Patent Publication Number: US-7212182-B2

Title: Drive circuit of TFTLCD

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority benefit of Taiwan application serial no. 91112054, filed on Jun. 05, 2002. 
   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention relates in general to a drive circuit of a thin-film transistor type liquid crystal display (TFTLCD), and more particularly, to a drive circuit that maintains a sufficient voltage to provide a normal operation of such drive circuit. 
   2. Description of the Related Art 
   Recently, the conventional widely used cathode ray tube (CRT) display has been gradually replaced by the flat panel display in small volume and light weight. Currently, the most popular flat panel display includes the liquid crystal display. The application of the liquid crystal display includes the low power products such as calculator, cellular phone, palm pilot, and the notebook computer, desktop computer, and even the wall television. To obtain an optimum display area of a liquid crystal display and to form a thinnest module, the technique of applying a drive integrated circuit (drive IC) has been intensively developed and studied. 
   There are two commonly used technique for applying the drive IC to the liquid crystal display. One is to connect a printed circuit board mounted with a drive IC to a liquid crystal panel, the other is to install a drive IC on a liquid crystal panel directly. The latter method is also called a chip on glass method (COG). 
   The former method has the disadvantages including the usage of expensive wiring board (normally polyamide), requirement of large amount of devices, and the need of additional equipment to complete the connection between the drive circuit and the liquid crystal panel. Moreover, when the terminal has a minute pitch, it further restricts the tape carrier pattern and the connection to the electrodes of the liquid crystal panel. 
   The chip on glass method is a technique for forming a compact display. The electrodes of pixels are formed on the thin-film transistor glass of the liquid crystal panel directly. The liquid crystal panel is patterned to install the drive circuit. The drive circuit is then connected to the liquid crystal panel. This method provides an improved yield and stability and the advantages of small volume and low cost. 
   However, in the conventional chip on glass structure, metal wires are used to the drivers. The resistance of the metal wires causes a significant voltage drop from one driver to the other. The voltage drop seriously affects the normal operation of the drivers. Therefore, a long strip flexible printed circuit board (FPC) is provided to directly connect each source driver. Therefore, the voltage drop caused by the connection via the metal wires can be obviated. This method requires a large area of the flexible printed circuit board. In addition, as the drive circuits include data lines and power lines, a multiple layer structure is required for the flexible printed circuit board. This method does not only increase the fabrication complexity of the flexible printed circuit board, but also increase the cost and the overall volume of the liquid crystal display. Moreover, as the fabrication process is more complex, the reliability is decreased. 
   SUMMARY OF INVENTION 
   The invention provides a drive circuit, in which each source driver comprises a charge pump to compensate the voltage drop caused by the metal wire for power transmission. The large area flexible printed circuit board used in the prior art is not required, so that the increased volume of the liquid crystal display is obviated. In addition, the drive circuit provided by the invention has a simple fabrication process, such that the reliability is increased. 
   The drive circuit provided by the invention comprises a plurality of source drivers connected in series. The neighboring source drivers are connected with data lines and source lines. The charge pump is disposed at the inputs of the power lines, such that the voltage pump caused by the resistance of the metal wires is compensated. The power is maintained at a certain level to provide a normal operation of the drive circuit. The drive circuit further comprises a capacitor connected to each of the power lines, such that the wave of the power transmitted by the power lines can be filtered and rectified. 
   The drive circuit further comprises a plurality of gate drivers, preferably connected in series. When similar problem of voltage drop occur to the gate drivers, each of the gate drivers may also comprise a charge pump to resolve such problem. The capacitors for rectifying and filtering the power can be formed on the thin-film transistor glass, such that the overall volume of the liquid crystal display is not increased thereby. In addition, the source drivers and the gate drivers are connected to the liquid display panel via a flexible printed circuit board with a small area to achieve its driving function. 
   Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a structure of a liquid crystal display. 
       FIG. 2  shows an embodiment of a drive circuit provided by the invention. 
       FIG. 3  shows the enlarge view of two neighboring source drivers. 
