Patent Document

CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of Korean Patent Application No. 2005-35773, filed on Apr. 28, 2005, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
       BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The embodiments of the present invention relate to an organic light emitting display and a method of driving the same. More specifically, the embodiments of the present invention relate to an organic light emitting display capable of reducing power consumption and memory requirements while controlling brightness in accordance with the requests of users and a method of driving this display.  
         [0004]     2. Discussion of Related Art  
         [0005]     Recently, various flat panel displays (FPD) having a lower weight and volume compared with cathode ray tubes (CRT) have been developed. In particular, light emitting displays having high emission efficiency, brightness, and response speed and large view angles are spotlighted.  
         [0006]     Light emitting displays are generally divided into organic light emitting displays using organic light emitting diodes (OLED) and inorganic light emitting displays using inorganic light emitting diodes. OLEDs include anode electrodes, cathode electrodes, and an organic emission layer positioned between the anode electrodes and the cathode electrodes to emit light by the combination of electrons and holes. The inorganic light emitting diode referred to as a light emitting diode (LED), unlike the OLED, includes an emission layer formed of inorganic material such as a PN-junction semiconductor material.  
         [0007]      FIG. 1  illustrates a conventional organic light emitting display. Referring to  FIG. 1 , the conventional organic light emitting display includes a display region  10 , a data driver  20 , and a scan driver  30 .  
         [0008]     The display region  10  includes pixels  11  each of which includes an OLED (not shown). The pixels  11  are formed in the regions partitioned by scan lines S 1  to Sn and data lines D 1  to Dm. The display region  10  receives power from a first power source ELVdd and a second power source ELVss from the outside. Each of the pixels  11  receives a scan signal, a data signal, the first power source ELVdd, and the second power source ELVss to display an image.  
         [0009]     The data driver  20  generates data signals. The data signals generated by the data driver  20  are supplied to the data lines D 1  to Dm in synchronization with scan signals to be transmitted to the pixels  11 .  
         [0010]     The scan driver  30  generates scan signals. The scan signals generated by the scan driver  30  are sequentially supplied to the scan lines S 1  to Sn.  
         [0011]     In the conventional organic light emitting display having the above structure, the larger the number of pixels  11  that emit light, the larger the amount of current that flows to the display region  10 . In particular, the larger the number of pixels  11  that display high gray scales among the pixels  11  that emit light, the larger the amount of current that flows to the display region  10 . Therefore, power consumption increases. Also, in the conventional organic light emitting display, the brightness of the light that is emitted corresponds only to data input from the outside and brightness cannot be changed responsive to the requests of users.  
         [0012]     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.  
       SUMMARY OF THE INVENTION  
       [0013]     Accordingly, embodiments of the present invention provide an organic light emitting display capable of reducing power consumption and memory requirements while controlling brightness in accordance with the requests of users and a method of driving the display.  
         [0014]     According to a first aspect of the present invention, there is provided an organic light emitting display including a data driver for supplying data signals corresponding to image data to data lines, a scan driver for sequentially supplying scan signals to scan lines and sequentially supplying emission control signals to emission control lines, a display region including one or more pixels receiving the data signals, the scan signals, and the emission control signals to display images, and a brightness controller for controlling the brightness of the display region. The brightness controller includes a first look up table in which first widths of the emission control signals corresponding to the image data of one frame period are stored and a second look up table in which change values for changing the widths of the emission control signals in accordance with outside input modes are stored.  
         [0015]     The brightness controller may include a data summing unit for summing the image data of one frame period to generate sum data and for generating at least two digital values including the uppermost bit of the sum data as control data, a mode selector for extracting the change values corresponding to the outside input modes, a controller for extracting the first widths corresponding to the values of the control data and for generating second widths of the emission control signals using the extracted first widths and change values, and the brightness control signal generator for generating brightness control signals corresponding to the second widths to transmit the brightness control signals to the scan driver. The scan driver controls the widths of the emission control signals in response to the brightness control signals. The first widths are selected so that the brightness of the display region is reduced accordingly as the values of the control data increase. The change values have change values of the widths of the emission control signals corresponding to the outside input modes. The controller generates the second widths by adding the first widths and the change values to each other or subtracting the change values from the first widths. The change values are selected as decimal values corresponding to the outside input modes. The controller generates the second widths by multiplying the first widths by the change values.  
