Abstract:
An improved Liquid Crystal Display (LCD) heating system is provided. Resistor meshes, in particular VCOM resistor heating meshes, in the pixel array improve capacity of the display heater (heating system) so that the display is heated faster from low temperatures. The resistor mesh design provides much lower resistance from one point in the pixel array to an edge than pixel-to-pixel horizontal VCOM resistance, thus reducing horizontal crosstalk. Further, the approach permits the display to be active during the warm-up process.

Description:
RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/732,277, filed on Nov. 30, 2012. The entire teachings of the above application are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    A liquid crystal display (LCD) has limited operating temperature range due to the characteristics of liquid crystal (LC). As the temperature decreases, LC response time increases dramatically because of the increased viscosity. Consequently, a LCD cannot operate properly at low temperatures. Three approaches or heating systems for maintaining proper display operating temperature have been employed in existing LCD technologies. These heating systems include: (i) an external heater attached to the Indium Tin Oxide (ITO) glass surrounding an active pixel array, (ii) an internal row line heater, and (iii) an internal common electrode line (VCOM) heater. Each type of heating system has various shortcomings. 
         [0003]    Typically, the external heater is attached to the Thin Film Transistor/Indium Tin Oxide (TFT/ITO) cover glass following display fabrication. Such an approach provides maintenance heating by conducting current through the LCD cover glass. Because the external heater provides heat along the edges of the LCD and through the cover glass(es), an external heater is generally inefficient and unable to rapidly warm the display during cold start conditions. The row line heater and the VCOM heater provide much higher efficiency and uniform heat close to the LC material and can be used during cold start conditions. 
         [0004]      FIG. 1  shows a diagram of a standard pixel array  100  with polysilicon row lines  110  and polysilicon VCOM lines  130 . The row lines  110  carry signals to control pixel transistors. C LC  is the capacitance between the pixel electrode and the ITO common plate with the liquid crystal in between. VCOM serves as one plate of the pixel storage capacitors (C stg ) and is tied to a DC voltage during display operation. The DC voltage is effectively shielded from the LC by the active layer which resides between the VCOM lines and LC material. The active layer also serves as another plate of the storage capacitor. 
         [0005]    The second approach is a row line heater which is an internal heater integrated into the active matrix architecture of the display. The row line heater can be used during cold start conditions to rapidly warm the LC material. The row line heater is located within the pixel array, very close to the LC, so that it can provide high efficiency, uniform heating inside of the LCD glass. 
         [0006]      FIG. 2  shows a row line heating scheme. Row line drivers  230 ,  232  drive current through polysilicon row lines  210  from one end to the other and supply heat to the pixel array  200 . Since the row lines  210  also control pixel  220  operation, a display cannot operate during row line heating. The warm-up time needed during a cold start limits their usage in various applications of this heating scheme. 
         [0007]      FIG. 3  shows a VCOM heating scheme. The VCOM heater utilizes the polysilicon VCOM lines  344  of the pixel array  300  as resistive heating elements. Two terminals, V 1   340  and V 2   342 , are tied to the proper DC voltages to control the current through the VCOM lines  344  and produce heat close to the LC. The DC voltage has no effect on the AC video voltages stored in storage capacitors (Cstg), and therefore there is no effect on the pixel voltage (of pixels  348 ). With the active layer to shield the LC from the heater voltage, it is possible to heat the display during normal operation without introducing visible image artifacts. The heater terminals V 1   340  and V 2   342  are preferably externally accessible to the display package so that the heater can be controlled separately from the operation of the display circuits. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention addresses problems in the art and provides an improved Liquid Crystal Display heating system. An example embodiment includes a two dimensional array of display elements disposed on a common semiconductor substrate, each display element (or a pixel) comprising at least a pixel transistor, a storage capacitor (Cst), and a pixel electrode. Each transistor is arranged to control an operation state of the pixel, and has at least a gate, a drain, and a source terminal. The drain terminal is coupled to a first plate of the storage capacitor. A plurality of row select lines is distributed to control a first plurality of gate terminals. A plurality of column lines is distributed to pass video voltages to the pixel electrodes through respective pixel transistors. A resistor heating mesh includes a plurality of horizontal common voltage lines and a plurality of vertical common voltage lines. Each horizontal common voltage line is arranged in an orientation parallel to the row select lines and independent of both the row select and column lines. Each horizontal common voltage line also is coupled to and integral with two or more display elements at least at a second plate of the storage capacitor in each display element. Each horizontal common voltage line further includes two horizontal terminals providing a first horizontal node and a second horizontal node. Each vertical common voltage line is arranged in an orientation parallel to the column lines and independent of both the row select and column lines. Each vertical common voltage line is also coupled to the horizontal common voltage lines to form the mesh VCOM of the present invention. Each vertical common voltage line further includes two vertical terminals providing a first vertical line node and a second vertical line node. 
