Abstract:
An organic light emitting diode (OLED) display includes a first and a second digital/analog current converters, a feedback unit and a compensating unit. The feedback unit includes the first and second feedback circuits for generating the first and second feedback currents, respectively. The compensating unit includes the first and second compensating circuits for outputting the first and second compensating voltages as the first and second reference voltages for the first and second digital/analog current converters in accordance with the first and second feedback currents, respectively. The luminance change of the first and second pixels is positively proportional to the first and second feedback current change. Therefore, the first and second compensating voltages are changed accordingly, and the first and second reference voltages are regulated so as to compensate for the luminance of the first and second pixels.

Description:
This application claims the benefit of Taiwan application Serial No. 93117565, filed Jun. 17, 2004, the subject matter of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates in general to an organic light emitting diode (OLED) display and luminance compensating method thereof, and more particularly to an OLED display, which utilizes the operational current of a dummy OLED to simulate the change of the real pixel current, and luminance compensating method thereof. 
   2. Description of the Related Art 
     FIG. 1  is a block diagram showing a circuit structure of a conventional OLED display. The OLED display  100  includes a data driver  110 , a pixel matrix  120  and a scan driver  130 . The pixel matrix  120  includes several red pixels (R_Pixels)  122 , several green pixels (G_Pixels)  124  and several blue pixels (B_Pixels)  126 , each of which includes an OLED (not shown in the figure). The data driver  110  includes a horizontal shift register  112 , a plurality of red digital/analog current converters R_DACs  114 , a plurality of green digital/analog current converters G_DACs  116 , and a plurality of blue digital/analog current converters B_DACs  118 . 
   The R_DAC  114 , G_DAC  116  and B_DAC  118  respectively receive the digital data R_Data, G_Data and B_Data from the horizontal shift register  112  and convert them into analog currents I R , I G  and I B  according to a reference voltage Vbias. These analog currents I R , I G  and I B  are respectively sampled and held by a red sample/hold unit (R_S/H)  115 , a green sample/hold unit G_S/H  117  and a blue sample/hold unit B_S/H  119 , and then data currents I DR , I DG  and I DB  are thus generated and outputted to the R_Pixel  122 , G_Pixel  124  and B_Pixel  126 . The scan driver  130  turns on control switches (not shown in the figure) contained in each row of the pixels  122 ,  124  and  126  in the pixel matrix  120  in a row-by-row manner such that the OLEDs in each row of the pixels  122 ,  124  and  126  emit light. 
   Because the luminance efficiency of the OLED attenuates with the usage time and the luminance attenuation degrees of the red, green and blue pixels are different, the OLED display usually cannot display the correct picture frames after a period of time. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to provide a display luminance compensating device and a method thereof, wherein an operational current of a dummy OLED in a feedback circuit is utilized to simulate the condition that the real pixel current attenuates with time, and then a feedback current is outputted accordingly. A compensating circuit generates a compensating voltage according to the feedback current, and regulates the data current inputted to the real pixel to compensate for the luminance of the real pixel such that the display can display the correct color frame. 
   The invention achieves the above-identified object by providing an organic light emitting diode display including a first digital/analog current converter, a second digital/analog current converter, a feedback unit and a compensating unit. The feedback unit includes a first feedback circuit for providing a first feedback current and a second feedback circuit for providing a second feedback current. 
   The compensating unit, electrically coupled to the feedback unit, includes a first compensating circuit and a second compensating circuit for outputting a first compensating voltage and a second compensating voltage as a first reference voltage and a second reference voltage for the first and second digital/analog current converters in accordance with the first and second feedback currents respectively. 
   Each of the first feedback circuit and the second feedback circuit includes a feedback current mirror circuit and a dummy OLED. The feedback current mirror circuit comprises a first PMOS transistor and a second PMOS transistor. The gate and the drain of the first PMOS transistor are electrically connected to each other. The drain of the first PMOS transistor is coupled to the dummy OLED. The drain of the second PMOS transistor is for outputting the first/second feedback current. 
