Patent Publication Number: US-8537079-B2

Title: Method and apparatus for power control of an organic light-emitting diode panel and an organic light-emitting diode display using the same

Description:
This application claims the benefit of U.S. provisional application Ser. No. 61/367,370, filed Jul. 23, 2010, 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 a method and an apparatus for power control and an organic light-emitting diode (OLED) display, and more particularly to a method and an apparatus for power control and an OLED display for adjusting a booster voltage provided to an OLED panel. 
     2. Description of the Related Art 
     Generally, an organic light-emitting diode (OLED) is a self-emissive display element that emits light by electrically exciting a luminous organic compound. The OLED has recently received attention and application in the field of the flat panel display, television screens, computer displays, and portable electronic device screens. The OLED, when used in a display, lends itself to several advantages over flat-panel displays, such as its self-emissive ability which retires the backlight of the LED, wider viewing angles, and improved brightness. 
     The OLED-based displays, however, have a problem related to power consumption. Because of different circuit characteristics in the OLED devices, some consume more power than others. In order to assure that an OLED-based display has sufficient power to display images, the booster voltage, or the power level, is usually set at a level sufficient to allow the OLED-based display to display all kinds of images. In this way, when low brightness levels are required based on the displayed image, the level of the booster voltage remains the same. As a result, excess voltage is applied to the OLED panel, which can also be referred to that excess power that is not as demanding is used, and heat is generated. That heat is undesirable because it causes the problem of power consumption for the OLED-based display. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a method and an apparatus for power control, and an organic light-emitting diode (OLED) display for adjusting a booster voltage provided from a voltage generator to an OLED panel, which can reduce power consumption for driving the OLED panel, and result in lower power loss. 
     According to an aspect of the present invention, a method is provided for power control of an OLED panel. The method includes the steps of: estimating a voltage value according to image content of an image, the voltage value indicative of a minimal required voltage allowing the OLED panel to display the image; and controlling a voltage generator to adjust a booster voltage provided to the OLED panel according to the estimated voltage value. 
     According to another aspect of the present invention, an OLED display is provided. The OLED display includes an OLED panel and a power control apparatus. The OLED panel has a number of OLED elements for displaying an image. The power control apparatus is for estimating a voltage value according to image content of the image. The voltage value is indicative of a minimal required voltage allowing the OLED panel to display the image. The power control apparatus is for controlling a voltage generator to adjust a booster voltage provided to the OLED panel according to the estimated voltage value. 
     According to another aspect of the present invention, an apparatus is provided for power control of an OLED panel. The apparatus includes a load current estimation circuit, an OLED current estimation circuit, and a power estimation circuit. The load current estimation circuit is for estimating a first current value according to image content of an image, the first current value relating to a voltage drop of the OLED panel associated with the image. The OLED current estimation circuit for estimating a second current value according to the image content of the image, the second current value relating to a maximal display voltage of the OLED panel associated with the image. The power estimation circuit is for estimating a voltage value according to a combination of the estimated first current value and second current value, the voltage value indicative of a minimal required voltage allowing the OLED panel to display the image, and for controlling a voltage generator to adjust a booster voltage provided to the OLED panel according to the estimated voltage value. 
     The above and other aspects of the invention will become better understood with regard to the following detailed description of the preferred but non-limiting embodiment(s). The following description is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a basic panel model of an active matrix organic light-emitting diode (AMOLED) panel. 
         FIG. 2  is a schematic diagram showing the relationship between the booster voltage provided to the panel model in  FIG. 1  and the corresponding currents associated with image content of the image. 
         FIG. 3  is a schematic diagram showing the relationship between the booster voltage provided to the panel model in  FIG. 1  and the corresponding currents associated with image content of the image according to an embodiment of the invention. 
         FIG. 4  is a flow chart showing a method for driving an OLED panel according to an embodiment of the invention. 
         FIG. 5  is a block diagram showing an OLED display according to an embodiment of the invention. 
         FIG. 6  is a block diagram showing an example of a power control apparatus of the OLED display in  FIG. 5  according to an embodiment of the invention. 
         FIG. 7  is a block diagram showing an example of a load current estimation circuit of the power control apparatus in  FIG. 6  in accordance with the equation EQ. 1. 
         FIG. 8  is a block diagram showing an example of an OLED current estimation circuit of the power control apparatus in  FIG. 6  in accordance with the equation EQ. 2. 
