Abstract:
A dual-voltage three-state buffer circuit controls a post driver circuit to operate in a three-state mode and includes a tri-state logic control module operated under a low supply voltage, a level shifter for receiving one or more inputs from the tri-state logic control module and operating with an output control circuit for controlling two differential outputs of the level shifter, and a post driver circuit driven by the two differential outputs of the level shifter, wherein the level shifter, the output control circuit, an the post driver circuit are operated under a high supply voltage, and wherein when the tri-state logic control module generates the inputs for putting the post driver circuit in a high impedance state, the output control circuit operates with the level shifter to turn off the PMOS and NMOS transistors of the post driver circuit while isolating the level shifter from a high supply voltage.

Description:
BACKGROUND 
   The present invention relates generally to integrated circuits, and more particularly, to an improved design of a dual-voltage three-state buffer circuit using a tri-state level shifter. 
   A conventional dual-voltage three-state buffer includes two level shifters to control a post driver circuit that is made of PMOS and NMOS transistors. The two level shifters translate lower voltage signals to higher voltage signals. The post driver circuit determines the output of the overall circuit by deciding which transistor is to be turned on or off. However, the time it takes for the PMOS and NMOS transistors to turn on or off is different, since PMOS transistors are usually slower to drive than NMOS transistors. The time required for a signal to output from each of the level shifters may also be different, since different input signals can create different paths for the signals to travel through, wherein some paths may take more time than the others. With all these timing differences, a cross-bar current can occur during the switching of the transistors in the post driver circuit, thereby degrading the performance for the circuit. In order for a conventional dual-voltage three-state buffer to solve such issues, unbalanced inverters are inserted between the level shifter outputs and the transistors of the post driver circuit. While this method reduces the cross-bar current of the post driver circuit, the inverters are extremely unbalanced, consume extra power, and require additional layout areas. 
   Desirable in the art of dual-voltage buffer designs are designs that provide less power consumption, smaller layout area, and better versatility. 
   SUMMARY 
   In view of the foregoing, this invention provides an improved design of a dual-voltage buffer circuit by implementing tri-state level shifter. In one embodiment, it has a tri-state logic control module operated under a low supply voltage, a level shifter for receiving one or more inputs from the tri-state logic control module and operating with an output control circuit for controlling two differential outputs of the level shifter, and a post driver circuit having a PMOS transistor and an NMOS transistor connected in series and driven by the two differential outputs of the level shifter, wherein the level shifter, the output control circuit, an the post driver circuit are operated under a high supply voltage, and wherein when the tri-state logic control module generates the inputs for putting the post driver circuit in a high impedance state, the output control circuit operates with the level shifter to turn off the PMOS and NMOS transistors of the post driver circuit while isolating the level shifter from a high supply voltage. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following descriptions of specific embodiments when read in connection with the accompanying figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  illustrates a conventional dual-voltage three-state buffer circuit comprised of a decoder with two level shifters. 
       FIG. 1B  presents a truth table of the conventional dual-voltage three-state buffer circuit. 
       FIG. 2A  illustrates a dual-voltage three-state buffer circuit in accordance with a first embodiment of the present invention. 
       FIG. 2B  presents a truth table of the dual-voltage three-state buffer circuit in accordance with the first embodiment of the present invention. 
       FIG. 3A  illustrates a dual-voltage three-state buffer circuit in accordance with a second embodiment of the present invention. 
       FIG. 3B  presents a truth table of the dual-voltage three-state buffer circuit in accordance with the second embodiment of the present invention. 
       FIG. 4  presents a diagram illustrating the relationship of various signals in accordance with the first embodiment of the present invention. 
   

   DESCRIPTION 
   The present invention provides a dual-voltage three-state buffer circuit with a tri-state level shifter that simplifies circuit design. As such, the invention reduces the layout area and power consumption by the dual-voltage circuit. 
