Patent Application
20080111799
The invention relates to a driver for an electroluminescent display panel, a display module comprising an electroluminescent display panel and such a driver, a display apparatus comprising the display module, and a method of driving an electroluminescent display.
U.S. Pat. No. 6,441,560 B1 discloses an active matrix display device which comprises an array of display pixels, also referred to as pixels, each comprising an electroluminescent display element and a pixel driving circuit. The pixel driving circuit controls the current through the display element based on a drive signal which is applied to the pixel during an address period and which is stored as a voltage on a storage capacitance connected to the pixel driving circuit. Each pixel includes an electro-optic adjustment circuit which is responsive to light produced by the display element during addressing and which is arranged to adjust, in the address period, the voltage signal stored on the capacitance in accordance with the light output level of the display element. The adjustment of the voltage signal on the capacitance compensates for the effects of ageing of the display elements so that a desired light output level from a display element for a given applied drive signal is substantially maintained, regardless of possible variations in the drive current level to light output level characteristics of individual display elements in the array. Although this prior art provides a behavior of the pixels less dependent on aging, it does not increase the life time of the display.
It is an object of the invention to provide a driver for an electroluminescent display which obtains a longer life-time of at least one set of the light emitting elements having a particular color.
A first aspect of the invention provides a driver as claimed in claim 1. A second aspect of the invention provides a display module as claimed in claim 9. A third aspect of the invention provides a display apparatus as claimed in claim 10. A fourth aspect of the invention comprises a method of driving an electroluminescent display as claimed in claim 12. Advantageous embodiments are defined in the dependent claims.
A driver in accordance with the first aspect of the invention comprises a data driver and a low-pass filter. The data driver receives a first set of input signals representing a first color to supply a first set of data signals to a first set of light emitting elements, respectively. The data driver further receives a second set of input signals representing a second, other, color to supply a second set of data signals to the second set of light emitting elements, respectively. Thus, for example, the input signals of the first set are the red input signals and the first set of data signals is supplied to red light emitting elements. And the input signals of the second set are the blue input signals, and the second set of data signals is supplied to blue light emitting elements.
A low pass filter is present to obtain a bandwidth of the second set of input signals which is smaller than the bandwidth of the first set of input signals. Thus, in the same example, the bandwidth of the data signals supplied to the blue light emitting elements is limited with respect to the bandwidth of the data signals supplied to the red light emitting elements. The effect of the low-pass filtering is that the second data has more averaged values and thus has less high peak levels than if the low-pass filtering is not present. Consequently, the currents through the second light emitting elements will be averaged and the life-time of the second light emitting element is increased. This is due to the non-linear ageing behavior of the material of the light emitting elements which causes the light emitting elements to age faster at higher currents at a same value of the multiplication of the current level with the period in time it is present.
It has to be noted that U.S. Pat. No. 6,583,775 B1 discloses an active matrix display of which the pixels comprise a light emitting element with a brightness value which depends on an amount of current supplied to the light emitting element. The light emitting elements are OLED's (organic light emitting diodes). A scanning line drive circuit selects the rows of pixels one by one, each during a row select period. A data line drive circuit supplies data signals to the selected pixels. The pixels comprise a pixel drive circuit which determines a level of the current dependent on the data received. At the start of a row select period, the light emitting elements start to emit with a brightness determined by the current. After the row select period, the light emitting elements continue emitting with this brightness, usually until after a frame period the same row of pixels is selected again and new data signals are received. It is also possible that the row of light emitting elements only produce light during a single row select period. Also in this application, due to the low-pass filtering or averaging in accordance with the invention, the peak current levels occur significantly less and the lifetime of the display will be increased.