         [0009]    And the example embodiment includes a heater driver connected to each heater terminal to supply a proper DC voltage so proper current flows through the polysilicon VCOM lines. This is accomplished by the heater driver being coupled to at least one first and second vertical line node and arranged to supply at least a first and second vertical common line voltage to the at least one first and second vertical line node, respectively. The first vertical common line voltage is supplied at the first vertical line node, and the second voltage is supplied at the second vertical line node. This produces a vertical common line voltage difference and a vertical common line current flow through the vertical common voltage line, and thereby heats the display elements couple thereto. 
         [0010]    The heater driver can further control a source voltage applied to the heater terminal, independently of voltages applied to control the row select and column lines, enabling heat to be applied directly to the display element while the display element is actively operating to display information. 
         [0011]    The heater driver can be further coupled to at least one first and second horizontal line node and arranged to supply at least a first and second horizontal common line voltage to the at least one first and second horizontal node, respectively. The first horizontal common line voltage is supplied at the first horizontal node, and the second horizontal common line voltage is supplied at the second horizontal node producing a horizontal voltage difference and a current flowing through the horizontal common voltage line, and thereby heating the display elements coupled thereto. 
         [0012]    The heater driver can further include a voltage divider arranged to supply voltage continuity between the at least one first vertical line node and the first horizontal line node. The horizontal and vertical common voltage lines of the resistor heating mesh can be formed of polysilicon. The vertical common voltage lines can be metal-strapped vertical common voltage lines and the horizontal common voltage lines are formed of polysilicon. The horizontal common voltage lines can be metal-strapped horizontal common voltage lines and the vertical common voltage lines can be formed of polysilicon. The row select lines can be metal row lines. 
         [0013]    The resistor heating mesh can reduce horizontal crosstalk and provide a resistance level from one point in the array of display elements to an edge of the array that is much less than a corresponding resistance level from one corresponding point in horizontal-only common heater (i.e., an internal common voltage line (VCOM) heater) line array of display elements to an edge of the horizontal-only common heater line array. The resistor heating mesh can be disposed adjacent to each of the transistor and pixel electrode in each display element. Each horizontal and vertical common voltage line can be located in a plane beneath an active layer of the pixel elements. 
         [0014]    The display elements can be used in at least one of: a digital camera, digital Single Lens Reflex (SLR) camera, night vision display, handheld video game display, mobile telephone, or video eyewear device. 
         [0015]    At least one of the row select line or column line can be controlled by a low power shift register. The low power shift register can includes a stage circuit, the stage circuit can include a single voltage node driven by a single transistor. The heating element is the polysilicon (resistive) VCOM line connected horizontally and vertically in a mesh style. The active layer serves as another plate of the storage capacitor. The active layer (pixel electrodes) shields the LC (Liquid Crystal) material from the DC voltage on the polysilicon VCOM. 
         [0016]    According to a yet further example embodiment, a display apparatus includes a LCD display coupled to a horizontal and a vertical heating driver. The horizontal heating driver and the vertical heating driver coupled to one or more VCOM resistor heating meshes. The resistor heating meshes improve the display heating system capacity so that the display is heated faster from low temperatures. 
         [0017]    Further, the resistor meshes provide a resistance level from one point in the pixel array to an edge that is much less than pixel-to-pixel horizontal VCOM resistance. This results in reduced horizontal crosstalk, an improvement over the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
           [0019]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0020]      FIG. 1  is a schematic diagram of a polysilicon row line and polysilicon VCOM pixel array. 
           [0021]      FIG. 2  is a schematic diagram of row line heating of the prior art. 
           [0022]      FIG. 3  is a schematic diagram of horizontal VCOM heating of the prior art. 
           [0023]      FIG. 4   a  is a schematic diagram of a metal row line and polysilicon VCOM pixel array of the present invention. 
           [0024]      FIG. 4   b  is a schematic illustration of the column line stack in the pixel array of  FIG. 4   a.    
           [0025]      FIG. 5  is a schematic diagram of a polysilicon VCOM resistor grid of the present invention. 
           [0026]      FIG. 6  is a schematic diagram of a horizontal heating with mesh VCOM according to the present invention. 
           [0027]      FIG. 7  is a schematic diagram of vertical heating with mesh VCOM according to the present invention. 
           [0028]      FIG. 8  is a schematic diagram of a four-sided heating with top and bottom edges at +1V and grounded side edges. 
           [0029]      FIG. 9  is a heater power map associated with heater configuration in  FIG. 8 . 
           [0030]      FIG. 10  is a 3-D heater power map associated with heater configuration in  FIG. 8 . 
           [0031]      FIG. 11  is a schematic diagram of four-sided heating with partially driven edges. 
           [0032]      FIG. 12  is a heater power map associated with heater configuration in  FIG. 11 . 
           [0033]      FIG. 13  is a 3-D heater power map associated with heater configuration in  FIG. 11 . 
           [0034]      FIG. 14  is a schematic diagram of four-sided heating with voltage drop from center. 
           [0035]      FIG. 15  is a heater power map associated with heater configuration in  FIG. 14 . 
