JP2008261944A - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transflective liquid crystal display securing satisfactory animation visibility and realizing satisfactory display characteristics in both a transmission part and a reflection part. <P>SOLUTION: The liquid crystal display is provided with a display panel DP, where pixels PX are disposed into a matrix shape, a light source BL illuminating the display panel, a first retardation plate RF1 disposed between one surface of the display panel and the light source and giving retardation to light, having a prescribed wavelength and a second retardation plate RF2 disposed on the other surface of the display panel, wherein each pixel is a transflective OCB liquid crystal pixel, having the reflection part PXr and the transmission part PXt which can be independently driven; and a cell gap dr of the reflection part is larger than 1/2 of a cell gap dt of the transmission part. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is excellent in moving image visibility and capable of realizing an image display suitable for a use environment.

A flat display device typified by a liquid crystal display device is widely used as a display device for a personal computer, a portable information terminal, a television, or a car navigation system, taking advantage of features such as light weight, thinness, and low power consumption. .
In this liquid crystal display device, a large number of pixels are arranged in a matrix, and display is performed by controlling the light transmittance of each pixel. Therefore, various types of liquid crystal display devices such as a transmissive liquid crystal display device, a reflective liquid crystal display device, or a transflective liquid crystal display device are known depending on the form in which light is used.

  In the transmissive liquid crystal display device, an illuminating device called a backlight is provided on the back surface of the display panel, and an image is displayed by controlling the amount of transmitted light. With this transmissive liquid crystal display system, it is possible to display a good image even in a dark place with little ambient light. However, the brightness is insufficient in places where ambient light is bright such as outdoors, and display visibility is reduced.

On the other hand, the reflective liquid crystal display device reflects the ambient light of the display device inside the display panel, and controls the amount of reflection to display an image. For this reason, although it is suitable for use in a place where the ambient light is bright such as outdoors, there is a problem that the display cannot be recognized in a dark place where the ambient light is low.
In a transflective liquid crystal display device, a reflection unit that displays an image by reflecting ambient light and a transmission unit that displays an image by transmitting backlight light are provided in one pixel. Accordingly, it is possible to achieve both a transmissive display function and a reflective display function, and thus an excellent display can be realized regardless of the environmental illuminance (see, for example, Patent Document 1).
JP 2005-62573 A

By the way, in the field of liquid crystal televisions and mobile phones, an OCB mode (Optically Compensated Birefringence) liquid crystal display panel that has a high-speed liquid crystal response required for moving image display and can realize a wide viewing angle is attracting attention. ing.
In the OCB mode liquid crystal display panel, the liquid crystal alignment needs to be changed from the spray alignment to the bend alignment when displaying an image. Therefore, a relatively large electric field is applied to the liquid crystal when the power is turned on. In addition, even if this liquid crystal alignment is once changed to bend alignment, if the voltage application state below the level where the energy of spray alignment and the energy of bend alignment antagonize or the state where no voltage is applied continues for a long time, the liquid crystal alignment is changed to spray alignment again. It has the property of reverse transition.
Therefore, in order to prevent reverse transition from bend alignment to spray alignment, for example, a driving method is known in which a large voltage is periodically applied to the liquid crystal every frame for displaying an image of one frame. For example, in a normally white liquid crystal display panel, since this voltage corresponds to a pixel voltage at which black display is performed, a so-called black insertion driving method is employed in which black display is inserted at a constant rate. Incidentally, this black insertion driving also improves the visibility, which is reduced by the influence of the retinal afterimage generated in the viewer's vision in moving image display, by the discrete impulse response of luminance.

  Thus, by combining an OCB mode liquid crystal display device with a transflective liquid crystal display device, it is possible to realize excellent video visibility and image display independent of environmental illuminance.

  However, in recent years, it has been desired that both the moving image visibility at the transmission part and the display characteristics of the reflection part be compatible.

  The present invention has been made in order to solve the above-described problems, and is a transflective liquid crystal that ensures good video visibility and realizes good display characteristics in both the transmissive part and the reflective part. An object is to provide a display device.

