Patent Grant
9928781
The present invention relates to an organic EL display device and a driving method, and particularly relates to a method of driving an organic EL display device that uses current-driven organic EL light emitting elements and the organic EL display device.
In an organic EL panel used in an organic EL display device, a larger current flows into the organic EL panel when screen luminance is higher. The flow of a larger current into the organic EL panel causes the organic EL panel to generate heat, which leads to shorter life of organic EL elements (light emitting elements). Overload control is therefore necessary. Moreover, since a heat dissipation mechanism is needed to cool the panel, a thinner panel cannot be realized.
For example, Patent Literature (PTL) 1 discloses a method of directly detecting power consumption from pixel data before actually displaying an image.
Japanese Unexamined Patent Application Publication No. 2007-156045
With the method disclosed in PTL 1, however, it is necessary to increase or decrease the gray level of input image data before actually displaying the image, or control the drive current in the image being displayed. In the case of increasing or decreasing the gray level of input image data before actually displaying the image, the operation is complex, and it takes time to display the image. In the case of controlling the drive current in the image being displayed, the luminance of the image being displayed changes, with which the viewer may feel uncomfortable.
The present invention has been made in view of the aforementioned problems, and has an object of providing an organic EL display device and a method of driving the organic EL display device that can prevent a decrease in life of light emitting elements.
To achieve the object stated above, an organic EL display device according to an aspect of the present invention includes: a plurality of display pixels arranged in a matrix; and an adjustment unit that adjusts a predetermined voltage applied to a power line connected to the plurality of display pixels, wherein each of the plurality of display pixels includes: a light emitting element; a first capacitance element that holds a first voltage used to cause the light emitting element to emit light; a drive transistor that causes the light emitting element to emit light, by supplying a current corresponding to the first voltage held in the first capacitance element to the light emitting element; and a second capacitance element that holds a second voltage different from the first voltage held in the first capacitance element, the second voltage being a next voltage to be held in the first capacitance element, the power line is connected to a drain electrode of the drive transistor or a cathode of the light emitting element, and the adjustment unit adjusts the voltage applied to the power line to be lower than the predetermined voltage, when a total sum of current of the plurality of display pixels in the case where a current corresponding to the second voltage is supplied to the light emitting element in each of the plurality of display pixels is greater than or equal to a threshold.
The organic EL display device, etc. according to the present invention can prevent a decrease in life of light emitting elements.
One aspect of an organic EL display device according to the present invention includes: a plurality of display pixels arranged in a matrix; and an adjustment unit that adjusts a predetermined voltage applied to a power line connected to the plurality of display pixels, wherein each of the plurality of display pixels includes: a light emitting element; a first capacitance element that holds a first voltage used to cause the light emitting element to emit light; a drive transistor that causes the light emitting element to emit light, by supplying a current corresponding to the first voltage held in the first capacitance element to the light emitting element; and a second capacitance element that holds a second voltage different from the first voltage held in the first capacitance element, the second voltage being a next voltage to be held in the first capacitance element, the power line is connected to a drain electrode of the drive transistor or a cathode of the light emitting element, and the adjustment unit adjusts the voltage applied to the power line to be lower than the predetermined voltage, when a total sum of current of the plurality of display pixels in the case where a current corresponding to the second voltage is supplied to the light emitting element in each of the plurality of display pixels is greater than or equal to a threshold.
With this structure, a decrease in life of light emitting elements can be prevented. In detail, the organic EL display device includes the adjustment unit, and is capable of writing a data signal voltage and displaying video independently of each other. This enables overload control so that the panel power due to the current flowing into the organic EL panel is less than a predetermined level, without lowering video quality. A decrease in life of the organic EL elements can thus be prevented.
For example, the organic EL display device may further include a control unit that controls each of the plurality of display pixels, wherein the control unit, in each of the plurality of display pixels: causes the second capacitance element to hold the second voltage, in a first period in a light emission period during which the light emitting element emits light; and copies the second voltage held in the second capacitance element to the first capacitance element to cause the first capacitance element to hold the second voltage as the first voltage, in a second period following an initialization period in a non-light emission period after the light emission period, the initialization period being a period during which the first capacitance element is initialized, and the adjustment unit adjusts the voltage applied to the power line to be lower than the predetermined voltage in the non-light emission period, when the total sum of current of the plurality of display pixels corresponding to the second voltage is greater than or equal to the threshold.
For example, the organic EL display device may display video by dividing one frame period of a video signal into a plurality of subframe periods, wherein a period between a light emission period during which the light emitting element emits light and a next light emitting period during which the light emitting element emits light corresponds to any of the plurality of subframe periods.
For example, the adjustment unit may adjust the voltage applied to the power line to be lower than the predetermined voltage by linearly changing the voltage for a predetermined time, when the total sum of current of the plurality of display pixels corresponding to the second voltage in the case where the current corresponding to the second voltage is supplied to the light emitting element in each of the plurality of display pixels is greater than or equal to the threshold.
For example, in the case where the drive transistor is p-type, the drive transistor may have the drain electrode connected to a first electrode of the first capacitance element, a gate electrode connected to a second electrode of the first capacitance element, and a source electrode connected to an anode of the light emitting element, and the power line may be connected to the cathode of the light emitting element.
For example, each of the plurality of display pixels may further include: a first switch that switches between conduction and nonconduction between a signal line for supplying a data signal voltage and a first electrode of the second capacitance element; a second switch that switches between conduction and nonconduction between the first electrode of the second capacitance element and the second electrode of the first capacitance element; a third switch that switches between conduction and nonconduction between a reference power line for supplying a reference voltage and the second electrode of the first capacitance element; and a fourth switch that switches between conduction and nonconduction between the source electrode of the drive transistor and the anode of the light emitting element, the reference power line may be connected to a second electrode of the second capacitance element, and the first switch, the second switch, the third switch, and the fourth switch may each be a p-type transistor.
For example, in the case where the drive transistor is n-type, the power line may be connected to the drain electrode of the drive transistor, and the drive transistor may have a source electrode connected to a first electrode of the first capacitance element and an anode of the light emitting element, and a gate electrode connected to a second electrode of the first capacitance element.
For example, each of the plurality of display pixels may further include: a first switch that switches between conduction and nonconduction between a signal line for supplying a data signal voltage and a first electrode of the second capacitance element; a second switch that switches between conduction and nonconduction between the first electrode of the second capacitance element and the second electrode of the first capacitance element; a third switch that switches between conduction and nonconduction between a reference power line for supplying a reference voltage and the second electrode of the first capacitance element; and a fourth switch that switches between conduction and nonconduction between the drain electrode of the drive transistor and the power line, the reference power line may be connected to a second electrode of the second capacitance element, and the first switch, the second switch, the third switch, and the fourth switch may each be an n-type transistor.
