Abstract:
An apparatus comprising a device and a resistor. The device generally comprises (i) a gate configured to receive an input voltage, (ii) a drain coupled to a first supply voltage, and (iii) a source coupled to an output. The resistive element is generally coupled between the source and a second supply voltage. The apparatus generally provides voltage tolerance between the input voltage and the output.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for implementing voltage tolerance generally and, more particularly, to a voltage tolerant (e.g., 5v) fail-safe input scheme using a source follower configuration. 
     BACKGROUND OF THE INVENTION 
     In general a five volt tolerant circuit is a circuit that is able to withstand five volts on the input without compromising reliability while power is being supplied. A five volt fail-safe circuit is a circuit that is able to withstand five volts on the input without compromising reliability while power is either being supplied to the circuit or not being supplied to the circuit. 
     In a standard CMOS process, NMOS devices are formed in a P-well which is inherently tied to a P type substrate. PMOS devices are formed in an N-well diffusion tub, which is isolated from the substrate and the wells of other PMOS devices. During normal operation, the N well diffusion is tied to VSS. However, to provide five volt fail safe protection, the well of the PMOS device is disconnected from VDD and tied to the PAD voltage whenever the pad rises above VDD. This will prevent the gate to well potential from ever exceeding VDD, even in a power down condition. 
     It would be desirable to implement a five volt tolerant and fail-safe input scheme using a source follower configuration. 
     SUMMARY OF THE INVENTION 
     The present invention concerns an apparatus comprising a device and a resistor (or active device biased to create a resistive element). The device generally comprises (i) a gate configured to receive an input voltage, (ii) a drain coupled to a first supply voltage, and (iii) a source coupled to an output. The resistive element is generally coupled between the source and a second supply voltage. The apparatus generally provides voltage tolerance between the input voltage and the output. 
     The objects, features, method and/or advantages of the present invention include implementing a circuit that may (i) provide voltage tolerance above a supply voltage (e.g., 5 volts), (ii) provide a fail-safe input scheme, (iii) implement a source follower configuration, and/or (v) be implemented with or without a native device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
     FIG. 1 is a diagram of an embodiment of the present invention; 
     FIG. 2 is a graph illustrating the output voltage as a function of the pad voltage; 
     FIG. 3 is a graph illustrating the voltage drop from pad to source (and pad to drain) with exemplary values of resistance; 
     FIG. 4 is an illustration of the output voltage versus the pad voltage with a 10 MΩ resistor; 
     FIG. 5 is a cross section of the device of FIG. 1 illustrating a channel while in a first state; 
     FIG. 6 is another cross section of the device of FIG. 1 illustrating a channel while in a second state; 
     FIG. 7 is a diagram of an alternate embodiment of the present invention; 
     FIG. 8 is a graph of the output voltage as a function of the input voltage for a PMOS device; 
     FIG. 9 is an illustration of the p-channel source follower implementation of FIG. 8; 
     FIG. 10 is a diagram of an alternate embodiment of the present invention; 
     FIG. 11 is a graph of a PMOS follower followed by an NMOS follower; 
     FIG. 12 is a diagram of another alternate embodiment of the present invention illustrating a parallel configuration; and 
     FIG. 13 is a diagram of another alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a diagram of a circuit  100  is shown in accordance with a preferred embodiment of the present invention. The circuit  100  is shown implementing an NMOS configuration. However, a PMOS configuration may also be implemented (to be described in more detail in connection with FIG.  7 ). The circuit  100  generally comprises a transistor MN 1  and a resistor R 1 . A drain of the transistor MN 1  is generally connected to a supply voltage (e.g., VDD). A source of the transistor MN 1  is generally connected to an output node (e.g., Vout) and a first side of the resistor R 1 . A second side of the resistor R is generally connected to a second supply voltage (e.g., VSS). The voltage VSS may be a ground voltage or a virtual ground voltage. A gate of the transistor MN 1  generally receives a pad voltage (e.g., Vpad). In one example, the resistor R 1  may be implemented as an active device. 
