Abstract:
Liquid crystal display (LCD) driver circuits, and corresponding driving methods, having selectable grayscale and bi-level modes, that also provide DC restore are presented, including an example embodiment driver circuit having selectable direct current (DC) restore voltage switches including a digital to analog converter, a high voltage video signal path including a high voltage video amplifier, a set of high voltage level switches, a high voltage capacitor and a low voltage video signal path including a low voltage video amplifier, a set of low voltage level switches, a low voltage capacitor. Advantages include, for some applications, a display operates in a bi-level mode saving power relative to operating in a grayscale mode, while also being able to offer full grayscale mode in other applications. Further, advantages of some example embodiments include an extended DC-restore mode providing a longer period of DC restore voltage.

Description:
RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/535,444, invented by Frederick P. Herrmann, filed on Sep. 16, 2011, entitled, “Power Saving Drive Mode For Bi-Level Video.” The entire teachings of the above application are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    In many liquid crystal display (LCD) configurations, and particularly those employing the commonly-used twisted nematic (TN) phase, the brightness of a pixel is modulated by the voltage applied across the liquid crystal (LC) cell. The voltage affects the degree to which the LC material rotates polarized light, which in turn controls how much light passes through an exit polarizer. In other words, a LCD is a passive device that acts as a light valve. The managing and controlling of data to be displayed is typically performed by one or more circuits, which are commonly referred to as display driver circuits or simply drivers. 
         [0003]    Grayscale can be achieved by driving varying analog voltages to LCD pixels. Analog video amplifiers are often used in the video signal path of LCD driven circuits. If the video signal source is digital, then one or more digital-to-analog converters (DACs) will typically be used to convert the digital video signal into a corresponding analog video signal. An important consideration in the design of video electronics is the power dissipation of these analog circuits because the DACs and amplifiers can account for a significant, or even dominant, portion of the system power budget. 
         [0004]    Some display applications require pixels driven to purely white or black, and do not use intermediate gray levels. Such purely white or black applications are referred to as bi-level video systems. With only one bit per pixel, these bi-level video systems can often be simpler to drive than grayscale systems, since the DAC and video amplifier and can often be replaced with a switch to select between the voltages associated with driving a LCD to black and white. 
         [0005]    Generally, LCDs do not work well with direct current (DC) voltages. A graph of transmission versus voltage applied to a LCD is shown in  FIG. 1 . High transmission occurs with zero voltage and low transmission with either positive or negative voltage. Therefore, to drive a LCD to black, a positive or negative voltage can be applied to the LCD. However, driving a LCD at a steady state DC voltage may damage the display by, for example, causing contaminants to plate on one side or the other of the LC cell. In order to prevent damage, the voltage applied to the LCD is generally flipped back and forth (alternated) between high-black and low-black, to preserve zero (0) DC voltage, also called DC restore. 
         [0006]    There are different scenarios for preserving zero volts DC (0 Vdc), as shown in the series of succeeding frames of  FIGS. 2A-2D . One scenario uses column inversion as shown in  FIG. 2A , where one frame is written with all the columns having alternating polarity, positive-negative, and positive-negative. In the next frame all the columns are written negative-positive, negative-positive. In the succeeding frame, all the columns are again written positive-negative, positive-negative. As shown in  FIG. 2B , frame inversion can be used where the first frame is written with all positives and the next frame is written with all negatives. The succeeding frame is again written with all positives. As shown in  FIG. 2C , pixel inversion can be used which produces a checkerboard like effect in the first frame and an inverted effect in the second frame. In the third frame, the checkerboard like effect matches that of the first frame. Lastly, as shown in  FIG. 2D , row inversion can be used where all the rows are alternating polarity, positive-negative, and positive-negative. In the next frame all the rows are written negative-positive, negative-positive. In the third frame, the rows are again written positive-negative, positive-negative. 
         [0007]    One approach to implementing an alternating current-coupled (AC-coupled) display driver circuit with one or more direct current-restore (DC-restore) switches integrated within a LCD is U.S. Pat. No. 7,138,993, by Frederick P. Herrmann, issued on Nov. 21, 2006, and assigned to Kopin Corporation of Taunton, Mass., the entire contents of which are hereby incorporated by reference. 
       SUMMARY 
       [0008]    Presented herein are corresponding methods and example embodiments of liquid crystal display (LCD) driver circuits having selectable grayscale and bi-level modes, that also provide DC restore. An example embodiment display driver circuit, and corresponding method for driving a display, having selectable grayscale and bi-level modes includes a digital to analog converter (DAC), video amplifier, set of level switches and enable circuit having a grayscale mode to enable the DAC and video amplifier, and a bi-level mode to enable a subset of the level switches and disable the DAC and video amplifier is presented. 
         [0009]    When operating an example embodiment of the driver circuit in a bi-level mode, power is conserved relative to operating in grayscale mode because the switches used in bi-level mode use less power than the DAC and video amplifier. 
         [0010]    The display driver circuit can include a high voltage level black switch, a low voltage level black switch, and a white voltage level switch. The white level voltage switch can be further comprised of a high voltage level white switch and a low voltage level white switch. 
