Abstract:
A fuse circuit configuration is described wherein a compensation capacitor counteracts a parasitic capacitor. The parasitic capacitor occurs between a connection point of a switching transistor and a fuse and ground. The compensation capacitor is connected to an evaluation circuit. In this manner, the negative effects caused by the parasitic capacitor are compensated for.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a fuse circuit configuration formed of a series circuit composed of a first switching transistor, a second switching transistor, and a fuse. An evaluation circuit is connected at a junction between the two switching transistors, it being possible to store the state of the fuse at the output of the evaluation circuit. Fuse circuit configurations such as fuse latches are preferably utilized for redundancy, chip identification and various other settings in conventional memories such as DRAMs, FeRAMs, flashes and so on.  
         SUMMARY OF THE INVENTION  
         [0003]    It is accordingly an object of the invention to provide a fuse circuit configuration that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which practically excludes the possibility of misevaluations due to parasitic capacitors.  
           [0004]    With the foregoing and other objects in view there is provided, in accordance with the invention, a fuse circuit configuration. The fuse circuit configuration contains a first switching transistor and a second switching transistor disposed in series with the first switching transistor and defines a series circuit with a junction point disposed between the first switching transistor and the second switching transistor. A fuse is connected to the second switching transistor. An evaluation circuit is connected to the junction point and has an output at which a status of the fuse can be tapped. A compensation capacitor is connected to the junction point and counteracts a parasitic capacitor occurring between a connection point between the second switching transistor and the fuse on a first side and ground on a second side.  
           [0005]    The object is inventively achieved in a fuse configuration of the above mentioned type by a compensation capacitor which is connected to the junction and which counteracts a parasitic capacitor arising between the connection between the second switching transistor and the fuse, on one side, and ground, on the other side. The compensation capacitor can simulate the layout of the connection between the second switching transistor and the fuse.  
           [0006]    In the inventive fuse circuit configuration, a compensation capacitor is provided at the junction between the two switching transistors. The compensation capacitor is initially charged to a high potential. The required charging current for the parasitic capacitor can thereby be drawn from the compensation capacitor without having to fear a tipping of the latch. For reasons of space, the compensation capacitor should not be proportioned too large.  
           [0007]    Rather, the compensation capacitor should have the same value (i.e. capacitance) as the parasitic capacitor. This can be accomplished by also simulating the layout of the connection between the second switching transistor and the fuse in the connection between the junction and the latch. In other words, the layout of the fuse configuration is repeated in the evaluation circuit to the greatest extent possible in order to generate the compensation capacitor that counteracts the parasitic capacitor.  
           [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a fuse circuit configuration. The fuse circuit configuration contains a a first switching transistor and a second switching transistor disposed in series with the first switching transistor and defines a series circuit with a junction point disposed between the first switching transistor and the second switching transistor. A fuse is connected to the second switching transistor. An evaluation circuit is connected to the junction point and has an output at which a status of the fuse can be tapped. A ground terminal is provided and a compensation capacitor is connected between the junction point and the ground terminal. The compensation capacitor delivers a charging current for a parasitic capacitor occurring between a connection between the second switching transistor and the fuse on a first side and the ground terminal on a second side.  
           [0009]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0010]    Although the invention is illustrated and described herein as embodied in a fuse circuit configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
       
    
    
       [0011]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a circuit diagram of a fuse circuit configuration according to the invention;  
         [0013]    [0013]FIG. 2 is a circuit diagram of a first exemplifying embodiment of a layout of a compensation capacitor;  
         [0014]    [0014]FIG. 3 is a circuit diagram of a second exemplifying embodiment of the layout of the compensation capacitor;  
         [0015]    [0015]FIG. 4 is a circuit diagram of an existing fuse circuit configuration;  
         [0016]    [0016]FIG. 5 is a graph showing signals in the fuse circuit configuration shown in FIG. 4;  
         [0017]    [0017]FIG. 6 is a circuit diagram corresponding to FIG. 4, with an additional parasitic capacitor; and  
         [0018]    [0018]FIG. 7 is a graph showing signals in the fuse circuit configuration of FIG. 6, with the additional parasitic capacitor.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 4 thereof, there is shown a representation of a known fuse latch. Between a chip supply voltage VDD and ground gnd there is a series circuit composed of a p-channel MOS field effect transistor p 1 , an n-channel MOS field effect transistor n 1 , and a fuse F 1 . The transistor p 1  is controlled by a signal FPUPn, whereas the transistor n 1  is controlled by a signal FPUN. A junction B between the two transistors p 1  and n 1  is connected to a latch L, which has an output OUT.  
