Abstract:
An input structure protects an Integrated Circuit (IC) against increases in the IC pad voltage when the supply voltage to the IC is tuned off. The input structure includes circuitry for transferring either a divided-down pad voltage or the positive supply voltage to a buffer circuitry. The buffer circuitry receives the voltage transferred thereto and lowers the pad voltage. The lowered pad voltage generated by the buffer circuitry is subsequently applied to the IC.

Description:
FILED OF THE INVENTION 
     The present invention relates to input structures and, more particularly, to input structures for protecting Integrated Circuits. 
     BACKGROUND OF THE INVENTION 
     Input structures for protecting the gate oxide of MOS (Metal Oxide Semiconductor) transistors connected to input pads are widely used in Integrated Circuits (IC). Such an input structure typically receives the voltage applied to an IC pad and supplies a reduced voltage to the IC thereby ensuring that the gate-to-source and gate-to-drain voltages of the IC transistors do not exceed a maximum allowable limit. 
     Currently known input structures fail to provide adequate protection for the gate oxide when no power is supplied to the IC and yet the pads of the IC continue to receive power. The problem is further compounded when ICs manufactured using deep submicron (e.g. 0.25 μm) CMOS technologies—where the gate-to-source and gate-to-drain voltages of an MOS transistor must remain below 3.5 volts—are used in a system requiring 5.5 volts to operate. When used in such a system, the IC must be able to withstand the application of 5.5 volts to its pads both when the power supply to the IC is on and when it is off. 
     FIG. 1 shows a known input structure  10 . Input structure  10  receives voltage Vin on pad  12  and supplies voltage Vout at the output terminal of inverter  22 . Input structure  10  suffers from contention, as described below. To force the voltage Vout to a low level when no voltage is applied to pad  12 , a user may place an external resistor (not shown) across pad  12  and the system ground. When such a resistor is used and a hiqh voltage is applied to pad  12  before tri-stating pad  12 , PMOS transistor  18  turns on, pulling node N 1  to a high voltage. At the same time, the external resistor pulls node N 1  to ground. Therefore, a contention develops between PMOS transistor  18  and the external transistor. If the pull-up capability of transistor  18  is greater than the pull-down capability of the external resistor, voltage Vout remains at the high level. 
     FIG. 2 shows known input structure  30 . Input structure  30  does not have the contention problem of input structure  10  but consumes too much DC power because PMOS transistor  36  is never completely turned off. 
     FIG. 3 shows known input structure  50 . Input structure  50  does not have the contention problem of input structure  10  nor does it have the excessive power consumption of input structure  30  but it suffers from a major disadvantage. To avoid the natural hysterisis in input structure  50 , PMOS transistors  56  and  58  must be made large to meet the required threshold high and low specifications and which, in turn, makes the input structure undesirably slow. 
     In yet other known input structures (not shown) the MOS transistors are formed using thick gate oxides to protect against the pad over-voltage when the supply voltage is tuned off. Therefore, an IC containing such an input structure requires a manufacturing process that supports both regular and thick gate oxide MOS transistors and is thus expensive. 
     Therefor, a need exists for an input structure for protecting the internal circuitry of an IC when the pads of the IC continue to receive power but the supply power to the IC is turned off, and which overcomes the known problems of the existing input structures discussed above. 
     SUMMARY OF THE INVENTION 
     An input structure for an Integrated Circuit (IC), in accordance with one embodiment of the present invention, includes a first group of transistors for dividing the IC pad voltage, a second group of transistors for transferring the IC supply voltage, and a transistor for coupling the first and the second groups of transistors. 
     The input structure further includes a buffer circuit which receives either the divided pad voltage or the transferred supply voltage and generates a voltage that is applied to the IC. 
     When the supply voltage to the IC is turned off, the first group of transistors divides the pad voltage and supplies it to the buffer circuit. During this time, the coupling transistor remains conducting to disable the second group of transistors. When the supply voltage to the IC is turned on, some of the transistors in the first group and the coupling transistor are turned off. Transistors in the second group transfer the supply voltage to the buffer circuit. 
     The buffer circuit includes a first inverter coupled to additional transistors to ensure that the gate-to-source and gate-to-drain voltage of the inverter transistors do not exceed a specified limit. A second inverter of the buffer receives the output voltage of the first buffer and supplies the voltage to the IC. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a known input structure. 
     FIG. 2 is a schematic diagram of another known input structure. 
     FIG. 3 is a schematic diagram of another known input structure. 
     FIG. 4 shows a schematic diagram of the input structure, in accordance with one embodiment of the present invention. 
