WO2005045798A1

WO2005045798A1 - Color display device

Info

Publication number: WO2005045798A1
Application number: PCT/IB2004/052350
Authority: WO
Inventors: Fransiscus J. Vossen; Roel Van Woudenberg; Nijs C. Van Der Vaart
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-11-10
Filing date: 2004-11-09
Publication date: 2005-05-19
Also published as: TW200529145A

Abstract

The invention relates to a color display device (6) comprising a display panel (2) with light emissive elements (30R,30G,30B) for different colors (R,G,B) and a drive element (8;80) to drive the light emissive elements (30R,30G,30B) in accordance with respective input data signals (IR,IG,IB). The display device (6) is arranged to map the respective data signals (IR,IG,IB) on respective driving ranges (OR,OG,OB) of the light emissive elements (30R,30G,30B).

Description

Color display device

The invention relates to a color display device comprising a display panel with light emissive elements for different colors and a drive element to drive said light emissive elements in accordance with respective input data signals.

Display devices employing light emissive elements deposited on or over a substrate are becoming increasingly popular. These light emissive elements may be light emitting diodes (LED's), incorporated in or forming display pixels that are arranged in a matrix of rows and columns. The materials employed in such LED's are suitable to generate light if a current is driven through these materials, such as particular polymeric (PLED) or small molecule organic (SMOLED) materials. Accordingly the LED's have to be arranged such that a flow of current can be driven through these light emitting materials. Typically passively and actively driven matrix displays are distinguished. For active matrix PLED (AMPLED) displays, the display pixels themselves comprise active circuitry such as one or more transistors. This active circuitry conveys the current to the PLED material to generate or emit light. Such a display is addressed row at a time and the light emitting elements emit light during the remainder or a fraction of the frame period. Color displays typically comprise red, green and blue light emissive elements arranged in a matrix of rows and columns. The appropriate elements are addressed by selecting a certain row and driving the columns with a data voltage in accordance with a data signal for the display. The red, green and blue light emissive elements are known to have different light efficiencies, i.e. the light output per electron is different for each color. In order to allow the use of equal data voltages for obtaining the same light output, the sizes of the drive transistors in the display pixels may differ from color to color. Typically large drive transistors are employed for the red light emissive elements. These transistors occupy a relatively large area of the display pixel. Moreover, as the light emitting elements suffer from ageing, i.e. degradation of the light emitting efficiency over time, which effect is especially observed for blue light emitting elements, the data-signals need to be manipulated. This manipulation complicates operation of the display device. EP 1 260 959 discloses a color active matrix OLED flat-panel display comprising a plurality of light emitting elements and associated control circuits connected to a programmable power supply for each color. A sensor senses the light output for each color of the light emitting elements and transmits a feedback signal to a display controller for programming the appropriate programmable power supply to modify the operational characteristics of the light emitting elements within the display over power lines in order to compensate for ageing of the display pixels. The modification of the operational characteristics of the light emitting element over the power lines of the display pixel is disadvantageous in that the voltage on the power lines is difficult to control as a considerable voltage drop is usually observed over the display. Moreover the application of a sensor enhances the complexity of the display device.

