Abstract:
An apparatus and method of communicating signals between a 2.5 volt internal circuit and both 3.3 and 5 volt external circuits using a P-well. The apparatus includes a circuit having a P-well control circuit and a number of NMOS transistors. The P-well control circuit is configured to receive a P-well control signal and an external signal, and in accordance therewith selectively generate a P-well voltage. The NMOS transistors are coupled to the P-well control circuit. At least one of the NMOS transistors has a bulk region configured to receive the P-well voltage. The NMOS transistors are further configured to receive a 5 volt signal and in accordance therewith selectively generate a 2.5 volt signal. The NMOS transistors are still further configured to receive a 3.3 volt signal and in accordance therewith selectively generate a 2.5 volt signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to input/output buffer circuits. In particular, the invention relates to input/output buffer circuits between a 2.5 volt circuit and a 3.3 or 5 volt circuit. 
     2. Description of the Related Art 
     The capability to support multiple power supplies (e.g., 5, 3.3, and 2.5 volts) and signaling specifications has been increasingly required for electronic interface circuits due to market need and process technology advancement. Because of design cycle reduction, development cycle reduction between different process technology generations, and the selection of different generation products from those available on the market to reduce the overall end-product cost, each new generation of interface circuit is desired to be able to work with the prior generation without causing any permanent damage or raising reliability concerns. This kind of device is called an over-voltage protection/tolerant interface circuit, an input/output (I/O) circuit, or a buffer circuit. 
     In the past, some over-voltage tolerant I/Os have been developed but they only work between two immediately successive generations of circuits. For example, many 3.3 volt powered I/O circuits are tolerant to the prior 5 volt generation. Many 2.5 volt powered I/O circuits are tolerant to 3.3 volt powered circuits. Thus for each generation, the common process technology may only support one prior generation of power supply. 
     One potential solution is to use a dual gate-oxide process. However, this may incur a higher cost and may be undesirable for most applications, in which lowering costs is often the driving design goal. 
     FIG. 1 shows a typical bi-directional I/O circuit without over-voltage tolerance circuits. Illustrated are an output enable signal node  40 , an output internal signal node  42 , an inverter  44 , a NAND gate  46 , a NOR gate  48 , a PMOS transistor  50 , an NMOS transistor  52 , an internal power supply node  54 , a ground node  56 , an external node  58 , a noninverting buffer  60 , and an input internal signal node  62 . When the output enable signal node  40  is high, the buffer circuit is output enabled and the signal at output internal signal node  42  can be sent to the external node  58 . When the output enable signal node  40  is low, the buffer circuit is output disabled and the external node  58  is in a high impedance state. If there is an incoming signal, it will be sent from the external node  58  to the input internal signal node  62 . 
     FIG. 2 shows an existing 3.3/5 volt tolerant I/O circuit that includes a floating N-well  80 , an output enable signal node  82 , an output internal signal node  84 , a NAND gate  86 , an inverter  88 , a NOR gate  90 , an internal power supply node  92 , a ground node  94 , an I/O power supply node  96 , an I/O ground node  98 , NMOS transistors  100 ,  112 ,  114 ,  116 ,  118  and  134 , PMOS transistors  102 ,  104 ,  106 ,  108 ,  110  and  138 , nodes  120 ,  122  and  124 , an external node  128 , an inverter  136 , a noninverting buffer  130 , and an input internal signal node  132 . The floating N-well  80  connects to the bulk nodes of the transistors  102 ,  104 ,  106 ,  108  and  110  that are exposed to 5 volts. 
     In the case of output disabled, nodes  120  and  122  are supposed to be high and low, respectively, to force transistors  106  and  118  into a high impedance state. If the external node  128  is driven from outside with 5 volts, transistor  116  protects the gate oxide of transistor  118  from the destructive 5 volts. On the other hand, node  120  is charged to 5 volts via transistor  108  and node  124  is charged to 5 volts via transistors  108  and  110 , turning off transistors  100  and  102 , so that the output of NAND gate  86  is isolated from the 5 volts on node  120 . In the meantime, the floating N-well  80  is charged to 5 volts minus V diode  via various parasitic source or drain P/N junctions, where V diode  is the voltage drop across these junctions. This ensures that the potential difference between any gate oxide of those transistors exposed to the 5 volts in the circuit is less than the allowable voltage. 
     In the case of output enabled, node  124  is charged to 0 volts via transistors  112  and  114 , so that the 5 volt tolerant circuits will not affect circuit performance in the normal output mode. 
