Abstract:
A voltage reference overshoot protection circuit senses unwanted ringing voltage levels in a driven device such as a backplane and controls the gate voltage to a voltage level control transistor such that a ringing output signal produced by an associated output driver is reduced in response to a control signal dependent on the ringing voltage level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to dampening circuits, and more particularly to a voltage reference overshoot protection circuit to dampen residual device, e.g. backplane, energy during switching operations. 
     2. Description of the Prior Art 
     Dampening circuits are well known in the art. The type of dampening circuit used is dependent upon the particular application. One application of dampening circuits is associated with unwanted ringing in a backplane. Recent advancements in backplane technology have made higher switching speeds possible. Modern trends indicate that improvements in switching speeds associated with backplanes will not only be desirable, but necessary, in order to accommodate the higher CPU speeds utilized in state of the art computers and related peripherals, among other devices. 
     One source of ringing in backplanes is associated with a low to high switching transition of a backplane driver voltage. Those skilled in the art of backplane technology have generally used edge rate control techniques to minimize the ringing in a backplane during a backplane driver output signal low to high switching transition. Edge rate control techniques are used to increase the rise and fall times of the backplane driver output signal. Although these known techniques have proven adequate in the past, such techniques are not able to accommodate the high speeds necessary to provide backplanes that are functional with higher speed computers, peripherals and other digital devices presently being designed and produced. These known edge rate control dampening techniques are problematic in that they reduce the bandwidth necessary to accommodate the higher data transmission rates and shorter propagation delay times generally associated with modern communication devices and protocols. 
     In view of the foregoing, a need exists for a dampening circuit capable of eliminating or substantially reducing unwanted ringing in devices such as backplanes, among other like devices. The bandwidth capabilities of such devices can then be optimized to accommodate higher data transmission speeds than that achievable using conventional edge rate control techniques generally known in the art. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a dampening circuit architecture capable of substantially reducing the ringing voltage injected into a driven device, e.g. backplane, associated with a driver circuit such as a backplane driver. One preferred embodiment of the present invention is a voltage reference overshoot protection circuit that reduces the unwanted ringing in a backplane. This is achieved without the aid of an internally controlled bias generator. 
     A simple open drain output driver is used to help summarize the present invention. In one embodiment, an open drain output driver is coupled to a supply voltage V TT  via a pull-up resistor connected to a backplane. Thus, when the open drain output driver is turned off, it will be pulled to a “high” state via the supply voltage V TT . If this low to high transition has a fast edge rate, unwanted ringing can occur in the backplane. 
     The present invention substantially reduces the unwanted ringing in the backplane as follows. One preferred embodiment of the present invention comprises a p-channel transistor having a gate voltage V REF , provided by an external voltage source. When the voltage provided by the open drain output driver rises above the gate voltage V REF , plus the voltage threshold of the p-channel transistor, the p-channel transistor will start to turn “on” and supply a gate voltage to an n-channel dampening transistor. As the ringing gets higher, more gate voltage is supplied to the n-channel dampening transistor, thereby pulling the open drain output driver voltage down and counteracting the undesirable ringing in the backplane caused by the open drain output driver overshoot. 
     In view of the foregoing descriptions, another preferred embodiment of the present voltage reference overshoot dampening circuit comprises: 
     a first transistor capable of receiving a reference voltage signal and configured to receive an oscillating voltage signal such that the first transistor can pass a current proportional to the oscillating voltage signal there through when the oscillating voltage signal exceeds a level determined by the reference voltage signal and a turn-on voltage signal associated with the first transistor; and 
     a second transistor configured to receive the oscillating voltage signal and the current passed by the first transistor such that the second transistor is capable of reducing the oscillating voltage signal in proportion to a current received from the first transistor. 
     Another preferred embodiment of the present voltage reference overshoot dampening circuit comprises: 
     a first transistor configured to receive a reference voltage and further configured to receive a ringing voltage such that the first transistor can generate a control voltage when the ringing voltage exceeds a level determined by the reference voltage and a turn-on voltage associated with the first transistor; and 
     a second transistor configured to receive the control voltage generated by the first transistor such that the second transistor is capable of reducing the ringing voltage received by the first transistor. 