       FIG. 4  shows gate drivers, each of which having a charge pump according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , a typical chip on glass type liquid crystal display with multiple chips is shown. The liquid crystal display comprises a liquid crystal panel  100 , a drive circuit (referred as  10  as shown in  FIG. 2 ), and a flexible printed circuit board  12  to connect the liquid crystal panel and the drive circuit. The liquid crystal panel  100  further comprises a circuit array substrate, that is, the thin-film transistor substrate  104 , a counter substrate  102  and a liquid crystal material filling the space between these two substrates  102  and  104 . The connection between the flexible printed circuit board  12  and the liquid crystal panel  100  is typically achieved using an electrically anisotropic conductive thin film  106 , for example. 
   As mentioned above, in the conventional drive circuit, the power transmission between the source drivers are performed by the metal lines. Such metal lines have a resistance that generates a significant voltage drop, so that the drive circuit cannot operate properly. The prior art provides a structure to resolve such problem. That is, using a long strip flexible printed circuit board to connect each source driver directly. Therefore, the power transmission between the source drivers is achieved without using the metal wires, and consequently, the problem of voltage drop is resolved. However, this structure requires a flexible printed circuit board with a large area that increase a great amount of fabrication cost, and further increase the overall volume of the liquid crystal display. Therefore, instead of using a flexible printed circuit board with a large area, the invention applies a flexible printed circuit board connected to one source driver and one gate driver only. The power transmission is executed with the metal wires, while the voltage drop caused thereby is compensated by pumping up the power source with a charge pump installed in each source driver. Therefore, the space occupied by the flexible printed circuit board is greatly saved, and the cost is reduced. It is appreciated that people of ordinary skill in the art may modify the size and the connection between to the printed circuit board according to specific requirement. For example, the flexible printed circuit board may be connected to more than one source driver and/or more than one gate driver. 
     FIG. 2  shows an embodiment of a drive circuit of a thin-film transistor type liquid crystal display provided by the invention, and  FIG. 3  shows an enlarged view of two neighboring source drivers as shown in  FIG. 2 . 
   Referring to  FIGS. 2 and 3 , the invention provides a drive circuit  10  using a flexible printed circuit board  12  to connect the drive circuit  10  to the liquid crystal panel ( 100  as shown in  FIG. 1 ). Between two neighboring source drivers  14 , there are a data line and a power line for power transmission. When a power is transmitted from one source driver  14  to the next source driver  14 , a voltage drop is inevitably caused by the resistance of the metal for forming the power line. This voltage drop is more and more significant for the later source drivers  14  such that the normal operation is affected. Therefore, from the second source driver  14 , that is, the source driver  14  right after the one connected to the flexibility printed circuit board  12  directly, a charge pump  24  is installed therein. The charge pump  24  is preferably located at an input of the power line  22  to compensate the voltage drop. Thereby, the voltage is pumped up to a certain level to provide a normal operation of each of the source drivers  14 . 
   In addition, the drive circuit  10  further comprises a capacitor  18  connected to each of the power lines  22  for rectifying and filtering the power transmitted thereby. The capacitors  18  are preferably formed on the thin-film transistor substrate  106  to save the space, so as to reduce the volume of the liquid crystal display. 
   The drive circuit  10  further comprises a plurality of gate drivers  16  connected to the flexible printed circuit board  12  in series. When similar problem of voltage drop occurs to these gate drivers  16 , the charge pumps  24  may also be installed in the gate drivers  16  to compensate the voltage drop and to maintain a normal operation. 
   Constructed as above, that is, by installing a charge pump in each of the source drivers that are not directly connected to the flexible printed circuit board, the voltage drop caused by the resistance of the power lines can be compensated. As a result, the power of each source driver is maintained at a certain level to provide a normal operation of the drive circuit. Therefore, without a large area of the flexible printed circuit board, the liquid crystal display can be operated normally. Furthermore, additional discrete capacitors used in other conventional method are also avoided. The invention thus provides a drive circuit in small volume, low cost and with high reliability. 
   Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.