Abstract:
A transient blocking unit (TBU) is a transistor circuit that is normally on, but rapidly and automatically switches to a high-resistance current blocking state when a current threshold is exceeded, thereby protecting a series connected load from over-voltage or over-current conditions. Process variation of transistor threshold voltage and on-resistance can cause undesirable variation of the TBU threshold current and/or of TBU resistance. Control of TBU threshold current and/or resistance is improved by providing for trimming the TBU during its fabrication to provide a one-time adjustment of the threshold current or resistance. Such trimming can be done with a resistive trimming circuit placed in series with the on-resistance of the relevant TBU transistor. Alternatively, a segmented TBU transistor having an on-resistance that is adjustable by way of wire bonding during fabrication can be employed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional patent application 60/962,221, filed on Jul. 26, 2007, entitled “Programmable Control IC for Circuit Protection”, and hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to suppression of electrical transients. 
       BACKGROUND 
       [0003]    A transient blocking unit (TBU) is an arrangement of transistors connected such that the TBU resistance is ordinarily low, but this resistance automatically and rapidly switches to a high value in response to an over-current condition. Due to this characteristic behavior, TBUs are applicable for protecting series connected loads from over-current or over-voltage conditions. 
         [0004]      FIG. 1  shows a typical example of a TBU. In this example, Q 1  and Q 2  are both depletion mode transistors (i.e., normally on). As I TBU  increases, passage of I TBU  through the series resistance of Q 1  and Q 2  provides gate voltages at Q 1  and Q 2  that tend to switch the circuit off. Below a well-defined current threshold, this tendency is negligible, and the series resistance of the TBU is low. Above this current threshold, positive feedback sets in, because gate voltages tend to increase as the transistors start to switch off. As a result, the TBU rapidly switches to a high-resistance current blocking state, thereby protecting its series-connected load. The example of  FIG. 1  is referred to as a uni-directional TBU because the device works as described above for one polarity of I TBU , but not for the other polarity of I TBU . 
         [0005]      FIG. 2  shows a typical example of a bi-directional TBU. This example can be understood as effectively being two uni-directional TBUs in series. More specifically, Q 1  and Q 2  of  FIG. 2  form the uni-directional TBU of  FIG. 1 , while Q 1  and Q 3  of  FIG. 2  form a second uni-directional TBU having opposite polarity. In this manner, protection can be provided for transients of either polarity. More specifically, the combination of Q 1  and Q 2  defines a first current threshold T 1 , and the combination of Q 1  and Q 3  defines a second current threshold T 2 , where T 1  and T 2  have opposite signs. U.S. Pat. No. 5,742,463 provides further description of uni-directional and bi-directional transient blocking units. Refinements of the basic TBU concept have also been considered in the art. For example, US 2006/0098363 describes a TBU approach where a core TBU is combined with a discrete high-voltage device. 
       SUMMARY 
       [0006]    As indicated above, the threshold current of a TBU depends on the series resistance of the depletion mode transistors when they are in a conducting state. This parameter is frequently referred to as the transistor on-resistance. For most applications, it is desirable to minimize the on-resistance, e.g., as considered in U.S. Pat. No. 5,869,865. However, the TBU application is unusual, since the basic TBU circuit would not function with transistors having zero on-resistance. Instead, for TBU fabrication, it is highly desirable that on-resistance be a well-controlled device parameter. 
         [0007]    However, it turns out in practice that device on-resistance is typically a relatively poorly controlled device parameter, and that this lack of control of device on-resistance has significant effects on TBU yield. TBUs having a threshold current that does not meet product specifications (e.g., 150 mA +/−20%) are rejected, thereby decreasing yield. Device on-resistance variation is a significant contributor to this yield issue. 