     FIG. 1A  illustrates a conventional dual-voltage three-state buffer  100  comprised of a decoder with level shifters  102  and  104 . The buffer  100  has two modes of operation: normal mode and tri-state mode. These modes of operation are controlled by an enable pin  106  and an input pin  108 . The normal mode allows the buffer  100  to output what is inputted from the input pin  108  to a pad  110 , if the enable pin  106  is set low. Tri-state mode&#39;s function is to turn off a post driver PMOS transistor  112  and a post driver NMOS transistor  114  to create high impedance at the output of the buffer  100 . 
   For this example, the enable pin  106  is expected to be low and the input pin  108  is expected to be high to illustrate the operation of the normal mode of the buffer  100 . A control logic block  116  in the buffer  100  is powered by a low voltage source VDD and is made up of an AND gate  118 , an OR gate  120 , and three inverters  122 ,  124  and  126 . The components within the control logic block  116  work together to provide the correct input signal for the level shifters  102  and  104 . The OR gate  120  takes in the high signal from the input pin  108  and the low signal from the enable pin  106  to provide a high signal for a node  128 . The AND gate  118  takes in the high signal from the input pin  108  and the high inverted enable signal from the inverter  122  to provide a node  130  with a high signal. The high signals from the nodes  128  and  130  then turn on NMOS transistors  134  and  132 , while the low signals at the gates of NMOS transistors  136  and  138  caused by the inverters  124  and  126  turn off the NMOS transistors  136  and  138 . With both NMOS transistors  132  and  134  turned on, nodes  140  and  142  are both pulled to low, and in return provide the gates of PMOS transistors  144  and  146  with low signals. The PMOS transistors  144  and  146  will turn on and pull the nodes  148  and  150  high due to the source voltage VDDIO. The high signal at the nodes  148  and  150  will turn off the PMOS transistors  152  and  154  and also be inverted to low signals before reaching the post driver transistors  112  and  114  by going through inverters  156  and  158 . The low signals at the gates of the transistors will turn the transistor  112  on and the transistor  114  off. This allows a node  160  to be pulled high by source voltage VDDIO, thereby giving a high output signal at the pad  110 . 
   To show how the circuit  100  operates during tri-state mode, both the enable pin  106  and the input pin  108  are now set to high. The two signals will first go through the control logic block  116 . In the control logic block  116 , the AND gate  118  takes in a low inverted enable signal from the inverter  122  and a high signal from the input pin  108  to give the node  130  a low signal. Similarly, the OR gate  120  takes in a high signal from the input pin  108  and the enable pin  106  to provide the node  128  with a high signal. The low signal at the node  130  will turn the NMOS transistor  132  off and the NMOS transistor  136  on after going through the inverter  124 . The high signal at the node  128  turns the NMOS transistor  134  on and the NMOS transistor  138  off after going through the inverter  126 . As a result, nodes  148  and  142  are pulled to low, thereby turning the PMOS transistors  152  and  146  on. The nodes  140  and  150  are pulled high by the source voltage VDDIO since the PMOS transistors  152  and  146  are turned on. The two high state nodes  140  and  150  help turn off the PMOS transistors  144  and  154 . With the node  148  pulled to low and the node  150  pulled to high, both signals are inverted after going through the inverters  156  and  158 . The output of the inverter  156  provides the gate of the transistor  112  with a high signal, thereby turning the transistor  112  off. The inverter  158  outputs a low signal for the gate of the transistor  114 , thereby turning the transistor  114  off. With both transistors  112  and  114  turned off, the circuit enters tri-state, and both the node  160  and the PAD  110  will have high impedance. 
     FIG. 1B  presents a truth table  162  of the conventional dual-voltage three-state buffer  100 . The truth table  162  shows the expected output signals for all three possible states with different combinations of enable or input signals. 
   While the buffer  100  reduces post driver cross-bar current and translates lower voltage to higher voltage with the use of two level shifters  102  and  104 , the inverter ratio that drives the gates of post driver transistors  112  and  114  are extremely un-balanced because of the timing issues. Furthermore, the level shifters  102 ,  104  and inverters  156 ,  158  increase the power consumption and layout area. This invites an improved design of the dual-voltage three-state buffer. 
     FIG. 2A  illustrates a dual-voltage three-state buffer circuit  200  in accordance with the first embodiment of the present invention. The circuit  200  includes only one level shifter and a plurality of pull-up and pull-down switches. 
