Patent Application
20060164408
1. Field of the Invention
The present invention relates to a display device, and particularly to a display device which maintains constancy of contrast by adjusting luminance according to ambient light.
2. Description of the Related Art
A recent display device, which is represented by a liquid crystal display (LCD) or an organic EL display using an organic EL element, has been increasingly reduced in size and thickness, and extended in life.
The organic EL element, in particular, is self-luminous and does not require a backlight necessary for the liquid crystal display. Accordingly, the organic EL element is optimal for reducing the thickness of the display device. Moreover, the organic EL element does not limit viewing angles. The organic EL display is therefore highly expected to be put into practical use as a next-generation display device.
By the way, the methods of driving the organic EL display is of two types: a passive type of a simple matrix structure and an active type using TFTs. The active type generally uses a circuit configuration shown
As shown in
At each intersection of the gate lines 1 and the drain lines 2, a selection TFT 4 is connected to the gate lines 1 and drain lines 2. A gate and a drain of the selection TFT 4 are connected to the gate lines 1 and drain lines 2, respectively. The source of the selection TFT 4 is connected to a storage capacitor 5 and the gate of a driving TFT 6.
A drain and a source of the driving TFT 6 are connected to the driving line 3 and an anode of an organic EL element 7, respectively. The opposite electrode of the storage capacitor 5 is connected to a capacitance line (not shown) extending in the row direction.
The gate lines 1 are connected to a not-shown vertical scanning circuit, and gate signals are sequentially applied to the gate lines 1 by means of the vertical scanning circuit. The gate signal is a binary signal which becomes on or off. The gate signal has a predetermined positive voltage when it becomes on, and has OV when it becomes off. The vertical scanning circuit turns on a gate signal of a predetermined gate line selected out of the plurality of gate lines 1 connected thereto. When the gate signal of the selected gate line 1 is turned on, all of the selection TFTs 4 connected to the selected gate line 1 are turned on, and the drain lines 2 and the gates of the driving TFTs 6 are connected to each other through the selection TFTs 4.
Data signals determined according to pictures for display are outputted from a horizontal scanning circuit (not shown) to the drain lines 2. The data signals are inputted to the gates of the driving TFTs 6 and charged in the storage capacitors 5.
Each of the driving TFT 6 connects the driving line 3 and the organic EL element 7 at a conductivity according to a magnitude of the data signal. As a result of the above, an electric current according to the data signal is supplied from the driving line 3 through the driving TFT 6 to the organic EL element 7, and the organic EL element 7 therefore emits light at a luminance level according to the data signal.
Each of the storage capacitors 5 forms a capacitance in conjunction with another electrode such as the dedicated capacitance line or the driving line 3 and is capable of storing the data signal for a certain period of time.
Even after the vertical scanning circuit selects another gate line 1 and the previously selected gate line 1 becomes deselected state to turn off the selection TFT 4, the data signal is kept stored by the storage capacitor 5 for one vertical scanning period. During that period, the driving TFT 6 maintains the same conductivity as above, and the organic EL element 7 can continue to emit light at the same luminance level.
As shown in
On the driving TFT 6, an interlayer insulating film 153 is formed, on which the drain lines 2 and driving lines 3 are arranged. The driving line 3 is connected to the drain 6D of the driving TFT 6 through a contact. On the drain lines 2 and driving lines 3, a planarization insulating film 154 is formed. On the planarization insulating film 154, the organic EL element 7 is arranged for each pixel.
The organic EL element 7 includes an anode 155 formed of a transparent electrode of indium tin oxide (ITO) or the like, a hole transport layer 156, a light emitting layer 157, an electron transport layer 158, and a cathode 159 made of metal such as aluminum, which are sequentially stacked. As a result of recombining holes injected into the hole transport layer 156 from the anode 155 and electrons injected into the electron transport layer 158 from the cathode 159, light is emitted. As indicated by an arrow in the drawing, this emitted light is transmitted through the glass substrate 151 from the transparent anode 155 side and radiated to the outside. The anode 155 and light emitting layer 157 are separately formed for each pixel, and the hole transport layer 156, electron transport layer 158, and cathode 159 are formed in common with all the pixels. This technology is described in Japanese Patent Laid-open Publication No. 2002-251167.
As described above, the organic EL element constituting each pixel of the organic EL display is a current-driven type light emitting element which emits light according to electric current flowing between the anode and the cathode.
In the conventional organic EL display, the organic EL element emits light based on a luminance level adjusted before product shipment.