         FIG. 9  is a block diagram showing a power control apparatus of the OLED display in  FIG. 5  according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic diagram showing a basic panel model of an active matrix organic light-emitting diode (AMOLED) panel. The panel model  100  includes a panel resistance  102 , a switch circuit  104 , and an OLED circuit  106 . The panel model  100  receives different voltages including power voltage Pvdd and cathode voltage Pvee. The voltage difference between the power voltage Pvdd and the cathode voltage Pvee represents the booster voltage of the OLED panel  100 , which can be provided from a voltage generator such as a booster. The panel resistance  102  represents a combinational resistance of various elements on the panel model  100 , such as a wiring for the power voltage Pvdd, a wiring for flexible printed circuit (FPC), a wiring for the cathode voltage Pvee, indium tin oxide (ITO) cathode plane, or electrical contact and so on. The switch circuit  104  includes a number of active switch elements such as thin film transistors (TFT). The active switch elements of switch circuit  104  receive a number of control voltages Vg converted from image data of an image, and conduct corresponding currents for the OLED circuit  106 . The OLED circuit  106  includes a number of OLED elements which correspondingly receive the currents from the switch circuit  104  and become energized to emit light as the image desired. It is apparent for those skilled in the art that the panel model  100  in  FIG. 1  is provided for exemplary illustration, and OLED panels are frequently more complex than that in  FIG. 1 . 
       FIG. 2  is a schematic diagram showing the relationship between the booster voltage provided to the panel model in  FIG. 1  and the corresponding currents associated with image content of the image. As shown in  FIG. 2 , the currents are arranged in an increasing order from left to right. The region  102 R represents a voltage drop Vload generated on the panel resistance  102 . The region  106 R represents a number of display voltages within a range of voltages V 1 -Vx, such as 3.10-3.70V. Each display voltage is indicative of a drive voltage transmitted to or a pixel data stored across an OLED element of the OLED circuit  106  when an image or a frame is to be displayed thereon, and the display voltage is related to a current flowing therethrough. The active switch elements, when receiving the control voltages and turned on, are operated at an saturation region in which each active switch element has a substantial fixed voltage drop of about 0.5V. Because the booster voltage is usually set at a relatively high level Lv 1  in a conventional practice, a high excess voltage Vex shown in region  104 R is applied across an active switch element, such as the drain-source junction of a TFT, of the switch circuit  104 , resulting in the problem of power consumption. Moreover, heat is generated due to the excess voltage Vex, resulting in accelerating the degradation of the components and shortening the lifetime of the OLED panel. 
       FIG. 3  is a schematic diagram showing the relationship between the booster voltage provided to the panel model in  FIG. 1  and the corresponding currents associated with image content of the image according to an embodiment of the invention. As shown in  FIG. 3 , the horizontal axis indicates the current in an increasing order from left to right. According to an embodiment of the invention, the booster voltage is set at a level associated with the image content and sufficient to allow the OLED panel to display the image, which can also be regarded as each frame or one frame of a video, while the level is adjusted with respect to different images containing different image content. In other words, as shown in  FIG. 3 , the booster voltage can be set at a lower level Lv 2  than the level Lv 1 , and the level Lv 2  of the booster voltage can be determined according to the image content of the image from different voltages of the OLED panel. For example, the level Lv 2  can be determined from three voltages measurable on the OLED panel: (1) the voltage drop Vload on the panel resistance  102 ; (2) the voltage drop on the switch circuit  104  which can be regarded as an offset voltage Vofs about 0.5V; and (3) a maximum display voltage Voled among the display voltages of the OLED elements of the OLED circuit  106 . In this way, the booster voltage can be adjusted according to the to-be-display image, so that excess voltage Vloss shown in region  108 R can be reduced, making it possible to reduce power consumption for driving the OLED panel, and to lessen power loss. 
       FIG. 4  is a flow chart showing a method for driving an OLED panel according to an embodiment of the invention. The method is used for power control of an OLED panel. The method includes the following steps. In step S 410 , a voltage value is estimated according to image content of an image, the voltage value indicative of a minimal required voltage allowing the OLED panel to display the image. In step S 420 , a voltage generator is controlled to adjust a booster voltage provided to the OLED panel according to the estimated voltage value. 
     According to the aforementioned method for driving the OLED panel, a voltage value, which is indicative of a minimal required voltage sufficient for the OLED panel to display the image, is estimated and used to adjust the booster voltage provided to the OLED panel. Using the voltage value indicative of the minimal required voltage means the booster voltage can be adjusted to an adequate level as a function of the image content, so as to reduce power consumption. Therefore, driving the OLED panel according to the embodiment can result in reduced power consumption. 
