RELATED APPLICATION  
       [0001]     The present invention claims priority of India Patent Application No. 1060/Del/2004, filed Jun. 8, 2004, which is incorporated herein in its entirety by this reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to an input buffer in the field of integrated circuits. Specifically, the invention pertains to a high-voltage tolerant input buffer circuit.  
       BACKGROUND OF THE INVENTION  
       [0003]     With the advent of sub-micron technology, the device dimensions are decreased so as to be suitable in low cost and low power applications. Also, circuit designing for standard protocols has become more challenging. Sub-micron technology devices cannot tolerate high-voltage because of reliability issues. The gate-oxide breakdown voltage and/or the punch-through between source and drain typically define the voltage of a particular technology. To meet the standard protocols&#39; electrical specifications, interface circuits must work at high voltages (e.g. 5V, 3.3 V etc) with high reliability. One notable problem in interfacing low-voltage circuitry with high-voltage circuitry is that if the voltage applied to the low-voltage circuitry gets too high, some devices may experience temporary or even permanent damage. The gate-oxide stress causes threshold voltage to fluctuate because of tunneling effect—moreover, device lifetime deteriorate.  
         [0004]     At the process level, the high-voltage tolerant transistors can be fabricated by increasing gate oxide and an extended drain scheme. These devices increase the fabrication cost because of extra masks required to make device level tune in the same CMOS baseline process. Another disadvantage is performance degradation.  
         [0005]      FIG. 1  shows a schematic diagram of a conventional input buffer  100  with an input IN and output OUT, for 3.3 volt devices. VDDS=3.3 volt.  
         [0006]      FIG. 2  is a schematic diagram of a 5V tolerant input buffer operating at 3.3V nominal voltage. VDDS=3.3 volt. All the devices are in 3.3V technology. IN is connected to the drain of MOSFET M 1, which translates the input signal to a lower voltage at node  1  for safe operation of the buffer  200 . When IN goes as high as 5V, node is clamped to (VDDS−V t ), so all the devices are safe. Because the substrate bias effect V t  of transistor M 1  is high, node  1  voltage is comparatively low. This may cause M2 to be in weak inversion or in strong sub-threshold region. So, the conventional input buffer  200  will consume DC power, which is more serious in 0.13 μm technology because V t  is less, when signal on pad is high. Moreover, this structure cannot be used when device is of 2.5V and is operating at 3.3V.  
         [0007]      FIG. 3  is another schematic of 5V tolerant input buffer  300  in 3.3V technology. VDDS=3.3 volt. MOSFET  2  and an NMOS are used to clamp high voltage at the input. To avoid turn ON of M 2  (because of difference {VDDS−V 2 }&gt;|VtM 2 |), a weak pull-up structure consisting of two series transistors  4  (PMOS) and  5  (NMOS) has been used. It will pull the node  120  to VDDS level provided node  110  is at (VDDS+Vt 5 ). It happens only when IN starts rising above VDDS. When IN reaches VDDS+|Vt 1 |, transistor  1  turns on and node  110  is charged to a voltage equal to IN. When IN rises to 5V, node  110  also gets charged to 5V. Transistor  3  remains OFF because the gate and source voltages are at the same level. Transistor  5  turns-on strongly and node  120  is pulled to VDDS (3.3V nominal). When the voltage at IN reaches ground level, node  110  discharges to (VDDS+|Vt 1 |) volt only through transistor  1 . Transistor  3  pulls node  110  to |Vt 3 | level so that transistor  5  (NMOS) is OFF.  
         [0008]     The circuit in  FIG. 3  cannot be used for 2.5V devices operating at 3.3V because the gates of  1  and  2  cannot be connected to VDDS (3.3 Volt) directly. Moreover, 5V cannot be directly applied to the gate of 2.5V devices because the gate-bulk voltage (Vgb) for NMOS ( 5 ) is significant (5.0V) to deteriorate the oxide.  
         [0009]     The circuit in  FIG. 4  is another 5V tolerant input buffer structure using 2.5V devices designed for 2.5V operation. VDDS=2.5 volt. This structure is able to tolerate input signal of 5V while operating safely. In normal mode LPN is connected to ground. Transistors M 1  and M 4  form a source follower structure where M 4  acts as a resistor. Node  1  never exceeds VDDS level. M 9  has been added to speed-up the buffer when IN makes transition from high to low, because the size of transistor M 4  is less (to reduce the dc power consumption in normal mode). Transistors M 6  and M 7  have been used to provide buffering at the output. This structure also works perfectly without stressing any device. But the buffer cannot be used for low power and 3.3 Volt operations. In normal mode it consumes dc current and an extra mode control signal is required.  
