KR20040021464A - Gamma voltage generating circuit and generating method of tft-lcd - Google Patents

Abstract

PURPOSE: A circuit and a method for generating a gray scale voltage of an LCD are provided to control a gray scale curve by using a variables resistor connected to a voltage distribution circuit. CONSTITUTION: A circuit for generating a gray scale voltage of an LCD includes a supply voltage source, a variable resistor, and a voltage distribution circuit. The supply voltage source is used for generating a constant voltage. The variable resistor is connected to the supply voltage source in order to drop the constant voltage. The voltage distribution circuit is connected to the variable resistor in order to distribute the dropped constant voltage and generate a desired gray scale voltage. The voltage distribution circuit includes a plurality of resistances.

Description

Gradient voltage generation circuit and method of liquid crystal display device {GAMMA VOLTAGE GENERATING CIRCUIT AND GENERATING METHOD OF TFT-LCD}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gray scale voltage generating circuit of a liquid crystal display device, and more particularly, to a gray scale voltage generating circuit capable of compensating for an error of a gray scale curve caused by a process variation.

The full-scale mass production of liquid crystal displays, the flagship product of flat panel displays, began in the early 1990s. In recent information society, the importance of display is more important as a visual information transmission medium. In particular, with the trend of light, thin, short, and small in all electronic products, the importance of low power consumption, thinness, light weight, high definition, and portability is increasing. As liquid crystal displays are not only capable of meeting these conditions of flat panel displays but also mass-produced displays, various new products are being created rapidly, and they can gradually replace existing cathode ray tubes (CRT). As a core component industry, the electronics industry is having a ripple effect over the semiconductor.

A schematic diagram of a system for driving a thin film transistor liquid crystal display (TFT-LCD) is shown in FIG. 1.

Digital image data output from a graphic controller of a computer is input to a timing controller 110 of the LCD. Since the TFT-LCD operates in a digital manner, when the input data is analog, an analog / digital conversion circuit for converting an analog signal into a digital signal should be used together with the timing controller 110. The timing controller 110 converts the input digital data into a digital signal in a form that the data driver IC 120 can process, and then the source and gate driver IC 130. Control signals such as various timing control signals required for driving are generated. In the analog power supply circuit 140, a gray voltage required for multi-level gray scale display is generated and supplied to the data driver IC 120. A voltage capable of turning the TFT on and off is also produced by the analog power supply circuit 140 and supplied to the gate driver circuit 130 to be sequentially output to the gate signal wiring 170. The inverter 160 supplies power to the CCFL used as the backlight 150 and adjusts the brightness of the LCD by adjusting the current of the backlight.

The gradation voltage generating circuit for generating the gradation voltage generates a gradation voltage to be applied to the data signal wiring 180 of the LCD panel and sends it to the source driving circuit.

In general, gradation means light and dark color, and gradation voltage refers to a voltage applied to the source electrode of the thin film transistor, that is, the data signal wiring 180, to represent gradation.

Hereinafter, a conventional gray voltage generator circuit will be described.

2 is a block diagram illustrating the internal circuit of the gray voltage generator.

As shown in FIG. 2, the conventional gray voltage generator circuit includes a plurality of resistors R ₁ to R _{n + 1} connected in series between a power supply voltage V _cc and a ground GND. Each of the resistors R ₁ to R _{n + 1} distributes the constant voltage supplied from the power supply voltage Vcc at a predetermined ratio to generate n gray voltages V _G1 to V _Gn at nodes between the resistors. In the normally-white mode, V _G1 corresponds to a black level, and V _Gn has a voltage level corresponding to a white level.

The gray voltages V _G1 to V _Gn are provided to the data driving circuit 120 of the liquid crystal display.

The data driver circuit 120 includes a voltage divider 210 including a plurality of resistors to select a liquid crystal driving voltage based on n gray voltages V _G1 to V _Gn input from the gray voltage generator circuit. Will print When the driving circuit is driven with k bits, 2 ^k liquid crystal driving voltages V ₁ to V ₂ ^k are output. Other components in the data driving circuit 120 except for the voltage divider 210 are not shown.

The relationship between the gray scale voltage applied to the liquid crystal cell and the light transmittance has a T-V (transmittance-voltage) characteristic as shown in FIG. 3A when the NW mode is normally white when the voltage is not applied. As can be seen from the TV curve, when the applied voltage is gradually increased at the maximum light transmittance (white state), the transmittance gradually decreases, and at a medium voltage, the transmittance changes slightly proportionally, and again, a high voltage is applied to the lowest transmittance. When it reaches the vicinity of (black state), even if the voltage changes, the transmittance hardly changes. Therefore, it is possible to express by the magnitude of the voltage to apply not only white and black display but also medium brightness display.