         [0011]    The DAC, video amplifier and set of level switches can be integrated in the same integrated circuit (IC). The set of level switches can be p-channel and n-channel metal-oxide semiconductor field-effect transistors (MOSFETs). The p-channel MOSFET can have a source terminal coupled to a high video reference voltage source. An n-channel MOSFET can have a terminal coupled to a low video reference voltage source. 
         [0012]    The display driver circuit can be further implemented with different display colors, such as primary colors red, green, and blue, each color having three or four associated switches because color display uses at least three times as many switches as monochrome (e.g., black and white). The display driver circuit can further include a high video signal path or sub-channel and a low video signal path or sub-channel in parallel between the DAC and liquid crystal display. Each high and low video sub-channel (or path or branch) can respectively include a video amplifier, a set of level switches, and a capacitor. 
         [0013]    A voltage DC restore mode or extended DC-restore mode can be enabled in the non-active video signal path. 
         [0014]    Further presented herein is a liquid crystal display (LCD) driver circuit having selectable direct current (DC) restore voltage switches including a digital to analog converter, a high voltage video signal sub-channel including a high voltage video amplifier, set of high voltage level switches, high voltage capacitor, and a low voltage video signal sub-channel including a low voltage video amplifier, set of low voltage level switches, low voltage capacitor. The high voltage path can further include a high voltage enable circuit having a high voltage grayscale mode that enables a high voltage view amplifier and disables high voltage level switches, and an extended DC restore that provides a longer period of DC restore using a set of low level voltage switches. The low voltage sub-channel can further contain a low voltage enable circuit having a low voltage grayscale mode enabling the low voltage video amplifier and disabling the set of low voltage level switches, and an extended DC restore mode enabling a longer period of DC restore using the set of high voltage level. 
         [0015]    A quiescent current of the high and low video amplifiers can be substantially the same. In grayscale modes, only one amplifier needs to be enabled at a time and thus supplied power during operation. The inactive amplifier can be powered down, so that the dual amplifier circuit uses no more power than a single amplifier circuit. This provides for power savings. DC restore mode can be enabled while the low voltage signal amplifier is active and the low voltage DC restore mode can be enabled while the high voltage video amplifier is active. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    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. 
           [0017]      FIG. 1  is a representative transmission versus voltage diagram. 
           [0018]      FIGS. 2A-2D  are diagrams showing successive frames using column inversion, frame inversion, pixel inversion and row inversion, respectively. 
           [0019]      FIG. 3  is a high-level schematic diagram of a circuit capable of selectable grayscale and bi-level mode operation. 
           [0020]      FIG. 4  is a schematic diagram of a circuit with a single amplifier capable of selectable grayscale and bi-level mode operation. 
           [0021]      FIG. 5  is a schematic diagram of a circuit with two sub-channels capable of selectable grayscale, bi-level mode and extended DC-restore mode operation. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    A description of example embodiments of the invention follows. 
         [0023]    Mobile electronic systems typically manage power carefully to prolong battery life and maximize the time between charges. It is common for such devices to have a “standby” or “sleep” mode which uses much less power than the normal operating mode. Other power-saving options may reduce performance or disable features. For example, many laptop computers may be configured to dim the screen and/or reduce CPU clock frequency when operating on battery power, and e-book readers may allow the user to disable wireless connectivity to conserve power. 
         [0024]    Different power management modes may have different display requirements. It may be advantageous for a display to operate in a bi-level video mode for some applications, while also being able to offer full grayscale in others. For example, bi-level text and simple graphics could provide status information in a standby mode. In another example, an e-book reading application could reduce power consumption by driving bi-level video for text, and switching to grayscale drive only when displaying pictures or illustrations. 
         [0025]      FIG. 3  shows a high-level schematic diagram of an example embodiment of a display driver circuit  10  constructed to enable both bi-level and grayscale modes. The display driver circuit  10  includes a DAC  12 , a video amplifier  13 , and a set of level switches  15   a - 15   d , receives a digital video signal  11  input and outputs analog video signal  17  to a display, such as a LCD. Enabling signal EN  14  enables the DAC  12  and video amplifier  13  when the driver circuit  10  is operating in the grayscale mode. In the bi-level mode, the DAC  12  and video amplifier  13  are disabled and the set of level switches  15   a - 15   d  is used to select the appropriate voltage level for driving black or white video. 
         [0026]    Color displays may also use multiple video inputs for separate red, green, and blue component signals. In the case of color displays, bi-level drive of the red, green, and blue primary colors can produce eight possible colors. 
         [0000]                                      TABLE 1                   Combinations of bi-level primary colors            Red   Green   Blue   Color               0 +   0 +   0 =   Black       1 +   0 +   0 =   Red       0 +   1 +   0 =   Green       1 +   1 +   0 =   Yellow       0 +   0 +   1 =   Blue       1 +   0 +   1 =   Magenta       0 +   1 +   1 =   Cyan       1 +   1 +   1 =   White               Where 0 means the respective color channel is driven to the dark state and 1 means it is driven to the bright state.            
For clarity, the following discussion continues to refer to single inputs or input pairs, such as for driving black and white, but the ideas and techniques described may be readily scaled for displays with multiple inputs.
 