One aspect of a driving method according to the present invention is a method of driving an organic EL display device including: a plurality of display pixels arranged in a matrix; and a power line that is connected to the plurality of display elements and to which a predetermined voltage is applied, each of the plurality of display pixels including: a light emitting element; a first capacitance element that holds a first voltage used to cause the light emitting element to emit light; a drive transistor that causes the light emitting element to emit light, by supplying a current corresponding to the first voltage held in the first capacitance element to the light emitting element; and a second capacitance element that holds a second voltage different from the first voltage held in the first capacitance element, the second voltage being a next voltage to be held in the first capacitance element, the power line being connected to a drain electrode of the drive transistor or a cathode of the light emitting element, and the method including: adjusting the voltage applied to the power line to be lower than the predetermined voltage, when a total sum of current of the plurality of display pixels in the case where a current corresponding to the second voltage is supplied to the light emitting element in each of the plurality of display pixels is greater than or equal to a threshold; and causing the drive transistor to supply the current corresponding to the second voltage held in the first capacitance element to the light emitting element.
An organic EL display device and a method of driving the organic EL display device according to one aspect of the present invention are described in detail below, with reference to drawings.
Each of the embodiments described below shows a specific example of the present invention. The numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, steps, the processing order of the steps, etc. shown in the following embodiments are mere examples, and do not limit the scope of the present invention. Of the structural elements in the embodiments described below, the structural elements not recited in any one of the independent claims representing the broadest concepts are described as optional structural elements. Note that each drawing is a schematic and does not necessarily provide precise depiction.
This embodiment describes the case of using organic EL elements as light emitting elements of an organic EL display device according to one aspect of the present disclosure.
1-1. Structure of Organic EL Display Device
An organic EL display device 1 illustrated in
The organic EL display panel 6 at least includes N (e.g. N=1080) gate signal lines and N lighting control lines arranged in parallel with each other, and M source signal lines orthogonal to the N gate signal lines and N lighting control lines (not illustrated). The organic EL display panel 6 also includes pixel circuits each including thin-film transistors and an organic EL element, at the respective intersections of the source signal lines and gate signal lines (not illustrated).
In the organic EL display panel 6 according to this embodiment, EL elements of three primary colors of red (R), green (G), and blue (B) are arranged in a matrix.
Color filters of red (R), green (G), and blue (B) can be formed in correspondence with the pixel positions. The color filters are not limited to RGB, and pixels of cyan (C), magenta (M), and yellow (Y) may be formed. Pixels of white (W) may be formed, too. In this case, pixels of R, G, B, and W are arranged in a matrix on the display screen.
The pixel aperture ratio of each of R, G, and B may be different. When the aperture ratio is different, the density of current flowing through the light emitting element (the organic EL element 68) of each of R, G, and B is different. Such different current density allows the light emitting element of each of R, G, and B to have the same degradation speed. With the same degradation speed, the organic EL display panel 6 has no white balance deviation.
Pixels of white (W) are formed according to need. In this case, pixels of R, G, B, and W constitute the display screen. The RGBW composition contributes to higher luminance. The RGBG composition is also possible.
The organic EL display panel 6 is colorized by mask evaporation. This embodiment is, however, not limited to such. For example, a blue-light emitting EL layer may be formed, with the emitted blue light being converted into light of R, G, and B by color conversion layers of R, G, and B (color change media (CCM)).
A circularly polarizing plate (circularly polarizing film) (not illustrated) may be placed on the light emission surface of the organic EL display panel 6. The circularly polarizing plate (circularly polarizing film) is an integrated combination of a polarizing plate and a phase film.
The display panel control circuit 2 generates, based on a display data signal S1, a control signal S2 for controlling the source driver IC (circuit) 5, and outputs the generated control signal S2 to the source driver IC (circuit) 5. The display panel control circuit 2 also generates, based on a synchronization signal included in the display data signal S1, a control signal S3 for controlling the gate driver IC (circuit) 3, and outputs the generated control signal S3 to the gate driver IC (circuit) 3.
The display data signal S1 is a signal indicating display data, and includes a video signal, a vertical synchronization signal, and a horizontal synchronization signal. The video signal is a signal designating each pixel value as gray level information per frame. The vertical synchronization signal is a signal for synchronizing vertical processing on the screen, based on which per-frame processing is timed. The horizontal synchronization signal is a signal for synchronizing horizontal processing on the screen.
The control signal S2 includes the video signal and the horizontal synchronization signal. The control signal S3 includes the vertical synchronization signal and the horizontal synchronization signal.
The gate driver IC (circuit) 3 drives each gate signal line in the organic EL display panel 6, based on the control signal S3 generated by the display panel control circuit 2.
The source driver IC (circuit) 5 drives each source signal line in the organic EL display panel 6, based on the control signal S2 generated by the display panel control circuit 2. In more detail, the source driver IC (circuit) 5 outputs a source signal (data signal voltage) to each pixel circuit, based on the video signal and the horizontal synchronization signal.
The adjustment unit 7 adjusts a predetermined voltage applied to a power line (EL cathode power line) connected to the plurality of display pixels, based on the display data signal S1 received via the display panel control circuit 2. This will be described in detail later.
The organic EL display device 1 may include, for example, a central processing unit (CPU), a storage medium such as read only memory (ROM) storing a control program, working memory such as random access memory (RAM), and a communication circuit, although not illustrated. In this case, the display data signal S1 is generated, for example, by the CPU executing the control program.
In this embodiment, the organic EL display panel 6 is connected with gate signal lines 84, 85, 86, and 87 controlled by a gate driver IC (circuit) 30 and a gate driver IC (circuit) 31.
The organic EL display panel 6 is also connected with source signal lines 88 controlled by the source driver IC (circuit) 5. The gate signal lines 84, 85, 86, and 87 and the source signal line 88 will be described later.
Although the gate driver IC (circuit) 3 is composed of the gate driver IC (circuit) 30 and gate driver IC (circuit) 31 which are respectively provided on the left and right of the organic EL display panel 6 in
Although this embodiment assumes the circuit 5 as a source driver IC, the circuit is not limited to a source driver IC formed by a semiconductor chip. As an example, a silicon wafer is used to form transistors, which is peeled off and transferred onto a glass substrate. As another example, a silicon wafer is used to form a transistor chip, which is mounted on a glass substrate by bonding to form a display panel. Alternatively, a source driver circuit may be formed directly on a glass substrate on which pixels have been formed, using a technology such as low-temperature polysilicon, high-temperature polysilicon, or TAOS.
