Abstract:
An off-chip driver circuit having a set of input terminals and an output terminal, a pull-up transistor having a controllable path connected between a first power supply and the output terminal of the off-chip driver circuit, a pull-down transistor having a controllable path connected between a second power supply and the output terminal of the off-chip driver circuit, a first controllable path for applying a first voltage at one of the input terminals to a control terminal of the pull-up transistor, the first controllable path functioning in response to voltages at the output terminal below a first value, a second controllable path for applying a second voltage greater than the first voltage to the control terminal of the pull-up transistor, the second controllable path functioning in response to voltages at the output terminal above the first value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electronic systems and, more particularly, to methods and apparatus for providing an off-chip driver circuit implemented in complementary metal oxide silicon (CMOS) technology which has a supply voltage less than the supply voltage of external circuits which may be connected to the output of the driver circuit. 
     2. History of the Prior Art 
     In modern day integrated circuits much attention is focused on the design of output driver circuits that must provide signals to various bus types having various loading conditions. An off-chip driver circuit should be designed not only to successfully drive logic levels relating to the supply voltage of the off-chip circuit but should also be protected against any high voltages which may occur when the off-chip driver circuit is disabled and its output terminal is coupled to an external circuit operating at a higher supply voltage. It is desirable to provide this protection while minimizing the number of transistors and therefore the chip area utilized by the off-chip driver. 
     FIG. 1 is a circuit diagram of a known off-chip driver circuit  10 . This circuit is described in U.S. Pat. No. 5,151,619 assigned to International Business Machines Corporation. The off-chip driver circuit  10  has first and second input terminals IN 1  and IN 2  which are connected to a pre-driver circuit (not shown). The off-chip driver circuit  10  is arranged to drive a signal received at the input terminals IN 1  and IN 2  to an output terminal OUT during an output mode. The off-chip driver circuit includes a first p-channel MOS transistor device  12  and a second n-channel MOS transistor device  14  which are serially arranged between a supply voltage Vdd and a point of reference potential Vss (which is typically at ground). The output terminal OUT is connected to a circuit node between the pull-up transistor  12  and the pull-down transistor  14 . A pass gate  16  is formed by a n-channel transistor device  18  having its gate connected to the supply voltage Vdd and its drain/source path connected between the input terminal IN 1  and the gate of the pull-up transistor  12 . The n-channel transistor  18  acts in parallel with a p-channel transistor device  20  having its drain/source path connected to the same nodes and its gate connected to the output terminal OUT. 
     The pull-down transistor  14  has its gate connected to the second input terminal IN 2 . The off-chip driver circuit  10  includes a control transistor device  22  which has its gate connected to a control voltage Vref (typically equal to the source voltage Vdd) and its drain/source path connected in series between the pass gate p-channel transistor  20  and the output terminal OUT. The p-channel transistor  20  and the control transistor  22  are formed in a common n-well  26 . An additional p-channel transistor device  24  has its gate connected to the terminal OUT to provide the supply voltage Vdd to bias the n-well  26  in certain conditions. 
     The off-chip driver circuit  10  has its output terminal OUT selectively connectable to an external circuit  28  which has a supply voltage Vcc and which is used in an input mode of the off-chip driver circuit  10  to supply signals to the chip via the output terminal OUT which is connected to an input signal line (not shown). 
     The voltage supply Vdd for the off-chip driver circuit  10  is typically about 3.3 volts+/−0.3 volts. However, the external circuit  28  may operate at a higher source voltage such as a conventional CMOS level of five volts. When used as an off-chip driver circuit, the circuit  10  should be capable of driving the output terminal OUT at zero volts to indicate a logical zero or 3.3 volts to indicate a logical one. However, when the circuit  10  is not driving out, it must be able to tolerate voltages as high as seven volts at the terminal OUT. 
     When the off-chip driver circuit is used in the output mode, the same signal level is applied at each of the first input terminal IN 1  and the second input terminal IN 2  to provide an output level at output terminal OUT. As is more fully discussed in the above-referenced U.S. Pat. No. 5,151,619, with the input terminals IN 1  and IN 2  low (typically ground), the voltage at the output terminal OUT is at the source voltage Vdd. With the input terminals IN 1  and IN 2  high (typically 3.3 volts), the voltage at the output terminal OUT is a low voltage (approximately ground). To disable the output mode of the off-chip driver circuit  10 , the pre-driver circuit which furnishes input signals to the first and second input terminals IN 1  and IN 2  is tristated by driving the first input terminal IN 1  high and the second input terminal IN 2  low. In this condition, both the pull-up p-channel transistor  12  and the pull-down n-channel transistor  14  are off. 