       FIG. 5  is a block diagram showing an OLED display according to an embodiment of the invention. The OLED display  500  includes an OLED panel  510 , a voltage generator  520 , and a power control apparatus  530 . The OLED panel  510  has a number of OLED elements for displaying an image and a number of switch elements, while each of the switch element and a corresponding OLED element are defined as a sub-pixel, which is well known in the art. In a practical example, the OLED panel can be a white OLED panel with RGBW color filters, or a side-by-side (SBS) OLED panel where R, G, B OLED materials are deposited in a manner of side-by-side to form patterned RGB sub-pixels, but this invention is not limited thereto. The voltage generator  520  provides a booster voltage Vboost to the OLED panel  510 . The booster voltage Vboost is for example represented by the voltage difference between the power voltage Pvdd and the cathode voltage Pvee shown in  FIG. 1 . The power control apparatus  530  estimates a voltage value Vs according to image content Lin of the image. The image content Lin can be for example image data of the image, or other information retrieved from a portion of image data or all the image data of the image. The voltage value Vs is indicative of a minimal required voltage allowing the OLED panel  510  to display the image. The minimal required voltage can be for example the booster voltage being set at the level Lv 2  in  FIG. 3 . According to the estimated voltage value Vs, the power control apparatus  530  controls the voltage generator  520  to adjust the booster voltage Vboost provided to the OLED panel  510 . For example, the power control apparatus  530  can use the voltage value Vs to modify the voltage setting of the voltage generator  520 , causing the voltage generator  520  to set the booster voltage Vboost at the level Lv 2 . 
       FIG. 6  is a block diagram showing an example of a power control apparatus of the OLED display in  FIG. 5  according to an embodiment of the invention. The power control apparatus  530  includes a load current estimation circuit  632 , an OLED current estimation circuit  634 , and a power estimation circuit  636 . The load current estimation circuit  632  estimates a first current value Iload according to the image content Lin. The first current value Iload is for example indicative of a load current flowing through the OLED panel associated with the image. The OLED current estimation circuit  634  estimates a second current value Ioled according to the image content Lin. The second current value Ioled is for example indicative of a maximal current flowing through one of the OLED elements of the OLED panel  510  associated with the image. 
     In a practical example, in order for the power control apparatus  530  in  FIG. 6  to estimate current values Iload and Ioled, the image content Lin can be the information of luminance values converted from the image data of the image. As to an AMOLED, the current flowing through a single pixel can be estimated as a value proportional to the square of its drive voltage, and linearly proportional to the luminance of its sub-pixels. In other words, in the example of  FIG. 6 , the luminance value of a sub-pixel, which is converted from the square value of a piece of raw image data, is served as the image content Lin and provided to the current estimation circuits  632  and  634  for current estimation. However, this invention is not limited thereto. The conversion from image data to luminance can be completed in another example where the gamma correction is taken into consideration. In addition to the luminance, the image content Lin can be other information which a person having ordinary skill in the art could use to estimate the current values. 
     As to the operation of the current estimation circuits  632  and  634 , estimating the current values Iload and Ioled involve a weighted summation which is related to the converted luminance values and a number of weighting values. Further description is provided as follows for illustration. 
     In an embodiment, the first current value Iload is estimated according to a number of sub-pixel luminance values and a number of corresponding weighting values. Each sub-pixel luminance value can be converted from a corresponding primary color sub-pixel data. For example, it is exemplified that the OLED panel  510  includes four kinds of primary color sub-pixel each including an OLED element and a corresponding active switch element, and the first current value Iload can be estimated in accordance with the equation EQ. 1 as follows: 
                     I   load     =       ∑       x   =   R     ,   G   ,   B   ,   W               ⁢       F     load   ,   x       ·       ∑     i   =     1   ⁢           ⁢   …   ⁢           ⁢   N                 ⁢     L     x   ,   i                     EQ   .           ⁢   1               
wherein I load  represents the first current value Iload; Lx,i(x=R, G, B, W)(i=1:N) represent four sub-pixel luminance values for N pixels; and Fload,x(x=R, G, B, W) represent four weighting values for the four sub-pixel luminance values.
 
       FIG. 7  is a block diagram showing an example of a load current estimation circuit of the power control apparatus in  FIG. 6  in accordance with the equation EQ. 1. The load current estimation circuit  632  includes four summation units  732   a , four multipliers  732   b , and a summation and output unit  732   c . The four summation units  632   a  receive and sum up four primary color sub-pixel data LR,i, LG,i, LB,i, LW,i, and transmit their results to the four multipliers  732   b , respectively. The four multipliers  732   b  multiply the results of the summation units  732   b  with four weighting values Fload,R, Fload,G, Fload,B, Fload,W, respectively. The summation and output unit  732  receives and sums up the results of the four multipliers  732   b , thus providing the first current value Iload. 