It is an object of the invention to provide an improved color display device reducing or even eliminating one or more of the disadvantages of the prior art. This object is achieved by providing a color display device, wherein the respective input data signals are mapped on respective driving ranges of the light emissive elements. Thus, the invention allows the driving ranges for the colors to be manipulated independently of each other by the drive element. The independent control at the drive element of the driving range for each particular color avoids e.g. the need to manipulate the voltages on the power supply lines in the display pixels. Further, if corrections are required due to e.g. ageing of the light emissive elements, the data signals themselves require no correction. This correction can be realised by manipulating the driving ranges. The independent mapping operation for each individual color ensures that efficient use can be made of all bits of the color specific data signals for each color, while corrections to compensate for ageing of the light emissive elements for a specific color do not influence the available driving ranges. In an embodiment of the invention the drive element comprises respective sub- elements for the respective colors, each of said sub-elements being coupled to receive respective control signals determining the respective driving ranges. In this way the control signals set at least the maximum driving signal for the respective colors and thus determine the maximum brightness. The control signals may be manually set for the respective colors independently, thereby allowing the user of the color display device to set or adjust the white point of the display panel, or may be controlled by the system, e.g. to maintain a proper white point. In an embodiment of the invention, the respective control signals are coupled to respective power supplies, the respective control signals being coupled to the respective power supplies. Preferably, the respective power supplies are adapted for providing respective voltages to said sub-elements substantially equal to or slightly higher than respective maximum voltages within the respective driving ranges. In this way the voltages of the power supplies are matched to the needs of the respective sub-element in terms of maximum voltage and margin. So, the voltage drop over each sub-element is reduced which is advantageous for the power consumption of the device. In an embodiment of the invention two or more of the sub-elements are integrated in a single drive element, typically a customized integrated circuit. This embodiment provides a cost effective arrangement for the color display device. In an embodiment of the invention the display device further comprises adaptive means for adjusting one or more of said respective driving ranges. These adaptive means may e.g. relate to buttons that can be manually operated, remote controllable means, or to hardware or software for automatically correcting for changing parameters, e.g. the ageing of specific light emissive elements requiring adjustment of the driving range from a control unit. It should be appreciated that the display device discussed above may constitute either a display module for mounting in an electric device product (the module, for example, comprising the display panel with corresponding control and driving circuitry) or an electric device product as such. Such an electric device product may e.g. relate to handheld devices such as a mobile phone, a Personal Digital Assistant (PDA) or a portable computer as well as to devices such as a monitor for a Personal Computer, a television set or a display on e.g. a dashboard of a car. The invention further relates to a method for controlling the light output of a color display device comprising a display panel with light emissive elements for different colors and a drive element to drive said light emissive elements in accordance with respective input data signals, the method comprising the step of independently mapping said respective input data signals on respective driving ranges of said light emissive elements. The invention will be further illustrated with reference to the attached drawings, which show preferred embodiments of the invention. It will be understood that the invention is not in any way restricted to these specific and preferred embodiments. In the drawings: Fig. 1 shows an electric device comprising a color display device; Fig. 2 shows a color display device with an active matrix display panel; Fig. 3 shows a part of a color display device according to the prior art; Fig. 4 shows a mapping characteristic of a display device comprising display pixels with driving transistors of equal size; Fig. 5 shows an alternative mapping characteristic of a display device comprising display pixels with driving transistors of different size; Fig. 6 shows a part of the color display device according to a first embodiment of the invention; Fig. 7 shows independent mapping characteristics accomplished by the embodiments of the invention; Fig. 8 shows a part of the color display device according to a second embodiment of the invention, and Figs. 9A and 9B show parts of the color display device according to a third embodiment of the invention.

