Abstract:
An apparatus and a method for controllably reducing power and heat dissipated by OLED display circuitry are disclosed. Image dependent drive voltage adjustments are made to reduce the power generated by and the heat dissipated by the OLED pixel circuitry. That extends the life span of the components of the OLED pixel circuitry and maintains their quality by reducing or eliminating the degradation caused by heat. The apparatus and the method of the present invention selectively reduce the voltage level provided to the drain of the transistor used to drive the OLED. The drive transistor&#39;s drain voltage level is controllably lowered during display intervals that require less than the brightest level of illumination.

Description:
RELATED APPLICATION  
       [0001]     The present application claims priority to the U.S. Provisional Patent Application No. 60/674,672, filed on Apr. 20, 2005. 
     
    
     FIELD OF INVENTION  
       [0002]     This invention relates to flat panel displays and more specifically to Organic Light Emitting Diode (OLED) type displays.  
       BACKGROUND OF THE INVENTION  
       [0003]     Flat panel displays including plasma, electroluminescent (EL), organic light emitting diode (OLED) and liquid crystal displays are used in a variety of products ranging from cell phones and personal digital assistants (PDA) to computers and televisions. Active Matrix Liquid Crystal Displays (AMLCD) are known in the art. In the active matrix displays, the row driver and the column driver are used to control the OLED pixel, and a capacitor is used to continue to drive the pixel when the drivers are not driving the pixel because they are driving other pixels. The AMLCD can produce about 16 million different colors by carefully controlling the voltages provided to each addressable pixel of the display by using digital to analog (D/A) converters on each display column.  
         [0004]     Active matrix OLED (AMOLED) are being developed and have shown promise to surpass the AMLCD because features such as the viewing angle, response time and power consumption are vastly improved in the AMOLED displays. However, a major problem with the OLED displays is that the OLED display elements degrade over time and output progressively less light. A factor which contributes to this degradation of the OLED display components is the heat dissipated by the circuitry that drives the OLED.  
         [0005]     The schematic in  FIG. 1  shows an example of the drive scheme presently employed for the sub-pixel  100  in an exemplary color AMOLED display. Three such sub-pixels  100  (one each for Red, Green, and Blue) are required for a color display. The drive voltage is supplied by the column driver integrated circuit (IC) chip (not shown), which applies a voltage to the drain of transistor T 1 . That voltage can be referred to as V Data  and is passed through T 1  to the gate of the transistor T 2  when the row driver voltage (referred to as V enable ) is asserted or raised to an on condition.  
         [0006]     Brightness levels (for example 256 brightness levels for each RGB sub-pixel) are controlled by varying the voltage of the column driver, which in turn controls the voltage to the gate of T 2 , which then supplies current through T 2  to energize the OLED to emit the desired brightness. The gate voltage of T 2  is held by capacitor C 1  so that when the row driver voltage is not asserted or switched to the off condition (in order to drive the next row of the display), T 2  continues to drive the OLED at the desired brightness level.  
         [0007]     Voltage V DD  connected to the drain of T 2  is usually set to a high value to supply adequate levels for anticipated maximum brightness levels. In the present art, when low brightness levels are required (based on the displayed image), voltage V DD  remains the same. As a result, excess voltage appears across the drain-source junction of transistor T 2  and heat is generated as the result of the power that is dissipated, which is equal to the product of the current flowing through T 2  and the voltage drop across the drain-source junction of transistor T 2 . That heat is undesirable because it results in the degradation of the components located near T 2 .  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention relates to methods and apparatus for displaying an image having one or more frames. The display can be an organic light emitting diode (OLED) display. The display includes an active matrix of pixels including OLED circuitry. The display also includes a voltage source for providing a voltage to the OLED driver and a detection circuit for determining the maximum brightness level associated with an image frame. A control circuit for adjusting the voltage provided to the OLED driver according to the maximum brightness level determined by the detection circuit is also provided.  
         [0009]     A thin film transistor (TFT) is used to couple the voltage source to the OLED and drive the OLED. At least enough voltage is provided by the voltage source to ensure that the TFT operates in the saturation mode and thereby acts as a current source for the OLED.  