         [0016]     According to another aspect of the present invention, there is provided an organic light emitting display including a data driver for supplying data signals to data lines, a brightness controller for controlling the brightness of a display region in response to image data of one frame period and outside input modes, a scan driver controlled by the brightness controller to generate emission control signals so that the brightness of the display region is controlled and to sequentially supply scan signals to scan lines, and the display region including pixels controlled by the data signals, the scan signals, and the emission control signals to generate light of predetermined brightness.  
         [0017]     According to another aspect of the present invention, there is provided a method of driving an organic light emitting display, the method including extracting one of the first widths of emission control signals using image data of one frame period, extracting one of the change values in response to outside input modes and generating second widths using the extracted first widths and change values, generating brightness control signals having the widths of emission control signals using the second widths, and generating the emission control signals in response to the brightness control signals.  
         [0018]     The first widths may be set so that the brightness of the display region is reduced as the values of the control data increase. The second widths are generated by adding the change values to the first widths or by subtracting the first widths from the change values. The second widths are generated by multiplying the change values by the first widths. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  illustrates a conventional organic light emitting display.  
         [0020]      FIG. 2  illustrates an organic light emitting display according to an embodiment of the present invention.  
         [0021]      FIG. 3  illustrates an example of a pixel illustrated in  FIG. 2 .  
         [0022]      FIG. 4A  illustrates waveforms that describe a method of driving the pixel illustrated in  FIG. 3 .  
         [0023]      FIG. 4B  illustrates waveforms that describe a method of driving the pixel illustrated in  FIG. 3 .  
         [0024]      FIG. 5  illustrates an embodiment of the brightness controller illustrated in  FIG. 2 .  
         [0025]      FIG. 6  illustrates an embodiment of the first look up table illustrated in  FIG. 5 .  
         [0026]      FIG. 7A  illustrates a first embodiment of the second look up table illustrated in  FIG. 5 .  
         [0027]      FIG. 7B  illustrates look up tables virtually generated by the second look up table illustrated in  FIG. 7A .  
         [0028]      FIG. 8A  illustrates a second embodiment of the second look up table illustrated in  FIG. 5 .  
         [0029]      FIG. 8B  illustrates a look up table virtually generated by the second look up table illustrated in  FIG. 8A .  
         [0030]      FIG. 9  is a graph illustrating brightness reduction curves in accordance with the look up tables illustrated in  FIGS. 7A and 8A .  
         [0031]      FIG. 10  illustrates another example of the pixel illustrated in  FIG. 2 .  
         [0032]      FIG. 11  illustrates waveforms that describe a method of driving the pixel illustrated in  FIG. 10 . 
     
    
     DETAILED DESCRIPTION  
       [0033]      FIG. 2  illustrates an organic light emitting display according to an embodiment of the present invention. The organic light emitting display of  FIG. 2  includes a display region  100 , a data driver  200 , a scan driver  300 , and a brightness controller  400 .  
         [0034]     The display region  100  includes pixels  110  each of which includes an OLED (not shown). The pixels  110  are formed in the regions partitioned by scan lines S 1  to Sn, emission control lines EM 1  to EMn, and data lines D 1  to Dm. The display region  100  receives power from a first power source ELVdd and a second power source ELVss located outside the organic light emitting display. Each of the pixels  110  receives a scan signal, an emission control signal, a data signal, power from the first power source ELVdd, and power from the second power source ELVss to display an image.  
         [0035]     The data driver  200  receives image data from the outside to generate data signals. The data signals generated by the data driver  200  are supplied to the data lines D 1  to Dm in synchronization with scan signals to be transmitted to the pixels  110 .  
