltage. In this manner, the voltage level at the connection between the transistors  232  and  236  when the transistor  232  is “off” (and at the connection between the transistors  234  and  238  with the transistor  234  is “off”) is approximately the internal operating voltage  250 . In other words, the reference voltage  256  is stepped up from the internal operating voltage  250  by the transistor  260  by about the same amount that the voltage level at the connection between the transistors  232  and  236  (and between the transistors  234  and  238 ) is stepped down from the reference voltage  256  by the transistor  236  (and the transistor  238 ). 
     The limiting voltage (the internal operating voltage  250  plus the gate-to-source threshold voltage of the bottom transistor  260 ) applied to the gates of the transistors  236  and  238  is greater than the limiting voltage (the internal operating voltage  218 , FIG. 2) applied to the gates of the transistors  206  and  208  (FIG.  2 ). The greater limiting voltage enables the voltage-limiting transistors  236  and  238  to be driven “on” more strongly than are the voltage-limiting transistors  206  and  208 . Therefore, the voltage level between the transistors  232  and  236  when the transistor  232  is “off” (and between the transistors  234  and  238  when the transistor  234  is “off”) is greater than the voltage level between the transistors  202  and  206  or  204  and  208  (FIG.  2 ). The greater voltage level between the transistors  232  and  236  and between the transistors  234  and  238  enables the switching transistors  232  and  234  to be driven from “off” to “on” more rapidly than are the transistors  202  and  204 . Care must still be taken to ensure that the drain-to-source voltage across the transistors  232  and  234  does not exceed the maximum permissible voltage for the transistors  232  and  234 . With this embodiment, however, the voltage level shifter  230  can operate more rapidly than can the voltage level shifter  200  (FIG.  2 ). 
     The voltage level shifters  200  (FIG. 2) and  230  (FIG. 3) have the advantage of operating with the lower internal operating voltages that are in use or coming into use with current and upcoming IC&#39;s while still shifting the digital signals to the higher external transfer voltages needed to maintain compatibility with legacy IC&#39;s. The voltage level shifter  230  has the further advantage over the voltage level shifter  200  of being able to operate at higher frequencies due to the higher limiting voltage, i.e. the reference voltage  256  (FIG.  3 ), as opposed to the internal operating voltage  218  (FIG.  2 ). 
     Presently preferred embodiments of the subject matter herein and its improvements have been described with a degree of particularity. This description has been made by way of preferred example. It should be understood that the scope of the claimed subject matter is defined by the following claims, and should not be unnecessarily limited by the detailed description of the preferred embodiments set forth above.