     However, this 3.3/5 volt tolerant I/O circuit may not work when the process technology moves to a 2.5 volt power supply. Based on the JEDEC extended 5 volt signaling specifications (JESD12-6), the maximum voltage on the external node is 5.5 volts. From the 2.5 volt, 0.25 micron fabrication processes and related electrical specifications, the minimum power supply is 2.3 volts and the absolute maximum supply voltage (destructive) is 3.25 volts. That is, the voltage drop across any drain-gate, gate-source or gate-bulk of a transistor cannot exceed 3.25 volts. This may also be true for drain-bulk or source-bulk regions due to higher surface doping concentration and shallow source or drain P/N junctions for deep submicron CMOS technology. 
     Thus, when the circuit of FIG. 2 is implemented at 0.25 micron fabrication process and powered by a 2.5 volt power supply, the biggest node-to-node voltage drop in the transistors, e.g., the drain-bulk voltage drop of transistor  116 , may not be tolerant to 5.5 volts. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these and other problems of existing circuits with a 2.5 volt powered I/O circuit that is tolerant to both 3.3 and 5 volt circuits. 
     According to one embodiment, an apparatus according to one embodiment of the present invention includes a buffer circuit for communicating signals between an internal circuit having a low power supply voltage and an external circuit having a power supply voltage above that of the internal circuit. The apparatus includes a P-well control circuit and a number of NMOS transistors. The P-well control circuit is configured to receive a P-well control signal and an external signal, and in accordance therewith selectively generate a P-well voltage. The plurality of NMOS transistors is coupled to the P-well control circuit. At least one of the plurality of NMOS transistors has a bulk region configured to receive the P-well voltage. The plurality of NMOS transistors is further configured to receive the external signal having a first potential, and in accordance therewith selectively generate an internal signal having a second potential less than the first potential. The plurality of NMOS transistors is still further configured to receive the external signal having a third potential, and in accordance therewith selectively generate the internal signal having the second potential. The third potential is between the first potential and the second potential. 
     According to another embodiment, a method according to another embodiment of the present invention protects an internal circuit having a low power supply voltage from a signal communicated by an external circuit having a power supply voltage above that of the internal circuit. The method includes the steps of receiving an external signal having one of a first potential and a second potential; generating a P-well voltage corresponding to the external signal; providing the P-well voltage to a bulk region of at least one NMOS transistor; and generating an internal signal having a third potential and corresponding to the external signal. The third potential is less than the second potential, which is less than the first potential. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of an existing input/output buffer circuit. 
     FIG. 2 is a circuit diagram of an existing overvoltage input/output buffer circuit. 
     FIG. 3 is a block diagram of one embodiment of the present invention. 
     FIG. 4 is a circuit diagram of a more specific embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure assumes the 2.5 volt power supply is EIA/JEDEC (EIA/JESD8-5) standard, with a normal range power supply whose minimum, typical and maximum voltages are 2.3, 2.5 and 2.7 volts, respectively. 
     FIG. 3 is a block diagram of a buffer circuit  150  according to one embodiment of the present invention. The buffer circuit  150  includes an N-well control circuit  152 , an N-well  154 , a PMOS circuit  156 , a P-well control circuit  158 , a P-well  160  and an NMOS circuit  162 . 
     The N-well control circuit  152  is configured to receive an N-well control signal  164  and a voltage  166 . The N-well control signal  164  controls generation of an N-well voltage  168  based on the voltage  166 . The N-well  154  supplies the N-well voltage  168  to the PMOS circuit  156 . The N-well  154  may also supply the N-well voltage  168  to the N-well control circuit  152  and the P-well control circuit  158 . 
     The P-well control circuit  158  is configured to receive a P-well control signal  190  and a voltage  172 . The P-well control signal  190  controls generation of a P-well voltage  174  based on the voltage  172 . The P-well  160  supplies the P-well voltage  174  to the NMOS circuit  162 . 
     The PMOS circuit  156  includes a number of PMOS transistors, at least one of which having a bulk region configured to receive the N-well voltage  168 . The NMOS circuit  162  includes a number of NMOS transistors, at least one of which having a bulk region configured to receive the P-well voltage  174 . 
     The NMOS and PMOS circuits  162  and  156  are configured to receive an external signal  176  and in accordance therewith generate an internal signal  178 . The external signal  176  is also provided to the P-well control circuit as the voltage  172 . The P-well  160  and the N-well  154  help protect the NMOS and PMOS circuits  162  and  156  from the external signal  176  when buffer circuit  150  operates in an input mode, as described below. 
     The NMOS and PMOS circuits  162  and  156  may be further configured to receive an output enable signal  170  and an output internal signal  180 , and in accordance therewith generate an output external signal  182  when buffer circuit  150  operates in an output mode, as described below. 