         [0008]    In practice, transistor threshold voltage variation is also an important contributor to TBU current threshold variation. In such cases, it is important to provide a match of on-resistance to threshold voltage to make TBU threshold current more consistent. For example, the TBU threshold current of the bi-directional TBU of  FIG. 2  is often roughly given by V t2 /R on1  or V t3 /R on1 , where V t2  and V t3  are the threshold voltages of Q 2  and Q 3  respectively, and R on1  is the on-resistance of Q 1 . Threshold voltages V t2  and V t3  can also vary significantly as a result of normal process variation. In such situations, it is important to match the on-resistance of Q 1  to the measured value of V t2  (or V t3 ) in order to improve TBU product yield. 
         [0009]    According to embodiments of the invention, this TBU threshold current yield issue is addressed by trimming the TBU during fabrication to adjust the current threshold. Here “trimming” is understood to refer to adjusting parameters of one or more devices of a TBU during TBU fabrication. Such trimming is often performed in connection with device and/or TBU characterization, where measured values from the characterization are used as inputs for the trimming. Trimming as practiced in embodiments of the invention entails making one-time adjustments to device parameters during fabrication, as opposed to providing components having parameter values that can be changed multiple times and/or after fabrication is complete (e.g., a variable resistor, etc.). 
         [0010]    In a first approach, a resistive circuit is added to the TBU in series with the pertinent transistor on-resistance. This additional resistive circuit has a resistance that can be adjusted during fabrication, to compensate for variations in transistor on-resistance and/or threshold voltage. In a second approach, a TBU transistor is fabricated as a segmented device having an on-resistance that depends on the number of transistor segments connected to terminals in final wire bonding. 
         [0011]    Trimming as described above can also be employed to adjust the resistance of the TBU when it is in its normal current conducting state, since process variation of this TBU resistance can be a significant problem in some situations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a prior art uni-directional transient blocking unit. 
           [0013]      FIG. 2  shows a prior art bi-directional transient blocking unit. 
           [0014]      FIG. 3  shows a uni-directional transient blocking unit in accordance with an embodiment of the invention. 
           [0015]      FIG. 4  shows a bi-directional transient blocking unit in accordance with an embodiment of the invention. 
           [0016]      FIG. 5  shows a bi-directional transient blocking unit in accordance with another embodiment of the invention. 
           [0017]      FIGS. 6   a - c  show segmented transistors suitable for use in connection with embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 3  shows a uni-directional transient blocking unit in accordance with an embodiment of the invention. In this example, a first depletion mode transistor Q 1  and a second depletion mode transistor Q 2  are connected in series with each other such that when I TBU  exceeds a first current threshold (T 1 ), transistors Q 1  and Q 2  automatically switch to a high impedance blocking state. In this example, trimming of the TBU during fabrication to adjust T 1  is provided by a resistive trimming circuit connected in series with Q 1  and Q 2 , where a resistance of the resistive trimming circuit can be selected during fabrication. 
         [0019]    More specifically, the resistive trimming circuit of  FIG. 3  includes resistors R 1  and R 2  connected in series, each of the resistors also being connected in parallel to a corresponding fuse. Here fuses F 1  and F 2  correspond to resistors R 1  and R 2  respectively. 
         [0020]    In this example, the resistance of the resistive circuit can be selected during fabrication according to whether or not F 1  and F 2  are set to an open or short state during fabrication, as indicated in the following table. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 F1 
                 F2 
                 resistance 
               