This causes a problem that, for example, in the open air, where ambient light intensity is high, contrast of the display area is reduced, and the display area is difficult to be observed.
Moreover, indoors or at night, where the display area has enough contrast, constant electric current is always supplied to the organic EL elements. This leads to problems that power consumption of the organic EL display cannot be reduced and the operating life of the organic EL elements cannot be extended.
The invention provides a display device that includes a display area having a plurality of pixels arranged on a substrate and displaying an image, a photosensor provided on the substrate and configured to measure ambient light intensity, and a luminance adjustment controller outputting a signal to adjust a contrast of the image displayed in the display area based on the measured ambient light intensity.
The invention also provides a display device that includes a display area having a plurality of pixels arranged on a substrate, a photosensor provided on the substrate and configured to measure ambient light intensity, a luminance adjustment controller outputting a signal to adjust luminance of the pixels based on the measured ambient light intensity, and display data correction circuit adjusting image data supplied to the pixels based on the signal outputted from the luminance adjustment circuit.
The invention further provides a display device that includes a display area having a plurality of pixels arranged on a substrate, an electroluminescent element disposed in each pixel and having a light emitting layer disposed between an anode and a cathode, a thin film transistor disposed in each pixel and driving a corresponding electroluminescent element, a photosensor provided on the substrate and configured to measure ambient light intensity, a luminance adjustment controller outputting a signal to adjust luminance of the electroluminescent elements based on the measured ambient light intensity, a first power source connected with the thin film transistor and supplying a first power source voltage to the thin film transistor, a second power source connected with the electroluminescent element and supplying a second power source voltage to the electroluminescent element, a voltage changing circuit changing a potential between the first and second power sources based on the signal outputted from the luminescent adjustment controller.
With reference to FIGS. 1 to 20, a detailed description will be given of embodiments of the present invention taking as an example an active matrix type organic EL display using TFTs.
FIGS. 1 to 9C are views explaining a first embodiment of the present invention and explain a case of adjusting luminance of a display area in a display data correction circuit.
A display device 20 includes a display area 21, a photo sensor 100, and a driver integrated circuit (IC) 50.
The display area 21 is formed of a plurality of display pixels 30 arranged in a matrix on an insulating substrate 10 of glass or the like. Each of the display pixels 30 includes an EL element having a light emitting layer between an anode and a cathode, a driving transistor to drive the EL element, and a selection transistor. Both of the driving and selection transistors are thin film transistors (hereinafter, referred to as TFTs).
On the substrate 10, a plurality of drain lines 2 and a plurality of gate lines 1 are arranged, and the display pixels 30 are arranged corresponding to individual intersections of the drain lines 2 and the gate lines 1. To be specific, the display pixels 30 are connected to sources of the driving TFTs, and the drains and gates of the driving TFTs are connected to the drain lines 2 and gate lines 3, respectively.
On the outside of the display area 21, along its side edges, a horizontal scanning circuit (hereinafter, referred to as an H scanner) 22, which sequentially selects the drain lines 2 extending in the column direction, and a vertical scanning circuit (hereinafter, referred to as a V scanner) 23, which supplies gate signals to the gate lines 1 extending in the row direction, are provided. Not-shown wires for transmitting various types of signals inputted into the gate lines 1 and drain lines 2 and the like are gathered in a side edge of the substrate 10 and connected to an external connection terminal 24.
The photo sensor 100 comprises a TFT provided on the same substrate (plane) as that of the display area 21. In the photo sensor 100, a photocurrent is obtained according to light irradiated when the TFT is off. In other words, the photo sensor 100 is to sense ambient light and detect a photocurrent according to the ambient light intensity.
The driver IC 50 includes a luminance adjustment controller 51 adjusting luminance and a display data correction circuit 53 outputting data signals Vdata to the display area 21. The driver IC 50 further includes a DC/DC converter 56 and applies driving voltage to the driving TFTs connected to the organic EL elements to cause the organic EL elements to emit light.
The luminance adjustment controller 51 of the first embodiment includes a reference voltage acquisition unit 52, and according to the ambient light intensity detected by the photo sensor 100, outputs a correction value for maintaining constant contrast of the display area 21.
In this embodiment, first, the ambient light intensity is detected by the photo sensor 100. The detected ambient light intensity is inputted into the luminance adjustment controller 51. The correction value with which predetermined contrast can be maintained for the present ambient light intensity is calculated.
