Computer graphics processing and selective visual display system – Display driving control circuitry – Display power source
Reexamination Certificate
1999-04-16
2003-10-28
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Display driving control circuitry
Display power source
C345S204000, C345S052000, C345S214000
Reexamination Certificate
active
06639590
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for controlling a liquid crystal display device which prevents deterioration of the liquid crystal by quickly removing the charge stored in each liquid crystal layer after, for example, turning off of the power supply, a device for driving the liquid crystal display device, the liquid crystal display device, and an electronic apparatus incorporating the liquid crystal display device.
2. Description of the Related Art
In general, an active matrix type liquid crystal display device mainly consists of an element array substrate having a switching element provided on each of a plurality of pixel electrodes arranged in a matrix, an opposite substrate having a color filter or the like formed thereon, and a liquid crystal filling the space between the two substrates. In this structure, a liquid crystal layer is formed by each pixel electrode, the opposite substrate and the liquid crystal filling the space therebetween.
In the structure described above, applying an ON (selection state) signal onto a switching element leads to a conductive condition of that switching element. As a result, a predetermined amount of charge is stored in the liquid crystal layer connected to that switching element. Even when the switching element is brought into an OFF state by applying an OFF (non-selection state) signal after storage of the charge, stored charge in the liquid crystal layer is maintained, provided that the liquid crystal layer has a sufficiently high resistance. When the amount of stored charge is controlled by driving the individual switching elements, a change occurs in the alignment state of the liquid crystal for each pixel, thus permitting display of a predetermined information. At this point, because charge needs to be stored for each liquid crystal layer only during a portion of the period, it is possible to achieve multiplex driving using in common with the scanning lines and the data lines for the plurality of pixels by selecting individual scanning lines in a time-sharing manner.
Applicable switching elements are broadly classified into three-terminal type switching elements, such as a thin-film transistor (TFT: Thin Film Transistor) or an MOS type transistor, and two-terminal type switching elements, such as a thin-film diode (TFD: Thin Film Diode) having a non-linear characteristic. These three-terminal type and two-terminal type switching elements, having a non-linear current-voltage characteristic, are referred to also as non-linear elements.
According to the structure in which a supply of driving signal is discontinued simultaneously with the turning-off of the power supply, when turning off the power supply for a liquid crystal display device, the electric field, which has been applied to the liquid crystal layer at the moment of the stoppage of the driving signal, remains as it is, and the liquid crystal layer turns into a state that a DC voltage is applied thereto. If a DC voltage is continuously applied onto the liquid crystal layer in this state, material properties of the liquid crystal vary, leading to deterioration such as a reduced resistivity, and hence to a reduced service life of the liquid crystal display device. Therefore, it is desirable to adopt a structure in which at the time of the turning-off of the power supply for a liquid crystal display device, the supply of driving signals is continued until the charge stored in the liquid crystal layer reaches a null level.
However, because the discharge time constant of the stored charge is dependent upon various factors such as resistance and size of the pixel electrode, the material for the liquid crystal and the distance between the substrates, there is a problem that the period of time required for the charge stored in the liquid crystal layer to reach null varies for every different pixel and every liquid crystal display device. This means that the period over which the driving signals must be maintained after turning off the power supply is inconsistent and, hence, leads to a secondary problem of difficulty encountered in designing a driving signal supply circuit.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a method for controlling a liquid crystal display device, wherein liquid crystal layers are quickly cleared of any residual charge without dependency of the clearing time on individual devices, thereby suppressing degradation of the liquid crystal. The invention also is aimed at providing a device for driving the liquid crystal display device, the liquid crystal display device, and an apparatus incorporating the liquid crystal display device.
With a view in achieving the aforementioned object, the first aspect of the present invention provides a method for controlling a liquid crystal display device of the type in which desired images are displayed through control of charge amounts in liquid crystal layers of the liquid crystal display device, the method which may consist of detecting turning-off of a power supply connected to the liquid crystal display device; and upon detection of the turning-off of the power supply, electrically connecting the liquid crystal layer to a fixed potential.
According to this control method, the liquid crystal layer is connected to a fixed potential such as a grounding potential upon detecting the turning-off of the power supply. The liquid crystal layers are thus quickly cleared of the charges at a constant rate. As a result, a DC voltage is never applied on the liquid crystal for a long period of time, thus permitting prevention of deterioration of the liquid crystal. It is also possible to set a period of time ending when the charge stored in the liquid crystal layer becomes null without depending upon factors such as the resistance and size of the pixel electrode of the liquid crystal display panel, the material for the liquid crystal, and the distance between the substrates and so on.
Further, in the aforementioned controlling method of the liquid crystal display device, it is desirable to electrically connect the signal line, which applies a voltage onto the liquid crystal layer, to the foregoing fixed potential upon detecting the turning-off of the power supply. Removal of charges from the liquid crystal layers can indirectly be achieved even by such a simple measure as to connect the signal lines to the fixed potential.
Further, in the aforementioned controlling method of the liquid crystal display device, it is desirable to electrically connect the signal line, which is electrically connected to the liquid crystal layer, to a predetermined voltage supplying line, and to connect the predetermined voltage supplying line to the fixed potential, upon supplying the turning-off of the power supply. Removal of charges from the liquid crystal layers can indirectly be achieved by means of simple arrangement and control, using a switch that first connects the predetermined voltage supplying line to the signal lines which supply a voltage to the liquid crystal layers and then connects the predetermined voltage supplying line to the fixed potential.
In the aforementioned method of controlling the liquid crystal display device, the arrangement is preferably such that the predetermined voltage supplying line includes a first voltage supplying line for supplying a positive voltage relative to the fixed potential and a second voltage supplying line for supplying a negative voltage relative to the same, and that the signal lines are alternately connected to the first and second voltage supplying lines. Since there are thus provided the supplying lines that supplies the voltages that are positive and negative relative to the fixed potential, and these two supplying lines are alternately connected to the signal line and at the same time these two lines are connected to the fixed potential, it is possible to remove the charge from the liquid crystal layer as the positive and negative potentials of the supplying lines converge from posi
Kawakami Hisanori
Takahashi Kotoyoshi
Nelson Alecia
Oliff & Berridg,e PLC
Saras Steven
Seiko Epson Corporation
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