Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-10-30
2001-10-02
Chang, Kent (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S094000, C345S208000
Reexamination Certificate
active
06297792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an apparatus for driving a liquid crystal display panel, a liquid crystal display apparatus, and an electronic apparatus and, more particularly, to an apparatus and method for driving an active matrix drive type of a liquid crystal display panel using a two-terminal type non-linear element such as a MIM (metal insulator metal) element having a bidirectional diode characteristic, to a liquid crystal display apparatus provided with the driving apparatus (liquid crystal display module), and to an electronic apparatus provided with the liquid crystal display apparatus.
2. Description of Related Art
Conventional active matrix drive type of liquid crystal display panels include those using a two-terminal type non-linear element such as a MIM element having a bidirectional diode characteristic, as well as those using TFT (thin-film transistor) elements. MIM elements, etc., have a sharper change at a threshold and therefore have the advantage of being simple in structure and being manufactured by a simpler process in comparison with TFT elements.
FIG. 15
shows a liquid crystal display panel using a MIM element as the above-mentioned two terminal type non-linear element. This liquid crystal display panel is constructed in such a manner that, as shown in
FIG. 15
, a liquid crystal layer and a MIM element layer are connected in series to form one pixel region at each of intersections of a plurality of data signal lines ( . . . , Xi−1, Xi, Xi+1 . . . ) and a plurality of scanning signal lines ( . . . , Yj−1, Yj, Yj+1 . . . ), which are respectively arranged on a pair of substrates so as to form a matrix. A scanning signal drive circuit
81
is connected to the scanning signal lines while a data signal drive circuit
82
is connected to the data signal lines. From the scanning signal drive circuit
81
, a scanning signal is supplied to each scanning signal line. From the data signal drive circuit
82
, a data signal is supplied to each data signal line. In each pixel region, therefore, the MIM element can be driven in an on-off manner if the potential difference between the scanning signal and the data signal at the pixel region is set in a certain level relationship with the threshold voltage of the MIM element. When the MIM element is turned on, the liquid crystal layer connected to the MIM element is charged to turn on the pixel region. After charging for a predetermined time period, the MIM element is turned off and set in a high-impedance state. This state of the MIM element and the resistance of the pixel region set to a sufficiently large value enable the charge on the liquid crystal layer to be retained, thereby maintaining the on state of the pixel region. As the time for selecting and charging one of the pixel regions (hereinafter referred to as “selecting period”), a part of the time period through which the pixel region is maintained in the on state may suffice, as described above. Therefore, this selecting period can be set with respect to each of the scanning signal lines in a time division manner, thus enabling matrix drive of the plurality of pixel regions sharing the scanning signal lines and the data signal lines.
As a typical example of such a drive method, a drive method called four-value drive method can be mentioned. A four-value drive method is a method of using a two-value scanning signal and a two-value data signal and inverting the polarities of the scanning signal and the data signal about the middle value of the data signal with the passage of every horizontal period, for example, and inverting the polarities about the middle value of the data signal with the passage of every vertical period with respect to each scanning signal line. This method can be practiced with a comparatively simple circuit arrangement.
As mentioned above, the liquid crystal display panel using the MIM element is constructed so that the MIM element and the liquid crystal layer are connected in series in each pixel region. Accordingly, the voltage applied to the liquid crystal layer immediately after each selecting period depends upon the voltage applied to the MIM element at the corresponding time. The voltage applied to the MIM element at the corresponding time, i.e., the voltage applied to the MIM element when charging of the liquid crystal layer is stopped substantially completely, depends upon the current-voltage characteristic of the MIM element. Therefore, an error can occur in the voltage applied to each of the MIM elements due to a variation in the current-voltage characteristics of the MIM elements. Such a voltage error cannot be canceled out in the four-value drive method or the like, in which the polarities of scanning and data signals are simply inverted about the middle value of the data signal with the passage of every vertical period to simply invert the polarity of the voltage applied to the liquid crystal layer. Consequently, an error of the above-described kind occurs between the pixel regions to cause a variation in the voltage applied to the liquid crystal layer in each pixel region, which results in a display unevenness or the like.
A drive method called charge and discharge method has been proposed as a drive method for achieving an improvement in display characteristics in comparison with the four-value drive method. This drive method comprises making a MIM element conductive by charging and making the MIM element conductive by discharging after overcharging opposite in polarity to the charging about the middle value of a data signal, and is arranged for driving in charging and discharging modes, as shown in FIGS.
16
(B) and
17
(C). In the charging mode, a first selecting voltage (VS
1
) is supplied to one scanning signal line to charge the liquid crystal layer by the voltage of the potential difference from a data signal. On the other hand, in the discharging mode, a voltage−VPRE which is a precharge voltage opposite in polarity to the first selecting voltage (VS
1
) about the middle value of the data signal is supplied to overcharge the liquid crystal layer. Successively, a second selecting voltage (VS
2
) opposite in polarity to the precharge voltage about the middle value of the data signal is supplied to discharge the overcharged liquid crystal layer. Therefore, if the amount of discharge is controlled by the data signal during the time period through which the second selecting voltage (VS
2
) is supplied, the displaying state of the pixel region can be controlled.
For example, if, as shown in FIG.
16
(A), a data signal having values VH/2 and −VH/2 is supplied to data signal line Xi in every horizontal period (the period indicated by 1H in FIG.
16
(A)), and if, as shown in FIG.
16
(B), a scanning signal having a selecting potential such as that described above is supplied to scanning signal line Yj, voltage VB
1
applied to the liquid crystal layer in the pixel region at the intersection of data signal line Xi and scanning signal line Yj immediately after the end of the selecting period in the charging mode is given by the following equation.
VB
1
=(
VS
1
+VH/2
−VON
)−K·(
VS
1
−VH/2) (1)
wherein K is a capacitance ratio expressed as CM/(CM+CL) if the capacitance of the MIM element is CM and the capacitance of the liquid crystal layer is CL; K·(VS
1
−VH/2) represents a shift of the liquid crystal layer voltage caused through capacitive coupling at the moment when the MIM element is turned off; and VON is the voltage applied to the MIM element when charging of the liquid crystal layer is stopped substantially completely.
In the discharging mode, after overcharging at precharge voltage−VPRE, the accumulated charge is discharged by the second selecting voltage VS
2
, so that the voltage applied to the liquid crystal layer after the end of the selecting period is VS
2
−VH/2−VON. Accordingly, the voltage VB
2
applied to the liquid crystal layer immediately before the end of
Chang Kent
Oliff & Berridg,e PLC
Seiko Epson Corporation
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