Abstract:
There is provided a digital-analog converter capable of easily extending the resolution that can easily extend the resolution by a simple circuit implementation when a 10-bit digital-analog converter is configured on the basis of an 8-bit digital-analog converter used in a display driving IC, and prevent an increase in area of the display driving IC. A digital-analog converter capable of easily extending the resolution according to an aspect of the invention includes a voltage dividing unit including a plurality of resistors connected in series with each other between operating voltage terminals to divide operating voltages into a plurality of voltages at connection nodes of the plurality of resistors, a decoder unit providing a plurality of switching signals including a first switching signal for selecting a voltage and a second switching signal for controlling the current, a switch unit selecting two voltages from the plurality of voltages divided by the voltage dividing unit according to the first switching signal, a comparing unit comparing the two voltages with each other selected by the switching unit to output a reference current corresponding to a difference between the two voltages, a current control unit controlling a feedback current according to the second switching signal of the decoder unit, and an adding unit transmitting the feedback current to the current control unit and adding one voltage between the two voltages selected by the switch unit and a feedback voltage determined according to the feedback current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 2006-111825 filed on Nov. 13, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a digital-analog converter capable of easily extending the resolution, and more particularly, to a digital-analog converter capable of easily extending the resolution that can easily extend the resolution by a simple circuit implementation when a 10-bit digital-analog converter is configured on the basis of an 8-bit digital-analog converter used in a display driving IC, and prevent an increase in area of the display driving IC. 
         [0004]    2. Description of the Related Art 
         [0005]    In recent years, the market for display devices, especially, digital TVs has grown. In order to achieve high display quality, high output resolution of driving ICs of the display devices has been required. 
         [0006]    In general, bit extension of the digital-analog converter used in the driving IC is performed to increase the output resolution of the driving IC. 
         [0007]      FIG. 1A  is a configuration view illustrating an example of a digital-analog converter according to the related art. 
         [0008]    Referring to  FIG. 1A , a digital-analog converter  10  according to the related art includes a first resistor array  11  that divides voltages by a plurality of resistors, a first switch array  12  that has a plurality of switches for selecting the divided voltages, a first buffer unit  13  that buffers the two voltages selected by the first switch array  12 , a second resistor array  14  that divides the voltages buffered by the first buffer unit  13  by a plurality of resistors, a second switch array  15  that selects the voltages divided by the second resistor array  14 , a decoder unit  16  that controls the operation of selecting the voltages by each of the first switch array  12  and the second switch array  15 , and a second buffer unit  17  that buffers the voltages selected by the second switch array  15  to output a final output voltage. 
         [0009]    The above-described digital-analog converter  10  according to the related art is obtained by adding the first buffer unit  13 , the second resistor array  14 , and the second switch array  15  to a converter with 8-bit resolution. However, problems occur with the bit extension in that an increase in the number of resistors and switches results in an increase of chip area, and an off-set voltage of the buffer unit is applied to the final output voltage to cause deterioration in off-set characteristics of the entire system. 
         [0010]      FIG. 1B  is a configuration view illustrating another example of a digital-analog converter according to the related art. 
         [0011]    Referring to  FIG. 1B , a digital-analog converter  20  according to the related art includes a first resistor array  21  that divides voltages by a plurality of resistors, a first switch array  22  that selects the voltages divided by the first resistor array  21  by a plurality of switches, a second resistor array  23  that divides the voltages divided by a plurality of resistors, a second switch array  24  that selects the voltages divided by the second resistor array  23  by a plurality of switches, a decoder unit  25  that controls the operation of selecting the voltages by the first switch array  22  and the second switch array  24 , and a buffer unit  26  that buffers the voltages selected by the second switch array  24 . 