     In the input mode, buffer circuit  150  receives the external signal  176  from an external node  184  and generates the internal signal  178  at an input internal node  186 . Buffer circuit  150 , and the NMOS and PMOS circuits  162  and  156 , operate at a low voltage. In a preferred embodiment this low voltage is approximately 2.5 volts. When the external signal  176  is above 2.5 volts, this may damage the circuits as described above. The N-well  154  and P-well  160  protect the NMOS and PMOS circuits  162  and  156  by adjusting the relative voltage potential between nodes of the circuit elements. 
     The N-well  154  and P-well  160  protect over a wide range of external signal voltages. In a preferred embodiment, the buffer circuit  150  may receive external signal voltages both at approximately 3.3 volts and approximately 5 volts. 
     In the output mode, buffer circuit  150  receives the output internal signal  180  from an output internal node  188  and the output enable signal  170 . The buffer circuit  150  then generates the output external signal  182  at the external node  184 . (Preferably internal node  186  and internal node  188  are not the same node.) The N-well control signal  164  and the P-well control signal  190  are based on the output enable signal  170 . The N-well control circuit  152  uses the N-well control signal  164  to control generation of the N-well voltage  168 . The P-well control circuit  158  uses the P-well control signal  190  to control generation of the P-well voltage  174 . In a preferred embodiment the output internal signal  180  and output external signal  182  are approximately 2.5 volts. 
     FIG. 4 is a circuit diagram showing more details of the buffer circuit  150 . The N-well control circuit  152  (see FIG. 3) includes a PMOS transistor  200 . The gate of transistor  200  is connected to an N-well control node  202  to receive the N-well control signal  164  based on the output enable signal  170  (see FIG.  3 ). The source of transistor  200  is connected to a voltage source node  204  which supplies the voltage  166  (see FIG.  3 ). The voltage source  204  preferably supplies a low voltage, most preferably approximately 2.5 volts. The bulk and drain of transistor  200  are connected to the N-well  154 . 
     The P-well control circuit  158  (see FIG. 3) includes PMOS transistors  206  and  208 , and NMOS transistors  210  and  212 . The source of transistor  206  is connected to the external node  184 . The bulk of transistor  206  is connected to the N-well  154 . The gate of transistor  206  is connected to a low voltage source, preferably the voltage source node  204 . 
     The drain of transistor  206  is connected to the source of the transistor  208 . The bulk of transistor  208  is connected to the N-well  154 . The gate and drain of transistor  208  are connected to the P-well  160 . 
     The gate and drain of transistor  210  are connected to the P-well  160 . The bulk and source of transistor  210  are connected to ground, preferably a ground node  214 . 
     The drain of transistor  212  is connected to the P-well  160 . The bulk and source of transistor  212  are connected to ground, preferably the ground node  214 . The gate of transistor  212  is connected to an output enable node  216  to receive the output enable signal  170  as the P-well control signal  190  (see FIG.  3 ). 
     The PMOS circuit  156  (see FIG. 3) includes transistors  218 ,  220 ,  224 ,  226 , and  228 . The NMOS circuit  162  (see FIG. 3) includes transistors  230 ,  232 ,  234 ,  236 ,  238 , and  240 . The source of transistor  224  is connected to an I/O power supply node  251 . The source of transistor  240  is connected to an I/O ground node  252 . The connections and functions of all these transistors will be explained below with reference to the input mode and output mode operation of the buffer circuit  150 . 
     Input Mode 
     In an input mode, the buffer circuit  150  receives the external signal  176  (see FIG. 3) at the external node  184  from an external circuit (not shown). The external signal  176  may be at a voltage above that of the power supplied to buffer circuit  150 , for example, approximately 3.3 or approximately 5 volts. The buffer circuit  150  then converts the external signal into a low voltage internal signal  178  (for example, 2.5 volts) (see FIG. 3) at the internal node  186  for communication with an internal circuit (not shown). 
     More specifically, when the output enable node  216  is low, inverter  242  and NOR gate  244  cause node  246  to go low. Similarly, NAND gate  248  and transistors  232 ,  220 , and  236  cause node  250  to go high. The low voltage at node  246  and the high voltage at node  250  cause transistors  224  and  240  to turn off. 