         [0010]     U.S. Pat. Nos. 5,952,848 and 6,236,236 are referred to for additional reference.  
         [0011]     Since for standard protocols, the voltage levels (usually 3.3V and 5V) are fixed, an input buffer is required which can tolerate signal of 5V at the receiver input and can be implemented with low-voltage technology.  
         [0012]     It is therefore desirable to have an input buffer circuit, which is capable of receiving a high voltage without experiencing degradation of gate oxide lifetime. It would further be desirable if such input buffer does not increase the process complexity and is implemented in the recent technology while working at higher supply voltage (e.g. 3.3 volt nominal).  
       SUMMARY OF THE INVENTION  
       [0013]     According to an embodiment of the present invention, structures and methods for a low-power input buffer enables low-voltage circuitry to be interfaced and operated with relatively high-voltage circuitry while minimizing the voltage across the gate oxide of transistors used in the input buffer. According to an embodiment of the present invention a stress-free circuit is achieved with fewer number of transistors while at the same time achieving large hysteresis (for better noise immunity) and providing high speed and low power. Device reliability is also improved.  
         [0014]     An embodiment of the present invention provides a high-voltage tolerant input buffer circuit which includes a first NMOS transistor having its source terminal connected to the input pin, its gate terminal connected to a first reference voltage and its drain terminal connected to a first output terminal; a second NMOS transistor having its gate terminal connected to the first reference voltage and its source terminal connected to the first output terminal, a first PMOS transistor having its gate terminal connected to the drain terminal of the second NMOS transistor, its drain terminal connected to a second reference voltage which is lower than the first reference voltage and its source terminal connected to a second output terminal; a second PMOS transistor having its drain terminal connected to the drain terminal of the second NMOS transistor, its source terminal connected to the second output terminal; and its gate terminal connected to a control voltage, and a third PMOS transistor having its drain terminal connected to the second output terminal, its source terminal connected to a supply voltage, and its gate terminal connected to the control voltage.  
         [0015]     The first output terminal is connected to the gate terminal of the lowermost NMOS transistor while said second output terminal is connected to the gate terminal of the topmost PMOS transistor and the control terminal is connected to the output of a complementary cascode structure which includes a plurality of transistors connected in series to provide feedback in order to improve speed of response.  
         [0016]     A second complementary cascode structure having its input terminals connected in parallel with the input terminals of the first complementary cascode structure provides the output signal at a reduced voltage level, to the internal circuit.  
         [0017]     The PMOS and NMOS transistors are small-sized transistors to speed up transition and reduce power dissipation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein::  
         [0019]      FIG. 1  is a schematic diagram of a conventional input buffer;  
         [0020]      FIG. 2  is a schematic diagram of a conventional 5V tolerant input buffer;  
         [0021]      FIGS. 3-4  are schematic diagrams of similar 5V tolerant input buffer;  
         [0022]      FIGS. 5-7  are schematic diagrams of 5V tolerant input buffer with low-voltage device according to the present invention (VDDS=3.3 volt);  
         [0023]      FIG. 8  shows internal node waveforms for low to high transition at IN;  
         [0024]      FIG. 9  shows internal node waveforms for high to low transition at IN;  
         [0025]      FIG. 10  shows delay plot for Schmitt-buffer at nominal case; and  
         [0026]      FIG. 11  shows max frequency of operation vs load plot. 
     
    
     DETAILED DESCRIPTION  
       [0027]     Two reference signals—one to protect PMOS and the other to protect NMOS—are used to avoid gate-oxide stress and V ds  stress for oxide degradation and hot carrier effect respectively. Reference signals are necessary to be incorporated in the high-voltage designs with very short channel devices. So a single reference block can be used for the whole IO ring in the chip. Reference signals are shared among different IOs in the ring. For the sake of clear understanding of the new circuit, reference block has not been described.  