         [0010]     Many embodiments of the invention are disclosed in the specification. One of ordinary skill in the art will appreciate that other embodiments are possible without deviating from the scope and spirit of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:  
         [0012]      FIG. 1  illustrates a schematic of a drive scheme for a sub-pixel in an active matrix OLED display;  
         [0013]      FIG. 2  illustrates an expanded schematic of a current source for an OLED;  
         [0014]      FIG. 3  graphically illustrates the relationship between the current flowing through the OLED drive transistor and the voltage across the OLED drive transistor for two different gate voltages applied to the OLED drive transistor;  
         [0015]      FIG. 4  illustrates an exemplary histogram of pixel brightness in an image to be displayed;  
         [0016]      FIG. 5  graphically illustrates the relationship between the brightness of the light emitted by the OLED and the gate voltage applied to the OLED drive transistor;  
         [0017]      FIG. 6  graphically illustrates the relationships between brightness, voltages, powers and currents associated with the OLED and the OLED drive transistor;  
         [0018]      FIG. 7  illustrates an exemplary block diagram of the display system of the present invention;  
         [0019]      FIG. 8  illustrates another exemplary block diagram of the display system of the present invention;  
         [0020]      FIG. 9  illustrates another exemplary block diagram of the display system of the present invention;  
         [0021]      FIG. 10  illustrates another exemplary block diagram of the display system of the present invention; and  
         [0022]      FIG. 11  illustrates another exemplary block diagram of the display system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The present invention provides apparatus and methods to control and the reduce power provided to the OLED through T 2  and thus reduces the heat dissipated by T 2 , thereby improving OLED life and preventing the degradation of the OLED circuitry elements. The power consumption of the OLED is minimized by reducing the OLED drive transistor drain voltage during display intervals that require less than full-scale worst case illumination. By reducing V DD  when displaying dim brightness levels, the power and heat generated across T 2  are reduced and that increases the OLED lifetime.  
         [0024]     The V DD  supply is common to all sub-pixels and pixels of the display. Because one sub-pixel could be displaying a dim brightness level while another sub-pixel displays a high brightness level, lowering the V DD  for dim sub-pixels is not feasible in a system in which V DD  is common to all sub-pixels. Therefore, the present invention uses a multiplexor associated with each pixel or sub-pixel to adjust the voltage only for that pixel or sub-pixel.  
         [0025]     OLED materials are current driven. That is, the brightness level of the light emitted by the OLED is determined by the current level passing through the OLED. Although a voltage appears across the OLED when a specific current is flowing through the OLED, this voltage is not the direct cause of the photon emission. The current level is the therefore controlling factor because the light emission from the OLED is due to the recombination of holes and electrons which are supplied by the current flow (electrons entering the OLED from the cathode side and holes entering from the anode side).  
         [0026]     The current flowing through the OLED is controlled by the thin film transistor (TFT) T 2 . In order to have optimum current control, TFT T 2  is biased in the saturation mode.  FIG. 2  is a detailed schematic of the OLED driver circuitry for the OLED D 1 . During operation, the line V Data  supplies a voltage to the gate of T 2  though T 1 , when T 1  is enabled by a high voltage supplied by the V enable  line. When the V enable  becomes low, the data voltage V Data  is retained on the gate of T 2  by capacitor C.  
         [0027]     The current ID, which flows through T 2  and D 1  is proportional to V Data . The power dissipated in the circuit comprised of T 2  and D 1  is the product of ID and V DD . V DD  is proportioned between T 2  and D 1 . In  FIG. 2 , the voltage across T 2  is designated by V D  and the voltage across D 1  is designated V OLED . The relationship between V DD , V D  and V OLED  is defined as: V DD =V D+V   OLED . The power dissipated in T 2  is the product of I D  and V D . Any power dissipated in T 2  is not only wasted power but it also causes OLED D 1  to heat up thereby shortening the life of D 1 . Therefore, it is beneficial to reduce the voltage drop V D  to a minimum.  
         [0028]     Referring to  FIG. 3 , the graph shows the relationship between the I D  (y axis) and the V D  (x axis) for different levels of voltages applied to the gate of T 2  (V G1  and V G2 ). V D  is shown to be V G −V th , where V th  is the threshold voltage of the TFT T 2 . For T 2  to be in saturation, and thus, a current generator, the relationship between V DD , V G  and V th  must be: V DD &gt;=V G −V th . One of ordinary skill in the art would appreciate that a lesser level of V DD  would be required if V G1  is applied to T 2  than if V G2  were applied to T 2 . It follows that the greater the current level required by the OLED, the higher V DD  level that must be applied to maintain T 2  as a current generator.  
         [0029]     One of ordinary skill in the art would appreciate that in practice V th  is not completely stable and can increase over the life of the OLED. OLED materials also increase in resistance and decrease in quantum efficiency as they age. To compensate for the increase in voltage requirements over the life of the display, V DD  is set to a high value and therefore, a high percentage of the total power (I D ×V DD ) is wasted in T 2  due to excessive V D . The present invention solves the problem of wasted power dissipation due to excessive voltage across T 2 . One of ordinary skill will appreciate from  FIG. 3  that T 2  can be in saturation for a wide range of V DD  values.  