           [0036]      FIG. 16  is a 3-D heater power map associated with heater configuration in  FIG. 14 . 
           [0037]      FIG. 17  is a schematic diagram of a metal row line and metal H-VCOM pixel array. 
           [0038]      FIG. 18  is a schematic diagram of a poly row line and metal V-VCOM pixel array. 
           [0039]      FIG. 19  is a schematic diagram of a heating with metal-strapped mesh VCOM. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    A description of example embodiments of the invention follows. 
         [0041]    Image artifacts associated with high-resistance VCOM lines are indicative of horizontal crosstalk, which can be caused by charge coupling from the column lines. The amount of coupling charge is different on each row depending on the pattern displayed on the LCD and is perceived as the horizontal crosstalk. The higher the resistance of the VCOM lines, the longer the RC time constant and the more visible the horizontal crosstalk. This becomes a problem for high resolution displays as such displays have more coupling from many columns and faster operation, and, thus, a limited time for charge to disperse. 
         [0042]    As described herein and in more detail below, example embodiments of the present invention provide improvements for heating an LCD display of a digital system or the like. In particular, example embodiments of the present invention improve the heating capacity of such LCDs so that the display can be heated faster from low temperatures. An example of a display heating system that may be improved by the present invention is described in U.S. Pat. No. 8,022,913 by Kun Zhang et al., issued Sep. 20, 2011, herein incorporated by reference in its entirety. It will be understood by those of skill in the art that other display heating systems may be similarly improved by the principles of the present invention. 
         [0043]    Traditionally, the row lines and the VCOM lines are both made of polysilicon and run horizontally, in parallel with each other, in the pixel array. Using a newly developed metal row line, polysilicon VCOM lines can be connected vertically in the pixel array, resulting in a resistor heating mesh.  FIG. 4   a  shows such a polysilicon VCOM resistor heating mesh  430  and metal row lines  410  in a pixel array  400 .  FIG. 5  shows a circuit diagram of such a polysilicon VCOM resistor heating mesh formed of polysilicon VCOM resistor grids  500 . When the edge points are tied together, the resistance from one point in the pixel array to an edge is much lower compared with the traditional pixel-to-pixel horizontal VCOM resistance. Consequently, the horizontal crosstalk is dramatically reduced. 
         [0044]    Traditional row line heating is not practical using the metal row lines because of low metal resistance and risk of electromigration. However, other VCOM heating schemes can be achieved with resistor heating mesh (also referred to herein as mesh VCOM) of the present invention. 
         [0045]    In  FIG. 4   a,  a two dimensional array  400  of display elements is disposed on a common semiconductor substrate. Each display element (or pixel) comprises a pixel transistor  451 , a storage capacitor (C ST ) and a pixel electrode  453  ( FIG. 4   b ). Each pixel transistor  451  is arranged to control operation state of the pixel. A pixel transistor  451  is formed of a gate, a drain and a source terminal. The drain is coupled to a first plate of the storage capacitor (C ST ). 
         [0046]    A plurality of metal row select lines  410  is distributed to control a first plurality of the transistor  451  gate terminals. A plurality of polysilicon column lines is distributed with the column lines connected to the source of the pixel transistors  451 . The column lines carry video signals that are passed to the pixel electrodes  453  through respective pixel transistors  451 . 
         [0047]    Continuing with  FIG. 4   a,  the resistor heating mesh  430  of the present invention includes a plurality of horizontal common voltage lines (polysilicon) and a plurality of vertical common voltage lines (polysilicon). Each horizontal common voltage line is arranged in an orientation that is parallel to the metal row select lines  410  and independent of both the row select lines  410  and the column lines. Each horizontal common voltage line is coupled to and integral with two or more display elements (pixels) at a second plate of the storage capacitor C ST  in each display element. Each horizontal common voltage line further includes two horizontal terminals that provide a first horizontal node and a second horizontal node. 
         [0048]    Each vertical common voltage line of mesh VCOM  430  is arranged in an orientation parallel to the column lines and independent of both the row select and column lines. Each vertical common voltage line is coupled to the horizontal common voltage lines to form the mesh structure/design. Each vertical common voltage line includes two vertical terminals providing a first vertical line node and a second vertical line node. These nodes may serve as heater terminals. 
         [0049]    As will be made clear below, a heater driver may be connected to each heater (mesh VCOM  430 ) terminal to supply a proper DC voltage that causes a proper current to flow through the polysilicon VCOM lines  430 . The current flow heats the display elements coupled to the vertical common voltage lines of mesh VCOM  430 . 
         [0050]    With reference to  FIG. 4   b,  shown is a vertical pixel structure or column line stack. The metal column  420  is connected to the source of the pixel transistor  451 . The vertical polysilicon VCOM lines of  430  are connected to the horizontal polysilicon VCOM lines  430  to form a mesh style VCOM. The vertical polysilicon VCOM lines  430  serve as one plate of storage capacitor C st  and can be used for heating. The active layer  440  (containing pixel electrodes  453 ) is the pixel transistor  451  drain and serves as the other plate of C ST . The active layer  440  shields the LC material from the DC voltage on the polysilicon mesh VCOM  430  lines. 
         [0051]    Heating a c×r Pixel Array With Mesh VCOM
       c=number of columns   r=number of rows   R H =pixel-to-pixel horizontal resistance   R V =pixel-to-pixel vertical resistance       
 