               
                   
                   
               
             
             
               
                   
                 open 
                 open 
                 R1 + R2 
               
               
                   
                 open 
                 short 
                 R1 
               
               
                   
                 short 
                 open 
                 R2 
               
               
                   
                 short 
                 short 
                 0 
               
               
                   
                   
               
             
          
         
       
     
         [0021]    Preferably, R 1  and R 2  have different values (e.g., R 1 =1Ω and R 2 =2Ω), so that different resistance values are obtained when only R 1  or only R 2  is providing resistance. In this example, possible values for the resistive circuit resistances are 0, 1, 2, and 3 Ohms. 
         [0022]    The resistance of the resistive trimming circuit is in series with the on-resistance of transistor Q 1 . Therefore, it contributes to the gate voltage of Q 2  in the same way that the on-resistance of Q 1  contributes. Accordingly, the resistance of the resistive trimming circuit can be selected during fabrication to compensate for transistor on-resistance and/or V t  variation, thereby improving the consistency of the TBU threshold current. 
         [0023]    For example, suppose the transistor on-resistance (R on ) as fabricated varies over a range from 2 to 6 Ohms. A nominal total resistance R 0  can be selected (e.g., 5Ω), and the resistance R t  of the trimming circuit can be selected during fabrication (in response to measurements of R on ) such that R=R t +R on  is as close as possible to the nominal total resistance R 0 . In this example, the total resistance R would vary over a range from 5 to 6 Ohms, thereby providing a substantial improvement in threshold current consistency. Embodiments of the invention can include resistive trimming circuits having one or more resistors, each in parallel with its corresponding fuse. 
         [0024]    In some cases, it is more important to provide an appropriate match of the on-resistance of Q 1  to measured characteristics of Q 2  than it is to make the effective on-resistance of Q 1  more uniform. For example, suppose the TBU current threshold is given by a function f (R on , Q 2   parm ), where R on  is the on-resistance of Q 1 , and Q 2   parm  are the relevant parameters of Q 2  (e.g., threshold voltage), and suppose that the parameters of Q 2  vary significantly from device to device. In practice, Q 2  is typically an NMOS transistor, and the threshold voltage of an NMOS depletion mode transistor is a relatively poorly controlled device parameter. In this situation, it is preferred to select the trimming resistance R t  such that the current threshold f(R t +R on , Q 2   parm ) is as uniform as possible, based on measured values of Q 2   parm . The flexibility in on-resistance provided by trimming can be exploited to provide either of these functions (i.e., making the effective R on  more uniform, or directly making the current threshold more uniform). 
         [0025]    In the example of  FIG. 3 , current flowing through the TBU as indicated on the figure encounters Q 2 , then Q 1 , then the resistive trimming circuit. It is also possible for the resistive trimming circuit to be disposed between Q 1  and Q 2 , such that current flowing through the TBU encounters Q 2 , the trimming circuit, and then Q 1 . In either case, the resistance provided by the resistive trimming circuit is in series with the pertinent transistor on-resistance (i.e., the on-resistance of Q 1 ), so operation of the circuit is as described above. In other words, the resistive trimming circuit X is in series with Q 1  and Q 2 , and the Q 2 -Q 1 -X and Q 2 -X-Q 1  sequences are both applicable. 