         [0028]     In one embodiment ( FIG. 6 ), the input buffer circuit includes an n-channel field effect transistor and a feedback circuitry to provide a safe voltage range for the input inverter. This feedback circuitry structure includes series transistors and two-reference signal and is responsive to the voltage applied at the output node. It is also responsible for pulling-up the voltage at the gate of p-channel MOS transistor in the input inverter for low power consumption and to provide a high impedance input. This circuit does not consume dc current under steady state.  
         [0029]     In another embodiment ( FIG. 5 ), the input buffer circuit includes a cascoded inverter. The cascading transistors are connected to the reference signals to avoid gate-oxide and junction stress while the inverter input is connected to the safe node, which receives the high-voltage pad signals after level translation. This structure speeds-up the circuit performance in terms of speed and hence reduces short-circuit current during transition.  
         [0030]     The invention also includes the cascoded structure for the input inverter, which is similar to the second embodiment. The output of this inverter is fed to the low-voltage level-translator to provide a core level output signal.  
         [0031]     The present invention is not dependent on the body effect of the field effect transistors. So it is not very sensitive to the process variations. Since very stable reference voltages have been used, a high-voltage supply can be input (max of 5 volt) and tolerated.  
         [0032]     The input buffer of the present invention is particularly suitable to provide a level-shifted output voltage in response to a high-voltage input signal while working at a little higher voltage (3.3±10% in the present invention) while using the advantage of low-voltage technology (2.5 volt in the present invention).  
         [0033]     A Schmitt version of the input buffer is also presented to receive a TTL level signal and translate it to the CMOS level signal. A 400 mV of hysteresis has been achieved in the worst case with a small modification in the main input buffer. Silicon result is also given for the hysteresis value.  
         [0034]     In this invention such an input buffer in 2.5V technology has been presented and described in detail below:  
         [0035]     A schematic diagram for new circuit is shown below in  FIGS. 5-6 .  FIG. 6  is an Schmitt version of new input buffer. It has been divided into three parts. Input block  3  is a normal Schmitt buffer with a level-shifter at the output, which converts 3.3-volt signal to the core level (1.2 V nominal) CMOS signal. Since an input buffer is normally used to drive core logic in the chip, level-shifter has been added to make it complete. Block  2  has been added to increase the speed of the Schmitt buffer (different threshold point for high and low transition of Schmitt buffer for input signal inherently makes it slow). Block  1  is responsible for the protection of input transistors in block  3  by level-shifting the input signal (which are propagated from pad).  
         [0036]     Input buffer  3  consists of two parts, input inverter and a VDDS (nominal 3.3 volt) to VDD (core level with nominal voltage 1.2 volt) level-shifter. All the transistors are 2.5-volt capable in 0.13 μm technology except transistors  26 ,  27  and inverter  30  which are 1.2-volt capable in 0.13 μm technology. Input inverter has two PMOS transistors  15  and  16  in series and two NMOS transistors  17  and  18  in series. Input inverter output E will swing from 0 to 3.3 volt (up to 3.6 volt in worst case) because it is directly connected to 3.3-volt power supply.  
         [0037]     Inverter formed by transistors  24  and  25  also provides buffering to the signal at E. E and F are complementary signals of same level (3.3 volt) and are inputs of level-shifter. E is connected to the gate of NMOS  29  through pass-gate  5  and F to the gate of NMOS  28  through pass-gate  4 . The two complementary outputs of level-shifter are pull-down by NMOSs  28  and  29 . Only one of these outputs G has been used to drive the output inverter  30  to provide final CMOS level output. Transistors  4  and  5  have been used to level-translate the signal at F and E respectively so that signal levels at the gate of transistors  28  and  29  are 0−(VDDS−Vt|4|). Transistors  26  and  27  are 1.2-volt 0.13 um PMOS. This structure is faster than the two-inverter structure (First inverter of 3.3 V and second inverter of 1.2 V in 0.13 um) used for level shifting. Since low-voltage transistors  26  and  27  are faster (because of smaller channel length) than  28  and  29 , they can be made smaller. Again transistors size ratio of PMOS ( 15  or  16 ) to NMOS ( 17  or  18 ) will decide the input inverter&#39;s threshold point and hence the V il  or V ih  level. Size of transistors  23  and  18  is tuned to achieve V ih  level for worst case (2.0 worst TTL level). V il  level is set by the input inverters pull-up and pull-down ratio. An extra NMOS can also be used as feedback between nodes F and E (like PMOS transistor  23 ) to set V il  level if necessary.  
