Power saving drive mode for bi-level video

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.

BACKGROUND

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.

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.

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.

Generally, LCDs do not work well with direct current (DC) voltages. A graph of transmission versus voltage applied to a LCD is shown inFIG. 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.

There are different scenarios for preserving zero volts DC (0 Vdc), as shown in the series of succeeding frames ofFIGS. 2A-2D. One scenario uses column inversion as shown inFIG. 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 inFIG. 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 inFIG. 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 inFIG. 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.

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

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.

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.

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.

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.

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.

A voltage DC restore mode or extended DC-restore mode can be enabled in the non-active video signal path.

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.

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.

DETAILED DESCRIPTION

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.

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.

FIG. 3shows a high-level schematic diagram of an example embodiment of a display driver circuit10constructed to enable both bi-level and grayscale modes. The display driver circuit10includes a DAC12, a video amplifier13, and a set of level switches15a-15d, receives a digital video signal11input and outputs analog video signal17to a display, such as a LCD. Enabling signal EN14enables the DAC12and video amplifier13when the driver circuit10is operating in the grayscale mode. In the bi-level mode, the DAC12and video amplifier13are disabled and the set of level switches15a-15dis used to select the appropriate voltage level for driving black or white video.

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.

TABLE 1Combinations of bi-level primary colorsRedGreenBlueColor0 +0 +0 =Black1 +0 +0 =Red0 +1 +0 =Green1 +1 +0 =Yellow0 +0 +1 =Blue1 +0 +1 =Magenta0 +1 +1 =Cyan1 +1 +1 =WhiteWhere 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.

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 inFIG. 3, grayscale and bi-level mode operation configurations for amplifier13and switches15a-15dare 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.

TABLE 2Switch and amplifier configurations for the circuit of FIG. 3ModeENKHWHWLKLGray scaleEnabledOpenOpenOpenOpenBi-HighBlackDisabledClosedOpenOpenOpenlevelWhiteDisabledOpenClosedOpenOpenLowBlackDisabledOpenOpenOpenClosedWhiteDisabledOpenOpenClosedOpen

FIGS. 4 and 5display example embodiments of display driver circuits that use one and two amplifiers per channel, respectively. The driver circuits ofFIGS. 4 and 5include switches to enable a DC restore mode. The schematic diagrams ofFIGS. 4 and 5contain p-channel and n-channel metal-oxide semiconductor field-effect transistors (MOSFETs) used as switches. These switches provide a functionality similar to the switches15a-15dofFIG. 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.

FIG. 4is a schematic diagram of an example embodiment display driver circuit20. The display driver circuit20includes a DAC22, in series with video amplifier23, the output of the video amplifier23coupled to a parallel node with two switches25hand25l, and in parallel with two capacitors, high video capacitor CH26hand low video capacitor CL26l. The display driver circuit20can be operated in at least two modes, grayscale mode and bi-level mode. For grayscale mode, enable signal EN24enables the DAC22, which converts the digital video signal21into a corresponding analog signal. The analog video signal is input into video amplifier23(enabled by enable signal EN24) for amplification. The amplified analog video signal is fed to a circuit node including switches25hand25l, parallel capacitors, CH26hand CL26l. Capacitors CH26hand CL26lprovide high and low video signals27hand27l, respectively, which are used to drive a LCD display.

Switch25his a p-channel MOSFET device having a gate terminal GP29hand a source terminal coupled to a high video voltage reference VVH28hsupply, and a drain terminal coupled to the output of video amplifier23. Switch25lis a n-channel MOSFET device having a gate terminal GN29l, a drain terminal coupled to the output of video amplifier23, and a source terminal coupled to a low video voltage reference VVL28lsupply.

In bi-level mode, the DAC22and video amplifier23of display driver circuit20are disabled and the set of level switches25hand25lare used to drive two reference voltage states, high and low. The high video reference VVH28his used for black when driving high video and white when driving low video, and similarly, the low video reference VVL28lis used for white with high video and black for low video. Put another way, when the voltage between the gate GP29hand source is more negative than the threshold voltage of p-channel MOSFET switch25hso that switch25his closed, the high video reference voltage VVH28his applied to drive the display to black in bi-level high mode. Similarly, when driver circuit20is operating in bi-level low mode and the voltage between the gate GN29land corresponding source is more positive than the n-channel threshold voltage, MOSFET switch25lis closed, low video reference voltage VVL28lis 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 circuit20. One benefit of the configuration illustrated inFIG. 4is that it includes only one amplifier and two switches.