     Yet another preferred embodiment of the present voltage reference overshoot dampening circuit comprises: 
     a p-channel transistor having a drain, a gate configured to receive a reference voltage and a source configured to receive a ringing voltage such that the first transistor can generate a variable drain voltage when the ringing voltage exceeds a level determined by the reference voltage and the p-channel transistor turn-on voltage; and 
     a first n-channel transistor having a gate configured to receive the variable drain voltage generated by the p-channel transistor and further having a source configured to receive the ringing voltage such that the first n-channel transistor is capable of changing the ringing voltage received by the p-channel transistor in response to a change in the variable drain voltage generated by the p-channel transistor. 
     Still another preferred embodiment of the present invention comprises: 
     means for receiving a ringing voltage and generating a variable control voltage when the ringing voltage exceeds a predetermined level such that the variable control voltage amplitude at a particular moment in time is dependent on the ringing voltage amplitude at the particular moment in time; and 
     means for receiving the variable control voltage and changing the ringing voltage amplitude at the particular moment in time in response thereto. 
     Yet another preferred embodiment of the present invention comprises: 
     a first transistor configured to receive a reference voltage and a ringing voltage such that the first transistor can generate a control current having a value dependent upon the ringing voltage amplitude when the ringing voltage amplitude exceeds a level determined by the reference voltage and a turn-on voltage associated with the first transistor; and 
     a second transistor configured to receive the ringing voltage and the control current generated by the first transistor such that the second transistor is capable of changing the ringing voltage amplitude in response to the control current received from the first transistor. 
     Thus, in one aspect of the invention, an overshoot protection circuit substantially reduces unwanted ringing in a device such as a backplane without the aid of an internally controlled bias generator. 
     In still another aspect of the invention, an overshoot protection circuit substantially reduces unwanted ringing in a device without impairment of “off” state current specifications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Other aspects and features of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 illustrates an overshoot protection circuit according to one preferred embodiment of the present invention; 
     FIG. 2 illustrates unwanted ringing in a backplane caused by an open drain backplane driver circuit; and 
     FIG. 3 is a diagram illustrating a reduction in the unwanted ringing of FIG. 2 where the reduction is attributed to the overshoot protection circuit shown in FIG.  1 . 
     While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1B illustrates an overshoot protection circuit  100  according to one preferred embodiment of the present invention. The overshoot protection circuit  100  helps dampen the energy left in a backplane  10  or other like device during a low to high output transition from a driver device such as the open drain output backplane driver transistor  12  depicted in FIG.  1 A. The present invention, although described in terms of a backplane driver  12 , is in no way to be interpreted as being limited in application to backplane driver devices. A simple open drain output backplane driver  12  and associated backplane  10  are used herein solely for purposes of explaining the present invention and to preserve clarity and brevity. Further, the backplane  10  itself is external to the present invention. 
     Looking again at FIG. 1B, the voltage reference overshoot protection circuit  100  accepts an external reference voltage V REF . The external reference voltage V REF , has a value dependent upon the particular application. In the instant case, the external reference voltage V REF , is 1-volt dc. The GTLP line of products, for example, available from Texas Instruments Incorporated of Dallas, Tex., use an external reference voltage V REF , of 1-volt dc and would find useful applications when combined with the present invention. Transistors  102 ,  104  and  106  as well as ballast resistor  108  are simply electrostatic discharge (ESD) protection devices added to provide increased reliability and are not necessary, although most preferable, to practice the present invention. 
     The simple open drain output driver transistor  12  shown in FIG. 1A is now used to help explain the structure and operation of the voltage reference overshoot protection circuit  100  illustrated in FIG.  1 B. When the open drain output driver transistor  12  is turned “off” via an input signal at input node  14 , a supply voltage V TT  at node  16 , (approximately 1.5-volts dc in the instant case), will pull the backplane  10  to a “high” state (approximately 1.5-volts dc). This will occur since the open drain output drive transistor  12  will act as an open circuit between the backplane  10  and the IC ground node  18 . If this “low” to “high” transition has a fast edge rate, ringing can occur in the backplane  10  during this transition. The voltage reference overshoot protection circuit  100  will operate to dampen the unwanted energy left in the backplane  10  during this transition as described herein below. If the ringing at the output of the open drain output driver transistor  12  goes above the reference voltage V REF , (1-volt dc), plus the voltage threshold of p-channel transistor  110 , then p-channel transistor  110  will start to turn “on” and supply a gate voltage to n-channel transistor  112 . 