     In an embodiment, the second current value Ioled is estimated according to a number of sub-pixel luminance values and a number of corresponding weighting values. As is similar to the aforementioned example of estimating the first current value Iload where the OLED panel  510  includes four kinds of primary color sub-pixel, the second current value Ioled can be estimated in accordance with the equation EQ. 2 as follows: 
                     I   oled     =       max       x   =   R     ,   G   ,   B   ,   W       ⁢     (       F     oled   ,   x       ·       max     i   =     1   ⁢           ⁢   …   ⁢           ⁢   N         ⁢     L     x   ,   i           )               EQ   .           ⁢   2               
wherein I oled  represents the second current value Ioled; Lx,i(x=R, G, B, W)(i=1:N) represent four sub-pixel luminance values for N pixels; and Foled,x(x=R, G, B, W) represent four weighting values for the four sub-pixel luminance values.
 
       FIG. 8  is a block diagram showing an example of an OLED current estimation circuit of the power control apparatus in  FIG. 6  in accordance with the equation EQ. 2. The OLED current estimation circuit  634  includes four maximum units  734   a , four multipliers  734   b , and a maximum and output unit  734   c . The four maximum units  734   a  receive and obtain four maximum from four primary color sub-pixel data LR,i, LG,i, LB,i, LW,i, and transmit their results to the four multipliers  734   b , respectively. The four multipliers  734   b  multiply the results of the maximum units  734   b  with four weighting values Foled,R, Foled,G, Foled,B, Foled,W, respectively. The maximum and output unit  734   c  receives and obtain a maximum from the results of the four multipliers  734   b , thus providing the second current value Ioled. 
     As to the implementation of the summation units  732   a , the maximum units  734   a , and the output unit  732   c  and  734   c , their circuit architectures can be implemented by logistic elements such as adders, flip-flops, and/or comparators connected in open and/or closed loops. It is apparent for those skilled in the art to implement such devices or circuits which perform the function based on equations EQ. 1 and EQ. 2. 
     Based on the equations EQ. 1 and EQ. 2, the OLED panel  510  is exemplified as including four kinds of primary color sub-pixel. However, this invention is not limited thereto. In another embodiment, the OLED panel  510  can also be implemented as one which includes three kinds primary color sub-pixel, such as RGB sub-pixels where R, G, B OLED materials are deposited thereon. In this way, each of the equations EQ. 1 and EQ. 2 can be accordingly modified as one where x=R, G, B. Besides, the weighting values for each primary color sub-pixel can also provide the flexibility to circuit design. For example, the weighting values can be the same or different or can be determined according to the aperture sizes or the layout areas of these primary color sub-pixels. 
     The power control apparatus mentioned in above examples is exemplified to deal with all the image data of the image, but this invention is not limited thereto. In another embodiment, instead of dealing with all the image data of the image, the power control apparatus can also deal with a portion of image data so as to determine the voltage value. For example, the power control apparatus can calculate a weighted summation and maximum of a portion of sub-pixel luminance values, and normalize the calculation results to estimate the first current value Iload and the second current value Ioled. It is apparent for those skilled in the art that the image content can be retrieved from a portion of image data or all the image data of an image. Any estimation approach of the minimal required voltage and any adjustment to the booster voltage as a function of the image content of the image are regarded as a practicable embodiment of the invention. 
     Referring to  FIG. 6 , the power estimation circuit  636  estimates a first voltage value V 1  indicative of the voltage drop Vload in  FIG. 1  according to the estimated first current value Iload, and estimates a second voltage value V 2  indicative of the maximal display voltage Voled according to the estimated second current value Ioled. In an embodiment, the power estimation circuit  636  includes two look-up tables  636   a  and  636   b . The power estimation circuit  636  determines the first voltage value V 1  according to the first current value Iload and the look-up table  636   a , and determines the second voltage value V 2  according to the second current value Ioled and the look-up table  636   b . In accordance with the resistive relationship between voltages and currents, i.e. V=R*I, the first voltage value V 1  and the second voltage value V 2  can be obtained in a manner of using multipliers each containing a gain indicative of the resistive relationship between voltage and current. To take nonlinear effect into account, conversion of voltage and current in this embodiment can be implemented as look-up tables which contain voltage values and current values establishing the corresponding relationship between voltages and currents, i.e. V=LUT(I). In a practical example, the look-up tables  636   a  and  636   b  can be obtained from experimental results and designed to meet different requirements. 
