1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an apparatus and a method for driving a liquid crystal display device that reduces the heat generated by a device with reliable operation.
2. Description of the Related Art
A liquid crystal display device controls the light transmissivity of liquid crystal cells in accordance with a video signal to display a picture. An active matrix type of liquid crystal display device having a switch device formed at each liquid crystal cell is advantageous for motion picture because the switch device can be actively controlled. The switch device used in the active matrix liquid crystal display device is usually a thin film transistor (hereinafter, referred to as “TFT”).
The liquid crystal display device, as shown in Formula 1 and 2, has a disadvantage in that its response speed is slow due to the unique characteristic of liquid crystal such as viscosity and elasticity thereof.
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δ            ⁢                                                  ⁢            ɛ            ⁢                                                                          V                  a                  2                                -                                  V                  F                  2                                                                                                      [                  FORMULA          ⁢                                          ⁢          1                ]            Here, τr represents a rise time when a voltage is applied to liquid crystal, Va represents an applied voltage, VF represents a Freederick Transition Voltage where a liquid crystal molecule starts a tilt motion, d represents a cell gap of a liquid crystal cell, and γ (gamma) represents the rotational viscosity of the liquid crystal molecule.
                              τ          f                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    [                  FORMULA          ⁢                                          ⁢          2                ]            Here, τf represents a fall time when the liquid crystal is restored to its original location by an elastic restitutive force after the voltage applied to the liquid crystal is turned off, and K represents the unique elastic modulus of liquid crystal.
The response speed of the liquid crystal of twisted nematic TN mode (which is most commonly used) might differ according to the physical properties and cell gap of a liquid crystal material, but conventionally, the rise time is 20˜80 ms and the falling time is 20˜30 ms. The response speed of the liquid crystal is longer than one frame period (NTSC: 16.67 ms). Because of this, the signal will be in the next frame before the voltage being charged in the liquid crystal cell reaches a desired voltage, as shown in FIG. 1. Thus, a motion blurring phenomenon is generated in a screen showing a motion picture.
Referring to FIG. 1, a liquid crystal display device of the related art could not express a desired color and brightness because the display brightness BL corresponding thereto does not reach the desired brightness when a data VD is changed from one level to another level. As a result, the liquid crystal display device has the motion blurring phenomenon in the motion picture, and has its picture quality dropped due to the deterioration of contrast ratio.
In order to overcome the slow response speed of the liquid crystal display device, U.S. Pat. No. 5,495,265 or PCT International Publication No. WO99/05567 has suggested a method of modulating a data in accordance with the existence or absence of the change of the data using a look-up table, hereinafter referred to as “high-speed driving method”. The high speed driving method modulates the data with the principle shown in FIG. 2.
Referring to FIG. 2, the high speed driving method modulates an input data VD into a pre-set modulated data MVD, and the modulated data MVD is applied to the liquid crystal cell to get the desired brightness MBL. The high speed driving method has the value of |Va2−VF2| in Formula 1 on the basis of the existence or absence of change of the data to get a desired brightness corresponding to the brightness value of the input data within one frame period. Accordingly, the liquid crystal display device using the high speed driving method compensates for the slow response time of liquid crystal by modulating the data value to ease the motion blurring phenomenon associated with a motion picture.
In other words, the high speed driving method modulates the data of the current frame to a pre-set modulated data if there is any change between the data when the data are compared between the previous frame and the current frame.
The modulated data needed in the high speed driving method is determined with the method shown in FIG. 3. Referring to FIG. 3, a modulated data determination method, in a step S1, applies a data voltage to a test piece liquid crystal display panel in relation to data with a designated difference, measures the change of brightness of the test piece liquid crystal display and changes the data voltage until it reaches to the target brightness within a desired time. Through this process, the first modulated data are determined, wherein the first modulated data reach the target brightness within the desired time in the data with a designated distance.
FIG. 4 represents an example of the first modulated data. In FIG. 4, the data of the leftmost column represents the data of the previous frame Fn−1 and the data of the uppermost row represents the data of the current frame Fn. The first modulated data of FIG. 4 include 17×17 numbers of modulated data which are determined with 17 data gaps.
In this way, after the first modulated data are determined, the modulated data determination method, in a step S2, automatically determines a second modulated data using a distance compensating method. Here, the second modulated data corresponds to each of 16 data in the gap between two adjacent first distance compensating data and are determined with a designated distance using software. The second modulated data have a linear relation with the first distance compensating data. The first modulated data and the second modulated data determined in the steps S1 and S2 are stored in a read only memory ROM in a step S3.
On the other hand, if all of the modulated data determined by the modulated data determination method of the related art are stored in the ROM, the capacity of the ROM must be large and a current flow when accessing the modulated data is large. Thus, the heat generation of the ROM increases and the reliability of operation is deteriorated. For example, the number of the total modulated data stored at the ROM is 256×256=65536 assuming that there are 256 gray levels. The modulated data is 1 byte (or 8 bits), thus the minimum capacity of the ROM to store the 65536 modulated data is 65536×8=524288 bits.