JP3052355B2 - Liquid crystal display - Google Patents

Description

The present invention relates to a liquid crystal display device, and more particularly to an active matrix type device.

(Conventional technology) As a conventional active matrix type liquid crystal display device, "Nikkei Electronics, September 10, 1984, p.211
−240 ”. FIG. 2 shows an equivalent circuit of this device. , Xm are arranged in the column direction, and a plurality of scanning lines Y1, Y2, ..., Yn are arranged in the row direction. At intersections of the signal lines X and the scanning lines Y, thin film transistors (hereinafter, referred to as TFTs) are arranged in a matrix, and TFTs 31,..., 3m are provided in a row direction in the drawing. The gate is connected to the scanning line Y, the source is connected to the signal line X, and the drain is connected to the pixel electrode 41. Further, a common potential _VCOM is applied to each counter electrode 42. Scan line Y selectively drives the selection pulse signal V _G is applied to the source of TFT, the signal line X inputs the video signal V _ID to the source of the driven TFT.

Here, the timing chart of each signal is as shown in FIG. Selection pulse signal V _G shown by a two-dot chain line, be between the high-level time T1 in one horizontal scanning period, the low level during the remaining time T2. Video signal V _ID indicated by dotted lines, on necessary to AC drive the liquid crystal, the polarity is reversed one frame consisting of first and second fields as a periodic.

When the selection pulse signal V _G becomes a high level, TFT is input video signal V _ID conducting, is applied to the liquid crystal for one pixel through the pixel electrode 41. Thus, as the voltage V _p of the pixel electrode 41 shown in solid line in FIG. 4, the same potential as the video signal V _ID. Thereafter, TFT is turned off when the selection pulse signal V _G to the low level, is accumulated according to the capacitance CP with the liquid crystal of one pixel. At the moment when the TFT is turned off, the voltage drops by the offset voltage ΔV due to the parasitic capacitance existing between the gate and the drain of the TFT. Common potential V
_COM is set so that the charges held in the liquid crystal are equal between the first field and the second field, as indicated by the alternate long and short dash line.

(Problems to be Solved by the Invention) However, the conventional liquid crystal display device has a problem that the screen brightness is different between the input side (left side in FIG. 2) and the output side (right side in FIG. 2) of the scanning line Y. there were. A resistance Rs and a capacitance Cs are parasitic on the scanning line Y, and a time constant Rs · Cs exists. This time constant
The influence of the rs · Cs, the waveform of the selection pulse signal V _G becomes different between the output side and the input side of the scanning lines Y. As FIG. 4, the selection pulse signal V _G1 of the output side when rounding the waveform by a constant Rs · Cs, languish edge when falling. Due to this effect, the selection pulse signal V _Gm falls and T
Shift voltage [Delta] V _m caused by capacitive coupling between the source and drain when FT4m is turned off is smaller than the shift voltage [Delta] V ₁ on the input side. This is because a current i flows between the source and the drain to cancel the voltage drop while the TFT 4m is turned off. As a result, the voltage V _p applied to the pixel electrode differs between the input side (V _p1 ) and the output side (V _pm ), and the video signal V _ID held in the liquid crystal of each pixel also differs. It will appear as a difference in brightness.
The magnitude of the current i depends on the impedance of the TFT when the TFT is turned on. If the impedance is small as in the case of a TFT made of polycrystalline silicon, this phenomenon appears remarkably.

The present invention has been made in view of the above circumstances, and has as its object to provide a liquid crystal display device capable of preventing a difference in screen luminance between an input side and an output side of a scanning line.

[Configuration of the invention]

(Means for Solving the Problems) In a liquid crystal display device of the present invention, liquid crystal is sandwiched between a pair of substrates, and a plurality of scanning lines and a plurality of scanning lines are provided on one of the pair of substrates. A plurality of signal lines formed so as to intersect lines, a thin film transistor connected to each of the scanning lines and the signal lines, and a pixel electrode connected to the thin film transistor, wherein one of the scanning lines
A time constant Rs · Cs represented by the wiring resistance Rs and the wiring capacitance Cs present per unit is a pixel writing represented by the resistance Rp and the capacitance Cp for one pixel of the liquid crystal when the thin film transistor is conducting. It is characterized by being smaller than the time constant Rp · Cp.

Also, the time constant Rs · Cs of the scanning line and the writing time constant Rp of the pixel
・ Time Rs required for write operation, which is the value obtained by adding Cp
Cs + Rp · Cp is preferably shorter than the selection period T required for selecting one scanning line.

(Operation) The magnitude of the shift voltage generated at the moment when the TFT is turned off is
Due to the influence of the scanning line time constant RsCs, it changes between the input side and the output side of the scanning line.By making this time constant RsCs smaller than the pixel writing time constant RpCp, The rate of change is reduced, and the occurrence of uneven brightness on the left and right sides of the screen is suppressed.