     In this embodiment, the power estimation circuit  636  can also include a combination unit  636   c , with which the power estimation circuit  636  can determine the voltage value Vs indicative of the minimal required voltage according to a combination of the estimated first voltage value V 1  and second voltage value V 2 . In another embodiment, the power estimation circuit  636  can further estimate an offset voltage value. The offset voltage value is indicative of an offset voltage such as the offset voltage Vofs shown in  FIG. 3  which allows the switch elements of the OLED panel  510  to operate in saturation mode. The offset voltage Vofs is for example about 0.5V. The offset voltage Vofs can be transmitted to the combination unit  636   c  (not shown in  FIG. 6 ) for making its combination estimation. As such, the power estimation circuit  636  can determine the voltage value Vs indicative of the minimal required voltage according to a combination of the first voltage value V 1 , the second voltage value V 2 , and the offset voltage value Vofs, thus providing a resulted voltage being set at the level Vp 2  as shown in  FIG. 3 . 
     Moreover, in another embodiment, the OLED display  500  can further include a driver integrated chip (IC) not shown in the drawings. The drive IC uses the image content Lin to drive the OLED panel  510 , so as to display the image. For example, the drive IC can convert the raw image data into corresponding control voltages such as the control voltages Vg shown in  FIG. 1 . In this embodiment, the power control apparatus of this embodiment can be implemented in the drive IC, but this embodiment is not limited thereto. The power control apparatus can also be implemented in another place capable of receiving the image content and controlling the voltage generator. 
       FIG. 9  is a block diagram showing a power control apparatus of the OLED display in  FIG. 5  according to another embodiment of the invention. In this embodiment, as is similar to the power control apparatus  530  in  FIG. 6 , the power control apparatus  1130  includes a load current estimation circuit  1134  and an OLED current estimation circuit  1134 , whose operation will not be repeated for the sake of brevity. The power control apparatus  1130  further includes a current limit circuit  1138 . The current limit circuit  1138  limits the estimated first current value Iload and the estimated second current value Ioled. The current limit circuit  1138  includes a dim factor determination unit  1138   a  and a current modification unit  1138   b . The dim factor determination unit  1138   a  determines a dim factor DF according to a ratio of a limit current value Imax and the first current value Iload. The limit current value Imax can be a user-defined value or can be adjusted to meet different requirements. The current modification unit  1138   b  modifies the first current value Iload and the second current value Ioled according to the dim factor DF. For example, the current modification unit  1138   b  can be a multiplier, which multiplies the first current value Iload and the second current value Ioled respectively with a gain represented by the dim factor DF, and produces the limited first current value Iload′ and the limited second current value Ioled′. Receiving the limited first current value Iload′ and the limited second current value Ioled′, the power estimation circuit  1136  determines the first voltage value V 1  and the second voltage value V 2  according to the look-up tables  1136   a  and  1136   b , and determines the voltage value Vs indicative of the minimal required voltage according to a combination of the estimated first voltage value V 1  and second voltage value V 2 . 
     Besides, the power requirements for OLED elements to maintain the same level of luminance may increase with age due to their degradation. To compensate for the degradation of OLED elements and the aging OLED panel, the power control apparatus of this embodiment can control the voltage generator to adjust the level of the booster voltage adequately in response to the measured current or the measured luminance or brightness of the OLED elements. For example, the embodiment can further include a detector and a compensation look-up table (no shown). The detector can be used to detect the measured current or the measured luminance or brightness of the OLED elements. The compensation look-up table can be used to provide the relationship between the booster voltage and the current flowing through the OLED element or the luminance measured therefrom, the stored content of which the power control apparatus can use to determine the voltage value indicative of the minimal required voltage. As compared with a conventional practice which initially sets booster voltage at a relatively high level, the embodiment using the detector and the compensation look-up table gradually increases the booster voltage provided to the OLED panel to compensate for the degradation of OLED elements and the aging OLED panel, so as to drive the OLED panel with reduced booster voltage and lower power loss. 
     According to the OLED display and the method for driving the same disclosed in the embodiment of the invention, the booster voltage provided to the OLED panel is adjusted, based on the image content, to an adequate level for allowing the OLED panel to display the image, so that excess voltage can be reduced. Therefore, lower power loss and higher power efficiency can be achieved. Moreover, the lifetime of the OLED display can be lengthened due to the reduced booster voltage. 
     While the invention has been described by way of example and in terms of the preferred embodiment(s), 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.