  The present invention is arranged between a display panel in which pixels are arranged in a matrix, a light source that illuminates the display panel, one surface of the display panel, and the light source, and gives a phase difference to light having a predetermined wavelength. A first retardation film and a second retardation film disposed opposite to the other surface of the display panel, wherein the pixel includes a reflection section and a transmission section that can be independently driven and controlled. A transflective OCB liquid crystal pixel having a cell gap of the reflective portion larger than ½ of a cell gap of the transmissive portion.

  According to the present invention, it is possible to obtain a transflective liquid crystal display device in which good moving image visibility is ensured and good display characteristics are realized in both the transmissive part and the reflective part.

[First Embodiment]
Hereinafter, a transflective OCB liquid crystal display device (hereinafter referred to as a liquid crystal display device) according to a first embodiment of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a diagram schematically showing a circuit configuration of the liquid crystal display device 1.
The liquid crystal display device 1 includes a liquid crystal display panel DP provided with a plurality of OCB liquid crystal pixels PX, a display control circuit CNT that controls the liquid crystal display panel DP, and a backlight BL that illuminates the liquid crystal display panel DP. The liquid crystal display panel DP has a structure in which a liquid crystal layer 4 is sandwiched between the array substrate 2 and the counter substrate 3.
The OCB liquid crystal pixel PX is a transflective liquid crystal pixel. Each OCB liquid crystal pixel PX includes a transmissive pixel PXt arranged in an odd-numbered row of the liquid crystal display panel DP and a reflective pixel PXr arranged in an even-numbered row. It is configured as a set. In other words, the transmission part and the reflection part of the OCB liquid crystal pixel PX are configured electrically independently in each pixel.
The display control circuit CNT controls the transmittance (reflectance) of the liquid crystal display panel DP by changing the liquid crystal driving voltage applied to the liquid crystal layer 4 from the array substrate 2 and the counter substrate 3.
The array substrate 2 includes a plurality of pixel electrodes PE, a plurality of gate lines G (G1 to Gm), a plurality of source lines X (X1 to Xn), a pixel switching element W, a gate driver 10, and a source driver 20. Each gate line G includes a transmission gate line Gt and a reflection gate line Gr.
The pixel electrodes PE are arranged in a matrix on a transparent insulating substrate such as glass. The gate line G is disposed along a row of the plurality of pixel electrodes PE. A transmissive gate line Gt and a reflective gate line Gr are connected to the OCB liquid crystal pixel PX, and drive the transmissive pixel PXt and the reflective pixel PXr, respectively. The source lines X (X1 to Xn) are arranged along the columns of the plurality of pixel electrodes PE. The pixel switching element W is disposed in the vicinity of the intersection position of the gate line G and the source line X. The gate driver 10 sequentially drives the plurality of gate lines G. The source driver 20 drives the plurality of source lines X while each gate line G is driven.

Each pixel switching element W is made of, for example, a polysilicon thin film transistor. In this case, the gate of the thin film transistor is connected to one gate line G, and the source and drain paths are connected between one source line X and one pixel electrode PE, respectively.
The gate driver 10 is configured using a polysilicon thin film transistor that is formed simultaneously in the same process as the pixel switching element W. The source driver 20 is an integrated circuit (IC) chip mounted on the array substrate 2 by COG (Chip On Glass) technology. Similarly to the gate driver 10, the source driver 20 can also be configured using a polysilicon thin film transistor.

The counter substrate 3 is disposed on a transparent insulating substrate such as glass, for example, and a color filter (not shown) made of red, green, and blue colored layers facing each of the plurality of pixel electrode PE columns, and the color filter The common electrode CE etc. which are arrange | positioned and oppose the whole some pixel electrode PE are included.
Each pixel electrode PE and common electrode CE constituting the transmissive pixel PXt is made of a transparent electrode material such as ITO, for example, and is disposed between the pixel electrode PE and the common electrode CE. The pixel electrode PE constituting each reflective pixel PXr is made of a reflective electrode material such as aluminum, for example, and is disposed between the pixel electrode PE and the common electrode CE corresponding to the electric field between the pixel electrode PE and the common electrode CE. The liquid crystal molecular alignment of the liquid crystal layer 4 is controlled. Then, the liquid crystal molecular arrangement of the liquid crystal layer 4 is controlled in accordance with the electric field from the pixel electrode PE and the common electrode CE. The pixel region of the pixel electrode PE, the common electrode CE, and the liquid crystal layer 4 constitutes the OCB liquid crystal pixel PX.