     The circuit  10  of FIG. 1 is designed to receive signals at the terminal OUT when in this disabled condition. With a voltage of zero volts at the output terminal OUT, the pass gate p-channel transistor  20  is turned on and passes the 3.3 volts present on the first input terminal IN 1  to the gate of the pull-up transistor  12 , turning the transistor  12  off. Thus, there is no leakage current through the pull-up transistor  12 . When a high voltage, for example five volts, is applied to the output terminal OUT by the external circuit  28 , the p-channel pass gate transistor  20  is turned off. However, the p-channel control transistor  22  is turned on because the voltage applied at its source exceeds the control voltage Vref (3.3 volts) at its gate. The path through the transistor  22  furnishes the voltage at the output terminal OUT to the gate of the pull-up p-channel transistor  12 , turning it off. In this condition, the voltages at the gate and the source of the p-channel transistor  12  are approximately the same; and, consequently, the oxide of the p-channel transistor  12  is not subject to any significant stress. Therefore, in the disabled condition when the likely extreme values of voltages are imposed by the external circuit  28 , the prior art circuit  10  of FIG. 1 works well. 
     However, problems arise both when voltages at the terminal OUT are at middle values between the extremes and during transition states. When the value of the voltage at the terminal OUT is in a range between the reference voltage (Vref) minus the p-channel threshold voltage (Vpt) and Vref plus Vpt, neither the pass gate transistor  20  nor the control transistor  22  is on. In this range, the voltage at the gate of the pull-up transistor  12  is not tracking either the supply voltage Vdd or the output voltage. This can cause leakage current to be referred to the input of the circuit  10  from the device  12 . This leakage can cause specification violation in certain applications such as PCI drivers where the input leakage must be below seventy microamperes with the input in a range from zero to five volts. 
     For example, in order to turn the control transistor  22  on, the voltage at the output terminal OUT must be at least a threshold voltage Vpt above the control voltage Vref at the gate of the control transistor  22 . If this condition is not satisfied, the control transistor  22  will remain off. If the voltage at the terminal OUT is slightly above or slightly below the reference voltage (normally 3.3 volts), the voltage is neither low enough to turn on the p-channel pass gate transistor  20  nor high enough to turn on the control transistor  22 . Thus, both the transistors  20  and  22  are off. The n-channel transistor  18  of the pass gate will try and pull up the gate of the p-channel transistor  12 , but it will be only able to pull it up to a threshold value Vnt below the voltage on the input IN 1  (about 2.6/2.7 volts). This is inadequate to reliably turn off the pull-up p-channel transistor  12 , and therefore there will be a leakage current through that transistor. Thus, with voltages at the terminal OUT closer than a threshold Vpt to the supply voltage Vdd, the prior art circuit  10  has a major disadvantage. 
     One solution to the problem is proposed in an article entitled “3.3 V-5 V Compatible I/O Circuit without Thick Gate Oxide” by Takahashi et al, IEEE 1992 Custom Integrated Circuits Conference 23.3.1-23.3.4. In that solution, different types of transistors are introduced to exclude undesirable leakage paths and to prevent oxide stress. This solution suffers from the disadvantage that it requires different fabrication techniques to produce the entire circuit, an expensive option. 
     Another problem displayed by the circuit  10  is that the p-channel device  12  does not provide any electrostatic discharge (ESD) clamping action for over-shoot conditions of any voltage present at the terminal OUT. Since the device  12  is held off under all over-shoot conditions of the voltage at the terminal OUT, the device  12  can not be used to clamp the output when electrostatic discharge occurs (in conventional output drivers the output devices and junction diodes are combined to reduce damage from electrostatic discharge). With this prior art circuit  10 , additional devices (not shown) must be added to eliminate problems caused by electrostatic discharge; and this addition results in the circuit  10  occupying additional silicon area. 
     The circuit  10  also includes a method for isolating the n-well of the p-channel devices when the P+/N− diodes would otherwise be forwarded biased resulting in leakage current. P-channel transistor device  24  acts as a switch between the positive supply Vdd and the well bias source. The device  24  is directly controlled by the voltage at the terminal OUT and is disabled when the input voltage at the OUT terminal comes within a threshold voltage Vpt of the 3.3 volt source voltage therefore eliminating the leakage path to the well before the diodes can be forwarded biased. Since conduction by the device  24  is controlled directly by the voltage at the terminal OUT, care must be taken to protect the gate of the device  24  from electrostatic discharge damage. Such protection also adds extra circuitry to the silicon. 