         [0012]    When compared with the digital-analog converter  10  according to the related art, which is shown in  FIG. 1A , the digital-analog converter  20  according to the related art increases a plurality of resistances of the plurality of resistors that are included in the second resistor array  23  to thereby remove the first buffer unit  13 . However, the second resistor array  23  and the second switch array  24  are added to extend the resolution of the digital-analog converter with 8-bit resolution. The addition of the second resistor array  23  and the second switch array  24  results in an increase in the number of resistors and switches. This increase results in an increase of chip area. Further, since the resistances of the resistors included in the second resistor array  23  are increased, a current loss may occur to reduce uniformity between output channels. 
       SUMMARY OF THE INVENTION 
       [0013]    An aspect of the present invention provides a digital-analog converter that can easily perform bit extension by a simple circuit implementation when a converter having 10 bits or more is configured on the basis of an 8-bit digital-analog converter, and prevent an increase in area of the driving IC. 
         [0014]    According to an aspect of the present invention, there is provided a digital-analog converter capable of easily extending the resolution, the digital-analog converter including a voltage dividing unit including a plurality of resistors connected in series with each other between operating voltage terminals to divide operating voltages into a plurality of voltages at connection nodes of the plurality of resistors, a decoder unit providing a plurality of switching signals including a first switching signal for selecting a voltage and a second switching signal for controlling the current, a switch unit selecting two voltages from the plurality of voltages divided by the voltage dividing unit according to the first switching signal, a comparing unit comparing the two voltages with each other selected by the switching unit to output a reference current corresponding to a difference between the two voltages, a current control unit controlling a feedback current according to the second switching signal of the decoder unit, and an adding unit transmitting the feedback current to the current control unit and adding one voltage between the two voltages selected by the switch unit and a feedback voltage determined according to the feedback current. 
         [0015]    The switch unit may select two voltages of the connection nodes adjacent to each other from the plurality of voltages divided by the voltage dividing unit. 
         [0016]    The decoder unit may include a first decoder decoding an external voltage selecting signal to provide the first switching signal, and a second decoder decoding an external current control signal to provide the second switching signal. 
         [0017]    The comparing unit may include a first comparator circuit receiving a first voltage having a higher voltage than the other voltage between the two voltages, and a second voltage having a lower value than the first voltage to output the reference current corresponding to the difference between the first voltage and the second voltage according to a resistance set beforehand, and a second comparator circuit receiving the first voltage to off-set unnecessary current components occurring in the first comparator circuit according to a resistance set beforehand. 
         [0018]    The first comparator circuit may include a first transistor group including a plurality of transistors each receiving and switching the first voltage and the second voltage from the comparing unit, a second transistor group including a plurality of transistors connected in series with the first transistor group and connected in parallel with each other to mirror the current switched by the first transistor group, a first resistor having a resistance set beforehand and connected between input terminals of the plurality of transistors of the first transistor group, a third transistor group including a plurality of transistors connected to both ends of the first resistor, respectively, to mirror the current switched by the first transistor group, and a fourth transistor group including a plurality of transistors receiving the switched current mirrored by the third transistor group, the transistors connected in parallel with each other and mirroring the current mirrored by the third transistor group to generate the reference current, and the second comparator circuit may include a fifth transistor group including a plurality of transistors each receiving and switching the first voltage from the comparing unit, a sixth transistor group including a plurality of transistors connected in series with the fifth transistor group, connected in parallel with each other, and mirroring the current switched by the fifth transistor group, a second resistor having a resistance value set beforehand and connected between input terminals of the plurality of transistors of the fifth transistor group, a seventh transistor group including a plurality of transistors connected to both ends of the second resistor, respectively, to mirror the switched current mirrored by the fifth transistor group, and an eight transistor group including a plurality of transistors receiving the switched current mirrored by the seventh transistor group, the transistors connected in parallel with each other and mirroring the current mirrored by the seventh transistor group to off-set necessary current components included in the reference current of the fourth transistor group. 
         [0019]    The current control unit may switch parallel paths of the feedback current according to the second switching signal, and mirrors a feedback current of the selected path using the reference current from the comparing unit so as to control the feedback current. 