Although this embodiment assumes each of the circuits 30 and 31 as a gate driver IC, the circuit is not limited to a gate driver IC formed by a semiconductor chip. As an example, a silicon wafer is used to form transistors, which is peeled off and transferred onto a glass substrate. As another example, a silicon wafer is used to form a transistor chip, which is mounted on a glass substrate by bonding to form a display panel. Alternatively, a source driver circuit may be formed directly on a glass substrate on which pixels have been formed, using a technology such as low-temperature polysilicon, high-temperature polysilicon, or TAOS.
1-2. Circuit Structure of Display Pixel
The display pixel according to Embodiment 1 includes transistors, capacitors, an EL element, etc. Although each of the transistors including drive transistors and switch transistors is described as a thin-film transistor (TFT) here, this is not a limitation. The transistor may be a FET, a MOSFET, a MOS transistor, a bipolar transistor, or the like. These are also basically thin-film transistors. Other examples include a varistor, a thyristor, a ring diode, a photodiode, a phototransistor, and a PLZT element.
The element is also not limited to a thin-film element, and may be a transistor formed in a silicon wafer. As an example, a silicon wafer is used to form transistors, which is peeled off and transferred onto a glass substrate. As another example, a silicon wafer is used to form a transistor chip, which is mounted on a glass substrate by bonding to form a display panel.
Both n-type and p-type transistors preferably use a lightly doped drain (LDD) structure.
Each transistor may be formed using any of high-temperature polycrystalline silicon (HTPS), low-temperature polysilicon (LTPS), continuous grain silicon (CGS), transparent amorphous oxide semiconductor (TAOS, IZO), amorphous silicon (AS), and infrared RTA (RTA: rapid thermal annealing).
Although all transistors in the display pixel are p-type in
Each switch transistor is not limited to a transistor, and may be, for example, an analog switch formed using both p-type and n-type transistors.
Each transistor preferably has a top-gate structure. With the top-gate structure, parasitic capacitance is reduced, and the gate electrode pattern of the top gate serves as a light shielding layer which shields light emitted from the EL element. This reduces transistor malfunctions and off-leakage current.
A process that can use copper wiring or copper alloy wiring as the wiring material of gate signal lines, source signal lines, or both gate signal lines and source signal lines is preferable. This reduces the wiring resistance of the signal lines, and makes it possible to realize a larger EL display panel.
Each gate signal line driven (controlled) by the gate driver IC (circuit) preferably has lower impedance. The same applies to the structure of the gate signal line.
The use of low-temperature polysilicon (LTPS) is particularly preferable. With low-temperature polysilicon, n-type and p-type transistors of a top-gate structure with low parasitic capacitance can be produced, and the copper wiring or copper alloy wiring process can be used as the process. The copper wiring preferably has a three-layer structure of Ti—Cu—Ti.
The wiring of gate signal lines, source signal lines, or the like preferably has a three-layer structure of Mo—Cu—Mo where Mo is molybdenum, in the case where the transistors are TAOS (transparent amorphous oxide semiconductor).
Each capacitor is formed or placed so as to overlap at least one of a source signal line and a gate signal line. This increases the freedom of layout, and secures a wider space between elements. Yields are thus enhanced.
An insulation film or an insulation film (planarizing film) made of an acrylic material is formed on each source signal line and gate signal line to ensure insulation, and a pixel electrode is formed on the insulation film.
A pixel circuit 60 illustrated in
The pixel circuit 60 is an example of one of the display pixels (light emitting pixels) arranged in a matrix. For example, as illustrated in
The source signal line 88 is an example of a signal line (source signal line) for supplying a data signal voltage (source signal).
The EL anode power line 81 (VTFT) is a high-voltage power line, connected to the drain electrode of the drive transistor, for determining the potential of the drain electrode of the drive transistor 66. For example, the EL anode power line 81 (VTFT) is 6 V.
The EL cathode power line 82 (VEL) is a low-voltage power line connected to the cathode (second electrode) of the organic EL element 68.
The reference power line 83 (VREF) is an example of a power line for supplying a reference voltage, and supplies a reference voltage VREF. The potential difference between the reference voltage VREF supplied by the reference power line 83 (VREF) and the anode voltage VTFT supplied by the EL anode power line 81 (VTFT) is set to a voltage greater than the threshold voltage (Vth) of the drive transistor 66, that is, “(threshold voltage Vth)<(anode voltage VTFT)−(reference voltage VREF)”. For example, the reference voltage VREF is 0 V in this embodiment.
The organic EL element 68 is an example of a light emitting element that emits light according to a current supplied from the drive transistor 66. A plurality of light emitting elements are arranged in a matrix. The organic EL element 68 has a cathode (second electrode) connected to the EL cathode power line 82, and an anode (first electrode) connected to the source (source electrode) of the drive transistor 66 via the switch 67. The voltage supplied to the EL cathode power line 82 is VEL, which is −6 V as an example.
The drive transistor 66 is a voltage-driven drive element that controls the supply of current to the organic EL element 68, and causes the organic EL element 68 to emit light by supplying the current corresponding to the voltage held in the capacitance element 65 to the organic EL element 68. In more detail, the drive transistor 66 has a drain electrode conducting with a first electrode of the capacitance element 65, a gate electrode conducting with a second electrode of the capacitance element 65, and a source electrode connected to the anode of the organic EL element 68 via the switch 67, as illustrated in
In a non-light emission period following the light emission period, for example, the switch 67 is turned off (nonconduction), and so the drive transistor 66 does not flow the current through the organic EL element 68. Thus, the drive transistor 66 does not cause the organic EL element 68 to emit light in the non-light emission period.
In this embodiment, the thin-film transistor (TFT) forming the drive transistor 66 is p-type.
Note that the thin-film transistor (TFT) forming the drive transistor 66 may be n-type, p-type, or a combination of n-type and p-type. Moreover, the channel layer of the thin-film transistor may be made of any of amorphous silicon, microcrystalline silicon, polysilicon, oxide semiconductor, organic semiconductor, etc. For example, the oxide semiconductor may be an oxide semiconductor material including at least one type selected from indium (In), gallium (Ga), and zinc (Zn). The oxide semiconductor has low off-current, has high electron mobility even in an amorphous state, and can be formed by a low-temperature process. For example, amorphous indium-gallium-zinc oxide (InGaZnO) may be used.
The capacitance element 65 is an example of a first capacitance element (display capacitor) for holding a first voltage used to cause the organic EL element 68 to emit light. The capacitance element 65 holds the voltage for determining the amount of current to be flown by the drive transistor 66. In detail, the first electrode (the electrode opposite to the node C) of the capacitance element 65 is connected between the drain electrode (the electrode on the EL anode power line 81 side) of the drive transistor 66 and the EL anode power line 81. The second electrode (the electrode on the node C side) of the capacitance element 65 is connected to the gate electrode of the drive transistor 66. The second electrode of the capacitance element 65 is connected to a first electrode (the electrode on the node A side) of the capacitance element 62 via the switch 63. The second electrode of the capacitance element 65 is connected to the reference power line 83 (VREF) for supplying the reference voltage VREF, via the switch 64.
