Abstract:
A semiconductor device capable of accurately controlling the current value is provided. In a semiconductor integrated circuit having a constant current circuit, the constant current circuit includes a plurality of constant current elements having a gate terminal and a source terminal in common. Branched drain terminals of the constant current element arranged on one end of the gate terminal and the source terminal are arranged to both the gate terminal and the source terminal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a circuit structure that uniformly controls a current value of each of channels in a constant current circuit using a plurality of constant current elements each of which outputs a desired current ratio.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 3 is a structural diagram showing the arrangement of N constant current elements in a conventional constant current circuit that outputs the same current. Up to now, in a constant current circuit using p-type or n-type MOS transistors which are a plurality of constant current elements each of which outputs a desired current ratio, as shown in FIG. 3, the constant current element of each channel completes a layout for each element. A gate terminal  1  and a source terminal  2  are commonly used in the respective constant current elements. Also, in an order from one end of the gate terminal  1  and source terminal  2 , a first drain terminal  6  that corresponds to a first output terminal channel  3 , a second drain terminal  7  that corresponds to a second output terminal channel  4  and an N-th drain terminal  8  that corresponds to an N-th output terminal channel  5  are so disposed as to be branched, respectively. Reference numeral  12  denotes a contact hole for wiring.  
           [0005]    [0005]FIG. 4 is a diagram showing the schematic structure of N conventional constant current circuits that output the same current. In FIG. 4, in order to understand the arrangement diagram more easily, the gate terminal  1  and the source terminal  2  are omitted from the structural diagram of FIG. 3, and only the connection of the drain terminals is shown. In this way, the constant current elements respectively having the second output terminal channel  4  and the second output terminal channel  5  are disposed adjacent to the completed layout of the constant current element having the first output terminal channel  3  and the first drain terminal  6 , and the same arrangement is made up to the following N-th constant current element. Also, in a conventional structural example, the constant current elements in which the drain terminal is not branched are arranged in parallel with each other (refer to Patent Document 1 is JP 9-73331 A (FIG. 6)).  
           [0006]    However, in the conventional structure as described above, a difference in the wiring resistance value occurs in portions that correspond to the gate terminal  1  and the source terminal  2  because there is a difference in the distance from the constant current source due to the arrangement of the respective constant current elements. As a result, there arises such a problem in that the current and the voltage which are supplied from the gate terminal  1  and the source terminal  2  are different among the respective current constant elements. In addition, because an influence of temperature due to other structural elements or the like is different depending on locations at which the respective constant current elements are arranged, a difference occurs in the current values of the respective channels due to an influence of the wiring resistance value and the temperature, resulting in such a problem in that the control of an accurate current value becomes difficult.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention has been made in order to solve the above-mentioned problems.  
           [0008]    To achieve the above object, according to the present invention, respective drain branched terminals of p-type or n-type MOS transistors which are constant current elements of a constant current circuit are disposed alternately for a plurality of pieces from both ends of a layout pattern. That is, in the case where N constant current elements exist in the constant current circuit, the drain terminals are disposed from one end thereof in the order of a first-channel drain terminal, a second-channel drain terminal and so on up to an N-th channel drain terminal, and the drain terminals are also disposed from a left end in the order of a first-channel drain terminal, a second-channel drain terminal and so on up to an N-th channel drain terminal in such a manner that the N-th channel branched drain terminals are adjacent to each other in the center.  
           [0009]    Also, as another means, the respective branched drain terminals of the p-type or n-type MOS transistors which are the constant current elements of the constant current circuit are disposed alternately for a plurality of pieces from one end of the layout pattern. That is, in the case where N constant current elements are disposed in the constant current circuit, the drain terminals are disposed from one end thereof in the order of a first-channel drain terminal, a second-channel drain terminal and so on up to an N-th channel drain terminal, and the drain terminals are again disposed in the order of a first-channel drain terminal, a second-channel drain terminal and so on up to an N-th channel drain terminal, so that in the case where the number of respectively branched drain terminals is M, the drain terminals are disposed alternately by M drain terminals.  
           [0010]    With the above arrangement, it is possible to unify an influence of wiring resistances which are added to the source terminal and the gate terminal by the element unit of the p-type or n-type MOS transistor which is each constant current element. In addition, it is possible to unify an influence of current values due to a change in temperature. Therefore, in the constant current circuit that outputs a plurality of desired current ratios, it is possible to improve a precision of the current value control of the respective channels.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    These and other objects and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a structural diagram showing the arrangement of constant current elements of a constant current circuit in accordance with an embodiment mode;  
         [0013]    [0013]FIG. 2 is a diagram showing the schematic structure of the constant current circuit in accordance with the embodiment mode;  
         [0014]    [0014]FIG. 3 is a structural diagram showing the arrangement of a conventional constant current circuit;  
         [0015]    [0015]FIG. 4 is a diagram showing the schematic structure of the conventional constant current circuit;  
         [0016]    [0016]FIG. 5 is a circuit structural diagram showing the constant current circuit using the constant current elements in accordance with the embodiment mode of the present invention;  
         [0017]    [0017]FIG. 6 is a circuit structural diagram showing a constant current circuit using the constant current elements in accordance with the embodiment mode;  
         [0018]    [0018]FIG. 7 is a structural diagram showing the arrangement of constant current elements of a constant current circuit in accordance with another embodiment mode; and  
         [0019]    [0019]FIG. 8 is a diagram showing the schematic structure of the constant current circuit in accordance with the embodiment mode. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Hereinafter, a description will be given of preferred embodiment modes of the present invention with reference to the accompanying drawings.  
