Patent Publication Number: US-2011057868-A1

Title: Liquid crystal display capable of switching common voltage

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a Liquid Crystal Display (LCD), and more particularly, to an LCD capable of switching the common voltage. 
     2. Description of the Prior Art 
     Since LCDs have the advantages of portability, low power consumption, and low radiation, LCDs have been widely used in various portable information products, such as notebooks, cell phones, personal digital assistants (PDA), flat panel televisions, etc. The LCD comprises a Thin Film Transistor (TFT) substrate, and a color filter substrate. A liquid crystal layer is sandwiched in between the TFT substrate and the color filter substrate. The rotation angle of the liquid crystal molecules in the liquid crystal layer can be controlled by means of controlling the drop voltage across the liquid crystal layer, so the transmittance of the liquid crystal layer changes as desired for displaying images. 
     Please refer to  FIG. 1 .  FIG. 1  is a schematic diagram illustrating a convention TFT LCD panel (display panel)  10 . The display panel  10  comprises a plurality of scan lines G 1 ˜Gm, a plurality of data lines S 1 ˜Sn, and a plurality of pixels. Each pixel comprises a transistor  12 , a storage capacitor  14 , and a liquid crystal capacitor  16 . A parasitic capacitor  18  exists between the gate and the drain of the transistor  12 . The pixel electrically connected to the scan line G 1  and the data line  51  is illustrated as below for example. The gate of the transistor  12  is electrically connected to the scan line G 1 . The source of the transistor  12  is electrically connected to the data line S 1 . The drain of the transistor  12  is electrically connected to the pixel electrode; that is, the first end of the storage capacitor  14  and the first end of the liquid crystal capacitor  16 . The liquid crystal capacitor is an equivalent capacitor formed by the liquid crystal layer sandwiched in between the two substrates (TFT substrate and the color filter substrate) of the display panel  10 . The voltage inputted to the first end of the liquid crystal capacitor  16  is referred as a pixel voltage. The storage capacitor  14  is utilized for storing the pixel voltage until the next data signal is inputted. The voltage inputted to the second end of the liquid crystal capacitor  16  is referred as a common voltage VCOM. Generally speaking, the voltage level of the voltage Vcst on the second end of the storage capacitor  14  is equal to the common voltage VCOM, but sometimes the voltage Vcst of the storage capacitor  14  is adjusted for obtaining the desired display characteristics. 
     Please refer to  FIG. 2 .  FIG. 2  is a waveform diagram illustrating the voltages of the display panel  10  in  FIG. 1 . When the scan-line voltage  22  goes up from Vgl to Vgh, the transistor  12  is turned on. The data-line voltage  24  charges the pixel electrode during the duty time Ton of the scan-line voltage  22 . The pixel voltage  26  goes up from Vdl to Vdh. After the duty time Ton of the scan-line voltage  22 , the scan-line voltage goes down to Vgl. Meanwhile, the transistor  12  is turned off so that the data line can not keep charging the pixel electrode. When the data-line voltage  24  goes down from Vdh to Vdl, the storage capacitor  14  keeps the pixel voltage at Vdh so that the pixel voltage  26  does not goes down to Vdl immediately. However, when the scan-line voltage  22  goes down from Vgh to Vgl, the pixel voltage  26  is reduced by a feed-through voltage ΔVp because of the coupling effect of the parasitic capacitor  18 , generating the flicker phenomenon of the TFT LCD. 
