Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-08-20
2004-03-30
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S082000, C315S169300
Reexamination Certificate
active
06714177
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-emitting display device that employs light-emitting elements such as organic EL (electroluminescent) elements and a driving method therefor.
2. Description of Related Art
In recent years, organic EL elements that are self-light-emitting elements employing organic compounds have been extensively studied, and dot matrix displays employing an organic EL element have been developed as well.
FIG. 1
shows an equivalent circuit of an organic EL element.
FIG. 2A
shows the current luminance properties of the organic EL element,
FIG. 2B
shows the voltage-current properties of the organic EL element, and
FIG. 2C
shows the voltage luminance properties.
As shown in
FIG. 1
, the organic EL element can be represented by a light-emitting element E having diode properties, and the parasitic capacitance C connected in parallel to the light-emitting element E and the resistance R connected in series with the light-emitting element E.
As shown in
FIGS. 2A through 2C
, the organic EL element emits light with luminance in proportion to current. In the case where the driving voltage is less than the predetermined light emission specifying voltage Vth, it allows current to hardly flow, resulting in practically no emission.
FIG. 3
shows a driving method of a prior art light-emitting element.
The driving method shown in
FIG. 3
is called the passive matrix driving method, in which the positive electrode lines A
1
through A
4
and the negative electrode lines B
1
through Bn (n is a natural number. Four positive electrode lines are used for ease of explanation) are arranged in a matrix (grid). To each intersection of the positive electrode lines and the negative electrode lines arranged in a matrix, light-emitting elements E
11
through E
4
n
are connected. Either one of the positive electrode lines or the negative electrode lines are selected for scanning at constant intervals of time and other lines are driven by the constant-current sources
21
through
24
, whereby light-emitting elements at arbitrary intersections are allowed for emitting light in synchronization with the scanning.
A voltage source may be used for the driving source, however, a current source may be preferably used to provide better reproducibility of luminance. This is because current luminance properties are more stable against changes in environmental temperature than voltage luminance properties, and current luminance properties of light-emitting elements have a linear proportionality.
In the case of
FIG. 3
, the driving source employs constant-current sources with the amount of constant current sufficient for the desired instantaneous luminance. Therefore, when the instantaneous luminance of light-emitting elements is desired to be equal to Lx, as shown in
FIGS. 2A through 2C
, the amount of constant current of a driving source is to be set to Ix. Also the voltage across both ends of the light-emitting element (hereinafter designated the light emission specifying voltage) becomes V
x
when light is emitted with desired instantaneous luminance (hereinafter designated a steady state of light emission).
There are two driving methods by means of said driving sources, namely, scanning negative electrode lines and driving positive electrode lines, and scanning positive electrode lines and driving negative electrode lines.
FIG. 3
shows the method of scanning negative electrode lines and driving positive electrode lines. The negative electrode line scan circuit,
1
, is connected to the negative electrode lines B
1
through Bn. The positive electrode line drive circuit
2
that comprises the current sources
21
through
24
and the drive switches
31
through
34
are also connected to the positive electrode lines A
3
through A
4
.
The negative electrode line scan circuit
1
performs scanning while sequentially switching the scan switches
11
through
1
n
over to the ground terminal sides at constant intervals of time, thereby providing negative electrode lines B
1
through Bn with ground potential (0V) in sequence. Furthermore, the positive electrode line drive circuit
2
controls the on and off of the drive switches
31
through
34
in synchronization with the switch scanning of said negative electrode line scan circuit
1
. This allows the positive electrode lines A
1
through A
4
to be connected with the constant-current sources
21
through
24
to supply driving current to light-emitting elements located at desired intersections. These negative electrode line scan circuit
1
and the positive electrode line drive circuit
2
are drive-controlled by means of a control circuit that is not shown.
For example, a case where the light-emitting elements E
11
and E
21
are lit is taken as an example. As shown in the drawing, when the scan switch
11
of the negative electrode line scan circuit
1
is switched to the ground side with the ground potential applied to the first negative electrode line B
1
, the drive switches
31
and
32
of the positive electrode line drive circuit
2
are preferably switched over to the sides of the constant-current sources to connect the constant-current sources
21
and
22
to the positive electrode lines A
1
and A
2
. By repeating the scanning and driving at a high speed, control is performed in a manner such that light-emitting elements at arbitrary positions are lit as if each light-emitting element emits light at the same time.
Other negative electrode lines B
2
through Bn except for negative electrode line B
1
that is being scanned are connected with the constant voltage sources
42
through
4
n
to apply a reverse bias voltage V
1
that has the same potential as the light emission specifying voltage V
x
. This prevents the light-emitting elements E
12
through E
1
n
and E
22
through E
2
n
, connected to the positive electrode lines A
1
and A
2
, emitting light accidentally.
The reverse bias voltage sources
41
through
4
n
, which provide the reverse bias voltage V
1
, are provided so that light-emitting elements connected to the intersections of the positive electrode lines A
1
and A
2
to be driven and the negative electrode lines B
2
through Bn not to be scanned (E
12
through E
1
n
and E
22
through E
2
n
in the case of
FIG. 3
) do not emit light accidentally. Accordingly, the voltage applied thereto is preferably set in a manner such that the voltage across both ends of the light-emitting element is equal to or less than the light emission threshold voltage Vth. However, the reverse bias voltage V
1
is best set to the light emission specifying voltage V
x
for the reason mentioned below. That is, letting V
1
=V
x
causes the voltage across both ends of the light-emitting element to become 0, and thus the current supplied by the drive source flows only into the light-emitting elements that are emitting light, thereby reproducing a desired luminance in accuracy.
As mentioned above referring to
FIG. 3
, the state of charge of each parasitic capacitance of each light-emitting element is as follows. The light-emitting elements E
11
and E
21
connected to the intersections of the positive electrode lines A
1
and A
2
to be driven and the negative electrode line B
1
to be scanned are forward charged. The light-emitting elements E
11
through E
1
n
and E
22
through E
2
n
connected to the intersections of the positive electrode lines A
1
and A
2
to be driven and the negative electrode lines B
2
, B
3
, and B
4
, which are not scanned, are not charged. The light-emitting elements E
31
and E
41
connected to the intersections of the positive electrode lines A
3
and A
4
not to be driven and the negative electrode line B
1
to be scanned are not charged. The light-emitting elements E
32
through E
3
n
and E
42
through E
4
n
, connected to the intersections of the positive electrode lines A
3
and A
4
, which are not driven, and the negative electrode lines B
2
, B
3
, and B
4
, which are not scanned, are reverse charged. (In the drawing, each light
Pioneer Corporation
Piziali Jeff
Shalwala Bipin
Sughrue & Mion, PLLC
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