Information
-
Patent Grant
-
6246180
-
Patent Number
6,246,180
-
Date Filed
Monday, January 31, 200024 years ago
-
Date Issued
Tuesday, June 12, 200123 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Philogene; Haissa
- Dinh; Trinh Vo
Agents
-
CPC
-
US Classifications
Field of Search
US
- 315 1693
- 315 206 R
- 315 1691
- 315 1692
- 315 291
- 315 307
- 315 241 R
- 345 55
- 345 76
- 345 211
- 345 204
-
International Classifications
-
Abstract
A drive unit for driving a corresponding one of organic EL elements of an active matrix EL display device includes a blanking switch for blanking the video signal stored in a storage capacitor in each frame period before the start of the next frame period. A drive transistor drives a corresponding EL element based on the correct current supplied for this If the video signal is a current signal, a transistor operating as a current-voltage converter is provided
Description
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an organic EL (electroluminescence) display device having an improved image quality and, more particularly, to a drive circuit for driving organic EL elements in an active matrix EL display device.
(b) Description of a Related Art
Flat-panel display devices now attract public attention due to their small thicknesses. Among other flat-panel display devices, an organic EL display device has an advantage of low power dissipation. In an EL display device, a plurality of EL pixels are arranged on a substrate in a matrix, each of the EL pixels having one or more of organic thin-film EL element. As a first generation of the EL display device, a simple matrix EL display device using a simple matrix driving scheme is now under development.
The simple matrix EL display device have “m” rows and “n” columns (m×n) for pixel elements, wherein each column is supplied with image data and each row is supplied with a scanning signal. An image is displayed on the screen by scanning the “m” rows periodically and sequentially with a constant cycle while supplying the “n ” columns with image data.
The simple matrix EL display device has a problem in that a larger dimension of a desired screen reduces the time length used for scanning each row of the EL elements, which causes a reduction of a mean luminance on the screen or an increase of power dissipation for a higher luminance.
Thus, a next generation EL display device using an active matrix driving scheme is expected to solve the above problem.
Patent Publication JP-A-9-305 139, for example, proposes an active matrix organic EL display device such as shown in FIG.
1
. The display device includes a plurality of EL pixels P
11
to Pmn arranged in a m×n matrix. An analog video signal Vs is amplified in a video amplifier, corrected with respect to the characteristics thereof in a V(voltage)/I(current) correction circuit, and then supplied to each of the EL pixels P
11
to Pmn. The video signal Vs is supplied to the EL pixels P
11
to Pmn intermittently in a time-division system by using a scanning control circuit, which receives a synchronizing signal and controls the timing for the scanning based on the synchronizing signal.
FIG. 2
shows one of the drive units for the EL pixels shown in FIG.
1
. Each pixel has an organic EL element
92
and a drive unit
91
for driving the EL element
92
. The drive unit
91
includes a transfer transistor
97
controlled by a control signal Cs for receiving the video signal Vs, a storage capacitor
96
for storing the video signal in a frame period until the video signal Vs for the next frame period is supplied, and a drive transistor
95
for driving a corresponding EL element
92
with a current corresponding to the video signal Vs stored in the storage capacitor
96
in each frame period.
When the video signal Vs is to be supplied to a pixel, the transfer transistor
97
is turned on to apply the video signal Vs to a storage capacitor
96
and the gate of the drive transistor
95
. The drain current of the drive transistor
95
is supplied to the organic EL element
92
as a cathode current thereof, thereby making the EL element
92
luminous during the frame period based on the drain current of the drive transistor
95
.
Each organic EL element
92
in each of the pixels P
11
to Pmn has a luminance based on the current supplied by the drive transistor
95
, whereby the luminance of the EL element
92
is controlled at a continuous gray-scale level based on the analogue video signal Vs.
FIG. 3
shows a timing chart of the drive unit
91
. When the transfer transistor
97
is ON due to an active level of the control signal Cs, the video signal Vs supplied through the signal line
98
is stored in the storage capacitor
96
for a single frame period and turns on the drive transistor
95
, which supplies a drive current I
EL
to the organic EL element
92
for luminescence based on the gate voltage stored by the storage capacitor
96
.
In the organic EL display device as described above, the luminescence of the organic EL element
92
during a single frame period is determined based on the video signal Vs received by the transfer transistor
97
. If a dark image succeeds a bright image based on the video signal at the changeover of the frame, as shown in
FIG. 3
, the potential on the signal line
98
which has changed from a high voltage for a frame period to a low voltage for the next frame period is abruptly applied to the storage capacitor. At this stage, the charge stored in the storage capacitor
96
returns toward the signal line
98
through the transfer transistor
97
, which received the next active level of the control signal Vs. In the changeover of the frame period, the gate voltage of the drive transistor
95
is affected by the gate voltage thereof during the precedent frame period, whereby the drive transistor
95
supplies a large current to the organic EL element
92
during the initial stage of the next frame period, thereby raising the luminance thereof above. the desired level, as shown in FIG.