     When the external circuit (not shown) applies a 5.5 volt external signal  176  (see FIG. 3) to external node  184 , transistor  226  charges node  250  to 5.5 volts. This turns transistor  224  off. Transistor  228  turns on and charges node  202  to 5.5 . Transistor  220  then turns off, protecting NAND gate  248  from the 5.5 volts at node  250 . Although transistor  232  is on as long as the voltage at node  204  is higher than that at P-well  160 , transistor  232  limits the voltage on the output node of NAND gate  248  to the voltage of node  204  minus the threshold voltage of transistor  232 . In other words, without considering the effect of transistor  220 , the high logic state of the output of NAND gate  248  is not affected by the voltage level of node  250 . Otherwise, transistor  232  turns off when the P-well  160  is charged to a voltage higher than that of node  204 . 
     The voltage of N-well  154  is therefore charged to 5.5 volts minus the P/N junction build-in voltage (about 0.2 volts) by all of the parasitic source or drain P/N junction potentials of PMOS transistors  224 ,  226 ,  228 ,  200 ,  220 ,  206 , and  208 . 
     The 5.5 volts on external node  184  also cause transistors  206  and  208  to turn on and transistor  210  to turn off, generating a P-well voltage  174  (see FIG. 3) at P-well  160 . (Transistor  212  is off because node  216  is low in the input mode.) By sizing the transistors  206 ,  208 ,  210  and other transistors that share the P-well  160 , the resulting P-well voltage  174  should be higher than 2.25 volts (5.5−3.25=2.25), so that the voltage drop between the drain and bulk of transistor  238  would be less than the allowed voltage of 3.25 volts. Other the other hand, assuming that the voltage supply node  204  supplies the minimum preferred voltage of 2.3 volts, the voltage drop on the gate oxide of these transistors is approximately 3.2 volts (5.5=2.3=3.2), which is also lower than 3.25 volts. 
     Given these constraints, transistors  206 ,  208  and  210  must be designed very carefully so that the P-well voltage  174  is between approximately 2.25 (5.5−3.25) and 3.25 volts when the external signal is at 5.5 volts. 
     Finally, transistors  218  and  230 , inverter  254  and noninverting buffer  256  receive the external signal  176  and generate the internal signal  178  (see FIG. 3) at internal node  186 . Suppose the internal power supply voltage (at node  204 ) is 2.5 volts and the output is disabled. If the voltage on external node  184  is 2.5 volts, then the voltage on the other terminal (source) of transistor  230  would be 2.5 volts minus its threshold voltage, i.e., 2.5−0.5=2.0 volts, which is lower than 2.5 volts. The function of inverter  254  and PMOS gate  218  is to pull up the internal line from 2.0 volts to 2.5 volts. 
     The choice of the 5.5 volt external signal in the above example is based on the maximum voltage allowed under JEDEC extended 5 volt signaling specifications (JESD12-6). A similar analysis results when an approximately 3.3 volt external signal is applied at the external node  184 . 
     Output Mode 
     In an output mode, buffer circuit  150  receives output internal signal  180  (see FIG. 3) at internal node  188  from the internal circuit (not shown). Buffer circuit  150  then generates output external signal  182  (see FIG. 3) at external node  184  for communication with the external circuit (not shown). Buffer circuit  150  uses N-well control signal  164  and P-well control signal  190  (both based on output enable signal  170 ) (see FIG. 3) to isolate circuit components not used in the output mode so that these unused components do not affect circuit performance. 
     More specifically, output enable node  216  is high, causing transistor  212  to discharge P-well  160  to ground. The internal circuit (not shown) applies the output internal signal  180  (see FIG.  3 ), preferably a 2.5 volt signal, to output internal node  188 . When output internal signal  180  is high, node  250  is low and node  246  is low, causing transistors  224  and  240  to supply a high signal from I/O power supply node  251  to external node  184 . When the output internal signal  180  is low, node  250  is high and node  246  is high, causing transistors  224  and  240  to supply a low signal from  1 / 0  ground node  252  to the external node  184 . 
     When the power supply (preferably connected to node  204 ) of buffer circuit  150  is approximately 2.5 volts, node  250  goes to a maximum potential of approximately 2.7 volts under transient conditions. Similarly, output node  184  is a maximum of approximately 2.7 volts under transient conditions as well. Under steady state conditions, node  250  is approximately 2.5 volts. This causes transistors  226 ,  228  and  206  to be off. Node  202  is discharged to ground by transistors  234  and  236 , so transistor  220  is on. The N-well  154  is charged to the power supply voltage of node  204  by transistor  200 . Thus, the N-well and P-well control circuits  152  and  158  (see FIG. 3) do not affect the output buffer performance, even as modified with the improvements of the present invention. 
     Simulation results of buffer circuit  150  give results in conformance with the above discussion. 
     It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that structures within the scope of these claims and their equivalents are covered thereby.