         [0027]    Because most LCDs need to periodically invert the video to prevent damaging the LC cells from prolonged exposure to a DC voltage, two reference voltage levels are used, high and low. To prevent damage in bi-level video mode operation, each reference voltage level (high and low) has a corresponding black and white voltage to drive the display to black or white respectively. In other words, to prevent damaging a LCD operating in bi-level video mode four voltage levels can be used to drive the display: high black (KH), high white (WH), low white (WH) and low black (KH). For the example embodiment shown in  FIG. 3 , grayscale and bi-level mode operation configurations for amplifier  13  and switches  15   a - 15   d  are summarized below in Table 2. Those of skill in the art will recognize that in cases where the high and low white voltage levels are the same only three switches are needed. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Switch and amplifier configurations for the circuit of FIG. 3 
               
             
          
           
               
                 Mode 
                 EN 
                 KH 
                 WH 
                 WL 
                 KL 
               
               
                   
               
             
          
           
               
                 Gray scale 
                 Enabled 
                 Open 
                 Open 
                 Open 
                 Open 
               
             
          
           
               
                 Bi- 
                 High 
                 Black 
                 Disabled 
                 Closed 
                 Open 
                 Open 
                 Open 
               
               
                 level 
                   
                 White 
                 Disabled 
                 Open 
                 Closed 
                 Open 
                 Open 
               
               
                   
                 Low 
                 Black 
                 Disabled 
                 Open 
                 Open 
                 Open 
                 Closed 
               
               
                   
                   
                 White 
                 Disabled 
                 Open 
                 Open 
                 Closed 
                 Open 
               
               
                   
               
             
          
         
       
     
         [0028]      FIGS. 4 and 5  display example embodiments of display driver circuits that use one and two amplifiers per channel, respectively. The driver circuits of  FIGS. 4 and 5  include switches to enable a DC restore mode. The schematic diagrams of  FIGS. 4 and 5  contain p-channel and n-channel metal-oxide semiconductor field-effect transistors (MOSFETs) used as switches. These switches provide a functionality similar to the switches  15   a - 15   d  of  FIG. 3 . The MOSFETs maybe integrated in the same integrated circuit (IC) as the DAC and amplifiers. Those with skill in the art will recognize that any type of switch, such as transistors other than MOSFETs, can be used as switches and may or may not be integrated in an IC chip. The switches enable a DC restore signal to be applied to the display. Many displays, such as those available from Kopin Corporation of Taunton Mass. use capacitively coupled video signals with switches for DC restore integrated in the display. 
         [0029]      FIG. 4  is a schematic diagram of an example embodiment display driver circuit  20 . The display driver circuit  20  includes a DAC  22 , in series with video amplifier  23 , the output of the video amplifier  23  coupled to a parallel node with two switches  25   h  and  25   l , and in parallel with two capacitors, high video capacitor C H    26   h  and low video capacitor C L    26   l . The display driver circuit  20  can be operated in at least two modes, grayscale mode and bi-level mode. For grayscale mode, enable signal EN  24  enables the DAC  22 , which converts the digital video signal  21  into a corresponding analog signal. The analog video signal is input into video amplifier  23  (enabled by enable signal EN  24 ) for amplification. The amplified analog video signal is fed to a circuit node including switches  25   h  and  25   l , parallel capacitors, C H    26   h  and C L    26   l . Capacitors C H    26   h  and C L    26   l  provide high and low video signals  27   h  and  27   l , respectively, which are used to drive a LCD display. 
         [0030]    Switch  25   h  is a p-channel MOSFET device having a gate terminal GP  29   h  and a source terminal coupled to a high video voltage reference VVH  28   h  supply, and a drain terminal coupled to the output of video amplifier  23 . Switch  25   l  is a n-channel MOSFET device having a gate terminal GN  29   l , a drain terminal coupled to the output of video amplifier  23 , and a source terminal coupled to a low video voltage reference VVL  28   l  supply. 
         [0031]    In bi-level mode, the DAC  22  and video amplifier  23  of display driver circuit  20  are disabled and the set of level switches  25   h  and  25   l  are used to drive two reference voltage states, high and low. The high video reference VVH  28   h  is used for black when driving high video and white when driving low video, and similarly, the low video reference VVL  28   l  is used for white with high video and black for low video. Put another way, when the voltage between the gate GP  29   h  and source is more negative than the threshold voltage of p-channel MOSFET switch  25   h  so that switch  25   h  is closed, the high video reference voltage VVH  28   h  is applied to drive the display to black in bi-level high mode. Similarly, when driver circuit  20  is operating in bi-level low mode and the voltage between the gate GN  29   l  and corresponding source is more positive than the n-channel threshold voltage, MOSFET switch  25   l  is closed, low video reference voltage VVL  28   l  is applied to drive the display to black in bi-level low mode. The configurations for the enablement and settings for the switches are summarized in Table 3 for display driver circuit  20 . One benefit of the configuration illustrated in  FIG. 4  is that it includes only one amplifier and two switches. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Switch and amplifier configurations for the system of FIG. 4 
               