     Please refer to  FIG. 3 .  FIG. 3  is a schematic diagram illustrating a conventional method of setting the voltage Vcst of the storage capacitor  14 . The effect of the feed-through voltage ΔVp is reduced in the display panel  10  by means of adjusting the voltage level of the common voltage. In addition, the voltage Vcst of the storage capacitor  14  is adjusted for obtaining the desired display characteristics. The common voltage VCOM and the voltage Vcst of the storage capacitor  14  are both provided by a printed circuit board (not shown in  FIG. 1 ) electrically connected to the display panel  10 . The voltage Vcst of the storage capacitor  14  is usually designed as a fixed voltage. If the voltage Vcst of the storage capacitor  14  is to be adjusted, the voltage Vcst has to be connected to the required voltage source through the resistor  31  of 0 ohm. The required voltage source includes the common voltage source VCOM, the ground GND, and the analog voltage source AVDD, wherein the printed circuit board divides the analog voltage AVDD for generating the gamma voltage. However, in this way, if the voltage Vcst of the storage capacitor is to be adjusted again, the resistor  31  of 0 ohm has to be removed by the welding torch at first, and then welded to the required voltage source again, wasting a lot of time and may causing a short-circuited problem or an open-circuited problem. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present invention to provide a liquid crystal display capable of switching a common voltage. 
     The present invention provides a Liquid Crystal Display (LCD). The LCD comprises a display panel, and a printed circuit board. The display panel comprises a plurality of scan lines, a plurality of data lines, and a plurality of pixels. Each pixel comprises a transistor, a storage capacitor, and a liquid crystal capacitor. The transistor has a control end electrically connected to a scan line, a first end electrically connected to a data line, and a second end. The storage capacitor has a first end electrically connected to the second end of the transistor, and a second end. The liquid crystal capacitor has a first end electrically connected to the second end of the transistor, and a second end electrically connected to a first voltage source. The printed circuit board is electrically connected to the display panel. The printed circuit board comprises a switcher. The switcher is utilized for switching the second end of the storage capacitor electrically connecting to the first voltage source or a second voltage source. 
     The present invention further provides an LCD. The LCD comprises a Thin Film Transistor (TFT) substrate, a color filter substrate, a printed circuit board, and a switcher. The color filter substrate is utilized for sandwiching a liquid crystal layer with the TFT substrate. The printed circuit board is utilized for providing a first common voltage to the color filter substrate and providing a second common voltage to the TFT substrate. The switcher is disposed on the printed circuit board. The switcher is utilized for switching a voltage level of the second common voltage. 
     The present invention further provides a printed circuit board. The printed circuit board is utilized for providing a common voltage of an LCD. The printed circuit board comprises a common voltage source, an analog voltage source, a ground, and a switcher. A voltage level of the analog voltage source is higher than a voltage level of the common voltage source. A voltage level of the ground is lower than the voltage level of the common voltage source. The switcher comprises three input ends, and an output end. The three input ends of the switcher are respectively electrically connected to the common voltage source, the analog voltage source, and the ground. The output end of the switcher is utilized for outputting a voltage of the common voltage source, a voltage of the analog voltage source, or a voltage of the ground to the LCD. 
     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 
         FIG. 1  is a schematic diagram illustrating a convention TFT LCD panel (display panel). 
         FIG. 2  is a waveform diagram illustrating the voltages of the display panel in  FIG. 1 . 
         FIG. 3  is a schematic diagram illustrating a conventional method of setting the voltage of the storage capacitor. 
         FIG. 4  is a schematic diagram illustrating a display of the present invention. 
         FIG. 5  is a schematic diagram illustrating a switcher according to a first embodiment of the present invention. 
         FIG. 6  is a schematic diagram illustrated a switcher according to a second embodiment of the present invention. 