Since there is a non-linear section in the TV characteristic as shown in FIG. The gradation voltage must be properly adjusted. If it is assumed that the operating voltage range of the liquid crystal is divided by the gray scale voltages (V _G1 to V _G9 ) of eight equal intervals, the transmittance characteristic for the gray scale voltage is not linear but is nonlinear. Because of the characteristics, the transmittance or screen brightness for each gray scale is distributed in a direction toward black and white without maintaining a constant interval, so that an expression of an intermediate gray scale is insufficient. Therefore, in actual applications, the gray scale voltage should be appropriately set as shown in FIG. 3A so that the transmittance for each gray scale maintains a constant interval. That is, the gray voltage interval should be narrowed at the portion where the slope of the TV curve is large and large at the gentle portion, so that the transmittance for each gray scale is constant. However, in practical applications, it is necessary to consider that the human visual characteristics easily distinguish the brightness difference on the dark screen, but the brightness difference on the bright screen is not well distinguished. Therefore, it is also necessary to adjust the linearity of transmittance for each gray scale in consideration of human visual characteristics. This series of adjustment processes is called gamma correction, and FIG. 3C shows the relationship between the gray scale and the transmittance of the gamma-corrected LCD driver circuit.

Conventional liquid crystal display devices always generate a constant gray scale voltage by a gray scale voltage generating circuit. However, the T-V characteristic of the liquid crystal display device is not always constant due to process variations that may occur during the manufacturing process of the liquid crystal display device, and may deviate from a predetermined value as shown in FIG. 4A. The curve 410 indicated by the dotted line is a predetermined value and the curve 420 indicated by the solid line is a curve when there is an error in the process.

When the T-V curve changes, the transmittance of the liquid crystal changes with respect to a predetermined voltage, so that the gray scale curve of the liquid crystal also moves up and down as shown in FIG. 4B. In other words, the transmittance at a specific gray scale voltage is larger or smaller than a predetermined value, so that a desired color is not accurately displayed.

As described above, the conventional gradation voltage generation circuit generates gradation voltage under the assumption that the liquid crystal cell always has a constant T-V characteristic. Therefore, even if an error occurs in the process and the T-V characteristic of the liquid crystal cell is changed, the gray scale voltage generation circuit generates a predetermined gray scale voltage regardless of this.

When a plurality of liquid crystal display devices are produced for the same reason as described above, liquid crystal display devices having different colors may be produced due to process errors.

The present invention has been proposed to solve the above problems, and an object of the present invention is to provide a gray scale voltage generation circuit capable of compensating for an error that may occur in a process.

Another object of the present invention is to provide a gray voltage generation method capable of compensating for errors due to process deviations.

1 is a configuration diagram of an entire system for driving a liquid crystal display device

2 is a circuit diagram showing a conventional gray scale voltage generation circuit and a data driving circuit.

3A to 3C are graphs showing transmittance-voltage characteristics and grayscale-transmission characteristics of liquid crystals.

4A and 4B are graphs showing changes in transmittance-voltage characteristics and grayscale-transmission characteristics of liquid crystals.

5A and 5B are circuit diagrams showing a gamma voltage generation circuit according to a first embodiment of the present invention.

6 is a circuit diagram showing a gamma voltage generating circuit according to a second embodiment of the present invention.

7 is a circuit diagram showing a gamma voltage generation circuit according to a third embodiment of the present invention.

8 is a graph showing the change of the grayscale curve by the gamma voltage generation circuit of the present invention.

<Description of Symbols for Main Parts of Drawings>

110: timing controller 120: data driver IC

130: gate driver IC 140: analog power circuit

150: backlight 160: inverter

170: data signal wiring 180: gate signal wiring

210: voltage distribution

In order to achieve the above object, the present invention provides a power supply voltage for generating a constant voltage; A variable resistor connected to the power supply voltage to drop the constant voltage; And a voltage distribution circuit connected to the variable resistor and distributing a voltage drop to generate a desired number of gray voltages.

The voltage distribution circuit preferably includes a plurality of resistors.

As another method for achieving the above object, the step of supplying a constant voltage; Dropping the constant voltage to a desired magnitude; And dividing the voltage drop constant voltage to generate a plurality of gray voltages.

The main difference between the present invention and the prior art is that the variable resistor is connected to the conventional voltage distribution circuit.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Since the data driving circuit connected to the gray scale voltage generating circuit is the same as the prior art, a description thereof will be omitted.

5A shows a gradation voltage generating circuit according to a first embodiment of the present invention.

The gradation voltage generating circuit according to the first embodiment of the present invention is constructed as follows.

