Patent Publication Number: US-8970642-B2

Title: Display device and driving method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2011-0120912 filed in the Korean Intellectual Property Office on Nov. 18, 2011, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The disclosed technology relates to a display device and a driving method thereof, and more particularly, to an organic light emitting diode (OLED) display, to which current consumption reduction technology is applied and a driving method thereof. 
     2. Description of the Related Technology 
     A display device has a display area with a plurality of pixels on a substrate in a matrix and has scan lines and data lines connected to each pixel to selectively apply data signals to the pixels, so as to display images. Display devices may, for example, be either a passive matrix light emitting display device or an active matrix light emitting display device depending on the method of driving the pixels. Many display devices are active matrix light emitting display devices in which unit pixels have high resolution, contrast, and operation speed. 
     Such display devices are used in personal computers, mobile phones, portable information terminals such as PDAs, etc., or in various other information devices. Common types of display technologies include, for example, a liquid crystal display (LCD), an organic light emitting diode (OLED) display, and a plasma panel (PDP). Recently, various light emitting display devices with low weight and volume, as compared to the cathode ray tubes, have been developed. In particular, OLED displays having high luminance efficiency, preferable luminance and viewing angles, and quick response speed are attracting attention. 
     In the OLED displays, a control method of automatically controlling current (Automatic Current Limit, hereinafter, referred to as ‘ACL’) to lower luminance on the display when the entire screen is lighted at high luminance by video signals in one frame, is used to reduce power consumption. The aforementioned ACL method includes summing all data values for a frame of data on an organic light emitting display panel to determine an average luminance value of the organic light emitting display panel, adjusting a light emission period depending on the luminance value, or changing the image data to control driving current. However, it is hard to apply the aforementioned ACL method because the data to be summed is different from the data for the data rendering technology which has been variously developed for display devices, or optical characteristics of image quality displayed after data rendering may not be guaranteed. Therefore, it is necessary to develop an improved ACL method to be integrated with the data rendering technology and applicable. 
     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 CERTAIN INVENTIVE ASPECTS 
     One inventive aspect is a display device. The device includes a display unit including a plurality of pixels connected to a plurality of scan lines, a plurality of light emission control lines, and a plurality of data lines, where each pixel is configured to emit light with a driving current corresponding to image data signals transmitted through the data lines during a light emission period based on light emission control signals transmitted through the light emission control lines. Each of the pixels includes a plurality of subpixels, each configured to display one of a plurality of colors. The device also includes a controller configured to convert external video signals to image data signals, output the converted signals to a data driver, generate light emission driving control signals for controlling the light emission duty ratio of the light emission control signals, and calculate the pixel-on-ratio for each subpixel to reduce the driving current for displaying images according to the pixel on ratio of the subpixel. 
     Another inventive aspect is a method of driving a display device, the device including a display unit including a plurality of pixels emitting light with driving current corresponding to image data signals during a light emission period depending on light emission control signals transmitted through a plurality of light emission control lines. The Method includes converting input video signals to first image data signals, calculating the pixel on ratio for each subpixel displaying one of a plurality of colors according to the first image data signals transmitted each frame, and reducing the driving current based on the pixel on ratio of the subpixels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a display device according to an exemplary embodiment. 
         FIG. 2  is a circuit diagram showing a pixel circuit of  FIG. 1 . 
         FIG. 3  is a block diagram showing in detail the configuration of a controller shown in  FIG. 1  according to an exemplary embodiment. 
         FIG. 4  is a block diagram showing in detail the configuration of the controller shown in  FIG. 1  according to another exemplary embodiment. 
         FIG. 5  is a block diagram showing the specific configuration of an automatic current limiter of  FIG. 4 . 
         FIG. 6  is a flow chart showing a method of automatically limiting current of a display device according to an exemplary embodiment. 
         FIG. 7  is a flowchart showing in detail the process at S 100  of  FIG. 6  according to an exemplary embodiment. 
         FIG. 8  is a graph showing the effect of reducing luminance by use of the method of automatically limiting current in a display device according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Various aspects are described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. 
     Further, in the embodiments, like reference numerals generally designate like elements throughout the specification representatively in a first exemplary embodiment and, in some cases, only elements other than those of the first exemplary embodiment are described. The drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals generally designate like elements throughout the specification. 
     Throughout this specification and the claims that follow, when it is described that an element is “coupled” to another element, the element may be “directly coupled” to the other element or “electrically coupled” to the other element through a third element. In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising,” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
       FIG. 1  is a block diagram of a display device according to an exemplary embodiment. Referring to  FIG. 1 , the display device includes a display unit  10  including a plurality of pixels  60 , a scan driver  20 , a data driver  30 , a light emission control driver  40 , a controller  50 , and a power supply for supplying a first voltage ELVDD and a second voltage ELVSS. 