In this embodiment, a second voltage held in the capacitance element 62 is copied to the capacitance element 65 which accordingly holds the second voltage as the first voltage under control of the display panel control circuit 2, in a second period following an initialization period for initializing the capacitance element 65 in the non-light emission period after the light emission period during which the organic EL element 68 emits light.
The capacitance element 62 is an example of a second capacitance element (write capacitor) for holding the second voltage that is different from the first voltage held in the capacitance element 65 and is the next voltage to be held in the capacitance element 65. The capacitance element 62 serves as memory for temporarily holding the next voltage to be held in the capacitance element 65. In detail, the first electrode (the electrode on the node A side) of the capacitance element 62 is connected to the source signal line 88 via the switch 61. The second electrode (the electrode on the node B side) of the capacitance element 62 is connected to the reference power line 83 (VREF) for supplying the reference voltage VREF.
In this embodiment, the second voltage is written to and held in the capacitance element 62 under control of the display panel control circuit 2, in a first period in the light emission period during which the organic EL element 68 emits light.
The switch 61 is an example of a first switch that switches between conduction and nonconduction between the source signal line 88 (signal line) for supplying the data signal voltage and the first electrode of the capacitance element 62. In detail, the switch 61 is a switching transistor having one of the drain and source terminals connected to the source signal line 88, the other one of the drain and source terminals connected to the first electrode of the capacitance element 62, and the gate connected to the gate signal line 84. In other words, the switch 61 has a function of writing to the capacitance element 62 (write capacitor) for temporarily holding the data signal voltage (data signal) corresponding to the video signal voltage (video signal) supplied via the source signal line 88.
The switch 63 is an example of a second switch that switches between conduction and nonconduction between the first electrode of the capacitance element 62 and the second electrode of the capacitance element 65. In detail, the switch 63 is a switching transistor having one of the drain and source terminals connected to the first electrode of the capacitance element 62, the other one of the drain and source terminals connected to the second electrode of the capacitance element 65, and the gate connected to the gate signal line 85. In other words, the switch 63 has a function of supplying (copying) the second voltage held in the capacitance element 62 to the capacitance element 65.
The switch 64 is an example of a third switch that switches between conduction and nonconduction between the reference power line 83 (VREF) for supplying the reference voltage VREF and the second electrode of the capacitance element 65. In detail, the switch 64 is a switching transistor having one of the drain and source terminals connected to the reference power line 83 (VREF), the other one of the drain and source terminals connected to the gate electrode of the drive transistor 66 and the second electrode of the capacitance element 65, and the gate connected to the gate signal line 86. In other words, the switch 64 has a function of supplying the reference voltage (VREF) to the second electrode of the capacitance element 65 and the gate electrode of the drive transistor 66.
The switch 67 is an example of a fourth switch that switches between conduction and nonconduction between the source electrode of the drive transistor 66 and the anode of the organic EL element 68. In detail, the switch 67 is a switching transistor having one of the drain and source terminals connected to the source electrode of the drive transistor 66, the other one of the drain and source terminals connected to the anode of the organic EL element 68, and the gate connected to the gate signal line 87.
In the pixel structure in this embodiment, a video signal voltage can be written to the pixel even in the state where current is supplied to the organic EL element 68, as illustrated in
In the current frame period, the gate driver IC (circuit) sequentially selects pixel rows, and the source driver IC applies a video signal to each selected pixel (display pixel). In the pixel, the voltage corresponding to the video signal is held in the capacitance element 62. In each blanking period of one frame, the voltage held in the capacitance element 62 is copied to the capacitance element 65. During this period, the display screen is kept in a non-display state.
In the next frame, the drive transistor 66 supplies current to the organic EL element 68, based on the voltage held in the capacitance element 65.
As described above, the pixel according to this embodiment includes the capacitance element 65 and the capacitance element 62 each of which holds a voltage based on a video signal.
Although this embodiment describes the case where the capacitance element 65 and the capacitance element 62 each of which holds a voltage based on a video signal are provided, this is not a limitation. For example, two memory circuits may be formed using transistors, etc., and each memory circuit may hold a voltage based on a video signal. Moreover, the gate capacitor of each MOS transistor may hold a voltage based on a video signal.
The features described above are equally applicable to the other embodiments in this description, and may be combined with the other embodiments. Although this embodiment describes the case where the switches 61, 63, 64, and 67 in the pixel circuit 60 are p-type TFTs, this is not a limitation, and the switches may be n-type TFTs.
1-3. Structure of Adjustment Unit
The following describes the adjustment unit 7 illustrated in
The adjustment unit 7 includes a current calculation unit 71, a voltage calculation unit 72, and a voltage change unit 73, and adjusts the predetermined voltage (cathode voltage VEL) applied to the EL cathode power line 82 connected to the plurality of display pixels.
The current calculation unit 71 calculates the current value flowing in the organic EL display panel 6 in the case where the data signal voltage based on the display data (video data) included in the display data signal S1 is supplied to the organic EL display panel 6.
In more detail, the current calculation unit 71 includes a first current value calculation unit 711, a second current value calculation unit 712, a third current value calculation unit 713, a weighting unit 714, a weighting unit 715, a weighting unit 716, and an addition unit 717.
The first current value calculation unit 711 calculates the current value (first current value) flowing in the organic EL display panel 6 in the case where a red data signal voltage which is the signal voltage of red data (Rdata) included in the display data (video data) is supplied to the organic EL display panel 6. Likewise, the second current value calculation unit 712 and the third current value calculation unit 713 calculate the respective current values (second current value and third current value) flowing in the organic EL display panel 6 in the case where a green data signal voltage of green data (Gdata) and a blue data signal voltage of blue data (Bdata) included in the display data (video data) are supplied to the organic EL display panel 6.
The weighting unit 714 weights the first current value calculated by the first current value calculation unit 711, and outputs the weighted first current value to the addition unit 717. The weighting is performed because of the difference of the color light emission efficiency of the organic EL element 68 depending on the material and the like. In this embodiment, for example, the weighting unit 714 weights the first current value by 2, i.e. multiplies the first current value by 2, and outputs the weighted first current value to the addition unit 717. Likewise, the weighting unit 715 weights the second current value calculated by the second current value calculation unit 712 and outputs the weighted second current value to the addition unit 717, and the weighting unit 716 weights the third current value calculated by the third current value calculation unit 713 and outputs the weighted third current value to the addition unit 717. In this embodiment, for example, the weighting unit 715 weights the second current value by 1, and the weighting unit 716 weights the third current value by 5.
