Abstract:
An in-cell capacitive touch panel is disclosed. The present invention utilizes a sensing unit that comprises a sensing liquid crystal capacitor and three transistors to detect touch events. A first transistor is connected to a first gate line and the sensing liquid crystal capacitor and controlled by the first gate line to charge the sensing liquid crystal capacitor. A second transistor together with a third transistor functions as a capacitance-current converter. The second transistor generates an output current according to the voltage of a first electrode of the sensing liquid crystal capacitor. A second gate line controls the third transistor to transfer the output current through a readout line to a readout unit that determines the touch positions. Thus, the in-cell capacitive touch panel of the present invention can use a simple-structure readout circuit to achieve superior readout accuracy and is adaptive to various sizes of touch panels.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a touch panel, particularly to an in-cell capacitive touch panel. 
     2. Description of the Related Art 
     The touch panel has been more and more popular recently. When touched by a finger or a stylus, a touch panel outputs an analog signal, and a controller converts the analog signal into a digital signal recognizable by a computer. The touch panel driver program processes the digital signal and controls a display card to present the touched position on the touch panel. 
     The current touch panels may be categorized into the resistive type, capacitive type, sonic type, infrared type and in-cell type. Among them, the in-cell type touch panel attracts much attention. The traditional resistive type or capacitive type touch panel needs an additional touch panel for detecting touch events on the display panel. The in-cell touch panel has the touch control function built inside the LCD cell without the additional touch panel and thus has advantages of lightweight, compactness and high optical performance. Therefore, the in-cell touch panel is highly valued. 
     At present, most of the in-cell touch panels belong to the optical sensation type, wherein photo sensors built inside the LCD cell sense brightness variation on the touch panel to detect whether there is a touch event. Refer to  FIG. 1  and  FIG. 2 . The photo sensor may be a TFT (Thin Film Transistor) sensor  10 , such as a photo sensor consisting of a photo TFT and a Readout TFT. The photo sensor may also be a p-i-n diode  12 . However, the background of the detected image varies with the situation of the touch panel, and the environmental brightness affects the detection of the photo sensor. To solve such a problem, the readout system has to possess a dynamic feedback and auto-calibration capability. Thus, the system becomes more complicated. So far, the problem still lacks an effective solution. 
     Refer to  FIG. 3   a  for a conventional in-cell capacitive touch panel. The in-cell capacitive touch panel has a plurality of sensing liquid crystal capacitors (Cslc)  14 . Each sensing liquid crystal capacitor  14  is cascaded to a reference capacitor (Cref)  16 . The capacitance variation of the sensing liquid-crystal capacitors  14  is used to detect touch events and determine touch points. Refer to  FIG. 3   b  for the structure of the sensing liquid-crystal capacitor  14 . The sensing liquid crystal capacitor  14  has an upper transparent substrate  141 , an upper metal layer  142 , a liquid crystal layer  143 , a lower metal layer  144 , and a lower transparent substrate  145  from top to bottom. The upper metal layer  142  functions as the electrode layer and provides a common voltage source (Vcom). The in-cell capacitive touch panel is exempted from the influence of environmental illumination and has a simpler readout system than the optical sensation type in-cell touch panel. However, the in-cell capacitive touch panel also has its own problems. For example, large-size in-cell capacitive touch panels are hard to fabricate because their capacitive sensors have pretty high parasitic capacitance. Further, the capacitive sensors can only reach a common level accuracy because of the high parasitic capacitance. Therefore, a high-resolution capacitive sensor is hard to achieve. 
     Accordingly, the present invention proposes a novel in-cell capacitive touch panel to solve the abovementioned problems. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide an in-cell capacitive touch panel, which uses a combination of a sensing liquid crystal capacitor and three transistors to function as a high-resolution sensing unit. The sensing unit is installed inside the LCD cell, has advantages of lightweight, small size and high optical performance and is adaptive to a large-size touch panel, whereby the present invention has a simple-structure readout unit and high readout accuracy, wherefore the present invention can effectively solve the problems of the conventional technologies. 
     To achieve the abovementioned objective, the present invention proposes an in-cell capacitive touch panel, which comprises a plurality of gate lines and a plurality of sensing units each connected to gate lines. Each sensing unit includes a sensing liquid crystal capacitor and three transistors. A first transistor is connected to a first gate line and the sensing liquid crystal capacitor and is controlled by the first gate line to charge the sensing liquid crystal capacitor and generate a reference voltage. A second transistor is connected to a first electrode of the sensing liquid crystal capacitor. A third transistor is connected to a second gate line and a readout line. The second transistor generates an output current to the third transistor according to the voltage of the first electrode of the sensing liquid crystal capacitor. The second gate line controls the third transistor to transfer the output current to the readout line. The readout unit receives the output current to detect touch events and find out touch positions. 
