Luminous display and its driving method

Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix

Reexamination Certificate

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Details

C345S078000

Reexamination Certificate

active

06351255

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a luminous display using luminous elements such as organic EL (electro-luminescence), and its driving method.
2. Description of the Related Art
Recently, attention has been paid to an organic EL display as a self-luminous type display. Development of organic materials has advanced, and its service life has increased. Furthermore, it is thin, and is high in luminescence, and it is low in power consumption including its back light. Hence, its screen is improved in definition and increased in size.
The organic EL is a capacitive element. Therefore, it suffer from a problem that, in a simple matrix drive system popularly employed as a matrix display drive method, the parastic capacitance of the luminous element is charged, and the resultant charge makes the luminescence of the element insufficient.
This problem will be described concretely below:
A drive method shown in
FIG. 6
is called “a simple matrix drive system. Anode lines A
1
through A
256
and cathode lines B
1
through B
64
are arranged in matrix. At the intersections of the anode lines and the cathode lines thus connected in matrix, luminous elements E
1
.
1
through E
256
.
64
are connected. The anode lines or the cathode lines are scanned at predetermined time intervals, while, in synchronization with this scan, the other lines are driven with constant currents
21
through
2256
which are employed as drive sources, so that the luminous elements at the desired (optional) intersections are caused to emit light. Each of the constant current sources
21
through
2256
supplies a constant current I.
In the case of
FIG. 6
, the luminous elements E
11
and E
12
are turned on. That is, the scanning switch
51
is switched over to 0V (side), and the cathode line B
1
is scanned.
For the remaining cathode lines B
2
through B
64
, the scanning switch
52
through
564
function, to apply reverse bias voltage Vcc (10V) to them B
2
through B
64
.
The application of the reverse bias voltage is to prevent current supplied from the constant current sources
21
through
2256
from being applied to the cathode lines which are not scanned, It is preferable that the value Vcc is substantially equal to the voltage value applied between the luminous elements to cause the luminous elements to emit light at a desired instantaneous brightness; that is, a voltage of the luminous element which are connected between a constant current source and ground.
The anode lines A
1
and A
2
are connected through drive switches
61
and
62
to the constant current sources
21
and
22
, and shunt switches
71
and
71
are kept opened. For the remaining anode lines A
3
through A
256
, the constant current sources are opened, and the shunt switches
73
through
7256
are at ground potential.
Accordingly, in the case of
FIG. 6
, the luminous elements E
1
.
1
and E
2
.
1
are biased forwardly, so that drive currents from the constant current sources flow as indicated by the arrows in
FIG. 6
, whereby only two luminous elements E
1
.
1
and E
2
.
1
emit light.
The operations of the scanning switches
51
through
564
, the drive switches
61
through
6256
, the shunt switch
71
through
7256
are controlled by a luminescence control circuit
4
+ to which luminous data are applied.
With the aid of the scanning switches
52
through
564
, reverse bias voltage is applied to first terminals of the luminous elements connected at the intersections of the cathode lines B
2
through B
64
and the anode lines A
1
and A
2
, while the constant current sources
21
and
22
supply a voltage, which is substantially equal to the reverse bias voltage, to the second (remaining) terminals thereof. Therefore, no current flows in the luminous elements. Accordingly, no parastic capacitances of the luminous elements are charged.
Reverse bias voltage is applied to the luminous elements connected at the intersections of the cathode lines B through B
64
and the anode lines A
3
through A
256
. Therefore, the parastic capacitances (the capacitors shaded) of the luminous elements are reversely charged as indicated in
FIG. 6
(the potential on the side of cathodes of the element being higher).
When, under the condition that the parastic capacitances are reversely charged, the cathode lines are scanned to cause the next luminous element to emit light, then the period of time required for the next luminous element to activate, and accordingly, it is impossible to perform a high speed scanning operation. This will be described with reference to
FIGS. 7A and 7B
.
FIGS. 7A and 7B
show only the luminous elements E
3
,
1
through E
3
,
64
connected to the anode line A
3
in FIG.
6
.
FIG. 7A
is for a description of the scanning of the cathode line B
1
, and
FIG. 7B
is for a description of the scanning of the cathode line B
2
. In this connection, let us consider the case where, when the cathode line B
1
is scanned, the light emission of the luminous element E
3
,
1
is not carried out, and when the cathode line B
2
is canned, the light emission of the luminous element E
3
,
2
is carried out.
As shown in
FIG. 7A
, in the case where, when the cathode line B
1
is scanned, the anode line A
3
is not driven, the luminous elements E
3
,
2
through E
3
,
64
(other than the luminous element E
3
,
1
) connected to the cathode line B
1
which is being scanned are charged as shown in
FIG. 7A
by the reverse bias voltage Vcc applied to the cathode lines B
2
through B
64
.
As shown in
FIG. 7B
, if, when the scanning is shifted to the cathode line B
2
, the anode line A
3
is driven to cause the luminous element E
3
,
2
to emit light, then not only the parastic capacitance of the luminous element E
3
,
2
is charged, but also current flows to the parastic capacitances of the luminous elements E
3
,
3
through E
3
,
64
connected to the other cathode lines B
3
through B
64
as indicated by the arrows; that is, those parastic capacitances are charged.
On the other hand, a luminous element has a characteristic that its luminescent brightness changes with a voltage across it. Hence, if the voltage across it is not increased to a predetermined value, the steady light emission (the light emission with a desired instantaneous brightness) thereof is not achieved.
In the case of the conventional drive method, as shown in
FIGS. 7A and 7B
, when the anode line A
3
is driven to cause the luminous element E
3
,
2
to emit light which is connected to the cathode line B
2
, then not only the parastic capacitance of the luminous element E
3
,
2
to be caused to emit light but also the other luminous elements E
3
,
3
through E
3
,
64
connected to the anode line A
3
are charged. Therefore, it takes time to charge the parastic capacitance of the luminous element E
3
,
2
to be caused to emit light; that is, it is impossible to quickly increase the voltage across the luminous element E
3
,
2
to a predetermined value which is connected to the cathode line B
2
.
Accordingly, the conventional method is disadvantageous in that the time required for a luminous element to emit light is slow, and it is impossible to perform a high speed scanning operation.
In order to solve this problem, the present Applicant has proposed the following drive method under Japanese Patent Application No. 38393/1996: As shown in
FIG. 8
, during the period of time between the accomplishment of a scanning operation and the shifting the scanning operation to the next cathode line, all the drive switch
61
through
6256
are turned off, all the scanning switches
51
through
564
and all the shunt switches
71
through
7256
are switched over to 0V side, so that the resetting operation with 0V is effected, whereby the parastic capacitances of the luminous elements are discharged. The proposed method functions as described above.
In the above-described conventional drive method, the parastic capacitances of the luminous elements E
3
,
2
through E
3
,
64
charged by the reverse bias voltage Vcc during the scanning of th

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