         [0020]    During the initializing of the fuse latch, the control signal FPUPn is initially held at a low level, i.e. active, thereby switching through the transistor p 1  and putting it in a conductive state. The junction B is thereby charged to the voltage VDD and held at this potential by the latch L. The potential gnd, which is the inverse thereof, is available at the output OUT.  
         [0021]    The control signal FPUPn is then deactivated, i.e. drawn to the potential VDD, and the signal FPUN is activated, i.e., raised to a high level. The transistor  1  is thereby switched through transiently. The potential at junction B is dependent upon whether the fuse F 1  is continuous, i.e. conductive, or blown, i.e. non-conductive. It is noted that an anti-fuse can also be used instead of the fuse F 1 . An anti-fuse is non-conductive, and it is conductive in the blown state.  
         [0022]    If the fuse F 1  is conductive, the junction B is drawn to the potential gnd, whereas the potential VDD is maintained when the fuse is non-conductive.  
         [0023]    The latch L stores this setting at its output OUT inverted. If the fuse F 1  is non-conductive, i.e. the junction B is at potential VDD, then the output OUT is at low potential gnd, whereas, when the fuse F 1  is conductive, the junction B is at potential gnd and the inverted potential VDD is available at the output OUT.  
         [0024]    [0024]FIG. 5 shows the characteristic of the signals FPUPn and FPUN as well as the potential profile at the output OUT given a conductive fuse f 1  and given a non-conductive fuse F 1 . An evaluation of the fuse information (conductive or non-conductive, i.e. not blown or blown for the fuse and blown and not blown for the anti-fuse, respectively) is obtained at the output OUT.  
         [0025]    In the existing fuse circuit configuration as represented in FIG. 4, parasitic capacitors can cause a misevaluation of the fuse information, which will be described more closely with reference to FIGS. 6 and 7.  
         [0026]    A parasitic capacitor Cp, which emerges automatically via surfaces or lines in the layout of the circuit configuration, is connected in parallel to the fuse F 1 . The parasitic capacitor Cp then effectuates a discharging of the junction B even when the fuse F 1  is non-conductive. Specifically, the parasitic capacitor Cp is charged by way of the conductive transistor n 1  given a non-conductive fuse F 1 , thereby discharging the junction B. When the junction B has been discharged, the high potential VDD stands at output OUT. That is, the latch L tilts, which leads to a faulty evaluation of the status of the fuse F 1 . This is recognizable in FIG. 7. At the output OUT the same evaluation is obtained when the fuse F 1  is conductive and when it is non-conductive. A misevaluation such as this makes the fuse circuit configuration unusable and unsuitable for use in memory configurations, for example.  
         [0027]    As is apparent from FIG. 1, in the fuse circuit configuration according to the invention, a compensation capacitor Cc is situated between the junction B, which connects the two switching transistors p 1  and n 1  to each other, and ground gnd. The compensation capacitor Cc has approximately the same magnitude as the parasitic capacitor Cp that arises between the junction B between the transistor n 1  and the fuse F 1 , on one side, and ground gnd, on the other side. When the fuse F 1  is non-conductive, i.e. blown, the charging current for the automatically occurring parasitic capacitor Cp is drawn from the compensation capacitor Cc by way of the junction B and the transistor n 1 . A discharging of the junction B can be prevented in this way, so that this remains at high potential. The inverse state, i.e. low potential, is then present at the output OUT, which must be the case when the fuse F 1  is non-conductive.  
         [0028]    A misevaluation of the status of the fuse F 1  at the output OUT of the latch L can thus be reliably prevented.  
         [0029]    [0029]FIGS. 2 and 3 represent advantageous developments of the compensation capacitor Cc. As mentioned above, the compensation capacitor Cc should optimally simulate the layout of the connection between the transistor n 1  and ground gnd by way of the fuse F 1 . To this end, the compensation capacitor Cc is constructed similarly to a printed board for the connection between the transistor n 1  and ground gnd with the aid of a metal piece that forms the fuse F 1 , which has a tapering on which a laser can rest and cut the tapering. In the exemplifying embodiment of FIG. 2 the complete metal piece of the fuse  1  is thus “imaged” onto the compensation capacitor Cc, so that overcompensation may occur when the fuse F 1  is blown.  
         [0030]    By contrast, in the exemplifying embodiment of FIG. 3, the compensation capacitor Cc simulates the layout of the circuit given an expected laser cut subsequent to blowing of the fuse F 1 . That is, the layout of the compensation capacitor Cc is reduced in that only the metal piece that remains after the laser cutting process is used for wiring.