     The display unit  10  includes a plurality of signal lines S 1 -S n , D 1 -D m , and EM 1 -EM n  and a plurality of pixels connected to the plurality of signal lines S 1 -S n , D 1 -D m , and EM 1 -EM n  and arranged in a matrix format. The signal lines S 1 -S n , D 1 -D m , and EM 1 -EM n  include a plurality of scan lines S 1 -S r , for transmitting scan signals, a plurality of data lines D 1 -D m  for transmitting data signals, and a plurality of light emission control lines EM 1 -EM n  for transmitting light emission control signals. The scan lines S 1 -S n  and the light emission control lines EM 1 -EM n  extend in a substantial row direction and are substantially parallel to each other, and the data lines D 1 -D m  extend in a substantially column direction and are substantially parallel to each other. 
       FIG. 1  illustrates only a pixel (PXiij)  60  formed at the point where the i-th scan line Si, the j-th data line Dj and the i-th light emission control line EMi cross each other. The pixel circuit PXiij includes a light emitting diode (for example, organic light emitting diode (OLED)). The light emitting diode is connected to the power supply for supplying the first voltage ELVDD and the second voltage ELVSS. Specifically, one end and the other end of OLED are electrically connected to the first voltage ELVDD and the second voltage ELVSS, respectively, and the OLED emits light depending on the current flowing between the two terminals. Here, the current flowing between the two terminals of the light emitting diode is referred to as driving current (Ioled). 
     Each pixel circuit generates and transmits driving current (Ioled) to the OLED depending on the image data signals, the first voltage ELVDD and the second voltage ELVSS, and the OLED emits light at brightness proportional to the driving current (Ioled). Here, the first voltage ELVDD may be a voltage higher than the second voltage ELVSS. 
     The scan driver  20  generates and transmits a plurality of scan signals to the scan lines S 1 -S n  depending on the scan driving control signals CONT 3  transmitted from the controller  50 , respectively. That is, the scan driver  20  applies scan signals to the display unit  10  every specific cycle (for example, horizontal synchronization signal Hsync cycle), which is controlled by the scan driving control signals CONT 3 . The plurality of scan signals are signals for transmitting signals for activating pixels to one of the plurality of scan lines to transmit the image data signals to the pixel circuit. 
     The data driver  30  receives a plurality of image data signals DATA 2  and DATA 2 ′ transmitted from the controller  50 , generates and transmits a plurality of image data signals to pixel row by pixel row through the plurality of data lines D 1 -D m . That is, the data driver  30  applies image data signals DATA 2  and DATA 2 ′ to the display unit  10  for every specific cycle (for example, vertical synchronization signal Vsync cycle), which is controlled by the data driving control signals CONT 2  transmitted from the controller  50 . 
     In this case, the image data signals applied by the data driver  30  may be the image data signal DATA 2  primarily converted from an external video signal DATA 1 , according to the exemplary embodiment of the method of ACL, or the image data signal DATA 2 ′ obtained by compensating for the luminance of the primarily converted image data signal DATA 2  again. 
     The light emission control driver  40  generates and transmits a plurality of light emission control signals to the light emission control lines EM 1 -EMn depending on the light emission driving control signals CONT 1  transmitted from the controller  50 . That is, the light emission control driver  40  applies light emission control signals to the display unit  10  for every specific cycle (for example, horizontal synchronization signal Hsync cycle), which is controlled by the light emission driving control signals CONT 1 . The plurality of light emission control signals are used for controlling the light emission duty of pixels on one of the plurality of light emission control lines. 
     That is, the light emission duty ratio of the plurality of light emission control signals is controlled by the light emission driving control signals CONT 1  including off duty width information of the pulse calculated to apply the ACL technology according to the exemplary embodiment. 
     The controller  50  receives image data signals DATA 1 , horizontal synchronization signals Hsync, vertical synchronization signals Vsync and main clock signals MCLK which are transmitted from the outside, and outputs image data signals DATA 2  and DATA 2 ′ converted to correspond to scan driving control signals CONT 3 , data driving control signals CONT 2 , light emission driving control signals CONT 1  and image data signals DATA 1 , required for displaying images in the display unit  10  depending on the image data signals DATA 1 . Here, the image data signals DATA 1  include a plurality of grayscale data for controlling luminance of each of the plurality of pixels. The image data signals DATA 1  correspond to the color display signals (RGB signals) corresponding to each color when the plurality of pixels included in the display unit  10  are composed of subpixels for displaying basic  3  primary colors R, G, and B, respectively. 
     If the structure of the plurality of pixels included in the display unit  10  is the PenTile structure in which the ratio of the different number of red, blue and green subpixels is 1:1:2 (RGBG), not an ordinary arrangement scheme of subpixels for displaying primary three colors of red, green and blue, the controller  50  outputs the image data signals DATA 2 , DATA 2 ′ which are the converted version of the image data signals DATA 1 , that is, RGB signals, to correspond to the PenTile structure. 
     Meanwhile, the controller  50  may include a data converter  51  and an automatic current limiter  52  in order to apply ACL technology to the PenTile structure of the display device as described above. 