The addition unit 717 outputs a total current value by adding (summing) the weighting units 714, 715, and 716, to the voltage calculation unit 72. The total current value is the current value flowing in the organic EL display panel 6 in the case where the data signal voltage based on the display data included in the display data signal S1 is supplied to the organic EL display panel 6.
Although this embodiment describes the case where the current calculation unit 71 calculates the total current value to facilitate understanding, this is not a limitation. Any value, magnitude, change amount, or the like may be used as long as it has proportionality or predetermined correlation to the current or power flowing in the display screen of the organic EL display panel 6.
In the case where the cathode voltage VEL and the anode voltage VTFT are fixed, for example, a current-correlation value is calculated from the display data (video data), and the current-correlation value and the cathode voltage VEL, the anode voltage VTFT, or the like are multiplied to obtain a power-correlation value. In the case where the cathode voltage VEL, the anode voltage VTFT, or the like is changed by the voltage change unit 73, etc., the current-correlation value and the changed cathode voltage VEL or anode voltage VTFT are multiplied to obtain the power-correlation value.
Not all display data (video data) forming the display screen may be used to calculate the current. As an example, display data (video data) corresponding to any pixels selected in the display screen may be used to calculate the current. The total current flowing in the display screen can be estimated by proportional multiplication on the selected pixels. This reduces the calculation time and the number of operations to calculate the current value.
As another example, display data (video data) whose gray level is not less than a predetermined gray level may be used to calculate the current. The value corresponding to the current may be calculated from the number of pixels of display data (video data) whose gray level is not less than the predetermined gray level. The voltage calculation unit 72 determines whether or not to change the predetermined voltage applied to the EL cathode power line 82, based on the total current value output from the addition unit 717. In the case where the predetermined voltage is to be changed, the voltage calculation unit 72 calculates a voltage to be applied to the EL cathode power line 82. In more detail, in the case where the total current value output from the addition unit 717 is greater than or equal to a predetermined threshold, the voltage calculation unit 72 determines that the predetermined voltage applied to the EL cathode power line 82 is to be changed, and calculates, as the voltage to be applied to the EL cathode power line 82, a voltage higher than the predetermined voltage.
The voltage change unit 73 changes the voltage (cathode voltage) to the voltage value calculated by the voltage calculation unit 72. For example, suppose the voltage of the EL cathode power line 82 is −6 V in the case where the current flowing in the organic EL display panel 6 is in a predetermined value range (normal range). In the case where the current flowing in the EL display panel becomes greater than or equal to the predetermined value, the voltage change unit 73 changes the voltage of the EL cathode power line 82 to −4 V. By changing the voltage of the EL cathode power line 82 from −6 V to −4 V, the potential difference between the EL anode power line 81 and the EL cathode power line 82 decreases, so that the power used by the EL display panel can be reduced.
In
For example, suppose the maximum value of gray level data is 255. When the gray level data applied to 10 pixels (display pixels) are all 16, the total sum is 16×10=160, and the lighting rate is 160/(255×10)=0.0627 (6.27%). When the gray level data applied to 10 pixels (display pixels) are all 255, the total sum is 255×10=2550, and the lighting rate is 2550/(255×10)=1.0 (100%). The lighting rate has a maximum value of 100%, and a minimum value of 0%. Here, the lighting rate needs to be weighted by the light emission efficiency of each color of light emitting element such as red (R), green (G), and blue (B).
In the organic EL display panel 6, the light emission efficiency differs among the EL elements of red (R), green (G), and blue (B). Accordingly, gray level data is multiplied by a weighting factor depending on the color of R, G, or B, to calculate added data on the display screen. In the case of R, G, B, and W (white), each of the four colors is weighted based on the light emission efficiency.
The light emission efficiency corresponding to the current flowing in the organic EL element needs to be converted. In the case where the gray level data is gamma-transformed, for example, the gray level data is subject to calculation with a gamma transformation coefficient, to obtain the current flowing through the organic EL element. The obtained current is weighted based on the light emission efficiency of the corresponding light emission color. For example, suppose the weighting factor of green (G) is 1. If the light emission efficiency of red (R) is ½ of green (G), the weighting factor of red (R) is 2. If the light emission efficiency of blue (B) is ⅕ of green (G), the weighting factor of blue (B) is 5.
A lighting rate of 0% indicates the state where the display screen is black and all pixels are unlit. In this state, ideally no current flows through the organic EL elements of the display screen. A lighting rate of 100% indicates the state where the display screen is white with the maximum luminance (maximum brightness). In this state, all pixels of the display screen are at the maximum gray level, and the current flowing in the display screen is the maximum current.
Basically, the current I flowing through an organic EL element and the luminance B of the organic EL element are proportional to each other. In the case where the gray level data and the current flowing through the organic EL element based on the gray level data are proportional to each other, the lighting rate can be calculated by multiplying the current flowing through the EL element of each light emission color by the corresponding weighting factor. The lighting rate is obtained by weighting the current flowing in the display screen based on the light emission efficiency of each of the light emission colors arranged in a matrix.
Let Ix be the current flowing in the display screen in a given image display state, and Im be the current flowing in the display screen in the white display state where the whole display screen has the maximum luminance (maximum brightness). Then, the lighting rate (%)=Ix/Im×100. Here, the cathode voltage and the anode voltage are constant (fixed).
Although the above describes the case where the current flowing in the display screen (or the current-correlation value) is obtained from the gray level data, this is not a limitation. For example, the current of each of R, G, and B flowing through the cathode line or anode line may be measured by an ammeter or the like, to perform weighting using the current of each of R, G, and B. The current flowing in the display screen may be measured, to calculate the lighting rate from the measured current. Moreover, a pickup resistor may be placed in series in the EL cathode power line 82 or the EL anode power line 81, to measure the voltage across the pickup resistor by a voltmeter and obtain the current flowing in the display screen.
With the current data obtained from the gray level data or the current obtained by direct current measurement, the consumed current (consumed power) flowing in the display screen can be calculated in the case where the anode voltage and the cathode voltage are fixed. The consumed current and the value obtained by weighting the video signal of R, G, B, or the like based on the EL element efficiency are correlated with each other.
The consumed current (consumed power) flowing in the display area is calculated. In the case of R, G, B, and W (white), each of the four colors is weighted based on the corresponding light emission efficiency.
Although the above describes the case where the lighting rate is calculated from the gray level data or the current flowing in the whole display screen, this is not a limitation. The lighting rate may be obtained by estimating or calculating the gray level data sum or current of the whole display screen from the gray level data sum or current of a part of the display screen (e.g. ¼ of the display screen). The lighting rate need not necessarily be calculated from the gray level data sum or current of one frame period. For example, the gray level data sum or current of each of a plurality of frames may be averaged or transfer-integrated to obtain the lighting rate.