The display data correction circuit 53 includes a gradation standard voltage generation circuit 54 and a gamma correction circuit 55. The gradation standard voltage generation circuit 54 divides a voltage between first and second reference voltages to obtain a plurality of gradation display voltages. The gamma correction is to correct a proportional relation between outputted luminance and an inputted signal raised to the power of gamma into a proportional relation between the outputted luminance and the inputted signal.
The first reference voltage with a lower potential corresponds to a maximum luminance level (white) of the EL element of the display pixel 30, and the second reference voltage with a higher potential corresponds to a minimum luminance level (black) of the EL element thereof. In the present specification hereinbelow, the first and second reference voltages are referred to as white and black reference voltages, respectively.
The correction value is inputted to the display data correction circuit 53 and set as the white reference voltage of the gradation standard voltage generation circuit 54. The gradation standard voltage generation circuit 54 divides the voltage between the white and black reference voltages for each RGB color to generate the plurality of gradation display voltages. The display data correction circuit 53 carries out D/A (digital-analog) conversion of the data signals to generate analog RGB data signals based on the plurality of gradation display voltages. The analog RGB data signals are further corrected by the gamma correction circuit 55. The data signals Vdata are outputted to the display area 21 to display an image. Thus, the display area 21 can perform gradation display based on the gradation display voltages.
In this embodiment, the correction value to obtain a predetermined contrast according to the ambient light intensity is calculated and set as the white reference voltage of the gradation standard voltage generation circuit 54.
The plurality of gate lines 1 extending in the row direction are arranged, and the plurality of the drain lines and driving lines 3 are arranged in the column direction in order for them to cross the gate lines 1. Each of the driving lines 3 is connected to a power source PV. The power source PV is a power source outputting, for example, a positive constant voltage.
At each intersection of the gate lines 1 and drain lines 2, a selection TFT 4 is connected to the gate lines 1 and drain lines 2. The gate and drain of the each selection TFT 4 are connected to the gate lines 1 and drain lines 2, respectively. The source of the each selection TFT 4 is connected to a storage capacitor 5 and a driving TFT 6.
The drain of the driving TFT 6 is connected to the driving line 3, and the source thereof is connected to an anode of an organic EL element 7. A cathode of the each organic EL element 7 is connected to a power source CV. The power source CV is a power source outputting, for example, a negative constant voltage. As long as the voltage of the power source PV is higher than that of the power source CV, the voltage of each power source may be either positive or negative. The opposite electrode of the storage capacitor 5 is connected to a capacitance line 9 extending in the row direction.
The gate lines 1 are connected to a not-shown V scanner, and gate signals are sequentially applied to the gate lines 1 by the V scanner. Each of the gate signals is a binary signal which becomes on or off. The gate signal has a predetermined positive voltage when it becomes on, and the gate signal is 0 V when it becomes off. The V scanner turns on the gate signal of a predetermined gate line selected out of the plurality of gate lines 1 connected thereto. When the gate signal is turned on, all of the selection TFTs 4 connected to the selected gate line 1 are turned on, and through the selection TFTs 4, the drain lines 2 and the gates of the driving TFTs 6 are connected.
The data signals Vdata determined according to pictures to be displayed are outputted from the H scanner 22 to the drain lines 2. The data signals Vdata are inputted to the gate of the individual driving TFTs 6 and are charged in the storage capacitors 5.
Each of the driving TFT 6 connects the driving line 3 and the organic EL element 7 with conductivities according to the magnitude of the data signal Vdata. Electric currents according to the data signal Vdata is then supplied from the driving line 3 through the driving TFT 6 to the organic EL element 7, and the organic EL element 7 emit light with luminance according to the data signal Vdata.
Each of the storage capacitors 5 forms a capacitance in conjunction with another electrode such as the dedicated capacitance line 9 or the driving line 3 and is capable of storing a data signal for a certain period of time.
The data signals Vdata are stored by the storage capacitors 5 during one vertical scanning period after the V scanner selects another gate line 1 and the previously selected gate line 1 becomes unselected state to turn off the selection TFTs 4. During that period, the driving TFTs 6 maintain the same conductivities as described above, and the organic EL elements 7 can continue to emit light at the same luminance.
In this embodiment, as shown in
In the display pixel 30, an insulating film (SiN, SiO2, or the like) 14 as a buffer layer is provided on an insulating substrate 10 made of quarts, alkali-free glass, or the like. On the insulating film 14, a semiconductor layer 63 made of a p-Si (poly-silicon) film is laminated. This p-Si film may be formed by laminating an amorphous silicon film followed by recrystallization by laser annealing or the like.