         FIG. 7  is a schematic diagram illustrating adjusting the voltage of the storage capacitor by means of the switcher. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 4 .  FIG. 4  is a schematic diagram illustrating a display of the present invention. In the present embodiment, the display is an LCD  40  for example. The LCD  40  comprises a printed circuit board  41 , a source driver  42 , a display panel  42 , a gate driver  44 , and a common voltage switcher  46 . The source driver  42  and the common voltage switcher  46  are disposed on the printed circuit board  41 . The gate driver  44  is disposed on the display panel  43 . The display panel  43  comprises a TFT substrate  431 , and a color filter substrate  432 . The TFT substrate  431  comprises a plurality of scan lines G 1 ˜Gm, a plurality of data lines S 1 ˜Sn, and a plurality of pixels  45 . Each pixel  45  comprises a transistor  451 , a storage capacitor  452 , and a liquid crystal capacitor  453 . The liquid crystal capacitor  453  is an equivalent capacitor formed by a liquid crystal layer (not shown in  FIG. 4 ) sandwiched in between the TFT substrate  431  and the color filter substrate  432 . The first end of the liquid crystal capacitor  453  is electrically connected to the transistor  451  of the TFT substrate  431 . The second end of the liquid crystal capacitor  453  is electrically connected to the color filter substrate  432 . The first end of the storage capacitor  452  is electrically connected to the first end of the liquid crystal capacitor  453 . The second end of the storage capacitor  452  is electrically connected to the common voltage switcher  46 . 
     In the present embodiment, the second end of the liquid crystal capacitor  453  is electrically connected to the common voltage source VCOM. The second end of the storage capacitor  452  can be electrically connected to the common voltage source VCOM, the ground GND, or the analog voltage source AVDD, through the common voltage switcher  46 . Therefore, the voltage Vcst of the storage capacitor  452  can be switched to be voltage VCOM, GND, or AVDD. Since the second end of the liquid crystal capacitor  453  is usually electrically connected to the common voltage source VCOM (generally speaking, the voltage level of the common voltage source VCOM is about 3.3V), a voltage source having a higher voltage level than the common voltage source VCOM and a voltage source having a lower voltage level than the common voltage source VCOM are selected to be the voltage sources for adjusting the voltage Vcst of the storage capacitor  451 . The ground GND and the analog voltage source AVDD are the voltage sources used in general LCDs. The analog voltage source AVDD is utilized for generating the gamma voltage in the LCD. The voltage level of the analog voltage source AVDD is about 8.5V, and the voltage level of the ground GND is 0V. 
     In the present embodiment, the display is illustrated to be an LCD for example. However, the operational principle of the present invention can be applied for various displays. For example, the display can be an Electrophoresis Display (EPD), an Organic Light-Emitting Diodes (OLED) display, a flexible display, or an LED display according to the requirement. 
     Please refer to  FIG. 5 .  FIG. 5  is a schematic diagram illustrating a switcher  461  according to a first embodiment of the present invention. The switcher  461  comprises a switcher  51 . The second end of the switcher  51  is electrically connected to the second end of the storage capacitor  452  (please refer to  FIG. 4 ). The first end of the switcher  51  is selectively electrically connected to the common voltage source VCOM, the ground GND, or the analog voltage source AVDD according to the requirement. Comparing with the conventional method of using the resistor of 0 ohm for connecting to the required voltage source, the voltage Vcst of the storage capacitor  452  is easily adjusted to be connected to the required voltage source by means of the switcher  51 . Hence, when some defects appear in the display panel, the voltage Vcst of the storage capacitor  452  can be switched to be equal to the voltages AVDD, VCOM, or GND according to the grades of the defects. 
     Please refer to  FIG. 6 .  FIG. 6  is a schematic diagram illustrated a switcher  462  according to a second embodiment of the present invention. The switcher  462  comprises three switches  61 ,  62 , and  63 . The first ends of the switches  61 ,  62 , and  63  are respectively electrically connected to the analog voltage source AVDD, the common voltage source VCOM, and the ground GND. The second ends of the switches  61 ,  62 , and  63  are all electrically connected to the second end of the storage capacitor  452 . The voltage Vcst of the storage capacitor  452  can be adjusted by mean of the switches  61 ,  62 , and  63 . For example, when the voltage Vcst of the storage capacitor  452  is to be adjusted to be the voltage AVDD, the switch  61  is turned on, and the switches  62  and  63  are turned off; when the voltage Vcst of the storage capacitor  452  is to be adjusted to be the voltage VCOM, the switch  62  is turned on, and the switches  61  and  63  are turned off; when the voltage Vcst of the storage capacitor  452  is to be adjusted to be the voltage GND, the switch  63  is turned on, and the switches  61  and  63  are turned off. As a result, when one of the switches  61 ,  62 , and  63  is turned on, the other two switches are turned off. 
