JP2007298799A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inhibit brightness slope from arising when adopting a frame reversal drive method as an AC drive system. <P>SOLUTION: A liquid crystal display device comprises: a control electrode drive circuit which applies a selected scanning voltage for turning on an active element to a control electrode of the active element of an selected pixel, and applies a non-selected scanning voltage for turning off the active element to the control electrode of the active element of a non-selected pixel; and a counter electrode drive circuit which applies a first level common voltage and a second level common voltage to the counter electrode in each frame, wherein the control electrode drive circuit applies the first level non-selected scanning voltage to the control electrode of the active element of the non-selected pixel when the first level common voltage is applied to the counter voltage, and applies the second level non-selected scanning voltage to the control electrode of the active element of the non-selected pixel when the second level common voltage is applied to the counter electrode. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device, and more particularly to a technique effective when applied to a scanning line driving circuit of a liquid crystal display device.

2. Description of the Related Art A TFT (Thin Film Transistor) type liquid crystal display module having a small liquid crystal panel is widely used as a display unit of a mobile device such as a mobile phone.
In general, since the life of a liquid crystal is deteriorated when a DC voltage is applied, it is necessary to drive the liquid crystal with an alternating current, and a common inversion driving method is known as one of the alternating current driving methods.
In this common inversion driving method, the electric field direction in the liquid crystal layer is a direction from the pixel electrode to the counter electrode (also referred to as a common electrode or a common electrode) (hereinafter referred to as positive writing), or from the counter electrode to the pixel. Switch alternately in the direction toward the electrode (hereinafter referred to as negative polarity writing) every screen (frame) (hereinafter referred to as frame inversion driving method) or every horizontal scanning period (hereinafter referred to as 1 line inversion driving method). It is done.

In a TFT liquid crystal display module for mobile phones, the frame inversion driving method in the common inversion driving method is adopted as an AC driving method in order to reduce power consumption.
However, this common inversion driving method has a problem that a luminance gradient occurs along the gate scan direction when displaying a halftone on the liquid crystal display panel.
The present invention has been made to solve the above-described problems of the prior art, and an object of the present invention is to provide a luminance gradient when a frame inversion driving method is employed as an AC driving method in a liquid crystal display device. It is an object of the present invention to provide a technique capable of preventing the occurrence.
The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

Of the inventions disclosed in this application, the outline of typical ones will be briefly described as follows.
(1) A plurality of pixels and a drive circuit that drives the plurality of pixels, each pixel of the plurality of pixels includes an active element, a pixel electrode to which a video voltage is applied via the active element, The drive circuit applies a selection scanning voltage for turning on the active element to the control electrode of the active element of the selected pixel, and the other electrode than the selected pixel. A control electrode driving circuit for applying a non-selection scanning voltage for turning off the active element to a control electrode of the active element of the non-selected pixel, a common voltage at a first voltage level for each frame, and a second voltage In the liquid crystal display device having a common electrode driving circuit for applying a common voltage at a voltage level to the common electrode, the control electrode driving circuit is common to the common electrode at the first voltage level. When a voltage is applied, a non-select scanning voltage of a first voltage level is applied to the control electrode of the active element of the non-selected pixel, and a common voltage of the second voltage level is applied to the counter electrode. Then, a non-selection scanning voltage of a second voltage level is applied to the control electrode of the active element of the non-selected pixel.

(2) In (1), the common voltage at the first voltage level is a voltage higher than the common voltage at the second voltage level, and the non-selection scanning voltage at the first voltage level is the second voltage. The voltage is higher than the level non-selection scanning voltage.
(3) In (1) or (2), the potential difference between the common voltage at the first voltage level and the common voltage at the second voltage level is ΔVcom, the non-selected scanning voltage at the first voltage level and the second voltage When the potential difference between the level and the non-selection scanning voltage is ΔVg, ΔVg is a voltage proportional to ΔVcom.
(4) In (1) or (2), a scanning line for supplying the selected scanning voltage and the non-selected scanning voltage is provided, and the common voltage at the first voltage level and the common voltage at the second voltage level are ΔVcom, the potential difference between the non-selection scan voltage at the first voltage level and the non-selection scan voltage at the second voltage level is ΔVg, and the capacitance of the parasitic capacitance formed between the pixel electrode and the scan line When the value is Cgst, ΔVg is a voltage proportional to ΔVcom and inversely proportional to Cgst.