     It is desirable to eliminate the range in which leakage current can occur in an off-chip driver circuit which receives input signals at its output terminal which input signals may have values greater than the internal source voltage of the driver circuit. 
     It is also desirable to provide protection against electrostatic discharge in an off-chip driver circuit without increasing the size of the circuit. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an off-chip driver circuit which overcomes the disadvantages of the prior art without significantly increasing the area required for the circuit. 
     This and other objects of the present invention are accomplished by an off-chip driver circuit having a set of input terminals and an output terminal, a pull-up transistor having a controllable path connected between a first power supply and the output terminal of the off-chip driver circuit, a pull-down transistor having a controllable path connected between a second power supply and the output terminal, a first controllable path for applying a first voltage at one of the input terminals to a control terminal of the pull-up transistor, the first controllable path functioning in response to voltages at the output terminal below a first value, and a second controllable path for applying a second voltage greater than the first voltage to the control terminal of the pull-up transistor, the second controllable path functioning in response to voltages at the output terminal above the first value. 
     These and other features of the invention will be better understood by reference to the detailed description which follows taken together with the drawings in which like elements are referred to by like designations throughout the several views. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a prior art off-chip driver circuit; and 
     FIG. 2 is a circuit diagram of an off-chip driver circuit in accordance with the present invention. 
    
    
     NOTATION AND NOMENCLATURE 
     Some portions of the detailed descriptions which follow are presented in terms of symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. The operations are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. 
     Further, the manipulations performed are often referred to in terms, such as adding or comparing, which are commonly associated with mental operations performed by a human operator. No such capability of a human operator is necessary or desirable in most cases in any of the operations described herein which form part of the present invention; the operations are machine operations. Useful machines for performing the operations of the present invention include general purpose digital computers or other similar devices. In all cases the distinction between the method operations in operating a computer and the method of computation itself should be borne in mind. The present invention relates to a method and apparatus for operating a computer in processing electrical or other (e.g. mechanical, chemical) physical signals to generate other desired physical signals. 
     The term “assert” as applied to a signal indicates that signal is active independent of whether the level of the signal is low or high. The term “de-assert” indicates that a signal is inactive. 
     DETAILED DESCRIPTION 
     Referring now to FIG. 2, there is illustrated a block diagram of an off-chip driver circuit  40  constructed in accordance with the present invention. The off-chip driver circuit  40  receives input at a pair of input terminals PDRIV and NDRIV and provides output at an output terminal Z when an enable signal EN is deasserted low. The output terminal Z may be connected to circuitry such as the circuit  28  illustrated in FIG. 1 which is also adapted to provide input to the circuit  40  at the terminal Z. As with the circuit of FIG. 1, the circuit  40  is adapted to provide output signals in an output mode of high (typically 3.3 volts) and low (typically ground) values at the output terminal Z depending on the condition of the input signals applied at the terminals PDRIV and NDRIV. When a high valued input signal (typically 3.3 volts) is applied at both of the terminals PDRIV and NDRIV, a n-channel transistor device MN 2  is enabled as is a n-channel device MN 1 ; and a p-channel transistor device MP 3  is disabled causing ground to be applied through the devices MN 1  and MN 2  at the output terminal Z. Alternatively, when a low valued input signal (typically ground) is applied at both of the terminals PDRIV and NDRIV, the n-channel transistor device MN 2  is disabled; and the p-channel transistor device MP 3  is enabled causing PWR (typically 3.3 volts) to be applied at the output terminal Z. This causes the circuit  40  to provide the two desired output signals at the output terminal Z. 
     In the mode in which the circuit  40  is not used to drive signals off of the chip but instead receives input signals at the terminal Z from external circuitry which are transferred at a terminal RIN to input circuitry (not shown), the terminals PDRIV and NDRIV are tristated by the application of disabling voltages (typically values of 3.3 volts and ground, respectively). At the same time, the signal EN is asserted providing a high value (typically 3.3 volts) at the gate of a n-channel transistor device MNENB. At the same time, the voltage supply (typically 5 volts) of the external circuit which is furnishing input signals at the terminal Z is maintained at a terminal VDDSEL. The high value EN signal enables a feedback path through the n-channel transistor device MNENB for response to the voltage at the terminal Z. The operation of the feedback path is described below. 