   Like the buffer  100 , the circuit  200  also switches between three different states with two modes of operation: normal mode and tri-state mode. Normal mode occurs when an enable pin  202  is set to low, and allows a PAD  204  to output the inverse signal of what was inputted into an input pin  206 . In order to activate the tri-state mode, the enable pin  202  will be set to high by an output enable signal. Regardless of the state of the input signal, the PAD  204  will have a high impedance. The circuit  200  essentially serves to translate lower voltage signals to higher voltage signals with the help of a tri-state level shifter. Only a tri-state logic control module  208  and other components before it are supplied by a lower voltage source VDD. All other components within the circuit  200  are powered by a higher voltage power supply VDDIO. 
   A three state level shifter, which is collectively represented by PMOS transistors  232 ,  238  and NMOS transistors  218 ,  220 , is connected between a high voltage output switch such as a PMOS transistor  234  and ground. A high voltage output control circuit, which may include the PMOS transistor  234 ,  240 ,  246 , and NMOS transistor  222  are powered by the high voltage power supply. The NMOS transistor  222  is connected to the gate of the PMOS transistor  234  on one end and the ground on the other with its gate controlled by node  224 . The level shifter is connected to a post driver circuit, which includes a PMOS transistor  242  and a NMOS transistor  250 , via an inverter  244  and a buffer  248 . The gate of the PMOS transistor  240  is further connected to the inverter  244 , and the gate of the PMOS transistor  246  is further connected to the buffer  248 . It is noted that the transistor  240  and  246  are connected in series, with the gate of the transistor  240  connected to the input of the inverter  244 , and with the gate of the transistor  246  tied to the input of the buffer  248 . As such, the high voltage output control circuit affects how signals travel from the differential outputs, i.e., nodes  228  and  236 , of the level shifter to the post driver circuit. 
   To illustrate how normal mode operates, a low signal is inputted into the enable pin  202  and a high signal is inputted into the input pin  206 . The two signals first go through the tri-state logic control module  208  that is powered by a low voltage source VDD. The tri-state logic control module  208  is made of several logic components: inverters  210  and  212 , and NAND gates  214  and  216 . These components work together to determine which of the pull-down transistors  218 ,  220 , and  222  are to be turned on or turned off. A node  224  simply has a high inverted signal of what is inputted to the enable pin  202 . This high signal at the node  224  will turn on the transistor  222 . A node  226  controls the switching of the transistor  218  and it has a high signal since the NAND gate  214  takes in the high signal from the node  224  and the low inverted signal of the input pin  206  from the inverter  210 . The high signal of the node  226  also turns on the transistor  218 . With both signals at the nodes  224  and  226  high, the NAND gate  216  provides the gate of the transistor  220  with a low signal, thereby turning it off. Since the transistors  218  and  222  are both turned on, the two differential output nodes  228  and  230  are both pulled low, thereby turning on the PMOS transistor  232  and a pull-up PMOS transistor  234 . This provides a straight path from source voltage VDDIO to a node  236 , thereby pulling it high. This in return also turns off the PMOS transistor  238 . With the node  228  pulled down, the PMOS transistor  240  is turned on. Since the gate of the post driver PMOS transistor  242  will have a high signal because of the inverter  244 , the transistor  242  will be turned off. The high signal at the node  236  turns off the PMOS transistor  246  and continues through a buffer  248  to turn on the post driver NMOS transistor  250 . Since the transistor  242  is turned off and the transistor  250  is turned on, the signal at the PAD  204  will be pulled low, which is the inverse of the input at the input pin  206 . 