(5) In (1) or (2), a common voltage having the first voltage level, including a video line that supplies the video voltage, a scanning line that supplies the selected scanning voltage, and the non-selected scanning voltage. And the common voltage at the second voltage level is ΔVcom, the potential difference between the non-selected scanning voltage at the first voltage level and the non-selected scanning voltage at the second voltage level is ΔVg, and the pixel electrode and the video line ΔVg is (1 + 2 × Cdst / Cgst) where Cdst is the capacitance value of the parasitic capacitance formed between and Cgst is the capacitance value of the parasitic capacitance formed between the pixel electrode and the scanning line. , And is a voltage proportional to ΔVcom.
(6) In any one of (1) to (5), the pixel includes a storage capacitor element formed between the pixel electrode and the common electrode.

The effects obtained by the representative ones of the inventions disclosed in the present application will be briefly described as follows.
According to the liquid crystal display device of the present invention, when the frame inversion driving method is adopted as the common inversion driving method, it is possible to prevent a luminance gradient from occurring.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In all the drawings for explaining the embodiments, parts having the same functions are given the same reference numerals, and repeated explanation thereof is omitted.
FIG. 1 is a block diagram showing a schematic configuration of a liquid crystal display module according to an embodiment of the present invention.
In the liquid crystal panel (PNL), a plurality of scanning lines (or gate lines) (G) and video lines (also called source lines or drain lines) (D) are provided in parallel.
A pixel portion is provided corresponding to a portion where the scanning line (G) and the video line (D) intersect. The plurality of pixel portions are arranged in a matrix, and each pixel portion is provided with a pixel electrode (PIX) and a thin film transistor (TFT). In FIG. 1, the number of subpixels of the liquid crystal panel (PNL) is 240 × 320 × 3.
A counter electrode (also referred to as a common electrode or a common electrode) (CT) is provided so as to face each pixel electrode (PIX) with the liquid crystal interposed therebetween. Therefore, a liquid crystal capacitor (Cpx) is formed between each pixel electrode (PIX) and the counter electrode (CT).
A liquid crystal panel (PNL) includes a glass substrate (first substrate) (GLASS) provided with pixel electrodes (PIX), thin film transistors (TFTs), and a glass substrate (second substrate) on which color filters and the like are formed. (Not shown) are stacked with a predetermined gap therebetween, and both substrates are bonded together with a sealing material provided in the form of a frame near the peripheral edge between the two substrates, and provided on a part of the sealing material. A liquid crystal is sealed and sealed inside the sealing material between the two substrates from the liquid crystal sealing port, and a polarizing plate is attached to the outside of both the substrates.
Since the present invention is not related to the internal structure of the liquid crystal panel, a detailed description of the internal structure of the liquid crystal panel is omitted. Furthermore, the present invention can be applied to a liquid crystal panel having any structure. For example, in the case of the vertical electric field method, the counter electrode is formed on the second substrate. In the case of the horizontal electric field method, the counter electrode is formed on the first substrate. A color filter may be provided on the first substrate.

In the liquid crystal display module shown in FIG. 1, a drive circuit (DRV) is mounted on a glass substrate (GLASS).
The driving circuit (DRV) includes a controller circuit 100, a source driver (drain driver) 130 for driving the video line (D) of the liquid crystal panel (PNL), and a gate for driving the scanning line (G) of the liquid crystal panel (PNL). A liquid crystal driving power source that generates a driver 140 and a power source voltage (for example, a common voltage (Vcom) supplied to the counter electrode (CT) of the liquid crystal panel (PNL)) necessary for displaying an image on the liquid crystal panel (PNL). A generation circuit 120 and a memory circuit (hereinafter referred to as RAM) 150 are included. In FIG. 1, FPC is a flexible wiring board.
Note that FIG. 1 illustrates the case where the drive circuit (DRV) is configured by one semiconductor chip. However, the drive circuit (DRV) includes, for example, a thin film transistor that uses low-temperature polysilicon for a semiconductor layer. And may be formed directly on a glass substrate (GLASS).
Similarly, a part of the circuit of the drive circuit (DRV) may be divided and the drive circuit (DRV) may be configured by a plurality of semiconductor chips. A thin film transistor using low-temperature polysilicon as a layer may be used to form directly on a glass substrate (GLASS).
Further, the drive circuit (DRV) or a part of the drive circuit (DRV) may be formed on the flexible wiring board instead of being mounted on the glass substrate (GLASS).