         [0036]     The scan driver  300  generates scan signals and emission control signals. The scan signals generated by the scan driver  300  are sequentially supplied to the scan lines S 1  to Sn. The emission control signals generated by the scan driver  300  are sequentially supplied to the emission control lines EM 1  to EMn. The scan driver  300  receives brightness control signals from a brightness controller  400  to generate emission control signals having widths or durations corresponding to the brightness control signals.  
         [0037]     The brightness controller  400  generates a brightness control signal using a sum of the image data received for one frame period and a mode input by a user from the outside (hereinafter, referred to as an outside input mode). The brightness control signal generated by the brightness controller  400  is input to the scan driver  300  to control the brightness of the display region  100 .  
         [0038]      FIG. 3  illustrates an example of the pixel  110  illustrated in  FIG. 2 . For convenience sake, in  FIG. 3 , the pixel  110  that is coupled to the nth scan line Sn, the nth emission control line EMn, and the mth data line Dm is illustrated.  
         [0039]     The pixel  110  of the organic light emitting display according to the present invention includes a first transistor M 1 , a second transistor M 2 , a third transistor M 3 , a storage capacitor Cst, and an organic light emitting diode OLED.  
         [0040]     A first electrode of the first transistor M 1  is coupled to the data line Dm and a second electrode of the first transistor M 1  is coupled to a gate electrode of the second transistor M 2  and one terminal of the storage capacitor Cst. The first electrode and the second electrode may, for example, signify the source electrode and the drain electrode of the transistor, respectively. A gate electrode of the first transistor M 1  is coupled to the scan line Sn. The first transistor M 1  is turned on when a scan signal is supplied to the scan line Sn. The first transistor M 1  supplies the data signal supplied to the data line Dm to the storage capacitor Cst. As a result, a voltage corresponding to the data signal is charged in the storage capacitor Cst.  
         [0041]     The gate electrode of the second transistor M 2  is coupled to one terminal of the storage capacitor Cst. A first electrode of the second transistor M 2  is coupled to the first power source ELVdd and the other terminal of the storage capacitor Cst and a second electrode of the second transistor M 2  is coupled to a second electrode of the third transistor M 3 . The second transistor M 2  supplies a current corresponding to the voltage charged in the storage capacitor Cst from the first power source ELVdd to the second electrode of the third transistor M 3 .  
         [0042]     A gate electrode of the third transistor M 3  is coupled to the emission control line EMn. The second electrode of the third transistor M 3  is coupled to the second electrode of the second transistor M 2  and a first electrode of the third transistor M 3  is coupled to an anode electrode of the OLED. In response to the emission control signal, the third transistor M 3  is turned on to supply the current supplied from the second transistor M 2  to the OLED. In the exemplary embodiment shown, the third transistor M 3  is of a different conduction type from the first and second transistors M 1 , M 2 . For example, when the first and second transistors M 1 , M 2  are PMOS transistors, the third transistor M 3  is an NMOS transistor. Therefore, the polarity of the emission control signal that is used to turn on the NMOS third transistor M 3  is opposite to the polarity of the scan signal that turns on the PMOS first and second transistors M 1 , M 2 . In alternative embodiments, the third transistor M 3  may be of the same conduction type as the first and second transistors M 1 , M 2 .  