   Each of the first and second feedback circuits includes a feedback current mirror circuit and a plurality of dummy OLEDs connected to each other in parallel. The feedback current mirror circuit includes a first PMOS transistor and a second PMOS transistor. The gate and the drain of the first PMOS transistor are electrically connected to each other. The drain of the first PMOS transistor is coupled to the dummy OLEDs. The drain of the second PMOS transistor is for outputting the first/second feedback current. 
   Each of the first and second compensating circuits includes a compensating current mirror circuit including a resistor, a first NMOS transistor and a second NMOS transistor. The gate and the drain of the first NMOS transistor are electrically connected to each other. The drain of the second NMOS transistor is connected to an operational voltage through the resistor. The drain of the second NMOS transistor Is for outputting the first/second compensating voltage. 
   The first digital/analog current converter and a second digital/analog current converter provide a first data current and a second data current to a first pixel and a second pixel. As soon as the luminance of the first and second pixels attenuates with time, the first and second feedback currents reduce with time, such that the first and second compensating voltages increase accordingly. The first and second compensating voltages respectively increase the first and second reference voltages so as to increase the first and second data currents. 
   The invention also achieves the above-identified object by providing a method of compensating for the luminance of a display having a first pixel and a second pixel. The method includes the steps of generating a first feedback current and a second feedback current, wherein the first feedback current and the second feedback current change is positively proportional to the luminance change of the first and second pixels; generating a first compensating voltage and a second compensating voltage in accordance with the first and second feedback currents; and adjusting the first and the second data currents in accordance with the first and the second compensating voltages, respectively, wherein the changes of the first and the second data currents are inversely proportional to the changes of the first and the second compensating voltages. 
   The step of generating the first and the second feedback currents includes the sub-steps of: providing a first operational current for a first dummy light emitting component and a second operational current for a second dummy light emitting component; and duplicating the first and second operational currents as the first and second feedback currents. This method utilizes a first current mirror circuit and a second current mirror circuit to provide the first and the second operational currents and to duplicate the first and second feedback currents. 
   Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a circuit structure of a conventional OLED display. 
       FIG. 2A  is a block diagram showing a circuit structure of a display according to a preferred embodiment of the invention. 
       FIG. 2B  shows a circuit structure of a pixel of  FIG. 2A . 
       FIG. 2C  shows a circuit structure of a feedback circuit of  FIG. 2A . 
       FIG. 2D  shows another circuit structure of the feedback circuit of  FIG. 2A . 
       FIG. 2E  shows a circuit structure of a compensating circuit of  FIG. 2A . 
       FIG. 3A  is a schematic illustration showing a relative position between the feedback circuit and the compensating circuit of  FIG. 2A , which are disposed on the display. 
       FIG. 3B  is a schematic illustration showing another relative position between the feedback circuit and the compensating circuit of  FIG. 2A , which are disposed on the display. 
       FIG. 4  is a flow chart showing a method of compensating for the luminance of the display according to the preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The main feature of the display luminance compensating device of the invention is to utilize an operational current of a dummy OLED in a feedback circuit to simulate the condition that the real pixel current attenuates with time, and then a feedback current is outputted accordingly. A compensating circuit generates a compensating voltage according to the feedback current as a reference voltage for a digital/analog current converter, regulates the data current inputted to the real pixel, and compensates for the luminance of the real pixel such that the display can display the correct color picture frames. 
     FIG. 2A  is a block diagram showing a circuit structure of a display according to a preferred embodiment of the invention. Referring to  FIG. 2A , the display  200  includes a data driver  210 , a pixel matrix  220 , a scan driver  230  and a luminance compensating device  235 . The data driver  210  includes a horizontal shift register  212 , R_DACs  214 , G_DACs  216 , B_DACs  218 , R_S/Hs  215 , G_S/Hs  217 , and B_S/Hs  219 . The pixel matrix  220  is located in the active region (not shown in the figure) and includes R_Pixels  222 , G_Pixels  224  and B_Pixels  226 . 