Display data and a display control signal are input to the controller circuit 100 from a microcomputer (Micro controller Unit) on the main body side or a graphic controller.
In FIG. 1, SI is a system interface and is a system in which various control signals and image data are input from an MCU or the like.
DI is a display data interface (RGB interface), and is a system (external data) in which image data formed by an external graphic controller and a data capturing clock are continuously input.
In this display data interface (DI), the image data is sequentially captured in accordance with the capture clock in the same manner as a drain driver used in a conventional personal computer.
The controller circuit 100 controls the display by sending the image data received from the system interface (SI) and the display data interface (DI) to the source driver 130 and the RAM 150.

FIG. 6 is a diagram showing voltage waveforms applied to the respective electrodes in the liquid crystal display module of the present embodiment or the conventional liquid crystal display module.
When the frame inversion driving method is adopted as the AC driving method, as shown in FIG. 6A, a common voltage (Vcom + ΔVcom) of the first voltage level is applied from the liquid crystal driving power generation circuit 120 to the counter electrode (CT). ) And the common voltage (Vcom) at the second voltage level are alternately applied every frame.
Correspondingly, as shown in FIG. 6B, the pixel electrode (PIX) has a negative gradation voltage (VsL) having a lower potential than the common voltage (Vcom + ΔVcom) of the first voltage level, A positive gradation voltage (VsH) having a higher potential than the common voltage (Vcom) at the second voltage level is applied.

FIG. 4 is a diagram for explaining a problem of a conventional TFT liquid crystal display module for a cellular phone. FIG. 5 is a diagram for explaining an AC driving method of a conventional liquid crystal display module.
If the gate scan direction is the direction from point A to point B in FIG. 4 and the voltage applied to the counter electrode (CT) is the second voltage level common voltage (Vcom), point A in FIG. Then, at the time of Ta, positive polarity writing, that is, positive gradation voltage (VsH) having a higher potential than the common voltage (Vcom) of the second voltage level is written to the pixel electrode (PIX). Thereafter, the pixel electrode (PIX) is in a floating state, but the potential of the pixel electrode (PIX) is VsH.
After that, when the voltage applied to the counter electrode (CT) is inverted to the common voltage (Vcom + ΔVcom) of the first voltage level, the pixel electrode (PIX) is in a floating state, so the potential of the pixel electrode (PIX) is (VsH + ΔVs).
At time Tb, negative writing, that is, negative gradation voltage (VsL) having a lower potential than the common voltage (Vcom + ΔVcom) at the first voltage level is written to the pixel electrode (PIX). Thereafter, the pixel electrode (PIX) is in a floating state, but the potential of the pixel electrode (PIX) is VsL.

On the other hand, at point B in FIG. 4, at the time point Tc, positive polarity writing, that is, positive polarity gradation voltage (VsH) higher than the second voltage level common voltage (Vcom) is applied to the pixel electrode (PIX). ). Thereafter, the pixel electrode (PIX) is in a floating state, but the potential of the pixel electrode (PIX) is VsH.
After that, when the voltage applied to the counter electrode (CT) is inverted to the common voltage (Vcom + ΔVcom) of the first voltage level, the pixel electrode (PIX) is in a floating state, so the potential of the pixel electrode (PIX) is (VsH + ΔVs).
At time Td, negative polarity writing, that is, negative gradation voltage (VsL) having a lower potential than the first voltage level common voltage (Vcom + ΔVcom) is written to the pixel electrode (PIX). Thereafter, the pixel electrode (PIX) is in a floating state, but the potential of the pixel electrode (PIX) is VsL.
Here, the voltage of (ΔVs) is expressed by the following equation (1).
ΔVs = (Cpxt + Cstgt) × ΔVcom / (Cpxt + Cstgt + Cgst + 2Cdst) <ΔVcom
.... (1)
Here, as shown in FIG. 2, Cpxt is the capacitance value of the liquid crystal capacitor (Cpx), Cstgt is the capacitance value of the holding capacitor (Cstg), and Cgst is the parasitic between the pixel electrode (PIX) and the scanning line (G). The capacitance value of the capacitance (Cgs), Cdst is the capacitance value of the parasitic capacitance (Cds) between the pixel electrode (PIX) and the video line (D).
2 is a diagram showing an equivalent circuit of the pixel shown in FIG. 1, and the storage capacitor element (Cstg) is a capacitor element formed by the pixel electrode (PIX) and the counter electrode (CT). .

