Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
2000-10-20
2004-11-16
Liang, Regina (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S092000, C345S206000
Reexamination Certificate
active
06819317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention refers to a liquid crystal display and drive method thereof, and particularly to a TFT active matrix display and drive method thereof for low power consumption.
2. Related Background Art
A prior art liquid crystal display is disclosed, for example, as a liquid crystal display to get a high definition display image in the Official Gazette of Japanese Patent Laid-Open NO. 133629/1998 discloses. Another example is disclosed in the Official Gazette of Japanese Patent Laid-Open NO. 113876/1997 where a polarity inversion circuit is connected to an opposite electrode to ensure stable operation and low power loss. The Official Gazette of Japanese Patent Laid-Open NO.104246/1995 discloses an active matrix liquid crystal drive for lower power consumption.
The following describes the prior art TFT active matrix drive system:
A line sequential scanning system is used to drive the TFT active matrix liquid crystal display, and one scanning pulse is applied to each scanning electrode for each frame time. About 1/60 second is appropriate as one frame time. These pulses are applied downwardly from the top of the panel to the bottom in sequence at differently timed intervals. Consequently, 480 gate wires are scanned in one frame in a liquid crystal display having a 640×480-dot pixel configuration, so the time range of the scanning pulse is about 35 microseconds.
Meanwhile, the liquid crystal drive voltages applied to liquid crystals for one-row pixels where scanning pulses are applied are simultaneously applied to the signal electrode in synchronization with scanning pulses. In the selection pixel where gate pulses are applied, the gate electrode voltage of the TFT connected to the scanning electrode is increased to turn on the TFT. In this case, liquid crystal drive voltage is applied the display electrode via the source and drain of the TFT to charge the pixel capacity comprising the liquid crystal capacity formed between the display electrode and opposite electrode formed on the opposite substrate, plus load capacity assigned to the pixel. This operation is repeated to allow liquid crystal application voltage to be applied repeatedly to the pixel capacity of the all panel surfaces for each frame time.
Since a.c. voltage is required to drive the liquid crystal, the voltage with the polarity inverted for each frame time is applied to the signal electrode. As a result, even if the image to be displayed does not change, much of the power to drive the panel is consumed to repeatedly charge or discharge, at every gate selection, the capacity at the crossing portion between scanning and signal lines or the capacity of the liquid crystal between the line and the opposite electrode formed on all surfaces of on the opposite substrate.
The Official Gazette of Japanese Patent Laid-Open NO.258168/1997 discloses a technology to solve said problem and to implement a liquid crystal display of lower power consumption.
The liquid crystal display disclosed in the Official Gazette of Japanese Patent Laid-Open NO.258168/1997 has the following components in each of the pixel areas enclosed by multiple scanning electrodes and multiple signal electrodes of a substrate; (1) a display data retention circuit connected to corresponding scanning electrodes and signal electrodes to capture and retain the display data from signal electrodes in response to scanning signals, (2) a switching element connected to the display data retention circuit wherein switching operation is controlled by said circuit, and (3) a display electrode connected to the switching element. Display electrode drive voltage is changed in response to the data retained by the display data retention circuit, thereby controlling pixel indications.
The display data retention circuit has a sampling TFT where the gate is connected to the corresponding scanning electrode and the drain is connected to the corresponding electrode, and a sampling capacitor connected to the sampling TFT source. The switching element has a switching TFT where the gate is connected to the source of the display data retention circuit and the source is connected to said display electrode. The sampling capacitor comprising said display data retention circuit and the drain of the switching TFT connected to the display electrode are connected to the common electrode.
The display data retention circuit sends to the sampling capacitor via the sampling TFT the display data signal voltage fed from the signal electrode in synchronism with the scanning signal to select the scanning electrode, and retains the pixel display data as voltage information.
The liquid crystal drive voltage controlling the light and dark pattern of the pixel is determined by a.c. voltage applied to the liquid crystal held closely between the display electrode and opposite electrode. When liquid crystal drive power voltage is applied to the opposite electrode, the voltage is applied to the liquid crystal if the switching TFT is on, but not applied to the liquid crystal if said switching TFT is off. This arrangement allows liquid crystal applied voltage of each pixel to be controlled by the display data signal voltage in the pixel.
In this case, the display data retention circuit can continue to retain the display data until voltage across the sampling capacitor as display data signal voltage is discharged below the threshold voltage of the switching TFT due to leakage of switching TFT or the like. Time until said discharge occurs depends on the leakage current value of the switching TFT and the capacity of the sampling capacitor. Normally, the TFT leakage current value is very small, but is sufficiently longer than 16.6 ms—a representative value of frame time. Moreover, liquid crystal drive voltage can be applied to all pixels in one operation from the opposite electrode. For pixels where display contents do not change, display can be maintained by application of liquid crystal drive voltage alone if the display data signal voltage is changed and the switching TFT is turned on or off. Scanning signal and display data signal voltage should be applied only when display contents are to be rewritten. This ensures excellent display while keeping low power consumption inside the panel.
However, said prior art has a problem that much is required to rewrite the image in response to changes of display contents.
Voltage across the sampling capacitor changes in response to changes of display contents, and this involves changes in the state of the switching TFT. In this case, if the switching TFT changes from OFF to ON state, the voltage of the display electrode will become the same as that of the common electrode immediately. Voltage will be applied to the liquid crystal to get the desired display.
However, if switching TFT is changed from the ON to OFF state, the display electrode is in the floating mode while voltage between the display electrode and opposite electrode is retained, so d.c. voltage will be applied to the liquid crystal between the display electrode and opposite electrode. The desired display cannot be obtained. This d.c. voltage is reduced by liquid crystal leakage, but the time constant for this reduction is long. Complete switching takes much time.
Although the TFT leakage current is very small, it is not zero. The voltage stored in the sampling capacitor cannot be retained for a long time. This makes it necessary to make up for the voltage reduced by leakage whenever required, even if there is no change in display contents. In other words, overwriting is sometimes necessary. When overwriting, the voltage of the sampling capacitor is changed by making up for it. However, if this change affects the state of the switching TFT, the image will change; this is not preferred. In other words, this requires the sampling capacitor voltage to be overwritten without changing the state of the switching TFT.
When overwriting, pulse signals are normally applied to the scanning electrode, and voltage corresponding to the display of pixels fo
Hoshino Minoru
Komura Shinichi
Satou Hideo
Liang Regina
Nguyen Jennifer T.
LandOfFree
Liquid crystal display and drive method thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Liquid crystal display and drive method thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal display and drive method thereof will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3314666