On the semiconductor layer 63, a gate insulating film 12 made of SiN, SiO2, or the like is laminated, on which a gate electrode 61 is formed of refractory metal such as chromium (Cr) and molybdenum (Mo). The semiconductor layer 63 includes a channel 63c, which becomes intrinsic or substantially intrinsic, located under the gate electrode 61. On the both sides of the channel 63c, a source 63s and a drain 63d as n+ type impurity diffused regions are provided to constitute the driving TFT 6. The selection TFT, for which a drawing is omitted, has the same structure.
On the entire surface of the gate insulating film 12 and the gate electrode 61, a SiO2 film, a SiN film, a SiO2 film in this illustrative order are sequentially stacked to laminate an interlayer insulating film 15. Contact holes are provided in the gate insulating film 12 and interlayer insulating film 15 in a manner that the contact holes correspond to the drain 63d and the source 63s, and the contact holes are then filled with metal such as aluminum (Al) to form drain electrodes 66 and source electrodes 68, respectively. The drain electrodes 66 and source electrodes 68 are configured to be in contact with the drain 63d and the source 63s, respectively. On a planarization insulating film 17, an anode 71 of ITO (indium tin oxide) or the like is provided as a display electrode. The anode 71 is connected to the source electrode 68 (or the drain electrode 66) through a contact hole provided in the planarization insulating film 17.
The organic EL element 7 includes a hole transport layer 72, a light emitting layer 73, and an electron transport layer 74 stacked on the anode 71 in this order, and further includes a cathode 75 formed of a magnesium-indium alloy. The cathode 75 is provided for the entire surface of the substrate 10 forming the organic EL display device 20 or the entire surface of the display area 21.
In the organic EL element 7, holes injected from the anode 71 and electrons injected from the cathode 75 are recombined inside the light emitting layer 73 to excite organic molecules forming the light emitting layer 73 and generate exciters. In a process of radiative deactivation of the exciters, light is radiated from the light emitting layer 73. The light is released from the transparent anode 71 through the transparent insulating substrate 10 to the outside for luminescence.
The cross-sectional view of
As shown in
Specifically, the photo sensor 100 is a TFT in which a gate electrode 101, an insulating film 102, and a semiconductor layer 103 made of a p-S film are stacked on the insulating substrate 10 and the semiconductor layer 103 includes a channel 103c, a source 103s, and a drain 103d.
In the p-Si TFT of such a structure, when incident light enters the semiconductor layer 103 from the outside while the TFT is off, electron-hole pairs are generated in a junction region between the channel 103c and the source 103s or between the channel 103c and drain 103d. These electron-hole pairs are drawn apart by an electric field of the junction region to generate a photovoltaic, whereby a photocurrent is obtained. The photocurrent is outputted from, for example, the source electrode 108 side.
In other words, the TFT is utilized as the photo sensor 100 by sensing the increase in photocurrent obtained when the TFT is off.
Here, the semiconductor layer 103 may be provided with a low concentration impurity region. The low concentration impurity region is provided in adjacent to the source 103s or the drain 103d on the channel 103c side and has a lower impurity concentration than that of the source 103s or the drain 103d. The low-concentration impurity region can reduce the electric field concentrated in an edge portion of the source 103s (or drain 103d). The width of the low concentration impurity region is, for example, about 0.5 to 3 μm.
In this embodiment, a low concentration impurity region 103LD is provided, for example, between the channel and the source (or between the channel and the drain) to form a so-called light doped drain (LDD) structure. In the LDD structure, the junction region contributing to generation of the photocurrent can be extended in the direction of the gate length L, thus facilitating generation of the photocurrent. Specifically, the low concentration impurity region 103LD may be provided at least on a side from which the photocurrent is taken out. Moreover, the LDD structure stabilizes an OFF characteristic (region for detection) of Vg-Id characteristics, thus the device can be made stable.
The above description has been given to the photo sensor 100 of the top gate structure. The photo sensor 100 may have the bottom gate structure where the gate electrode 101 is arranged under the semiconductor layer 103. The drawing shows only the TFT as the photo sensor 100. However, the TFT may be connected to a detection circuit and convert the photocurrent into voltage for detection when necessary.
A description will be given of contrast with reference to
As shown in
Contrast CR, the self-emitted light luminance Lel, and the reflected light luminance Lref have a relation expressed by the following equation.