         [0026]    Trimming in accordance with principles of the invention is also applicable to bi-directional TBUs.  FIG. 4  shows an example of such a bi-directional transient blocking unit. The circuit of  FIG. 4  can be understood as a modified version of the bi-directional TBU of  FIG. 2 , where a resistive trimming circuit of the kind described in connection with  FIG. 3  is added. The example of  FIG. 4  also shows biasing elements RB 1  and RB 2 , which can be resistors and/or diodes disposed to prevent substantial current flow to or from the gate of Q 1 . Such biasing elements are known in connection with TBUs, and therefore need no further description here. 
         [0027]    In this example, a first depletion mode transistor Q 1 , a second depletion mode transistor Q 2 , and a third depletion mode transistor Q 3  are connected in series with each other such that when I TBU  exceeds a first current threshold (T 1 ), transistors Q 1  and Q 2  automatically switch to a high impedance blocking state, and such that when I TBU  exceeds a second current threshold (T 2 ), transistors Q 1  and Q 3  automatically switch to a high impedance blocking state, where thresholds T 1  and T 2  have opposite polarity. 
         [0028]    Thresholds T 1  and T 2  can be adjusted during fabrication by blowing none, some or all of fuses F 1 , F 2 , F 3 , and F 4 . For example, if R 2 =2R 1 , R 3 =4R 1  and R 4 =8R 1 , then the resistive trimming circuit can provide any resistance selected from the set {0, R 1 , 2R 1 , 3R 1 , 4R 1 , 5R 1 , 6R 1 , 7R 1 , 8R 1 , 9R 1 , 10R 1 , 11R 1 , 12R 1 , 13R 1 , 14R 1 , 15R 1 } according to which fuses are open or short after fabrication is complete. The resistance provided by the resistive trimming circuit is in series with the on-resistance of Q 1 , and can therefore be set to compensate for variations in Q 1  on-resistance as described above. 
         [0029]    In some preferred embodiments of the invention, the resistive trimming circuit is made symmetric with respect to Q 1  (e.g., R 1 =R 4 =1Ω, R 2 =R 3 =2Ω). Furthermore, in such embodiments, the resistance of the resistive trimming circuit is set during trimming to be disposed as symmetrically as possible relative to Q 1 . This configuration is preferred because it tends to reduce asymmetry in TBU characteristics. Having series resistances with different values on either side of Q 1  in the circuit of  FIG. 4  causes the circuit to start to turn off at different current values, depending on the polarity of I TBU . Although this asymmetry has a relatively small effect on TBU threshold, because Q 2  or Q 3  tend to start switching off before Q 1  as I TBU  increases, it is still often preferred to minimize this effect. 
         [0030]    However, the option of trimming the resistance on both side of Q 1  to different values may in some circumstances be useful. For example, it can be used to trim the turn-off characteristic of the TBU to ensure that for both current directions the TBU turns off at the same absolute current level in situations where transistors Q 2  and Q 3  are not exactly matched. This is another embodiment of the invention. 
         [0031]    The preceding examples show resistive trimming circuits that include resistors and fuses. The invention can also be practiced with other kinds of resistive trimming circuits. For example,  FIG. 5  shows a bi-directional transient blocking unit having a resistive trimming circuit that includes one or more resistors and wire bonding contact pads connected in alternating series. 
         [0032]    More specifically, contact pads  502 ,  504  and  506  are connected in alternating series with resistors R 3  and R 4 . Similarly, contact pads  508 ,  510 , and  512  are connected in alternating series with resistors R 1  and R 2 . The resistance of the resistive trimming circuit in this example is selected during fabrication by selecting which of contact pads  502 ,  504 , and  506  is connected to lead  520 , and by selecting which of contact pads  508 ,  510 , and  512  is connected to lead  530 . These connections of leads to pads can be made by conventional techniques, such as wire bonding. 
         [0033]    The adjustability provided in this manner can be appreciated by example. Suppose resistance values are as follows: R 1 =R 4 =X, R 2 =R 3 =2X. Then the resistances that can obtained by the various bonding combinations are as given in the following table: 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 Lead 520 connection 
                 Lead 530 connection 
                 Total R 
               