     FIG. 5A shows a cross-sectional view of one of the transistors of FIG.  4 . 
     FIG. 5B shows a schematic view of the transistor of FIG.  5 A. 
     FIG. 6 shows the computer simulation results of the voltage level at one of the nodes of the input structure of FIG.  4 . 
     FIG. 7 shows the computer simulation results of the voltage levels of the two of the nodes of the input structure of FIG. 4 when the positive supply voltage is turned off and the voltage applied to the pad is ramped up and then ramped down. 
     FIG. 8 shows the computer simulation results of the voltage level of the output signal of the input structure of FIG. 4 when the positive supply voltage is turned on and the voltage applied to the pad is ramped up and then ramped down. 
     FIG. 9 shows the computer simulation results of the voltage levels at two of the nodes of the input structure of FIG. 4 when the positive supply voltage is turned on and the voltage applied to the pad is ramped up and then ramped down. 
     FIG. 10 shows a schematic diagram of the input structure, in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A transistor schematic diagram of input structure  100  of an Integrated Circuit (IC), in accordance with one embodiment of the present invention, is depicted in FIG.  4 . Input structure  100  includes circuit  200  for generating voltage signal Vpwr which is equal to either the positive supply voltage Vdd or to a divided-down voltage of pad  102 . Input structure  100  also includes circuit  300  for generating voltage signal Vin which is applied to the IC. 
     Circuit  200  includes transistor groups  220 ,  240  and coupling transistor  206 . Transistor group  220 , in turn, includes PMOS transistors  208 ,  212  and  214  and NMOS transistors  210  and  216 . Transistor group  240  includes NMOS transistors  202  and PMOS transistor  204 . The operation of circuit  200  when voltage supply Vdd is turned off (i.e. Vdd is at 0 volts) is described next. 
     When Vdd is at 0 volts and a high voltage (e.g. 5.5 volts) is applied to pad  102 , a current discharge path forms between pad  102  and supply voltage Vdd through transistors  208 ,  210 ,  212 ,  214  and  216 . The source and bulk terminals of PMOS transistor  208  are connected to pad  102  and the gate and the drain terminals of PMOS transistor  208  are connected to the drain terminal of transistor  210 . Therefore, PMOS transistor  208  turns on and provides slightly less than one PMOS threshold voltage across its drain-to-source terminals. In other words, PMOS transistor  208  operates in a subthreshold region. 
     NMOS transistor  210  has its gate and drain terminals connected together. The source terminal of NMOS transistor  210  is connected to the source terminal of PMOS transistor  212  and the substrate terminal of NMOS transistor  210  is connected to the voltage supply Vss. NMOS transistor  210  also operates in a subthreshold region and thus provide slightly less than one NMOS threshold voltage drop across its drain-to source terminals. 
     The gate, drain and bulk terminals of NMOS transistor  212  are connected together. FIG. 5A shows a cross-sectional view  400  of PMOS transistor  212 . As seen from FIG. 5A, the drain region  402  of transistor  212  is connected to N +  region  404 . Source region is separated from drain region  402  by a P-N junction diode formed between source  402  and N-well  408 . In other words, a p-n junction diode  222  exists in parallel between the source and drain terminals of PMOS transistor  212 , as shown in FIG.  5 B. Consequently, the voltage drop across the drain-to-source terminals of transistor  212  is equal to one base-to-emitter voltage (i.e., Vbe) of a bipolar transistor. Because the threshold voltage of an MOS transistor has a positive temperature coefficient (i.e. the threshold voltage increases with rising temperature) and the base-to-emitter voltage of a bipolar transistor has a negative temperature coefficient (i.e. the base-to-emitter voltage decreases with rising temperature), the voltage at the drain terminal of PMOS transistor  212  exhibits a reduced sensitivity to temperature variations. 
     The combined transistors  208 ,  210  and  212  provide a voltage at the drain terminal of transistor  212  that is lower than that of pad  102  by a sum consisting of slightly less than one NMOS threshold voltage (because transistor  212  operates just below the threshold region), slightly less than one PMOS threshold voltage (because transistor  214  operates just below the threshold region), and a bipolar base-to-emitter voltage. 
     PMOS transistor  206  has a source terminal that is coupled to the gate terminal of PMOS transistor  212  and a gate terminal that is coupled to supply voltage Vdd. The drain terminal of transistor  206  is connected to the gate terminal of PMOS transistor  204  and to the drain terminal of NMOS transistor  202 . When the supply voltage Vdd is at zero volts and the voltage applied to pad  102  is greater than four MOS transistor threshold voltages and a bipolar transistor base-to-emitter voltage, the voltage passed from the source terminal to the drain terminal of PMOS transistor  206  switches off transistor  204  whose source terminal receives supply voltage Vdd. The gate and drain terminals of NMOS transistor  202  respectively receive Vdd and Vss supply voltages. Therefore, NMOS transistor  202  remains off when supply voltage Vdd is at 0 volts. 
