Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-09-19
2002-02-26
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S076000, C345S089000, C345S156000, C345S156000
Reexamination Certificate
active
06351077
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving EL display device, a driving circuit for implementing the driving method, and an EL display device comprising the driving circuit.
2. Description of the Related Art
Techniques of forming a TFT (thin film transistor) on a substrate have been widely progressing in recent years, and development of applications thereof to an active matrix type display device are advancing. In particular, a TFT using a polysilicon film has a higher electric field effect mobility than a TFT using a conventional amorphous silicon film, and high speed operation is therefore possible. As a result, it becomes possible to perform pixel control, conventionally performed by a driving circuit external to the substrate, by the driving circuit formed on the same substrate as a pixel.
This type of active matrix display device has been in the spotlight because of the many advantages which can be obtained by incorporating various circuits and elements on the same substrate, such as reduced manufacturing cost, display device miniaturization, increased yield, and higher throughput.
Presently, active matrix EL display devices with EL elements as self-light-emitting elements are actively researched. An EL display device is also referred to as an organic EL display (OELD) or an organic light emitting diode (OLED).
Unlike a liquid crystal display device or the displays, an EL display device is of a self-light-emitting type. An EL element is structured such that an EL layer is sandwiched between a pair of electrodes. The EL layer typically has a laminated structure. A laminated structure of “a hole transporting layer/a light emitting layer/an electron transporting layer” proposed by Tang, et al. of Eastman Kodak Co. is a typical laminated structure. This structure has very high light emitting efficiency, and thus, most of EL display devices that are now under research and development adopt this structure.
Other than this, the laminated structure may be a hole injecting layer/a hole transporting layer/a light emitting layer/an electron transporting layer, or, a hole injecting layer/a hole transporting layer/a light emitting layer/an electron transporting layer/an electron injecting layer laminated in this order on a pixel electrode. A fluorescent pigment or the like may be doped in an EL layer.
When predetermined voltage is applied from a pair of electrodes to the EL layer structured as described in the above, recombination of carriers in the light emitting layer is caused to emit light. It is to be noted that light emission by an EL element may be herein referred to as driving of an EL element.
Color display methods of an EL display device are roughly divided into four: a method where three kinds of EL elements emitting R (red), G (green), and B (blue) light, respectively, are formed; a method where EL elements emitting white light are combined with a color filter of R, G, and B; a method where EL elements emitting blue or blue-green light are combined with a fluophor (fluorescent color conversion layer: CCM); and a method where EL elements corresponding to R, G, and B are superimposed on a transparent electrode used as a cathode (an opposing electrode).
Generally, the luminance of red light emission is lower than the luminance of blue and green light emission in many organic EL materials. When an organic EL material having such light emitting characteristics is used for an EL display device, the luminance of red in a displayed image is low. Further, since the luminance of red light emission is lower than the luminance of blue and green light emission, a method is conventionally adopted where orange light the wavelength of which is a little shorter than that of red light is used as red light. However, in this case also, the luminance of red itself of an image displayed on the EL display device is low, and an image which is intended to be displayed in red is displayed in orange. As a result, only a display device, which has unbalanced luminance of red, green, and blue light emission and unsatisfactory white balance, can be provided.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problems, and an object of the present invention is to provide a driving method and a driving circuit for realizing an EL display device with excellent white balance.
A method of driving an EL display device according to the present invention is now described. In the driving method according to the present invention, in view of the lower luminance of red light emission of the EL light emitting layer, by suppressing the luminance of a green image and the luminance of a blue image, the luminance of a red image, the luminance of a green image, and the luminance of a blue image are well-balanced, which makes it possible to improve the white balance. It is to be noted that the present invention can be applied not only to EL light emitting elements using an EL light emitting layer which emits white light and a color filter but also to EL light emitting elements using an EL light emitting layer which emits red light, an EL light emitting layer which emits green light, and an EL light emitting layer which emits blue light.
It is to be noted that here, for the sake of simplicity, a case where an original image signal inputted from the external is 6 bit digital data is described. First, reference is made to
FIG. 1
, which shows the luminance of red (R) light emission, the luminance of green (G) light emission, and the luminance of blue (B) light emission of EL light emitting elements with respect to gray-scale levels of the 6 bit digital data. It is to be noted that luminance of 64 (=2
6
) gray-scale levels can be obtained from the 6 bit digital data. Further, it is to be noted that, though a case where 6 bit digital data is inputted is described herein, the driving method according to the present invention can also be applied to a case where n bit digital data is inputted (n is a natural number).
B
Rmax
, B
Gmax
, and B
Bmax
are the maximum values of the luminance of red light emission, the luminance of green light emission, and the luminance of blue light emission, respectively (here, in the case of 64 gray-scale levels). It is to be noted that, for the sake of convenience, a case where B
Gmax
=B
Bmax
=2B
Rmax
is assumed.
As shown in
FIG. 1
, when the gray-scale level is at maximum (64), the luminance of red light emission, the luminance of green light emission, and the luminance of blue light emission take the maximum values B
Rmax
, B
Gmax
, and B
Bmax
, respectively. However, since the maximum value B
Rmax
of the luminance of red light emission is half of the maximum value B
Gmax
of the luminance of green light emission or half of the maximum value B
Bmax
of the luminance of blue light emission, if the display is carried out with them being as they are, the maximum luminance varies and the white balance is unsatisfactory.
FIGS. 2 and 3
are conceptual views of the method of driving an EL display device according to the present invention. In the method of driving an EL display device according to the present invention, n bit digital data having red, green, and blue image information (gray-scale information) are converted into (n+1) bit digital data, respectively. Here, a case where 6 bit digital data are converted into 7 bit digital data is described as an example. First, digital data conversion carried out in the driving method according to the present invention is described with reference to FIG.
3
.
Data conversion of 6 bit digital data having red image information is shown in
FIG. 3R
, data conversion of 6 bit digital data having green image information is shown in
FIG. 3G
, and data conversion of 6 bit digital data having blue image information is shown in FIG.
3
B.
First, data conversion of 6 bit digital data having red image information (gray-scale information) (
FIG. 3R
) is described. R
0
(=1) is added below R
1
that is the least significant bit among the 6
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Philogene Haissa
Semiconductor Energy Laboratory Co,. Ltd.
LandOfFree
EL display device and driving method thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with EL display device and driving method thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and EL display device and driving method thereof will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2946035