In
In detail, the adjustment unit 7 adjusts the voltage applied to the EL cathode power line 82 to be higher than the predetermined voltage (VEL) (so that the potential difference between the anode voltage and the cathode voltage is lower), when the total sum of current (or the lighting rate) corresponding to the second voltage in the case where the current corresponding to the second voltage is supplied to the organic EL element 68 in each of the plurality of display pixels is greater than or equal to a threshold. In more detail, the voltage change unit 73 does not change the cathode voltage steeply, but changes the cathode voltage along straight line L1 having a constant gradient as illustrated in
Thus, the adjustment unit 7 may adjust the voltage applied to the EL cathode power line 82 to be lower than the predetermined voltage, when the total sum of current or gray level (the lighting rate) corresponding to the second voltage in the case where the current corresponding to the second voltage is supplied to the organic EL element 68 in each of the plurality of display pixels is greater than or equal to the threshold.
In this embodiment, when the total sum of current or gray level (the lighting rate) corresponding to the second voltage is greater than or equal to the threshold, the adjustment unit 7 adjusts the voltage applied to the EL cathode power line 82 to be lower than the predetermined voltage, in the non-light emission period of the pixel circuit 60. The timing of adjustment will be described in detail later.
In this way, the adjustment unit 7 performs overload control so that the panel power due to the current flowing into the organic EL display panel 6 is less than a predetermined level. A decrease in life of the organic EL elements 68 can thus be prevented. The current corresponding to the applied voltage (gray level signal voltage) is caused to flow through the organic EL element 68 by the drive transistor 66. If the current flowing through the display pixels (organic EL elements 68) constituting the organic EL display panel 6 is large, the organic EL display panel 6 generates heat, which leads to shorter life (degradation) of the organic EL elements 68.
The adjustment unit 7 (the voltage change unit 73) changes the voltage VEL of the EL cathode power line 82 of a pixel circuit 60a (the voltage VEL of a voltage source 73a) to the voltage value calculated by the voltage calculation unit 72.
The pixel circuits 60a are arranged in a matrix in the organic EL display panel 6, as illustrated in
1-4. Operation of Display Pixel
The following describes the method of driving the pixel circuit illustrated in
With the structure of the pixel circuit 60 illustrated in
(Write Process)
In this embodiment, first, the switches 63 and 64 are maintained nonconducting (off), the switch 67 is maintained conducting (on), and the drive transistor 66 supplies the current corresponding to the voltage held in the capacitance element 65 to the organic EL element 68 so that the organic EL element 68 emits light (light emission period).
In the first period (write period) in the light emission period, the switch 61 is rendered conducting (on), to cause the capacitance element 62 to hold the next voltage (video signal voltage Vsig) to be held by the capacitance element 65. In other words, in this embodiment, the display panel control circuit 2 causes the second voltage (the next video signal voltage) to be written to and held in the capacitance element 62 in the light emission period (in the first period) during which the organic EL element 68 emits light, in each of the plurality of pixel circuits 60. The aforementioned operation is performed from the top pixel row to bottom pixel row of the screen sequentially. In detail, the gate driver circuit 3 performs shift operation to sequentially select the pixel row position to which the video signal is applied.
Thus, in the pixel circuit 60 in this embodiment, in the first period in the light emission period during which the organic EL element 68 emits light according to the voltage (video signal voltage) held in the capacitance element 65, the voltage (video signal voltage) to be held in the capacitance element 65 in the next light emission period can be written to and held in the capacitance element 62 beforehand. In other words, the pixel circuit 60 according to this embodiment can write the next video signal voltage (video data) to the pixel circuit 60 even during the light emission of the organic EL element 68. Hence, the next image display can be written to each pixel while holding the image display state of the display screen.
(Reset Process)
In this embodiment, in the non-light emission period during which the switch 67 is nonconducting (off), the switch 64 is rendered conducting (on) while maintaining the switches 61 and 63 nonconducting (off), to input the voltage VREF to one end of the capacitance element 65. The reset process of initializing the capacitance element 65 is carried out in this way.
(Copy Process)
In this embodiment, in the second period following the reset process in the non-light emission period during which the switch 67 is nonconducting (off), the switch 64 is rendered nonconducting (off) while maintaining the switch 63 nonconducting (off), and then the switch 63 is rendered conducting (on). As a result, the first electrode of the capacitance element 62 and the second electrode of the capacitance element 65 are connected, and so the next video signal voltage (Vsig) held in the capacitance element 62 can be copied (written) to the capacitance element 65. The video signal voltage held in the capacitance element 62 in each pixel is concurrently copied to the capacitance element 65. In the next frame, an image is displayed according to the video signal voltage copied to the capacitance element 65.
(Voltage Adjustment Process)
Although
For example, the reset process, the copy process, and the voltage adjustment process are performed in the blanking period. During these operations, the shift operation of the gate driver circuit is stopped. Moreover, these operations are performed simultaneously on all pixels of the display screen.
The adjustment unit 7 calculates the current flowing in the organic EL display panel 6, from the display data signal S1 of one frame or subfield. Based on the calculation result, the adjustment unit 7 changes (adjusts) the anode voltage to be lower than the predetermined voltage. This enables overload control so that the panel power due to the current flowing into the organic EL display panel 6 is less than a predetermined level. The adjustment unit 7, in combination with the pixel circuit 60 in this embodiment, can perform the anode voltage change as overload control in the non-light emission period before video display, without lowering video quality.
(Light Emission Process)
In this embodiment, the switch 67 is rendered conducting (on) while maintaining the switches 61, 63, and 64 nonconducting (off). This enables the organic EL element 68 to emit light according to the next video signal voltage (Vsig) held in the capacitance element 65.
By repeatedly executing the write process, the reset process, the copy process, the voltage adjustment process, and the light emission process in the aforementioned manner, video (e.g. a moving image) can be displayed on the plurality of display pixels in this embodiment.
In the light emission process, by changing the switch 67 from nonconduction (off) to conduction (on) simultaneously in all display pixels of the organic EL display panel 6, the frame display can be switched simultaneously in all display pixels. Thus, the display according to the current video signal voltage and the display according to the next video signal voltage can be kept from being mixed.
1-5. Advantageous Effects
As described above, the organic EL display device 1 according to this embodiment can prevent a decrease in life of the organic EL elements 68.
In detail, in a conventional organic EL display device, when performing overload control so that the panel power due to the current flowing into the organic EL panel is less than a predetermined level, the voltage of the power line needs to be decreased during video display. This causes lower video quality such as display unevenness or flicker.