     The n-channel transistors  114  and  116  are biased “on” during this transition and appear as short circuits between the p-channel transistor  110  and the gate of n-channel transistor  112 . Although not necessary to practice the present invention, a further explanation setting forth the purpose and operation of n-channel transistors  114 ,  116  will be provided herein following further details of operation for the present voltage reference overshoot protection circuit  100 . 
     As the above described ringing in the backplane  10  gets even higher, more voltage is supplied to the gate of n-channel transistor  112  since the p-channel transistor  110  is then turned “on” even harder allowing more current to pass through current limiting resistor  120  which raises the voltage on the n-channel transistor  112  gate. As the n-channel transistor  112  is turned “on”, it will then pull the output of the open drain output driver transistor  12  downward, thereby counteracting the undesirable overshoot caused by the fast edge rate signal presented to the backplane  10  during the turn-on of open drain output driver transistor  12 . Thus, the stronger the ringing, the harder n-channel transistor  112  is turned “on” to help counteract the overshoot. In other words, n-channel transistor  112  provides a path for the unwanted energy left in the backplane  10  to subside during the low to high transition at the output of the output driver transistor  12 . 
     Although not necessary to the present invention, n-channel transistor  114  is provided to set the voltage reference overshoot protection circuit  100  in an “off” condition during periods of inactivity. Transistor  114  will open the gate bias path to transistor  112  when transistor  114  is in its “off” condition, e.g. during a leakage current test to determine a value of “off” state leakage current I OFF  for the overshoot protection circuit  100 . Current limiting resistor  120  will then pull the gate of transistor  112  to a “low” state since resistor  120  is tied to the common IC ground  18 . Resistor  120  also functions as a current limiting resistor to control the voltage bias on the gate of transistor  112  during normal operation as described herein before. 
     Switching transistor  116  also is not necessary to the present invention and is provided to enhance operation of the voltage reference overshoot protection circuit  100  during tri-state operating conditions. When the open drain output of open drain output driver transistor  12  is in its tri-state condition, n-channel transistor  116  is in its “off” state. During this period of time, transistor  116  will open the voltage path to the dampening transistor  112  such that the current limiting resistor  120  will again pull the gate of transistor  112  to a “low” state, thereby turning transistor  112  “off” and reducing unwanted high impedance output current, I OZ  from flowing during non-operational periods. 
     FIG. 2 illustrates unwanted ringing in a backplane caused by an open drain backplane driver circuit such as the one shown in FIG.  1 A. The broken line represents the output voltage signal presented at the output of the open drain output driver transistor  12 . Since the voltage swing of the output signal is so large, the received signal, e.g. backplane signal, oscillates at unwanted levels having voltage swings almost as large as  15  that presented by the output driver transistor  12 . 
     FIG. 3 is a diagram illustrating a reduction in the unwanted ringing depicted in FIG. 2 wherein the reduction is attributed to the overshoot protection circuit  100  shown in FIG.  1 B. The voltage reference overshoot protection circuit  100  substantially reduces the unwanted large ringing voltage swings such that the received signal, e.g. backplane signal, remains significantly above 1.2-volts dc at all times while the output driver signal is “high.” 
     In view of the above, it can be seen the present invention presents a significant advancement in the art of dampening circuits and associated methods. The present invention reduces propagation delay times to allow higher rates of data flow and processing by optimizing the useable bandwidth of the dampening circuit. This is accomplished since the rise and fall times of the open drain output driver are not increased to accommodate the requisite dampening functions. Further, the preferred embodiments set forth herein also accommodate I OFF  leakage current requirements associated with backplane drivers and other like devices. Additionally, the preferred embodiments set forth herein also accommodate reduction of unwanted high impedance output current associated with backplane drivers among others. Finally, the present invention substantially improves the operational reliability of receiver devices, e.g. backplanes, since the received signal voltage levels are never allowed to drop below the requisite voltage levels necessary to provide uninterrupted, stable and reliable operation of associated transistors and other like devices. 
     This invention has been described in considerable detail in order to provide those skilled in the damping circuit art with the information need to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow. For example, while the embodiments set forth herein illustrate particular types of transistors, the present invention could just as well be implemented using other transistor types, e.g. bipolar, HBT, among others. Further, while particular embodiments of the present invention have been described herein with reference to structures and methods of voltage control, the present invention shall be understood to also parallel structures and methods of current control as defined in the claims.