Patent Publication Number: US-6211693-B1

Title: Testability circuit for cascode circuits used for high voltage interface

Description:
This application claims priority under 35 USC § 119 (e)u) of Provisional Application No. 60/068,582 filed Dec. 23, 1997. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to testability circuits. More specifically, this invention relates to testability for cascode circuits used as high voltage interfaces. 
     BACKGROUND OF THE INVENTION 
     Newer technology devices, e.g., CMOS devices, which utilize lower operating voltages, are becoming pervasive. Nevertheless, there is still a need to interface with older technology devices. These older devices typically have higher operating ranges, e.g., 4.5 V to 5.5 V which are generally higher than a single transistor can tolerate. In some cases, buffer circuits, designed to tolerate this higher voltage, are used to interface with these older devices. These high voltage tolerant buffer circuits, however, are inappropriate for some newer CMOS devices that are required to use level shifting to interface to the higher voltage levels. 
     Some of the lower voltage devices contain level shift outputs to interface with devices having higher operating voltages. These level shift outputs use transistors that tolerate five volts. Level shifting can also be accomplished using cascode transistors to maintain acceptable voltage stress levels on the switching transistors. If, however, a cascode transistor is faulty, e.g., has a drain to source short, then the switching transistor will, generally, fail due to voltage stress. Conventional cascode design does not allow a faulty cascode device to be readily detected. Thus, although a conventional cascode configuration will likely perform appropriately during testing, it will fail prematurely in the field due to voltage overstress. 
     SUMMARY OF THE INVENTION 
     Therefore, a need has arisen for a testability circuit for cascode devices used in high voltage interface circuits. A testability circuit according to the present invention enables a determination to be made as to whether internal cascode transistors are operating properly before they enter the field. 
     According to one embodiment of the present invention, a level shifting circuit having a testability network is disclosed. The level shifting circuit includes a cascode configuration of active devices and a bias network arranged to bias the cascode configuration. The cascode configuration includes at least a first switching transistor and a second switching transistor and a first cascode transistor and a second cascode transistor arranged to limit voltage stress on said first and second switching transistors. A test network is connected to the cascode configuration and the bias network such that it provides an indication of a short circuit in one or both of the cascode transistors. 
     According to another embodiment of the present invention a testability circuit for cascode devices is disclosed. The testability circuit includes at least one test transistor pair, and a biasing transistor pair. Each test transistor pair comprises a first test transistor and a second test transistor. The drain of the first test transistor is connected to the source of the second test transistor, and the source of the first test transistor is connected to the drain of the second test transistor. The biasing transistor pair comprises a first biasing transistor and a second biasing transistor connected to the first and second test transistors of one test transistor pair to limit voltage stress on said first and second test transistors. When one of said test transistor pairs is connected to a cascode circuit arrangement, the testability circuit according to this embodiment provides an indication of a fault in said cascode circuit arrangement. 
     The above-described embodiments of the present invention provide various technical advantages. For example, the testability circuit according to the above-described embodiments provides the technical advantage of allowing the cascode devices to be tested. In addition, the testability circuits according to the above-described embodiments increase the reliability of level shifting circuits which use cascode devices helping to ensure that such level shifting circuits do not fail prematurely in the field. Other technical advantages are apparent to one skilled in the art from the following figures, description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and objects of the present invention, and the manner of attaining them is explained in detail in the following DETAILED DESCRIPTION OF THE INVENTION when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 represents a circuit schematic of a test circuit for a level shifting circuit according to one embodiment of the present invention; 
     FIG. 2 represents a circuit schematic of a level shifting circuit with which a test circuit according to one embodiment of the present invention may be used; 
     FIG. 3 represents a circuit schematic of a test circuit for multiple level shifting circuits according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Transistors in cascode circuit arrangements, among other things, are effectively used to limit voltage stress on switching devices. In such circuit arrangements, a problem arises if a cascode transistor has a fault because the switching device will be subjected to whatever voltage stress the cascode device was designed to limit. A test circuit according to the embodiments of the present invention enables internal transistors to be tested for faults, e.g., drain to source shorts before they are sent to the field and thereby improve circuit reliability in the field. The test circuit according to the embodiments of the present invention will be explained in conjunction with a cascode configuration used in a level shifting circuit. It should, however, be understood that test circuits according to the embodiments of the present invention are useful in conjunction with a number of different circuit arrangements. 