     The data converter  51  changes the image data signals DATA 1  input from the outside to the image data signals DATA 2  through the PenTile algorithm in order to apply the image data signals DATA 1  to the PenTile structure. This is to convert the image data signals DATA 1 , that is, RGB signals, to correspond to the PenTile structure. In this case, the image data signals DATA 2  are the signals primarily converted through the algorithm (for example, L6 algorithm) of the data converter  51 . That is, for example, an image data signal transmitted to the first RGB pixel is converted to be transmitted to the RG 1  pixel, and an image data signal transmitted to the second RGB pixel is converted to be transmitted to the BP 2  pixel. 
     According to the ACL technology, the image data signals DATA 1  may primarily be converted to be modulated to image data signals DATA 2 , and may then be subject to the compensation value calculation process of RGB data once again in order to reduce luminance above a predetermined level of reference luminance. The controller  50  may thus output compensated image data signals DATA 2 ′. 
     The automatic current limiter  52  receives the image data signals DATA 2  converted to be applied to the PenTile structure, and calculates the pixel on ratio for each subpixel with the received image data signals. It is not necessary that the automatic current limiter  52  includes a pixel on ratio calculator structure for calculating the pixel on ratio, provided that the controller  50  includes the pixel on ratio calculator structure. 
     The automatic current limiter  52  applies the power consumption reduction scheme selected for the image data signals DATA 2  converted in the data converter  51  according to an exemplary embodiment of a driving method of a display device. 
     It is possible to convert the image data signals DATA 2 ′ compensated by reducing luminance above the predetermined level of reference luminance to correspond to the image data signals converted in the data converter  51  according to the exemplary embodiment. According to another exemplary embodiment, it is possible to calculate an off duty width for light emission control of the light emission control signals by using the image data signals DATA 2  converted to be applied to the PenTile structure. That is, the greater the off duty width of the light emission control signals is, the longer the period of time is, in which light emission of each pixel in the display unit  10  is blocked, and the smaller power consumption is. The automatic current limiter  52  may calculate the aforementioned off duty width. The specific configuration and resulting functions of certain embodiments of the controller  50  will be described hereinafter with reference to the drawings. 
     Referring to  FIG. 1 , the controller  50  is constructed to include the data converter  51  and the automatic current limiter  52 , but the configuration is not limited thereto. Of course, other structures may be further included for generating control signals in order to externally receive video signals, synchronous signals, and clock signals to display images, although not shown in  FIG. 1 . 
       FIG. 2  is a circuit diagram showing the pixel  60  circuit of  FIG. 1 . The circuit diagram shown in  FIG. 2  is an exemplary circuit diagram of a pixel (PXiij)  60  at a point where the i-th pixel row and the j-th pixel column meet among a plurality of pixels constructing the display unit  10  constructed in a matrix with n pixel rows and m pixel columns. 
     Referring to  FIG. 2 , the pixel(PXiij)  60  is connected to i-th scan line, the i-th light emission control line EMi and the j-th data line Dj, and includes an organic light emitting diode (OLED), a driving transistor M 1 , a capacitor Cst, a switching transistor M 2  and a light emission control transistor M 3 . 
     The driving transistor M 1  includes a gate connected to the switching transistor M 2 , one end connected to the first voltage ELVDD, i.e., driving voltage, source, and the other end connected to an anode electrode of the OLED. In further detail, the other end of the driving transistor M 1  is connected to one end of the light emission control transistor M 3  and connected to the OLED through the light emission control transistor M 3  as well. The driving transistor M 1  delivers driving current (Ioled) of which the scale is different depending on the voltage across the gate and the other end to OLED. 
     The switching transistor M 2  includes a gate connected to the scan line Si, one end connected to the data line Dj, and the other end connected to the gate of the driving transistor M 1 . The switching transistor M 2  transmits data voltage depending on the corresponding image data signal Vdata[j] applied to the data line Dj to the gate of the driving transistor M 1  when the switching transistor M 2  is turned on in response to the scan signal scan[i] applied to the scan line Si. 
     The capacitor Cst includes one electrode connected to the gate of the driving transistor M 1  and the other electrode connected to the first voltage ELVDD source. The data voltage transmitted to the gate of the driving transistor M 1  is applied to the one electrode of the capacitor Cst through the switching transistor M 2 , and the first voltage ELVDD is applied to the other electrode thereof. Therefore, the value of a voltage as much as the difference in the voltage across both electrodes of the capacitor Cst is charged and is maintained after the switching transistor M 2  is turned off. 
     The light emission control transistor M 3  includes a gate connected to the light emitting signal line EMi, one end connected to the other end of the driving transistor M 1 , and the other end connected to the anode electrode of the OLED. The light emission control transistor M 3  receives the light emission control signal EM[i] through the light emitting signal line EMi and is thus selectively turned on, such that the light emission control transistor M 3  serves to supply current (Ioled) flowing across the driving transistor M 1  to the OLED. According to the exemplary embodiment, the light emission control transistor M 3  is controlled to be turned on/off depending on the predetermined light emission control signals established to have a duty width in an off duty width calculated according to the algorithm of ACL scheme to correspond to the output image data signals applied to the PenTile scheme. Therefore, the light emitting time for displaying images in the OLED is adjusted. 