               
                   
               
             
             
               
                 502 
                 508 
                 3X 
               
               
                 502 
                 510 
                 5X 
               
               
                 502 
                 512 
                 6X 
               
               
                 504 
                 508 
                  X 
               
               
                 504 
                 510 
                 3X 
               
               
                 504 
                 512 
                 4X 
               
               
                 506 
                 508 
                 0 
               
               
                 506 
                 510 
                 2X 
               
               
                 506 
                 512 
                 3X 
               
               
                   
               
             
          
         
       
     
         [0034]    Here the total R value is the total resistance provided by the resistive trimming circuit in the main TBU current path (i.e., through transistors Q 2 , Q 1 , and Q 3  in series). Even though R 1 , R 2 , R 3 , and R 4  are always in the circuit in view of their connections to the gates of transistors Q 2  and Q 3 , they are only relevant if the main TBU current flows through them. For example, when lead  520  is connected to pad  504  or to pad  506 , there is no significant voltage drop across R 3  because the gate current of transistor Q 3  is negligible. Thus, R 3  does not contribute to the on-resistance in this situation, as indicated in the preceding table. This example shows trimming circuits having two resistors and three pads in alternating sequence. This approach is applicable to one or more resistors in alternating series with two or more contact pads. 
         [0035]    In this example, resistors R 1 , R 2 , R 3 , and R 4  are low value resistors, which can conveniently be fabricated by patterning one of the metal layers of the transistor fabrication process. Other kinds of resistors are also applicable (e.g., polysilicon resistors). Assuming typical levels of process variation in a numerical example, the threshold current standard deviation can be reduced from 11.2% to 4.9% following the approach of  FIG. 5 . Further reduction of threshold current standard deviation can be obtained by providing more resistance options in the resistive trimming circuit (e.g., by increasing the number of resistors in series) and/or by choosing different resistance values in the trimming circuit. 
         [0036]    The preceding examples show trimming approaches based on providing a resistive trimming circuit suitable for making one-time adjustments of a resistance in series with the on-resistance of the relevant transistor (i.e., Q 1  of the figures). Another approach is to fabricate Q 1  such that its on-resistance can be altered during later stages of fabrication. For example,  FIGS. 6   a - b  show a top view of a segmented transistor suitable for use in connection with an embodiment of the invention. 
         [0037]    In this example, the transistor of  FIG. 6   a  has a device-level source terminal  604  and a device-level drain terminal  606 . It also includes two or more segments  610 , each segment having a corresponding source and drain. The segment sources are referenced by  612 , and the segment drains are referenced by  614 . Thus each segment can be regarded as a source-drain pair. A common gate  602  controls current flow in each source drain pair. The segments can have the same width or different widths (the case of different widths is shown). 
         [0038]    The on-resistance of the final device can be adjusted by selecting some or all of the source-drain pairs to be connected to the device level terminals  604  and  606 . By adding more and/or larger segments in parallel, the on-resistance can be adjusted. For example, the configuration of  FIG. 6   a  shows connection of two segments to the device terminals with bonds  608   a - d , and the configuration of  FIG. 6   b  shows connection of three segments to the device terminals with bonds  608   a - f . This approach can be regarded as providing a transistor having a width that is adjustable at a late stage of fabrication. Such width adjustment is helpful for TBU fabrication, because on-resistance depends on transistor width. Individual characterization of the transistor segments may or may not be performed in the course of trimming the TBU, depending on how well controlled the parameters of individual segments are. 
         [0039]      FIG. 6   c  shows an alternative segmented transistor approach that reduces the number of wire bonds required to select the transistor on-resistance. This example is similar to the example of  FIG. 6   b , except that all segment drains  614  are connected together by a device level drain terminal  620 . Selective wire bonding of segment sources  612  to device level source terminal  604  can be employed to select the transistor on-resistance. In this case, as in the examples of  FIGS. 6   a - b , the resulting device has one or more segments connected in parallel to the device level source and drain terminals, thereby providing for adjustment of the on-resistance. In this example, the roles of source and drain can be reversed (i.e., all segment sources connected together, and segment drains selectively wire bonded). 
         [0040]    One or more of the transistors of a TBU can be segmented transistors as on  FIGS. 6   a - b . In such cases, it is typically preferred for the center transistor of a bi-directional TBU to be segmented (e.g., Q 1  on  FIGS. 2 ,  4 , and  5 ). 
         [0041]    One aspect of the invention is a method for TBU fabrication including trimming the TBU during fabrication to adjust the TBU current threshold. Another aspect of the invention is a TBU circuit including means for trimming the TBU during fabrication to adjust the TBU current threshold. One kind of means for trimming described above, by example, is a resistive trimming circuit. Such a resistive trimming circuit can be any circuit that provides a resistance R trim  in series with a pertinent transistor on-resistance, where the resistance R trim  can be set to one of several values by a one-time adjustment during fabrication. For example, the resistors+fuses approach of  FIGS. 3 and 4 , and the selective bonding approach of  FIG. 5 , are both “means for trimming” in this sense. 
         [0042]    Another means for trimming, also described above by example, is a TBU transistor having a fab-adjustable on-resistance. For example, the segmented transistor of  FIGS. 6   a - b  has a fab-adjustable on-resistance. Other approaches for providing transistors having a fab-adjustable on-resistance are also applicable “means for trimming” for practicing the invention. 
         [0043]    The preceding description has been by way of example as opposed to limitation, and so many details shown and/or described are not essential for practicing the invention. For example, bi-directional TBUs are shown having Q 2  and Q 3  being N-channel MOSFETs, and having Q 1  being a P-channel JFET. This configuration is preferred, but not required, and embodiments of the invention can be practiced with any combination of transistor types that provides the basic TBU functionality as described above. 
         [0044]    The preceding description has mainly focused on the situation where trimming of a TBU is performed to adjust the TBU threshold current (i.e., the current value at which it turns off). It is also possible to employ any or all of the trimming techniques or means for trimming described above in order to adjust the resistance provided by the TBU when it is in its normal conducting state. Hereinafter, this resistance is referred to as the “TBU resistance”.