In the embodiment in accordance with the invention as claimed in claim 2, the driver comprises a first set of pixel drivers which supply a first set of currents to the first set of light emitting elements of the display. The driver further comprises a second set of pixel drivers which supply a second set of currents to the second set of light emitting elements of the display. The first set of currents is determined by the first set of data signals and the second set of currents is determined by the second set of data signals. The low-pass filter low-pass filters the second set of input image signals to obtain a set of low-pass filtered image signals which are supplied to the data driver instead of the second set of input signals. Thus the bandwidth of the second set of input image signals has been made smaller than the bandwidth of the first set of input image signals. Consequently, the luminance values of the second set of light emitting elements are averaged and thus have lower peak values than the luminance values of the first set of light emitting elements.
In the embodiment in accordance with the invention as claimed in claim 3, the low-pass filter is a spatial low-pass filter which low-pass filters the data signal of the same set of light emitting elements of at least one adjacent pixel in the same frame period. Usually, this spatial low-pass filtering or averaging is obtained by determining a weighted sum of the data signal of the present pixel and the data signal of at least one spatially neighboring pixel. Preferably, the spatial neighboring pixel or pixels are preceding and/or succeeding pixels in the same row such that no line memories are required.
In the embodiment in accordance with the invention as claimed in claim 4, the low-pass filter is a two-dimensional filter which averages the data signals of pixels in the same row (usually extending in the horizontal direction) and in previous and/or next row(s) (usually vertically offset with respect to the present pixel). Although in this embodiment at least one line memory is required, the spatial low-pass filtering may further reduce the peak values in the current.
In the embodiment in accordance with the invention as claimed in claim 5, the low-pass filter is a temporal filter. Such a filter usually determines a weighted sum of the present data signal and the data signal at the same position of a previous frame or frames and/or a spatially neighboring data signal of a previous frame or of previous frames. The temporal filter comprises one or more frame memories to store the data signal of the previous frame or of the previous frames, respectively.
In the embodiment in accordance with the invention as claimed in claim 6, the light emitting elements are organic light emitting diodes, further referred to as OLED's. Such polymer and small molecule organic light emitting diodes have opened a new path to make high quality displays. The advantages of these displays are the self-emissive technology, the high brightness, the near-perfect viewing angle, and the fast response time. For large displays, an active matrix construction is required to reduce the power consumption, for small displays also passive matrix is possible. In present OLED displays, the lifetime of the blue OLED material is much shorter than that of the red and green OLED materials. In the embodiment in accordance with the invention as claimed in claim 7, the low-pass filtering is performed on the data signal for the blue pixels. The lower average currents through the blue OLED's results in an increased lifetime of the blue pixels. Thus, the lifetime of the blue pixels becomes more equal to the lifetime of the red and the green pixels and the lifetime of the display increases. It has been found that the low-pass filtering of only the blue data signal does not significantly deteriorate the quality of the displayed image. The human eye appears to resolve a lower resolution for blue light than for red and green light.
In an embodiment in accordance with the invention as claimed in claim 8, the first set of data signals is high frequency boosted with a high frequency boosting filter. This compensates for the resolution decrease, if present, caused by a relatively strong low-pass filtering of the second set of data signals.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
In the drawings:
By way of example, in
Each one of the pixel driving circuits PD1 receives a select signal from an associated select electrode SE, a data signal RD1 from an associated data electrode DE, a power supply voltage VB from an associated power supply electrode PE, and supplies a current I1 to its associated LED PL1. Each one of the pixel driving circuits PD2 receives a select signal from its associated select electrode SE, a data signal BD1 from its associated data electrode DE, a power supply voltage VB from its associated power supply electrode PE, and supplies a current 12 to its associated LED PL2. Although for the same groups of pixels 10 the same references are used to indicate the same elements, the value of signals, voltages and data may be different.
