JP2516351B2 - Driving method of active matrix type liquid crystal panel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] A gate voltage for turning on a switching element such as a thin film transistor is applied in correspondence with the polarity of a data voltage by applying a data voltage with its polarity inverted every predetermined period such as a frame. Then, the display luminance is changed to prevent uneven display brightness.

[Industrial applications]

The present invention relates to a method for driving an active matrix liquid crystal panel that can improve display quality.

In an active matrix type liquid crystal panel, scan bus lines and data bus lines are arranged at right angles, liquid crystal cells are connected to the intersections thereof through switching elements such as thin film transistors, and the on / off of the switching elements is controlled. Since the data voltage is applied to the liquid crystal cell, there is an advantage that the problem of the drive duty ratio does not occur even if the display capacity is increased. Further, by providing a color filter corresponding to a liquid crystal cell, full-color display becomes possible, and it can be applied to a portable television receiver or the like. There is a demand for further improving the display quality of such an active matrix type liquid crystal panel.

[Conventional technology]

The active matrix type liquid crystal panel has a configuration in which a liquid crystal cell is connected via a thin film transistor (hereinafter abbreviated as TFT) at an intersection of a scan bus line and a data bus line arranged orthogonally, and a data voltage applied to the data bus line is provided. Is to invert the polarity every frame and sequentially apply a gate voltage for turning on the TFT to the scan bus line.

FIG. 6 is a diagram for explaining the operation of the conventional example. The gate voltage applied to the scan bus line is TF as shown in (a).
It is composed of a voltage Vgon for turning on T and a voltage Vgoff for turning off, and the polarity of the data voltage is periodically inverted as shown in (b).

For example, when a voltage Vgon is applied to a scan bus line when a positive data voltage + Vd is applied to a data bus line, the TFT connected to the scan bus line
Is turned on, and the data voltage + Vd is applied to the liquid crystal cell via the TFT. Next, the voltage Vgof is applied to the scan bus line.
When f is applied, the TFT is turned off, and the liquid crystal cell voltage is reduced by ΔV due to capacitive coupling with the gate capacitance of the TFT. This liquid crystal cell voltage is held by the capacitance of the liquid crystal cell until the next cycle. Then, in the next cycle, the polarity of the data voltage is inverted, and when the voltage Vgon is applied to the scan bus line, the negative data voltage −Vd is applied to the liquid crystal cell. Therefore, the polarity of the liquid crystal cell voltage is periodically inverted as shown in (c) of FIG.

FIG. 7 is a diagram for explaining the connection configuration of the liquid crystal cell. The drain of the TFT 23 is connected to the data bus line 21, the gate is connected to the scan bus line 22, and the source is connected to the liquid crystal cell 24. Cg is the gate capacitance of the TFT 23, Cc is the liquid crystal cell capacitance, R is the equivalent resistance of the scan bus line 22, and C is the equivalent capacitance.

FIG. 8 is an operation explanatory diagram when the TFT is turned on and off.
(A) shows the case where the TFT 23 is turned on, and the liquid crystal cell capacitance Cc has a gate voltage Vg via the gate capacitance Cg of the TFT 23.
gon is applied, and the data voltage Vd is applied via the TFT 23 in the ON state. Further, (B) shows a case where the TFT 23 is shifted from the ON state to the OFF state, and the gate voltage is changed from Vgon to Vgoff. Rt represents the resistance in the process of shifting the TFT 23 from the on state to the off state. Also (C)
Indicates that the gate voltage is Vgoff and the TFT 23 is completely off.

As TFT23 shifts from on to off, the liquid crystal cell voltage becomes Only changes. This corresponds to ΔV in FIG. 6 (c). For this change ΔV, the common voltage Vc
The positive and negative polarities of the liquid crystal cell voltage are set so as to be symmetrical after being corrected by (see (c) of FIG. 6).

[Problems to be solved by the invention]

Due to the equivalent resistance R and the equivalent capacitance C of the scan line 22, the gate voltage output from the driver gradually becomes blunt. At a position close to the driver, the gate voltage falls sharply from Vgon to Vgoff,
The TFT on state of FIG. 7 (A) is instantly changed to the TFT off state of FIG. 7 (C). However, at a position far from the driver, the falling edge becomes gentle due to the rounding of the waveform, so the data voltage Vd passes through the resistor Rt to the liquid crystal cell capacitance Cc via the process of FIG. 7B. Since the voltage is continuously applied, the change in the liquid crystal cell voltage becomes small. In other words, in the process in which the gate voltage falls from Vgon to Vgoff, the TFT 23 continues to be on until the gate voltage drops to the threshold voltage Vth of the TFT 23,
In this case, the change ΔV ′ in the liquid crystal cell voltage is: Becomes Since Vgon> Vth, the change amount of the liquid crystal cell voltage ΔV> ΔV ′, and the change amount of the liquid crystal cell voltage at a position far from the driver becomes small.

