Abstract:
Provided are a trimming circuit which does not need a dedicated terminal to which a current for cutting a fuse is input, and also a semiconductor device including the trimming circuit. The trimming circuit includes: an input terminal connected to a pad which is an external terminal of an internal circuit; a fuse provided between a power supply terminal and an output terminal; and a diode provided between the input terminal and the output terminal. The trimming circuit performs trimming by applying, to the pad, such a voltage that the diode is biased in the forward direction.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2011-116032 filed on May 24, 2011, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a trimming circuit to be used for function setting or characteristics adjustment of a circuit in a semiconductor integrated circuit. 
     2. Description of the Related Art 
     In a circuit of a semiconductor device, such as a voltage detection circuit or a voltage setting circuit, a resistor circuit for setting a voltage is provided with a trimming circuit for resistors in order to adjust the voltage (see, for example, Japanese Patent Application Laid-open No. 2009-212415). 
       FIG. 5  is a circuit diagram of a conventional trimming circuit. 
     Description is given of the case of cutting a fuse F 1  in a conventional trimming circuit  1 . 
     A decode circuit  2  outputs a voltage of high level to a gate of an NMOS transistor N 1 , and outputs a voltage of low level to gates of NMOS transistors N 2  to N 4 . The NMOS transistor N 1  is turned ON and the NMOS transistors N 2  to N 4  are turned OFF. At this time, a current flows from a pad TRIM, which is a current supply terminal for fuse cutting, to a ground terminal via a diode D 1 , the fuse F 1 , and the NMOS transistor N 1 . The fuse F 1  is cut by this current, and a resistor R 1  is added in series into a resistor circuit. Further, a current also flows from the pad TRIM to the ground terminal via a diode D 2 , a fuse F 2 , a resistor R 5 , and the NMOS transistor N 1 . However, the current flows through the current limiting resistor R 5 , and hence the fuse F 2  is not cut by this current. 
     In this way, a transistor to be turned ON is selected by the decode circuit  2  so that a desired fuse may be cut, thereby being capable of adding a desired resistor into the resistor circuit. 
     The conventional trimming circuit, however, needs a terminal (pad TRIM) to which a current for cutting a fuse is input. Therefore, there is a problem in that the chip size increases. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problem, and provides a trimming circuit which does not need a terminal dedicated for trimming, and also provides a semiconductor device including the trimming circuit. 
     In order to solve the above-mentioned problem, a trimming circuit according to an exemplary embodiment of the present invention includes: an input terminal connected in common to a pad which is an external terminal of an internal circuit; a first output terminal connected to a resistor circuit; a first fuse which is provided between a first power supply terminal and the first output terminal and is connected in parallel to one resistor of the resistor circuit; and a first diode provided between the input terminal and the first output terminal. 
     A trimming circuit according to another exemplary embodiment of the present invention further includes: a second output terminal connected to the resistor circuit; a second fuse which is provided between a second power supply terminal and the second output terminal and is connected in parallel to another resistor of the resistor circuit; and a second diode provided between the input terminal and the second output terminal. 
     According to the trimming circuits of the exemplary embodiments of the present invention, a terminal to which a current for cutting a fuse is input can serve also as a terminal of an internal circuit. Therefore, a terminal dedicated for trimming is unnecessary, and hence the chip size can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a circuit diagram illustrating a trimming circuit according to a first embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating another example of the trimming circuit according to the first embodiment of the present invention; 
         FIG. 3  is a circuit diagram illustrating still another example of the trimming circuit according to the first embodiment of the present invention; 
         FIG. 4  is a circuit diagram illustrating a trimming circuit according to a second embodiment of the present invention; and 
         FIG. 5  is a circuit diagram of a conventional trimming circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention are described with reference to the accompanying drawings. 
     (First Embodiment) 
       FIG. 1  is a circuit diagram illustrating a trimming circuit according to a first embodiment of the present invention. 
     A trimming circuit  10  includes a fuse  11 , a diode  12 , a current limiting resistor  13 , an input terminal  17 , and an output terminal  18 . 
     The fuse  11 , the diode  12 , and the current limiting resistor  13  are connected in series between a power supply terminal and the input terminal  17 . The output terminal  18  is connected between the fuse  11  and the diode  12 . 
     The input terminal  17  is connected to a pad  16  which is an external terminal of an internal circuit  15 . The output terminal  18  is connected between resistors  21  and  22  of a resistor circuit  20  including the resistors  21  and  22  and resistors  23  and  24 . Therefore, the fuse  11  is connected in parallel to the resistor  21 . 
     Next, the operation of the trimming circuit  10  is described. 
     First, the case of cutting the fuse  11  is described. To the pad  16 , a voltage equal to or higher than a voltage obtained by adding a threshold voltage of the diode  12  to a power supply voltage is applied. The diode  12  is biased in the forward direction, thereby allowing a current to flow. Accordingly, a current flows from the pad  16  to the power supply terminal via the fuse  11 . The fuse  11  is cut by this current. 
