Abstract:
The protection circuit of the present invention addresses the problem of indeterminate logic levels caused by loss of one of the power supplies in a two-power-supply CMOS integrated circuit. The circuit of the present invention replaces the typical scheme of power supply sequencing to fix the problem. The circuit disclosed herein detects the state of the core voltage and disables the output drivers when the core voltage is detected as being off. The disabled drivers are put into a high impedance state, thereby eliminating the potential for damage and eliminating the need for power supply sequencing. The invention also protects against the sudden loss of the integrated circuit core voltage, VDD, power supply during normal operation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to the field of integrated circuits. In particular, this invention relates to power supplies for integrated circuits and to an improved and simplified method for providing a stable source of power for a complementary metal oxide semiconductor (CMOS) integrated circuit utilizing a split-rail or dual power supply.  
           [0003]    2. Related Art  
           [0004]    The field of integrated circuitry is a rapidly developing field of technology. Integrated circuits are continually being made smaller with the attendant requirements of increasing both device speed and circuit density. The miniaturized devices built within and upon a semiconductor substrate are spaced very closely together and the integrated circuit density, that is, the number of integrated circuits per unit of surface area, continues to increase significantly. The highest integrated circuit density is achieved using Field Effect Transistors (FETs). A FET is a device having a source, gate, and drain arranged such that when a high logic signal voltage is applied to the gate, current may pass from the source to the drain. Similarly, a FET does not allow current to pass between the source and the drain when a low logic signal is applied to the gate.  
           [0005]    As the integrated circuit density increases, the amount of power dissipated by the integrated circuits on the substrate increases proportionally. The amount of power dissipation is a concern because complicated heat sinks and circuit packaging may be required to prevent the chip temperature from rising above its rated operational temperature limit. Further, many devices containing integrated circuits typically operate on stored power. One example is a portable computer operating on battery power. As power dissipation increases, battery life decreases, and the shorter the operational life of the electronic device. Therefore, reducing the power consumption for a given integrated circuit density is important to the design of integrated circuits.  
           [0006]    One way to decrease this power consumption is to reduce the voltage at which the circuits operate. However, decreasing the operational voltage level creates a compatibility problem because some integrated circuits are designed to operate at predetermined, specific voltage levels. For example, some circuits may interface with low voltage circuits, and these same circuits may need to operate at higher voltage levels to operate electromechanical devices. Also, there are many existing integrated circuits that cannot have their operating voltage altered, and yet, new, lower voltage circuits must interact with them. Therefore, in order to lower the voltage of integrated circuits so that they dissipate less power, and still permit their interaction with different or existing hardware, some form of interface circuitry is required.  
           [0007]    The related art has provided a variety of interface circuits for translating lower voltage levels into higher voltage logic levels and vice versa. This is primarily due to the fact that logic voltage levels implemented in integrated circuits have been steadily decreasing as technologies evolve.  
           [0008]    In general, complementary metal oxide semiconductor (CMOS) integrated circuits require more than one power supply per chip. Such designs are known in the art as “split rail designs.” For instance, a split rail design is utilized when the internal or core chip voltage, VDD, operates at a different voltage level than the input/output (I/O) interface voltage, or the output driver voltage, OVDD. The integrated circuit core voltage, VDD, is limited by the integrated circuit technology or power dissipation requirements of the chip, as well as the driver output voltage, OVDD.  
           [0009]    Split rail designs create many challenges that must be addressed by both integrated circuit designers and system designers. For a typical split rail integrated circuit to operate properly, both of the power supplies must be in their powered-up state. Numerous problems can occur when one supply is “off” while the other is “on.” Problems can also occur when the sequence in which the two supplies are powered up or powered down becomes critical.  
           [0010]    One example of such a problem occurs when the integrated circuit core voltage, VDD, is in an off state and the output drivers are powered up via the output driver voltage, OVDD. In this situation, the output drivers will have lost all the control signals from the integrated circuit core which are derived from the integrated circuit core voltage, VDD. With no control signals to the drivers, the drivers&#39; output stages may try to pull the output pad both up and down at the same time. This scenario is characterized by a high crossover current effect from the output driver voltage, OVDD, to ground, which can be multiplied by hundreds of drivers throughout the chip thereby causing permanent equipment damage.  