3
. This causes malfunction of the EL display device such as a deteriorated image or a poor contrast on the screen.
Patent Publication JP-A-4-247491 describes a drive circuit, which superimposes a blanking signal onto the scanning lines in an active matrix EL display device. In the described drive circuit, however, the blanking signal is supplied during each horizontal scanning period. Thus, this configuration does not solve the above problem caused by the function of the active matrix drive circuit in each frame period or a vertical scanning period.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to provide a drive circuit for driving an organic EL element in an organic EL display device, which is capable of solving the above problem to improve an image quality on the screen.
In short, the present invention provides, in one embodiment thereof, a drive circuit for driving an organic EL element in an EL display device, wherein a blanking transistor is provided in parallel to the storage capacitor which supplies a gate voltage to a drive transistor for driving the organic EL element in each frame period. The blanking transistor receives a blanking signal for switch-on thereof, the blanking signal being made active for a specified time length just before the start of the next frame period. Thus, the storage capacitor is subjected to blanking of the precedent video signal, whereby the influence by the precedent vide signal on the organic EL element can be eliminated in the next frame period for improvement of the image quality.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a circuit diagram of a conventional active matrix organic EL display device.
FIG. 2
is a circuit diagram of one of the EL elements shown in FIG.
1
.
FIG. 3
is a timing chart of the drive unit shown in FIG.
2
.
FIG. 4
is a drive unit for driving an organic EL element in an organic EL display device according to a first embodiment of the present invention.
FIG. 5
is a top plan view of the layout for the drive unit of FIG.
4
.
FIG. 6
is a block diagram of the organic EL display device including the drive unit of FIG.
4
.
FIG. 7
is a timing chart of the drive unit of FIG.
4
.
FIG. 8
is a circuit diagram of a drive unit in an organic EL display device according to a second embodiment of the present invention.
FIG. 9
is a timing chart of the drive unit of FIG.
8
.
FIG. 10
is a circuit diagram of a drive unit in an organic display device according to a third embodiment of the present invention.
PREFERRED EMBODIMENTS OF THE INVENTION
Now the present invention is more specifically described with reference to the accompanying drawings wherein similar constituent elements may be designated by similar reference numerals.
Referring to
FIG. 4
, a drive unit, generally designated by reference numeral
11
, is implemented as a unit element of an active matrix drive circuit for driving an EL display device according to an embodiment of the present invention. The drive unit
11
drives an organic EL element
12
disposed adjacent to the drive unit
11
. The drive unit
11
includes a source line
13
, a ground line
14
, a drive transistor
15
, a storage capacitor
16
, a transfer transistor
17
shown by a symbol of switch, a signal line
18
, a control signal line or scanning line
19
and a blanking transistor
20
.
The organic EL element
12
and the drive transistor
15
are connected in series between the source line
13
and the ground line
14
. The transfer transistor
17
has a drain connected to the signal line
18
and a source connected to the gate of the drive transistor
15
. The blanking transistor
20
and the storage capacitor
16
are connected in parallel between the gate of the drive transistor
15
and the ground line
14
. The transfer transistor
17
has a gate connected to the control signal line
19
, and the blanking transistor
20
has a gate connected to a blanking signal line
21
which receives a blanking signal. The blanking signal is used for blanking the video signal for the frame at the end of the frame period, or before the start of the following frame period.
Referring to
FIG. 5
, the arrangement of the drive unit
11
is shown therein. The transistors
15
,
17
and
20
are implemented as n-channel thin-film transistors (TFTs). The control signal line
19
is connected to the gate of the transfer transistor
17
, the source-drain path of which is connected between the signal line
18
and the storage capacitor
16
. The blanking signal line
21
is connected to the gate of the blanking transistor
20
, the source and drain of which are connected to the ground line and the signal path between the storage capacitor
16
and the gate of the drive transistor
15
, respectively. The storage capacitor
16
is connected between the ground line
14
and the gate of the drive transistor
15
, the drain of which is connected to a corresponding EL element
12
disposed adjacent to the drive unit
11
.
In the drive circuit shown in
FIGS. 4 and 5
, the organic EL element
12
is supplied with a drive current I
EL
by the drive transistor
15
. The blanking transistor
20
is controlled by the blanking signal to drain the charge stored across the storage capacitor
16
to the ground line
14
for a specified time interval at the end of a frame period.