     Below, the embodiments are described in detail in cooperation with the attached drawings to make easily understood the objectives, technical contents, characteristics and accomplishments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing a conventional TFT sensor circuit; 
         FIG. 2  is a diagram schematically showing a conventional p-i-n diode sensor circuit; 
         FIG. 3   a  is a diagram schematically showing a capacitive sensing circuit of a conventional in-cell touch panel; 
         FIG. 3   b  is a diagram schematically showing the structure of a conventional sensing liquid crystal capacitor; 
         FIG. 4  is a diagram schematically showing a single sensing unit according to the present invention; 
         FIG. 5  is a diagram showing the relationship of timing and operations of a sensing unit according to the present invention; 
         FIG. 6   a  and  FIG. 6   b  are diagrams respectively showing the circuit of a sensing unit and the waveform of the gate signal in stage t 1  of the sensing unit according to the present invention; 
         FIG. 7   a  and  FIG. 7   b  are diagrams respectively showing the circuit of a sensing unit and the waveform of the gate signal in stage t 2  of the sensing unit according to the present invention; 
         FIG. 8   a  and  FIG. 8   b  are diagrams respectively showing the circuit of a sensing unit and the waveform of the gate signal in stage t 3  of the sensing unit according to the present invention; 
         FIG. 9   a  and  FIG. 9   b  are diagrams respectively showing the circuit of a sensing unit and the waveform of the gate signal in stage t 4  of the sensing unit according to the present invention; 
         FIG. 10  is a diagram schematically showing the structure of a sensing liquid crystal capacitor having a protrusion on the transparent substrate corresponding to the sensing liquid crystal capacitor according to the present invention; and 
         FIG. 11  is a diagram schematically showing the structure of a sensing liquid crystal capacitor having color resistors stacked on the color filter corresponding to the sensing liquid crystal capacitor according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The in-cell capacitive touch panel of the present invention comprises a display unit. The display unit contains a plurality of data lines and a plurality of gate lines. The data lines and gate lines cross each other to form a pixel array. The display unit also has a plurality of sensing units arranged thereinside. Each pixel of the display unit has a pixel TFT (Thin Film Transistor), a pixel electrode, a pixel capacitor, and a storage capacitor. Each sensing unit is electrically coupled to a readout line and at least one gate line of the display unit and further comprises a sensing liquid crystal capacitor and three TFTs for detecting touch events. The number of the sensing units is determined by the required resolution of the touch panel. The number of the sensing units may be equal or unequal to the number of the pixels. When the number of the sensing units is equal to the number of the pixels, the sensing units are preferably one-by-one corresponding to the pixels. Below, one embodiment of a single sensing unit is used to demonstrate the technical features of the present invention, and the detailed structure of the pixel is omitted for simplicity. 
     Refer to  FIG. 4 . Each sensing unit  20  is arranged between two adjacent gate lines (the (n−1)th gate line and the nth gate line). In  FIG. 4 , the sensing unit  20  is arranged between a first gate line  22  and a second gate line  24  and electrically coupled to the first gate line  22  and the second gate line  24 . Each sensing unit  20  includes a sensing liquid crystal capacitor (Cslc)  28  and three TFTs—a first transistor (T 1 )  30 , a second transistor (T 2 )  32  and a third transistor (T 3 )  33 . The gate and drain of the first transistor  30  are connected to the first gate line  22 , and the source of the first transistor  30  is connected to a first electrode of the sensing liquid crystal capacitor  28 . The first gate line  22  controls the first transistor  30  to charge the sensing liquid crystal capacitor  28  and generate a reference voltage (Vp) at a node P. The voltage level of the first electrode of the sensing liquid crystal capacitor  28  is equal to the voltage level of the node P. A second electrode of the sensing liquid crystal capacitor  28  is connected to a first bias source Vbias 1 . The gate of the second transistor  32  is connected to the first electrode of the sensing liquid-crystal capacitor  28  and the source of the first transistor  30 . The drain and source of the second transistor  32  are respectively connected to the drain of the third transistor  33  and a second bias source Vbias 2 . The gate and source of the third transistor  33  are respectively connected to the second gate line  24  and a readout line  26 . The second transistor  32  and the third transistor  33  control the conduction state of the second transistor  32  according to the variation of the reference voltage Vp (i.e. the voltage variation of the first electrode of the sensing liquid crystal capacitor  28 ). The second bias source Vbias 2  generates an output current to the third transistor  33  and then to the readout line  26 . A touch on the touch panel varies the capacitance of the sensing liquid crystal capacitor  28  and the reference voltage Vp, which further makes the second transistor  32  generates an output current  21  to the third transistor  33 . Thus, the second gate line  24  controls the third transistor  33  to transfer the output current  21  to the readout line  26  and then to a readout unit (not shown in the drawings). The readout unit determines the touch position according to the variation of the output current  21 . The first bias source Vbias 1  and the second bias source Vbias 2  may be connected to an identical voltage source or different voltage sources, preferably to a common voltage source Vcom of the display unit. 
     Refer to  FIG. 5  for the relationship of the timing and the operations of the sensing unit. An operation cycle of the sensing unit  20  includes four stages of t 1 , t 2 , t 3  and t 4 , wherein the signals of the gate lines  22  and  24  respectively vary the states of the three transistors  30 ,  32  and  33 : 
     In stage t 1 , the first transistor  30  is turned on, and the third transistor  33  is turned off. 
