JPS63175889A - Driving of active matrix type liquid crystal panel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Summary] A data voltage is applied with the polarity reversed at every predetermined period such as every frame, and the gate voltage for turning on a switching element such as a thin film transistor corresponds to the polarity of the data voltage. This prevents uneven display brightness.

[Industrial application field]

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

In an active matrix liquid crystal panel, scan canvas lines and data bus lines are arranged perpendicularly, and a liquid crystal cell is connected to the intersection through a switching element such as a thin film transistor, and the on/off of the switching element is controlled. , which applies a data voltage to the liquid crystal cell,
There is an advantage that even if the display capacity is increased, the problem of drive duty ratio does not occur. Further, by providing a color filter corresponding to the liquid crystal cell, full color display is possible, and it can be applied to portable television receivers and the like. It is desired to further improve the display quality of such active matrix liquid crystal panels.

[Conventional technology]

An active matrix liquid crystal panel has a structure in which a liquid crystal cell is connected via a thin film transistor (hereinafter abbreviated as TPT) to the intersection of orthogonally arranged scan canvas lines and a data bus line, and the data voltage applied to the data bus line The polarity is reversed every frame, and the gate voltage for turning on the TPT is sequentially applied to the scan line.

FIG. 6 is an explanatory diagram of the operation of the conventional example, in which the gate voltage applied to the scan canvas line is TP
The voltage V gon that turns T on and the voltage V g that turns it off
OFF, and the polarity of the data voltage is periodically reversed, as shown in (b).

For example, when a positive data voltage +Vd is applied to the data bus line, the voltage V go is applied to the scan bus line.
When n is applied, the TPT connected to the scan canvas line is turned on, and the data voltage +Vd is applied to the liquid crystal cell via the TPT. Next, when the voltage Vgoff is applied to the scan canvas line, the TPT is turned off, and the liquid crystal cell voltage decreases by Δ■ due to capacitive coupling with the gate capacitance of the TPT. This liquid crystal cell voltage is then 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 reversed, and when the voltage Vgon is applied to the scan canvas line, a negative data voltage -Vd is applied to the liquid crystal cell. Therefore, the polarity of the liquid crystal cell voltage is periodically reversed, as shown in FIG. 6(C).

FIG. 7 is an explanatory diagram of the connection configuration of the liquid crystal cell, and TPT23
The drain is connected to the data bus line 21, the gate is connected to the scan canvas line 22, and the source is connected to the liquid crystal cell 24.

Further, Cg is the gate capacitance of the TPT 23, Cc is the liquid crystal cell capacitance, R is the equivalent resistance of the scan canvas line 22, and C is the equivalent capacitance.

FIG. 8 is an explanatory diagram of the operation when TPT is turned on and off,
(A) shows the case where the TPT 23 is in the on state, and the gate voltage V gon is applied to the liquid crystal cell capacitance Cc via the gate capacitance Cg of the TPT 23, and the TPT in the on state is
Data voltage Vd is applied via PT23. Also (B
) shows the case where the TPT 23 is shifted from the on state to the off state, and the gate voltage is changed from V gon to Vgoff.

Further, Rt represents the resistance in the process of transition of the TPT 23 from the on state to the off state. Further, (C) shows a state in which the gate voltage becomes Vgoff and the TPT 23 is completely turned off.

As the TPT 23 shifts from on to off, the liquid crystal cell voltage changes by the following amount. This corresponds to ΔV in (C1) in FIG. 6. Regarding this change ΔV, the common voltage Vc
(See (C) in FIG. 6), the positive polarity voltage and negative polarity voltage of the liquid crystal cell voltage are set to be symmetrical.

[Problem that the invention seeks to solve]

Due to the equivalent resistance R and equivalent capacitance tC of the scan canvas line 22, the waveform of the gate voltage output from the driver becomes gradually rounded. At a position close to the driver, the gate voltage drops sharply from V gon to V gof f, so that from the TPT on state in FIG. 7(A),
This results in an instantaneous transition to the TPT off state (C).

However, at a position far from the driver, the fall is gradual due to the rounding of the waveform, so the data voltage Vd passes through the process shown in FIG. 7B to the liquid crystal cell capacitor Icc via the resistor Rt. Since the voltage is applied continuously, the amount of change in the liquid crystal cell voltage becomes small. In other words, in the process of the gate voltage falling from Vgon to Vgoff,
The TPT 23 will continue to be in the on state until the gate voltage drops to the threshold voltage vth of the TPT 23, and the change ΔV' in the liquid crystal cell voltage in that case will be as follows. Since Vgon > V th, the amount of change in the liquid crystal cell voltage is Δ■>ΔV', and the amount of change in the liquid crystal cell voltage at a position far from the driver is small.