In the FLA frame of FIG. 6, the voltage applied to the counter electrode (CT) is a common voltage (Vcom) of the second voltage level, and the pixel electrode (PIX) and the counter electrode (CT) after the positive writing. (V1) is expressed by the following equation (2).
V1 = VsH-Vcom (2)
Further, when the frame is inverted from the FLA frame to the FLB frame in FIG. 6, the voltage applied to the counter electrode (CT) is the common voltage (Vcom + ΔVcom) at the first voltage level, and the pixel electrode (PIX before negative writing) ) And the counter electrode (CT) is represented by the following equation (2).
V2 = VsH + ΔVs− (Vcom + ΔVcom)
= VsH−Vcom + (ΔVs−ΔVcom)
.... (3)
As can be seen from the above equations (2) and (3), the potential difference between the pixel electrode (PIX) and the counter electrode (CT) is greater before frame inversion than after frame inversion (V1> V2).
Here, as shown in FIG. 5, at the point A in FIG. 4, the time during which the voltage V1 is applied to the liquid crystal is longer than the time during which the voltage V2 is applied to the liquid crystal, and at the point B in FIG. The time during which the voltage V2 is applied to the liquid crystal is longer than the time during which the voltage V1 is applied to the liquid crystal.
Therefore, the effective voltage at point A in FIG. 4 is higher than the effective voltage at point B in FIG. As a result, as shown in FIG. 4, when a halftone is displayed, a luminance gradient occurs such that the start side of the gate scan is bright and the end side of the gate scan is dark.

In this embodiment, the non-selective scanning voltage applied to the gate of the thin film transistor (TFT) from the gate driver 140 via the scanning line (G) is converted into an alternating current in synchronization with the common voltage applied to the counter electrode (CT). It is characterized by doing.
In this embodiment, as shown in FIG. 6C, the non-selection scanning voltage when the common voltage (Vcom) of the second voltage level is applied to the counter electrode (CT) is set to the second voltage level ( The scanning voltage of (Vgl) and the non-selection scanning voltage when the common voltage (Vcom + ΔVcom) of the first voltage level is applied to the counter electrode (CT) are the scanning voltage of (Vgl + ΔVgl) of the first voltage level.
In general, ΔVs when the voltage fluctuation of the pixel electrode (PIX) due to the video voltage is sufficiently small and the writing / holding characteristics are not problematic is expressed by the following equation (4).
ΔVs = (Cpxt + Cstgt) × ΔVcom / (Cpxt + Cstgt + Cgst + 2Cdst)
+ Cgst × ΔVgl / (Cpxt + Cstgt + Cgst + 2Cdst)
.... (4)
In order to prevent the luminance gradient described above, ΔVs = ΔVcom may be set. ΔVgl when ΔVs = ΔVcom is satisfied is expressed by the following equation (5).
ΔVgl = (1 + 2 × Cdst / Cgst) × ΔVcom
(5)

As a result, as shown in FIG. 3, the effective voltage at the point A in FIG. 4 and the effective voltage at the point B in FIG. 4 can be made substantially equal. It is possible to prevent a luminance gradient from occurring, such that the start side is bright and the gate scan end side is dark.
FIG. 3 is a diagram for explaining the AC drive system of the embodiment of the present invention. In FIG. 3, VgH is a gate of a thin film transistor (TFT) from the gate driver 140 via the scanning line (G). The selective scanning voltages are sequentially applied.
As described above, in this embodiment, the non-selection scanning voltage applied from the gate driver 140 to the gate of the thin film transistor (TFT) is converted into an alternating current in synchronization with the common voltage applied to the counter electrode (CT). When a common voltage (Vcom + ΔVcom) of the first voltage level is applied to CT), a non-selection scanning voltage of ΔVgl is applied to the gate of the thin film transistor (TFT), thereby preventing a luminance gradient from occurring. it can.

As is clear from the above equation (5), the non-selection scanning voltage of ΔVgl is a voltage proportional to ΔVcom, and is a voltage inversely proportional to the capacitance value (Cstgt) of the storage capacitor (Cstg). Alternatively, the voltage is proportional to (1 + 2 × Cdst / Cgst). Here, Cgst is a capacitance value of the parasitic capacitance (Cgs) between the pixel electrode (PIX) and the scanning line (G), and Cdst is a parasitic capacitance (Cds) between the pixel electrode (PIX) and the video line (D). ) Capacity value.
In the above description, the case where the present invention is applied to an IPS liquid crystal display device has been described. However, the present invention is not limited to this, and the present invention is represented by an equivalent circuit shown in FIG. The present invention can also be applied to a liquid crystal display device having a pixel to be used, for example, a TN liquid crystal display device or a VA liquid crystal display device.
The present invention is particularly effective for a liquid crystal display device in which the capacitance value of the pixel capacitance (Cpx) or the capacitance value of the holding capacitance (Cstg) is relatively small. In the equivalent circuit shown in FIG. ) And the video line (D), the parasitic capacitance (Cds) may not exist.
As mentioned above, the invention made by the present inventor has been specifically described based on the above embodiments. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Of course.
The present invention is not limited to a liquid crystal display device, but can be applied to a display device that periodically switches voltage levels.