           [0011]    This problem has been solved in the related art by requiring power supply sequence rules that dictate that the internal chip voltage VDD must power up before the driver voltage OVDD. This ensures that there are valid logic levels at the “DATA” and “ENABLE” inputs of the driver before the driver is powered up. A power-down sequence is also required such that the OVDD is powered down before VDD for the same reasons described in the power-up sequence.  
           [0012]    There is also the case wherein there is a sudden loss of VDD after both supplies are up and running. This can be due to the VDD supply failing and causing the high current condition in the output drivers. For this final case, there must be some intelligence in the system that forces OVDD off if there is a sudden and permanent loss of VDD.  
           [0013]    This level of sophistication is not always available in power supplies used to power integrated circuits. The solution this invention provides places no restrictions on power supply sequence or the sudden loss or failure of a power supply. This small amount of circuitry added to the driver design is noninvasive, and therefore will not affect driver performance.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention solves these problems in the related art by breaking the series path through the driver&#39;s output stage when the core voltage is detected to be “off.” The conduction path between OVDD and GND is broken, thereby eliminating the potential for damage and eliminating the need for power supply sequencing requirements. The invention also protects against the sudden loss of the VDD power supply during otherwise normal operation.  
           [0015]    The present invention provides a semiconductor chip, comprising: a first plurality of circuits connected to a first voltage contact having a first voltage potential, and to a ground contact; a second plurality of circuits connected to a second voltage contact having a second voltage potential, and to said ground contact; a detection circuit connected to said first voltage contact, said second voltage contact, and said ground contact, and having an output node; said detection circuit adapted to operate by detecting the loss of voltage potential at said first voltage contact; said detection circuit adapted to pull said output node to said second voltage potential when said first voltage potential and said second voltage potential are both present; and said detection circuit adapted to pull said output node to ground potential when said first voltage potential is not present.  
           [0016]    Another aspect of the invention is to provide a semiconductor interface circuit for translating lower voltage logic levels into higher voltage logic levels, comprising: a first voltage contact; a second voltage contact; a ground contact; a sensing circuit which monitors the voltage levels at the first and second voltage contacts; and a detection circuit having an output node, and adapted to force said output node to ground potential, only when a second voltage source is connected to said second voltage contact and no voltage source is connected to said first voltage contact; and wherein the semiconductor interface circuit is adapted to enter a high impedance state when said output node is pulled to said ground potential at the ground contact.  
           [0017]    The present invention also presents a method of protecting circuitry in a semiconductor chip, comprising: providing a first plurality of circuits connected to a first voltage contact and a ground potential contact; providing a second plurality of circuits connected to a second voltage contact and said ground potential contact; providing a detection circuit connected to said first voltage contact and said second voltage contact, and having an output node, said detection circuit operating by forcing said output node to said ground potential contact only when a second voltage source is connected to said second voltage contact and no voltage source is detected at said first voltage contact; and connecting at least one of said second plurality of circuits to said output node of said detection circuit.  
           [0018]    The present invention further provides a two-supply input/output protection circuit comprising: a driver input stage having a data input and an enable input; a detection stage having a pre-drive stage comprising a NAND gate and a NOR gate, said detection stage operationally connected to an output of said input stage; an output stage operationally connected to an output of the pre-drive stage; and a loss detection stage having an inverter stage and a voltage bias stage, said inverter stage operationally connected to said driver input stage, and said voltage bias stage operationally connected to said output stage.  
           [0019]    The present invention additionally provides a two-supply input/output protection circuit comprising: a driver input stage having a data input and an enable input; a detection stage having a pre-drive stage comprising a NAND gate and a NOR gate, said detection stage operationally connected to an output of said input stage; an output stage operationally connected to an output of the pre-drive stage; a loss detection stage having an inverter stage and a voltage bias stage, said inverter stage operationally connected to said driver input stage, and said voltage bias stage operationally connected to said output stage; and an inverting levels translation circuit for detecting the loss of a first voltage potential. 