Also, the time Rs · Cs + Rp ·
By making Cp shorter than the selection period T, more reliable writing becomes possible.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the liquid crystal display device according to the present embodiment, the time constant RsCs existing in one scanning line is the resistance Rp when the TFT is conducting.
It is characterized in that it is smaller than the writing time constant Rp · Cp of the pixel represented by the following formula and the capacitance Cp for one pixel of the liquid crystal.

FIG. 1 shows the time constant Rs · Cs of the scanning line and the write time constant Rp.
A value (ΔV ₁ −ΔV _m ) / ΔV ₁ indicating a ratio Rs · Cs / Rp · Cp to Cp and a difference between the shift voltage ΔV ₁ on the input side of the scanning line and the shift voltage ΔV _m on the output side. Is shown in FIG. When the time constant ratio Rs ・ Cs / Rp ・ Cp becomes 1 or more, ΔV
_It can be seen that the value of ₁ −ΔV _m has increased dramatically. From this, the scan line time constant Rs / Cs becomes the write time constant.
By setting to be smaller than rp · Cp, it is possible to reduce the difference between the shift voltage [Delta] V _m on the output side and the shift voltage [Delta] V ₁ on the input side. Such time by forming the scan line to have a constant ratio of voltage V _p of the pixel electrode changes out with the input side (V _p1) and an output side (V _pm) decreases, uneven brightness of the screen Is suppressed.

Generally, in order to realize a large capacity in a liquid crystal display device, it is necessary to operate a TFT at a high speed. Therefore, a process for improving a transfer characteristic of the TFT is used. As described above, the shift voltage ΔV between the input side and the output side of the scanning line is significantly different as the resistance Rp when the TFT is conductive is smaller. According to this embodiment, the parasitic resistance of the scanning line
By setting Rs and parasitic capacitance Cs to be smaller, it is possible to prevent a large difference in the shift voltage ΔV, and to obtain a screen without uneven brightness even when a high-speed TFT is used. Can be. Thus, a large-capacity and high-quality liquid crystal display device can be realized.

Also, the scanning line time constant Rs and Cs and the writing time constant Rp and Cp
Is set to be smaller than the period for selecting one scanning line, that is, one horizontal scanning selection period, so that the gate is connected to one scanning line. All TFTs ensure that the video signal V
It becomes possible to write the _ID .

〔The invention's effect〕

As described above, according to the liquid crystal display device of the present invention,
The time constant RsCs of the scanning line is the writing time constant of the pixel represented by the resistance Rp when the TFT is conducting and the capacitance Cp for one pixel of the liquid crystal
Since it is smaller than Rp / Cp, the rate at which the shift voltage generated in the pixel electrode potential changes between the output side and the input side of the scanning line when the TFT is turned off is reduced, suppressing unevenness in screen brightness and improving image quality. Is achieved.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a voltage ratio (ΔV ₁ −ΔV _m ) / ΔV ₁ for explaining setting of a time constant ratio Rs · Cs / Rp · Cp in a liquid crystal display device according to an embodiment of the present invention. FIG. 2 is a circuit diagram showing an equivalent circuit of a liquid crystal display device to which the present invention can be applied, FIG. 3 is a timing chart showing waveforms of drive signals of the device, and FIG. 6 is a timing chart showing a change in a shift voltage ΔV generated between the output side and the output side. Y1 scanning line, X1 to Xm signal line, 31 to 3m TFT, 41
…… Pixel electrode, 42 …… Counter electrode, Cp …… Capacity of liquid crystal, Δ
V ₁ , ΔV _m ... shift voltage, V _ID ... common potential, V _p , V _p1 , V _pm
…… Pixel potential, V _G , V _G1 , V _Gm …… Selection pulse signal, Rs ……
Wiring resistance, Cs: Wiring capacitance.

Continued on the front page (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/1362 G02F 1/1343 G02F 1/133 G09F 9/30 G09G 3/36

Claims (2)

(57) [Claims] A liquid crystal is sandwiched between a pair of substrates, and a plurality of scanning lines and a plurality of signals formed so as to intersect the plurality of scanning lines on one of the pair of substrates. Line, a thin film transistor connected to each of the scanning lines and the signal line, and a pixel electrode connected to the thin film transistor, wherein a wiring resistance Rs and a wiring capacitance Cs existing per one of the scanning lines are provided. Is the resistance Rp when the thin film transistor is conducting and the capacitance of one pixel of the liquid crystal.
A liquid crystal display device characterized by being smaller than a pixel writing time constant Rp · Cp represented by Cp. 2. The time required for a write operation, Rs · Cs + Rp · Cp, which is a value obtained by adding the time constant Rs · Cs of the scan line and the write time constant Rp · Cp of the pixel, selects one of the scan lines. 2. The liquid crystal display device according to claim 1, wherein the selection period is shorter than a selection period T required for the operation.