Abstract:
A liquid crystal display includes a glass substrate, a plurality of pixels formed on the glass substrate for displaying an image according to gamma voltages, a voltage divider installed on a printed circuit board, the voltage divider including a resistor and a thermistor coupled in series with the resistor for generating gamma voltages for the pixels, and a driver IC chip coupled to the pixels and the voltage divider for controlling the voltage divider to generate gamma voltages to the pixels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD having a voltage divider with a thermistor.  
         [0003]     2. Description of the Prior Art  
         [0004]     A conventional liquid crystal display (LCD) comprises an upper transparent substrate, a lower transparent substrate, and liquid crystal molecules are filled between the two transparent substrates. Please refer to  FIG. 1 , which is a schematic diagram of a lower transparent substrate of a conventional LCD having a chip-on-glass (COG) module structure. The lower transparent substrate, such as a glass substrate  12 , comprises a plurality of pixels  14  formed on the glass substrate  12  for displaying an image according to gamma voltages, a voltage divider  20  installed on a printed circuit board (PCB)  19  for generating gamma voltages corresponding to a gamma value for the pixels  14 , a driver IC chip  16  installed on the glass substrate  12  and coupled between the voltage divider  20  and the pixels  14  for controlling the voltage divider  20  to generate the gamma voltages, a flexible printed circuit (FPC)  22  for electrically connecting the PCB  19  and the glass substrate  12 , and an anisotropic conductive film (ACF)  18  coupled between the driver IC chip  16  and the glass substrate  12  for adhering the driver IC chip  16  to the glass substrate  12 . The ACF  18  is a kind of macromolecule material, and serves as media for conduction and interface adhesion of the driver IC chip  16  to the glass substrate  12 . The voltage divider  20  comprises a plurality of serially connected resistors  21 ,  23 ,  25 ,  27 ,  29  all of which have constant resistance, and constant gamma voltages are respectively outputted between two adjacent resistors.  
         [0005]     Please refer to  FIG. 2 , which is a relation diagram between the voltages applied to a pixel  14  and the transmittance of the pixel  14  for a normally white operation mode, where an abscissa represents the voltages, and an ordinate represents the transmittance. The relation between the voltages and the transmittance of the LCD is changed by the temperature. As the LCD operates in a normal temperature environment, the transmittance is varied with the voltages according to a V-T curve  22 . As the LCD operates in a higher temperature environment, the transmittance is varied with the voltages according to a V-T curve  26 . However, as the LCD operates in a lower temperature environment, the transmittance is varied with the voltages according to a V-T curve  24 .  
         [0006]     According to the V-T curve  22 , if a first gamma voltage V 1  is applied to the pixel  14 , the pixel  14  has a first transmittance L 1  when the LCD operates in a normal temperature environment. However, when the LCD operates in a higher temperature environment, the first gamma voltage V 1  is corresponding to a second transmittance L 2  according to the V-T curve  26 . Similarly, when the LCD operates in a lower temperature environment, the first gamma voltage V 1  is corresponding to a third transmittance L 3  according to the V-T curve  24 . Consequently, the LCD will display different image when receiving the same gray value data in different temperature of environments.  
         [0007]     A thermal sensor and a programmable gamma value IC are introduced to the LCD to overcome the above-mentioned problem. The thermal sensor senses the temperature of the LCD, and the programmable gamma value IC selects and provides a set of appropriate gamma voltages corresponding to one of a plurality of gamma values of the programmable gamma value IC according to the temperature sensed by the thermal sensor.  
         [0008]     Indeed, the installation of the thermal sensor and the programmable gamma value IC solves the problem. However, the LCD having the thermal sensor and the programmable gamma value IC costs high.  
       SUMMARY OF THE INVENTION  
       [0009]     It is therefore a primary objective of the claimed invention to provide an LCD having a voltage divider with a thermistor to overcome the above-mentioned problems.  
         [0010]     According to the claimed invention, the LCD includes a glass substrate, a plurality of pixels formed on the glass substrate for displaying an image according to gamma voltages, a voltage divider comprising a resistor and a thermistor coupled in series with the resistor for generating gamma voltages for the pixels, and a driver IC chip coupled to the pixels and the voltage divider for controlling the voltage divider to generate gamma voltages to the pixels.  