It is a block diagram which shows schematic structure of the liquid crystal display module by which this invention is employ | adopted. It is a figure which shows the equivalent circuit of the pixel shown in FIG. It is a figure for demonstrating the alternating current drive system of the Example of this invention. It is a figure for demonstrating the problem of the conventional liquid crystal display module of the TFT system for mobile phones. It is a figure for demonstrating the alternating current drive system of the conventional liquid crystal display module. It is a figure which shows the voltage waveform applied to each electrode in the liquid crystal display module of the Example of this invention, or the conventional liquid crystal display module.

Explanation of symbols

100 controller circuit 120 liquid crystal drive power generation circuit 130 source driver 140 gate driver 150 memory circuit PNL liquid crystal panel D video line (source line or drain line)
G Scan line (or gate line)
TFT Thin film transistor PIX Pixel electrode CT Counter electrode (common electrode or common electrode)
GLASS glass substrate DRV driving circuit Cpx liquid crystal capacitance Cstg holding capacitance Cgs parasitic capacitance between pixel electrode (PIX) and scanning line (G) Cds parasitic capacitance between pixel electrode (PIX) and video line (D)

Claims (6)

A plurality of pixels;
A drive circuit for driving the plurality of pixels,
Each pixel of the plurality of pixels includes an active element,
A pixel electrode to which a video voltage is applied through the active element;
A counter electrode facing the pixel electrode;
The driving circuit applies a selective scanning voltage for turning on the active element to the control electrode of the active element of the selected pixel, and applies it to the control electrode of the active element of a non-selected pixel other than the selected pixel. A control electrode driving circuit for applying a non-selection scanning voltage for turning off the active element;
In a liquid crystal display device having a common electrode driving circuit that applies a common voltage of a first voltage level and a common voltage of a second voltage level to the common electrode for each frame.
When the common voltage of the first voltage level is applied to the counter electrode, the control electrode driving circuit applies a non-selected scanning voltage of the first voltage level to the control electrode of the active element of the non-selected pixel. And applying a non-selective scanning voltage of a second voltage level to the control electrode of the active element of the non-selected pixel when the common voltage of the second voltage level is applied to the counter electrode. A characteristic liquid crystal display device. The common voltage of the first voltage level is a voltage having a higher potential than the common voltage of the second voltage level;
2. The liquid crystal display device according to claim 1, wherein the unselected scanning voltage at the first voltage level is a voltage having a higher potential than the unselected scanning voltage at the second voltage level.   The potential difference between the common voltage at the first voltage level and the common voltage at the second voltage level is ΔVcom, and the potential difference between the non-selected scanning voltage at the first voltage level and the non-selected scanning voltage at the second voltage level is ΔVg. 3. The liquid crystal display device according to claim 1, wherein ΔVg is a voltage proportional to ΔVcom. A scanning line for supplying the selected scanning voltage and the non-selected scanning voltage;
ΔVcom is a potential difference between the common voltage at the first voltage level and the common voltage at the second voltage level, and ΔVg is a potential difference between the non-selection scanning voltage at the first voltage level and the non-selection scanning voltage at the second voltage level. 2. The voltage according to claim 1, wherein when a capacitance value of a parasitic capacitance formed between the pixel electrode and the scanning line is Cgst, ΔVg is a voltage proportional to ΔVcom and inversely proportional to Cgst. Item 3. A liquid crystal display device according to Item 2. A video line for supplying the video voltage;
A scanning line for supplying the selected scanning voltage and the non-selected scanning voltage;
ΔVcom is a potential difference between the common voltage at the first voltage level and the common voltage at the second voltage level, and ΔVg is a potential difference between the non-selection scanning voltage at the first voltage level and the non-selection scanning voltage at the second voltage level. When the capacitance value of the parasitic capacitance formed between the pixel electrode and the video line is Cdst, and the capacitance value of the parasitic capacitance formed between the pixel electrode and the scanning line is Cgst, ΔVg is The liquid crystal display device according to claim 1, wherein the voltage is proportional to ΔVcom (1 + 2 × Cdst / Cgst).   The liquid crystal display device according to claim 1, wherein the pixel includes a storage capacitor element formed between the pixel electrode and the common electrode.

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