Referring to
FIG. 6
, an organic EL display device generally designated by numeral
100
includes a plurality of EL pixels
10
arranged in a m×n matrix (“m” rows by “n” columns) on a substrate, each of the EL pixels
10
including a drive unit
11
and an organic EL elements
12
shown in FIG.
4
. Each of the “m” source lines
13
disposed for each row of the EL pixels
10
is connected in common with the other source lines
13
to a DC power source
31
. Each of the “n” signal lines
18
disposed for each column of the EL pixels
10
is connected to a terminal of a corresponding one of signal drivers
32
, whereas each of the “m” control signal lines (scanning lines)
19
disposed for each row of the EL pixels
10
is connected to a terminal of a corresponding one of control drivers
33
. In addition, each of the blanking signal lines
21
disposed for each row of the EL pixels
10
is connected to a terminal of a corresponding one of blanking signal drivers
34
. These drivers
32
,
33
and
34
are controlled by an overall control circuit (not shown) for an active matrix driving scheme.
The signal drivers
32
supply video signals as either voltage signals or current signals while the control drivers
33
sequentially supply scanning signals to the respective control signal lines
19
one by one. The blanking signal drivers
34
sequentially supply the blanking signals to the blanking signal lines
21
one by one in synchrony with the clock signal which drives the control drivers
33
.
Referring to
FIG. 7
in addition to
FIG. 4
, in operation of the EL display device, a control signal which is active during a specified interval in each frame period is supplied through the control signal line
19
to turn on the transfer transistor
17
, while an analogue video signal Vs, such as shown in
FIG. 7
, is supplied through the signal line
18
. Thus, the video signal Vs is stored in the storage capacitor
16
and supplied to the gate of the drive transistor
15
. The drive transistor
15
supplies a drive current to the organic EL element based on the gate voltage of the drive transistor
15
, or the video signal stored in the storage capacitor
16
, in each frame period.
The drive current I
EL
for driving the EL element
12
corresponds to the gate voltage of the drive transistor
15
applied by the storage capacitor
16
. The EL element
12
operates at a luminance corresponding to the drive current and continues the luminance, after the control signal is made inactive as shown in
FIG. 12
to turn OFF the transfer transistor
17
.
At the end of each frame period, an active level of the blanking signal is supplied to the gate of the blanking transistor
20
, which turns ON to discharge the storage capacitor
16
for blanking the stored video signal. As a result, the gate voltage of the drive transistor
15
is made zero at the end of the frame period, which makes the drive current I
EL
zero.
The blanking signal is then made inactive at the start of the next frame period when the control signal is made active for the next frame period. Thus, the gate voltage of the drive transistor
15
at the start of the next frame period is determined only by the video signal at the start of the next frame, as shown in
FIG. 7
, whereby the drive current I
EL
for the EL element is determined only by the video signal for the next frame period. Accordingly, the luminance of the EL element
12
is determined for each frame by the video signal at the each frame.
The pulse duration and the timing of the blanking signal is determined so that the drive current dose not fluctuate at the changeover of the frame period. The blanking signal blanks the video signal for the frame period, and thus may reduce the mean luminance of the EL element in the frame. Since the organic EL element is a spontaneous luminous element, the reduction of the luminance on the screen can be compensated by raising the luminance power for the EL element at a uniform rate and thus is not serious for the function of the display unit. The organic EL display device of the present embodiment can achieve a higher contrast on the screen.
In the organic EL display unit
100
of the present embodiment, each EL pixel operates at an accurate luminance during each frame period substantially without fluctuation, whereby the image achieved on the screen is based on the accurate gray-scale level. Thus, a higher contrast can be achieved on the screen even if the video signal involves a higher-speed movement for the image or a higher-speed luminance change.
Referring to
FIG. 8
, a drive unit of a drive circuit according to a second embodiment of the present invention is different from the first embodiment in that the video signal is supplied as a current signal compared to the first embodiment wherein the video signal is supplied as a voltage signal. The drive transistor
55
, the storage capacitor
56
and the blanking transistor
60
as well as the connection thereof are similar to those in the first embodiment.
The drive unit of the present embodiment includes a first transfer transistor
62
having a drain connected to the signal line
58
and a gate connected to the control signal line
54
, a converting transistor
61
having a drain connected to the source of the first transfer transistor
62
, a source connected to the ground line
59
, and a gate connected to the drain thereof, a second transfer transistor
57
having a drain connected to the source of the first transfer transistor
62
, a source connected to the gate of the drive transistor
55
, and a gate connected to the control signal line
54
.