Fig. 1 shows an electric device 1 comprising a display device 6 with a display panel 2 having a plurality of light emitting elements or display pixels 3 arranged in a matrix of rows 4 and columns 5. Fig. 2 shows a schematical illustration of the display device 6, comprising the display panel 2 as shown in Fig. 1. The display panel 2 comprises a row selection circuit 7 and a drive element or column driver IC 8. Information or data, such as (video)images, received via line 9 and to be presented on the display panel 2 is input to the control unit 10 which information or data is subsequently transmitted by the control unit 10 to the appropriate parts of the drive element 8 via line 11 as a digital data signal. The selection of the rows 4 of the display pixels 3 is performed by the row selection circuit 7 via selection lines 12. Data are written to the display pixels 3 from the drive element 8 via data lines 13. Moreover the control unit 10 may control the power supply of the display pixels 3 via power lines 14. Fig. 3 shows a part of a color display device 6 according to the prior art. The display pixels 3 comprise light emissive elements 30 for red (R), green (G) and blue (B), indicated as 30R, 30G and 30B respectively. In Fig. 3 a color specific RGB data signal is received at a 6-bit wide databus at the drive element 8 over the line 11. The drive element 8 has a power supply input 31 for receiving a power supply voltage of e.g. 8V. The 6-bit color specific data signal, i.e. data signals are received for R, G and B individually, is distributed to the appropriate parts of a data register 32 such that the drive element 8 may drive or program the light emissive elements 30R, 30G and 30B of each display pixel 3 with an analogue signal over the lines 13 corresponding to a particular light emission of the light emissive elements 30R, 30G, 30B. The data register 32 as shown in Fig. 3, has 64 registers numbered from 1 to 64. Data register outputs Rl, Gl, Bl are coupled via data lines 13 to respective light emissive elements 3 OR, 30G, 30B. Typically more than one drive element 8 and/or larger drive elements 8 are employed as most display panels 2 comprise more than 64 emissive elements in a row 4. Fig. 4 shows a transfer or mapping characteristic 40 from digital to analogue for a drive element 8 driving display pixels 3 having driving transistors T (not shown) of equal size. Horizontally the value I of the digital 6-bit color specific data signal input received via the line 11 is shown. Vertically the analogue driving range output O in Volts on the line 13 of the drive element 8 is displayed. The red, green and blue emissive elements 30 have their maximum light output at different drive voltages indicated with R, G and B respectively in Fig. 4. To establish the maximum light output, the blue element requires e.g. a drive voltage of 4 Volts, the red element 5 Volts and the green element 3 Volts. These numbers are determined by the efficiency of the light emissive materials. Clearly, from Fig. 4 it can be observed that for the red emissive elements 30R full use is made of the 6 available bits (i.e. 64 available light output gradation levels from 0 - 63), whereas for the green emissive elements 30G only 39 levels are used assuming a linear scale. For the blue emissive element 30B only 52 gradation levels can be used. As a result, the coarseness of the gradation of the light output levels is larger for green and blue than it is for red. It should be appreciated that alternatively the mapping characteristic 40 may be non-linear, e.g. to include a gamma correction in the device 6. In such a case additional problems arise as the gamma-characteristics are in fact different for each color. Fig. 5 shows an alternative mapping characteristic 40' of a display device comprising emissive elements 30R, 30G, 30B with driving transistors T of different size for the respective colors R,G,B. If the display device 6 is optimised, the drive element 8 may have an equal driving range O for the respective colors R, G and B. The different behaviour of the emissive elements 30R, 30G and 30B mentioned above is accounted for by the different sizes of the driving transistors T for each color R,G or B of an emissive element 30. In such a case the mapping characteristics 40' for the respective colors R,G,B coincide, i.e. the color specific driving range is the same for each color. In case of ageing or drift, the data specific signal may be manipulated to balance these effects, however at the cost of a less efficient use of the driving range. Fig. 6 shows a part of the color display device 6 according to a first embodiment of the invention. A sub-element 8R, 8G and 8B is provided for each color R, G and B to perform driver functions for the respective emissive elements 30R, 30G and 30B individually. The sub-elements 8R, 8G and 8B have respective control inputs 50R, 50G and 50B for receiving a control signal from a control-element 51 that may include a digital-to- analogue converter. The control element 51 is controlled via lines 52. The sub-elements 8R, 8G and 8B are powered by a common power supply 310 coupled to respective power supply inputs 31. The display pixel 3 is conventionally controlled via the row selection lines 12, the lines 13 and the power line 14, as discussed with reference to Fig. 2. The driving transistors T for the emissive elements 30R, 30G and 30B may have substantially the same dimensions. It is noted that the impression may occur that application of the invention requires more drive elements 8 than the three drive sub-elements 8R, 8G and 8B shown in Fig. 5 as opposed to a single drive element 8 in Fig. 3. However, the arrangement does not require an increase of the number of drive elements 8 as will be clear by the following simple example. In a W-VGA panel 2, 768 columns 5 (see Fig. 1) of three colors are present, i.e. a total of 768 * 3 = 2304 sub-pixel columns are present. Typically a drive element 8 has 384 outputs, so that 768 * 3 / 384 = 6 drive elements 8 are required. In the prior art arrangement of Fig. 3, this is accomplished by connecting the first drive element 8 to columns 1-128, corresponding to in total 384 sub-pixel columns, up to the sixth drive element 8 for the last 128 columns. In the arrangement according to an embodiment of the invention, the first drive sub-element 8R may drive the red emissive elements 3 OR of all first 384 sub-pixel columns, the second the green elements 30G, and the third the blue emissive elements 30B, etc. Thus, the amount of drive elements 8 required may remain the same as in the prior art. Next the operation of the color display device 6 shown in Fig. 6 will be described with reference to Fig. 7. Digital color specific data-signals IR, IG and IB are received