   The tri-state mode can be activated by a high output enable signal at the enable pin  202 , thereby creating a high impedance to the output at the PAD  204 , regardless of the input signal at the input pin  206 . To show how the tri-state mode operates, both the enable pin  202  and the input pin  206  will be set to high. The operation begins by having the two signals enter the tri-state logic control block  208  to determine which of the pull-down switches are to be turned on or off. Because of the inverter  212 , the node  224  will have a low signal, which turns the transistor  222  off. The NAND gate  214  provides the node  226  with a high signal, after taking in the two low signals from the inverters  210  and  212 . The node  226  with a high signal turns on the transistor  218 . The NAND gate  216  takes in the high signal at the node  226  and the low signal at the node  224 , thereby providing a high signal to the gate of the transistor  220  and turning the transistor  220  on. This immediately pulls both the nodes  228  and  236  low and then turning the transistors  240  and  246  on. The gate of the switch/PMOS transistor  234  will be pulled high from the direct path connected to source voltage VDDIO. This helps to shut off high voltage power from the entire level shifter. The transistor  242  is turned off, after the low signal at the node  228  goes through the inverter  244 . The transistor  250  will also turn off since the low signal from the node  236  continues through the buffer  248 . With both transistors  242  and  250  turned off, the PAD  204  will have a very high impedance. 
     FIG. 2B  presents a truth table  252  in accordance with the first embodiment of the present invention. The truth table  252  shows the expected output signals for all three possible states with different combinations of enable or input signals. 
     FIG. 3A  illustrates a dual-voltage three-state buffer circuit  300  in accordance with the second embodiment of the present invention. The buffer circuit  300  has only one level shifter and several pull-up and pull-down switches. 
   Similar to the buffer  200 , the buffer  300  also switches between three different states with two modes of operation: normal mode and tri-state mode. Normal mode occurs when an enable pin  302  is set to low and it allows a PAD  304  to output the inverse signal of what is inputted into an input pin  306 . In order to activate the tri-state mode, the enable pin  302  will be set to high by an output enable signal. Regardless of the state of the input signal, the PAD  304  will have a high impedance. The dual-voltage three-state buffer circuit  300  also provides a function to translate lower voltage signals to higher voltage signals with the help of the tri-state level shifter. Only a tri-state logic control module  308  and other components before it are supplied by a lower source voltage VDD. All other components within the buffer  300  are powered by a higher source voltage VDDIO. 
   PMOS transistors  336 ,  338 , collectively representing a high voltage output switch, are connected in parallel between a high voltage power supply and a level shifter. The level shifter, which is collectively represented by PMOS transistors  332 ,  342  and NMOS transistors  318 ,  322 , is connected between the PMOS transistors  336 ,  338  and ground. An output control circuit, which is collectively represented by PMOS transistors  334 ,  330 , NMOS transistor  320 , and output switches such as the PMOS transistors  336 ,  338 , asserts controls over the differential outputs  328  and  340  of the level shifter. The NMOS transistor  320  is connected to the gate of the PMOS transistors  336 ,  338 . The level shifter is connected to a post driver circuit, which includes a PMOS transistor  346  and a NMOS transistor  350 , via an inverter  348  and a buffer  352 , respectively. The gate of the PMOS transistor  340  is further connected to the inverter  348 , and the gate of the PMOS transistor  344  is further connected to the buffer  352 . 
   To illustrate how the normal mode of the buffer  300  operates, the enable pin  302  is set to low and the input pin  306  is set to high. The signals first arrive at the tri-state logic control block  308 . The tri-state logic control block  308  has the same function as the tri-state logic control block  208  used in the first embodiment as illustrated in  FIG. 2A . The inverters  310  and  312 , and the NAND gates  314  and  316  works together in the tri-state logic control block  308  to provide commands for pull-down switches NMOS transistors  318 ,  320  and  322 . The low signal at the enable pin  302  is inverted by the inverter  312 , thereby giving a node  324  a high signal, which then turns on the transistor  320 . The NAND gate  314  takes in the high signal at the node  324  and the low inverted input signal from the inverter  310  to provide a node  326  with a high signal. This in return also turns on the transistor  318 . The NAND gate  316  takes in the two high signals from the nodes  326  and  324  to give the gate of the transistor  322  a low signal, thereby turning it off. With both the transistors  318  and  320  switched on, nodes  328  and  330  are both pulled to low, thereby turning on PMOS transistors  332 ,  334 ,  336  and  338 . When the transistors  332  and  338  are turned on, they provide a path for source voltage VDDIO to pull a node  340  to high. The high signal at the node  340  in effect will turn off the PMOS transistors  342  and  344 . With a low signal at the node  328 , when the gate of a post driver PMOS transistor  346  receives a high signal because of an inverter  348 , the transistor  346  will be turned off. A NMOS transistor  350  will be turned on since the high signal at the node  340  simply continues through a buffer  352 . When the transistor  350  is turned on, it helps pull the signal at the PAD  304  to low. As such, the output signal of the buffer  300  becomes the inverse of the input signal. 