     Please refer to  FIG. 7 .  FIG. 7  is a schematic diagram illustrating adjusting the voltage Vcst of the storage capacitor by means of the switcher  46 . In the fabrication of the LCD panel, if the TFT substrate and the color filter substrate are assembled incorrectly, the Photo Spacer (PS) can not move back to the original location when the LCD panel is flapped, generating the mura  71  (means the color is not uniform) as shown in the panel (A) of  FIG. 7 . The voltage Vcst of the storage capacitor of the panel (A) is equal to the voltage GND. If the voltage Vcst of the storage capacitor of the panel (A) is adjusted to be equal to the voltage AVDD, since the liquid crystal molecules change the rotation angle because of the lateral electrical field, the mura  71  of the panel (A) can be reduced as shown in the part  72  of the panel (B), so the image quality of the LCD is improved. The similar situations are shown in the panel (C) and the panel (D). In the panel (C), the voltage Vcst of the storage capacitor is equal to the voltage AVDD, and the phenomenon of the light leakage is generated in the edge between the pixel and the data line (shown as the part  73 ). If the voltage Vcst of the storage capacitor is adjusted to be equal to the voltage GND at the time, the liquid crystal molecules change the rotation angle because of the lateral electrical field generated by the voltage Vcst of the storage capacitor and the data line. In this way, the phenomenon of the light leakage is improved as shown in the panel (D). In addition, in the panel (E), the voltage Vcst of the storage capacitor is equal to the voltage VCOM. The panel (E) is normally white when the voltage is not inputted. If the two ends of the storage capacitor of a pixel is shorten-circuited due to the contamination during the fabrication, there is no voltage drop across the two ends of the liquid crystal capacitor of the pixel so the pixel becomes a bright point  75 . As shown in the panel (F), if the voltage Vcst of the storage capacitor is adjusted to be equal to the voltage GND, a voltage drop is generated between the two ends of the liquid crystal capacitor of the pixel so the pixel becomes a dark point  76 . Comparing with the bright point  75 , the dark point  76  is unapparent. Thus, in the example of the panels (E) and (F), the repair process for the bright points can be easily executed by means of adjusting the voltage Vcst of the storage capacitor. In the above-mentioned illustration, the different defects of the panel can be improved respectively by means of adjusting the voltage Vcst of the storage capacitor to be equal to the voltages AVDD, VCOM, or GND. However, it is most important that since the voltage Vcst of the storage capacitor can be immediately adjusted by means of the switcher of the present invention, the grades of the defects can be easily compared when the voltage Vcst of the storage capacitor is adjusted. In this way, the voltage Vcst of the storage capacitor can be immediately adjusted according to the grades of the defects. 
     In conclusion, the present invention provides a liquid crystal display capable of switching the common voltage. The liquid crystal display includes a display panel and a printed circuit board. The display panel includes a plurality of scan lines, a plurality of data lines, and a plurality of pixels. Each pixel includes a transistor, a storage capacitor, and a liquid crystal capacitor. The first ends of the storage capacitor and the liquid crystal capacitor are electrically connected to the transistor. The second end of the liquid crystal capacitor is electrically connected to a common voltage source. The printed circuit board includes a switcher for switching the second end of the storage capacitor electrically connecting to common voltage source, an analog voltage source, or a ground. In the liquid crystal display of the present invention, since, the voltage on the second end of the storage capacitor can be immediately adjusted by means of the switcher, the different defects can be easily improved, causing a great convenience. 
     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.