         [0020]    The current control unit may include a switch group including a plurality of switches individually connected to the plurality of parallel paths and receiving the feedback currents, the switch group switching the plurality of switches according to the second switching signal from the decoder unit to switch the paths of the feedback currents, and a current mirror unit mirroring the feedback current of the path that is switched by each of the switches of the switch group. 
         [0021]    The current mirror unit may include a ninth transistor group including a plurality of transistors connected in series with each other to mirror the reference current from the comparing unit, and a tenth transistor group including a plurality of transistors connected in parallel with the switches of the switch group, connected in parallel with each other, and each receiving the reference current mirrored by the ninth transistor group. 
         [0022]    The digital-analog converter may be integrated into a driving IC for a display device. 
         [0023]    The adding unit may add the second voltage selected by the switch unit and the feedback voltage to output a driving voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0025]      FIGS. 1A and 1B  are configuration views illustrating a digital-analog converter according to the related art. 
           [0026]      FIG. 2  is a configuration view illustrating a digital-analog converter according to an exemplary embodiment of the present invention. 
           [0027]      FIGS. 3A and 3B  are configuration views illustrating a comparing unit that is used in the digital-analog converter according to the exemplary embodiment of the present invention. 
           [0028]      FIG. 4  is a configuration view illustrating a current control unit that is used in the digital-analog converter according to the exemplary embodiment of the present invention. 
           [0029]      FIG. 5  is a graph illustrating resolution extension of the digital-analog converter according to the exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, in description of operation principles associated with the embodiments of the present invention, detailed description of a known art or construction is omitted because it may obscure the spirit of the present invention unnecessarily. 
         [0031]      FIG. 2  is a configuration view illustrating a digital-analog converter according to an exemplary embodiment of the invention. 
         [0032]    Referring to  FIG. 2 , a digital-analog converter  100  according to an exemplary embodiment of the present invention includes a voltage dividing unit  110 , a switch unit  120 , a comparing unit  130 , a current control unit  140 , a decoder unit  150 , and an adding unit  160 . 
         [0033]    The voltage dividing unit  110  includes a plurality of resistors R 0  to R 255  that are connected in series with each other between operating voltage terminals VH and VL. The number of the plurality of resistors R 0  to R 255  is in proportion to the number of bits of resolution of the digital-analog converter. The voltage dividing unit  110  includes 256 (2 8 ) resistors on the basis of eight bits. 
         [0034]    The switch unit  120  includes a plurality of switches that switch voltages divided by the plurality of resistors R 0  to R 255  included in the voltage dividing unit  110 . Though not shown, the plurality of switches that are connected in parallel with on each other, and each of the switches switches each of the voltages divided by the plurality of resistors R 0  to R 255 . Therefore, preferably, the number of the plurality of switches is the same as the number of the plurality of resistors R 0  to R 255 . 
         [0035]    The comparing unit  130  receives a first voltage V 1 out and a second voltage V 2 out from the switch unit  120 , and outputs a reference current Iref that corresponds to a difference between the first voltage V 1 out and the second voltage V 2 out. The configuration of the above-described comparing unit  130  will be described in detail with reference to  FIG. 3 . 
         [0036]    The current control unit  140  includes a switch group  141  and a current mirror unit  142 . The switch group  141  includes a plurality of switches (not shown) that are connected in parallel with each other. The current mirror unit  142  receives the reference current Iref from the comparing unit  130  and a feedback current that is selected by the switch group  141 . The detailed configuration of the above-described current control unit  140  will be described below with reference to  FIG. 4 . 
         [0037]    The decoder unit  150  includes a first decoder  151  and a second decoder  152 . The first decoder  151  provides a first switching signal for controlling the operation of selecting the voltages by the switch unit  120 . The second decoder  152  provides a second switching signal, by which the current control unit  140  controls the current control of the switch group  141 . 
         [0038]    Preferably, the adding unit  160  may be composed of an amplifier. More preferably, the adding unit  160  may be composed of an operational amplifier (op amp). The adding unit  160  receives the second voltage V 2 out through a positive input terminal (+) and an out voltage through a negative input terminal (−). Here, the output voltage of the adding unit  160  is fed back to the adding unit  160  through a feedback resistor Rf. 