         [0038]     The gate of PMOS  16  is connected to VL reference signal and gate of NMOS  17  is connected to the VH reference signal. The typical value for VH and VL is 2.5 V and 0.7 V. When pull-down (NMOS structure of  17  and  18 ) is off and pull-up (PMOS structure of  15  and  16 ) is on, E will be at 3.3 volt for typical case. Because of cascading of  17  and  18 , V ds , (drain to source voltage) of these two transistors will be less than 2.5 volt and V dg  (drain to gate voltage) of  17  is approximately 0.8 volt. In worst case it will be 1.1 volt when VDDS is 3.6 volt. When pull-down is on and pull-up is off, E is at 0 volt. Again transistors  15  and  16  are free from V ds  stress. Gate to drain voltage of  16  is only 0.7 volt. In any case Vgb (gate to bulk voltage) of  16  and  17  are 2.6 volt and 2.5 volt respectively.  
         [0039]     In fact, reference voltages VL and VH are dependent on supply voltage VDDS but the variation is small and it helps in making the junction and the gate-bulk voltages almost constant. For example, when VL is 0.7 volt for nominal case (3.3 volt), Vgb voltage for transistor  16  is 2.6 volt. When VDDS goes to 3.6 volt, VL increases to 0.8 volt to make Vgb 2.8 volt, which is acceptable for 2.5-volt device. Gate voltages of  15  and  18  are also limited by block  1  so that these devices are also safe from any kind of stress. For the sake of clarity, blocks  1  and  2  have been shown again in  FIG. 7 .  
         [0040]     Block  1  has been added to level-translate the input signal. NMOS  10  is directly connected to the input with gate connected to VH (2.5 volt nominal). Level-translated signal A is the input of the block  2  and block  3  (Schmitt buffer). Max value for logic high at A will be (VH−vt 10 ) which is less than VH so transistors  22  and  18  are also safe from stress. NMOS  11  is connected between nodes A and B with gate connection to VH.  11  will pass signal at A to B without level-translation. When logic high value at IN is 5.0 volts, V dg  (drain to gate voltage) for  10  is approximately 2.5 volts and device  10  is not stressed.  10  and  11  will pass logic low (0 volt). When input IN is at logic high (3.0 to 5.0 volt, in case of TTL input worst value may be 2.0 volt), A and B will have values VH−vt 10  . It may be 2.0 volt if VH−vt 10  is greater than 2.0 volt. PMOS  12  will turn-off immediately because node C is at VL when IN was at logic low. When node A and B rises to VH−vt 10 , NMOSs  22  and  18  turn on, pulling node D and E towards zero. PMOSs  19  and  15  are still ON because transistors  14  and  13  are OFF and C is at VL. As soon as D drops below (VDDS−|vt 13 |),  13  turns on and starts charging node C rapidly which turns-off PMOSs  19  and  15  which further increases the speed of pulling-down of node D and E (block  2 ). Transistors  19 ,  20 ,  21  and  22  are small sized transistors, which have been added to speed-up the transition. Since node E is being pulled-up by PMOS  23  (in block  3 ), it takes long time to go to logic low as compared to that at node D. So addition of these four small size transistors have made the turn-off of PMOSs  19  and  15  very fast and hence reduced the crowbar current during transition because of short transition period to make circuit efficient for power and speed. Transistor  14  is required to turn-off transistor  12  to stop steady current from VDDS to VL because  12  will turn-on when C will rise to VDDS. As soon as D falls below VDDS−|vt 13 |, C starts rising and when difference [V(C)−V (D)] crosses |vt 14 |, also turns-on and starts charging node B to VDDS. So device  14  will not let transistor  12  turn-on. Transistor  11  will not pass VDDS at B to A because NMOS  11  gate is connected to VH. This will not let the devices  22  and  18  to get stressed. The transition of these nodes has been shown in  FIG. 8 .  
         [0041]      FIG. 8  shows internal node waveforms for low to high transition at IN. All the important internal nodes waveforms have been shown. Point C represents the point after which transition is very fast because of block  2 . It is clear that node D makes transition much faster than node E. It is also clear that when A has made transition to VH−vt 10  , node C and hence node B makes full transition. If feedback point was E instead of D, it would have taken long time for the node E to make transition.  