     FIG. 1 illustrates a test circuit  10  for use with a level shifting circuit  20  shown in FIG.  2 . Level shifting circuit  20  performs the function of allowing higher voltage devices to be interfaced to lower voltage devices and is comprised of a bias network  21 , and a switching network  22 . Switching network  22  of level shifting circuit  20  utilizes cascode transistors Q 2  and Q 3  in a cascode topology to limit the voltage stress on switching transistors Q 1  and Q 4 . Bias network  21  sets the threshold voltages for transistors Q 1 , Q 2 , Q 3  and Q 4 . The bias voltages are chosen to protect the individual transistors from overstress, keep the test circuitry at leakage current levels, and maximize output switching performance. For one particular embodiment of the circuit in FIG. 2, assume V CCHV  is 5 V and the maximum transistor stress level is 3.3 V. Optimal performance is obtained with maximum Vgs on cascade transistors Q 2  and Q 3 . The bias voltages would then be F=3.3 V and G=(5−3.3)=1.7 V. A check is required to determine whether these voltages are compatible with the test circuitry. 
     The source of switching transistor Q 1  is connected to the high voltage power supply, V CCHV , the drain of transistor Q 1  is connected to the source of cascode transistor Q 2 , and the gate of switching transistor Q 1  ties to an earlier stage predriver circuit operating between V CCHV  and voltage G. The drain of transistor Q 2  is connected to the drain of cascode transistor Q 3  and the gate of transistor Q 2  is connected to the bias network at contact point G. The source of transistor Q 3  is connected to the drain of switching transistor Q 4  and the gate of transistor Q 3  is connected to bias network  21  at contact point F. The gate switching transistor of Q 4  ties to an earlier stage predriver circuit operating between voltage F and ground. In one particular embodiment of level shifting circuit, switching transistors Q 1  and Q 4  are PMOS and, NMOS transistors respectively, and cascode transistors Q 2  and Q 3  are PMOS and NMOS transistors respectively. It should be recognized, however, that the particular level shifting circuit arrangement and the choice of specific types of transistors is for example purposes only. Other circuit configurations and transistor types are possible. 
     Bias network  21  activates cascode transistors, Q 2  and Q 3 , and regulates the drain to source voltage drop for Q 2  and Q 3  so that they limit the voltage range which switching transistors Q 1  and Q 4  are subjected to. In the embodiment shown in FIG. 2, bias network  21  comprises a series of resistors  12 ,  13 , and  14  connected between high voltage power supply V CCHV  and ground. Resistors  12 ,  13 , and  14  divide the high voltage power supply voltage, V CCHV , and thus provide contact points F and G having voltages appropriate for operation of transistors Q 2  and Q 3 . As can be seen in FIG. 2, the voltages from contact points F and G are used to drive the gates of cascode transistors, Q 3  and Q 2  respectively. Although bias network  21  is shown as a resistor network in FIG. 2, bias network  21  could also be comprised of, e.g., transistors that may be configured to provide a similar type of resistor network. Typically, node B is the only node available on a bond pad for output from switching network  22 , thus necessitating the use of a test circuit. 
