Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-07-19
2002-11-26
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169300, C345S204000, C345S084000, C345S077000
Reexamination Certificate
active
06486607
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit and system for driving an organic thin-film electroluminescent (EL) display to emit light, and particularly to a circuit and system for driving the organic thin-film EL element to emit light at a specified constant driving current.
2. Description of Related Art
The light-emitting luminance of the organic thin-film EL elements varies when the driving current flowing into the element varies. To control the uniformity of luminance of organic thin-film EL element, the driving current flowing into the element must be controlled and maintained at a specified constant current level among the organic thin-film EL elements.
FIG. 1
shows a prior art driving circuit. In
FIG. 1
, a constant current supply
13
intends to change the driving current, which is supplied from a power supply
11
to a light-emitting element
12
. It should be noted that the light-emitting element
12
emits light when a switch
14
is open as indicated by solid line, and ceases to emit light when the switch
14
is closed as indicated by dotted line.
FIG. 2
shows another prior art driving circuit. In this configuration, a high resistance
15
, which is inserted in series between a light-emitting element
12
and a power supply
11
, intends to control the driving current flowing through the light-emitting element
12
to be a constant. It should be noted that the light-emitting element
12
emits light when a switch
16
is located at a position indicated by solid line, and ceases to emit light when the switch
16
is changed to another position indicated by dotted line.
The organic thin-film EL element can be modeled as an equivalent circuit composing a diode
32
and a parasitic capacitor
31
connected in parallel, as shown in FIG.
3
. The parasitic capacitor
31
within the equivalent circuit always causes a response problem, especially in a matrix of organic thin-film EL elements. The organic thin-film EL elements cannot emit light normally unless a voltage difference between both ends exceeds a specified forward voltage V
f
. The forward voltage V
f
of LED is as low as +1.5 V to +2 V and also relatively stable. On the other hand, the forward voltage of the organic thin-film EL is as high as +5 V to 12 V and also greatly vanes in accordance with luminance, temperature and time passage. Besides, the parasitic capacitance effect is more severe in an organic thin-film EL element than in a LED due to a higher forward voltage V
f
. The forward voltage V
f
has to rise above the specified voltage value for luminance and the rise time is depended on the total charging time of all the parasitic capacitors parasitizing in the organic thin-film EL elements. Normally, the power supply is required to boost to a V
cc
voltage potential higher than the forward voltage V
f
in order to drive the organic thin-film EL element to emit light.
FIG. 4
shows a prior art driving system
40
for driving luminous elements. In
FIG. 4
, the prior art driving system
40
is constructed with a matrix arrangement of the number of N×M (only 6×5 organic thin-film EL elements appear in FIG.
4
), in which the cathode-scanning unit consists of N number of cathode scanning lines. The cathodes of organic thin-film EL elements are connected to the switches
7
1
to
7
n
through the cathode scanning line X
1
to X
n
for selecting a power potential V
B
or a ground potential. The anode data-driving unit consists of M number of anode data-driving lines. The anode data-driving lines Y
1
to Y
m
are individually connected to the switches
11
1
to
11
m
with constant current supplies
10
1
to
10
m
and ground. The prior art driving system
40
causes the luminous elements at an arbitrary intersection to emit light by selecting and scanning one of the anode lines and the cathode lines sequentially at fixed time intervals.
Accordingly, the prior art driving system
40
always causes problems once used in driving a matrix of organic thin-film EL elements for luminance. The main problem is that the scanning speed will be slowed down due to the parasitic capacitors described above. When the organic thin-film EL is used as a luminous element, this problem becomes more severe since the organic thin-film EL has a large capacitor to generate a surface emission. The above problem is more severe when the number of the luminous elements increases since the organic thin-film EL will to accumulate all the parasitic capacitors. Furthermore, the parasitic capacitors of all luminous elements connected to the anode lines have to be charged, and the current sources for driving the luminous elements connected to each anode line must be designed large enough to satisfy the appropriated response time. This requirement for generating large current sources is detrimental from the aspect of miniaturization of the circuit.
FIG. 5
is a timing chart of the driving system shown in FIG.
4
.
FIG. 5
shows the parasitic capacitor problem in the switching operations of the switches
7
i−1
,
7
i+1
,
7
i+1
, and
11
j
. The potential of Y
j
data electrodes cannot increase at once due to the parasitic capacitance in the reverse bias direction of at least (n−1) pixels. A delay time t
d
occurs until a forward bias is applied to the pixel D(i, j) for light emitting. In addition, the current source
10
j
will limit the increasing rate of the potential of the Y
j
data electrodes and results in a larger delay time t
d
.
FIG. 6
shows a current response when an input voltage pulse is applied to an organic thin-film EL element. In
FIG. 6
, a curve
61
represents the organic thin-film EL element current response, and a curve
62
represents the voltage pulse. It is clear that the rise time is longer than the fall time. This indicates that the time for capacitance discharge is shorter than the time for capacitance charge in the organic thin-film EL element. The advantage of a shorter capacitance discharge time can be used to develop a fast response driving circuit for an organic thin-film EL display. In the prior art driving system shown in
FIG. 4
, a constant current source
10
j
is connected to a set of parallel organic thin-film EL elements, D(l, j) through D(n, j), following to the ground potential in D(i,j) and to reverse power potential in rest of D(l to i−1, j) and D(i+1 to n, j). Normally, the constant current source is sourcing a magnitude of current to light up an organic thin-film EL element. It should be noted that the parasitic capacitors in parallel could enhance the parasitic capacitance effect compared to that of a single organic thin-film EL element. The current source limits the current and worsens the response to emit light of the scanned organic thin-film EL element D(i, j) due to the above parasitic capacitance effect when a power potential is applied. Several methods to improve the response to emit light in prior art organic thin-film EL display driving system is proposed in U.S. Pat. No. 6,201,520 and No. 5,844,368. However, the above methods do not really resolve the existent problems.
SUMMARY OF THE INVENTION
The object of the present invention is to resolve the problems and disadvantages of the related art. The present invention provides a driving circuit for driving an organic thin-film EL element to emit light. Furthermore, a driving system organized by the driving circuits of the present invention is applied to drive an organic thin-film display.
In a first embodiment of the present invention, a driving circuit for driving an organic thin-film EL element comprises an anode-scanning switch, an organic thin-film EL element, a constant current source and a cathode data-driving switch. The anode-scanning switch is connected to a power potential while being scanned and connected to a ground potential otherwise. The organic thin-film EL element is connected to the anode-scanning switch. The constant current source is connected to the organic thin-film EL element. One end of the cathode dat
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