CR=1+Lel/Lref
The reflected light luminance Lref has a proportional relation with the ambient light intensity, and the higher the ambient light intensity, the larger the reflected light luminance Lref is. If the self-emitted light luminance Lel of the organic EL element is constant at this time, the self-emitted light luminance Lel is reversed with a magnitude of the reflected light luminance Lref. This means that the contrast is reduced and has a characteristic indicated by a solid line a of
In this specification, luminance L (L1, L2 or L3) necessary for keeping the contrast CR constant in certain ambient light is referred to as necessary luminance L.
In addition, the contrast CR satisfies the following relation.
CR=(Lel(white)+Lel(black)+Lref)/(Lel(black)+Lref)=1+Lel(white)/(Lel(black)+Lref)
where Lel(white) denotes luminance of white, and Lel(black) denotes luminance of black.
At the time of product shipment, adjustment has been carried out in order that enough contrast CR can be obtained indoors (i.e., that black can be sufficiently observed as black). The Lel(black) is low enough, and this value does not change even in the open air. Specifically, the Lel(black) is around 0 (anywhere either indoors or in the open air) independent of the value of the Lref.
The contrast CR is a difference between the Lel(white) and Lel(black), and the Lel(black) is low enough and close to 0 independent of the reflected light luminance Lref as described above. When the contrast CR is reduced, therefore, an increase in the Lel(white) enables the contrast CR to be maintained constant.
Meanwhile, the photo sensor 100 outputs a photocurrent according to the ambient light as described above. Specifically, the photo sensor 100 includes analog and digital outputs corresponding to the ambient light, and the relation between the photocurrent and the ambient light can be obtained by measuring the characteristics of the photo sensor 100 beforehand.
In this embodiment, the necessary luminance L is calculated according to the ambient light, and the reference voltage determining the Lel(white) is corrected. Using the data signal Vdata thus obtained, the value of the gate voltage VG of the driving TFT 6 can be adjusted as shown in
With reference to
With reference to
First, the light intensity is detected by the photo sensor 100. For example, analog values of current and voltage according to the light intensity are detected and inputted into the luminance adjustment controller 51.
In the luminance adjustment controller 51, the necessary luminance L to maintain constant contrast is obtained based on the current and voltage values according to the ambient light-CR characteristic chart (
Next, the necessary luminance L is inputted in the reference voltage acquisition unit 52. Between the reference voltage of the gradation standard voltage generation circuit 54 and the luminance, there is a relation as shown in
With reference to
First, the light intensity is detected by the photo sensor 100. For example, in the case of certain ambient light, the on/off state of the photo sensor 100 is detected, and the signal (1/0) thereof (photo sensor output) is inputted into the luminance adjustment controller 51.
In the luminance adjustment controller 51, the necessary luminance L to maintain substantially constant contrast is obtained based on the input signal according to the ambient light-CR characteristic chart (
Next, in the reference voltage acquisition unit 52, the correction value Vsig corresponding to the necessary luminance L is obtained according to the characteristic chart as shown in
With reference to
First, the light intensity is detected by the photo sensor 100. The photo sensor 100 in this case changes, depending on luminance, with regard to timing when it becomes on, and the area of the pulse waveform during the on-state is integrated to obtain an analog value.
Specifically, the pulse waveform is inputted into the luminance adjustment controller 51 as shown in
In the luminance adjustment controller 51, the necessary luminance L for maintaining constant contrast is obtained based on the input signal (analog DC waveform) according to the ambient light-CR characteristic chart (
Next, in the reference voltage acquisition unit 52, the correction value Vsig corresponding to the necessary luminance L is obtained according to the characteristic chart as shown in
In the first embodiment, the display data correction circuit 53 includes the gradation standard voltage generation circuit 54 and the gamma correction circuit 55. The correction value Vsig outputted as described above is inputted into the gradation standard voltage generation circuit 54.
As shown in
In this circuit, the black reference voltage is fixed, and the white reference voltage of the gradation standard voltage generation circuit 54 is set to be the correction value Vsig.
The gradation standard voltage generation circuit 54 generates gradation display voltages between the corrected white reference voltage (Vsig) and the black reference voltage (fixed value).
For example, when the white reference voltage is reduced, only the white level is reduced (3V to 2V) as shown in
The correction is to change the white reference voltage, and black-and-white gradations are obtained by dividing the voltage range between the white and black reference voltages by resistors. Even when the white reference voltage is changed, therefore, the correction to maintain constant contrast can be performed without reducing the number of gradations.