     One embodiment of test circuit  10  is shown in FIG.  1 . Test circuit  10  is comprised of two test transistors Q 6  and Q 7 , and two biasing transistors Q 5  and Q 8 . The drains of test transistors Q 6  and Q 7  are connected to a common node and the sources of test transistors Q 6  and Q 7  are connected to a common node. The drain of a first biasing transistor Q 5  is connected to V CCHV , and the source of transistor Q 5  is connected to the drains of test transistors Q 6  and Q 7 . The source of a second biasing transistor Q 8  is connected to the sources of test transistors Q 6  and Q 7 , and the drain of transistor Q 8  is connected to ground. The gates of the biasing transistors Q 5  and Q 8  are connected to contact points F and G of bias network  21  so that transistors Q 5  and Q 8  are biased to conduct current if one of test transistors Q 6  or Q 7  is activated. 
     In this configuration, test transistor Q 6  and Q 7  control current flow through test circuit  10  by acting as switches controlled by the voltages at contact points C and A respectively. Biasing transistors Q 5  and Q 8  serve the dual purposes of setting the threshold voltages for test transistors Q 6  and Q 7  and limiting the voltage stress on test transistors Q 6  and Q 7 . In this embodiment of test circuit  10 , biasing transistor Q 5  and test transistor Q 6  are NMOS transistors and biasing transistor Q 8  and test transistor Q 7  are PMOS transistors. It should be recognized, however, that the type of transistors used for Q 5 , Q 6 , Q 7 , and Q 8  is determined by a number of factors, including for example, the type of transistor used for cascode transistors Q 2  and Q 3  and the specifications of the particular circuit with which test circuit  10  is being used. 
     The operation of test circuit  10  can best be understood with reference to FIGS. 1 and 2. As stated above, test circuit  10  provides an indication of a fault, e.g., a drain to source short, in either of cascode transistors Q 2  or Q 3 . That is, as long as cascode transistors Q 2  and Q 3  are functioning normally, the voltage at contact point C is not high enough to turn on Q 6 , and the voltage at contact point A is not low enough to turn on Q 7 . Therefore, as long as Q 2  and Q 3  function normally, test transistors Q 6  and Q 7  remain, essentially, open circuits and therefore, no current flows through test circuit  10 . This is verified as follows. If the turn-on voltage of the NMOS transistor is 0.6 V and G is at the aforementioned 1.7 V then E would have to be greater than (1.7+0.6)=2.3 V for current to flow. If the current flows through Q 6  then the voltage at C would have to be greater than (2.3+0.6)=2.9 V. Since Q 3  acts as a source follower the voltage at C is equal to F−0.6 V=2.7 V which is less than the calculated 2.9 V required to sustain current flow in the test circuit. In a like manner, it can be shown that current is not flowing through Q 7  so the test circuit is not loading V DDHV . In this specific example the margins are very tight, and some adjustment of the voltage levels at F and G would be desirable to guarantee the test circuit doesn&#39;t draw current. 
     If, however, there is a drain to source short in one of cascode transistors Q 2  or Q 3 , test circuit  10  will provide an indication of such short. For example, if Q 2  has a drain to source short, the voltage at contact point A is pulled to ground when Q 4  is on. If the voltage at contact point A is pulled to ground, then Q 7  turns on and current flows through test circuit  10  from V DDHV  to ground. The test circuit is designed such that this current is large enough to be detected and thus the current flow provides an indication of a fault in Q 2  or Q 3 . If Q 3  has a drain to source short, the voltage at contact point C is pulled to V CCHV  when Q 1  is on. If the voltage at contact point C is pulled to V CCHV , then Q 6  turns on also causing current to flow through test circuit  10 . Therefore, test circuit  10  enables a fault in an internal cascode device to be detected at the unit test stage and thereby enhances the reliability of circuits incorporating such cascode transistors. 
     In another embodiment, a plurality of test transistor pairs, Q 6   1  and Q 7   N  could be grouped with regulating transistors Q 5  and Q 8 . As shown in FIG. 3, the test transistor pairs are connected such that they are connected to regulating transistors Q 5  and Q 8  at common nodes. In this arrangement, each test transistor pair is used to test for faults in one particular pair of cascode devices. The plurality of test transistor pairs provide for testing of multiple cascode output transistor circuits. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the intended scope as defined by the appended claims.