         [0021]    [0021]FIG. 1 is a structural diagram showing the arrangement of constant current elements in a constant current circuit in accordance with this embodiment mode. A gate terminal  1  and a source terminal  2  in the constant current elements which is n-type MOS transistors or p-type MOS transistors are commonly employed in all of channels. The respective branched drain terminals are arranged in the order of a first drain terminal  6 , a second drain terminal  7  and an n-th drain terminal from each of a left end and a right end of the gate terminal  1  and the source terminal  2  toward the center. In this way, the N-th branched drain terminals  8  which correspond to the N-th channel are adjacent to each other in the center. Then, the drain terminals are structured so as to be connected to a first output terminal channel  3 , a second output terminal channel  4  and an N-th output terminal channel  5  which are output terminals of the respective channels.  
         [0022]    The simplification of this arrangement diagram corresponds to a structural diagram shown in FIG. 2. FIG. 2 shows only the connection of the drain terminals in which the source terminal  1  and the gate terminal  2  are omitted from the structural diagram shown in FIG. 1.  
         [0023]    Even in the case where a variation in the values of a current and a voltage which are supplied to the respective constant current elements occurs due to the wiring resistance of the commonly used source terminal  1  and the gate terminal  2 , in the respective constant current elements, an influence of the variation in the current value and the voltage value of the source terminal  1  and the gate terminal  2  can be unified. In this embodiment mode, the current values and the voltage values are ideally unified to values of the current and the voltage which are supplied from the source terminal  1  and the gate terminal  2  to the N-th drain terminal  8 . In this way, a variation in the current value among the respective channels can be suppressed. Also, it is possible to reduce a variation in an influence of a temperature which is caused by other structural elements due to a difference of the arrangement of the respective constant current elements. That is, a variation in the current value among the channels due to the temperature change can also be suppressed.  
         [0024]    [0024]FIG. 7 is a structural diagram showing the arrangement of the constant current elements in a constant current circuit in accordance with another embodiment mode. The gate terminal  1  and the source terminal  2  in the constant current elements which are the n-type MOS transistors or the p-type MOS transistors are made common in all of the channels. The respective drain terminals which are branched in M pieces are arranged in the order of a first drain terminal  14 , a second drain terminal  15  and an n-th drain terminal from the left end of the gate terminal  1  and the source terminal  2  toward the right end thereof. In this way, the respective drain terminals are arranged in turn up to an M-th branched drain terminal in such a manner that the first branched N-th drain terminal  16  and the second branched first drain terminal  14  which correspond to the N-th channel, are adjacent to each other. Then, those drain terminals are structured so as to be connected to the first output terminal channel  3 , the second output terminal channel  4  and the N-th output terminal channel  5  which are the output terminals of the respective channels.  
         [0025]    The simplification of this arrangement diagram corresponds to a structural diagram shown in FIG. 8. FIG. 8 shows only the connection of the drain terminals in which the source terminal  1  and the gate terminal  2  are omitted from the structural diagram shown in FIG. 7.  
         [0026]    Even in the case where a variation in the values of a current and a voltage which are supplied to the respective constant current elements occurs due to the wiring resistance of the commonly used source terminal  1  and gate terminal  2 , in the respective constant current elements, an influence of the variation in the current value and the voltage value of the source terminal  1  and the gate terminal  2  can be unified. In this embodiment mode, the current values and the voltage values are ideally unified to values of the current and the voltage which are supplied from the source terminal  1  and the gate terminal  2  to the N-th drain terminal  8 . In this way, a variation in the current value among the respective channels can be suppressed. Also, it is possible to reduce a variation in an influence of a temperature which is caused by other structural elements due to a difference of the arrangement of the respective constant current elements. That is, a variation in the current value among the channels due to the temperature change can also be suppressed.  
         [0027]    Embodiments of the constant current circuit using the constant current elements structured as described above are shown in FIGS. 5 and 6. FIG. 5 is a diagram showing the embodiment of the constant current circuit using a p-type MOS transistor as a constant current element, and FIG. 6 a diagram showing the embodiment of the constant current circuit using an n-type MOS transistor as a constant current element.  
         [0028]    Referring to FIGS. 5 and 6, reference numeral  9  denotes a constant current source, reference numeral  10  denotes a constant current element p-type MOS transistor, and reference numeral  11  denotes a constant current element n-type MOS transistor. In addition, reference numeral  12  denotes a contact hole for conducting wiring, and reference numeral  13  denotes a bear hole for conducting wiring. As shown in the figure, the constant current elements of the constant current circuit are structured, thereby being capable of providing a semiconductor device having a constant current circuit with high precision.  
         [0029]    As described above, according to the present invention, it is possible to unify the current value and the voltage value due to a distribution of both the wiring resistance which is added to the source terminal and the wiring resistance which is added to the gate terminal by the element unit of the p-type or n-type MOS transistor which is each of the constant current element. Further, it is possible to unify an influence of the current value due to a change in temperature. Accordingly, it is possible to control the current values of the respective channels more accurately, and it is possible to supply a constant current having matching among the respective channels with high precision.  
         [0030]    The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiment modes and embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.