When the constant voltage is applied from the power supply voltage Vcc to the gradation voltage generating circuit, the voltage is first dropped to V _A via the variable resistor R _x . The voltage drop constant voltage V _A is distributed by a voltage distribution circuit including _{n + 1} resistors R ₁ to R _{n + 1} to generate n gray voltages V _G1 to V _Gn . The gray voltages V _G1 to V _Gn are input to the data driver driving circuit to implement color.

The voltage distribution circuit may use resistors connected in series as shown in FIG. 5A, or may connect and connect the resistors connected in series as shown in FIG. 5B in parallel.

As the resistance value of the variable resistor is changed, the voltage value at the node A is changed, and thus the gradation voltage output at each node of the voltage distribution circuit is changed.

For example, a larger voltage drop occurs in the variable resistor than when the variable resistor has a large resistance value. Therefore, as the resistance value of the variable resistor increases, the voltage value at the node A decreases, and the voltage generated in the voltage distribution circuit divides the gray level voltage generated. When the gray scale voltage decreases, the transmittance increases in the NW mode (normally white mode).

6 shows a gradation voltage generating circuit according to a second embodiment of the present invention. The case of generating even-numbered gradation voltages is shown.

As shown in series, the resistors connected in series are connected to the variable resistors R _x1 and R _x2 and connected in parallel again. By adjusting the R _x1 and R _x2 are the voltage V _B and V _C at node B and C is determined, the V _B and V _C is the voltage division by the resistor connected to the variable resistance _{(R 1 ~ R n + 1} ) Thus, gray voltages (V _G1 to V _Gn ) are generated.

7 shows a gradation voltage generating circuit according to a third embodiment of the present invention.

As shown, n + 1 variable resistors _Rx1 to _{Rxn + 1} are connected to the power supply voltage Vcc in series. Examples 1 and 2 of the is applied from the power supply voltage is allocated to the voltage the voltage drop is constant ratio through a variable resistor for each gray level, but the voltage is generated the third embodiment when n of the variable resistor (R _x1 ~ R _xn case ₊₁ ) can be adjusted manually to generate the optimal gradation voltage (V _G1 ~ V _Gn ).

When the above circuit is used, the variable resistor is added as compared with the first embodiment, but the gray scale voltage can be accurately adjusted to display the optimum image.

8 shows a change in the gray scale curve according to the change in the resistance value of the variable resistor.

Increasing the resistance of the variable resistor causes a large voltage drop in the variable resistor, thereby reducing the voltage applied to the resistance of each node, thereby reducing the magnitude of the gradation voltage generated at each node. When the gray voltage decreases, the transmittance of the liquid crystal cell increases, so that the curve 810 moves downward. On the contrary, when the resistance value of the variable resistor is reduced, the voltage drop decreases in the variable resistor, thereby increasing the voltage applied to the resistance of each node, thereby increasing the magnitude of the gradation voltage generated at each node. As the gray scale voltage increases, the transmittance of the liquid crystal cell decreases, so that the curve 830 moves upward.

If the gray scale curve is the same as the dotted line 820, the most desirable image quality can be adjusted. By adjusting the variable resistor, the gray scale curve can be moved up and down to coincide with the dotted line to set the desired value.

According to the present invention made as described above, when the TV characteristic of the liquid crystal module is out of a predetermined value and a change occurs in the gray scale curve due to an error in the manufacturing process, the gray scale curve is adjusted by adjusting the variable resistor connected to the voltage distribution circuit. By matching the desired value, the effect of compensating the error can be achieved.

While many details are set forth in the foregoing description, they should be construed as illustrative of preferred embodiments rather than to limit the scope of the invention. For example, the voltage distribution circuit can be arranged in any form as long as it can generate only n gray voltages. Therefore, the scope of the invention should not be defined by the described embodiments, but should be defined by the claims and the equivalents of the claims.

Claims (5)

A power supply voltage for generating a constant voltage; A variable resistor connected to the power supply voltage to drop the constant voltage; And A voltage distribution circuit connected to the variable resistor for distributing a voltage drop constant voltage to generate a plurality of gradation voltages; Gray voltage generator circuit of the liquid crystal display device comprising a. The gray voltage generator circuit of claim 1, wherein the voltage divider circuit comprises a plurality of resistors. A power supply voltage for generating a constant voltage; A plurality of variable resistors connected to the power supply voltage for voltage-sharing the constant voltage; Gray voltage generation circuit comprising a. Supplying a constant voltage; Dropping the constant voltage to a desired magnitude; And Dividing the voltage drop constant voltage to generate a plurality of gray voltages; Gray voltage generation method of the liquid crystal display device comprising a. 5. The method of claim 4, wherein the ratio of dividing the voltage drop constant voltage is adjustable.