   The R_DAC  214 , G_DAC  216  and B_DAC  218  respectively receive digital data R_Data, G_Data and B_Data from the horizontal shift register  212  and convert them into analog currents I R , I G  and I B  according to reference voltages V R , V G  and V B . These analog currents I R , I G  and I B  are respectively sampled and held by the R_S/H  215 , G_S/H  217  and B_S/H  219 , and then data currents I DR , I DG  and I DB  are generated and outputted to the R_Pixel  222 , G_Pixel  224  and B_Pixel  226 . The scan driver  230  simultaneously turns on control switches S 1 , S 2 , and S 3  contained in each row of the R_Pixel  222 , G_Pixel  224  or B_Pixel  226  in the pixel matrix  220  in a row-by-row manner, as shown in  FIG. 2B , such that the data current I D (=I DR , I DG  or I DB ) can flow into the OLED as an operational current I P  for enabling the OLED to emit light. At the same time, the capacitor C is charged by a voltage drop (Va−Vb). In the next scanning period, the switches S 1  and S 2  are turned off and the switches S 3  and S 4  are turned on such that a current generated by the voltage Vdd can subsequently serve as the operational current I P  for enabling the OLED to emit light. Because the voltage drop (Va−Vb) is kept by the capacitor C, the operational current I P  is substantially the same as the data current I D . 
   The luminance compensating device  235  includes a feedback unit  240  and a compensating unit  250 . The feedback unit  240  includes a red feedback circuit  242 , a green feedback circuit  244  and a blue feedback circuit  246  for outputting feedback currents I FR , I FG  and I FB , respectively. As shown in  FIG. 2C , each of the feedback circuits  242 ,  244  and  246  includes a feedback current mirror circuit  241  and a dummy OLED  245 . The feedback current mirror circuit  241  includes a PMOS (P-typed Metal Oxide Semiconductor) transistor P 1  and a PMOS transistor P 2 . The gate G 1  and the drain D 1  of the transistor P 1  are electrically connected to each other. The dummy OLED  245  is electrically connected to the drain D 1  of the transistor P 1  through a resistor R 1 . In addition, the sources S 1  and S 2  of the transistors P 1  and P 2  are connected to the operational voltage VDD. When the drain D 1  of the transistor P 1  outputs the operational current I O  (=I OR , I OG  or I OB ), the drain D 2  of the transistor P 2  outputs the feedback current I F  (=I FR , I FG  or I FB ), wherein the feedback current I F  is substantially equal to the operational current I O . The invention utilizes the operational current I O  flowing through the dummy OLED  245  to simulate the condition that the real pixel current I P  attenuates with time. 
   Of course, each of the feedback circuits  242 ,  244  and  246  may include a feedback current mirror circuit  241  and a plurality of OLEDs  247  emitting light of the same color and connected to each other in parallel, as shown in  FIG. 2D . These OLEDs  247 , connected to each other in parallel, are. connected to the drain D 1  of the transistor P 1  through a resistor R 2 . The operational current I O′  (I OR′ , I OG′  or I OB′ ) generated by using the same color OLEDs connected to each other in parallel is the sum of the currents flowing through the OLEDs  247 . Because the current attenuation degrees of the OLEDs  247  of the same color in the real pixel matrix  220  are different, the operational current I O′  can simulate an average current attenuation degree of several OLEDs  247  of the same color in the better manner. 
   The compensating unit  250  includes a red compensating circuit  252 , a green compensating circuit  254  and a blue compensating circuit  256  for respectively outputting compensating voltages V CR , V CG  and V CB  as reference voltages V R , V G  and V B  for R_DAC  214 , G_DAC  216  and B_DAC  218  according to the feedback currents I FR , I FG  and I FB . As shown in  FIG. 2E , each of the compensating circuits  252 ,  254  and  256  is a compensating current mirror circuit, which includes a NMOS transistor N 3  and a NMOS transistor N 4 . The gate G 3  and drain D 3  of the transistor N 3  are electrically connected to each other. The feedback current I F  is inputted to the drain D 3  of the transistor N 3 . The drain D 4  of the transistor N 4  outputs a compensating voltage V C  (=V CR , V CG  or V CB ), and the drain D 4  of the transistor N 4  is connected to the operational voltage V DD  through a resistor R 3 . According to the current mirror principle, the current  13  flowing through the resistor R 3  is equal to the feedback current I F . Therefore, the compensating voltage V C  is equal to (V DD −I F ×R 3 ). 