     The OLED includes an anode electrode connected to the other end of the light emission control transistor M 3 , and a cathode electrode connected to the second voltage ELVSS source. The OLED emits light at an intensity different depending on the driving current (Ioled) corresponding to the data signal Vdata[j] supplied by the driving transistor M 1  through the light emission control transistor M 3 , in order to display images. 
     The OLED may emit light in one color among the basic primary colors. Exemplary basic colors may include three primary colors of red, green and blue, and a desired color may be displayed according to a spatial sum or a temporal sum of these three primary colors. The display device according to the exemplary embodiment is a display device in the PenTile structure in which one pixel displays a spatial sum of the basic primary colors of red, first green, blue, and second green. Therefore, the pixel structure shown in  FIG. 2  illustrates the circuit structure of a subpixel displaying one among, for example, two, three, four, or more basic primary colors. 
     The driving transistor M 1 , the switching transistor M 2  and the light emission control transistor M 3  may be a p-channel field effect transistor (FET). However, the driving transistor M 1 , the switching transistor M 2  and the light emission control transistor M 3  are not limited thereto, and at least one of M 1 , M 2  and M 3  may be an n-channel field effect transistor. Connection among the transistors M 1 , M 2 , and M 3 , the capacitor Cst and the OLED may be varied, provided that the circuit element can carry out the same role. The pixel  60  shown in  FIG. 2  is an exemplary pixel of the display device, and a different type of pixel including at least two transistors or at least one capacitor may be used. 
       FIG. 3  and  FIG. 4  are block diagrams showing in detail an exemplary configuration of the controller  50  shown in  FIG. 1  according to an exemplary embodiment. 
     That is,  FIG. 3  shows a configuration of the controller  50  to carry out the ACL scheme appropriately applied to the PenTile structure of the display, provided that the ACL scheme is to adjust the off duty ratio of the light emission control signals. 
     First, referring to  FIG. 3 , the controller  50  includes the data converter  51  which receives the initial external image data signals DATA 1  which are RGB signals to convert the signals to the image data signals DATA 2  corresponding to the PenTile type in which the pixel structure in the display unit is an RGBG structure. Since the ACL scheme according to the exemplary embodiment of  FIG. 3  is to adjust the off duty width of the light emission control signals, the controller  50  of  FIG. 3  includes the automatic current limiter  52  for carrying out calculation with the ACL algorithm to be appropriate for the PenTile technology. 
     Although not shown in  FIG. 3 , the automatic current limiter  52  includes a structure for receiving the image data signals DATA 2  converted in the data converter and calculating the pixel on ratio of a pixel, a first operation unit structure for executing an algorithm for calculating the off duty ratio of the light emission control signals to correspond to the pixel on ratio, and a second operation unit configured to develop luminance equations, calculate the compensated luminance value from the luminance data of summed image data to correspond to a luminance equation selected on the basis of pixel on ratio for each subpixel, and compensate for the image data signals DATA 2 . The structure for calculating the pixel on ratio is not included in the automatic current limiter  52  as shown in  FIG. 3 , but may be disposed as an independent constituent element belonging to the controller  50  according to another exemplary embodiment. 
     The image data signals converted in the data converter  51  are modulated so that the RGB signals can be appropriate for the PenTile structure, and the pixel on ratio (POR) of a pixel is found with reference to the data signals modulated in the structure for calculating the pixel on ratio of the pixel in the automatic current limiter  52 . 
     The pixel on ratio of the pixel is found, by finding and summing up the pixel on ratio depending on the image data signals for each subpixel displaying the basic primary colors per frame. 
     The pixel on ratio for each subpixel is a ratio of the number of subpixels activated in a turned-on state to the entire number of subpixels displaying the basic primary colors, respectively, in one frame. That is, it is possible to decide on/off for each subpixel for each basic primary color in the structure for calculating pixel on ratio according to a digital signal to find the pixel on ratio for each subpixel. For example, since the converted image data signals DATA 2  are the RG 1 BG 2  signals applied to the PenTile structure, it is possible to find the pixel on ratio (PORr) for the red (R) signals, the pixel on ratio (PORg 1 ) for the first green (G 1 ) signals, the pixel on ratio (PORb) for blue (B) signals, and the pixel on ratio (PORg 2 ) for the second green (G 2 ) signals, respectively. The pixel on ratio (PORr) for the red (R) signals is a ratio for the number of subpixels for displaying red signals activated and turned on to the entire number of subpixels displaying the red signals per frame. The same concept is applied to the pixel on ratios for the remaining basic primary color signals. 
     Thereafter, the pixel on ratio for each subpixel is summed up to find the pixel on ratio of the entire pixels per frame. In this case, the pixel on ratio of the subpixel displaying green is the average of the pixel on ratio of the subpixel displaying the first green and the pixel on ratio of the subpixel displaying the second green. This can be expressed as the following Equation 1.
 
POR=POR r +POR b +(POR g 1+POR g 2)/2,  (Equation 1)
 
wherein
         POR: pixel on ratio of the entire pixels   PORr: pixel on ratio for the red (R) signals   PORb: pixel on ratio for the blue (B) signals   PORg 1 : pixel on ratio for the first green (G 1 ) signals   PORg 2 : pixel on ratio for the second green (G 2 ) signals       

     In general, since it is not necessary to apply ACL technology in order to reduce power consumption if image display is in a low luminance domain, the case of calculating the pixel on ratio (POR) of a pixel as described above for this embodiment will be limited to the case that image brightness is implemented above a reference level of luminance (ACL_START_STEP) to which a predetermined ACL scheme is applied. 