       1. Horizontal Heating: Heater Driver on Left/Right Edges 
       [0056]    
       
         
           
             
               R 
               
                 ARRAY 
                 , 
                 H 
               
             
             = 
             
               
                 R 
                 ROW 
               
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         [0057]    A vertical VCOM connection does not affect horizontal heating. As shown in the horizontal heater driver  600  of  FIG. 6 , uniform current I H  flows through the horizontal resistors R H . Heater power is dependent of the resistivity of the VCOM lines. In small pixel displays, the horizontal VCOM lines are typically narrow to maximize the display aperture, and, thus, a more resistive VCOM that produces lower heater power. 
       2. Vertical Heating: Heater Driver on Top/Bottom Edges 
       [0058]    
       
         
           
             
               R 
               
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             = 
             
               
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               Heater 
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         [0059]    As shown in  FIG. 7 , a vertical heater driver  700  provides a uniform current I V  through the vertical resistors R V  in a manner that is similar to that of the horizontal heater  600  of  FIG. 6 . Nevertheless, the vertical heater  700  has more advantages in color displays and wide aspect ratio displays. 
         [0060]    In a square monochrome display, R ROW =c·R H ; R COL =r·R V ; and c/r=1 
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         [0061]    In a square color display of same resolution; 
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         [0000]    and c/r=3 
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         [0062]    In a wide-screen monochrome display of aspect ratio 16:9, c/r=16/9 
         [0000]    
       
         
           
             
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         [0063]    In a wide-screen color display of aspect ratio 16:9; c/r=3×16/9 
         [0000]    
       
         
           