     The transistor MN 1  is generally configured in a source-follower arrangement with the resistive element R 1  to provide voltage tolerance (e.g., five volt tolerance) and/or fail-safe (e.g., five volt) protection. Such configurations may be especially useful for low voltage differential input applications. 
     In the example illustrated in FIG. 1, the transistor MN 1  is shown implemented as an NMOS transistor. The transistor MN 1  is generally connected in a source follower configuration. The voltage Vpad can range, in one example, from 0V to 5V. The transistor MN 1  is designed to limit the voltage to not exceed the supply voltage VDD (in one example 3.3v) across the gate oxide. 
     When the voltage Vpad equals the threshold voltage Vt of the NMOS device MN 1 , the transistor MN 1  turns on and current flows through the resistor R 1  (e.g., Ron). As current flows through the resistor R 1 , the output voltage Vout rises. The output voltage Vout is generally limited to the voltage Vpad—the threshold voltage Vt, since any voltage on the source greater than Vpad—Vt will shut off the NMOS device MN 1 . When the voltage Vpad reaches VDD+Vt, the NMOS device MN 1  moves into a linear mode (e.g., slope  120  in FIG.  2 ), and the output voltage Vout is limited to the voltage VDD−(VDD* [Ron/(Ron+R 1 )]). Even though the gate of the NMOS device MN 1  may reach voltages in excess of the supply voltage VDD and the bulk of the NMOS device MN 1  is tied to the voltage VSS, the voltage across the gate oxide will not normally exceed the supply voltage VDD. 
     FIG. 3 is a graph illustrating the voltage drop from the pad to drain ( 3   a ) and pad to source ( 3   b ) with exemplary values of the resistance. FIG. 3 a  illustrates that as the pad voltage ramps from 0V to 5V, the drop across the gate and drain of the input device ranges from −Vdd (−3.65v in this case) to 1.7v. In FIG. 3 b  illustrates that as the pad voltage ramps from 0V to 5V, the voltage drop from the gate to the source of the input devices ranges from −0.5V to 1.7V. Therefore, at no time is a voltage of greater than VDD dropped across the gate oxide. 
     FIG. 4 is an illustration of the output voltage versus the pad voltage with a 10 MΩ resistor. The output voltage is shown ranging from 0v to around 3.3v. When x=0.37479 at the line  132 , a linear voltage increase begins. The linear increase generally ends when x=4.1912 at the line  134 . 
     Referring to FIG.  5  and FIG. 6, cross sections of the device MN 1  are shown. When the gate to source voltage (e.g., Vgs) is less than the drain to source voltage (e.g., vds)+the threshold voltage Vt, a pinched off channel  140  forms between the source  150  and the drain  160 . The voltage at the source  150  will generally be equal to the voltage of the gate  170  minus the threshold voltage Vt. The voltage at the drain  160  will generally be equal to the supply voltage VDD. Thus, the voltage at the base of the gate oxide will be graded from the supply voltage VDD to Vgs−Vt. Therefore, the oxide will never see a voltage greater than the supply voltage VDD. 
     When the gate  170  rises a threshold voltage Vt above the supply voltage VDD, a resistive channel (e.g.,  145  of FIG. 6) forms. The voltage across the channel  145  is graded from the supply voltage VDD on the drain side and VDD−(VDD*[Ron/(Ron+R 1 )]) on the source side. 
     As long as the resistance R 1  is large, the source remains close to the supply voltage VDD. Thus, if the maximum allowable supply voltage VDD is 3.3V, the input may be driven to 6.6V without oxide reliability concerns. Therefore, the circuit  100  limits the output voltage Vout to less than the supply voltage VDD. Limiting the output may (i) protect devices downstream of the output Vout and (ii) limit the voltage across the input NMOS device MN 1  to less than VDD. 
