Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-09-26
2003-11-18
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C315S169300
Reexamination Certificate
active
06650308
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic EL display device using organic EL (electro-luminescence) elements, and a method for driving the same.
2. Description of the Related Art
In a luminescent display device using organic EL elements, the organic EL elements are arranged in a matrix pattern. The organic EL elements are illuminated by, for example, successively scanning rows of elements in a column direction by means of a scanning line driving circuit (row driving circuit) and selectively supplying a driving current to the elements in a specified row selected by the row driving circuit. The driving current is supplied by means of a data line driving circuit (column driving circuit). Such an organic EL display device has been attracting public attention as a self-emissive display device which does not require a backlight.
FIG. 1
is a diagram illustrating a conventional passive matrix type organic EL display device. A plurality of organic EL elements
1
are arranged in a matrix pattern to form an organic EL panel
2
. For the sake of simplicity, each organic EL element
1
is shown in
FIG. 1
to be composed only of a diode. However, each organic EL element
1
includes a parasitic capacitor, arranged in parallel with the element
1
, which has a very large electrostatic capacitance with respect to the current flowing through the element
1
.
As illustrated in
FIG. 1
, a column of organic EL elements
1
are connected by their anodes to each data line
3
(
3
a,
3
b,
3
c,
3
d,
3
e,
etc.). The data lines
3
are connected to a column driving circuit
5
. A row of organic EL elements
1
are connected by their cathodes to each scanning line
4
(
4
a,
4
b,
4
c,
4
d,
etc.). The scanning lines
4
are connected to a row driving circuit
6
. The data lines
3
can be selectively connected to the ground level via shunt switches
7
(
7
a,
7
b,
7
c,
7
d,
7
e,
etc.). In the column driving circuit
5
, each signal current source
8
is connected to a data line
3
via drive switches
9
(
9
a,
9
b,
9
c,
9
d,
9
e,
etc.). Each scanning line
4
is connected to a scanning switch
10
of a plurality of scanning switches (
10
a,
10
b,
10
c,
10
d,
etc.) of the row driving circuit
6
. The scanning lines
4
are selectively connected to either a power source V
2
or the ground level via the scanning switches
10
. The drive switches
9
of the column driving circuit
5
, the scanning switches
10
of the row driving circuit
6
and the shunt switches
7
are controlled by a control circuit
11
.
In the conventional organic EL display device with such a configuration, the control circuit
11
receives image display data and controls the row driving circuit
6
to successively scan the scanning lines
4
. While a scanning line
4
is selected, the column driving circuit
5
supplies a predetermined current as the driving current to a particular selected data line
3
. In this way the organic EL element
1
, which is connected to the selected scanning line
4
and the selected data line
3
, is illuminated. For example, while the row driving circuit
6
is scanning the scanning line
4
b,
the organic EL elements
1
connected to the data lines
3
b
and
3
c,
among the organic EL elements
1
connected to the scanning line
4
b,
can be illuminated by controlling the row driving circuit
6
to switch the scanning switch
10
b
to the ground side and switching the scanning switches
10
a,
10
c,
10
d,
etc., connected to the other scanning lines
4
a,
4
c,
4
d,
4
e,
etc., to the power source V
2
side. The column driving circuit
5
applies a power source V
1
from the signal current source
8
to the data lines
3
b
and
3
c
by turning OFF the shunt switches
7
b
and
7
c
and turning ON the driving switches
9
b
and
9
c.
The column driving circuit
5
then connects the data lines
3
a,
3
d
and
3
e
to a ground by turning ON the shunt switches
7
a,
7
d,
7
e,
etc., and turning OFF the driving switches
9
a,
9
d
and
9
e.
At the same time the scanning line
4
b
is at the ground potential. In this way, the driving current supplied from the signal current source
8
to the data lines
3
b
and
3
c,
based on the potential difference between the power source V
1
and the ground, flows through the organic EL elements
1
connected between the data lines
3
b
and
3
c
and the scanning line
4
b.
In this way the elements
1
are illuminated.
The elements
1
which are connected to the data lines
3
b
and
3
c
and to the other scanning lines
4
a,
4
c,
4
d,
4
e,
etc., have their cathodes connected to the power source V
2
via the scanning switches
10
a,
10
c,
10
d,
10
e.
In this way the power source V
1
is applied to the anodes of the elements
1
via the data lines
3
b
and
3
c
while the power source V
2
is applied, as a reverse bias, to the cathodes of the elements
1
via the scanning lines
4
a,
4
c,
4
d,
4
e,
etc. Since the voltages of the power source V
1
and the power source V
2
are set at similar levels, there is no voltage difference applied between the anode and the cathode of such elements
1
. Consequently, the elements
1
are not illuminated.
The organic EL elements
1
which are connected to the scanning line
4
b
and to the other data lines
3
a,
3
d,
3
e,
etc., have their anodes and cathodes both grounded, and there is no voltage difference between them. Consequently, such elements
1
are not illuminated.
The power source V
2
is applied to the cathodes and the ground potential is applied to the anodes of the organic EL elements
1
which are connected between the other data lines
3
a,
3
d,
3
e,
etc., and the other scanning lines
4
a,
4
c,
4
d,
etc. Consequently, a voltage difference in the opposite direction is applied to the elements
1
. Therefore, a current does not flow through such elements
1
, and the elements
1
are not illuminated. However, since a voltage difference in the opposite direction is applied to the elements
1
, the parasitic capacitors of the elements
1
are charged in an opposite direction to the direction in which the parasitic capacitors of the illuminated elements
1
are charged.
In a case where the data lines
3
a,
3
d
and
3
e,
which have not been driven in the previous scanning step, are driven in the next scanning step, in other words in a case where the data lines
3
a,
3
d
and
3
e,
which have not been driven while scanning the scanning line
4
b,
are driven when the scanning operation proceeds to the scanning line
4
c,
a current of course flows through the organic EL elements
1
that are connected to the scanning line
4
c
and are to be illuminated. A current also flows through the organic EL elements
1
that are not connected to the scanning line
4
c
but have been charged in the reverse direction in the previous scanning step so as to cancel out the reverse charge. Therefore, it takes a long time to charge the organic EL elements
1
to be illuminated, and the current cannot be raised quickly.
In view of this, in the prior art, when the scanning operation by the row driving circuit
6
proceeds from the scanning line
4
b
to the next scanning line
4
c,
all of the driving switches
9
a,
etc., of the column driving circuit
5
are turned OFF. At the same time all of the scanning switches
10
a,
etc., of the row driving circuit
6
and all of the shunt switches
7
a,
etc., are connected to a ground or the power source. As a result the charge stored in the organic EL elements
1
is discharged. In this way, selected organic EL elements
1
are illuminated by applying a constant pixel current to the selected organic EL elements
1
after discharging all of the parasitic capacitors. The unnecessary charging of the organic EL elements
1
is consequently avoided.
While the current-voltage characteristics of the organic EL element
1
are conceptually close to those of a light emitting diode, the voltage at which the current rises is as high as about 5 to 10 V for the organic EL element
1
, whe
McGinn & Gibb PLLC
Mengistu Amare
NEC Corporation
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