     Next, the operation of the trimming circuit  10  in normal use is described. In normal use, a voltage equal to or lower than the power supply voltage is applied to the pad  16 . At this time, the diode  12  is biased in the reverse direction, and hence no current flows. Therefore, the fuse  11  is not cut. Then, the pad  16  functions only as a terminal of the internal circuit  15 . 
     Note that, the trimming circuit  10  is configured so that the fuse  11  is cut through the application of a voltage equal to or higher than the power supply voltage to the pad  16 , but the trimming circuit  10  may be configured as illustrated by a trimming circuit  30  of  FIG. 2 . 
     In the trimming circuit  30 , a fuse  31 , a diode  32 , and a current limiting resistor  33  are connected in series between an input terminal  37  and a ground terminal. An output terminal  38  is connected between the fuse  31  and the diode  32 . 
     The input terminal  37  is connected to a pad  36  which is an external terminal of an internal circuit  35 . The output terminal  38  is connected between the resistor  23  and the resistor  24 . That is, the fuse  31  is connected in parallel to the resistor  24  in order to trim the resistor  24 . 
     In the trimming circuit  30  of  FIG. 2 , in order to cut the fuse  31 , a voltage equal to or lower than a voltage obtained by subtracting a threshold voltage of the diode  32  from a ground voltage is applied to the pad  36 . 
     Alternatively, as illustrated in  FIG. 3 , both the trimming circuit  10  and the trimming circuit  30  may be provided. 
     Note that, the current limiting resistor  13  and the current limiting resistor  33  are provided for improving noise resistance of the pad  16  and the pad  36 , respectively. Therefore, if there is no problem on the noise resistance of the pads, the current limiting resistor  13  and the current limiting resistor  33  do not need to be provided. 
     As described above, according to the trimming circuit of the present invention, the terminal to which a current for cutting the fuse is input can serve also as a terminal of the internal circuit. Therefore, a terminal dedicated for trimming is unnecessary, and hence the chip size can be reduced. 
     (Second Embodiment) 
       FIG. 4  is a circuit diagram illustrating a trimming circuit according to a second embodiment of the present invention. 
     A trimming circuit  40  includes a fuse  41 , a diode  43 , a diode  44 , a fuse  42 , an input terminal  47 , an output terminal  48 , and an output terminal  49 . 
     The fuse  41 , the diode  43 , the diode  44 , and the fuse  42  are connected in series between a power supply terminal and a ground terminal. The input terminal  47  is connected between the diode  43  and the diode  44  and also connected to a pad  16  which is an external terminal of an internal circuit  15 . The output terminal  48  is connected between the fuse  41  and the diode  43  and also connected to a resistor circuit  20 . The output terminal  49  is connected between the diode  44  and the fuse  42  and also connected to the resistor circuit  20 . 
     The fuse  41  is connected in parallel to the resistor  21  in order to trim the resistor  21  of the resistor circuit  20 . The fuse  42  is connected in parallel to the resistor  24  in order to trim the resistor  24  of the resistor circuit  20 . 
     Next, the operation of the trimming circuit  40  is described. 
     First, the case of cutting the fuse  41  is described. To the pad  16 , a voltage equal to or higher than a voltage obtained by adding a threshold voltage of the diode  43  to a power supply voltage is applied. The diode  43  is biased in the forward direction, thereby allowing a current to flow. Accordingly, a current flows from the pad  16  to the power supply terminal via the fuse  41 . The fuse  41  is cut by this current. 
     Next, the case of cutting the fuse  42  is described. To the pad  16 , a voltage equal to or lower than a voltage obtained by subtracting a threshold voltage of the diode  44  from a ground voltage is applied. The diode  44  is biased in the forward direction, thereby allowing a current to flow. Accordingly, a current flows from the ground terminal to the pad  16  via the fuse  42 . The fuse  42  is cut by this current. 
     Next, the operation of the trimming circuit  40  in normal use is described. In normal use, a voltage equal to or lower than the power supply voltage and equal to or higher than the ground voltage is applied to the pad  16 . At this time, the diode  43  and the diode  44  are biased in the reverse direction, and hence no current flows. Therefore, the fuse  41  and the fuse  42  are not cut. Then, the pad  16  functions only as a terminal of the internal circuit  15 . 
     In the trimming circuit  40 , in order to adjust a resistance value of the resistor circuit  20 , one of the fuse  41  and the fuse  42  is cut. In this case, a circuit including a fuse which is left uncut functions an ESD protection circuit for the pad  16 . Therefore, the diode of the trimming circuit  40  also has the function of the ESD protection circuit, and hence the ESD protection circuit is not necessarily provided for the pad  16 . That is, the number of circuits is reduced, and the chip size can be reduced more. 
     As described above, according to the trimming circuit of the present invention, the terminal to which a current for cutting a fuse is input can serve also as a terminal of the internal circuit. Therefore, a terminal dedicated for trimming is unnecessary, and hence the chip size can be reduced. Besides, the diode of the trimming circuit also has the function of an ESD protection circuit, and hence the chip size can be reduced more.