     Because the gate terminal of PMOS transistor  214  receives supply voltage Vdd, when the supply voltage Vdd is at zero volts, the voltage at the source terminal of PMOS transistor  214  is passed to its drain terminal which generates voltage signal Vpwr. The source terminal of PMOS transistor  214  is connected to the drain terminal of NMOS transistor  212  while the drain terminal of PMOS transistor  214  is connected to the drain terminals of PMOS transistors  204  and NMOS transistor  216 . 
     The gate and drain terminals of NMOS transistor  216  are connected together and the source terminal of NMOS transistor  216  receives supply voltage Vdd. Therefore, transistor  216  stays conductive when voltage Vdd is at 0 volts. 
     Consequently, when Vdd is at 0 volts and a high voltage is applied to pad  102 , each of transistors  208 ,  210 ,  212 ,  214  and  216  conducts current I 1  which flows from pad  102  to voltage supply Vdd. The magnitude of current I 1  is such that PMOS transistor  208  and NMOS transistor  210  operate in their respective subthreshold region. The operation of circuit  200  when voltage supply Vdd is turned on is described next. 
     When supply voltage Vdd is turned on, NMOS transistor  202  turns on. Consequently, voltage Vss is applied to the gate terminal of PMOS transistor  204 , turning on PMOS transistor  204 . Therefore, signal voltage Vpwr receives the full Vdd supply voltage. Also, when supply voltage Vdd is turned on, PMOS transistor  214  and NMOS transistor  216  are both in a non-conducting state and, therefore, the voltage signal Vpwr is unaffected by the transistors in the transistor group  220 . Consequently, when voltage supply Vdd is switched on, the current discharge path from pad  102  to voltage supply Vdd is shut off. Voltage signal Vpwr is applied to circuit  300 , which is described next. 
     Circuit  300  includes NMOS transistors  302 ,  306 ,  310 ,  314  and PMOS transistors  304 ,  308  and  312 . Circuit  300  acts as a buffer transferring a level-shifted voltage of pad  102  to the IC. 
     As stated above, to operate reliably, the gate-to-source and gate-to-drain voltages of all transistors in input structure  100  must be smaller than a maximum allowable limit (e.g. 3.4 volts). Supply voltage Vdd is never increased above the maximum allowable limit. Therefore, when the voltage applied to pad  102  is 0, the gate-to-source and gate-to-drain voltages of all the transistors in input structure  100  stay within the allowable limit. Application of voltage signal Vpwr to circuit  300  ensures that the maximum allowable gate-to-source and gate-to-drain voltages are not exceeded when a high voltage is applied to pad  102 . 
     Voltage signal Vpwr is applied to the source and substrate terminals of transistor  304  whose gate terminal is connected to pad  102 . Therefore, the voltage across the gate-to-source region of transistor  304  is always maintained below the maximum allowable limit. To ensure that the voltage across the gate-to-drain regions of transistor  304  stays below the allowable limit, the drain terminal of transistor  304  is connected to the source terminal of transistor  306  whose gate and drain terminals are supplied with voltage signal Vpwr. Because no DC current flows through transistor  306 , transistor  306  operates in a subthreshold region and, therefore, the voltage across the drain and source regions of transistor  306  is smaller than an NMOS transistor threshold voltage. Consequently, even when a high voltage (e.g. 5.5 volts) is applied to pad  102 , the voltages across the gate-to-source and gate-to-drain regions of transistor  304  stay within the allowable limit. 
     The drain terminal of transistor  302  is connected to pad  102 . Voltage signal Vpwr is applied to the gate terminal of transistor  302 . Therefore, the voltage signal PADint of the source terminal of transistor  302  is at a transistor threshold voltage below that of its drain terminal. The source terminal of transistor  302  is connected to the gate terminals of transistors  308  and  310  which together form an inverter. 
     The voltage of the source terminal of transistor  308  (i.e. the drain terminal of transistor  310 ) is applied to the gate terminals of transistors  312  and  314 . The source terminals of transistors  312  and  314  are respectively connected to supply voltages Vdd and Vss. The drain terminals of transistors  312  and  314  are connected together and provide voltage signal Vin which is applied to the IC. 
     When the voltage at pad  102  is at a low level (i.e. at Vss), and voltage supply Vdd is turned on, voltage signal Vin is also at the Vss voltage level. Similarly, when the voltage at pad  102  is at a high level (i.e. 5.5 volts), and voltage supply Vdd is turned on, voltage signal Vin is at the Vdd voltage level. Thus, when the voltage at pad  102  is at a high level and supply voltage Vdd is turned off, voltage signal Vin is also at the Vss voltage level, but all the transistors in input structure  100  experience allowable gate-to-source and gate-to-drain voltages. 
     In one embodiment of the present invention, the CMOS process used to manufacture input structure  100  requires that the gate-to-source and gate-to-drain voltages not to exceed 4.3 volts. Accordingly, the transistor sizes of circuit  200  and circuit  300  are as follows: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 transistor no. 
                 channel width (μm) 
                 channel length (μm) 
               
