Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-02-13
2001-08-21
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S205000, C345S097000, C345S090000
Reexamination Certificate
active
06278426
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display apparatus. More particularly, the present invention relates to a liquid crystal display apparatus driven by thin-film transistors.
BACKGROUND OF THE INVENTION
An active matrix liquid crystal display which is driven by a thin film transistor (TFT) is used for a notebook-type personal computer in which the thinness of the display and low power consumption are critical. At present, the TN-type liquid crystal is used for the active matrix liquid crystal display. However, there are some problems. These include its field angle dependence and delayed response to motions. To resolve these problems, various liquid crystal materials and display modes have been studied.
An anti-ferroelectric liquid crystal which does not have a threshold value as a liquid crystal provides a wide field angle because the liquid crystals move within a cell plane. Its spontaneous polarization is able to increase the speed of liquid crystal motion by means of an electromagnetic field control. This is effective in increasing optical response speed and the resulting display performance.
Assume that an anti-ferroelectric liquid crystal is driven by the active matrix technique. When there are many pixels, that is, many scanning lines, the response speed (several tens of microseconds) of the liquid crystal is faster than the time frame cycle. However, compared to the time required for selecting one line, it is slower. As a result, when the TFT is turned on, the liquid crystal is not able to move freely but instead, the liquid crystal molecules try to move by the voltage charged when the TFT is turned off. While this is not a problem for a normal TN-type liquid crystal, a problem was observed for an anti-ferroelectric liquid crystal. That is, anti-ferroelectric liquid crystal molecules move by spontaneous polarization. Electrons, charged to the storage capacitance of the liquid crystal when the TFT is turned on, flow into the liquid crystal side when the TFT is turned off. As a result of this phenomenon the pixel voltage decreases.
Because of this phenomenon, the voltage applied to the liquid crystal decreases, decreasing the contrast and response speed, causing poor display properties. Also, the amount of voltage decrease from the original signal voltage of the pixel is dependent on the response speed of the liquid crystal, the resistivity of a conducting transistor, and writing time. A non-uniform display was observed when temperature distribution from back lighting exists within the screen or when the gate has a large resistivity and the gate pulse width changes due to the distance between the gate and the power supply end. It is understood this is due to the phenomena mentioned above.
This phenomenon was observed for different liquid crystals, besides anti-ferroelectric liquid crystals. Such crystals have a large induction and change the induction levels due to the liquid crystal molecule movement, such as cholesteric liquid crystals and the like.
As described, a problem with the prior art is that the liquid crystal cannot respond completely within the selected time when the matrix is driven. Therefore the display is degraded.
Another problem was observed in the liquid crystal display using, for example, the TN-type liquid crystal and the like, which requires that the drive force be derived from an alternating current frequency source to eliminate flicker (e.g., 60 Hz). Therefore, even when the same display image is kept, the display is driven by alternating current, consuming unnecessary power.
SUMMARY OF THE INVENTION
The object of the present invention is to prevent the degradation of the display and to provide a liquid crystal apparatus which is capable of reducing power consumption.
In a preferred embodiment the present invention includes a liquid crystal display apparatus having a plurality of pixel portions. A plurality of scanning lines are coupled to the pixel portions for supplying scanning signals to the plurality of pixel portions. A first substrate is included having a plurality of signal lines for supplying display signals to the plurality of pixel portions.
Also included is a second substrate facing the first substrate, and a liquid crystal layer arranged between the first substrate and the second substrate. Means are included for applying an electromagnetic field to the liquid crystal layer at positions corresponding to selected pixel portions, wherein each of the pixel portions further includes switching means coupled to the signal lines.
Holding means are included and coupled to the switching means for holding a display signal voltage supplied from the signal line via the switching means. A voltage supply means is coupled to the liquid crystal layer for supplying a voltage corresponding to the display signal voltage held by the holding means. The liquid crystal display apparatus also includes a pixel electrode connected to the output of the voltage supply means. Finally, control means are provided for changing the impedance state of the pixel electrode by controlling the output impedance of the voltage supply means.
The liquid crystal layer is formed of liquid crystal with spontaneous polarization wherein the voltage decreases with just “scanning time”.
In another preferred embodiment the control means places the pixel electrode into a low impedance state for a predetermined period of time to supply the display signal to the liquid crystal layer, and then places the pixel electrode into a high impedance state.
According to the apparatus incorporating the principles of the present invention, even after the scanning period is finished, the signal held by the holding means can supply the signal to the pixel electrode. With this configuration, a desired voltage can be applied to the liquid crystal layer for a longer period of time than the scanning period. Therefore, the liquid crystal can respond fast enough to prevent the degradation of contrast, response speed, and resulting degradation of display performance. Also, after supplying the voltage from the pixel electrode, the pixel electrode can be in the high impedance state, thus reducing the power consumption.
In a further embodiment of the present invention, a plurality of voltage supply means can be formed. With this configuration, the pixel electrodes can be switched to control the period during which it is in the low impedance state. By doing this, when the same display image is kept, the liquid crystal can be driven by alternating current without supplying the scanning signal or display signal; thus the power consumption is reduced.
In a further preferred embodiment the liquid crystal display apparatus incorporating the principles of the present invention includes a plurality of pixel portions, and a plurality of scanning lines coupled to the pixel portions for supplying scanning signals to the plurality of pixel portions. A first substrate is included having a plurality of signal lines for supplying display signals to the plurality of pixel portions. A second substrate is provided facing the first substrate.
A liquid crystal layer is arranged between the first substrate and the second substrate. Also included are means for applying an electromagnetic field to the liquid crystal layer at positions corresponding to selected pixel portions, wherein each of the pixel portions further includes a switching element and a first capacitor coupled to the switching element for holding a display signal voltage supplied from the signal line via the switching element.
A first transistor is included having a gate coupled to the first capacitor. A second transistor is included having a gate and having a source or drain coupled to the source or drain of the first transistor. A second capacitor is provided coupled between the gate of the first transistor and the gate of the second transistor. Also included is a pixel electrode coupled to the liquid crystal layer and to one of the sources or drains of the first transistor and the source or drain of the second transistor for su
Anderson Kill & Olick P.C.
Hjerpe Richard
Kabushiki Kaisha Toshiba
Nguyen Francis
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