         [0043]      FIGS. 4A and 4B  illustrate waveforms that describe a method of driving the pixel  110  illustrated in  FIG. 3 . The brightness controller  400  controls brightness using the widths of the emission control signals EMl. The width of a signal is the duration of the signal. The brightness controller  400  generates a sum data by summing the image data of one frame period. When the value of the sum data is small, the brightness controller  400  selects the widths of the emission control signals EMl large so that the pixels  110  emit light for a sufficient time. Conversely, when the value of the sum data is large, the brightness controller  400  selects the widths of the emission control signals EMl small so that the brightness of the pixels  110  can be limited. The brightness controller  400  selects the widths of the emission control signals EMl large, also, when a user inputs an input mode not to limit the brightness of the display region  100  and selects the widths of the emission control signals EMl small, also, when the user inputs an input mode to limit the brightness of the display region  100 . Because in the pixel  110  illustrated in  FIG. 3 , the third transistor M 3  that is turned on by the emission control signals EMl is an n-type transistor, when the widths of the emission control signals EMl are large, the emission period of the OLED during one frame period  1 F becomes longer. Therefore, when the widths of the emission control signals EMl are large, a larger amount of current flows to the OLED during one frame period  1 F causing the pixel  110  to emit light for a longer time. When the value of the sum data is small or the user inputs the input mode not to limit the brightness of the display region  100 , the widths of the emission control signals EMl are selected equal to a first period T 1  as illustrated in  FIG. 4A . During the first period T 1  where the emission control signals EMl are supplied, the third transistor M 3  is turned on so that a predetermined current is supplied from the second transistor M 2  to the OLED. Therefore, the OLED emits light during the first period T 1 .  
         [0044]     When the value of the sum data is large or the user inputs the input mode to limit the brightness of the display region  100 , the brightness controller  400  selects the widths of the emission control signals EMl equal to a second period T 2  smaller than the first period T 1  as illustrated in  FIG. 4B  so that the brightness of the pixels  110  is limited. During the second period T 2  when the emission control signals EMl are supplied, the third transistor M 3  is turned on so that a predetermined current is supplied from the second transistor M 2  to the OLED. Therefore, the OLED emits light. In the case of the second period T 2 , because the widths of the emission control signals EMl are smaller than those of the first period T 1 , the portion of one frame period  1 F during which the OLED emits light is reduced. Therefore, a smaller amount of current flows to the OLED and the brightness of the display region  100  is limited to a predetermined value. The scan signals SS and the emission control signals EM are generated by the scan driver  300  and the data signals DATA are generated by the data driver  200  in response to a vertical synchronizing signal Vsync and a horizontal synchronizing signal Hsync.  
         [0045]      FIG. 5  illustrates an exemplary embodiment of the brightness controller  400  illustrated in  FIG. 2 . The brightness controller  400  includes a data summing unit  410 , a first look up table  420 , a mode selector  430 , a second look up table  440 , a controller  450 , and a brightness control signal generator  460 .  
         [0046]     The data summing unit  410  sums the image data input for one frame period  1 F to generate the sum data. The data summing unit  410  transmits at least two digital values (hereinafter, referred to as control data) including the uppermost bits, or the two most significant bits, of the sum data to the controller  450 . In one exemplary embodiment, the values of the upper 5 bits of the sum data are transmitted. That is, the control data include the values of 5 bits. When the value of the sum data is large, it means that a large number of image data having brightness values no less than a predetermined brightness are included. When the value of the sum data is small, it means that a small number of image data having brightness values no less than the predetermined brightness are included.  
         [0047]     The first look up table  420  stores the first widths EW 1  (first data) of the emission control signal corresponding to the values of the control data. The first widths EW 1  of the emission control signal are widths of the emission control signals EMl that control the emission times of the pixels  110 . The first widths EW 1  of the emission control signal stored in the first look up table  420  are selected so that the brightness of the display region  100  is reduced as the values of the control data increase.  
         [0048]     The mode selector  430  extracts change values EWd (second data) from the second look up table  440  in accordance with an outside input mode input by a user to transmit the change values EWd to the controller  450 .  
         [0049]     The second look up table  440  stores at least one change value EWd that includes the change data on the widths of the emission control signals EMl in accordance with the outside input mode. The outside input mode values stored in the second look up table  440  are selected to control the brightness of the display region  100  in accordance with the requests of users.  
         [0050]     The controller  450  extracts the first widths EW 1  of the emission control signal from the first look up table  420  using the control data received from the data summing unit  410 . The controller  450  receives the change values EWd from the mode selector  430 . The controller  450  generates the second widths EW 2  (third data) of an emission control signal using the first widths EW 1  of the emission control signal and the change values EWd. The second widths EW 2  of the emission control signal are obtained by modifying the first widths EW 1  of the emission control signal by the change values EWd and are signal widths of the emission control signals EMl generated by the scan driver  300 .  
