JP2608403B2 - Driving method of active matrix type liquid crystal panel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] A data voltage is applied with a polarity inverted every predetermined period such as every frame or the like, and a common voltage applied to a common bus line to which a liquid crystal cell is commonly connected is changed to a direction of a scan bus line. By applying a voltage gradient along, the correction is performed so that the positive and negative symmetry of the liquid crystal cell voltage is obtained regardless of the distance from the driver, thereby preventing the occurrence of display luminance unevenness.

[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, a scan bus line and a data bus line are arranged orthogonally, and a liquid crystal cell is connected to the intersection via a switching element such as a thin film transistor via an element, and the on / off of the switching element is controlled. In addition, 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. 3 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 FIG.
It consists of a voltage Vgon for turning on T and a voltage Vgoff for turning off T. As shown in (b), the polarity of the data voltage is inverted every cycle such as every frame.

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, when the polarity of the data voltage is inverted and the voltage Vgon is applied to the scan bus line, the negative data voltage -Vd is applied to the liquid crystal cell. Accordingly, the polarity of the liquid crystal cell voltage is periodically inverted as shown in FIG.

FIG. 4 is an explanatory diagram of 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. The ground side of the liquid crystal cell 24 is connected to a common bus line. Here, Cg indicates the gate capacitance of the TFT 23, Cc indicates the liquid crystal cell capacitance, R indicates the equivalent resistance of the scan bus line 22, and C indicates the equivalent capacitance.

FIG. 5 is a diagram for explaining the operation 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. (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
The resistance component of the TFT 23 in the process of changing to Vgoff is indicated by Rt. In (C), the gate voltage becomes Vgoff and the TFT 23
Indicates a state where the switch is completely turned off.

As TFT23 shifts from on to off, the liquid crystal cell voltage becomes Only change. This corresponds to ΔV in FIG. 3 (c). For this change ΔV, the common voltage Vc
(See (c) of FIG. 3) is corrected by applying to the common bus line, so that the positive voltage and the negative voltage of the liquid crystal cell voltage are set to be symmetric.

[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 transition from the TFT on state in FIG. 5A to the TFT off state in FIG. 5C is instantaneous. However, at a position far from the driver, the fall becomes gentle due to the rounding of the waveform, so that the data voltage Vd is applied to the liquid crystal cell capacitance Cc via the resistor Rt through the process of FIG. Since the liquid crystal cell voltage is continuously applied, the change in the liquid crystal cell voltage is reduced. 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. 3 (c) indicates the liquid crystal cell voltage at a position near the driver, and the dotted line indicates the liquid crystal cell voltage at a position 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 bus line, the far-point liquid crystal cell voltage cannot be corrected. There is a disadvantage that flicker occurs due to different polar liquid crystal cell voltages.

The threshold voltage Vth of the TFT 23 changes depending on the data voltage Vd, and can be expressed by the following _equation: 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 data voltage is applied is lower than that when the positive data voltage is applied, and the change ΔV ′ in the liquid crystal cell voltage is as shown by the dotted line in FIG. In addition, the voltage becomes particularly small during the negative data voltage application period, which also causes display luminance unevenness.

6 (A) and 6 (B) are explanatory diagrams of the occurrence of uneven brightness. The horizontal axis represents the liquid crystal cell voltage V, and the vertical axis represents the transmitted light or reflected light intensity B. (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 at 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 quality indication.

[Means for solving the problem]

The driving method of the active matrix type liquid crystal panel of the present invention will be described with reference to FIG.
In a driving method of an active matrix type liquid crystal panel 1 in which a liquid crystal cell 3 is connected via a switching element 2 to an intersection of D1 to Dm and scan bus lines G1 to Gn, a data bus is connected to the data bus lines D1 to Dm. The polarity of the data voltage applied from the driver 5 is periodically inverted, and a gate voltage is sequentially applied from the scan bus driver 4 to the scan bus lines G1 to Gn, and the common bus lines CB1 to CBm to which the liquid crystal cells 3 are connected in common. , A common voltage having a voltage gradient along the scan bus line direction is applied from the power supply circuit 6.

[Action]

The change in the liquid crystal cell voltage varies along the scan bus line direction in accordance with the rounding of the waveform of the gate voltage output from the scan driver 4, and the common voltage correspondingly changes in the scan bus line direction. Therefore, the symmetry of the positive and negative polarities of the liquid crystal voltage is maintained, and it is possible to prevent the occurrence of display luminance unevenness.

〔Example〕

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

FIG. 1 is an explanatory view of an embodiment of the present invention, wherein 1 is a liquid crystal panel, 2 is a switching element (hereinafter referred to as TFT), 3 is a liquid crystal cell, 4 is a scan bus driver, 5 is a data bus driver, 6 Is a power supply circuit that outputs a common voltage, G1 to Gn are scan bus lines, D1 to Dm are data bus lines, CB1 to CBm are common bus lines, Rm is a resistor for adjustment, and R1 to R (m-1) are common This is a resistor that connects between bus lines.

TF is placed on one of the glass substrates (not shown)
T2, scan bus lines G1 to Gn, and data bus lines
D1 to Dm and the display electrodes of the liquid crystal cell 3 are formed, and on the other glass substrate (not shown), common bus lines CB1 to CBm facing the display electrodes and resistors R1 to R (m-1) Is formed, and a liquid crystal is filled between the glass substrates as a display medium. Note that the resistance Rm for adjustment may also be formed on a glass substrate, and the resistance value may be adjusted by laser light or the like.