         [0011]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic diagram of an LCD according to the prior art.  
         [0013]      FIG. 2  is a relation diagram between gamma voltages applied to a pixel of the LCD shown in  FIG. 1  and the transmittance of the pixel.  
         [0014]      FIG. 3  is a schematic diagram of an LCD of a first embodiment according to the present invention.  
         [0015]      FIG. 4  is a relation diagram between resistance and temperature of an ACF of the LCD shown in  FIG. 3 .  
         [0016]      FIG. 5  is a schematic diagram of an LCD of a second embodiment according to the present invention.  
         [0017]      FIG. 6  is an enlarged side view of a driver IC chip, an ACF and a glass substrate of the LCD shown in  FIG. 5 .  
         [0018]      FIG. 7  is another enlarged side view of a driver IC chip, an ACF and a glass substrate of the LCD shown in  FIG. 5 .  
         [0019]      FIG. 8  is a schematic diagram of the resistance of the thermistor according to the present invention.  
         [0020]      FIG. 9  is an enlarged side view of a driver IC chip, an NCF and a glass substrate of the LCD shown in  FIG. 5 .  
         [0021]      FIG. 10  is another enlarged side view of a driver IC chip, an NCF and a glass substrate of the LCD shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0022]     Please refer to  FIG. 3 , which is a schematic diagram of a lower transparent substrate of an LCD of a first embodiment according to the present invention. The lower transparent substrate, such as a glass substrate  12 , comprises the pixels  14 , the driver IC chip  16 , the ACF  18 , and a voltage divider  40  installed on a printed circuit board  19 .  
         [0023]     Different from the voltage divider  20  of the conventional LCD, the resistance of the resistors  21 ,  23 ,  25  of the voltage divider  20  being all constant, the voltage divider  40  of the LCD according to the present invention comprises a plurality of resistors  21 ,  23 , and a thermistor  42  coupled in series with the resistors  21 ,  23  to replace the resistor  25 .  
         [0024]     The thermistor  42  varies its resistance as the temperature of the LCD rises. Accordingly, the gamma voltages the voltage divider  40  generates for the pixels  14  vary for fitting in with the V-T curve  26  shown in  FIG. 2  as the temperature of the LCD rises. For example, the voltage divider  40  in the normal temperature environment generates the first gamma voltage V 1 , which is disposed along the first curve  22  and corresponds to the first luminance L 1 , but generates in the high temperature environment a lower voltage V 2  as the first gamma voltage, which is still corresponds to the first luminance L 1  according to the curve  26 . Consequently, the luminance of the pixels  14  of the LCD keep unchanged with the rising temperature.  
         [0025]     The thermistor  42  also varies its resistance as the temperature of the LCD drops. Accordingly, the gamma voltages the voltage divider  40  generates for the pixels  14  vary for fitting in with the V-T curve  24  shown in  FIG. 2  as the temperature of the LCD drops. For example, the voltage divider  40  in the normal temperature environment generates the first gamma voltage V 1 , which is disposed along the first curve  22  and corresponds to the first luminance L 1 , but generates in the low temperature environment a higher voltage V 3  as the first gamma voltage, which is still corresponds to the first luminance L 1  according to the curve  24 . Consequently, the luminance of the pixels  14  of the LCD keep unchanged with the dropping temperature. Because a higher resistance is needed for generating the gamma voltage as the temperature rising, the thermal coefficient of resistivity of the thermistor  42  is positive.  
         [0026]     According to the first embodiment, the voltage divider  40  comprises only one thermistor  42  and the thermistor  42  is coupled in series with the resistors  21 ,  23 . However, a voltage divider of an LCD of the present invention can be designed to comprise more than one thermistor and these thermistors can be coupled in series with the resistors  21 ,  23 .  
         [0027]     As the media for conduction and interface adhesion of the driver IC chip  16  to the glass substrate  12 , the volume of the ACF  18  sandwiched between the driver IC chip  16  and the glass substrate  18  is expanded with the rising temperature, and the ACF  18  has in equivalence a varied resistance. Please refer to  FIG. 4 , which is a relation diagram between resistance and temperature of the ACF  18 , where an abscissa represents the temperature, and an ordinate represents the resistance. It can be seen in  FIG. 4  that a resistance-temperature curve  11  of the ACF  18  is approximately linear and the resistance increases as the temperature rises. Therefore, the ACF  18  is suitable to compose the thermistor  42  with positive thermal coefficient of resistivity.  