         [0051]     The controller  450  subtracts the change values EWd from the first widths EW 1  of the emission control signal to generate the second widths EW 2  of the emission control signal. As a result, the second widths EW 2  of the emission control signal will be smaller as the first widths EW 1  of the emission control signal get smaller and as the change values EWd get larger. The predetermined widths by which the emission control signals EMl are to be reduced are stored in the second look up table  440  as the change values EWd. On the other hand, when the brightness of the display region  100  is to be increased in accordance with the outside input mode, the controller  450  adds the first widths EW 1  of the emission control signal and the change values EWd to each other to generate the second widths EW 2  of the emission control signal. In this case, the second widths EW 2  of the emission control signals are selected to be larger than the first widths EW 1  by the amount of the change values EWd.  
         [0052]     In another embodiment, the controller  450  multiplies the first widths EW 1  of the emission control signal by the change values EWd to generate the second widths EW 2  of the emission control signal. In this embodiment, the change values EWd stored in the second look up table  440  are decimal values that are the ratios of the second widths EW 2  of the emission control signal to be generated to the first widths EW 1  of the emission control signal. When the brightness of the display region  100  is to be increased, the change values EWd are larger than 1. When the brightness of the display region  100  is to be limited, the change values EWd are decimal values no more than 1. When the change values EWd are decimal values no more than 1, the second widths EW 2  of the emission control signal will be smaller as the first widths EW 1  of the emission control signal become smaller and as the change values EWd become smaller. The second widths EW 2  of the emission control signal generated by the controller  450  are transmitted to the brightness control signal generator  460 .  
         [0053]     The brightness control signal generator  460  generates brightness control signals corresponding to the second widths EW 2  of the emission control signal received from the controller  450 . The brightness control signals generated by the brightness control signal generator  460  are input to the scan driver  300 . The scan driver  300  that receives the brightness control signals generates the emission control signals EMl having widths determined by the brightness control signals.  
         [0054]      FIG. 6  illustrates an embodiment of the first look up table  420  illustrated in  FIG. 5 . The contents stored in the first look up table  420  may vary according to the resolution and size of the display region  100 .  
         [0055]     The first widths EW 1  of the emission control signal corresponding to the values of the upper 5 bits of the sum data, that form the control data, are stored in the first look up table  420 . The first widths EW 1  of the emission control signal become smaller as the values of the control data get larger so that the brightness and the resulting power consumption can be limited within a certain range. When the control data have at least one value including a minimum value, the first widths EW 1  of the emission control signal are maintained uniform. The control data have a value including the minimum value when the values of the upper 5 bits of the sum data are limited to 0, 1, 2, 3, 4, or 5. In other words, when the upper 5 bits of the sum data are “00000”, “00001”, “00010”, “00011”, or “00100”then the control data have a value including the minimum value.  
         [0056]     When the control data have a value no more than 4 (less than or equal to 4), the first widths EW 1  of the emission control signal amount to 325 periods of the horizontal synchronizing signal Hsync and brightness is not limited. In the case where the control data have at least one value including the minimum value as described above, then the first widths EW 1  of the emission control signal are not limited and contrast does not deteriorate when dark images are displayed. Therefore, it is possible to display images with a desirable contrast using a low value for the control data.  
         [0057]     When the control data have values no less than 5 (greater than or equal to 5), the first widths EW 1  of the emission control signal are gradually reduced as the values of the control data increase. In the case where the control data have at least one value larger than the minimum value of 4, the first widths EW 1  of the emission control signal are reduced and the brightness is reduced so that it is possible to maintain power consumption within a certain range. Limiting the brightness of the display region  100  makes it possible to prevent the eyes of a user from getting tired when the user watches a screen for a long time. Because the values of the control data increase as the number of pixels that display high gray scales increases, the ratio of limiting the brightness increases.  