   The tri-state mode occurs when the enable pin  302  is set to a high state. The input pin is also set to high, thereby helping to show how the buffer  300  operates in tri-state mode. Once again, the signals enter the low voltage tri-state logic control block  308  to determine when pull-down switches are to be opened or closed. The node  324  will carry a low signal since the inverter  312  inverts the high enable signal from the enable pin  302 , and this low signal also turns off the transistor  320 . The NAND gate  314  will take in the low signals from the node  324  and the inverter  310  to provide the node  326  with a high signal, thereby turning the transistor  318  on. The NAND gate  316  also takes in a high signal at the node  326  and a low signal at the node  324 , thereby giving the gate of the transistor  322  a high signal and turning the transistor  322  on. With the transistors  318  and  322  turned on, the nodes  328  and  340  are quickly pulled to low, thereby turning on the transistors  334  and  344  and allowing a high signal from the source voltage VDDIO to reach the gates of transistors  336  and  338 . This turns off both the transistors  336  and  338 , thereby shutting off power from the level shifter. The low signal at the node  328  turns the transistor  346  off after the signal goes through the inverter  348 , while the low signal at the node  340  turns the transistor  350  off after the low signal continues through the buffer  352 . With transistors  346  and  350  turned off, the output at the PAD  304  will have a high impedance. 
     FIG. 3B  presents a truth table  354  in accordance with the second embodiment of the present invention. The truth table  354  shows the expected output signals for all three possible states with different combinations of enable or input signals. 
     FIG. 4  presents a diagram  400  illustrating the relationship between output and input signals in accordance with the first embodiment of the present invention. With reference to  FIGS. 2A and 4 , the relationship is essentially between the output signal from the PAD  204  when the input signal of the input pin  206 , as well as the enable signal of the enable pin  202  from the first embodiment are being changed. 
   A curve  402  is the enable signal; it will remain at low state until 70 ns into the plot. A curve  404  shows the changes of the input signal. In this embodiment, the state of input signal is being changed around every 20 ns. The high state of the input signal is 1.2 volts, and low state is 0 volts. With reference to  FIGS. 2A and 4 , this low voltage results from the input signal entering the buffer  200  before going through a level shifter. A curve  406  shows the response of the output signal during the changes of the curve  402  (enable signal) and the curve  404  (input signal). During the first 70 ns, the enable signal has no affect on the response of the output signal. Whenever the input signal is at a low state, the output signal would be at high and vice versa. The high state for the output signal is around 3.3 volts and the low state is 0 volts since all components after the tri-state control logic block  208  in  FIG. 2A  are supplied by the higher voltage source. As mentioned in the description of  FIG. 2A , the input signal and output signal are inverse of each other if the enable signal is at a low state. However, when the enable signal turns high at 70 ns, the output signal drops out of existence since no signal can exit through the PAD  204 . The input signal can still be changed and would not affect the output signal. 
   This invention provides a solution to the cross-bar current issue while reducing the switching power and the pre-driver layout area by implementing a single tri-state level shifter. This invention features only one level shifter and some extra pull-up and pull-down switches. The differential outputs  228  and  230  of the level shifter control a pair of transistors  242 ,  250  in the post driver circuit to determine the output signals. Such differential output of the level shifter provides a self-generated time differential to prevent cross-bar current from occurring during switching. 
   This invention saves switching power and pre-driver layout area by removing an entire level shifter and a plurality of timing balancing inverters. As a result, the post-driver switching power can be reduced by about 50 percent. Moreover, the proposed dual-voltage three-state buffer circuit is compatible with all existing technologies. 
   The above illustration provides many different embodiments or examples for implementing different features of the new designs of the dual-voltage three-state buffer circuit. Specific examples of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.