TABLE 3Switch and amplifier configurations for the system of FIG. 4ModeENGPGNGray scaleEnabledHLBi-levelHighBlackDisabledLLWhiteDisabledHHLowBlackDisabledHHWhiteDisabledLL

FIG. 5is a schematic diagram of a further example embodiment display driver circuit30. The display driver circuit30includes a DAC32feeding parallel high and low video paths (also referred to herein as circuit branches or sub-channels)34hand34l. Each video sub-channel can include a video amplifier,33hand33l, feeding a node with a set of two level switches, level switch set35a,35band set35c,35d, and a respective high or low capacitor CH36hand CL36l.

The example embodiment of display driver circuit30can 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.

Grayscale mode operates in one of two sub-modes, high video or low video, in which one of the respective sub-channels, high video34hor low video34l, is enabled using a corresponding enable signal, ENH or ENL. The DAC32converts a digital video signal31into a corresponding analog signal fed to the parallel sub-channel node. For high video grayscale mode, enable signal ENH enables video amplifier33hto amplify an analog video signal received from a DAC32. The amplified analog video signal is fed to a sub-channel circuit node including a set of level switches35aand35band high capacitor CH36h. Capacitor CH36hprovides high video signal37hto drive a LCD.

For low video grayscale mode, enable signal ENL enables video amplifier33L to amplify an analog video signal received from a DAC32. The amplified analog video signal is fed to a sub-channel circuit node including a set of level switches35cand35dand high capacitor CL36l. Capacitor CL36lprovides high video signal37lto drive a LCD.

Switches35aand35dare p-channel MOSFET devices each having a gate terminal GPH39aand GPL39d, a source terminal coupled to a high video voltage reference VVH38hsupply, and a drain terminal coupled to the output of a respective video amplifier33hand33l. Switches35band35care n-channel MOSFET devices having gate terminals GNH39band GNL39c, a drain terminal coupled to the output of a respective video amplifier33hand33l, and a source terminal coupled to a low video voltage reference VVL38lsupply.

In bi-level mode, the DAC32and video amplifiers33hand33lof display driver circuit30are disabled and the set of level switches25a-25dare used to drive two reference voltage states, high and low. The high video reference VVH38his used for black when driving high video and white when driving low video, and similarly, the low video reference VVL38lis used for white with high video and black for low video. Put another way, when the voltage between the gate GPH39aand source is more negative than the threshold voltage for MOSFET switch35aso that switch35ais closed, the high video reference voltage VVH38his applied to drive the display to black in bi-level high mode. Similarly, when driver circuit30is operating in bi-level low mode and the voltage between the gate GNL39cand corresponding source is more positive than the threshold voltage, MOSFET switch35cis closed, low video reference voltage VVL38lis applied to drive the display to black.

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, VVH38hor VVL38l, using the same switching techniques describes above with reference to the level set of switches25aand25binFIG. 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 circuit30.

TABLE 4Switch and amplifier configurations for the system of FIG. 5ModeENHENLGPHGNHGPLGNLGray scaleHighEnabledDis-HLL*LabledLowDis-EnabledHH*HLabledBi-HighBlackDis-Dis-LLL*LlevelabledabledWhiteDis-Dis-HHL*LabledabledLowBlackDis-Dis-HH*HHabledabledWhiteDis-Dis-HH*LLabledabled*Indicates state for DC restore of inactive channel.

Although it requires more circuitry, a two-amplifier configuration, an example embodiment of which is illustrated inFIG. 5, is useful when driving larger displays with greater load capacitance because each amplifier, for example video amplifiers33hand33l, sees the load of only one of the high or low video signals, such as high and low video signals37hand37l, but not both, as is the case in a single amplifier configuration. Further, the quiescent current of the two amplifiers, such as video amplifiers33hand33l, 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.

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 andFIG. 5, when GPL39dis set to L while driving high video, setting the left side of CL36lto VVH38hprovides for DC restore. Similarly, GNH39bcan be set to H when driving low video to set the left side of CHto VVL38lto provide DC restore.

Two transistors with gates GNH39band GPL39dcan be used for DC restore in the double amplifier configuration of driver circuit30, whether or not bi-level mode is supported. With the example embodiment of driver circuit30, there are two amplifiers per channel, and coupling capacitors, such as CH36hand CL36l, 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.