         [0058]     In order to prevent the brightness from being excessively limited, the maximum ratio for limiting the brightness is selected as 34% so that the brightness is no less than 34% of maximum brightness even when the pixels  110  that display high gray scales occupy most of the area of the display region  100 . That is, when at least one value of the control data is the maximum value, the first widths EW 1  of the emission control signal are no less than a predetermined width. The look up table  420  in this case may be applied to moving images. The range at which the brightness is limited when the images displayed by the organic light emitting display are moving images is different from the range at which the brightness is limited when the images displayed by the organic light emitting display are still images. For example, in the case of the still images, the maximum ratio of limiting the brightness may be 50%.  
         [0059]      FIG. 7A  illustrates a first embodiment of the second look up table  440  illustrated in  FIG. 5 . The contents stored in the second look up table  440  may vary in accordance with the resolution and size of the display region  100 .  
         [0060]     The second look up table  440  stores the change values EWd corresponding to the outside input mode values received from the mode selector  430 . The change values EWd are the degree to which the widths of the emission control signals EMl are to be reduced. The second widths EW 2  of the emission control signal are generated by subtracting the change values EWd from the first widths EW 1  of the emission control signal. At least two outside input modes may be set and four outside input modes are selected for convenience sake according to the first embodiment of the present invention. For example, the outside input mode in which the brightness of the display region  100  is maximally limited is referred to as a super power saving mode and is selected as 0. The change value EWd when the outside input mode is 0 is selected as the value corresponding to the 70 periods of the horizontal synchronizing signal Hsync. The outside input mode 1 is referred to as a power saving mode and the change value EWd is selected as the value corresponding to the 40 periods of the horizontal synchronizing signal Hsync. The outside input mode 2 is referred to as a normal mode and the change value EWd is selected as the value corresponding to the 10 periods of the horizontal synchronizing signal Hsync. Finally, the outside input mode 3 is referred to as a bright mode and the change value EWd is selected as 0. As described above, the change values are reduced as the outside input mode values increase. Because the change value EWd is 0 when the outside input mode value is 3, that is the maximum value among the four outside input mode values, the brightness of the display region  100  is not limited. When the outside input mode is 3, the second widths EW 2  of the emission control signal are selected to be equal to the first widths EW 1  of the emission control signal. In such a case, the first widths EW 1  of the emission control signal are not reduced and the contrast of the display region  100  is maintained. With outside input mode 3, or bright mode, it is possible to display images with a desirable contrast.  
         [0061]     When the outside input modes are no more than 2, the second widths EW 2  of the emission control signal are reduced from the first widths EW 1  of the emission control signal by the change values EWd so that the brightness of the display region  100  is limited. For example, when the outside input mode is 2, the number of periods of the horizontal synchronizing signal Hsync for the second widths EW 2  of the emission control signal has 10 periods fewer than the horizontal synchronizing signal Hsync corresponding to the first widths EW 1  of the emission control signal. When the second widths EW 2  of the emission control signal are smaller than the first widths EW 1  of the emission control signal as described above, the widths of the emission control signals EMl generated by the scan driver  300  are selected to be smaller. Therefore, the brightness of the display region  100  is reduced so that it is possible to maintain power consumption in a certain range and to prevent the eyes of a user from getting tired. Also, according to the present invention, the outside input modes are at least two and may vary so as to satisfy the requests of users. When the number of outside input modes stored in the second look up table increases, as illustrated in  FIG. 7B , virtual look up tables corresponding to the number of outside input modes are generated. Therefore, a plurality of virtual look up tables are generated using one first look up table  420  so that it is possible to variably set the brightness of the display region  100 . The virtual look up tables make it possible to save memory used for the look up table.  
         [0062]      FIG. 7B  illustrates look up tables virtually generated by the second look up table illustrated in  FIG. 7A .  
         [0063]     When four outside input modes 0 to 3 are selected, three. virtual look up tables are generated corresponding to outside input modes 0, 1, and 2 &lt;Mode0&gt;, &lt;Mode1&gt;, &lt;Mode2&gt;. As explained before, the second widths EW 2  of the emission control signal are generated by subtracting the change values EWd from the first widths EW 1  of the emission control signal. Therefore, in the case of the outside input mode 3 where the change value EWd is 0, the second widths EW 2  of the emission control signal in accordance with the control data are selected to be equal to the first widths EW 1  of the emission control signal. The look up table corresponding to outside input mode 3 is the same as the first look up table corresponding to outside input mode 0 &lt;Mode0&gt;.  