   When the luminance of R_Pixel  222 , G_Pixel  224  and B_Pixel  226  attenuates with time, the luminance of the OLED  245  in the feedback circuits  242 ,  244  and  246  also attenuates with time. That is, the operational currents I OR , I OG  and I OB  attenuate with time such that the duplicated feedback currents I FR , I FG  and I FB  also attenuate with time. According to the above-mentioned equation: the compensating voltage V C =V DD −I F ×R 3 , the decreases of the feedback currents I FR , I FG  and I FB  increase the compensating voltages V CR , V CG  and V CB , and thus increase the reference voltages V R , V G  and V B . Because the reference voltages V R , V G  and V B  are increased, the analog currents I R , I G  and I B  are also increased. Hence, the data currents I DR , I DG  and I DB  are also increased to compensate for the luminance of the R_Pixel  222 , G_Pixel  224  and B_Pixel  226 . 
   The feedback unit  240  and the compensating unit  250  are disposed on a display panel  300  of the display  200 , as shown in  FIG. 3A . Alternatively, the feedback unit  240  is disposed on the display panel  300  while the compensating unit  250  is disposed on a printed circuit board  310  of the display  200 , and the printed circuit board  310  is connected to the display panel  300  through a flexible circuit board  320 , as shown in  FIG. 3B . 
     FIG. 4  is a flow chart showing a method of compensating for the luminance of the display according to the preferred embodiment of the invention. First, in the step  400 ; the feedback circuits  242 ,  244  and  246  generate the operational currents I OR , I OG  and I OB  flowing through the red, green and blue OLEDs  245 . Next, in the step  410 , the feedback currents I FR , I FG  and I FB  are duplicated using the feedback current mirror circuit  241  according to the operational currents I OR , I OG  and I OB . Obviously, when the pixel luminance of the R_Pixel  222 , G_Pixel  224  and B_Pixel  226  attenuates with time, the operational currents I OR , I OG  and I OB  of the OLED  245  in the feedback circuits  242 ,  244  and  246  also attenuate with time. The duplicated feedback currents I FR , I FG  and I FB  also attenuate with time. Hence, the operational currents I OR , I OG  and I OB  can be used to simulate the condition that the pixel currents I P  in the real pixels  222 ,  224  and  226  attenuates with time. In the step  420 , the compensating voltages V CR , V CG  and V CB  are generated using the compensating circuits  252 ,  254  and  256  according to the feedback currents I FR , I FG  and I FB . The compensating circuits  252 ,  254  and  256  are the above-mentioned compensating current mirror circuits, for example. According to the current mirror principle, the compensating voltage V C  is equal to (V DD −I F ×R 3 ). Therefore, when the feedback currents I FR , I FG  and I FB  attenuate with time, the compensating voltages V CR , V CG  and V CB  are increased with time. Finally, the data currents I R , I G  and I B  are regulated using the compensating voltages V CR , V CG  and V CB  as the reference voltages V R , V G  and V B  for R_DAC  214 , G_DAC  216  and B_DAC  218 . When the compensating voltages V R , V G  and V B  are increased with time, the data currents I R , I G  and I B  are also increased with time in order to compensate for the luminance attenuations of the R_Pixel  222 , G_Pixel  224  and B_Pixel  226 . 
   According to the preferred embodiment, the advantage of the display luminance compensating device of the invention is to utilize the simple feedback circuit design to output the feedback current and to simulate the condition that the current of the real pixel attenuates with time. In addition, the compensating circuit outputs the compensating voltage, which is increased as the feedback current is decreased, as the reference voltage for the digital/analog current converter in order to effectively compensate for the luminance attenuation caused by the pixel current attenuation. Performing the luminance compensations on the red, green and blue pixels simultaneously can keep the same luminance performance after a period of time with respect to the same picture frame, and thus lengthen the lifetime of the OLED display. 
   While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.