     So long as any input voltage at the terminal Z is below the value of the power source PWR (typically 3.3 volts) plus a threshold value Vpt of a p-channel device, the voltage PWR at its gate terminal disables a p-channel device MP 6 . As long as the device MP 6  is disabled, ground is applied to the gate of a p-channel transistor device MP 7 A through a path including a n-channel transistor device MN 9  and a n-channel transistor device MN  16 . The device MN 9  is held on by the voltage PWR at its gate terminal. The device MN 16  is a weak n-device biased on by a value NG at its gate sufficient to allows it to conduct only weakly. The leakage current during this tristate mode is typically limited to below 70 uA when being driven by a high voltage driver between 0.4 and 2.7 volts. 
     Ground at the gate of the device MP 7 A enables that device allowing it to transfer the value (typically 3.3 v.) furnished at the terminal PDRIV to a node PGATE. The high value at the node PGATE is applied at the gate terminal of the device MP 3  and holds the device MP 3  off positively. 
     For voltages at terminal Z below the source value PWR minus a threshold value Vpt, a p-channel transistor device MP 7  is also ON aiding the maintenance of 3.3 volts at the node PGATE. For voltages above 3.3 volts minus Vpt, the device MP 7  is disabled. 
     Any input signal at the output terminal Z is applied at the source terminal of the p-channel transistor device MP 6 . If the input signal rises to a value greater than PWR plus a threshold value Vpt (approximately 4 volts), it causes the device MP 6  to conduct and raise the voltage at a node LKFBSW to just less than that at the terminal Z. As this voltage rises, the device MP 7 A begins to turn off. 
     A voltage greater than the source value PRW at the node LKFBSW assures that n-channel transistor device MN 9  with the voltage PWR at its gate terminal conducts. However, since the weak device MN 16  has its gate biased by a value NG sufficient to allow it to conduct only weakly, the voltage at the node between the devices MN 16  and MN 9  rises from ground. As this voltage rises, a n-channel device MNPG 3  turns on. Not only does the device MP 7 A begin to turn off as the voltage at its gate rises, the device MP 7  is also disabled by the high voltage at the terminal Z which is applied through a resistor R RIN  at its gate terminal. As the device MP 7  turns off and the device MP 7 A begins to turn off, the voltage at the terminal PDRIV which has been holding the device MP 3  off is gradually isolated from the node PGATE. 
     However, as the device MP 7 A is turning off, the device MNPG 3  is turning on due to the rise of the voltage at its gate. The feedback path enabling device MNENB has a high value at its gate and is also on, so the voltage at the source terminal of a n-channel device MNPG 1  is lowered. The device MNPG 1  has PWR applied at its gate terminal so its also turns on lowering the voltage at the source terminal of a n-channel device MNCLMP. The device MNCLMP turns on lowering the voltage at the gate of a p-channel transistor device MPSEL 1 . The low voltage at its gate turns on the device MPSEL 1  furnishing essentially the value VDDSEL (five volts) at the drain of a p-channel device MPSEL 2  which has the voltage PWR applied at its gate. The device MPSEL 2  similarly turns on and furnishes essentially five volts at the node PGATE. 
     At essentially the same time, the device MP 7 A has completed turning off as its gate terminal has risen toward Z above the value PWR plus Vpt. This disables one path from the input terminal PDRIV to the node PGATE. An n-channel device MN 8  is also off because its gate and source are each at the value PRW. Consequently, the node PGATE becomes isolated from the terminal PDRIV just as the five volts VDDSEL is applied at the node PGATE. 
     The voltage at the node PGATE is applied at the gate of the p-channel transistor device MP 3  which has the voltage PWR at its source and a value just over four volts at its drain. This turns the device MP 3  off allowing the voltage at the terminal Z to rise. 
     Thus, as the voltage at the terminal Z rises above PWR plus Vpt (above approximately 4 volts), the device MP 6  functions as a voltage level detector causing the devices MP 7  and MP 7 A to turn off and the devices MPSEL 1  and MPSEL 2  to simultaneously turn on. Turning off the devices MP 7  and MP 7 A removes the lower voltage level PWR (3.3 volts) from the gate of the device MP 3  while turning on the devices MPSEL 1  and MPSEL 2  simultaneously applies 5 volts at that gate terminal. This arrangement guarantees that the device MP 3  stays off during the transition of the voltage at the terminal Z in contrast to the prior art. The device MP 7 A acts to ensure that the voltage on node PGATE does not leak down and turn on the device MP 3 . This is accomplished in the manner described by maintaining the voltage PWR from the terminal PDRIV at the node PGATE through the devices MP 7  and MP 7 A while the voltage at terminal Z is low and then turning off first the device MP 7  and then the device MP 7 A as the voltage rises. The device MP 7 A is turned off just as the higher voltage VDDSEL is applied to the node PGATE. This guarantees that the node PGATE is positively controlled so that the device MP 3  will remain off until the voltage on Z exceeds the normal range of the input signal (e.g., a threshold voltage Vpt above five volts). In this manner, the leakage present in this middle range in the prior art circuit of FIG. 1 is eliminated. 