       
    
    
       [0020]    The foregoing and other features of the invention will be apparent in the following and more particular description of the embodiments of the invention as illustrated in the accompanying drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    Several embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein:  
         [0022]    [0022]FIG. 1 depicts a schematic diagram of a typical two power supply output driver circuit of the related art;  
         [0023]    [0023]FIG. 2 depicts a schematic diagram of a circuit of the present invention that detects the loss of the VDD supply and forces the node N 1  (i.e., the gate of NFET T 11 ) to a logic zero or ground potential; and  
         [0024]    [0024]FIG. 3 depicts a schematic diagram of a VDD detection levels translator circuit of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Referring to the drawings, FIG. 1 illustrates an example of the related art problem discussed supra. The off-chip driver circuit  100  shown in FIG. 1 illustrates a typical two-power-supply output driver circuit. The off-chip driver circuit  100  includes an input and levels translation stage  110 , a pre-drive stage  120 , and an output stage  130 . In this example, the integrated circuit&#39;s internal core voltage, VDD, is nominally about 2.5 volts, while the output driver voltage, OVDD, is nominally about 3.3 volts. A first input, labeled “DATA”, is the off-chip driver input and a second input, labeled “ENABLE”, is used to switch the output stage  130  into a high impedance state.  
         [0026]    During normal operation, 2.5 volt logic data from the integrated circuit core is present at the DATA and ENABLE inputs to the input and levels translation stage  110  of the off-chip driver circuit  100 . Buffer circuits B 1  and B 2  pass the data to buffer circuits B 3  and B 4 , respectively, which convert, or level shift, the nominal 2.5 volt logic to nominal 3.3 volt logic. From this point, the remaining circuitry is a typical 3.3 volt driver consisting of the pre-drive stage  120  and the output stage  130 . The pre-drive stage  120  is a NAND/NOR pre-drive circuit used to control the rate of change of the driver circuit  100  output current (di/dt), and also to fundamentally perform the required Boolean function of NANDing/NORing the DATA and ENABLE inputs.  
         [0027]    During operation, when the 3.3 volt supply, OVDD, powers up before the 2.5 volt supply, VDD, or when there is a sudden loss of the 2.5 volt supply, VDD, during otherwise normal operation, the logic levels at the DATA and ENABLE inputs to the driver circuit  100  become indeterminate. Because the off-chip driver circuit  100  output devices, transistors T 9 , T 10  and T 11 , are powered by the 3.3 volt supply, the off-chip driver circuit  100  is capable of supplying current from 3.3 volts to ground through transistors T 9 , T 10 , and T 11 .  
         [0028]    One method of preventing this situation from occurring is to detect the loss of the 2.5 volt supply, and force a logic zero at node N 1  (i.e., the gate of transistor T 11 ). Typically, the gate of transistor T 11  is connected directly to the OVDD supply. This series output device, transistor T 11 , whose gate is hard-wired to the output power supply, OVDD, reduces the drain-to-source and drain-to-gate potentials when a high(er) output voltage is applied in a tri-state condition. This is done for reasons of enhanced overall device reliability and life span.  
         [0029]    The idea behind the present invention is to convert the hard-wired connection of the gate of transistor T 11  (i.e., node N 1 ), and actively bias it by a detection circuit which will force OVDD volts when the VDD supply is active, and force GND potential when the loss of VDD is detected. This will ensure that the device stack of the output stage  130  no longer has a connection to ground when VDD is not present. The power up/down duration is insignificant relative to the operating lifetime of the driver, and hence, will not significantly degrade reliability. This scheme can be used in lieu of the tri-stating means disclosed in U.S. patent application Ser. No. 09/541,196 entitled “Two-supply Protection Circuit” which is incorporated herein by reference. The use of either of these approaches will disable the off-chip driver from sinking unwanted current during a power supply on or off sequence, or when the VDD supply is lost while OVDD is present. However, the present invention&#39;s scheme doesn&#39;t inhibit any current from OVDD to PAD as the tri-stating disclosure.  
         [0030]    There are multiple methods to effect the goal of blocking the short circuit current when VDD is inactive and OVDD is active. One such known method is to connect node N 1  (FIG. 1) of the off-chip driver to VDD. Then, when VDD is active, NFET device T 11  will be turned on and the path to ground is enabled. When VDD is inactive, off-chip driver NFET device T 11  is turned off and the path to ground is disabled.  