A select driver SD supplies the select signals to the select electrodes SE. A data driver DD receives the input image signals FR and FB to supply the data signals RD1 and BD1 to the data electrodes DE. In a full color display, the data driver may further receive the data signal FG to supply the data signal GD1 to further pixels 10 (not shown) which have another color than both the pixels which receive the data signals RD1 and BD1. In the embodiment shown in
The de-gamma circuit DG processes the input image signal IV to remove the pre-gamma correction from it. Such a pre-gamma correction is usually present and was originally intended to pre-compensate for the gamma of a cathode ray tube. Thus, the corrected signals RG, GG, BG are present in the linear light domain. Consequently, advantageously, the low-pass filtering and the high-frequency boosting filtering are performed in the linear light domain. The gamma circuit GA processes the filtered signals RF, GF, BF to add a pre-gamma correction fitting the display panel 1 used.
The low-pass filter LPF and the high frequency boost filter HF may be part of a standard video scaler which scales the input image component signals R, G, B. Also the input image component signals R, G may be low-pass filtered, but the result should be that the bandwidth of the low-pass filtered signals BF and FB is smaller than the bandwidth of the output signals RF, FR and GF, FG. The de-gamma circuit DG and the gamma circuit GA may be implemented as well known lookup tables. If the de-gamma circuit DG and the gamma circuit GA are not present, the input image component signal B is low-pass filtered and fed to the data driver DD, directly. Further, if the high frequency boosting filter HF is also not present the input image component signals R and G are fed directly to the data driver DD.
In
The timing controller TC receives the synchronization signal SY associated with the input image signal IV and supplies the control signal CR to the select driver SD and the control signal CC to the data driver DD. The control signals CR and CC synchronize the operation of the select driver SD and the data driver DD such that the output signals FR, FG, FB are presented to the data electrodes DE after the associated row of pixels 10 has been selected. Usually, the timing controller TC controls the select driver SD to supply select voltages to the select electrodes (also commonly referred to as address lines) SE to select (or address) the rows of pixels 10 one by one. In practice, more address lines per display row (which is a row of pixels 10) may be used, for example to control the duty cycle of the currents I1, I2 supplied to the LED's PL1, PL2, respectively. It is possible to select more than one row of pixels 10 at a same time. The timing controller TC controls the data driver DD to supply the data signals RD1 and BD1 in parallel to the selected row of pixels 10. The effect of the low-pass filter LPF will be elucidated with respect to
The display panel 1 is defined to comprise the pixels 10. In a practical embodiment, the display panel 1 may also comprise all or some of the driver circuits DD, SD and TC. This combination of driver circuits and display panel is often referred to as display module. This display module can be used in many display apparatuses, for example in television, computer display apparatuses, game consoles, or in mobile apparatuses such as PDA's (personal digital assistant) or mobile phones.
It is possible to perform the low-pass filtering on the data signals BD1. However, this has the drawback that the filtering is not performed in the light linear domain (i.e. on values directly describing the desired light output or luminance). The signals BD1 from the data driver DD are not in the light linear domain, because the pixel circuits have a non-linear transfer function. Some video scalers already work in the linear light domain, and thus can be used also for the low-pass filtering. The video scalers supply the signals FR and FB to the data driver DD. Also the high frequency boost filter HF may be repositioned to process the signals RD1 and/or GD1 instead of the signals RG and/or GG.
The data driver DD, the optional gamma circuit GA, the optional high frequency boosting filter HF, the low-pass filter LPF, and the optional de-gamma circuit DC are collectively indicated by the data processor DR.
The operation of the circuit is elucidated in the now following. When a row of pixels 10 is selected by an appropriate voltage on the select electrode SE with which this row of pixels 10 is associated, the transistor T1 is conductive. The data signal D which has a level indicating the required light output of the LED PL is fed to the control electrode of the transistor T2. The transistor T2 gets an impedance in accordance with the data level, and the desired current I starts to flow through the LED PL. After the select period of the row of pixels 10, the voltage on the select electrode SE is changed such that the transistor T1 becomes a high resistance. The data voltage D which is stored on the capacitor C is kept and drives the transistor T2 to still obtain the desired current I through the LED PL. The current I will change when the select electrode SE is selected again and the data voltage D is changed.