     The value at the terminal Z may rise until its value is a threshold amount Vpt above the five volts applied at the gate terminal of the device MP 3 . When the voltage at the terminal Z reaches this point, the device MP 3  turns on and provides a drain to the source terminal of the device MP 3  (held at the voltage PWR) for current generated by the voltage at the terminal Z. The device MP 3  begins clamping the voltage at the terminal Z to limit its swing. In addition, the device MP 3  provides a path for discharging any electrostatic voltage which might be applied to the circuit without requiring additional circuitry as is required in the prior art. 
     Once the voltage at the terminal Z goes above five volts plus a threshold Vpt, device MP 3  will turn on and start to clamp the output signal. As the voltage on the terminal Z rises higher, the device MP 3  conducts more strongly as the voltages Vds and Vgs are continually trying to increase. The same effect will be seen during electrostatic discharge testing when the device MP 3  also acts as a clamp. 
     On the other hand, when the voltage at terminal Z drops below PWR plus Vpt, the device MP 6  is disabled. This causes the gate of the device MNPG 3  to drop disabling the path through the devices utilized to provide an enabling voltage at the gate of the device MPSEL 1  by which five volts is transferred to the node PGATE. At the same time, the devices MP 7  and MP 7 A are enabled so that PWR is again furnished at PGATE disabling the device MP 3 . Thus the drop of the potential at the terminal Z switches the voltage at the node PGATE from 5 volts to 3.3 volts. When the off-chip driver circuit  40  is enabled (EN is deasserted low), the feedback path through the device MNENB is turned off; and the device MP 3  acts as a normal clamping device (i.e., clamps when the voltage at the terminal Z exceeds the 3.3 volt supply by a p-channel threshold value Vpt). 
     The present invention obviates a number of problems of the prior art. The invention tolerates voltages above the maximum value of the voltage source PWR (typically 3.6 volts) at the terminal Z. While the voltage at the output terminal may rise as high 6-7 volts with a clamping circuit as in the prior art circuit of FIG. 1, the typical forms of clamping to the voltage of the source PWR cannot be used because the p+ and n-well junction diodes cannot ever be forward biased. The well must be isolated when the output is tristated and is acting to receive input signals. All source-to-drain voltages on output transistors must be limited not to exceed a specified maximum voltage. All gate-to-drain/source/bulk voltages also must be limited. This is accomplished in the present invention. 
     A p-channel transistor device MP 4  and a p-channel transistor device MP 5  are utilized to provide biasing for the well of the various devices in the circuit  40  to eliminate the possibility of diode action through the well at high input voltages. When a low voltage is applied to the node PGATE, the device MP 4  is enabled and transfers the value PWR to bias the well. When, however, the voltage at the node PGATE is five volts, the device MP 4  is disabled while the device MP 5  is enabled. This disconnects the lower value PWR from the well and isolates the well from the voltage at the terminal Z. At the same time, it connects the five volts available at VDDSEL to the well to eliminate forward biasing of the diode between the drain of the device MP 4  and the well. 
     The n-channel devices MN 1  and MN 2  are protected by having two devices in series. Since the input value at Z is divided across the devices MN 1  and MN 2 , the values across any two terminals of the transistor devices are kept below breakdown values. With this configuration the voltage on the source of MN 1  will never exceed the voltage PWR, therefore protecting MN 2  from the high voltage condition. MN 1  protects itself by allowing its source to rise when the output rises thus keeping the drain/source voltage below the voltage PWR. With its gate tied to the voltage PWR, the gate to drain/source voltage limit may not be exceeded. 
     Clamping to the voltage PWR cannot occur when the device is tristated (EN is asserted high) and the input at the terminal Z is between zero and five volts but can occur when the driver is enabled (e.g. over-shoot on output reflections). In order to clamp under these conditions, the over-voltage detection is turned off by lowering the voltage at the gate of the device MNENB. This will cause the output to clamp when the voltage on the drain of the device MP 3  exceeds a p-channel threshold (e.g. 3.3 plus Vpt volts). Therefore, output clamping and electrostatic discharge protection will occur by the operation of the device MP 3  when the output is driven high and by MN 2  when the output is driven low. 
     Although the present invention has been described in terms of a preferred embodiment, it will be appreciated that various modifications and alterations might be made by those skilled in the art without departing from the spirit and scope of the invention. The invention should therefore be measured in terms of the claims which follow.