The display control circuit CNT includes a frame memory 13, a set value memory 15, and a drive control unit 16.
The frame memory 13 stores a display signal extracted from the digital video signal. The drive controller 16 sequentially outputs a source signal and a control signal generated from a display signal stored in the frame memory 13 based on a synchronization signal input from an external signal source (not shown). Here, the source signal generated by the drive control unit 16 is a display signal for the transmissive pixel PXt and the reflective pixel PXr, and is converted to include a black signal for black insertion. Further, the drive control unit 16 generates a gate signal that is a control signal for the gate driver 10 based on a synchronization signal input from an external signal source. Further, the drive control unit 16 outputs a synchronization signal for controlling the blinking of the backlight BL based on the synchronization signal input from the external signal source.

The set value memory 15 stores the respective black insertion rates for the transmissive pixel PXt and the reflective pixel PXr as setting data.
The gate driver 10 drives the plurality of gate lines G1 to Gm so that the plurality of switching elements W are conducted in units of rows. The source driver 20 outputs pixel voltages to the plurality of source lines X1 to Xn in a period in which the switching elements W in each row are turned on by driving the corresponding gate lines G. The drive control unit 16 converts an externally input video signal so as to include a black insertion signal, and controls operation timings of the gate driver 10 and the source driver 20 with respect to the conversion result.

FIG. 2 is a diagram schematically showing the configuration of the source driver.
The control signal CTX is supplied from the drive control unit 16 to the source driver 20 and is used to assign the display signal DATA, which is a video signal or a non-video signal, to the pixels PX in one row (line) to each of the plurality of source lines X. The control signal CTX includes a horizontal start signal STH, a horizontal clock signal CKH, a strobe signal STB, and a polarity signal POL. The horizontal start signal STH is a pulse generated every predetermined period. The horizontal clock signal CKH is a pulse generated by the number of source lines in each predetermined period. The strobe signal STB is a pulse generated with a predetermined time delay from the start signal STH. The strobe signal STB is used to output the pixel voltage, which is the conversion result of the display signal DATA for one line of pixels PX, to the source lines X1 to Xn in parallel. The polarity signal POL is a signal for inverting the polarity of the pixel voltage every horizontal scanning period and every vertical scanning period.

The source driver 20 includes a shift register 21, a sampling & load latch 22, a digital / analog (D / A) conversion circuit 23, and an output buffer circuit 24.
The shift register 21 shifts the horizontal start signal STH in synchronization with the horizontal clock signal CKH, and controls the timing at which the display signal DATA is sequentially serial-to-parallel converted. The sampling & load latch 22 sequentially latches the display signal DATA for one line of the pixels PXt and PXr under the control of the shift register 21 and outputs them in parallel. The digital / analog (D / A) conversion circuit 23 converts the display signal DATA into an analog pixel voltage. The output buffer circuit 24 outputs the analog pixel voltage obtained from the D / A conversion circuit 23 to the source lines X1 to Xn.

  The operations of the gate driver 10 and the source driver 20 described above are executed for the non-video signal for black insertion for one frame and the video signal for one frame. By changing the holding period of the pixel voltage corresponding to the video signal based on the setting data of the setting value memory 15 with respect to the holding period of the pixel voltage corresponding to the non-video signal for black insertion, the non-video for one frame period The black insertion rate, which is the ratio of the pixel voltage holding period corresponding to the signal, can be changed for the pixels PXt and PXr.

  Further, as shown in FIG. 3, the liquid crystal display panel DP includes a second polarizing plate POL2, a second retardation plate RF2, and the other outer surface side of the liquid crystal display panel DP, which are disposed on one outer surface side of the liquid crystal display panel DP. Are provided with a first retardation plate RF1 and a first polarizing plate POL1. Further, a backlight BL for illuminating the liquid crystal display panel DP is provided on the back surface of the liquid crystal display panel DP.