256 analog voltages (gradation display voltages) for gradation display generated by the gradation standard voltage generation circuit 54 are outputted for each RGB color as the data signal Vdata through the gamma correction circuit 55 and drain signal lines to the display pixels 30 within the display area 21.
In the aforementioned example, the description has been given of the case where the white reference voltage is changed by the correction value Vsig. In addition thereto, the gamma characteristics used in the gamma correction may be changed.
In some cases, even the same color (for example, red) observed by the same user may look different indoors and outdoors. The gamma correction is to correct visibility of the gradations between black and white. It is therefore conceivable that the gamma characteristics may be changed due to the ambient light (reflected light). Accordingly, holding different gamma characteristics corresponding to the correction values Vsig, the gamma correction can be performed using a gamma characteristic suitable for that case, after the adjustment of the white reference voltage is performed according to the ambient light intensity.
The luminance adjustment by the first embodiment can be applied not only to the organic EL display of two transistor type (
Moreover, the luminance adjustment can be applied an organic EL display of a type (hereinafter, referred to as a digital duty driving type) in which a light emission period changes in proportion to a reference voltage. In the case of the digital duty driving type, the light emission period of the organic EL element changes with the reference voltage. In other words, each element has its emission height (luminance while emitting light) being constant, but the entire luminance of the display area can be changed by the reference voltage. Setting the white reference voltage to be the correction value Vsig therefore enables the contrast to be maintained constant.
Furthermore, the above description has been given taking as an example the organic EL display in which the display area 21 is composed of the display pixels 30 using the organic EL elements, but the display device is not limited to this. The display device 20 including pixels with driving TFTs formed of low-temperature polysilicon, such as LCD, can be implemented in a similar way. Specifically, only with the display device 20 replaced with the LCD or the like in
Next, a description will given of a case as a second embodiment where luminance of a device is adjusted by a value of a power source CV which supplies one of power source voltages of the driving TFT with reference to FIGS. 10 to 20. The second embodiment is mainly suitable for an organic EL display device of the digital duty drive type.
The organic EL display includes a display area 21, a photo sensor 100, and a driver integrated circuit 50.
The display area 21 and the photo sensor 100 are the same as those of the first embodiment, and details thereof are omitted.
The driver IC 50 includes a luminance adjustment controller 51 adjusting luminance and a display data correction circuit 53 outputting data signals Vdata to the display area 21. The driver IC 50 further includes a DC/DC converter 56 and applies a driving voltage to the driving TFTs connected to the organic EL elements to cause the organic EL elements to emit light.
The luminance adjustment controller 51 of the second embodiment includes a CV value calculation unit 57 and outputs a correction value to maintain constant contrast of the display area 21 according to ambient light intensity sensed by the photo sensor 100.
The luminance adjustment controller 51 includes a voltage changing circuit 58 within the DC/DC converter 56, which supplies a power source voltage of the driving TFTs driving the organic EL elements. The correction value outputted from the luminance adjustment controller 51 is inputted into the voltage changing circuit 58, and the power source voltage applied to the driving TFTs is changed to adjust the contrast of the display area 21.
The display data correction circuit 53 performs digital/analog (D/A) conversion of the data signals, and analog RGB data signals generated using the plurality of gradation display voltages are corrected in a gamma correction circuit 55. The data signals Vdata are outputted to drain lines 2, thus displaying an image.
An equivalent circuit diagram of the organic EL display device 20 is the same as that of the first embodiment (
The driving TFT 6 and the organic EL element 7 are connected in series between the power sources PV and CV. A driving current which flows through the organic EL element 7 is supplied from the power source PV through the driving TFT 6 to the organic EL element 7. The light emitting layer of the organic EL element 7 emits light according an amount of the driving current.
In the second embodiment, the power sources PV and CV are generated by the DC/DC converter 56. The power source PV is fixed, and the power source CV can be varied by the power source changing circuit 58. Details of the power source changing circuit 58 are described later. In this embodiment, the ambient light intensity is detected by the photo sensor 100, and a correction value to maintain predetermined contrast is calculated by the luminance adjustment controller 51. The correction value is inputted to the power source changing circuit 58, and the power source CV is changed according to the correction value. Upon the power source PV and the corrected power source CV being applied between the driving TFT 6 and the organic EL element 7, the organic EL element 7 emits light according to the potential difference thereof, and the display area 21 can maintain predetermined contrast.