         [0039]      FIGS. 3A and 3B  are configuration views illustrating a comparing unit that is used in the digital-analog converter according to the exemplary embodiment of the present invention. 
         [0040]    Referring to  FIG. 3A , the comparing unit  130  includes a first operational transconductor amplifier (hereinafter, simply referred to as an OTA)  131  and a second OTA  132 . The first OTA  131  receives the second voltage V 2 out through a negative input terminal (−) and the first voltage V 1 out through a positive input terminal (+). The second OTA  132  receives the first voltage V 1 out through a positive input terminal (+) and a negative input terminal (−). 
         [0041]    Referring to  FIG. 3B , each of the first OTA  131  and the second OTA  132  may be composed of a comparator circuit that includes a plurality of transistors. A first comparator circuit that forms the first OTA  131  includes a first transistor group, a second transistor group, a first resistor R 1 , a third transistor group, and a fourth transistor group. The first transistor group includes a plurality of transistors MP 7  and MP 8 , each of which receives and switches the first voltage V 1 out and the second voltage V 2 out. The second transistor group includes a plurality of transistors MP 9  to MP 12  and MN 9  to MN 12  that are connected in series with the first transistor group MP 7  and MP 8  and connected in parallel with each other so as to mirror the current that is switched by the first transistor group MP 7  and MP 8 . The first resistor R 1  has a resistance set beforehand and is connected between input terminals of the plurality of transistors of the first transistor group MP 7  and MP 8 . The third transistor group includes a plurality of transistors MN 7  and MN 8  that are connected to both ends of the first resistor R 1 , respectively, to mirror the current that is switched by the first transistor group MP 7  and MP 8 . The fourth transistor group includes a plurality of transistors MN 14  and MP 17  to MP 20  that receive the switched current that is mirrored by the third transistor group MN 14  and MP 17  to MP 20 . Further, the plurality of transistors MN 14  and MP 17  to MP 20  are connected in parallel with each other to mirror the current mirrored by the third transistor group MN 7  and MN 8  to generate the reference current. 
         [0042]    In the same manner, a second comparator circuit that forms the second OTA  132  includes a fifth transistor group, a sixth transistor group, a second resistor R 2 , a seventh transistor group, and an eighth transistor group. The fifth transistor group includes a plurality of transistors MP 1  and MP 2 , each of which receives and switches the first voltage from the comparing unit. The sixth transistor group includes a plurality of transistors MP 3  to MP 6  and MN 3  to MN 6  that are connected in series with the fifth transistor group MP 1  and MP 2  and connected in parallel with each other so as to mirror the current that is switched by the fifth transistor group MP 1  and MP 2 . The second resistor R 2  has a resistance set beforehand and is connected between input terminals of the plurality of transistors MP 1  and MP 2  of the fifth transistor group MP 1  and MP 2 . The seventh transistor group includes a plurality of transistors MN 1  and MN 2  that are connected to both ends of the second resistor R 2 , respectively, to mirror the current that is switched by the fifth transistor group MP 1  and MP 2 . The eighth transistor group includes a plurality of transistors MN 13  and MP 13  to MP 16  that receive the switched current that is mirrored by the seventh transistor group MN 1  and MN 2 . Further, the plurality of transistors MN 13  and MP 13  to MP 16  are connected in parallel with each other to off-set unnecessary current components that are included in the reference current of the fourth transistor group MN 14  and MP 17  to MP 20 . 
         [0043]    The above-described reference numerals N and P of the transistor groups refer to an N-type transistor and a P-type transistor, respectively. 
         [0044]      FIG. 4  is a configuration view illustrating a current control unit that is used in the digital-analog converter according to the exemplary embodiment of the present invention. 
         [0045]    Referring to  FIG. 4 , the current control unit  140  includes the switch group  141  that includes the plurality of switches, and the current mirror unit  142 . 
         [0046]    The switch group  141  includes a plurality of switches S 1  to S 3  that are connected in parallel with each other. Each of the switches S 1  to S 3  receives the feedback current If from the adding unit  160 . The number of the plurality of switches increases according to the number of bits for the resolution extension. Preferably, in case of two bits, the switch group  141  may include three switches S 1  to S 3 . 
         [0047]    The current mirror unit  142  may be composed of a current mirror circuit that includes a plurality of N-type transistors. 