     In stage t 2 , the first transistor  30  is turned off, and the third transistor  33  is turned off. 
     In stage t 3 , the first transistor  30  is turned off, and the third transistor  33  is turned on. 
     In stage t 4 , the first transistor  30  is turned off, and the third transistor  33  is turned off. 
     Below are described the details of each stage. 
     Refer to  FIG. 6   a  and  FIG. 6   b . In stage t 1 , the first gate line  22  (n−1) and the second gate line (n)  24  respectively have a high voltage level Vgh and a low voltage level Vgl, which turns on the first transistor  30  and turns off the third transistor  33 . At this moment, the high voltage level Vgh of the first gate line  22  (n−1) charges the sensing liquid crystal capacitor  28  via the first transistor  30  to generate a reference voltage Vp at the node P. 
     Refer to  FIG. 7   a  and  FIG. 7   b . In stage t 2 , the voltage of the first gate line  22  (n−1) rapidly descends from the high voltage level Vgh to the low voltage level Vgl (as shown in  FIG. 7   b ), and the voltage of the second gate line (n)  24  still maintains at the low voltage level Vgl. Because of the coupling effect, the first electrode of the sensing liquid crystal capacitor  28  has a voltage variation, which controls the second transistor  32  to generate an output current to the third transistor  33 . The voltage variation ΔVp of the reference voltage Vp can be expressed by Equation (1): 
                     Δ   ⁢           ⁢     V   p       =           C   gs         C   gs     +     C   slc         ·   Δ     ⁢           ⁢     V   g               (   1   )               
wherein Cslc is the capacitance of the sensing liquid crystal capacitor  28 , and Cgs is the gate-source capacitance of the first transistor  30 . When there is a touch events (such as a user touches the sensing unit), the capacitance Cslc of the sensing liquid crystal capacitor  28  varies. From Equation (1), it is known that Vp varies also. Thus, the conduction state of the second transistor  32  varies also, which further varies the output current  21  flowing to the third transistor  33 .
 
     Refer to  FIG. 8   a  and  FIG. 8   b . In stage t 3 , the voltage of the second gate line (n)  24  increases to the high voltage level Vgh, which turns on the third transistor  33 . At this moment, both the second transistor  32  and the third transistor  33  turn on, and the third transistor  33  controls the output current  21  to flow to the readout line  26 , and the readout unit determines the touch position. From the above description, it is known that the combination of the second transistor  32  and the third transistor  33  can be regarded as a capacitance-current converter, which transforms the variation of the reference voltage Vp into the variation of an output current. The variation of the output current is then transferred to the readout line  26  via the third transistor  33 . 
     Refer to  FIG. 9   a  and  FIG. 9   b . In stage t 4 , the voltages of the first gate line  22  (n−1) and the second gate line (n)  24  both descend to the low voltage level Vgl. At this moment, both the first transistor  30  and the third transistor  33  turn off until next sampling time. 
     To optimize the present invention and achieve a higher sensing accuracy, the gate-source capacitance Cgs of the first transistor  30  is designed to be slightly greater than or about equal to the capacitance Cslc of the sensing liquid crystal capacitor  28  in the array design according to Equation (1). Further, the cell gap in the sensing liquid crystal capacitor  28  is decreased to increase the ratio of the cell gap variation to the cell gap in the cell design. Refer to  FIG. 10 . In one embodiment of the present invention, the sensing liquid crystal capacitor  28  comprises a first transparent substrate  34 , a color filter  36  on the first transparent substrate  34 , a conductive electrode layer  38  on the color filter  36 , a second transparent substrate  40 , and a conductive electrode layer  42  on the second transparent substrate  40 . The second transparent substrate  40  is formed of TFT substrates. A protrusion  44  is formed in between the color filter  36  and the conductive electrode layer  38  corresponding to each sensing liquid crystal capacitor. Thus, the cell gap in the sensing liquid crystal capacitor (Cslc)  28  is decreased from d to S. Alternatively, color resists are stacked on the color filter  36  of each sensing liquid crystal capacitor. For example, a first color resist  46  and a second color resist  48  are stacked on the color filter  36 , as shown in  FIG. 11 . Thus, the cell gap in the sensing liquid crystal capacitor (Cslc) is decreased from d to S. For example, the cell gap in the sensing liquid crystal capacitor is reduced to less than 1.5 μm. The smaller the cell gap, the more sensitive the sensing liquid crystal capacitor. Therefore, the sensing unit is more likely to detect the touch events. 
     In the present invention, the capacitive sensing units are installed inside the LCD cell and have advantages of lightweight, small size and high optical performance. The technology of the present invention applies to large-size touch panels and has superior readout accuracy and simpler readout circuit structure. Therefore, the present invention can effectively solve the conventional problems. Further, the protrusion design of the present invention can further promote the accuracy of detecting touch events. 
     The embodiments described above are to demonstrate the technical contents and characteristics of the present invention to enable the persons skilled in the art to understand, make, and use the present invention. However, it is not intended to limit the scope of the present invention. Therefore, any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.