The solid line in FIG. 6 (c) shows the liquid crystal cell voltage near the driver, and the dotted line shows the liquid crystal cell voltage far from the driver. Therefore, even if the near-point liquid crystal cell voltage is corrected by the common voltage Vc applied to the common ground point, the far-point liquid crystal cell voltage cannot be corrected, causing unevenness in display brightness along the scan bus line direction, and There is a drawback that flicker occurs due to different polar liquid crystal cell voltages.

Further, the threshold voltage Vth of the TFT 23 changes depending on the data voltage Vd and can be expressed by Vth = Vd + Vth ₀ (3). Vth ₀ is a threshold voltage that does not depend on the data voltage.

Therefore, the threshold voltage when the negative polarity data voltage is applied is lower than that when the positive polarity data voltage is applied, and the variation ΔV ′ of the liquid crystal cell voltage is as shown by the dotted line in (c) of FIG. In particular, it becomes particularly small during the application period of the negative polarity data voltage, which also causes unevenness in display brightness.

9 (A) and 9 (B) are explanatory views of the occurrence of uneven brightness, in which the horizontal axis represents the liquid crystal cell voltage V and the vertical axis represents the intensity B of transmitted light or reflected light. (A) shows the case of binary display,
(B) shows the case of full color (gradation) display. In the case of binary display, as shown in (A), black is selected for a liquid crystal cell voltage equal to or lower than the threshold, and white is selected for a liquid crystal cell voltage equal to or higher than the saturation threshold. ) And the far point (indicated by solid circles or black circles) can be set to be displayed with substantially the same luminance.

On the other hand, when gradation display is performed, the liquid crystal cell voltage between the black threshold and the white saturation threshold is used as shown in FIG. In the case of white display, the brightness at the near point (dotted circle) having an effective value larger than the effective value of the liquid crystal cell voltage near the saturation threshold is almost the same as that at the far point. However, when the far point (black circle) is displayed in black,
The brightness of the near point (dotted circle) having an effective value larger than the effective value of the liquid crystal cell voltage at the far point is close to white. Therefore, when performing black display, uneven brightness occurs in which the brightness on the side close to the driver increases, which degrades display quality.

The present invention prevents the occurrence of uneven brightness as described above,
The purpose is to improve the display quality.

[Means for solving problems]

A method of driving an active matrix type liquid crystal panel of the present invention will be described with reference to FIG. 1. As shown in FIG. 1A, the data voltage applied to the data bus line is + Vd,
Like −Vd, it is inverted every predetermined period such as every frame F. Also, as shown in (b) and (d), two types of gate voltage to be applied to the scan bus line, Vgonp and Vgonm, are prepared, and when the data voltage has a positive polarity, it has Vgonp and a negative polarity. In this case, the voltage is switched to Vgonm lower than Vgonp and applied, and the gate voltage Vgoff for turning off the switching element is set to the same value in the positive polarity period and the negative polarity period of the data voltage, When the gate voltage shown in (b) is applied to the switching element corresponding to the liquid crystal cell at a position close to the driver, the voltage of the liquid crystal cell becomes that shown in (c), or the switching element corresponding to the liquid crystal cell at a position far from the driver. (D) is applied to the gate voltage having a blunt waveform, and the voltage of the liquid crystal cell becomes that shown in (e).

[Action]

By setting the gate voltage Vgonm in the period in which the negative polarity data voltage −Vd is applied to be lower than the gate voltage Vgonp in the period in which the positive polarity data voltage + Vd is applied,
Particularly, during the period of applying the negative polarity data voltage, the change amount ΔV ′ of the liquid crystal cell voltage at a position closer to the driver (see FIG. 1C) and the change amount ΔV ″ of the liquid crystal cell voltage at a position far from the driver (first
(See (e) in FIG. 2) is approximated, and the effective values of the liquid crystal cell voltages are made substantially equal to each other regardless of the distance from the driver to prevent uneven display brightness.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a block diagram of an embodiment of the present invention, in which 1 is a liquid crystal panel, 2 is a scan bus line, 3 is a data bus line, 4 is a TFT, 5 is a liquid crystal cell, 6 is a scan bus driver, and 7 is data. A bus driver, 8 is a control circuit, 9 is a voltage conversion circuit, and 10 is a gate voltage switching circuit. The control circuit 8 operates in response to the horizontal synchronizing signal and the vertical synchronizing signal,
The data clock signal DCLK is added to the data bus driver 7, the shift data GD is added to the shift register (not shown) in the scan bus driver 6, and the shift clock signal is added.
Add GCLK to the shift register. The switching signal SWCC is applied to the voltage conversion circuit 9 and the gate voltage switching circuit 10 according to the vertical synchronizing signal.

The voltage conversion circuit 9 outputs the voltage DD according to the display data and switches its polarity by the switching signal SWC. That is, the polarity of the data voltage is switched for each frame. This data voltage is applied to the data bus driver 7, is accumulated for one horizontal scanning line according to the data clock signal DCLK, and is followed by the data bus line 3 for line scanning.
The data voltage is output to.

In addition, the gate voltage switching circuit 10 is changed by the switching signal SWC.
The voltage Vgonp and Vgonm are switched, and the voltage output by switching is applied to the scan bus driver 6.
In addition, the scan bus driver 6 has a voltage for turning off the TFT4.
Vgoff is added.