As shown in
On the other hand, by increasing the self-emitted light luminance Lel or the intensity of self-emitted light of the organic EL element according to the ambient light intensity, the contrast of the display area 21 can be maintained constant (a solid line b in
Additionally, the photo sensor 100 has an analog output for ambient light, and a relation between ambient light and a photocurrent can be obtained by measuring the characteristic of the photo sensor 100 beforehand. In other words, when the contrast decreases, certain constant contrast can be maintained by changing the voltage applied between the driving TFT 6 and the organic EL element 7 and increasing the self-emitted light luminance Lel. In the second embodiment, the power source PV is fixed, and the power source CV is changed.
With reference to
In
As an example, if CV1 (dashed line a) is changed into CV2 (dashed line b), the operating point rises (from x1 to x2). The organic EL element 7 can therefore operate in a region having large Id, and the self-emitted light luminance Lel can be increased.
As shown in
A description will be given of the luminance adjustment controller 51 of the second embodiment with reference to
With reference to
First, the light intensity is detected by the photo sensor 100. For example, analog values of current and voltage according to the light intensity are detected and inputted into the luminance adjustment controller 51.
In the luminance adjustment controller 51, necessary luminance L to maintain constant contrast is obtained based on the current and voltage values according to the ambient light-CR characteristic chart (
Next, in the CV value calculation unit 57, a CV value corresponding to the necessary luminance L is obtained according to the characteristic chart shown in
In this embodiment, the calculated CV value is further converted into a signal which can be passed to the voltage changing circuit 58, and is then outputted. As the correction value, therefore, the value converted for passing, not the CV value itself, is outputted, which is hereinafter described as a correction value SOP. For example, in the case of
Furthermore, in the case where the CV value obtained in the CV value calculation unit 57 can be passed, without the change, as the voltage value of the power source CV, the CV value may be outputted as the correction value without being converted into the correction value SOP.
With reference to
First, the amount of light is detected by the photo sensor 100. For example, in the case of certain ambient light, an on/off state of the photo sensor 100 is detected, and the signal (1/0) thereof is inputted into the luminance adjustment controller 51.
In the luminance adjustment controller 51, the necessary luminance L to maintain substantially constant contrast is obtained based on the input signal according to the ambient light-CR characteristic chart (
Next, in a CV value calculation unit 52, a CV value corresponding to the necessary luminance L is obtained according to the characteristic chart as shown in
With reference to
First, the amount of light is detected by the photo sensor 100. The photo sensor 100 in this case changes in on time depending on luminance, and the area of a pulse section in the on time is integrated to obtain the analog value.
Specifically, the pulse waveform is inputted into the luminance adjustment controller 51 as shown in
In the luminance adjustment controller 51, the necessary luminance L to maintain constant contrast is obtained based on an analog value according to the ambient light-CR characteristic chart (
Next, in the CV value calculation unit 57, a CV value corresponding to the necessary luminance L is obtained according to the characteristic chart as shown in
Similar to the case shown in
The luminance adjustment controller 51 obtains a CV value (analog value) corresponding to the necessary luminance L according to the characteristic chart shown in
Herein, when the input of the voltage changing circuit 58 is analog, the output as the correction value SOP should be analog. However, when the TFT constituting the voltage changing circuit 58 and the TFT constituting the photo sensor 100 have different characteristics to an analog value, matching thereof is required. The above CV value (analog value) is a value to which the matching has been applied, and is outputted as the correction value SOP.
Similar to the case shown in
The luminance adjustment controller 51 obtains a CV value (analog value) corresponding to the necessary luminance L according to the characteristic chart shown in
The conversion to match the correction value SOP with the TFT constituting the voltage changing circuit 58 is then carried out, and the analog value obtained by the conversion is outputted as the correction value SOP.
FIGS. 18 to 20 are circuit diagrams showing the voltage changing circuit 58. The voltage changing circuit 58 of this embodiment is provided within the DC/DC converter 56 and supplies the power sources PV and CV of the driving TFT 6 and the organic EL element 7 as shown in
Specifically, as shown in FIGS. 18 to 20, the voltage changing circuit 58 is a circuit including a series regulator SR provided with a regulator IC 81, switching TFTs 82, and resistors R. The voltage changing circuit 58 is configured in order that each resistor R can be switched on and off depending on the correction value SOP. Herein, the regulator IC 81 outputs a signal ADJ which determines a maximum CV value.