On the other hand, the organic EL display device 1 according to this embodiment includes the adjustment unit 7, and is capable of writing a data signal voltage and displaying video independently of each other. Moreover, the adjustment unit 7 changes the cathode voltage or the like during the blanking period. The video signal is copied to the capacitance element 62 sequentially, and the video signal held in the capacitance element 62 is copied to the capacitance element 65 concurrently in the blanking period. This enables overload control so that the panel power due to the current flowing into the organic EL panel is less than a predetermined level, without lowering video quality or causing flicker when changing the cathode voltage or the like. A decrease in life of the organic EL elements 68 can thus be prevented.
The adjustment unit 7 calculates the current flowing in the organic EL display panel 6, from the display data signal S1 of one frame or subfield. Based on the calculation result, the adjustment unit 7 changes the anode voltage to be lower than the predetermined voltage. This enables overload control so that the panel power due to the current flowing into the organic EL display panel 6 is less than a predetermined level. Moreover, the adjustment unit 7 can perform the anode voltage change as overload control in the non-light emission period before video display. This has the advantageous effect of preventing lower video quality.
Furthermore, since the organic EL display device 1 according to this embodiment can perform overload control, no heat dissipation mechanism to cool the organic EL display panel 6 is needed. This has the advantageous effect of realizing a thinner organic EL display panel 6.
While Embodiment 1 describes the case where the thin-film transistors forming the switch 61, switch 63, switch 64, drive transistor 66, and switch 67 are p-type, this is not a limitation, and the thin-film transistors may be n-type. Embodiment 2 describes the case where the thin-film transistors forming the switch 61, switch 63, switch 64, drive transistor 66, and switch 67 are n-type. The following mainly describes the differences from Embodiment 1.
2-1. Circuit Structure of Display Pixel
A pixel circuit 60A illustrated in
The pixel circuit 60A is an example of one of the display pixels (light emitting pixels) arranged in a matrix. For example, as illustrated in
The initialization power line 613 (VINI) is an example of a power line for supplying a voltage VINI (also referred to as an initialization voltage VINI) for initializing the voltage between the source and gate of the drive transistor 66a, i.e. the voltage of the capacitance element 65a. The reference power line 614 (VREF2) is an example of a power line for supplying a reference voltage VREF2.
The organic EL element 68 is an example of a light emitting element that emits light according to a current supplied from the drive transistor 66a. A plurality of light emitting elements are arranged in a matrix.
The drive transistor 66a is a voltage-driven drive element that controls the supply of current to the organic EL element 68, and causes the organic EL element 68 to emit light by supplying the current corresponding to the voltage held in the capacitance element 65a to the organic EL element 68. In this embodiment, the thin-film transistor (TFT) forming the drive transistor 66a is n-type.
The capacitance element 65a is an example of a first capacitance element (display capacitor) for holding a first voltage used to cause the organic EL element 68 to emit light. The capacitance element 65a holds the voltage for determining the amount of current to be flown by the drive transistor 66a. In detail, the first electrode (the electrode opposite to the node D) of the capacitance element 65a is connected between the source electrode (on the EL cathode power line 82 side) of the drive transistor 66a and the anode of the organic EL element 68. The second electrode (the electrode on the node D side) of the capacitance element 65a is connected to the gate electrode of the drive transistor 66a. The second electrode of the capacitance element 65a is also connected to the initialization power line 613 (VINI) via the switch 611.
In this embodiment, a second voltage held in the capacitance element 62 is copied to the capacitance element 65a which accordingly holds the second voltage as the first voltage under control of the display panel control circuit 2, in a second period following an initialization period for initializing the capacitance element 65a in the non-light emission period after the light emission period during which the organic EL element 68 emits light.
The capacitance element 62 is an example of a second capacitance element (write capacitor) for holding the second voltage that is different from the first voltage held in the capacitance element 65a and is the next voltage to be held in the capacitance element 65a. The capacitance element 62 serves as memory for temporarily holding the next voltage to be held in the capacitance element 65a. In detail, the first electrode (the electrode on the node A side) of the capacitance element 62 is connected to the source signal line 88 via the switch 61a. The second electrode (the electrode on the node B side) of the capacitance element 62 is connected to the reference power line 83 (VREF) for supplying the reference voltage VREF.
In this embodiment, the second voltage is written to and held in the capacitance element 62 under control of the display panel control circuit 2, in a first period in the light emission period during which the organic EL element 68 emits light.
The switch 61a is an example of a first switch that switches between conduction and nonconduction between the source signal line 88 (signal line) for supplying the data signal voltage and the first electrode of the capacitance element 62.
The switch 63a is an example of a second switch that switches between conduction and nonconduction between the first electrode of the capacitance element 62 and the second electrode of the capacitance element 65a.
The switch 64a is an example of a third switch that switches between conduction and nonconduction between the reference power line 83 (VREF) for supplying the reference voltage VREF and the second electrode of the capacitance element 65a.
The switch 67a is an example of a fourth switch that switches between conduction and nonconduction between the drain electrode of the drive transistor 66a and the EL anode power line 81 (VTFT).
The switch 611 switches between conduction and nonconduction between each of the first electrode of the capacitance element 65a and the source electrode of the drive transistor 66a and the initialization power line 613 (VINI). In detail, the switch 611 has one of the drain and source terminals connected to the initialization power line 613 (VINI), the other one of the drain and source terminals connected to first electrode of the capacitance element 65a and the source electrode of the drive transistor 66a, and the gate connected to the Init line 617. In other words, the switch 611 has a function of supplying the initialization voltage VINI to the second electrode of the capacitance element 65a and the source electrode of the drive transistor 66a.
The switch 612 switches between conduction and nonconduction between the reference power line 614 (VREF2) for supplying the reference voltage VREF2 and each of the gate electrode of the drive transistor 66a and the second electrode of the capacitance element 65a. In detail, the switch 612 has one of the drain and source terminals connected to the reference power line 614 (VREF2), the other one of the drain and source terminals connected to the gate electrode of the drive transistor 66a and the second electrode of the capacitance element 65a, and the gate connected to the gate signal line 87a. In other words, the switch 612 has a function of supplying the reference voltage (VREF2) to the gate electrode of the drive transistor 66a and the capacitance element 65a to reset the drive transistor 66a and the capacitance element 65a.
2-2. Structure of Adjustment Unit
The adjustment unit 7 has the same structure as that illustrated in
In this embodiment, the adjustment unit 7 (the voltage change unit 73) changes the voltage VTFT of the EL anode power line 81 of a pixel circuit 60b (the voltage VTFT of a voltage source 73b) to the voltage value calculated by the voltage calculation unit 72.