         [0042]     In the second case when the input makes transition from high to low, A and B will be low and NMOSs  22  and  18  will turn off immediately. Transistor  12  will start discharging node C. Since PMOS  14  is on initially, node C will be discharged through PMOS  14  also. As soon as C drops below VDDS−|vt 19 |, transistors  19  and  15  turns ON and node D gets charged to VDDS rapidly. This fast charging stops current from VDDS to VL by turning  13  OFF. Transistors  14  will also turn-off and node C will discharge only to VL. If somehow node C goes up or down to VL, it will be discharged or charged to VL again by  12  because gate of  12  is at logic low (0 volt). So the steady state value of C is VL and it will not cause stress to transistors  19  and  15 . The internal nodes waveform has been shown in  FIG. 9  for this case.  
         [0043]      FIG. 9  shows internal node waveforms for high to low transition at IN. It is clear that transition of node E starts at point C. Block  1  introduces a small delay for high to low transition. This is the penalty, which will have to be paid for a stress free device operation higher voltage. After point C NMOS  22  turns-off and nodes A and B starts falling rapidly at the same time. Initially C starts falling fast because of two paths one through  14  and other through  12 . When  14  turns off node C discharges through  12 . Since transistor  12  is small (intentionally made) it delays the charging of node D and E. This is not a limitation of the design. If reference block has the capability to sink large current, then  12  can be made larger in size and speed can be improved further.  
         [0044]     In this way block  1  protects the input transistors and along with block  2  it makes transition fast and reduces the crowbar current during transition.  
         [0000]     Simulation Results:  
         [0045]     Simulation results for the 5-volt tolerant Schmitt buffer in 0.13 um, 2.5 V CMOS process are provided below.  
         [0046]     For performance, delay plot for the Schmitt buffer is shown in the  FIG. 10 . Data has been obtained for the nominal case and 3.3 V over temp range −40 to 125 degrees Celsius for 32× (32 times of the cap of 1× drive inverter) load.  
         [0047]     In  FIG. 11  maximum frequency of operation of Schmitt buffer has been plotted against load. Input clock has been assumed to have rise and fall time (measured from 0% to 100% of supply) as 20% of the total period and on/off period as 30% of the total period. This characteristic of clock is good enough to emulate the real data signal for maximum number of transitions for a given frequency. For the output to be considered as real waveform, it has been assumed that output must reach at-least 90% of VDDS for logic high and must be below 10% of VDDS for logic low.  
         [0048]      FIG. 10  shows a delay plot for Schmitt-buffer at nominal case.  
         [0049]      FIG. 11  shows maximum frequency of operation vs load plot. The Y-axis is maximum frequency in MHz and X-axis is load in pf.  
                                                           TABLE 1                           Hysteresis data for the Schmitt                LOT NO   VIL   VIH   VHYST                            1   1.084   1.516   0.432           2   1.05   1.47   0.42           3   1.089   1.526   0.437           4   1.083   1.518   0.435           5   1.06   1.46   0.40           6   1.097   1.523   0.432                        
         [0050]     The data given above in the table is for ambient temperature and 3.3 volt. The worst values obtained for VIL and VIH are 0.903 V and 1.693 V respectively (not shown in the table). So it is clear that even for different lots, VIL and VIH values are according to the TTL specification with enough margins.  
         [0051]     While there have been described above the principles of the present invention in conjunction with specific components, circuitry and bias techniques, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.
The present invention provides a high-voltage tolerant input buffer circuit including a first NMOS transistor having its source terminal connected to the input pin, its gate terminal connected to a first reference voltage and its drain terminal connected to a first output terminal; a second NMOS transistor having its gate terminal connected to said first reference voltage and its source terminal connected to said first output terminal; a first PMOS transistor having its gate terminal connected to the drain terminal of said second NMOS transistor, its drain terminal connected to a second reference voltage lower than said first reference voltage and its source terminal connected to a second output terminal; a second PMOS transistor having its drain terminal connected to the drain terminal of said second NMOS transistor, its source terminal connected to said second output terminal, and its gate terminal connected to a control voltage; and a third PMOS transistor having its drain terminal connected to said second output terminal, its source terminal connected to a supply voltage, and its gate terminal connected to said control voltage.