     That is, the pixel on ratio for the entire pixels found as described above is a pixel on ratio corresponding to the grayscale data level of image data actually input to the display device. Therefore, assuming the grayscale data level of the image data currently input is N, the level N is between the maximum grayscale data level (for example, maximum grayscale level  255 ) and the reference grayscale data level (ACL_START_STEP) at which ACL technology begins to be applied. That is, if the gray level (N) of the image data currently input is lower than the reference grayscale data level (ACL_START_STEP) to result in low luminance, it may not be necessary to apply the ACL technology for reducing power consumption and light emission control may not be carried out. 
     Following calculation of the pixel on ratio of the pixel in the automatic current limiter  52 , the off pulse width of the light emission control signals is calculated on the basis of the calculation. It is possible to find the off pulse width (ACWE) of the light emission control signals with the following Equation 2, but calculation thereof is not necessarily limited thereto, and any algorithm may be applicable, provided that the pulse width of the light emission control signals is calculated to correspond to the pixel on ratio.
 
ACWE=ACWE — 0+(ACWE_MAX*2)*{(POR n _ACL_START_STEP)/(255_ACL_STARTSTEP)}  (Equation 2)
         ACWE: off pulse width of a light emission control signal in which the current pixel on ratio is reflected   ACWE_ 0 : predetermined off pulse width default value of light emission control signals depending on the specification of a display device   ACWE_MAX: maximum predetermined value of the off pulse width of light emission control signals   n_ACL_START_STEP: luminance value of the current image corresponding to the pixel on ratio calculated depending on the image data signals currently input   255_ACL_START_STEP: entire luminance value of the image (for example, 255 luminance values)       

     ACWE of the light emission control signals calculated with the algorithm, for example, Equation 2, may reflect the ratio in which the pixels are activated in implementing currently input images, to control the off duty and control brightness. That is, as the pixel on ratio (POR) of the display unit displaying the currently input images increases, the luminance value of the actual image by light emission rises as compared to the entire luminance value, so that ACWE of the light emission control signals increases. As a result, the period of light emission by the OLED and the amount of light emission is reduced to contribute to power consumption. 
     The controller  50  in  FIG. 3  includes a signal controller  53  and a light emission driving control signal generator  54  in addition to the data converter  51  and the automatic current limiter  52 . 
     The signal controller  53  controls images of the display unit  10 , and receives image data signals, vertical synchronization signals Hsync, horizontal synchronization signals Vsync and main clock signals MLCK to control the images to be implemented in the display unit  10 . 
     The light emission driving control signal generator  54  receives the off duty ratio (off pulse width) of the light emission control signals corresponding to the pixel on ratio of the pixel calculated in the automatic current limiter  52 , generates and transmits the light emission driving control signals CONT 1  for controlling the light emission driver to the light emission driver thus to control light emission of the display unit  10 . 
     The light emission driving control signal generator  54  in  FIG. 3  is a signal generator which generates and transmits light emission driving control signals, generates and transmits various control signals transmitted to a driving IC circuit, for example, scan driving control signals, data driving control signals, etc., as well. 
     In various exemplary embodiments, the controller  50  of the display unit includes the signal controller  53  for controlling image display, and the light emission driving control signal generator  54  for generating and transmitting a plurality of driving control signals. Therefore, although not shown in  FIG. 4 , the controller  50  of  FIG. 4  as another embodiment may include the aforementioned signal controller and the driving control signal generator. 
     That is, the configuration of the controller  50  shown in  FIG. 4  is a configuration for modulating image data signals for reducing luminance as an ACL scheme appropriately applied to the PenTile structure. Although not shown in  FIG. 4 , it is natural that the controller  50  of  FIG. 4  includes a signal controller and a driving control signal generator. Referring to  FIG. 4 , the controller  50  includes a data converter  510  and an automatic current limiter  520 . 
     The data converter  510  of  FIG. 4  receives initial external image data signals DATA 1  that are the RGB signals, as in  FIG. 3 , and converts the signals to image data signals DATA 2  corresponding to the pixel structure of the display unit. If the display unit  10  of the display device is in the PenTile structure having the subpixel arrangement type of RGBG, the image data signals DATA 2  are the signals modulated from RGB signals to the RG 1 /BG 2  signals which are image data signals applicable to the PenTile structure. 
     The automatic current limiter  520  of  FIG. 4  reduces luminance in a manner of re-compensating for and outputting image data signals, unlike the ACL scheme according to the exemplary embodiment of  FIG. 3 . That is, the automatic current limiter  520  receives the image data signals DATA 2  converted in the data converter  510 , selects a reference value for compensating for data on the basis of the pixel on ratio of the pixel of the display unit corresponding to the image data signals currently input, re-compensates for the image data signals DATA 2  to output the compensated image data signals DATA 2 ′. Particularly, an ACL algorithm which can be integrated with the data rendering technology is provided because the pixel on ratios for each subpixel implementing basic primary colors of a pixel are calculated and compared to determine the compensated reference value for the input image data signals. 
