This invention is based on Japanese Patent Application No. 2004-067895, the content of which is incorporated by reference in its entirety.
1. Field of the Invention
The invention relates to a display device, particularly to a display device having a pixel portion.
2. Description of the Related Art
In a liquid crystal display device, when a DC voltage is kept applied to a liquid crystal of a pixel portion for a long time, a lag phenomenon called “burn-in” occurs. Therefore, the liquid crystal display device need use a driving method of inversing a potential of a pixel electrode relative to a potential of a common electrode in a predetermine cycle. One of such driving methods of the liquid crystal display device is a DC driving method of applying a DC voltage to the common electrode. As this DC driving method, a line inversion driving method of inverting the pixel potential relative to a potential of the common electrode to be applied with a DC voltage in each of horizontal periods has been known. This technology is proposed in “Introduction to liquid crystal display engineering,” pp. 101-103, written by Yasoji Suzuki and published by the Nikkan Kogyo Shimbun, Ltd., at Nov. 20, 1998. It is noted that one horizontal period means a period of writing an image signal to all the pixel portions arrayed along one gate line.
A liquid crystal display device using a dot inversion driving method of inverting each of image signals relative to a potential COM of a common electrode in each of adjacent pixel portions A to F has been proposed, too.
In the conventional driving methods described above, however, since a curve of transmittances of a liquid crystal layer relative to image signal voltages is highly steep as shown in
The invention provides a liquid crystal display device that includes a plurality of drain lines, a plurality of gate lines arranged substantially normal to the drain lines, a plurality of pixel electrodes connected to corresponding drain lines, a common electrode, a liquid crystal layer disposed between the pixel electrodes and the common electrode, and a plurality of supplemental capacitors each provided for corresponding pixel electrodes. Each of the supplemental capacitors includes a first electrode connected to a corresponding pixel electrode and a second electrode. The device also includes a plurality of supplemental capacitance lines each connected to corresponding second electrodes and configured to receive supplemental voltages, and a correction circuit correcting image signals supplied to the pixel electrodes through the drain lines and the supplemental capacitors when a start voltage that is a voltage of a pixel electrode at a start of receiving a supplemental voltage or an end voltage that is a voltage of the pixel electrode at an end of receiving the supplemental voltage falls within a range of voltage applied to the pixel electrode that corresponds to a change in a capacitance of the liquid crystal.
The invention also provides a method of controlling a liquid crystal display device. The method includes providing a liquid crystal display device that includes a gate line, a first pixel electrode and a second pixel electrode connected to the gate line, a common electrode, a liquid crystal layer disposed between the first and second pixel electrodes and the common electrode, a first supplemental capacitor provided for the first pixel electrode, a second supplemental capacitor provided for the second pixel electrode, a first supplemental capacitance line connected to the first supplemental capacitor and configured to receive a first supplemental voltage, and a second supplemental capacitance line connected to the second supplemental capacitor and configured to receive a second supplemental voltage. supplying the first supplemental voltage to the first supplemental capacitance line after completion of applying an image signal to the first pixel electrode. The method also includes supplying the second supplemental voltage to the second supplemental capacitance line after completion of applying another image signal to the second pixel electrode, and performing gamma correction to the another image signal based on a transmittance characteristic of the liquid crystal layer observed during the supplying of the first supplemental voltage.
Embodiments of the invention will be described with reference to the drawings.
Each of the pixel portions 3-1a to 3-1d and 3-2a to 3-2d includes a liquid crystal layer 31, a transistor 32, and a supplemental capacitance 33. The liquid crystal layer 31 is provided between a pixel electrode 34 and a common electrode 35.
Drains of the transistors 32 in the pixel portions 3-1a and 3-2a are connected with the drain line D1, and drains of the transistors 32 in the pixel portions 3-1b and 3-2b are connected with the drain line D2. In similar manners, drains of the transistors 32 in the pixel portions 3-1c and 3-2c are connected with the drain line D3, and drains of the transistors 32 in the pixel portions 3-1d and 3-2d are connected with the drain line D4. Sources in all the pixel portions are connected with the pixel electrodes 34, respectively.
One electrode 36 of the supplemental capacitance 33 in each of the pixel portions is connected with the pixel electrode 34. Another electrodes 37-1a and 37-1c of the supplemental capacitance 33 in the pixel portions 3-1a to 3-1c are connected with the supplemental capacitance line SC1-1, and another electrodes 37-1b and 37-1d of the supplemental capacitance 33 in the pixel portions 3-1b and 3-1d are connected with the supplemental capacitance line SC2-1. In similar manners, another electrodes 37-2a and 37-2c of the supplemental capacitance 33 in the pixel portions 3-2a and 3-2c are connected with the supplemental capacitance line SC1-2, and another electrodes 37-2b and 37-2d of the supplemental capacitance 33 in the pixel portions 3-2b and 3-2d are connected with the supplemental capacitance line SC2-2.