The solid line in FIG. 6C 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 ground point, the far-point liquid crystal cell voltage cannot be corrected, and display brightness unevenness occurs along the scan canvas line direction. There is a drawback that flickering occurs due to the difference in polarity liquid crystal cell voltage.

Further, the threshold voltage vth of the TFT 23 changes depending on the data voltage Vd, and is expressed as follows: V th = V d + V t h o
・It can be expressed as (3). Note that Vtho is a threshold voltage that does not depend on the data voltage.

Therefore, the dark value voltage when a negative polarity data voltage is applied is lower than when a positive polarity data voltage is applied, and the change ΔV' in the liquid crystal cell voltage is shown by the dotted line in (C) in FIG. As such, it becomes particularly small during the negative polarity data voltage application period, and this also causes display brightness unevenness.

FIGS. 9A and 9B are explanatory diagrams of the occurrence of brightness unevenness, where the horizontal axis shows the liquid crystal cell voltage V, and the vertical axis shows the intensity B of transmitted light or reflected light. In addition, (A) shows the case of 2 (i display,
(B) shows the case of full color (gradation) display. In the case of binary display, as shown in (A), by selecting the liquid crystal cell voltage below the dark value for black and selecting the liquid crystal cell voltage above the saturated dark value for white, the dotted line circle near the point ) is also the far point (
(indicated by a solid circle or a black circle) can also be set so that they can be displayed at approximately the same brightness.

On the other hand, when performing gradation display, as shown in (B), a liquid crystal cell voltage between the black darkness value and the white saturation darkness value is used, and the far point (solid circle) is used. ) is displayed as white, the luminance at the near point (dotted line circle), where the effective value is larger than the effective value of the liquid crystal cell voltage near the saturated dark value, is almost the same as that at the far point. However, when the far point (black circle) is displayed in black, the luminance of the near point (dotted line circle), which has an effective value larger than the effective value of the liquid crystal cell voltage at the far point, becomes close to white.

Therefore, when performing black display, the brightness on the side closer to the driver is thicker (ie, brightness unevenness occurs), which deteriorates display quality.

The present invention aims to improve display quality by preventing the occurrence of brightness unevenness as described above.

[Means for solving problems]

The method for driving an active matrix liquid crystal panel of the present invention will be explained with reference to FIG. 1. As shown in FIG. 1, the data voltage applied to the data bus line is +Vd,
-Vd, it is inverted at every predetermined period, such as every frame F. Furthermore, as shown in (b) and (d), the gate voltages applied to the scan canvas line are V gomp and V g
Onm and Vgonp are available during the period when the data voltage has positive polarity, and Vgonp during the period when the data voltage is negative polarity.
Gate voltage Vgoff for switching and applying Vgonm lower than onp and turning off the switching element
is the same regardless of the polarity of the data voltage, and the gate voltage shown in (bl) is applied to the switching element corresponding to the liquid crystal cell located near the driver, and the voltage of the liquid crystal cell is (
The voltage of the liquid crystal cell becomes as shown in (C), and when the gate voltage with the distorted waveform shown in (d) is applied to the switching element corresponding to the liquid crystal cell located far from the driver, the voltage of the liquid crystal cell becomes (
It will be as shown in e).

[Effect]

By setting the gate voltage V gonm during the period in which the negative polarity data voltage -Vd is applied to be lower than the gate voltage V gonp in the period in which the positive polarity data voltage terminal Vd is applied, the negative polarity data voltage is particularly applied. The change in liquid crystal cell voltage near the driver during the period Δv'(
(see Figure 1 (C)) and the variation ΔV' in the liquid crystal cell voltage at a far position (see Figure 1 (e)), the effective value of the liquid crystal cell voltage can be calculated regardless of the distance from the driver. The display brightness is made almost equal to prevent uneven display brightness.

〔Example〕

Embodiments of the present invention will be described in detail below 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 canvas line, 3 is a data bus line, 4 is a TPT, 5 is a liquid crystal cell, 6 is a scan canvas driver, and 7 is a data bus line. A bus driver, 8 a control circuit, 9 a voltage conversion circuit, and 10 a gate voltage switching circuit. The control circuit 8 operates in response to the horizontal synchronization signal and the vertical synchronization signal, and transmits the data clock signal DCLK to the data bus driver 7.
In addition, the shift data GD is sent to the scan canvas driver 6.
A shift register (not shown) within the shift register and a shift clock signal GCLK are applied to the shift register. Further, a switching signal SWC is applied to the voltage conversion circuit 9 and the gate voltage switching circuit 10 in accordance with the vertical synchronization signal.