         [0030]     Display images vary dramatically based on their application. Over the entire life of a display the images will sometimes be bright, dark or in between. A histogram like the one shown in  FIG. 4  plots the number of pixels that are displaying brightness settings 0-255. This exemplary histogram for a specific image shows no pixels are illuminating above about the brightness setting of 232. In the case of this image, the maximum current requirement for the brightest pixels in the image is less than full brightness requirement. Therefore, the required maximum gate voltage for T 2  is lower, and thus, V DD  can be reduced without T 2  falling out of saturation. The V DD  will result in reduced power and heat production and increased OLED life.  
         [0031]     In an 8 bit color system, each color has 0 to 255 steps of brightness. One of ordinary skill in the art will appreciate that the human eye response is logarithmic and thus the 255 steps are not linear but instead follow a logarithmic scale. Therefore, the 50% intensity point of OLED emission is at approximately data setting  181  (step  181 ). The data setting of 232 produces a brightness of about 82%. The more pictures or video frames that fall into the “reduced drive voltage” mode, the greater occurrences of power saving which in turn leads to longer OLED life, if the apparatus and methods of the present invention are used.  
         [0032]      FIG. 5  shows the relationship between the OLED brightness (y axis) and the V Data  (w axis), which is provided to the gate of T 2  through T 1 . As shown in  FIG. 5 , the higher the voltage that is applied to the gate of T 2 , the higher the light emitted by the OLED.  
         [0033]     Referring to  FIG. 6 , several characteristics of an exemplary display system of the present invention are shown, in which no pixels have a brightness level above 232 (line  2 ). Line  1  illustrates that the level of the current flowing through the OLED and the level of brightness of the OLED are directly proportional. Lines  3  and  4  show the power dissipated by T 2  for two different exemplary V DD  voltages, 13 volts and 8 volts respectively. As can be seen from lines  3  and  4 , dropping the voltage V DD  from 13 v to 8 v reduces the power dissipation in transistor T 2  from about 75% to about 25%. This 66.7% power reduction leads to less heat and therefore longer OLED life.  
         [0034]      FIG. 7  illustrates an embodiment of the OLED display system of the present invention. The OLED display system includes the row driver and the column driver for driving the display 60 pixels, which are well known in the art. The highest brightness detection circuit  30  is coupled to the digital to analog circuit  40 , which in turn is coupled to the multiplexor  50 . The OLED display system  200  includes a frame buffer  20  to store the RGB image. The data coming into the frame buffer  20  memory is screened for the highest brightness setting for each RGB input by the highest brightness detection circuit  30 . V DD  is then altered to accommodate only the highest brightness setting for the frame, which will be used on the next display period, and is synchronized with the buffer memory. The digital to analog converter circuit  40  is used to convert the highest brightness setting detected by the detection circuit  30  into a voltage value. The multiplexor  50  is then used to provide a proper portion of the V DD  to the pixel or sub-pixel.  
         [0035]      FIG. 8  illustrates another embodiment of the OLED display system of the present invention. Unlike typical OLED displays that have one global V DD  connection to all pixels and all sub_pixels, in this embodiment a separate V DD  connection is used for each color such as V DD     —   R (red), V DD     —   G (Green), V DD     —   B (Blue) and V DD     —   W (white).  
         [0036]      FIG. 9  illustrates another embodiment of the OLED display system of the present invention. One issue that arises is that the new V DD  value when presented will affect the image currently being displayed on the OLED screen, because the OLED retains the image until re-written. To solve this problem, V DD  is split into rows. A multiplexor (MUX) is used for each row to select between V DD  (frame_n) and V DD  (frame_n+ 1 ). The new V DD  value will be presented on a row by row basis as the maximum brightness for each row. In this embodiment, the highest brightness detection circuit  30  is replaced by the row-highest brightness detection circuit  34  for detecting the highest brightness setting for each row of display pixels instead of the highest brightness setting for the entire pixel.  
         [0037]      FIG. 10  illustrates another embodiment of the OLED display system of the present invention. In this embodiment, the change of V DD  occurs after n-successive frames. The nFrame highest brightness detection  36  detects the highest brightness setting only for selected frames of the image. The highest value for the V DD  will then be used and when a higher value is sensed incoming, the V DD  will use the new value immediately (i.e. switching to a higher value will not affect image brightness). This scheme will reduce voltage to V DD  conservatively (as the display has shown n frames of reduced maximum brightness levels) and switch back quickly without affecting the image brightness.  
         [0038]      FIG. 11  illustrates another embodiment of the OLED display system of the present invention. In this embodiment, the V DD  is switched during an intermediate black frame.