over the lines 1 IR, 1 IG and 1 IB from the control unit 10 (see Fig. 2) for each particular color R, G and B of the light emissive elements 30 at respective drive sub-elements 8R, 8G and 8B. So, sub-element 8R has outputs Rl, R2, R3, etc. coupled to respective red emissive elements 3 OR, sub-element 8B has outputs Gl, G2, G3, etc. coupled to respective green emissive elements 30G, and sub-element 8B has outputs Bl, B2, B3, etc. coupled to respective blue-emissive element 30B. An example of a circuit within a display pixel 3 is shown. The invention may also be combined with alternative circuits used within a display pixel 3 for driving the light emissive elements 30R, 30G, 30B. The display device 6 is arranged to map the color specific data signals IR, IG and IB independently to respective color specific driving ranges OR, OG and OB suitable for driving the light emissive elements 30R, 30G and 30B. The display device 6 is thus adapted to provide mapping characteristics 40R, 40G and 40B for the respective colors and allows individual control over these mapping characteristics. This means that the display panel 2 can be initially optimised during fabrication to correct for (average) production spread of efficiencies of the emissive elements 30 and drive transistors T without losing resolution (bits) in the signal for the individual colors R, G and B. As was noted above, it should be appreciated that the mapping characteristics 40 may be non-linear. In the embodiment of Fig. 6 the mapping characteristics 40R, 40G and 40B as shown in Fig. 7 are obtained by feeding a control signal VR, VG and VB to the respective drive sub-elements 8R, 8G and 8B. The control signals determine the color specific maximum driving signal VR, VG and VB of the color specific driving range OR, OG and OB. This maximum driving signal sets the maximum light output of the emissive element 30 for each color R, G, B such that this maximum light output can be controlled for each color individually. The maximum driving signals may e.g. be at VR=5V; VG=3V and VB=4V. Clearly from Fig. 7 it can be observed that all bits of the input signals IR, IG and IB are used, in contrast to the mapping characteristic of Fig. 4, wherein the entire input bit-range was only used for the red emissive element. As a result the red, green and blue colors have the same number of gradations. The 5 V supply for red is gradated with the full set of 64 levels. The same is true for the green drive voltage where the 64 levels are stretched over 3 V. For blue, the 64 levels are stretched over a 4V driving range. Moreover, this approach also allows the use of non-linear transfer characteristics without side effects. Alternatively if the drive transistors T have different sizes the emissive elements 30R, 30G and 30B may all be set with a color specific driving signal at a common maximum of e.g. 5 V, but still the color driving range OR, OG and OB can be controlled independently for each color R, G and B. The independent control of the color specific driving ranges OR, OG and OB allows a user to set the white point and/or may e.g. enable adaptation of the load of the display panel 2. Considering the issue of degradation of the light emissive elements 30, if e.g. the blue emissive element 30B degrades, the maximum driving signals VR and VG may be decreased without influencing the gradation of the blue light output, such that the white point remains the same. This is possible due to the invention, wherein the color specific driving range OR, OG, OB can be controlled independently for each color R,G,B. The maximum driving signals VR, VG and VB in Fig. 6 may be adjusted by adaptive means 54. These adaptive means may be a unit for detecting e.g. degradation (or any other relevant parameter) of the emissive elements 30R, 30G, 30B and for automatically stabilizing the white point by controlling the respective driving ranges OR, OG, OB. The adaptive means may also be a unit having a user interface for allowing a user to change color settings like the white point by controlling the respective driving ranges OR, OG, OB. These adaptive means 54 may partially be present in the control unit 10. The user interface may include a button influencing the control element 51 over lines 52. The maximum driving signals VR, VG and VB may also be manipulated by a remote control. The power required by the drive sub-elements 8R, 8G and 8B of the embodiment shown in Fig. 6 is supplied by the power supply 310. The voltage of the power supply 310 should be at least equal to the highest maximum driving signal, in this case VR=5V. As such a high voltage is not needed by the drive sub-elements 8B and 8G power dissipation may be unnecessarily high for these drive sub-elements 8B and 8G. The embodiment shown in Fig. 8 is adapted to reduce power dissipation, which is especially important for wireless devices 1. The difference with respect to the embodiment shown in Fig. 7 is that each drive sub-element 8R, 8G and 8B has its own power supply input 31 R, 3 IG and 3 IB coupled to respective power supplies 310R, 310G and 310B. The power supply inputs 31R, 31G and 3 IB are linked with a control input 50R, 50G, 50B respectively. The power supply 310R is e.g. at a voltage of 5-6 Volts; 310G at 3-4 Volts and 310B at 4-5 Volts if the driving transistors T in the display pixels 3 are of equal size. As the voltages are matched to the requirements for the specific drive sub-elements 8R, 8G and 8B, the power dissipation is reduced. Clearly the voltages should be sufficient to reach the maximum voltages VR, VG and VB. Therefore a margin of e.g. 1 Volt may be employed. Finally in Figs. 9A and 9B more elegant arrangements for the color display device 6 are shown, wherein the drive sub-elements 8R, 8G and 8B are implemented as a single customized integrated circuit 80. Identical reference numbers have been used to identify similar parts of the display devices 6. Fig. 9A shows an arrangement with a common power supply 310, which operates in the same way as the embodiment of Fig. 6. The integrated circuit 80 in Fig. 9B is equipped with three power supply inputs 31R, 31G and 3 IB. This arrangement operates in the same way as the embodiment of Fig. 8. The user can determine the preferred white point of the display panel 2 by e.g. changing (within a preset range) the power supply voltage 31 OR, 310G, 310B. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. Color display device (6) comprising a display panel (2) with light emissive elements (30R,30G,30B) for different colors (R,G,B); and a drive element (8;80) to drive said light emissive elements (30R,30G,30B) in accordance with respective input data signals (IR,IG,IB) mapped on respective driving ranges (OR,OG,OB) of said light emissive elements (30R,30G,30B).