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

Further, the gate voltage switching circuit 10 switches between the voltages Vgonp and Vgonm using the switching signal SWC, and the switched output voltage is applied to the scan canvas driver 6.
added to. Also, the scan canvas driver 6 has a TPT
A voltage Vgoff is applied which turns 4 off.

The scan bus driver 6 sequentially applies gate voltages to the scan canvas lines 2 in synchronization with the horizontal synchronization 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 canvas driver 6 to the scan canvas line 3 is V gomp,
During a period in which a data voltage of negative polarity is output from the data bus driver 7 to the data bus line 3, the gate voltage applied from the scan canvas driver 6 to the scan canvas line 3 becomes Vgonm (< Vgonp).

FIG. 3 is an explanatory diagram of the operation of the embodiment of the present invention, (al,
(bl, (C) is the gate voltage applied to the j, j+1.3+2th scan line, (d) is the data voltage, (el is the output voltage of the gate voltage switching circuit 10, and Fl is the positive polarity. In the data voltage application period, F2 indicates a negative data voltage application period, +V and -Vl indicate, for example, black display level voltages, and +V2 and -F2 indicate white display level voltages.

The black display level voltage (1) and the white display level voltage (2) are determined by the voltage-luminance characteristics of the liquid crystal cell, as shown in FIG. In the case of a liquid crystal cell voltage of F2 or higher, a white display is obtained.

In the positive polarity data voltage application period F1, the gate voltage is switched to Vgonp, and in the negative polarity data voltage application period F2, it is switched to Vgonm (<Vgonp), and the scan bus driver 6 to 1+] Applied to scan canvas line 2. During this period when the gate voltage of V gonp or V gom is not applied,
A voltage Vgoff is applied to turn off TPT4.

If the threshold voltage Vtho in the above equation (3) is O, then Vgonp>V 21 Vgor++w> -V
(Then, TPT4 can be turned on. That is, the voltages V gonp and V gonm that are switched and output in accordance with the switching signal SWC in the gate voltage switching circuit IO
can be selected depending on the data voltage V 2 r V I .

FIG. 5 is an explanatory diagram of the liquid crystal cell voltage distribution according to the embodiment of the present invention, and the variation Δ■ in the liquid crystal cell voltage during the positive polarity data voltage application period when the data voltage terminal vd is applied is the change in the gate voltage waveform. The influence of the accent is small, 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 when the data voltage -Vd is applied is greatly affected by the rounding of the gate voltage waveform, and in the conventional example, becomes larger. On the other hand, according to the present invention, the change in the far point ΔV
IT and the change in periapsis Δ■° are approximately equal. Therefore, occurrence of display brightness unevenness can be prevented.

〔Effect of the invention〕

As explained above, in the present invention, the gate voltage V g for turning on the TPT during the positive polarity data voltage application period is
T onp during the negative polarity data voltage application period.
The gate voltage V gonm for turning on the PT is lowered so that almost the same voltage is applied to liquid crystal cells near and far from the driver, excluding the effect of rounding of the gate voltage waveform. This has the advantage of preventing display brightness unevenness. Also, regardless of the distance from the driver, the positive polarity liquid crystal cell voltage and the negative polarity liquid crystal cell voltage can be corrected to be symmetrical using the common voltage Vc, so flickering can also be prevented. There is.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the principle of the present invention, FIG. 2 is a block diagram of an embodiment of the present invention, and FIG. 3 is an explanatory diagram of the operation of an embodiment of the present invention.
FIG. 4 is a voltage-luminance characteristic curve diagram, FIG. 5 is an explanatory diagram of the liquid crystal cell voltage distribution according to the embodiment of the present invention, FIG. 6 is an explanatory diagram of the operation of the conventional example, and FIG. 7 is an explanatory diagram of the liquid crystal cell connection configuration. Figure 8 (A) ~
9(C) is an explanatory diagram of the operation when TPT is turned on and off, and FIGS. 9(A) and 9(B) are explanatory diagrams of occurrence of brightness unevenness. +Vd, -Vd are data voltages, V gomp, V g
onm is the gate voltage for turning on the TFT, Vgof
f is a gate voltage for turning off the TFT.

Claims (1)

[Claims] A liquid crystal cell (5) is connected to the intersection of the data bus line (3) and the scan bus line (2) via a switching element (4).
) is connected to an active matrix liquid crystal panel, in which the polarity of the data voltage applied to the data bus line (3) is periodically reversed to turn on the switching element (4). The value of the gate voltage during the period when the data voltage is positive (
The value (Vgo
A method for driving an active matrix liquid crystal panel, characterized in that the gate voltage (Vgoff) for turning off the switching element is made the same regardless of the polarity of the data voltage.