             
          
           
               
                   
                 Mode 
                   
                 EN 
                 GP 
                 GN 
               
               
                   
                   
               
             
          
           
               
                   
                 Gray scale 
                 Enabled 
                 H 
                 L 
               
             
          
           
               
                   
                 Bi-level 
                 High 
                 Black 
                 Disabled 
                 L 
                 L 
               
               
                   
                   
                   
                 White 
                 Disabled 
                 H 
                 H 
               
               
                   
                   
                 Low 
                 Black 
                 Disabled 
                 H 
                 H 
               
               
                   
                   
                   
                 White 
                 Disabled 
                 L 
                 L 
               
               
                   
                   
               
             
          
         
       
     
         [0032]      FIG. 5  is a schematic diagram of a further example embodiment display driver circuit  30 . The display driver circuit  30  includes a DAC  32  feeding parallel high and low video paths (also referred to herein as circuit branches or sub-channels)  34   h  and  34   l . Each video sub-channel can include a video amplifier,  33   h  and  33   l , feeding a node with a set of two level switches, level switch set  35   a ,  35   b  and set  35   c ,  35   d , and a respective high or low capacitor C H    36   h  and C L    36   l.    
         [0033]    The example embodiment of display driver circuit  30  can be operated in at least three modes, grayscale, bi-level, and extended DC-restore. While grayscale and bi-level modes are mutually exclusive, extended DC restore is not. 
         [0034]    Grayscale mode operates in one of two sub-modes, high video or low video, in which one of the respective sub-channels, high video  34   h  or low video  34   l , is enabled using a corresponding enable signal, ENH or ENL. The DAC  32  converts a digital video signal  31  into a corresponding analog signal fed to the parallel sub-channel node. For high video grayscale mode, enable signal ENH enables video amplifier  33   h  to amplify an analog video signal received from a DAC  32 . The amplified analog video signal is fed to a sub-channel circuit node including a set of level switches  35   a  and  35   b  and high capacitor C H    36   h . Capacitor C H    36   h  provides high video signal  37   h  to drive a LCD. 
         [0035]    For low video grayscale mode, enable signal ENL enables video amplifier  33 L to amplify an analog video signal received from a DAC  32 . The amplified analog video signal is fed to a sub-channel circuit node including a set of level switches  35   c  and  35   d  and high capacitor C L    36   l . Capacitor C L    36   l  provides high video signal  37   l  to drive a LCD. 
         [0036]    Switches  35   a  and  35   d  are p-channel MOSFET devices each having a gate terminal GPH  39   a  and GPL  39   d , a source terminal coupled to a high video voltage reference VVH  38   h  supply, and a drain terminal coupled to the output of a respective video amplifier  33   h  and  33   l . Switches  35   b  and  35   c  are n-channel MOSFET devices having gate terminals GNH  39   b  and GNL  39   c , a drain terminal coupled to the output of a respective video amplifier  33   h  and  33   l , and a source terminal coupled to a low video voltage reference VVL  38   l  supply. 
         [0037]    In bi-level mode, the DAC  32  and video amplifiers  33   h  and  33   l  of display driver circuit  30  are disabled and the set of level switches  25   a - 25   d  are used to drive two reference voltage states, high and low. The high video reference VVH  38   h  is used for black when driving high video and white when driving low video, and similarly, the low video reference VVL  38   l  is used for white with high video and black for low video. Put another way, when the voltage between the gate GPH  39   a  and source is more negative than the threshold voltage for MOSFET switch  35   a  so that switch  35   a  is closed, the high video reference voltage VVH  38   h  is applied to drive the display to black in bi-level high mode. Similarly, when driver circuit  30  is operating in bi-level low mode and the voltage between the gate GNL  39   c  and corresponding source is more positive than the threshold voltage, MOSFET switch  35   c  is closed, low video reference voltage VVL  38   l  is applied to drive the display to black. 
         [0038]    Alternating between high and low sub-modes for both grayscale and bi-level modes provides an amount of DC-restore to a LCD. Extended DC-restore mode can perform DC-restore for an extended time period, which is useful in some applications. In extended DC-restore mode, when one of the sub-channels is enabled and active, the inactive sub-channel is set to a DC level, for example video reference voltage, VVH  38   h  or VVL  38   l , using the same switching techniques describes above with reference to the level set of switches  25   a  and  25   b  in  FIG. 4 . Extended DC-restore mode allows the inactive capacitor almost the entire line period to perform DC-restore, whereas in DC-restore mode DC-restore is performed only during a retrace period, such as a horizontal retrace period. The configurations for the enablement and settings for the switches are summarized in Table 4 for display driver circuit  30 . 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Switch and amplifier configurations for the system of FIG. 5 
               