     Therefore, the automatic current limiter  520  of  FIG. 4  may include a structure for calculating the pixel on ratio of pixel, particularly the pixel on ratio for each subpixel as in  FIG. 3 , or the structure may be included in the controller  50 , independently of the automatic current limiter  520 . 
     Specifically, the detailed configuration of the automatic current limiter  520  of  FIG. 4  will be described with reference to the detailed block diagram shown in  FIG. 5 . The automatic current limiter  520  of  FIG. 5  consists of a luminance calculation development unit  521 , a luminance data summation unit  522 , and a data compensator  523 . The luminance calculation development unit  521  receives the image data signals DATA 2  converted in the data converter  510  and develops calculation of luminance Y and Y′ on the basis of the input data. The luminance values are calculated according to coefficient calculation selected by a user as in the following example of Equation 3.
 
 Y=Kr 1 Yr+Kg 1 Yg+Kb 1 Yg+Kg 2 Yg,  
 
 Y′=bKr 1 Yr+bKg 1 Yg+bKb 1 Yb+bKg 2 Yg   (Equation 3)
 
     In this case, Kr 1 , Kg 1 , Kb 1 , Kg 2 , bKr 1 , bKg 1 , bKb 1  and bKg 2  are coefficients depending on OLED material characteristics, and Yr, Yg and Yb are basic primary color R, G, B signal data of the image data signals DATA 2 , respectively. 
     The luminance Y is an equation developed for compensating for ordinary luminance, and the luminance Y′ is an equation developed for automatically limiting current depending on the material characteristics of OLED. Therefore, depending on exemplary embodiments, diversified equations for the luminance Y′ may further be developed in the luminance calculation development unit  521 . 
     Equation 3 is for an exemplary embodiment, and implementations are not limited thereto, and any equation capable of calculating a luminance value depending on the image data signals DATA 2  may be used. For example, a weighted data equation may be used. 
     The luminance data summation unit  522  determines which will be applied between the luminance Y and Y′ developed in the luminance calculation development unit  521  to add image data and thus to find luminance data. The luminance data summation unit  522  adds entire luminance data (Ytot) per frame, and calculates the average (Yavg) thereof to determine resulting compensated luminance (ΔY). When the ACL technology is applied to the PenTile structure, the compensated luminance (ΔY) is determined on the basis of the pixel on ratio for each of R, G and B subpixels calculated in the structure for calculating the pixel on ratio of pixel (not shown). For example, calculation of luminance Y or luminance Y′ is selected according to the resulting decision following comparison of Pixel On Ratio of the blue (B) signal (PORb) with the Pixel On Ratio of other basic color signals. 
     In the example of Equation 3, since the luminance Y′ reflects the coefficient b depending on the OLED material characteristics in the Equation 3 as compared to the luminance Y, it is determined to sum up the image data with the equation of luminance Y′ if the pixel on ratio (PORb) of the blue (B) signal is greater than that of the other primary color signals, that is, the pixel on ratio (PORr) of the red (R) signal or the pixel on ratio ((PORg 1 +PORg 2 )/2) of the green (G) signal. 
     If the pixel on ratio (PORb) of the blue (B) signal is smaller than or equal to the pixel on ratio of the other primary color signals, it is determined to sum up image data with the equation of luminance Y. That is, the greater pixel on ratio (PORb) for the blue (B) signal per frame than pixel on ratios of other primary colors implies that displayed image is relatively dark as compared to the case otherwise. Therefore, the characteristics for the subpixels to output the blue signals must be taken into account in compensating for luminance of input image data. Therefore, it is possible to determine summation of image data with the equation of the luminance Y′. 
     The luminance data summation unit  522  sums up image data with the determined luminance Y or Y′ to find luminance data, and calculates the average luminance data (Yavg) of the entire display unit per frame. In general, a look-up table is stored in the driving IC circuit, in which luminance values (ΔY) compensated depending on luminance values are calculated. It is possible to determine compensated luminance values (ΔY) depending on the average luminance data (Yavg) calculated in the luminance data summation unit  522  with the look-up table. The compensated luminance value (ΔY) is greater if the pixel on ratio (PORb) of the blue (B) signal per frame is greater than the pixel on ratio of the other primary colors. 
     The look-up table for calculating the compensated luminance values (ΔY) may be generally stored in a memory of a driving IC, and may be stored in the manner of multiple time program ROM (MTP) which can be erased and written multiple times or one time program ROM (OTP) which can be erased and written only once. 
     The data compensator  523  finds compensated data signals, each corresponding to red, blue, first green and second green signals of the image data signals DATA 2  with the compensated luminance values (ΔY) determined in the luminance data summation unit  522 . 