Furthermore, H switches (n-channel transistor) 4a to 4d for driving (scanning) the drain lines D1 to D4 and drain lines in five or more columns (not shown) and an H driver 5 are provided on the substrate 1. The H switch 4a corresponding to the pixel portion 3-1a (drain line D1) is connected with an image signal line VIDEO 1, and the H switch 4b corresponding to the pixel portion 3-1b (drain line D2) is connected with an image signal line VIDEO2. Although the H switch denotes an H switch in this embodiment, the H switch can be a transfer gate formed of an H switch and a p-channel transistor or other means.
Furthermore, a V driver 46 for driving (scanning) the gate line G1 in the first row, the gate line G2 in the second row, and gate lines in the third or more rows (not shown in
A drive IC 9 is provided outside the substrate 1. This drive IC 9 supplies a positive potential HVDD, a negative potential HVSS, a start signal STH, and a clock signal CKH to the H driver 5. The IC 9 supplies a positive potential VVDD, a negative potential VVSS, a start signal STV, a clock signal CKV, and an enable signal ENB to the V driver 46. The IC 9 supplies a positive potential VSCH, a negative potential VSCL, and a clock signal CKVSC to a potential supply circuit 7.
Next, the enable signal ENB turns H level, and thus all the three signals (the signals of the shift register circuit portions 461a and 461b and the enable signal ENB) inputted to the AND circuit portion 462a become H level. Therefore, a signal of H level is supplied from the AND circuit portion 462a to the gate line G1. Next, the enable signal ENB turns L level, so that a signal of L level is supplied from the AND circuit portion 462a to the gate line G1 and the gate line G1 retains L level for one frame period. Then, the clock signal CKV2 turns L level.
Next, the clock signal CKV 1 turns H level again, and thus a signal of H level is inputted from the shift register circuit portion 461c to the AND circuit portions 462b and 462c. Then, the enable signal ENB turns H level again, and thus all the three signals (the signals of the shift register portions 461b and 461c and the enable signal ENB) inputted to the AND circuit portion 462b become H level. Therefore, a signal of H level is supplied from the AND circuit portion 462b to the gate line G2. Then, the enable signal ENB turns L level, and thus a signal of L level is supplied from the AND circuit portion 462b to the gate line G2 and the gate line G2 retains L level for one frame period. Then, the clock signal CKV1 turns L level.
Next, in similar manners to the AND circuit portions 462a and 462b, synchronized with the clock signals CKV1 and CKV2, signals of H level from the shift register circuit portions 461d to 461f are sequentially inputted to the AND circuit portions 462c to 462e. Thus, in similar manners to the gate lines G1 and G2, synchronized with the enable signals ENB, signals of H level from the AND circuit portions 462c to 462e are sequentially supplied to the gate lines G3 to G5. Then, synchronized with the enable signals ENB, signals of L level from the AND circuit portions 462c to 462e are sequentially supplied to the gate lines G3 to G5, and the gate lines G3 to G5 are retained L level for one frame period. It is noted that as shown in
Furthermore, the AND circuit portions 462b to 462e for supplying signals to the second or more gate lines sequentially input signals of H level to potential supply circuit portions 47a to 47d. When inputted with the input signal of H level, a potential supply circuit portion 47a supplies a H level potential VSCH to the supplemental capacitance line SC1-1 and a L level potential VSCL to the supplemental capacitance line SC2-1. Even when the input signal to the potential supply circuit portion 47a turns L level, the H level potential VSCH and the L level potential VSCL are still supplied to the supplemental capacitance line SC1-1 and the supplemental capacitance line SC2-1 respectively, being retained for one frame period. Then, the potentials supplied to these supplemental capacitance lines are inverted and retained for one frame period again. The potential supply circuit portions 47b to 47d shown in
In such a manner, the high level potentials VSCH and the low level potentials VSCL from the potential supply circuit portions 47a to 47d are sequentially supplied to the supplemental capacitance lines SC1-1 to SC1-4 and the supplemental capacitance lines SC2-1 to SC2-4 respectively, at the same timings as the timings of the H level signals supplied to the gate lines G2 to G5.
The input terminal of the AND circuit portion 462a is inputted with the output signals of the shift register circuit portions 461a and 461b and the enable signal ENB. Each of the AND circuit portions 462b and the following AND circuit portions is also inputted with the output signals of the two shift register circuit portions shifted by one portion from the previous shift register circuit portions and the enable signal ENB. The output terminals of the AND circuit portions 462a to 462e are connected with the gate lines G1 to G5, respectively. The V driver 46 has the potential supply circuit 47 therein, and the potential supply circuit 47 has the potential supply circuit portions 47a to 47d. The potential supply circuit portions 47a to 47d are provided corresponding to the gate lines G1 to G4, respectively. The potential supply circuit portion corresponding to the gate line G5 is not shown for simplification of the drawing.
The potential supply circuit portion 47a corresponding to G1 is inputted with the output signal of the AND circuit portion 462b the output terminal of which is connected with the gate line G2. That is, in this embodiment, the potential supply circuit portion connected with the supplemental capacitance line corresponding to a predetermined gate line is inputted with the output signal of the AND circuit portion the output terminal of which is connected with the next gate line. Furthermore, each of the potential supply circuit portions 47b to 47d has the same circuit structure as that of the potential supply circuit portion 47a.