               
                   
               
             
             
               
                 202 
                 8 
                 2 
               
               
                 204 
                 35 
                 1 
               
               
                 206 
                 3 
                 1 
               
               
                 208 
                 8 
                 1 
               
               
                 210 
                 8 
                 1 
               
               
                 212 
                 8 
                 1 
               
               
                 214 
                 8 
                 1 
               
               
                 216 
                 1 
                 35 
               
               
                 302 
                 15 
                 1 
               
               
                 304 
                 18 
                 1 
               
               
                 306 
                 1 
                 4 
               
               
                 308 
                 18 
                 1 
               
               
                 310 
                 3 
                 1 
               
               
                 312 
                 14 
                 1 
               
               
                 314 
                 6 
                 1 
               
               
                   
               
             
          
         
       
     
     In the above embodiment, supply voltage Vdd may vary between 3.0 to 3.6 volts and the pad  102  voltage may vary between 0 and 5.5 volts. 
     Transistor  216  whose channel width and channel length are 1 μm and 25 μm respectively operates in a linear mode and accordingly acts as a resistor. By selecting in transistor  216  the above channel dimensions, the bias current through transistors  208 ,  210 ,  212  and  214  is set. The voltage drop across each of transistors  208 ,  210 ,  212  and  214  is determined by, in part, by their respective channel dimensions as well as by the current flow through them. 
     FIG. 6 shows the computer simulation results of the generated voltage signal Vpwr of input structure  100  when pad  102  is at 5.5 volts and supply voltage Vdd is at 0 volts, using the transistor sizes as shown in the above table. As seen from FIG. 6, voltage signal Vpwr is at 2.7 volts when Vdd is at 0 volts. Voltage signal Vpwr and supply voltage Vdd are at the same potential when supply voltage Vdd is between 1.6 and 2.7 volts (not shown). 
     FIG. 7 shows the computer simulation results of the voltage signals Vpwr and PADint when the voltage at pad  102  is ramped up from 0 to 3.3 volts and then ramped down form 3.3 volts to 0 volts and when supply voltage Vdd is at 0 volts. As seen from FIG. 7, signal Vpwr rises from 0 to 1.88 volts when the voltage at pad  102  increases from 0 to 3.3 volts. Similarly, when the voltage at pad  102  decreases from 3.3 to 0 volts, signal Vpwr also decreases from 1.88 volts to 0 volts. 
     FIG. 8 shows the computer simulation results of the voltage signal Vin when supply voltage Vdd is at 1.62 volts and when the voltage at pad  102  is ramped up from 0 to 1.62 volts and then ramped down form 1.62 volts to 0 volts. As seen from FIG. 8, during the ramp-up, signal Vin reaches 1.62 volts and then goes to 0 during the pad voltage ramp-down. 
     FIG. 9 shows the computer simulation results of the voltage signals Vpwr and PADint when supply voltage Vdd is at 1.62 volts and when the voltage at pad  102  is ramped up from 0 to 3.3 volts and then ramped down form 3.3 volts to 0 volts. As seen from FIG. 9, signal Vpwr remains at 1.62 volts (at the Vdd voltage) while voltage signal PADint increases during the ramp-up and decreases during the ramp-down. 
     It is understood from the above discussion that depending on the IC manufacturing process, the specified range of supply voltage Vdd and pad voltage  102 , more or fewer transistors than that shown in transistor groups  220  and  240  may be required for proper operation of input structure  100 . 
     FIG. 10 shows input structure  500 , in accordance with another embodiment of the present invention. Identical reference numerals in FIGS. 4 and 10 refer to identical elements. Input structure  500  is similar to input structure  100  except that input structure  500  contains PMOS transistor  218 , disposed between transistors  210  and  212 . PMOS transistor  218  enables input structure  500  to operate with a wider range of pad  102  voltages. Accordingly, the range of allowable pad  102  voltages for input structure  500  is greater than that of input structure  100 . 
     The exemplary embodiments of the invention disclosed above are illustrative and not limiting. The inventions is not limited by the number or type of transistors disposed in transistor group  220  of FIG. 4; nor is the invention limited by the number or type of transistors disposed in transistor group  240 . For example, by varying the number of transistors in transistor group  220 , the range of operable Vdd supply voltage for circuit  100  is varied. Other embodiments of this invention are possible within the scope of the appended claims.