     That is, since the pixel structure of the display unit according to the exemplary embodiment is the PenTile structure, it is possible to find compensated data signals for each of the basic primary color data signal of the image data signals DATA 2  converted to correspond to the structure. For example, if the luminance range is 256 grayscales, the equation for compensating for the basic primary color data signals of the image data signals DATA 2  with the compensated luminance values (ΔY) is the following Equation 4.
 
 R′=R (1−(Δ Y/ 256))
 
 G 1′= G 1(1−(Δ Y/ 256))
 
 B′=B (1−(Δ Y/ 256))
 
 G 2′= G 2(1−(Δ Y/ 256))  (Equation 4)
 
     Since Equation 4 is an exemplary compensation equation of data signals with the compensated luminance values (ΔY), it is natural that the equation for compensating for the basic primary color data signals is not limited thereto. 
     The data compensator  523  compensates for the image data signals DATA 2  of the RG 1 BG 2  signals with compensated luminance values for which the pixel on ratio of subpixels is considered according to Equation 4, to output compensated image data signals DATA 2 , for example, R′G 1 ′B′G 2 ′ signals. 
     If the pixel on ratio (PORb) for the blue (B) signal per frame is greater than the pixel on ratios of the other primary colors, the compensated luminance value (ΔY) is greater, so that the data values of the basic primary colors of each compensated image data signal DATA 2 ′ are compensated to be small. Therefore, the effect in which luminance is reduced in the image output with the compensated image data signals DATA 2 ′ may be expected. 
       FIG. 8  is a graph in which luminance is reduced when image data signals are adjusted, to which the ACL scheme is applied with the automatic current limiter  520 , as described in  FIG. 4  and  FIG. 5 .  FIG. 8  is a graph showing luminance (vertical axis) of output images with respect to input image data brightness (horizontal axis). Since it is unnecessary to adjust image data signals input by applying the ACL scheme in a low luminance domain, the adjustment is carried out at the brightness above a predetermined reference point for applying the ACL scheme. 
     Referring to  FIG. 8 , line Y denotes extraction of compensation values for luminance data and resulting compensation of input data with an ordinary equation Y. Line Y′ denotes extraction of compensation values for luminance data depending on the equation (Y′) developed considering the case where ratios accounted by the pixel on ratio (PORb) of the blue (B) signal per frame is great and resulting compensation of input data. 
     Therefore, since the luminance of the final output image is reduced because the line Y′ is lower than the line Y, overall power consumption is advantageously reduced as a result. To say it again, the line Y exhibits an increase in power consumption and a reduction in life span, which is a disadvantage, due to bright image output because of high dependence on the red (R) signal or the green (G) signal. On the contrary, the line Y′ exhibits reduced power consumption to result in long product life spans because of high dependence on the blue (B) signal. 
     That is, if the line Y′ is applied, it is the case of a frame with more blue (B) signals. Although the same power is actually applied, the image looks dark because of low luminance (brightness) of blue. Therefore, Y′, not Y, is introduced because of small perceived loss although ACL technology is applied more to a screen of frames with more blue (B) signals. 
       FIG. 6  and  FIG. 7  illustrate the process of automatically calculating current in a display device according to an exemplary embodiment. The process of S 10  to S 12  in  FIG. 6  illustrates the ACL scheme of the display device according to the exemplary embodiment described with reference to  FIG. 3 . The process of S 20  to S 25  illustrates the ACL scheme of the display device according to another exemplary embodiment described with reference to  FIG. 4  and  FIG. 5 . 
       FIG. 7  illustrates the process of S 100  of  FIG. 6  according to the exemplary embodiment 2 in more detail. Referring to  FIG. 6 , the controller  50  of the display device receives image data signals DATA 1  from the outside and converts them to image data signals DATA 2  to be applied to the RGB data rendering technology (S 1 ). 
     The pixel on ratio for each subpixel displaying basic primary colors per frame is calculated (S 2 ). In this case, the pixel on ratio of the entire pixels can be found as well. According to the exemplary embodiment, the display unit may be in the PenTile structure. Therefore, it is possible to calculate the pixel on ratio for each subpixel displaying images depending on red, first green, blue, and second green signals. In this case, the pixel on ratio may be collected as digital signal data depending on subpixel on/off and calculated. 
     Subsequently, it is selected which scheme of ACL technology according to the exemplary embodiment is applied (S 3 ). That is, it is determined which to apply between the light emission period control followed by the process of S 10  to S 12 , or data compensation followed by the process of S 20  to S 25 . 
     In S 3 , it is possible to determine, in advance, the time at which the ACL scheme according to the exemplary embodiment is applied on the basis of the pixel on ratio calculated in S 2 . That is, it is not necessary to apply the ACL scheme in a low grayscale domain not higher than a predetermined brightness because driving current corresponding to the low grayscale domain is relatively low. Therefore, the ACL scheme is selected and applied, provided that a reference point for applying ACL is predetermined to compare the grayscale depending on the image data signals with the reference point for applying ACL, and the result is above the reference point for applying ACL. 
     When carrying out ACL through light emission period control according to the exemplary embodiment 1, the off duty width of the light emission control signals is calculated on the basis of the pixel on ratio of the pixel calculated in S 2  (S 10 ). The equation for calculating the off duty width of the light emission control signals will not be described because it was provided in Equation 2. 