  The first polarizing plate POL1 and the second polarizing plate POL2 control the polarization state of the light that has passed through them. The first polarizing plate POL1 and the second polarizing plate POL2 applied here have an absorption axis and a transmission axis orthogonal to each other in a plane orthogonal to the light traveling direction. Such a polarizing plate extracts light having a vibrating surface in one direction parallel to the transmission axis, that is, light having a linearly polarized light state, from light having a vibrating surface in a random direction.

The first retardation plate RF1 is arranged so that the slow axis forms a predetermined angle (acute angle) with respect to the absorption axis (or transmission axis) of the first polarizing plate POL1 in the plane. For example, a λ / 4 plate having a function of converting linearly polarized light transmitted through the polarizing plate POL1 into elliptically polarized light having a predetermined ellipticity (= amplitude in the minor axis direction / amplitude in the major axis direction) or circularly polarized light is included.
Similarly, the second retardation plate RF2 is arranged so that the slow axis forms a predetermined angle (acute angle) with respect to the absorption axis (or transmission axis) of the second polarizing plate POL2 in the plane. The linearly polarized light transmitted through the second polarizing plate POL2 has a function of converting it into elliptically polarized light or circularly polarized light having a predetermined ellipticity.
With this configuration, the present liquid crystal display device generates different modulation factors (gradation applied voltage characteristics) between the transmission part and the reflection part.
The first retardation plate RF1 and the second retardation plate RF2 each include a retardation layer having a slow axis and a fast axis that are orthogonal to each other. In discussing birefringence, the slow axis corresponds to an axis having a relatively large refractive index, and the fast axis corresponds to an axis having a relatively small refractive index. The slow axis coincides with the vibration plane of the extraordinary ray. The fast axis coincides with the plane of vibration of ordinary rays. When the refractive index of ordinary ray and extraordinary ray is set to no and ne, respectively, and the thickness of the retardation plate along the traveling direction of each ray is set to d, the retardation value Δn · d (nm) of the retardation layer is (Ne · d−no · d) (that is, Δn = ne−no).
Therefore, a desired retardation value can be obtained by appropriately selecting the retardation layers of the first retardation plate RF1 and the second retardation plate RF2. In particular, according to this embodiment, the retardation value of the retardation layer in the transmission part and the retardation value of the retardation layer in the reflection part are set differently. Specifically, normally, the retardation values of the retardation layers of the first retardation plate RF1 and the second retardation plate RF2 are set to be equal. In this embodiment, as will be described later, this is twice the cell gap dr of the reflecting portion. Is set to be larger than the cell gap dt of the transmissive portion, the retardation value of the retardation layer of the second retardation plate RF2 that does not contribute to the display of the reflective portion is reduced, and conversely contributes to the display of the reflective portion. The retardation value of the retardation layer of the first retardation plate RF1 is set large. Specifically, the retardation value of the retardation layer of the first retardation plate RF1 and the second retardation plate RF2 is usually set to 30 nm. In this embodiment, the retardation value of the retardation layer of the first retardation plate RF1 is set. The retardation value was set to 40 nm, and the retardation value of the retardation layer of the second retardation plate RF2 was set to 20 nm.

  Further, in FIG. 3, by setting the cell gap dt of the transmissive part to a value slightly different from twice the cell gap dr of the reflective part, a difference is provided in the optical path length between the transmissive part and the reflective part. It is possible to cause a difference in the retardation value with the reflecting portion, thereby shifting the voltage at which the modulation rate is maximized. In this embodiment, the cell gap dt of the transmission part is set to 4 μm, and the cell gap dr of the reflection part is set to 3 μm. That is, the optical path length of the transmissive part is 4 μm, whereas the optical path length of the reflective part is 6 μm, which is 2 μm longer than the transmissive part.