         [0064]     In the cases of the outside input modes 0, 1, and 2 &lt;Mode0&gt;, &lt;Mode1&gt;, &lt;Mode2&gt;, the virtual look up tables in which the second widths EW 2  of the emission control signal obtained by subtracting the change values EWd from the first widths EW 1  of the emission control signal are stored are generated according to the respective modes. In  FIG. 7B , in order to show that the look up tables are virtual, the three virtual look up tables are dot-lined. In this case, the display region  100  has four brightness reduction curves. Although the brightness reduction curves corresponding to all the outside input modes are not included, a plurality of brightness reduction curves corresponding to the number of outside input modes are generated based on the brightness reduction curve generated by the first look up table. Therefore, it is possible to satisfy various requests of users while saving memory.  
         [0065]      FIG. 8A  illustrates a second embodiment  440 ′ of the second look up table  440  illustrated in  FIG. 5 . The contents stored in the second look up table  440 ′ may vary according to the resolution and size of the display region  100 .  
         [0066]     The second look up table  440 ′ stores the change values EWd corresponding to the outside input modes received from the mode selector  430 . The change values EWd are the ratios of the second widths EW 2  of the emission control signal with respect to the first widths EW 1  of the emission control signal. The second widths EW 2  of the emission control signal generated by the controller  450  are obtained by multiplying the first widths EW 1  of the emission control signal by the change values EWd shown in  FIG. 8A . The change values EWd are greater than 1 when the brightness of the display region  100  is to be increased and are decimal values no more than 1 when the brightness of the display region  100  is to be limited. According to the exemplary embodiment shown, the change values EWd are selected as the decimal values no more than 1 to limit the brightness of the display region  100 . As the change values EWd become smaller, the second widths EW 2  of the emission control signal also become smaller.  
         [0067]     For convenience sake, the second embodiment is selected to include four outside input modes. For example, the outside input mode in which the brightness of the display region  100  is maximally limited is referred to as a super power saving mode and is selected as 0. The change value EWd for the outside input mode 0 is selected as 0.5. The outside input mode 1 is referred to as a power saving mode and its change value EWd is selected as 0.7. The outside input mode 2 is referred to as a normal mode and its change value EWd is selected as 0.9. Finally, the outside input mode 3 is referred to as a bright mode and its change value EWd is selected as 1. As the outside input mode values increase, the change values EWd also increase. Because the change value EWd is 1 when the outside input mode is 3, which is the maximum value among the four outside input mode values, the brightness of the display region  100  is not limited with outside input mode 3. When the outside input mode is 3, the second widths EW 2  of the emission control signal are selected to be equal to the first widths EW 1  of the emission control signal. In such a case, the first widths EW 1  of the emission control signal are not reduced and the contrast of the display region  100  does not deteriorate. Therefore, it is possible to display images with desirable contrast.  
         [0068]     When the outside input modes are no more than 2, the second widths EW 2  of the emission control signal are obtained by multiplying the first widths EW 1  of the emission control signal by the change values EWd smaller than 1 so that the brightness of the display region  100  is limited. For example, when the outside input mode is 2, the second widths EW 2  of the emission control signal are obtained by multiplying the first widths EWI of the emission control signal by 0.9. When the second widths EW 2  of the emission control signal are smaller than the first widths EW 1  of the emission control signal, the widths of the emission control signals EM 1  generated by the scan driver  300  are selected to be small. Therefore, the brightness of the display region  100  is reduced so that it is possible to maintain power consumption in a certain range and to prevent tiring the eyes of a user. Also, according to the present invention, the outside input modes are at least two and may be set to a different value in order to satisfy the requests of users. When the number of outside input modes stored in the second look up table increases, as illustrated in  FIG. 8B , virtual look up tables are generated to correspond to the number of outside input modes. Therefore, a plurality of virtual look up tables are generated using the first look up table  420  so that it is possible to vary the brightness of the display region  100 . Therefore, it is possible to save the memory used for the look up table.  
