Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-12-21
2001-03-13
Chang, Kent (Department: 2778)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S099000, C345S209000, C345S211000
Reexamination Certificate
active
06201522
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to circuitry for driving an active or passive matrix liquid crystal display (LCD) or the like, and more particularly, to a circuit and method which reduce the amount of power required for driving columns of the LCD display matrix.
2. Description of the Background Art
LCD displays are used today in a variety of products, including hand-held games, hand-held computers, and laptop
otebook computers. These displays are available in both gray-scale (monochrome) and color forms, and are typically arranged as a matrix of intersecting rows and columns. The intersection of each row and column forms a pixel, or dot, the density and/or color of which can be varied in accordance with the voltage applied thereto in order to define the gray shades of the liquid crystal display. These various voltages produce the different shades of color on the display, and are normally referred to as “shades of gray” even when speaking of a color display.
It is known to control the image displayed on the screen by individually selecting one row of the display at a time, and applying control voltages to each column of the selected row. The period during which each such row is selected may be referred to as a “row drive period”. This process is carried out for each individual row of the screen; for example, if there are 480 rows in the array, then there are typically 480 row drive periods in one display cycle. After the completion of one display cycle during which each row in the array has been selected, a new display cycle begins, and the process is repeated to refresh and/or update the displayed image. Each pixel of the display is periodically refreshed or updated many times each second, both to refresh the voltage stored at the pixel as well as to reflect any changes in the shade to be displayed by such pixel over time.
LCD displays used in computer screens require a relatively large number of such column driver outputs. Color displays typically require three times as many column drivers as conventional “monochrome” LCD displays; such color displays usually require three columns per pixel, one for each of the three primary colors to be displayed. Thus, a typical VGA (480 rows×640 columns) color liquid crystal display includes 640×3, or 1,920 column lines which must be driven by a like number of column driver outputs.
The column driver circuitry is typically formed upon monolithic integrated circuits. Assuming that an integrated circuit can be provided with 192 column output drivers, then a color VGA display screen requires 10 of such integrated circuits (10×192=1,920). One of the goals of circuit designers is to reduce the power consumption of such integrated circuits, both to minimize power drain on the batteries supplying such power and to reduce the power dissipated within the integrated circuit, and hence reduce the temperature at which such integrated circuit operates.
Integrated circuits which serve as column drivers (or “source drivers”) for active matrix LCD displays generate different output voltages to define the various “gray shades” on a liquid crystal display. These varying analog output voltages vary the shade of the color that is displayed at a particular point, or pixel, on the display. The column driver integrated circuit must drive the analog voltages onto the columns of the display matrix in the correct timing sequence. A preferred circuit for generating such analog voltages is described in co-pending U.S. patent application Ser. No. 183,474, filed Jan. 18, 1994, entitled “INTEGRATED CIRCUIT FOR DRIVING LIQUID CRYSTAL DISPLAY USING MULTI-LEVEL D/A CONVERTER” and assigned to the assignee of the present application.
Liquid crystal displays (LCD's) are able to display images because the optical transmission characteristics of liquid crystal material change in accordance with the magnitude of the applied voltage. However, the application of a steady DC voltage to a liquid crystal will, over time, permanently change and degrade its physical properties. For this reason, it is common to drive LCDs using drive techniques which charge each liquid crystal with voltages of alternating polarities relative to a common midpoint voltage value. It should be noted that, in this context, the “voltages of alternating polarities” does not necessarily require the use of driving voltages that are greater than, and less than, ground potential, but simply voltages which are above and below a predetermined median display bias voltage. The application of alternating polarity voltages to the pixels of the display is generally known as inversion.
Thus, driving a pixel of liquid crystal material to a particular gray shade actually involves two voltage pulses of equal magnitude but opposite polarity relative to the median display bias voltage. The driving voltage applied to any given pixel during its row drive period of one display cycle is typically reversed in polarity during its row drive period on the next succeeding display cycle. Thus, for a given pixel located in a particular row, the voltage applied thereto might be +6 volts on a first display cycle and −6 volts on the next display cycle. Over time, the average voltage to which the pixel is driven is a median bias point halfway between the positive and negative voltages; in the example set forth above, the median bias voltage is zero volts or ground.
Assuming that a pixel is initially charged to +6 volts during a first display cycle, then during the following display cycle, the column driver circuit that drives the column which intersects the corresponding row where such pixel is located must drive the pixel from its prior value of +6 volts all the way down to −6 volts, a negative transition of 12 volts. On the third display cycle, the column driver circuit will need to drive the same pixel from −6 volts back to the initial +6 volts (or to some other voltage above the median bias voltage if information is to be updated), a positive transition of as much as 12 volts. The same is true for the other 639 pixels (or the other 1,919 pixels, in the case of a color display) located in the same row, as well as for the pixels located in the other 479 rows. These relatively large voltage transitions result in significant power usage which must be sourced by the column driver circuits.
While the most trivial inversion scheme would be one in which every pixel on the display is first driven to its positive value during a first display cycle, and then driven to its negative value during the second display cycle, this scheme may cause the LCD to alternately display two slightly different images, which could be perceived by the viewer as a flicker in the display. Thus, more complex row inversion schemes are commonly employed to reduce or eliminate any such flickering. Typically, a row inversion technique is used such that, during a display cycle, the driving voltages applied to the columns of the array will alternate in polarity between successive row drive periods. Thus, if the pixels in a first row are driven with positive voltages during the first row drive period, then the pixels in the adjacent second row will be driven with negative voltages during the second row drive period, and so forth. During the next display cycle, the polarities are reversed. Hence, during the second display cycle, the pixels in the first row are driven with negative voltages during the first row drive period, and the pixels in the adjacent second row are driven with positive voltages during the second row drive period, and so forth.
When the row inversion scheme described above is used, a given column driver may, for example, need to establish +6 volts on its associated column during the first row drive period of the first display cycle, and then need to establish −6 volts on the same column during the immediately following row drive period. Thus, the column driver must transition from +6 volts to −6 volts, and
Erhart Richard Alexander
Harder Gerald T.
Cahill Sutton & Thomas P.L.C.
Chang Kent
National Semiconductor Corporation
LandOfFree
Power-saving circuit and method for driving liquid crystal... 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 liquid crystal..., 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 liquid crystal... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2513343