FIG. 4 is a diagram showing the modulation rates of the transmission part and the reflection part, which will be described in more detail.
In the transmission part, the light from the backlight BL passes through the first retardation plate RF1, the liquid crystal display panel DP, and the second retardation plate RF2. In this light path, as shown in (1) of FIG. 4, the retardation value of the retardation layer of the first retardation plate RF1 and the retardation of the second retardation plate RF2 so that the modulation rate becomes maximum near 0V. The retardation value of the retardation layer is set respectively.
In the reflector, light from the surroundings travels with the second retardation plate RF2, the liquid crystal display panel DP, the liquid crystal display panel DP, and the second retardation plate RF2. Therefore, by setting the cell gap dr of the reflection part to be slightly thicker than 1/2 times the cell gap dt of the transmission part, the retardation value of the reflection part is made larger than the retardation value of the transmission part. The retardation value of the retardation layer of the two retardation plate RF2 is set.

Note that the cell gap dt and the cell gap dr are preferably in the relationship of Expression (1).
dt / 2 <dr−dt / 2 <dr (1)
With such a configuration, the modulation factor curve of the reflecting portion does not become the maximum in the vicinity of 0 V, and the modulation factor curve is shifted as shown in (2) of FIG. Then, for example, the cell gap dr of the reflecting portion and the second retardation plate RF2 are designed so that the maximum value of the modulation factor is in the vicinity of the critical voltage Vc where the energy of the spray orientation and the energy of the bend orientation antagonize.
In addition, as shown in FIG. 4, it is possible to apply a voltage independently to the transmission part and the reflection part of the pixel of this liquid crystal display device.

FIG. 5 is a time chart showing the driving of the reflection part and the transmission part during white display.
The transmission pixel PXt is supplied with a voltage Vbt representing black by a modulation rate for a predetermined period of one frame period, and thereby black display is performed. The transmission pixel PXt is supplied with a voltage 0 V representing white with a modulation rate for the remaining period of one frame period, thereby displaying white.

  On the other hand, the voltages Vc and −Vc representing white with a modulation rate are alternately applied to the pixel PXr of the reflection unit for each frame period, and white display is thereby performed.

  In this way, the black portion is not driven for the reflective portion, and the impulse-type drive (black insertion drive) is performed for the transmissive portion, thereby preventing the reverse transition of the liquid crystal and high light utilization efficiency and transmission in the reflective portion. It is possible to improve the visibility of moving images in the section.

Further, in the transmissive portion, the power consumption can be further reduced by controlling the lighting / non-lighting of the backlight BL corresponding to the black display period in one frame period.
The black display period in the above-described embodiment may be uniformly provided in all pixel rows, or may be set sequentially.

[Second Embodiment]
The second embodiment is different in that black insertion driving is performed for both the transmissive part and the reflective part in the display operation of the liquid crystal display device. Accordingly, the same parts as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
In the second embodiment, as shown in FIG. 7, black insertion driving is performed for both the transmissive part and the reflective part, but the maximum modulation rate is antagonized by the spray orientation energy and the bend orientation energy in the reflective part. Therefore, it is not necessary to set the black insertion period so long in the transmissive part.

  Note that the black display period length in the reflection mode and the black display period length in the transmission mode can be changed based on the setting data in the setting value memory 15. That is, the black insertion rate in the reflection mode and the transmission mode can be designated. Note that it is desirable that the black display period in the reflection mode is shorter than the black display period in the transmission mode from the viewpoint of improving the light utilization efficiency.

In addition, the presence / absence of black insertion driving at the reflecting portion may be switched.
For example, whether or not to perform black insertion driving at the reflecting portion (whether or not black insertion is performed) may be controlled according to user settings.
Alternatively, whether or not there is a lot of motion in the input video is automatically detected by comparing the frames, and if there is a lot of motion, the black insertion drive at the reflecting portion may be executed in order to improve the visibility of the moving image. .
Furthermore, an external light sensor may be provided in a part of the display panel or MDL, and when the intensity of light received by the sensor is equal to or higher than a certain value, black insertion driving at the reflecting portion may be performed.

In this way, in order to enable switching of the presence or absence of black insertion driving in the reflection section, the reflection section performs black insertion driving between a white voltage and a black voltage equal to or higher than Vc.
Then, when switching is made so that the black insertion drive is performed in the reflection section, the black display period length in the reflection mode may be determined based on the data set in the set value memory 15.
However, in the black insertion period, it is necessary to make the potential of the pixel voltage PE transition to a large pixel voltage for black insertion in a short time, so that the usable black insertion rate is limited according to the value of the critical voltage Vc. May be.

