Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-07-24
2004-09-14
Wu, Xiao (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S100000
Reexamination Certificate
active
06791523
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to an electro-optical panel, a method for driving the electro-optical panel, an electro-optical device including the electro-optical panel, and electronic equipment including the electro-optical panel.
2. Description of Related Art
Conventional electro-optical devices, such as active-matrix liquid-crystal display panels, typically include an element substrate, a counter substrate opposed to the element substrate, and a liquid crystal interposed between the two substrates. The element substrate typically includes a plurality of data lines, a plurality of scanning lines, a matrix of pixel electrodes at locations corresponding to the intersections of the data lines and the scanning lines, and thin-film transistors (hereinafter referred to as TFTs) respectively arranged for the pixel electrodes. Typically, the counter substrate includes a counter electrode and a color filter.
FIG. 18
shows an equivalent circuit of a pixel used in a conventional electro-optical panel. As shown, the pixel includes a TFT
1
, a capacitor
2
of the liquid crystal and a storage capacitor
3
. Both capacitors
2
and
3
are connected to a drain electrode of the TFT
1
. The TFT
1
is configured with the gate electrode thereof being connected to a scanning line
4
and with the source electrode thereof being connected to a data line
5
. The liquid-crystal capacitor
2
is formed of the liquid crystal interposed between the pixel electrode and the counter electrode.
When a scanning signal (a selection voltage) is applied to the TFT
1
through the scanning line
4
in this circuit, the TFT
1
is turned on. When an image signal is applied to the pixel electrode through the data line
5
with the TFT
1
in a conductive state, a predetermined charge is stored in the liquid-crystal capacitor
2
between the pixel electrode and the common electrode. When the TFT
1
is turned off in response to a non-selective voltage applied subsequent to storage of the charge, the charge at the liquid-crystal capacitor
2
is maintained. When the amount of charge to be stored is controlled using the TFT
1
, the liquid crystal varies the orientation state pixel by pixel, thereby displaying predetermined information.
Since an off resistance of the TFT
1
is finite, charge stored in the liquid-crystal capacitor
2
is gradually discharged with time. The storage capacitor
3
is arranged to increase a time constant of discharge. Charge storage characteristics of the liquid-crystal capacitor
2
are thus improved. As a result, a contrast ratio of the panel is improved, and vertical cross-talk is controlled.
SUMMARY OF THE INVENTION
Since the field frequency of the image signal is 60 Hz in a NTSC system, the electro-optical panel is typically driven at 60 Hz. Depending on applications, the electro-optical panel has a low field frequency, for example, as low as 15 Hz or 30 Hz, or a high frequency, for example, as high as 120 Hz or 240 Hz.
To write the voltage of the data line
5
to the above-referenced pixel, the TFT
1
is turned on during a selective period and the voltage of the data line
5
is written to the liquid-crystal capacitor capacitance
2
through a time constant determined by the liquid-crystal capacitor, the storage capacitor, and the on resistance of the TFT
1
. With the storage capacitor
3
added to the liquid-crystal capacitor
2
, write time required to write the voltage on the pixel is prolonged.
With the storage capacitor
3
added, the voltage of the data line
5
is not sufficiently written to the liquid-crystal capacitor
2
when the field frequency is high and the selective period is short. On the other hand, with the storage capacitor
3
not added, holding a write voltage is difficult when the field frequency is low with a long holding period.
In the conventional electro-optical panel, typically the storage capacitor is determined to be compatible with both the selective period and the holding period. Varying these periods in accordance with the field frequency has not been contemplated.
The invention has been developed in view of the above problem, and it is an object of the present invention to assure the writing of the voltage to the pixel and the holding of the written voltage at the pixel even if the selective period and the holding period are varied.
To achieve the above object, an electro-optical panel of one exemplary embodiment of the invention includes a first substrate having a plurality of scanning lines and a plurality of data lines formed thereon, a second substrate opposed to the first substrate, and an electro-optical material interposed between the first substrate and the second substrate. The first substrate includes a plurality of control lines respectively formed corresponding to the scanning lines. A first switching element, which is arranged at each intersection of one of the scanning lines and one of the data lines, is controlled for an on and off operation in response to a scanning signal supplied through the scanning line. The first switching element is connected between the data line and a pixel electrode. A second switching element, which is arranged at each intersection of one of the scanning lines and one of the data lines, is controlled for an on and off operation in response to a control signal supplied through the control line. The second switching element is connected between the pixel electrode and a storage capacitor.
The electro-optical panel controls the second switching element in an on and off operation in response to the control signal, thereby connecting the storage capacitor to the pixel electrode or disconnecting the storage capacitor from the pixel electrode as necessary. An electro-optical material is switched based on whether or not the storage capacitor is connected to the electro-optical material capacitance format of the pixel electrode. A time constant involved in feeding a voltage from the data line to the pixel electrode through the first switching element is determined by an on resistance of the first switching element and the pixel capacitance value. A time constant involved in holding the voltage at the pixel is determined by the off resistance of the first switching element and the pixel capacitance value. The pixel capacitance value is modified depending on whether the storage capacitor is connected to the electro-optical material capacitance. By generating the control signal in accordance with the write period and the holding period, the voltage of the data line is reliably written on the pixel, and the voltage written on the pixel is reliably held. In this way, even if the field frequency is dynamically changed, high image quality is maintained.
Preferably, in another exemplary embodiment of the invention, an electro-optical panel further includes a first converter which converts input image data into point-at-a-time image data, a second converter which converts the point-at-a-time image data into line-at-a-time image data, a data line signal supplier which supplies each data line with a data line signal that is generated in accordance with the line-at-a-time image data, and a scanning line driver which generates a scanning signal for successively selecting the scanning lines and supplies the scanning lines with the scanning signal. Since the electro-optical panel includes the drivers for driving the data lines and the scanning lines, no driver circuits need to be arranged external to the electro-optical panel. A compact design is thus implemented in an apparatus that incorporates the electro-optical panel.
An electro-optical device according to another exemplary embodiment of the invention includes the above-referenced electro-optical panel and a control signal generator which generates the control signal in accordance with a field frequency. In this electro-optical device, the second switching element is controlled to disconnect the storage capacitor when the field frequency is high and the write period is short. When the field frequency is high and the holding
Fujita Shin
Ozawa Tokuro
Nguyen Kevin M.
Oliff & Berridg,e PLC
Seiko Epson Corporation
Wu Xiao
LandOfFree
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