Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-12-20
2004-08-17
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S090000
Reexamination Certificate
active
06778163
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a liquid crystal display device designed to have lower swing voltage to a data line in order to reduce power consumption. Additionally, the present invention relates to a driving circuit, to a driving method, and to electronic devices having the liquid crystal display device.
2. Description of Related Art
In recent years, liquid crystal display devices (LCD) have been used widely for various information processing devices, flat-screen TVs, and the like as display devices to replace cathode ray tubes (CRT).
These liquid crystal display devices can be classified into various types depending on the driving method and so on. An active-matrix-type LCD device, in which pixels are driven by switching elements, can be arranged as follows. Specifically, an active-matrix-type LCD device can include pixel electrodes arranged in a matrix, an element substrate provided with switching elements connected to each of the pixel electrodes, a counter substrate on which counter electrodes are formed to face the pixel electrodes, and liquid crystal sandwiched between both of these substrates.
In this arrangement, when an on-voltage is applied to a scanning line, the switching element connected to the scanning line becomes conductive. In the conductive state, if the voltage signal corresponding to a gray scale (density) is applied to an element electrode via a data line, the charge corresponding to the voltage signal is stored in a liquid crystal capacitor in which the liquid crystal is sandwiched between the element electrode and counter electrode. After the charge is stored, even if an off-voltage is applied to the scanning line to make the switching element nonconductive, the charge stored in the liquid crystal is maintained by the capacitance of the liquid crystal capacitor itself, in addition to the accompanying storage capacitor.
In this manner, by driving each switching element and controlling the amount of charge to be stored according to the gray scale, the orientation of the liquid crystal changes. Thus, the gray level is changed for every pixel, thereby making it possible to perform display as desired.
Also, in recent years, a scheme has been proposed to arrange D/A converters for every data line to convert gray scale data indicating the gray level of a pixel into an analog signal. With this scheme, image data is digitally processed immediately before it is output to the data line, thus deterioration of the display quality due to variations in analog circuit characteristics is prevented, thereby making it possible to obtain a high quality display.
For performing gray-scale display, it is necessary to apply a voltage with a range corresponding to values from the minimum gray level to the maximum gray level to the pixel electrodes in two separate ways, namely, positive polarity and negative polarity. Accordingly, the swing voltage between the minimum value and the maximum value which is required to be applied to a pixel electrode becomes greater than the swing of the logic level of CMOS circuits and so on.
SUMMARY OF THE INVENTION
However, increasing the swing voltage applied to the pixel electrode inevitably results in an increase in the swing voltage applied to the data line. If the swing voltage applied to the data line is increased, electrical power is wastefully consumed by a parasitic capacitance on the data line. Such a result is contrary to the demands generally made on liquid crystal devices for lowering the power consumption.
Also, when the swing voltage applied to the data line is increased, the output swing voltage from the D/A converter needs to be increased. Thus, the composition of the D/A converter becomes large, or a separate level shifter becomes necessary to amplify the output voltage.
Accordingly, the present invention is made in view of the foregoing, and an object of the invention is to keep the swing voltage applied to various signals, especially a data line, small, thereby providing a liquid crystal device, a driving circuit, a driving method, and electronic devices which are intended to reduce power consumption.
In order to accomplish the above-described object, in a liquid crystal device according to a first aspect of the present invention, there is provided a liquid crystal device including a scanning line to which an on-voltage is applied and then an off-voltage is applied, a liquid crystal capacitor having a liquid crystal sandwiched between a counter electrode and a pixel electrode, a D/A converter applying a voltage, which corresponds to gray scale data indicating a gray level and to a writing polarity of the liquid crystal, to a data line when an on-voltage is applied to the scanning line, and a switching element inserted between the data line and the pixel electrode, the switching element being turned on when the on-voltage is applied to the scanning line, and being turned off when an off-voltage is applied.
The liquid crystal device can further include a storage capacitor having one terminal connected to the pixel electrode, wherein, when the writing polarity during the period when the on-voltage is applied to the scanning line is equivalent to positive-polarity writing, the voltage of the other terminal is shifted to a high level when the off-voltage is applied to the scanning line, and when the writing polarity during the period when on-voltage is applied to the scanning line is equivalent to negative-polarity writing, the voltage of the other terminal is shifted to a low level when the off-voltage is applied to the scanning line.
With this arrangement, when on-voltage is applied to the scanning line, the switching element connected to the scanning line can be turned on, thereby the charge corresponding to the applied voltage is stored to the liquid crystal capacitor and storage electrode. When the switching element is turned off thereafter, the voltage of the other terminal of the storage capacitor shifts, and the voltage of one terminal of storage capacitor is raised by that amount (or lowered). At the same time, the amount of charge raised (or lowered) is distributed to the liquid crystal capacitor, thus the voltage effective value corresponding more than (or less than) the applied voltage to the data line is applied to the liquid crystal capacitor. In other words, when compared with the swing voltage applied to the pixel electrode, the swing voltage of the voltage signal applied to the data line is kept small. Thus, wasteful power consumption by parasitic capacitor on the data line is kept small, thereby making it possible to reduce power consumption. Additionally, enlarging the D/A converter is prevented, or level shifter for enlarging the output voltage of a D/A converter becomes unnecessary, thereby making it possible to narrow the pitch of a data line so as to achieve high precision.
Here, in the first aspect of the present invention, it is preferable to have the arrangement that in the case where the writing polarity is one of positive polarity writing and negative polarity writing, the display device further can further include a first power feeding line which is fed with a first voltage during a preset period, and which is fed with a second voltage which is higher than the first voltage during a set period after the preset period, a second power feeding line which is fed with a third voltage which is higher than the second voltage during the preset period, and which is fed with a fourth voltage which is lower than the third voltage and higher than the second voltage during the set period, and a selector to select one of the first and second power feeding lines during the preset period, and to select the other one of the first and second power feeding lines during the set period, wherein the D/A converter generates a supply voltage to the data line using the corresponding voltage selected by the selector during the preset period and the set period.
If the D/A converter is arranged such that in the case of using a first voltage during preset period, it uses a fou
Oliff & Berridg,e PLC
Patel Nitin
Seiko Epson Corporation
Shankar Vijay
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