Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-09-04
2001-08-07
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
Reexamination Certificate
active
06271816
ABSTRACT:
1. Technical Field
This invention relates to electronic circuits. More particularly, this invention relates to electronic circuits for driving active matrix (thin-film transistor) liquid crystal displays.
2. Description of Related Art
With recent progress in various aspects of active matrix (thin-film transistor) liquid crystal display (LCD) technology, the proliferation of active matrix displays has been spectacular in the past several years. Active matrix displays are used today in a great variety of electronic products, including notebook computers, and color versions of active matrix displays are now commonplace.
In an active matrix display, row and column electrodes form a matrix, and at the intersection of each row and column electrode is a display cell. The display cell typically comprises one transistor or switch. For a monochromatic display, each display cell would correspond to a single gray-scale pixel or dot of the display. For a color display, a grouping of three display cells (typically, one red, one green, and one blue) nearby each other would correspond to a single color pixel or dot of the display. For example, a color VGA display has a resolution of 480 rows and 640 columns of color pixels. Since three cells are needed for each color pixel, 640×3=1,920 column electrodes are typically present, along with 480 row electrodes. Naturally, higher resolution displays require more row and column electrodes, and displays are nowadays becoming increasingly higher in resolution.
An active matrix display is operated by applying a select voltage to a first row electrode to activate the gates of the first row of cells, and then applying in parallel appropriate analog display voltages to every one of the column electrodes to charge each cell in the first row to a desired level. Next, a select voltage is applied to a second row electrode to activate the gates of the second row of cells, and then applying in parallel appropriate analog display voltages to every one of the column electrodes to charge each cell of the second row to the desired level. And so on for the rest of the rows of the display matrix.
Column drivers (or source drivers) are very important circuits in the design of an active matrix display. The column drivers receive digital display data and control and timing signals from a display controller chip, convert the digital display data to analog display voltages, and drive the analog display voltages onto column electrodes of the display. The analog display voltages vary the shade of the color that is displayed at a particular pixel of the display.
Column drivers are typically formed upon integrated circuit chips. For example, assuming one integrated circuit chip can provide 192 column drivers, then a color VGA display would require 10 such integrated circuits to drive the 1,920 column electrodes of the display. The power consumed by these column driver chips is typically significant and typically causes a substantial power drain on batteries supplying the power in a notebook (laptop) computer. This power drain is a problem which reduces the amount of time a notebook computer may be powered by a charged battery.
LCD technology is able to display images because optical characteristics of the liquid crystal material are sensitive to voltages applied across it. However, the steady application of a near constant voltage across an LCD cell will, over time, degrade the properties and characteristics of the material in that cell. Therefore, LCDs are typically driven using techniques which alternate the polarity of the voltages applied across a cell. These voltages of “alternating polarity” may be voltages above or below a predetermined midpoint voltage (which may be non-zero).
Conventional implementations of the above described technique of applying voltages of alternating polarity typically result in large voltage transitions whenever the polarity is changed. Such large voltage transitions result in significant usage of power which is typically provided by the column driver circuits.
Display Inversion
There are several inversion schemes possible to implement the above described technique of applying voltages of alternating polarity. A first, and perhaps simplest, inversion scheme may be called “display inversion.” In display inversion, every cell in the display is driven to a positive voltage (with respect to the midpoint voltage) during a first display cycle, and then every cell is driven to a negative voltage (with respect to the midpoint voltage) during a second display cycle, and continuing by alternating between the first and second display cycles.
One drawback with the display inversion scheme is that the LCD may alternately display two different images; this alternation between two images being perceived by the viewer as a flicker in the display.
Row Inversion
A second inversion scheme may be called “row inversion” or “line inversion.” In row inversion, the driving voltages applied by the column drivers will alternate in polarity between successive rows of the display. Thus, a first row of pixels would be driven to positive voltages, a second adjacent row of pixels would be driven to negative voltages, and so on (alternating between positive and negative).
In addition, on the subsequent display cycle, the first row would be driven to negative voltages, the second row would be driven to positive voltages, and so on. Thus, inversion between alternating display cycles also occurs in the row inversion scheme.
A drawback with the row inversion scheme is that between successive row drive periods, the column drivers must typically alternate between driving positive and negative voltages. This alternation between positive and negative voltages results in the consumption of significant amounts of power by the column drivers. (In comparison, in the display inversion scheme, the column drivers need to oscillate between positive and negative voltages only once per display cycle, instead of once per row drive period.)
Pixel Inversion
A third inversion scheme may be called “pixel inversion” or “dot inversion.” In pixel inversion, the driving voltages applied by adjacent column drivers will alternate. Thus, during a row drive period, a first column would be driven to a positive voltage, a second column (adjacent to the first) would be driven to a negative voltage, a third column (adjacent to the second) would be driven to a positive voltage, and so on.
In addition, during the row drive period for the next row, the first column would be driven to a negative voltage, the second column would be driven to a positive voltage, the third column would be driven to a negative voltage, and so on. Thus, inversion between alternating rows also occurs in the pixel inversion scheme. Finally, inversion between alternating display cycles also occurs in the pixel inversion scheme.
The pixel inversion scheme typically suffers from the same drawback as discussed above with respect to the row inversion scheme. This is because the pixel inversion scheme includes row inversion, so the pixel inversion scheme also results in a significant drain of power as the column drivers alternate polarities between row drive periods.
Back Plane Switching
For optimal performance of the display, due to characteristics of the liquid crystal material in an active matrix display, column drivers typically need to drive voltages ranging between ±6 volts with respect to the midpoint voltage. This voltage range would typically preclude the use of integrated circuits manufactured with small dimension processes because those processes typically support operation only at about 5 volts or less. Chips are less efficiently produced by larger dimension processes. However, in order to avoid needing to use larger dimension processes, a technique called back plane switching may be used.
The back plane switching technique is typically used in conjunction with row inversion. In back plane switching, a bias voltage is driven onto the back plane of the active matrix display. The back plane bias voltage is driven
Jeong Deog-Kyoon
Kim Gyudong
Fenwick & West LLP
Hjerpe Richard
Laneau Ronald
Silicon Image Inc.
LandOfFree
Power saving circuit and method for driving an active matrix... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Power saving circuit and method for driving an active matrix..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Power saving circuit and method for driving an active matrix... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2497559