Common bus lines CB1 to CBm are formed in parallel with and opposite to the data bus lines D1 to Dm, and the common bus lines CB1 to CBm are connected by resistors R1 to R (m-1) such as resistance wiring. . By outputting the negative common voltage Vc1 from the power supply circuit 6, the current I flows through the circuit of the resistors R1 to Rm, and the common bus line near the scan bus driver 4 is connected.
For CB1, the potential becomes large with a negative polarity, but for the common bus line CBm on the far side, the resistances R1 to R (m−
Due to the voltage drop according to 1), the potential becomes a small negative potential.

FIG. 2 is a diagram for explaining the operation of the embodiment of the present invention.
As shown in the figure, the common voltage Vc1 applied to the far-point common bus line is compared with the common voltage Vc1 applied to the near-point common bus line. It has a gradient. In this case, when the adjusting resistor Rm is increased, the voltage gradient indicated by the dotted line is obtained, and the common voltage applied to the far common bus line becomes Vc2 '. That is, the voltage gradient of the common voltage can be adjusted by adjusting the resistance Rm.

FIG. 2B shows a data voltage and a gate voltage. The data voltage is switched between + Vd and −Vd for each frame F, and the data voltage is a voltage for turning on the TFT. Vgon and a voltage Vgoff for turning off, and a data voltage + Vd is applied to the data bus line Di (i = 1, 2,... M).
Is applied, and the scan bus line Gj (j = 1, 2,... N)
When a gate voltage Vgon is applied to the
Is turned on, a data voltage + Vd is applied to the liquid crystal cell 3, and the data voltage + Vd is held by the capacitance of the liquid crystal cell 3 until the next cycle.

FIG. 2C shows the data voltage Vd, the gate voltage Vg and the liquid crystal cell voltage for the near-point liquid crystal cell, and FIG. 2D shows the gate voltage Vg for the near-point liquid crystal cell. Although there is no rounding, the waveform of the gate voltage Vg for the liquid crystal cell at the far point becomes blunt as shown in (D) due to the equivalent resistance R and the equivalent capacitance C (see FIG. 4) of the scan bus line Gj. .

Further, the voltage of the liquid crystal cell at the near point shifts from the voltage at the time when the gate voltage Vg is applied by a variation ΔV according to the equation (1), and the liquid crystal cell voltages of the positive polarity and the negative polarity become symmetric. Thus, the common voltage Vc1 is set. The voltage of the liquid crystal cell at the far point is calculated from the voltage at the time when the gate voltage Vg is applied,
Shift by the change amount ΔV ′ according to the equation (2), and ΔV> Δ
Since the voltage V ′ is V ′, the common voltage Vc2 for making the liquid crystal cell voltages of the positive polarity and the negative polarity symmetrical is compared with the absolute value.
c1> Vc2. That is, the potential of the common bus line CB1 near the scan bus driver 4 is Vc1, and the potential of the common bus line CBm far from the scan bus driver 4 is Vc2, and a voltage gradient as shown in FIG. . In that case,
The voltage can be set to a desired voltage gradient by the adjusting resistor Rm.

In the above-described embodiment, in order to give a voltage gradient to the common voltage, the resistances R1 to R (m-
1) shows a case where the connection is made via a high-resistance wiring such as 1). However, a configuration in which the common bus lines CB1 to CBm are grouped in units of a plurality and connected therebetween by a high-resistance wiring is also possible. . In that case, without connecting with high resistance wiring,
It is also possible to generate and apply a common voltage with a voltage gradient from the power supply circuit 6.

〔The invention's effect〕

As described above, the present invention corrects the decrease in the shift amount of the liquid crystal cell voltage due to the rounding of the gate voltage waveform by using the common voltage having a voltage gradient, and the symmetry is obtained regardless of the distance from the driver. Since the liquid crystal cell voltage can be improved, there is an advantage that the occurrence of display luminance unevenness and flicker can be prevented.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an embodiment of the present invention, and FIGS.
(D) is an explanatory view of the operation of the embodiment of the present invention, FIG. 3 is an explanatory view of the operation of the conventional example, FIG. 4 is an explanatory view of the connection configuration of the liquid crystal cell, and FIGS. FIGS. 6A and 6B are explanatory diagrams of the operation by turning on and off, and FIGS. 6A and 6B are diagrams illustrating the occurrence of uneven brightness. 1 is a liquid crystal panel, 2 is a switching element, 3 is a liquid crystal cell, 4 is a scan bus driver, 5 is a data bus driver, 6 is a power supply circuit, G1 to Gn are scan bus lines, and D1 to
Dm is a data bus line, CB1 to CBm are common bus lines, R1
RRm is a resistance.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-52-9392 (JP, A) JP-A-60-66236 (JP, A)

Claims (1)

(57) [Claims] A switching element (2) is provided at an intersection of a data bus line (D1 to Dm) and a scan bus line (G1 to Gn).
In the method for driving an active matrix type liquid crystal panel to which a liquid crystal cell (3) is connected via a liquid crystal cell, the polarity of a data voltage applied to the data bus lines (D1 to Dm) is periodically inverted. An active matrix type liquid crystal panel characterized by applying a common voltage having a voltage gradient along the scan bus line direction to common bus lines (CB1 to CBm) to which liquid crystal cells (3) are commonly connected. Drive method.