             
          
           
               
                 Mode 
                 ENH 
                 ENL 
                 GPH 
                 GNH 
                 GPL 
                 GNL 
               
               
                   
               
             
          
           
               
                 Gray scale 
                 High 
                 Enabled 
                 Dis- 
                 H 
                 L 
                 L* 
                 L 
               
               
                   
                   
                   
                 abled 
               
               
                   
                 Low 
                 Dis- 
                 Enabled 
                 H 
                 H* 
                 H 
                 L 
               
               
                   
                   
                 abled 
               
             
          
           
               
                 Bi- 
                 High 
                 Black 
                 Dis- 
                 Dis- 
                 L 
                 L 
                 L* 
                 L 
               
               
                 level 
                   
                   
                 abled 
                 abled 
               
               
                   
                   
                 White 
                 Dis- 
                 Dis- 
                 H 
                 H 
                 L* 
                 L 
               
               
                   
                   
                   
                 abled 
                 abled 
               
               
                   
                 Low 
                 Black 
                 Dis- 
                 Dis- 
                 H 
                 H* 
                 H 
                 H 
               
               
                   
                   
                   
                 abled 
                 abled 
               
               
                   
                   
                 White 
                 Dis- 
                 Dis- 
                 H 
                 H* 
                 L 
                 L 
               
               
                   
                   
                   
                 abled 
                 abled 
               
               
                   
               
               
                 *Indicates state for DC restore of inactive channel. 
               
             
          
         
       
     
         [0039]    Although it requires more circuitry, a two-amplifier configuration, an example embodiment of which is illustrated in  FIG. 5 , is useful when driving larger displays with greater load capacitance because each amplifier, for example video amplifiers  33   h  and  33   l , sees the load of only one of the high or low video signals, such as high and low video signals  37   h  and  37   l , but not both, as is the case in a single amplifier configuration. Further, the quiescent current of the two amplifiers, such as video amplifiers  33   h  and  33   l , need not be greater than the quiescent current needed for only one amplifier, because only one amplifier is active at any time and the inactive amplifier may be disabled. 
         [0040]    Another benefit of the two-amplifier configuration is that it allows one half of the channel to perform DC restore while the other half is active. Referring to Table 4 and  FIG. 5 , when GPL  39   d  is set to L while driving high video, setting the left side of C L    36   l  to VVH  38   h  provides for DC restore. Similarly, GNH  39   b  can be set to H when driving low video to set the left side of C H  to VVL  38   l  to provide DC restore. 
         [0041]    Two transistors with gates GNH  39   b  and GPL  39   d  can be used for DC restore in the double amplifier configuration of driver circuit  30 , whether or not bi-level mode is supported. With the example embodiment of driver circuit  30 , there are two amplifiers per channel, and coupling capacitors, such as C H    36   h  and C L    36   l , are not tied together on their left sides. When one of the amplifiers is active, the other is disabled, and a separate switch can set separately the DC level on the left side of each coupling capacitor. Therefore, implementing bi-level mode therefore can be achieved with a net increase of only two transistors. 
         [0042]    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.