         [0048]    That is, the current mirror unit  142  may be composed of a current mirror circuit that includes a first transistor group and a second transistor group. The first transistor group includes a plurality of N-type transistors MN 15  and MN 16  that receive and mirror the reference current Iref from the comparing unit  130 . The second transistor group includes a plurality of N-type transistors MN 17  to MN 22  that are connected in series with the plurality of switches S 1  to S 3  of the switch group  141  and receive the reference current that is mirrored by the first transistor group MN 15  and MN 16 . 
         [0049]    Preferably, the analog-digital converter according to the exemplary embodiment of the present invention is integrated into a driving IC that drives a display device. 
         [0050]      FIG. 5  is a graph illustrating resolution extension of the digital-analog converter according to the exemplary embodiment of the present invention. 
         [0051]    Referring to  FIG. 5 , the resolution extension of the digital-analog converter according to the exemplary embodiment of the present invention can be shown. 
         [0052]    Hereinafter, the operation and effect of the present invention will be described in detail. 
         [0053]    Referring to  FIGS. 2 to 4 , the voltage dividing unit  110  that is used in the digital-analog converter  100  according to the exemplary embodiment of the present invention receives operating voltages of the operating voltage terminals VH and VL. That is, the plurality of resistors of the voltage dividing unit  110  that are connected in series with each other are connected between the operating voltage terminals to divide the operating voltages. Each of the divided voltages is applied to the switch unit  120 . The switch unit  120  includes the plurality of switches that are connected in parallel with each other. Each of the plurality of switches receives each of the divided voltages to select the two voltages V 1 out and V 2 out according to the first switching signal of the first decoder  151 . 
         [0054]    Preferably, the two voltages V 1 out and V 2 out are the first voltage V 1 out and the second voltage V 2 out that are selected by the adjacent switches to each other. Further, preferably, the first voltage V 1 out has a higher value than the second voltage V 2 out. 
         [0055]    The above-described operation is a conversion process to obtain 8-bit resolution. In order to obtain the resolution of 10 bits or more, which is larger than the 8-bit resolution, an additional conversion process is required as follows. 
         [0056]    Referring to  FIG. 3A , the first voltage V 1 out and the second voltage V 2 out are input to the positive terminal (+) and the negative terminal (−) of the comparing unit  130 , respectively. More particularly, the comparing unit  130  includes the first OTA  131  and the second OTA  132 . The first voltage V 1 out is input to the positive terminal (+) of the first OTA  131 , and the second voltage V 2 out is input to the negative terminal (−) of the first OTA  131 . Further, the first voltage V 1 out is input to each of the positive terminal (+) and the negative terminal (−) of the second OTA  132 . The comparing unit  130  compares the first voltage V 1 out with the second voltage V 2 out and outputs the reference current Iref that corresponds to a difference between the first voltage V 1 out and the second voltage V 2 out. 
         [0057]    Referring to  FIG. 3B , the first OTA  131  and the second OTA  132  include the first and second resistors R 1  and R 2 , respectively, which have resistances set beforehand. The comparing unit  130  outputs the reference current Iref corresponding to the difference between the first voltage V 1 out and the second voltage V 2 out according to the resistances. 
         [0058]    That is, the reference current Iref can be represented by the following equation. 
         [0000]        Iref =( V 1out− V 2out)× Gm    
         [0000]    (here, Gm refers to the reciprocal of the resistance.) 
         [0059]    The above-described reference current Iref is transmitted to the current control unit  140 . 
         [0060]    Referring to  FIG. 4 , the switch group  141  of the current control unit  140  includes the switches S 1 , S 2 , and S 3  that are connected in parallel with each other. 
         [0061]    The second decoder  152  outputs the second switching signal to the switch group  141 . According to the second switching signal, each of the first to third switches S 1 , S 2 , and S 3  of the switch group  141  performs an on and off operation. 
         [0062]    Here, the second decoder  152  may be a 2-bit decoder, and thus the second switching signal may be a 2-bit switching signal. That is, the second switching signal may be divided into a first bit switching signal b 1  having one bit of 0 or 1 and a second bit switching signal b 2  having the other bit of 0 or 1. The first bit switching signal b 1  controls the on and off operation of the first switch S 1 , and the second bit switching signal b 2  controls the on and off operation of the second and third switches S 2  and S 3  at the same time. 
         [0063]    Meanwhile, the adding unit  160  transmits the feedback current If to the switch group  141 . According to the paths of the feedback current If, the feedback current If is transmitted to the switch group  141  through the feedback resistor Rf. 
         [0064]    The above-described feedback current If is obtained by the following equation. 
         [0000]    
       
