Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-11-08
2001-04-24
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C345S044000, C345S048000, C345S077000, C345S084000
Reexamination Certificate
active
06222323
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a display device displaying information by illuminating a plurality of light-emitting elements, more particularly to a driving method of a display device employed in portable terminals and the same display device.
BACKGROUND OF THE INVENTION
In recent years, organic electro-luminescent (EL) elements have been arrayed in a matrix, which has been positively tested as a display panel. A driving method of this display panel employing the organic EL elements is disclosed as “a simple matrix method” in the Japanese Patent Application Unexamined Publication No. H06-301355.
FIG. 10
illustrates a structure and a driving method of a conventional display device.
In
FIG. 10
, the display device comprises display section
106
, anode control circuit
107
and cathode control circuit
108
. At each intersection of anodes “a
1
-am” and cathodes “c
1
-cn” arrayed in a matrix, light-emitting elements—formed of organic EL elements—“L
1
,
1
-Lm,n” are placed. The cathodes are scanned and driven at a given interval, and then the anodes are selectively driven being synchronized with this cathode-driving so that an arbitrary light-emitting element is selectively illuminated. Further, a reverse bias voltage or a voltage not more than a threshold value for illumination is applied to non-selected elements thereby avoiding erroneous lighting thereof (cross talk) due to leak current.
A driving method of the conventional display device is described hereinafter with reference to FIG.
10
.
FIG. 10
illustrates a case where “L
1
,
1
” and “L
2
,
1
” among the light-emitting elements L
1
,
1
-Lm,n are selected to be lit. Anode lines “a
1
” and “a
2
” are coupled to current sources J
1
and J
2
by closing switches “Sa
1
” and “Sa
2
”, and cathode line “c
1
” is coupled to ground potential (GND) by switch “Sc
1
”, thereby running forward-bias-current to elements L
1
,
1
and L
2
,
1
and lighting these two elements.
Anode lines “a
3
-an” are coupled to ground potential by switches Sa
3
-Sam, and cathode lines “c
2
-cn” are coupled to power supply voltage Vcc by switches Sc
2
-Scn. Forward-bias-voltage produced both the ends of the two elements L
1
,
1
and L
2
,
1
is referred to as Vf at lighting the two elements. Then the voltage applied to both the ends of non-lit elements takes either one of two values, i.e. “−Vcc” and “Vf−Vcc”. The value of Vcc is set at a value so that the value of “Vf−Vcc” cannot be more than the threshold value of illumination, whereby non-selected elements are prevented from being erroneously lit.
However, this driving method produces two bias voltages at the non-lit elements. The elements having different bias voltages store different amount of charges in each parasitic capacitance of respective elements. Then when these non-lit elements are driven simultaneously, the elements biased at “−Vcc” light at a lower brightness than the elements biased at “Vff−Vcc”. As a result, uneven brightness is observed between these elements.
The Japanese Patent Application Unexamined Publication No. H09-232074 teaches the following driving method which overcomes this problem: A reset period is reserved at switching the cathode to be driven, and during the reset period, switches Sa
1
, Sa
2
, and Sc
2
, Sc
3
, Sc
4
-Scn are switched so that these switches are coupled to ground potential as shown in broken lines in FIG.
10
. This discharges charges stored in each parasitic capacitance of respective non-lit elements. This reset period can equal respective charges stored in each parasitic capacitance of the elements just before the elements are driven. As a result, uneven brightness due to a difference between stored charges can be avoided.
This method, however, discharges the stored charges once out of every parasitic capacitance at switching the cathodes to be driven, and charges every parasitic capacitance again at driving the elements, thereby consuming a large amount of power. The charges stored by applying a reverse-bias-voltage, in particular, do not contribute at all to lighting the element, i.e. they just waste electric power.
This power consumption due to the reverse-bias-voltage is detailed hereinafter in a more specific way. In the display device shown in
FIG. 10
, let us assume the following case: where
Parasitic capacitance of respective element: C (F)
Power supply voltage of reverse-bias-voltage: Vcc (V)
Frame frequency (a frequency for driving the cathodes in one cycle): Fv (Hz)
A static data is displayed on the display section, and a number of elements to be lit on a cathode “ca” (1≦a≦n) is “m
on
”, then the number of elements to which the reverse-bias-voltage Vcc is applied is (n−1)×(m−m
on
), those elements are coupled to the cathodes except “ca” and coupled to anodes except the anodes of lit-elements.
Since those elements own parasitic capacitance “C” respectively, the energy “W” (J) supplied from the power supply to respective parasitic capacitances during the driving period of cathode “ca” is expressed as follows:
W
=(½)·
C
·(
Vcc
2
)·(
n−
1)·(
m−m
on
) (1)
The supplied energy “W” is discharged during the reset period, and charged by the power supply at the next scanning of the cathodes.
This control method discussed above can keep the non-selected elements at non-lit status. However, in an actual environment where this display device is used, external lights such as lamps and other light sources are also available. The elements reflect those external lights thereby producing reflection lights. The cathode lines are, in particular, formed of metal and thus produces a large amount of reflection lights. Under the strong external light such as sunlight, the difference between the illumination light and the reflection light becomes small, thereby lowering a contrast. As a result, pattern recognition of text data and the like becomes poor.
In order to overcome this disadvantage, a filter layer for limiting the external lights is often disposed on the surface of the display device. This measure decreases the influence of the external lights as well as increases an actual brightness responding to both of an attenuation factor in the filter layer and a desirable display brightness. A luminescent brightness of the conventional display device is determined with reference to a brightness visible enough even under intense external lights. Therefore, the display device illuminates with more brightness than it is required in a room or in the night where relatively weak external lights are available. The display thus becomes hard to see in a dark place, and consumes unnecessary power. This is a critical problem for display devices employed in battery-operated portable apparatuses among others.
As such, according to the conventional driving method, a power source for applying a reverse bias voltage supplies energy responsive to a number of non-lit elements at every scanning of cathodes. In this case a display pattern with a small number of lit-elements consumes a lot of power for charging/discharging each parasitic capacitance. This power basically does not contributes to lighting the elements, and just blocks the efforts of reducing power consumption.
SUMMARY OF THE INVENTION
The present invention addresses the problems discussed above, and aims to provide a driving method for reducing power consumption in a display device employing light-emitting elements as well as to provide the display device per se.
The driving method of the present invention is employed in the following display device: The display device having a plurality of light-emitting elements which include: (a) cathodes comprising a plurality of stripe lines, (b) anodes across the cathodes and comprising a plurality of stripe lines, and (c) a light-emitting layer between the cathodes and anodes.
The driving method comprises the steps below:
(1) First, illuminate a first light-emitting element coupled to a first cathode;
(2) Second, in order to illuminate a second light-emitting ele
Yamashita Akihiro
Yamashita Hideaki
Matsushita Electric - Industrial Co., Ltd.
Philogene Haissa
Ratner & Prestia
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