In the above configuration, the converting transistor
61
and the drive transistor
55
, when coupled together through the second transfer transistor
57
, form a current mirror wherein the converting transistor
61
and the drive transistor
55
are a reference transistor and an output transistor, respectively.
FIG. 9
shows a timing chart for the drive unit of the present embodiment. In operation, when the control signal is active in a frame period, the first transfer transistor
62
passes the current video signal, which is converted by the converting transistor
61
into a voltage video signal. The voltage video signal is then transferred through the second transfer transistor
57
to the storage capacitor
56
and the gate of the drive transistor
55
, which operate in association for supplying the video signal to the EL element
52
, similarly to the first embodiment.
The blanking transistor
60
is activated at the end of the frame period to blank the video signal in the frame period for preparing reception of the next video signal for the following frame period.
In the present embodiment, similarly to the first embodiment, the organic EL element operates from the start of the frame period at the luminance corresponding to the video signal supplied for the same frame due to the blanking of the precedent frame video signal. In addition, a higher contrast can be achieved on the screen even if the video signal involves therein a higher-speed movement for the image or a higher-speed luminance change.
In addition, even if the transistor characteristics of the drive transistor
55
may vary in the present embodiment due to the variations in the fabrication process, the current mirror formed by the converting transistor
61
and the drive transistor
55
allows the drive unit to operate at the accurate luminance so long as the transistor characteristics vary similarly for both the transistors
61
and
55
. Thus, a higher accuracy for the luminance and a more improved image quality can be achieved in the present embodiment compared to the first embodiment.
Referring to
FIG. 10
, a drive unit of a drive circuit according to a third embodiment is similar to the first embodiment except that the storage capacitor
71
in the present embodiment is implemented by a parasitic capacitance formed between the drain and the source of the blanking transistor, or between the drain and the ground. In this configuration, the occupied area for the drive unit can be reduced compared to the first embodiment, which allows a larger space for the organic EL element in each EL pixel and raise the luminescence of the each EL pixel.
The blanking transistors in the above embodiments may be disposed at any location, or may be changed from the n-channel transistor to a p-channel transistor together with corresponding modifications. The transfer transistor and the blanking transistor may be of any circuit element so long as these transistors have a switching function.
In the above embodiment, each EL pixel has a single EL element. However, the EL pixel may have a plurality of, typically three, EL elements depending on the color function of the EL display unit.
Since the above embodiments are described only for examples, the present invention is not limited to the above embodiments and various modifications or alterations can be easily made therefrom by those skilled in the art without departing from the scope of the present invention.
Claims
- 1. An organic EL display device comprising a plurality of EL elements arranged in a matrix, and a drive circuit including a plurality of drive units each disposed for a corresponding one of said EL elements,each of said drive units including a transfer switch, activated by a scanning signal, for transferring an analogue video signal during an active level of the scanning signal in a single frame period, a storage capacitor for storing the video signal transferred by said transfer switch, a drive transistor, controlled by the video signal stored by said storage capacitor, for supplying a current to a corresponding one of said EL elements, and a blanking switch, responsive to a blanking signal, for discharging charge stored in said storage capacitor, said blanking signal being active substantially at an end of the frame period.
- 2. The organic EL display device as defined in claim 1, wherein said drive unit further includes another transfer switch, activated by the scanning signal, for receiving a current signal from a signal line disposed for a column of the EL elements, and a converting transistor for converting the current signal to the video signal.
- 3. The organic EL display device as defined in claim 2, wherein said another transfer switch and said drive transistor form a current mirror.
- 4. The organic EL display device as defined in claim 1, wherein said storage capacitor is implemented by a parasitic capacitance between a drain and a source of said blanking switch.
- 5. The organic EL display device as defined in claim 1, wherein each of said drive transistor, said transfer switch and said blanking switch is implemented by a thin-film transistor.
- 6. The organic EL display device as defined in claim 1, wherein said drive circuit uses an active matrix driving technique.
- 7. A method for driving an organic EL display device including a plurality of EL elements arranged in a matrix, said method comprising the steps of:consecutively transferring analogue video signals based on scanning signals in a single frame period, storing the video signal in storage capacitors and supplying currents to the EL elements based on the video signals stored in the storage capacitors in the single frame period, blanking the video signals stored in the storage capacitors at an end of the frame period for preparing transfer of the analogue video signals for a next frame period.
Priority Claims (1)
Number |
Date |
Country |
Kind |
11-021579 |
Jan 1999 |
JP |
|
US Referenced Citations (8)
Foreign Referenced Citations (2)
Number |
Date |
Country |
4-247491 |
Sep 1992 |
JP |
9-305139 |
Nov 1997 |
JP |