         
           
             if 
             = 
             
               
                 ( 
                 
                   
                     V 
                      
                     
                         
                     
                      
                     1 
                      
                     out 
                   
                   - 
                   
                     V 
                      
                     
                         
                     
                      
                     2 
                      
                     
                         
                     
                      
                     out 
                   
                 
                 ) 
               
               
                 4 
                  
                 R 
                  
                 
                     
                 
                  
                 f 
               
             
           
         
       
     
         [0065]    The paths of the feedback current If are respectively input to the first to third switches S 1 , S 2 , and S 3  of the switch group  141 , and selected according to the first bit switching signal b 1  and the second bit switching signal b 2 . 
         [0066]    The feedback current If is in proportion to the reference current Iref according to the plurality of transistors of the current mirror unit  142  that are connected in parallel with each other. 
         [0067]    The relationship between the reference current Iref and the feedback current If that is selected on the basis of the first bit switching signal b 1  and the second bit switching signal b 2  is shown in Table as follows. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 b1 
                 b2 
                 If 
               
               
                   
                   
               
             
             
               
                   
                 on 
                 on 
                 3Iref 
               
               
                   
                 off 
                 on 
                 2Iref 
               
               
                   
                 on 
                 off 
                 Iref 
               
               
                   
                 off 
                 off 
                 x 
               
               
                   
                   
               
             
          
         
       
     
         [0068]    That is, the current mirror unit  142  mirrors the current having the same value as the reference current Iref. When the first switch S 1  is selected and the second and third switches S 2  and S 3  are not selected, the feedback current If that is equal to the reference current Iref is mirrored according to the plurality of transistors. When the first switch S 1  is not selected and the second and third switches S 2  and S 3  are selected, the feedback current If that is twice as large as the reference current Iref is mirrored according to the plurality of transistors. 
         [0069]    The adding unit  160  adds a feedback voltage corresponding to the above-described feedback current If and the second voltage V 2 out to output an output voltage Vout. The output voltage Vout can be obtained by the following equation. 
         [0000]        V out= V 2out+( b 1 ·Iref )+(2 b 1 ·Iref ) 
         [0070]    In this way, the voltage between the first voltage V 1 out and the second voltage V 2 out that are adjacent to each other is divided to extend the resolution. 
         [0071]    Referring to  FIG. 5 , it can be seen that the resolution is extended by dividing the voltage between the first voltage V 1 out and the second voltage V 2 out. 
         [0072]    Here, the voltage between the first voltage V 1 out and the second voltage V 2 out is divided into four voltages by using the second decoder  152 , which is the 2-bit decoder, the switch group  141  having the three switches S 1 , S 2 , and S 3 , and the current mirror unit  142  having the plurality of switches. However, when the number of bits of the second decoder  152 , the number of switches of the switch group  141 , and the number of transistors of the current mirror unit  142  are increased, the voltage between the first voltage V 1 out and the second voltage V 2 out can be divided to four or more voltages to thereby easily extend the resolution. Further, the divided voltages except for four voltages may be used as correction voltages, thereby obtaining accurate resolution. 
         [0073]    As set forth above, according to exemplary embodiments of the invention, in the resolution extension of the digital-analog converter that is used in the driving IC of the display, an increase in chip area can be prevented, the configuration of the circuit required for the resolution extension can be simplified, and correction of the resolution is performed to provided accurate resolution. 
         [0074]    While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.