         [0069]      FIG. 8B  illustrates look up tables virtually generated by the second look up table  440 ′ illustrated in  FIG. 8A .  
         [0070]     When four outside input modes 0, 1, 2, and 3 are selected, three virtual look up tables are generated &lt;Mode0&gt;, &lt;Mode1&gt;, &lt;Mode2&gt;. Because the second widths EW 2  of the emission control signal are generated by multiplying the first widths EW 1  of the emission control signal by the change values EWd, in the case of the outside input mode 3 where the change value EWd is 1, the second widths EW 2  of the emission control signal in accordance with the control data are selected to be equal to the first widths EW 1  of the emission control signal. Then, the look up table for this outside input mode is the same as the first look up table. For the outside input modes 0, 1, and 2, the virtual look up tables, in which the second widths EW 2  of the emission control signal generated by multiplying the first widths EW 1  of the emission control signal by the change values EWd are stored, are generated in accordance with the respective outside input modes. In this case, the display region  100  has four brightness reduction curves. While not all the lookup tables are shown, a plurality of brightness reduction curves corresponding to the number of outside input modes are generated based on the brightness reduction curve generated by the first look up table. Therefore, it is possible to satisfy various requests of users while saving memory.  
         [0071]      FIG. 9  is a graph illustrating brightness reduction curves in accordance with the second look up tables  440 ,  440 ′ illustrated in  FIGS. 7A and 8A .  
         [0072]     The maximum brightness of the display region  100  is reduced as the effective emission areas increase in the corresponding outside input modes. The effective emission areas are the areas of the pixels  110  that emit light with brightness no less than predetermined brightness. In outside input mode 3 where the brightness of the display region  100  is selected to be large, the brightness of the display region  100  is limited by the emission control signals EMl whose widths are equal to the first widths EW 1  of the emission control signal. Because the effective emission areas increase as the values of the control data increase, the ratio of limiting the brightness of the display region  100  increases. In outside input modes 0, 1, and 2, the brightness is more limited than outside input mode 3 by a predetermined value. As a result, the brightness reduction curves for these outside input modes are below the curve of the outside input mode 3 by the same predetermined value. That is, according to embodiments of the present invention, the first widths EW 1  of the emission control signal corresponding to the different outside input modes are changed with respect to the first width EW 1  corresponding to the outside input mode 3, in order to generate the plurality of brightness reduction curves corresponding to the outside input modes 0, 1, and 2. As described above, the display region  100  has different brightness reduction curves corresponding to at least two outside input modes.  
         [0073]     On the other hand, the pixel  110  of the organic light emitting display according to an alternative embodiment of the present invention may have the circuit  110 ′ illustrated in  FIG. 10 . In the circuit of the alternative pixel  110 ′ conduction type of the third transistor M 3  that is turned on by the emission control signal EMl may be the same as the conduction type of the first and second transistors M 1 , M 2 . For example, the first, second, and third transistors M 1 , M 2 , M 3  may be all PMOS. In this case, the operation processes illustrated in  FIG. 11  are the same as the operation processes of the pixel  110  illustrated in  FIGS. 3, 4A , and  4 B excluding that the OLED emits light in the periods where the emission control signals EMl are not being applied. Therefore, a detailed description of the operation of this alternative pixel  110 ′ is omitted.  
         [0074]     According to the organic light emitting display of the present invention and the method of driving the same, it is possible to variably set the outside input modes in response to the requests of users and to change the brightness of the display region while saving memory. Also, when the brightness of the display region is limited, it is possible to reduce power consumption and to prevent the eyes of a user from getting tired. When the brightness is not being limited, it is possible to achieve a desirable contrast for the display region.  
         [0075]     Although exemplary embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.

Technology Category: 3