Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-05-08
2001-09-11
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C345S102000, C345S103000
Reexamination Certificate
active
06288496
ABSTRACT:
TECHNICAL ART
The present invention relates to a system and method for driving an organic EL display which is constructed using an organic compound and has applications in the fields of information display panels used on audio equipment, automotive measuring instrument panels, displays for displaying moving images and freeze-frame pictures, household electrical appliances, car and bicycle electrical equipment, etc.
BACKGROUND ART
In recent years, an organic EL device has been intensively studied and put to practical use. The organic EL device is basically built up of a tin-doped indium oxide (ITO) or other transparent electrode, a triphenyldiamine (TPD) or other hole transporting layer laminated on the transparent electrode, an organic light emitting layer formed of a fluorescent material such as an aluminum quinolinol complex (Alq
3
) and laminated thereon, and a metal electrode (electron injecting electrode) provided on the organic light emitting layer and formed of a material having a low work function, for instance, Mg. Such a device now attracts attention as displays for use on household electrical appliances, car and bicycle electric equipment, etc., because a luminance of as high as several hundred to tens of thousands of cd/m
2
is obtained at a voltage of about 10V.
Such an organic EL device has a structure wherein an organic layer such as a light emitting layer is sandwiched between a scanning (common line) electrode that usually provides an electron injecting electrode and a data (segment line) electrode that usually provides a hole injecting electrode (transparent electrode), and formed on a transparent (glass) substrate. Electroluminescent displays are generally broken down into a matrix display wherein scanning electrodes and data electrodes are arranged in a matrix form to display information such as images and characters in the form of an assembly of dots (pixels), and a segment display comprising independently provided display units each having predetermined shape and size.
The segment type display may be driven in a static driving mode where the display units are independently driven. For the matrix display, on the other hand, a dynamic driving mode is used, wherein scanning lines and data lines are usually driven in a time division fashion. The dynamic driving mode is classified into two driving modes, one wherein the electron and hole injecting electrodes are driven as scanning and data lines, respectively, and the other wherein the electron and hole injecting electrodes are driven as data and scanning lines, respectively.
The organic EL device may be expressed in terms of an equivalent electrical circuit, as shown in FIG.
8
. In
FIG. 8
, the organic EL device is represented in the form of a parallel circuit comprising a diode element D and a parasitic capacity Cp, and so has a parasitic capacity. Therefore, when organic EL devices are arranged and connected together as shown in
FIG. 9
for instance, the respective parasitic capacities of the organic EL devices (pixels) connected to scanning lines are added up. Thus, a time constant is provided by the sum of the parasitic capacities (e.g., EL
1
+EL
4
+EL
7
+ . . . ) and pull-up resistance components connected to those electrodes or resistance components such as on-resistance components of push-pull switching elements when a push-pull circuit is used.
Here, the matrix circuit constructed as shown in
FIG. 8
is built up of switching elements SW
11
to SW
13
for driving scanning lines COM
1
to COM
3
(connecting them to the ground side or opening them), resistance components R
1
to R
3
(e.g., push-up resistance components or push-pull resistance components when a push-pull circuit is used) for stabilizing the scanning lines COM
1
to COM
3
at a given potential (power source potential) when these switching elements SW
11
to SW
13
are in non-operation (off), organic EL devices (pixels) EL
1
to EL
9
, capacity components of these pixels EL
1
to EL
9
, data lines SEG
1
to SEG
3
connected to the other ends of the pixels EL
1
to EL
9
, and switching elements SW
21
to SW
23
for connecting these data lines to the driving power source or ground side.
When matrix circuit is driven in a time division fashion, the scanning electrode COM
1
which reaches an L level upon turned on at a time t11 is turned off at a time t12, as shown in
FIG. 10
for instance, so that when the scanning line goes back to an H level, there is a delay time T
d
due to the time constant defined by the parasitic capacities and the resistance components such as pull-up resistance components. This delay time T
d
overlaps the on-time T
on
of the next scanning line COM
2
at the time t12 to tn (t13, t14 . . . ) with the result that although depending on the data line condition, some pixels at the scanning line emits light for this delay time irrespective of being a non-selected pixel.
As shown in
FIG. 11
as an example, when eyeing a certain group of pixels on the matrix, a pixel G appears, which gives rise to false light emission halfway between a lighting (driving) pixel L and a non-lighting (non-driving) pixel D or is brighter than in non-light emission state. Such false light emission makes contrast worse or is perceived as anomalous light emission, resulting in considerable drops of the quality of the display or a disturbance factor in images.
The case where electron and hole injecting electrodes are driven as scanning and data lines, respectively, has been explained. However, it is understood that when electron and hole injecting electrodes are driven as data and scanning lines, too, similar phenomena arise.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide an organic EL display driving system and method which enables an organic EL display to be driven with neither a contrast lowering nor a false light emission phenomenon yet in simple construction.
When the matrix type display is driven, there is a delay time due to CR components in the organic EL device and the driving circuit, as already explained. For this reason, overlapping light emission occurs between a certain driving line and the next driving line, resulting in a contrast lowering and an anomalous light emission phenomenon.
For an organic EL device, it is common to use a driving system for driving the scanning electrode side at a constant voltage. From the pull-up resistance R of a scanning electrode or the on-resistance R of a switching element corresponding to the pull-up resistance when a push-pull circuit is used and the combined capacity C of one scanning line corresponding to the sum of the parasitic capacity of the organic EL device, therefore, the time transition of the scanning electrode from a selected voltage (ground potential) to a non-selected voltage (power source voltage: E) is represented by the following equation:
vc
=
E
(
1
-
e
-
1
CR
t
)
(
2
)
The then equivalent circuit is shown in FIG.
12
. The voltage value Vc is a transient voltage after a switch SW is turned off at t=0 from the state where it is initially turned on.
Now consider the driving circuit for the organic EL display in further detail. In the matrix circuit of
FIG. 9
, when a scanning electrode COM
2
is driven following COM
1
, the corresponding pixels EL
4
and EL
5
are selected and lit on. Here assume that the pixel EL
4
is faintly lit up during the selection time for the pixel EL
5
(false light emission).
As shown in
FIG. 13
where V
th
signifies the forward threshold voltage of the organic EL device, even when the scanning electrode is in a non-selection state, a current continues to run through the organic EL device EL
4
until the voltage applied in the forward direction of the organic EL device EL
4
becomes lower than the threshold voltage V
th
. As a result, the device EL
4
emits light and so becomes brighter than other non-emitting device (false light emission). This is irrespective of whether data electrodes SEG
1
to SEG
3
are driven in a constant voltage or current mode. The threshold voltage V
th
is
Furukawa Hirotada
Kaita Yoshio
Saitoh Yoshihiro
Suzuki Mitsunari
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Philogene Haissa
TDK Corporation
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