The signal inputted into the switching TFT 82 is the correction value SOP outputted from the luminance adjustment controller 51. In the case of the two-step adjustment circuit, the inputted correction value SOP is the correction value SOP (1/0) shown in
The signal inputted to the each TFT 82 is the correction value SOP (1/0) outputted from the luminance adjustment controller 51 shown in
As an example, the voltage changing circuit 58 is configured in order that: when the resistors R1 and R2 are off, the luminance can be 80 cd/m2; when the resistors R1 and R2 are on and off, respectively, the luminance can be 150 cd/m2; and when the resistors R1 and R2 are off and on respectively, the luminance can be 250 cd/m2 (for those resistance values, R1=R2). As a result of sensing ambient light, suppose the luminance adjustment controller 51 calculates that luminance of 80 cd/m2 is required. The luminance adjustment controller 51 then calculates such a CV value that the above luminance can be obtained and further converts the CV value to the correction value SOP, outputting SOP1=0 and SOP2=0. The two resistors of the multi-step adjustment circuit are both disconnected, and the corresponding CV value can be obtained. This CV value is supplied to the power source CV, and the corrected voltage is therefore applied to the organic EL elements 7, the luminance of which is 80 cd/m2.
In a similar way, when SOP1=1 and SOP2=0 are inputted, the luminance of the organic EL element 7 is 150 cd/m2. When SOP1=1 and SOP2=1 are inputted, the luminance of the organic EL elements 7 is 250 cd/m2.
In the drawing, three-step adjustment using two resistors is described. In the case of multi-step adjustment circuit, the CV value can be changed in steps according to the number of the resistors connected. Accordingly, with more resistors connected, finer luminance adjustment can be achieved.
Herein, in the case where the correction value SOP takes two values (1/0), the luminance adjustment controller 51 shown in
This multi-step adjustment circuit has the same structure as that shown in
According to embodiments of the present invention, first, the organic EL display is provided with the photo sensor and the luminance adjustment controller, whereby luminance can be adjusted according to ambient light sensed by the photo sensor. This enables the display area to maintain constant contrast even when a surrounding environment thereof changes. Moreover, the amount of electric current is adjusted according to the ambient light, whereby it is possible to provide the organic EL display which achieves lower power consumption and longer operating life.
Secondly, the data signal outputted to the display area is adjusted by the correction value outputted from the luminance adjustment controller. This enables the display area to maintain constant contrast even when light intensity of the environment thereof varies.
Thirdly, the white reference voltage of the gradation standard voltage circuit is set to be the correction value outputted from the luminance adjustment controller to adjust the data signal, and luminance of the display area can be thereby adjusted. Moreover, in this case, the luminance adjustment can contribute to electric current of power consumption (P=V×I), and therefore the power consumption can be reduced. Furthermore, different gamma characteristics corresponding to ambient light are held, and gamma correction is performed by a gamma characteristic corresponding to the correction value. This enables correction of intermediate gradations between black and white.
Fourthly, setting the white reference voltage of the gradation standard voltage circuit to be the correction value allows the luminance of the display area to be adjusted without reducing the number of gradations. The luminance Lel(black), if enough contrast of the display area is obtained indoors before product shipment or the like, is low enough even in the open air, and changes thereof cannot affect the contrast. On the other hand, an increase in the luminance Lel(white) can increase the contrast. In other words, the white reference voltage is changed to increase the luminance Lel(white) and enables the display area to maintain constant contrast even in the open air with plenty of reflected light.
Fifthly, the voltage applied to the thin film transistor and the EL element is adjusted by the correction value outputted from the luminance adjustment controller. The display area can therefore maintain constant contrast even when the light intensity of its environment varies. Moreover, a value of the power source CV is changed. Accordingly, the luminance adjustment can be directly reflected on the power consumption and, in particular, can contribute to both a voltage and an electric current in power consumption (P=V×I). In the case of using the display device indoors without increasing the luminance, a large effect can be therefore obtained on reduction of power consumption.
Sixthly, the value of the power source CV of the voltage changing circuit is changed by the correction value, and the display device can therefore operate in a region with a large electric current.
Seventhly, the photo sensor is a TFT and can be arranged on the substrate as that of the display area. Accordingly, the photo sensor can sense light intensity equivalent to the ambient light received by the display area. The luminance can be therefore adjusted according to the ambient light intensity in order that the luminance can be increased when it is bright and is reduced when it is dark.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-205258 | Jul 2004 | JP | national |