     The circuit  100  may be implemented with a standard enhancement NMOS device. However, the circuit  100  can be modified by implementing the transistor MN 1  as a native NMOS device. A native device is a device with a threshold voltage at or near 0 volts. While not all processes have native devices available, when a native device is available, a significant advantage over the standard NMOS device may result. In particular, the same discussion above applies to the NMOS native configuration. However, since the threshold voltage of the native device is at or near 0 volts, the output voltage Vout=Vpad, up to the point where the pad voltage Vpad=VDD. Thus, the voltage is passed cleanly through to the output voltage Vout without any level shifting (e.g., the output in FIG. 4 would reach 3.5 volts). 
     Referring to FIG. 7, a circuit  100 ′ is shown implementing an alternate embodiment of the present invention. The circuit  100 ′ generally implements a transistor MP 1  in a configuration that is generally a complement to the NMOS configuration described in connection with FIG.  1 . 
     When implementing a PMOS transistor MP 1 , when the voltage Vpad is 0 volts, the output voltage Vout is clamped at Vpad+Vt. As the pad voltage Vpad is ramped, the output voltage Vout continues to be Vpad+Vt until Vout reaches VDD−(VDD* [R 1 /(Ron+R 2 ). This voltage will be approximately equal to the supply voltage VDD as long as the resistance Ron is large with respect to R 2 . Since the output voltage Vout is clamped at a voltage below the supply voltage VDD, no devices down stream will see input voltages greater than the supply voltage VDD even if the pad voltage Vpad rises to 5 volts. FIG. 8 illustrates the voltage output Vout as a function of the voltage Vpad for a PMOS device. FIG. 9 illustrates another graph of a p-channel source follower input. 
     The NMOS device can have a similar fail safe mode if a triple well structure is used. A triple well process architecture isolates the P type well of the NMOS device from the substrate by encapsulating it in an N type diffused region. Because both the P well and surrounding N type diffusion can be isolated from the substrate, the P type well can be floated in the same manner as previously described with the N Well of the PMOS device. This allows the NMOS device to be 5V fail safe. 
     The level shifting effect may be cancelled if a complementary configuration is implemented. For example, if the threshold voltages of the NMOS and PMOS devices are roughly the same, the level shift effect can be canceled out by running the output of a the PMOS configuration into the input of the NMOS configuration, or vise versa. FIG. 10 illustrates an example of the circuit  100 ′ connected in series with the circuit  100  to provide such connection in level shifting. FIG. 11 illustrates a simulation of the voltage output Vout as a function of the pad voltage Vpad. The remaining offset is not a concern for applications that are based on a differential input. 
     A further advantage can be obtained by combining one NMOS-PMOS combination circuit described above in parallel with a PMOS-NMOS circuit. FIG. 12 illustrates such a configuration. By putting the PMOS first, 5V tolerance is more easily achieved in most processes because the NWELL of the PMOS device can be floated. In a triple well process either device may be placed first. The order may be determined by the designer based on the input range specification trying to be met. 
     Furthermore, the output of each of the circuits  100  and  100 ′ can be run through an analog multiplexer  142  that is controlled by a comparator  144 . FIG. 13 illustrates such a configuration. Such a circuit will generally extend the output range of the source followers  100  and  100 ′. 
     The various signals of the present invention are generally “on” (e.g., a digital HIGH, or 1) or “off” (e.g., a digital LOW, or 0). However, the particular polarities of the on (e.g., asserted) and off (e.g., de-asserted) states of the signals may be adjusted (e.g., reversed) accordingly to meet the design criteria of a particular implementation. Additionally, inverters may be added to change a particular polarity of the signals. 
     It will be apparent to those skilled in the relevant art(s) that certain nodes of transistors and other semiconductor devices may be interchanged and still achieve the desired electrical characteristics. The node interchanging may be achieved physically and/or electrically. Examples of transistor nodes that may be interchanged include, but are not limited to, the drain and source of field effect transistors. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.