Thus, the adjustment unit 7 can perform overload control so that the panel power due to the current flowing into the organic EL display panel 6 is less than a predetermined level (see
2-3. Operation of Display Pixel
In this embodiment, with the structure of the pixel circuit 60A illustrated in
(Write Process)
In the write process, the switches 61a and 67a are conducting (on), and the switches 63a, 64a, 611, and 612 are nonconducting (off), as illustrated in
Thus, the pixel circuit 60A according to this embodiment can write the next video signal voltage (video data) to the capacitance element 62 in the pixel circuit 60A even during the light emission of the organic EL element 68.
(Reset Process)
First, as illustrated in
Next, as illustrated in
By providing the period during which the switch 611 is nonconducting in this way, the flow of through current between the EL anode power line 81 and the initialization power line 613 can be prevented. In other words, without the period for setting the state in
Next, as illustrated in
Next, as illustrated in
(Copy Process)
In this embodiment, in the second period following the reset process in the non-light emission period during which the switch 67a is nonconducting (off), the switch 63a is rendered conducting (on) while maintaining the switches 61a, 67a, 611, 611, and 612 nonconducting (off). As a result, the first electrode of the capacitance element 62 and the second electrode of the capacitance element 65a are connected, and so the next video signal voltage (Vsig) held in the capacitance element 62 can be copied (written) to the capacitance element 65a.
(Voltage Adjustment Process)
Although
The adjustment unit 7 calculates the current flowing in the organic EL display panel 6, from the display data signal S1 of one frame or subfield. Based on the calculation result, the adjustment unit 7 changes (adjusts) the anode voltage to be lower than the predetermined voltage. This enables overload control so that the panel power due to the current flowing into the organic EL display panel 6 is less than a predetermined level. The adjustment unit 7, in combination with the pixel circuit 60A in this embodiment, can perform the cathode voltage change as overload control in the non-light emission period before video display, without lowering video quality.
For example, the reset process, the copy process, and the voltage adjustment process are performed in the blanking period. During these operations, the shift operation of the gate driver circuit is stopped. Moreover, these operations are performed simultaneously on all pixels of the display screen.
(Light Emission Process)
In this embodiment, only the switch 67a is rendered conducting (on). This enables the organic EL element 68 to emit light according to the next video signal voltage (Vsig) held in the capacitance element 65a.
By repeatedly executing the write process, the reset process, the copy process, the voltage adjustment process, and the light emission process in the aforementioned manner, video (e.g. a moving image) can be displayed on the plurality of display pixels in this embodiment.
In the light emission process, by changing the switch 67a from nonconduction (off) to conduction (on) simultaneously in all display pixels of the organic EL display panel 6, the frame display can be switched simultaneously in all display pixels. Thus, the display according to the current video signal voltage and the display according to the next video signal voltage can be kept from being mixed.
2-4. Advantageous Effects
As described above, the organic EL display device 1 according to this embodiment can prevent a decrease in life of the organic EL elements 68.
In detail, the organic EL display device 1 according to this embodiment includes the adjustment unit 7, and is capable of writing a data signal voltage and displaying video independently of each other. This enables overload control so that the panel power due to the current flowing into the organic EL panel is less than a predetermined level, without lowering video quality. A decrease in life of the organic EL elements 68 can thus be prevented.
The adjustment unit 7 calculates the current flowing in the organic EL display panel 6, from the display data signal S1 of one frame or subfield. Based on the calculation result, the adjustment unit 7 changes the cathode voltage to be lower than the predetermined voltage. This enables overload control so that the panel power due to the current flowing into the organic EL display panel 6 is less than a predetermined level. Moreover, the adjustment unit 7 can perform the anode voltage change as overload control in the non-light emission period before video display. This has the advantageous effect of preventing lower video quality.
Furthermore, since the organic EL display device 1 according to this embodiment can perform overload control, no heat dissipation mechanism to cool the organic EL display panel 6 is needed. This has the advantageous effect of realizing a thinner organic EL display panel 6.
[Variations]
Although Embodiments 1 and 2 describe the case where the organic EL display device 1 expresses a video signal as one frame in one frame period, this is not a limitation. The organic EL display device 1 may express a video signal using a plurality of subfields obtained by dividing one frame period into a plurality of subfield periods (subframe periods). In other words, the period between the light emission period (first light emission period) during which the organic EL element 68 emits light and the next light emission period (second light emission period) during which the organic EL element 68 emits light may be one frame period or a subfield period (frame period).
For example, a video signal of one frame is divided into a plurality of subfields, and each subfield is classified by luminance (brightness). Each subfield may be classified by, for example, the bit position of video data. For example, in the case where the video signal is 8 bits, 8 subfields constitute one frame. The source driver IC outputs the voltage value obtained by weighting the bit to the source signal line in each subfield. In this case, the index value of each pixel row can be obtained by calculating the number of bits “1”.
In detail, the organic EL display device according to the present disclosure includes the pixel circuit 60 in
This prevents lower video quality caused by mixed display of two subfields as in conventional cases. Not only video quality can be improved, but also the amount of heat generation of the organic EL display panel 6 in the organic EL display device 1 can be reduced.
[Variation 2]
While Variation 1 describes the case where one frame is composed of a plurality of subframes, this is not a limitation. The gray level (luminance) of each pixel may be expressed by being divided into a plurality of subfields (subframes), as illustrated in
While the organic EL display device has been described above by way of embodiments, the present disclosure is not limited to these embodiments. Other modifications obtained by applying various changes conceivable by a person skilled in the art to the embodiments and any combinations of the structural elements in different embodiments without departing from the scope of the present disclosure are also included in the scope of one or more aspects.
The present invention can be used for an organic EL display device and a method of driving the organic EL display device, and particularly for a FPD display device such as a television illustrated in
1 organic EL display device
2 display panel control circuit
3, 30, 31 gate driver IC (circuit)
5 source driver IC (circuit)
6 organic EL display panel
7 adjustment unit
60, 60A, 60a, 60b pixel circuit
61, 61a, 63, 63a, 64, 64a, 67, 67a, 611, 612 switch
62, 65, 65a capacitance element
66, 66a drive transistor
68 organic EL element
71 current calculation unit
72 voltage calculation unit
73 voltage change unit
73
a, 73b voltage source
81 EL anode power line
82 EL cathode power line
83, 614 reference power line
84, 84a, 85, 85a, 86, 86a, 87, 87a gate signal line
88 source signal line
613 initialization power line
616 control line
617 Init line
711 first current value calculation unit
712 second current value calculation unit
713 third current value calculation unit
714, 715, 716 weighting unit
717 addition unit
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-012480 | Jan 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/006372 | 12/22/2014 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2015/111118 | 7/30/2015 | WO | A |
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