The current I has to be supplied by the power supply electrode PE which receives the power supply voltage VB via a resistor Rt. The resistor Rt represents the resistance of the power supply electrode towards the pixel 10 shown. It has to be noted that other pixels 10 associated with the same power supply electrode PE may carry current too, this current is denoted by Io. Both the currents Id and lo flow through the resistor Rt and thus cause a voltage drop in the power supply electrode PE. The pixel driving circuit PD will only function correctly if the voltage Vp across the series arrangement of the main current path of the transistor T2 and the LED L is sufficiently high to obtain the current I.
The pixel driving circuit PD may have another construction than shown in
To elucidate the effect of different levels of the current 12 on the aging of the light emitting element PL2 it is assumed that, due to the changing image content, without the low-pass filtering the current 12 through a particular light emitting element PL2 has alternatively the value Lh and L1, while another light emitting element PL2 always receives the current L. Because the light emitting element PL2 ages especially fast for high current levels, the total aging caused by the currents Lh and Ll (
The lifetime LT of polymer materials depends on the time T a luminance LU is generated by it:
LT˜LU−p/T
wherein p is a power factor which depends on the material. It has to be noted that the relation between the luminance LU and the current 12 is approximately linear. With a typical power factor value of 1.6, the lifetime LT1 for the particular light emitting element PL2 driven in accordance with
LT1˜(2*Lh−1,6)/T
LT2˜(3*Lh1,6)/T
Based on this insight, the present invention introduces the low-pass filter LPF. The low-pass filter LPF averages levels of the current 12 and thus limits the occurrence of high peak values of this current. The low-pass filtering is especially relevant if the light emitting elements PL2 age faster than the light emitting elements PL1. The lifetime of the display is increased because the light emitting elements PL2 are driven with lower peak currents.
Further, the differential aging of the light emitting elements PL2 becomes less because sharp transitions in the current 12 are smoothed and consequently no large aging difference occurs between the adjacent pixels A and B. Thus, because of the low-pass filtering, large luminance variations from pixel to pixel are decreased and the differential ageing, which is currently a large problem in OLED displays, is reduced. The reduction is obtained in displays with all types of organic LED materials (polymer as well as small molecule OLED), and also with a power factor p smaller than one. Note that also small molecule materials are known for which the power factor is larger than one. Furthermore, the differential aging can also be decreased in other displays such as inorganic electroluminescent display and plasma displays.
From the equations defining the lifetimes LT1 and LT2 it becomes clear that the lifetime LT2 is longer than the lifetime LT1 for all display panels for which holds that the power factor p is larger than 1. In OLED displays, the blue pixels have the shortest lifetime and thus the blue data signal is low-pass filtered to enlarge the lifetime of the blue pixels and thus the lifetime of the display panel. Since the human visual system can resolve less resolution in the blue part of the visible spectrum, the loss of resolution for the blue data is not or almost not perceived by viewers.
This low-pass filter determines for each pixel a level of the low-pass filtered input image signal BF which is the sum of one quarter the level of the corrected signal BG of the previous pixel, one quarter the level of the corrected signal BG of the adjacent pixel of the previous pixel, one line earlier, one quarter of the level of the corrected signal BG of the adjacent pixel of the present pixel, one line earlier, and one quarter of the level of the corrected signal BG of the present pixel.
Alternatively, in another preferred embodiment the multipliers C10 to C13 may multiply with the factors ½, ⅙, ⅙, and ⅙, respectively. However many other selections of coefficients may provide useful low-pass filter characteristics.
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.
For example, many more low-pass filters than the embodiments elucidated with respect to
Alternatively, analog low-pass filters may be used. The invention can also be applied in other displays wherein ageing effects occur, such as for example, inorganic electroluminescent displays or plasma displays.
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. Table 1, Filter coefficients used in experiments.
indicates data missing or illegible when filed
| Number | Date | Country | Kind |
|---|---|---|---|
| 04102282.3 | May 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB05/51534 | 5/11/2005 | WO | 00 | 11/16/2006 |