     The information about the off duty width (off duty ratio) of the light emission control signals calculated in S 10  is reflected in the light emission driving control signals generated in the light emission driving control signal generator  54  of the controller  50 . That is, the light emission driving control signal generator  54  of the controller  50  generates light emission driving control signals including the information about the off duty ratio calculated by reflecting the pixel on ratio of the pixel (S 11 ). 
     The light emission driving control signals are transmitted from the controller  50  to the light emission control driver  40 . Each of the plurality of pixels included in the display unit  10  receives light emission control signals of which the off duty ratio is predetermined depending on the light emission driving control signals, and then emits light. The light emission period is controlled to correspond to the pixel on ratio of the pixel (S 12 ). That is, as the pixel on ratio of the pixel increases, luminance (brightness) of the displayed images will increase. As a result, the off duty width of the light emission control signals is controlled to be set great. Accordingly, the light emission period is reduced and luminance is reduced thus to reduce power consumption. 
     When carrying out ACL through compensation of data signals according to the exemplary embodiment 2, a luminance equation is developed (S 20 ). That is, it is possible to develop a luminance equation, which includes a coefficient corresponding to dependence on the basic primary color signals of subpixels displaying each of basic primary colors, and corresponding to the material characteristics of the OLED. The luminance equation is not limited to any specific equation, but was exemplified in the aforementioned Equation 3 as an equation including the coefficient reflecting material characteristics of a pixel. When selectively applying the luminance equation depending on the pixel on ratio for each subpixel, driving current may be automatically limited because data signals are calculated for compensating for reduced luminance of the image data signals. The process of S 100  for calculating luminance-reduced compensated image data signals from the image data signals will hereinafter be described with reference to the flow chart of  FIG. 7 . 
     Following development of a luminance equation, the calculated pixel on ratio for each subpixel calculated in S 2  is compared (S 21 ). For example, as described with reference to  FIG. 5 , the pixel on ratios of the subpixels displaying the other basic color signals are compared on the basis of the pixel on ratio (PORb) of the blue subpixel displaying blue (B) signal. 
     Specifically referring to  FIG. 7 , pixel on ratios (PORr, PORg 1 , PORb, PORg 2 ) for the subpixels displaying each basic primary color calculated in image data signals DATA 2  converted to be applied to the PenTile structure, are acquired (S 101 ). 
     The pixel on ratio (PORb) of the blue subpixel is compared with the pixel on ratio (PORx) of subpixels displaying the other basic color signals (S 102 ). In this case, the pixel on ratio (PORx) of the subpixels displaying the other primary color signals is the pixel on ratio (PORr) of the subpixel displaying the red (R) signals or the pixel on ratio ((PORg 1 +PORg 2 )/2) of the subpixel displaying the green (G) signals. 
     The luminance equation Y is used if the pixel on ratio (PORb) of the blue subpixel is smaller than or equal to the pixel on ratio (PORx) of subpixels displaying the other primary color signals (S 103 ). The luminance equation Y′ is used if the pixel on ratio (PORb) of the blue subpixel is greater the pixel on ratio (PORx) of subpixels displaying the other primary color signals (S 104 ). As illustrated in Equation 3, the luminance equation Y′ includes the coefficient b in order to improve dependence on the blue signal as compared to the luminance equation Y. 
     As described above, following determination of the luminance equation on the basis of the pixel on ratio (PORb) of the blue subpixel, image data signals are summed up according to the luminance equation to obtain luminance data (S 105 ). The luminance data for one frame is summed up to calculate the average value and to obtain the compensated luminance values (ΔY) depending on the average luminance data through the look-up table. With the compensated luminance values (ΔY) calculated through the process of S 100 , each basic primary color data signal of the image data signals DATA 2  is compensated and output in S 24  of  FIG. 6 . As described above, the image data signals DATA 2 ′ compensated in the controller  50  are output and transmitted to the data driver, to display images at a luminance compensated with a corresponding data voltage (S 25 ). 
     In the exemplary embodiment shown in  FIG. 6 , different application of ACL schemes in the display device was described, respectively, but the present invention is not limited thereto, and both of ACL schemes in the exemplary embodiments 1 and 2 may be applied. That is, both of the methods may be simultaneously applied, one of which is to find the pixel on ratio for each subpixel displaying basic primary colors to correspond to the PenTile structure, followed by finding the off pulse width of the light emission control signals on the basis of the pixel on ratio to control the light emission period and the duration of light emission, and the other of which is to compare the pixel on ratio for each subpixel to select an appropriate luminance equation, followed by finding a compensated luminance value to compensate for image data signals. 
     The drawings and the detailed description described above are examples and are provided to explain various aspects, and the scope of the present invention is not limited thereto. Therefore, it will be appreciated to those skilled in the art that various modifications are made and other equivalent embodiments are available. Those skilled in the art can omit some of the constituent elements described in the present specification without deterioration in performance thereof or can add constituent elements to improve performance thereof. Further, those skilled in the art can modify the sequence of the steps of the method described in the present specification depending on the process environment or equipment.