         [0031]    While this simple approach of the related art will solve the conductance problem, it introduces three others. First, the conductance of the transistor T 11  is greatly reduced due to the reduced overdrive supplied by gate potential VDD. To compensate for this, the size of transistor T 11  must be greatly increased at the sacrifice of area and output capacitance. Second, and more importantly, the use of VDD in the off-chip driver NFET stack (T 9 , T 10 , T 11 ) means that any designer who wants to reduce the core VDD for power-consumption reasons impacts the output characteristics of the off-chip driver. This can cause problems in applications in which VDD is taken to relatively low levels (inevitably below those anticipated) and/or in which tracking between VDD and OVDD is not managed well. Third is the issue of reliability. The use of VDD as a transistor gate bias will increase the transistor&#39;s drain-to-gate bias during both normal operation and tri-state modes. This will seriously degrade the reliability of the device. Hence, for these reasons another means must be employed.  
         [0032]    According to the present invention, a novel method of detecting the loss of the 2.5 volt supply and forcing the node N 1  (i.e., the gate of NFET device T 11 ) to a logic zero or ground is presented in FIG. 2. The circuit  200  of FIG. 2 will be inserted into the off-chip driver circuit  100  of FIG. 1, and connected to node N 1 , in place of the existing OVDD connection to node N 1 , shown in FIG. 1 in the output stage  130 .  
         [0033]    In FIG. 2, levels translating circuit B 7  employs an inverting configuration. The levels translating circuit B 7  may include the inverting levels translation circuit  300  of FIG. 3 (discussed infra), or levels translating circuit B 7  may include any other suitable levels translating circuit.  
         [0034]    The input  210  of B 7  is connected to the internal core voltage supply, VDD. The output  220  of B 7  will achieve GND potential when VDD is present, and will achieve OVDD potential when VDD is not present (see FIG. 2). Transistors TN 1  and TP 1  are configured as a selector circuit. Transistor TN 1  is an NFET device with its drain connected to GND, source connected to output node N 1 , and gate connected to the output of levels translator circuit B 7 . Transistor TP 1  is a PFET device with its drain connected to OVDD, its source connected to the output node N 1 , and its gate connected to the output of levels translating circuit B 7 . The n-well of transistor TP 1  is connected to OVDD.  
         [0035]    During operation, when VDD is present, levels translating circuit B 7  outputs a GND potential, thereby turning transistor TN 1  off and turning on transistor TP 1 , which propagates OVDD to node N 1 . In the opposite case, when VDD is not present, levels translating circuit B 7  outputs OVDD potential thereby turning off transistor TP 1  and turning on TN 1 , which then propagates GND to node N 1 .  
         [0036]    An inverting levels translation circuit  300 , which is used for detecting the loss of VDD, is illustrated in FIG. 3. This inverting levels translation circuit  300  is represented in FIG. 2 by element B 7 . Operation of the inverting levels translation circuit  300  is as follows.  
         [0037]    Transistor TN 20  operates as a diode, resulting in the gate of transistor TN 21  being biased to the difference of OVDD-Vt volts (e.g., 3.3v−0.5v=2.8v). Transistor TN 20  can be supplemented by adding additional diode connected NFETs in series with it to reduce the gate bias point of transistor TN 21 . In normal functional mode, where VDD is powered up, the application of VDD to the gate of transistor TN 22  causes transistor TN 22  to conduct, thereby forcing a logic zero at node Out, which in turn causes transistor TP 21  to conduct forcing node N 2  to OVDD volts thereby turning off transistor TP 22  and transistor TN 21 .  
         [0038]    In the failure mode, defined as a loss of VDD, VDD is essentially connected to ground. Hence, transistor TN 22  is inactive, thereby releasing node Out and transistor TN 21  is now active and discharges node N 2  to ground, which in turn activates transistor TP 22 , forcing node Out to OVDD and turns off transistor TP 21 .  
         [0039]    It should be noted that other levels translation or detection schemes can be employed to drive the control line of the bias select circuitry.  
         [0040]    Still another approach is to connect the output of the levels translator circuit B 7  directly to the gate of transistor T 11  of the off-chip driver  100  and thereby eliminate the pass gate configuration of FIG. 3. This approach will certainly work logically, but circuit performance may be impacted as the ability of the levels translator circuit B 7  to maintain a solid OVDD potential may be comprised during switching activity of the off-chip driver.  
         [0041]    While embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.