             
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         [0064]    Assuming R H =R V , a vertical heater, such as vertical heater  700 , in a color display can provide three times more heat than that of a monochrome display of the same resolution. A vertical heater in a wide-screen display can provide (16/9) 2 ≅3.16 times more heat than that of a horizontal heater. The vertical heater  700  provides the most advantage in a wide-screen color display where the vertical heater power is 9.5 times more than the horizontal heater  600  power. 
       3. Four-Sided Heating 
       [0065]    As illustrated in  FIG. 8 , Heater drivers  800  can be placed on all four sides allowing more heat around the pixel array. Heater power is non-uniform and depends upon how the voltage drivers are arranged about and coupled to the subject pixel array. For example, a 128×128 mesh of 1Ω resistors was simulated with different four-sided heating schemes, and the power maps of the pixel array were produced. In one such example, the heating drivers  800  supplied the top and bottom edges with +1V, and the side edges are grounded as shown in  FIG. 8 . Such a configuration causes non-uniform heating with very high currents in the corners of the VCOM mesh, as illustrated in temperature maps  900  and  1000  of  FIGS. 9 and 10 , respectivley. Such high heat would cause local LC clearing. 
         [0066]    As an alternative, heater drivers  1100  can be connected partially to the edges as shown in  FIG. 11 . Heater power  1200 ,  1300  is more evenly distributed around the pixel array but there are still very high power spikes at the edges where the heater driver  1100  is disconnected from the mesh, as shown in  FIGS. 12 and 13 , respectively. 
         [0067]    To avoid such big power spikes, voltage dividers  1450  can be used for voltage continuity between the four edges of the pixel array as shown in  FIG. 14 . In one embodiment, the voltage dividers  1450  are non-linear resistors. Simulations show more evenly distributed heat maps  1500 ,  1600  than the heat maps  1200 ,  1300  of the pixel arrays using discontinuous driven edges (e.g., heater driver  1100 ), as shown in  FIGS. 15 and 16 . Although the improved performance is useful, such a configuration ( 1400 ) is not practical because the resistor divider  1450  is part of the resistor heating mesh. A non linear voltage divider  1450  is needed to prevent big spikes at the edges of the heater driver plates. This adds a lot of design complexity and reduces the application flexibility. 
         [0068]    The VCOM mesh may be all polysilicon, such as VCOM mesh  430  shown in  FIG. 4 , or may be polysilicon in one dimension (i.e., the horizontal or vertical dimension) and metal in the other dimension. For example,  FIG. 17  shows a diagram of a pixel array  1700  with metal row lines  1710  and horizontally metal-strapped mesh VCOM  1730 . In another example,  FIG. 18  shows a diagram of a pixel array  1800  with polysilicon row lines  1810  and vertically metal-strapped mesh VCOM  1830 . 
         [0069]    In the case of a metal-strapped VCOM, a high density heat array can be achieved by applying DC voltages to the metal VCOM lines. Such an approach can yield much more heat density than a traditional horizontal VCOM heater ( FIG. 3 ). 
         [0070]    High density vertical heating with DC voltages V 1  and V 2  is shown in  FIG. 19 . Uniform current I V  flows through segments  1964  of vertical polysilicon VCOM resistor heating mesh  1900  from a vertical common voltage line  1961  with a higher voltage potential V 1  to a vertical common voltage line  1963  with a lower voltage potential V 2 . A number of horizontal VCOM lines can be left unconnected to the VCOM resistor heating mesh  1900 , but strapped with metal for uniform appearance of the pixel array. The resistance between the two heater lines  1961  and  1963  decreases linearly with the square product of pixel segments  1964  since the resistors are connected in parallel. The heater power increases as many times as the resistance decreases. Depending on the pixel size and design, the heat density and the heater power can be optimized by changing the number of these segments  1964 . 
         [0071]    For example, assume n=number of segments in the pixel array, R eq  =equivalent resistance, and V 1 −V 2 =V, 
         [0072]    For a pixel array with only top and bottom heater drivers, 
         [0000]    
       
         
           
             
               n 
               = 
               1 
             
             ; 
           
         
       
       
         
           
             
               
                 R 
                 eq 
               
               = 
               R 
             
             ; 
           
         
       
       
         
           
             
               Heater 
                
               
                   
               
                
               Power 
             
             = 
             
               
                 
                   V 
                   2 
                 
                 
                   R 
                   eq 
                 
               
               = 
               
                 
                   V 
                   2 
                 
                 R 
               
             
           
         
       
     
         [0073]    For two-segmented array (as shown in  FIG. 19 ), 
         [0000]    
       
         
           
             
               n 
               = 
               2 
             
             ; 
           
         
       
       
         
           
             
               
                 R 
                 eq 
               
               = 
               
                 
                   
                     R 
                     2 
                   
                   // 
                   
                     R 
                     2 
                   
                 
                 = 
                 
                   R 
                   4 
                 
               
             
             ; 
           
         
       
       
         
           
             
               Heater 
                
               
                   
               
                
               Power 
             
             = 
             
               4 
                
               
                 
                   V 
                   2 
                 
                 R 
               
             
           
         
       
     
         [0074]    For n-segmented array, 
         [0000]    
       
         
           
             
               
                 R 
                 eq 
               
               = 
               
                 R 
                 
                   n 
                   2 
                 
               
             
             ; 
           
         
       
       
         
           
             
               Heater 
                
               
                   
               
                
               Power 
             
             = 
             
               
                 n 
                 2 
               
                
               
                 
                   V 
                   2 
                 
                 
                   R 
                    
                   
                       
                   
                 
               
             
           
         
       
     
         [0075]    Such an approach yields much higher heat density, particularly when combined with vertical common voltage line heating in a wide-aspect ratio LC display. For example, a four-segmented metal VCOM wide pixel array yields roughly 28 times more heat than a polysilicon VCOM square array. The horizontal metal VCOM will also further reduce the horizontal crosstalk because of the low-resistivity of the metal. 
         [0076]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.