2. Color display device (6) according to claim 1, said drive element (8) comprising respective sub-elements (8R,8G,8B) for the respective colors (R,G,B), each of said sub-elements (8R,8G,8B) being coupled to receive respective control signals determining the respective driving ranges (OR,OG,OB).

3. Color display device (6) according to claim 2, the respective sub-elements (8R,8G,8B) being coupled to respective power supplies (310R,310G,310B), the respective control signals being coupled to the respective power supplies (310R,310G,310B).

4. Color display device (6) according to claim 3, the respective power supplies (31 OR, 310G, 310B) being adapted for providing respective voltages to said sub-elements (8R,8G,8B) substantially equal to or slightly higher than respective maximum voltages within the respective driving ranges (OR,OG,OB).

5. Color display device (6) according to claim 2, wherein two or more of said sub-elements (8R,8G,8B) are integrated in a single drive element (80).

6. Color display device (6) according to claim 1, wherein said device (6) further comprises adaptive means (54) for adjusting one or more of said respective driving ranges (OR,OG,OB).

7. Color display device (6) according to claim 1, wherein said display panel (2) is an active matrix display with display pixels (3) comprising drive transistors (T) of substantially equal size.

8. Method for controlling the light output of a color display device (6) comprising a display panel (2) with light emissive elements (30R,30G,30B) for different colors (R,G,B) and a drive element (8; 80) to drive said light emissive elements (30R,30G,30B) in accordance with respective input data signals (IR,IG,IB), the method comprising the step of independently mapping said respective input data signals (IR,IG,IB) on respective driving ranges (OR,OG,OB) of said light emissive elements (30R,30G,30B).