The scan canvas driver 6 sequentially applies a gate voltage to the scan bus line 2 in synchronization with the horizontal sync signal, and a positive data voltage is output from the data bus driver 7 to the data bus line 3 in accordance with the switching signal SWC. During this period, the gate voltage applied from the scan bus driver 6 to the scan bus line 3 is Vgonp, and during the period when the negative data voltage is output from the data bus driver 7 to the data bus line 3, the scan bus driver 6 The gate voltage applied to scan line 3 is Vgon
It becomes m (<Vgonp).

FIG. 3 is a diagram for explaining the operation of the embodiment of the present invention.
(A), (b) and (c) are gate voltages applied to the j, j + 1, j + 2nd scan bus lines, (d) is a data voltage, and (e) is an output voltage of the gate voltage switching circuit 10. .
The F1 is positive data voltage application period, F2 denotes a negative polarity data voltage application period, + V ₁ and -V _1, for example the black display level voltage, + V ₂ and -V ₂ represents a white display level voltage.

This black display level voltage V ₁ and white display level voltage are V
_As shown in FIG. 4, ₂ is determined by the voltage-luminance characteristics of the liquid crystal cell. When the liquid crystal cell voltage is V ₁ or less, black display is shown, and when the liquid crystal cell voltage is V ₂ or more, white display is shown. Becomes

During the positive polarity data voltage application period F1, the gate voltage is
It is switched to Vgonp, and is switched to Vgonm (<Vgonp) in the negative polarity positive data voltage application period F2, and is sequentially applied to the scan bus line 2 from the scan bus driver 6. During the period when the gate voltage of Vgonp or Vgonm is not applied, the voltage Vgoff for turning off the TFT 4 is applied.

If the threshold voltage Vth ₀ in the above equation (3) is 0,
If Vgonp> V ₂ and Vgonm> −V ₁ , the TFT 4 can be turned on. That is, in the gate voltage switching circuit 10, the voltages Vgonp and Vgonm that are switched and output according to the switching signal SWC can be selected according to the data voltages V ₂ and V ₁ .

FIG. 5 is an explanatory diagram of the liquid crystal cell voltage distribution of the embodiment of the present invention, in which the variation ΔV of the liquid crystal cell voltage during the positive polarity data voltage application period in which the data voltage + Vd is applied depends on the rounding of the waveform of the gate voltage. There is little effect, and the near and far points are almost the same. However, the change in the liquid crystal cell voltage during the negative polarity data voltage application period in which the data voltage −Vd is applied is greatly affected by the rounding of the waveform of the gate voltage. Is bigger. On the other hand, according to the present invention, the variation Δ of the far point is
V ″ and the variation ΔV ′ of the near point are substantially equal to each other. Therefore, it is possible to prevent the occurrence of display luminance unevenness.

〔The invention's effect〕

As described above, according to the present invention, the gate voltage Vgonp for turning on the TFT during the positive polarity data voltage application period is different from the gate voltage for turning on the TFT during the negative polarity data voltage application period. By lowering the voltage Vgonm to eliminate the influence of the rounding of the waveform of the gate voltage, almost the same voltage is applied to the liquid crystal cell near the driver and the liquid crystal cell far from the driver to prevent uneven display brightness. There is an advantage that can be prevented. Further, regardless of the distance from the driver, the positive voltage liquid crystal cell voltage and the negative voltage liquid crystal cell voltage can be corrected to be symmetrical by the common voltage Vc,
There is an advantage that flicker can be prevented.

[Brief description of drawings]

1 is an explanatory view of the principle of the present invention, FIG. 2 is a block diagram of an embodiment of the present invention, FIG. 3 is an operation explanatory view of the embodiment of the present invention,
FIG. 4 is a voltage / luminance characteristic curve diagram, FIG. 5 is an explanatory diagram of liquid crystal cell voltage distribution of the embodiment of the present invention, FIG. 6 is an explanatory diagram of operation of a conventional example, FIG. 7 is an explanatory diagram of liquid crystal cell connection configuration, Figure 8 (A) ~
FIG. 9 (C) is an operation explanatory diagram when the TFT is turned on and off, and FIGS. 9 (A) and 9 (B) are explanatory diagrams of occurrence of uneven brightness. + Vd and −Vd are data voltages, Vgonp and Vgonm are gate voltages for turning on the TFT, and Vgoff is gate voltage for turning off the TFT.

Claims (1)

(57) [Claims] 1. A method of driving an active matrix type liquid crystal panel, wherein a liquid crystal cell (5) is connected to an intersection of a data bus line (3) and a scan bus line (2) through a switching element (4). During the period in which the polarity of the data voltage applied to the data bus line (3) is periodically inverted and the gate voltage for turning on the switching element (4) is the positive polarity of the data voltage. The gate voltage (Vgoff) for turning off the switching element (4) is set to be lower than the value (Vgonm) in the negative polarity period than the value (Vgonp) and the positive polarity period of the data voltage. A method of driving an active matrix type liquid crystal panel, which is characterized in that the same value is set in the negative polarity period.