The supplemental capacitance lines SC1-1 and SC2-1 are connected with the potential supply circuit portion 47a, and the supplemental capacitance lines SC1-2 and SC2-2 are connected with the potential supply circuit portion 47b. These potential supply circuit portions 47a and 47b have functions of supplying the H level potential VSCH and the L level potential VSCL to the supplemental capacitance lines SC1-1 and SC2-1, and SC1-2 and SC2-2, respectively, alternately in each of one frame periods. It is noted that one frame period means a period of writing image signals to all the pixel portions forming the display portion 2. The shift register portion 461 has a function of driving the potential supply circuit 47 so as to sequentially supply signals from the potential supply circuit 47 to a pair of the supplemental capacitance lines SC1-1 and SC2-1 along the first gate line G1 to a pair of supplemental capacitance lines (not shown) along the last gate line.
In this embodiment, the potential of the supplemental capacitance line is changed by ΔV after the image signal is inputted to the pixel electrode 34. The potential of the pixel electrode 34, which is at the same potential as the electrode 36 of the supplemental capacitance 33, changes by the amount of (CSC/CALL)×ΔV, so that a voltage applied between the pixel electrode 34 and the common electrode 35, that is, a voltage applied to the liquid crystal layer, changes. In this embodiment, a display can be made even with an image signal of low potential by using a supplemental capacitance coupling, thereby lowering voltages. It is noted that CALL means all the capacitance in the pixel, and is a sum of the capacitance CSC of the supplemental capacitance 33, the liquid crystal capacitance CLC, and other capacitances in the pixel (e.g. parasitic capacitance).
However, the dielectric constant of the liquid crystal changes when a voltage is applied to the liquid crystal, and thus the liquid crystal capacitance changes. Therefore, even when the potential of the supplemental capacitance line is changed by ΔV, sometimes the potential of the pixel electrode 34 do not change by (CSC/CALL)×ΔV. This is shown as the liquid crystal capacitance as a function of the voltage (C-V curve) in
Therefore, when at least one of voltages applied to the pixel electrode 34 at the start and end of changing the potential of at least one supplemental capacitance line, preferably a pair of supplemental capacitance lines, more preferably all the supplemental capacitance lines SC1-1, SC2-1, SC1-2, SC2-2 and so on lies within the transition region R, a correction circuit 19 compensates the image signal with (CSC/CALL)×ΔV corresponding to the liquid crystal capacitance CLC at the end of the changing.
In the first embodiment, a smooth grayscale display can be obtained by a driving method of changing the potential of the pixel electrode after the image signal is supplied thereto, as described above. This can realize a high quality display and reduce power consumption.
Furthermore, since the potential supply circuit 47 is set in the V driver 46 and the potential supply circuit portions 47a to 47d are sequentially driven by using signals for sequentially driving the gate lines G2 to G5, a circuit size can be reduced and a yield can be improved.
In the first embodiment, the potential supply circuit portion corresponding to the predetermined gate line is driven by inputting an output signal of the AND circuit portion, the output terminal of which is connected with the next gate line, to the potential supply circuit portion corresponding to the predetermined gate line. Therefore, the output signal from the next shift register circuit portion to the predetermined portion is outputted after the output signal of the shift register circuit portion for driving the predetermined gate line is outputted. Accordingly, either the H level potential VSCH or the L level potential VSCL can be easily supplied to each of the pair of the supplemental capacitance lines corresponding to the predetermined gate line after the completion of writing the image signal to the pixel portions arrayed along the predetermined gate line.
In the conventional driving method, an input voltage to a pixel electrode is almost equal to an effective voltage to the liquid crystal layer. In a driving method using supplemental capacitance coupling in this invention, however, the potential of the pixel electrode itself is changed by changing the potential of the supplemental capacitance line after the image signal is inputted to the pixel electrode, and the liquid crystal capacitance is also changed by the change of the potential of the pixel electrode. Therefore, the input voltage to the pixel electrode is different from the effective voltage applied to the liquid crystal layer, and it is difficult to measure the effective voltage finally applied to the liquid crystal layer although it is possible to calculate the effective voltage. Since a calculated value differs among setting methods of CALL, the accuracy lowers.
Therefore, this embodiment uses a gamma correction circuit performing gamma correction by relying on the relation between an input voltage applied to the liquid crystal layer before the potential of the supplemental capacitance line is changed and a transmittance of the liquid crystal finally obtained after the potential of the pixel electrode is changed using the supplemental capacitance coupling, without using the effective voltage to the liquid crystal. This gamma correction circuit can be provided either inside or outside the substrate. The structure and the driving method thereof are the same as those of the first embodiment.
This invention is not limited to the above embodiment. For example, another shift register 8 supplying signals to the plurality of supplemental capacitance lines sequentially can be provided as shown in
Number | Date | Country | Kind |
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2004-067895 | Mar 2004 | JP | national |
Number | Name | Date | Kind |
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5748169 | Okumura et al. | May 1998 | A |
7068292 | Morita | Jun 2006 | B2 |
20030038768 | Sakashita | Feb 2003 | A1 |
Number | Date | Country | |
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20050212743 A1 | Sep 2005 | US |