[Third Embodiment]
In the third embodiment, the connection mode between the liquid crystal pixel PX and the source line X is different from that in the first embodiment. Accordingly, the same parts as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
FIG. 6 is a diagram illustrating the connection between the liquid crystal pixel, the gate line, and the source line.
The OCB liquid crystal pixel PX is a transflective liquid crystal pixel, and includes a transmissive pixel PXt arranged in an odd-numbered row of the liquid crystal display panel DP and a reflective pixel PXr arranged in an even-numbered row. That is, the transmission part and the reflection part of the OCB liquid crystal pixel PX are independently configured in one pixel.
The OCB liquid crystal pixel PX is connected with a transmissive gate line Gt and a reflective gate line Gr, and drives the transmissive pixel PXt and the reflective pixel PXr, respectively. Further, a transmission source line Xt and a reflection source line Xr are connected to the OCB liquid crystal pixel PX to drive the transmission type pixel PXt and the reflection type pixel PXr, respectively.

  According to this configuration, since the gate line G and the source line X are provided independently for the transmissive pixel PXt and the reflective pixel PXr, respectively, the pixel PXt and the pixel PXr are driven independently. Each display period of the transmission part and the reflection part can be determined independently.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

1 is a diagram schematically showing a circuit configuration of a liquid crystal display device. The figure which shows the structure of a source driver schematically. Sectional drawing which shows the structure of a liquid crystal display device. The figure which shows the modulation factor of a transmission part and a reflection part. The time chart which shows the drive of the reflection part at the time of white display, and a transmission part. The figure which shows the connection of a liquid crystal pixel, a gate line, and a source line. The time chart which shows the drive of the reflection part at the time of white display, and a transmission part.

Explanation of symbols

  2 ... array substrate, 3 ... counter substrate, 4 ... liquid crystal layer, 7 ... gradation reference voltage generation circuit, 10 ... gate driver, 20 ... source driver, 23 ... D / A conversion circuit, 23 '... D / A conversion unit , PE ... pixel electrode, CE ... common electrode, PX ... liquid crystal pixel, DP ... display panel, POL1 ... first polarizing element, POL2 ... second polarizing element, BL ... backlight, CNT ... display control circuit, DP ... liquid crystal display Panel, Vc ... critical voltage, Vs ... pixel voltage, X ... source line, Xt ... transmission source line, Xr ... reflection source line, G ... gate line, Gt ... transmission gate line, Gr ... reflection gate line, W: Pixel switching element.

Claims (6)

A display panel in which pixels are arranged in a matrix, and
A light source for illuminating the display panel;
A first retardation plate that is arranged between one surface of the display panel and the light source and gives a phase difference to light of a predetermined wavelength;
A second retardation plate disposed opposite to the other surface of the display panel,
The pixel is a transflective OCB liquid crystal pixel having a reflective part and a transmissive part that can be independently driven and controlled, and the cell gap of the reflective part is larger than ½ of the cell gap of the transmissive part. A characteristic liquid crystal display device. The second retardation plate is configured to obtain a maximum transmittance when a predetermined voltage greater than 0 V is applied to the reflecting portion.
The liquid crystal display device according to claim 1, wherein the first retardation plate is configured to obtain a maximum transmittance when a voltage in the vicinity of 0 V is applied to the transmission portion. Drive control means for driving the reflection part and the transmission part;
The liquid crystal display device according to claim 1, wherein a black insertion rate when the drive control unit drives the reflection unit is smaller than a black insertion rate when the transmission unit is driven.   4. The liquid crystal display device according to claim 3, wherein the drive control unit performs black insertion driving for the transmission part and does not perform black insertion driving for the reflection part. 5.   The liquid crystal display device according to claim 1, wherein gradation application voltage characteristics are different between the reflection portion and the transmission portion.   6. The black insertion rate when driving the reflection unit and the transmission unit is adjusted according to a gradation applied voltage characteristic between the reflection unit and the transmission unit. The liquid crystal display device described.

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