         [0028]     Please refer to  FIG. 5 , which is a schematic diagram of a lower transparent substrate of an LCD of a second embodiment according to the present invention. The difference between the LCDs of the first embodiment and the second embodiment is the formation of the voltage divider  60 .  
         [0029]     The ACF  18  comprises a layer of resin  62  and a plurality of conductive metal particles  64  blended with the resin  62 , as shown in  FIG. 6  and  FIG. 7 , which are an enlarged side view of the driver IC chip  16 , the ACF  18 , and the glass substrate  12 . The ACF  18  is 25 microns in thickness and the conductive particles  64 - 74  have a particle diameter of 3˜5 microns.  
         [0030]     Taking advantage of the ACF  18  that its resistance varies with the rising temperature, as shown in  FIG. 6 , the voltage divider  60  uses dummy bumps  88 ,  90 ,  92  of the driver IC chip  16 , dummy pads  76 ,  78 ,  80  formed on the glass substrate  12 , and the conductive particles  64 , which are respectively coupled between the dummy bumps  88 ,  90 ,  92  and the dummy pads  76 ,  78 ,  80 , wherein the interconnecting lines  82  and  84  of the driver IC  16  respectively connect the dummy bumps  88  and  90  and connect the dummy bumps  90  and  92 . Furthermore, the dummy pad  76  is connected with the resistor  21  and the dummy pad  80  is connected with the resistor  23 , so as to form a thermistor  61 , which is shown in  FIG. 5 . Therefore, the luminance of the image displayed on LCD does not change with the rising temperature.  FIG. 7  depicts another bonding structure formed between the dummy bumps  88 ,  90 ,  92  of the IC driver  16  and the dummy pads  76 ,  78 ,  80  on the glass substrate  12 , and no interconnecting line is needed.  
         [0031]     The resistance of the thermistor  61  can be adjusted by the resistance of the connections between the dummy bumps  88 ,  90 ,  92  and the dummy pads  76 ,  78 ,  80 . For example, the resistance of the connection between one dummy bump and one dummy pad, R COG , is about 5-10 ohms, and the thermistor  61  of the voltage divider  60  can have resistance of a multiple of 5-10 ohms by forming a plurality of connections between the dummy bumps and the dummy pads. Furthermore, the connections between the FPC  22  and the PCB  19  (also known as: film on board, FOB) and between the FPC  22  and the glass substrate  12  (also known as: film on glass, FOG) performed by using an ACF respectively have resistance R FOB  and R FOG . As shown in  FIG. 8 , the resistance of the thermistor  61  is a sum of R FOB , R COG , and R FOG  and is varied with the operational temperature of the LCD. The thermistor  61  is coupled in series with the resistor  23  having a constant resistance.  
         [0032]     The ACF of the present invention may be replaced by a non-conductive film (NCF), which only comprises a layer of resin  62 , and the dummy pads  76 ,  78 ,  80  and the dummy bumps  88 ,  90 ,  92  are connected by surface contact, as shown in  FIG. 9  and  FIG. 10 . Due to the expansion property of the NCF, the thermistor composed of the NCF also has a positive thermal coefficient of resistivity, which is higher than a thermal coefficient of the thermistor composed of the ACF.  
         [0033]     In contrast to the prior art, the present invention can provide an LCD having a voltage divider having a thermistor, which can be composed of a dummy bump of a driver IC chip of the LCD, a conductive particle of an ACF or NCF used to adhere the driver IC chip to a glass substrate of the LCD, and a dummy pad installed on the glass substrate. Therefore, gamma voltages the voltage divider generates for a plurality of pixels of the LCD vary with the operational temperature of the LCD. Consequently, the luminance of the pixels of the LCD corresponding to a gamma voltage keeps unchanged with the rising or falling temperature. The present invention is not limited to the ACF or NCF for bonding the driver IC chip to the glass substrate, and any other conductive glue materials which have their volumes varied with temperature of the LCD can be applied. The present invention is not limited to a thermistor with positive thermal coefficient of resistivity, either. According to display characteristics of the LCD, a thermistor with negative thermal coefficient of resistivity may be used for generating gamma voltages.  
         [0034]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.