Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-11-05
2004-01-20
Lee, Wilson (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S092000, C345S082000
Reexamination Certificate
active
06680580
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the priority benefit of Taiwan application serial no. 91121105, filed on Sep. 16, 2002.
BACKGROUND OF INVENTION
1. Field of Invention
The present invention generally relates to a light emitting device display technique, and more particularly, to a driving technique of the active matrix organic light emitting diode (AMOLED), so as to increase the driving voltage of the light emitting device as well as the stability with regard to the time passed by.
2. Description of Related Art
Accompanying the development of high technology, video products, especially the digital video or image devices, have become a popular product in our daily life. Within these digital video or image devices, display devices are an important element for displaying the related information. The users can read the information from the display further to control the device operation.
In order to comply with modern life, the size of the video or image device is getting thinner and lighter. The conventional Cathode Ray Tube (CRT) display occupies a large capacity and consumes more electricity. Therefore, complying with photoelectron and semiconductor manufacturing technologies, the panel display device has been developed and has become a common used display product, like the LCD or the active matrix organic light emitting diode display.
LCD technology has developed for several years, so it is hard to have a breakthrough now. However, the active matrix organic light emitting diode (AMOLED) display technology, a newly developed technology, will be main stream of the display device accompanying LCD in the future. The major feature of the AMOLED display is using TFT technique to drive the organic light emitting diode, and the driving IC is installed on the panel directly, so as to fulfill the requirement of being light/thin/short/small in volume and reducing cost. The AMOLED display can be applied on the medium or small size panel in cellular phone, PDA, digital camera and palm game player, portable DVD player and the automobile global positioning system, which can even be implemented in a large size panel like computer and plane TV in the future.
The digital display is characterized by a display screen composed of multiple pixels in a matrix arrangement manner. In order to control individual pixels, a specific pixel is commonly selected via a scanning line and a data line, and an appropriate operating voltage is also provided, so as to display the display information corresponding to this pixel.
FIG. 1
schematically shows a sketch map of an AMOLED circuit that drives a corresponding pixel in the prior art. Referring to
FIG. 1
, the driving circuit comprises transistors
100
and
102
. The transistor is such as the thin film transistor (TFT). A gate of the transistor
100
connects to the scanning line and receives a scanning voltage Vscan at an appropriate point of time, and a source of the transistor
100
also receives a digital data voltage Vdata sent from the data line at this point of time. A drain of the transistor
100
connects to a gate of the transistor
102
. Generally speaking, the source and the drain of the transistor are swappable. The case shown in the present invention is only exemplified here for description. In addition, a storage capacitor
106
is connected in between the gate of the transistor
102
and a grounded voltage. The drain of the transistor
102
connects to a voltage source V
DD
, the source of the transistor
102
further connects to an anode of the organic light emitting device
104
, and a cathode of the organic light emitting device
104
connects to a corresponding low voltage V
SS
.
The operation principle of the driving circuit shown in
FIG. 1
mentioned above is described as follows. When the gate of the transistor
100
is activated by receiving the Vscan provided by the scanning line, the digital data voltage Vdata is input into the gate of the transistor
100
, so as to activate the transistor
102
. Meanwhile, the voltage source V
DD
flows into the organic light emitting device
104
via the transistor
102
, and makes it emit the light. The transistor
102
is also generally called a driving device. When the circuit is operating, the scanning line clock Vscan is input into the transistor
100
with a pre-determined frequency, and the time period between its clock pulses is also called a frame. A pre-determined image data block is input into the corresponding pixel during a time period of the frame. When the scanning line clock pulse Vscan activates the transistor
100
, the transistor
102
is also subsequently activated by the digital data voltage, and the digital data voltage Vdata is also stored in the storage capacitor
106
, so as to maintain the activation of the transistor
102
.
Therefore, the conventional organic light emitting device
104
always stays in the activation state in any of the frames. The variance only exists in the fact that the conventional voltage Vdata has different display gray scales in different frames. In other words, the light emitting device of the TFT-AMOLED always makes it stay at the emitting state in the conventional design. Conventionally, such emitting method complies with the image display effect and is able to avoid the picture flicking. In order to have the light emitting device continuously be driven, relatively, the transistor
102
must maintain its activation state.
However, when the light emitting device
104
, such as the organic light emitting diode, is operated for a long time period, there is a driving current continuously flowing through the light emitting device
104
. Therefore, its characteristic such as the driving voltage V
OLED
increases over time. Thus, the light emitting state of the light emitting device, such as the variances of the brightness and color, are impacted as shown in FIG.
2
. The relationship between the effect caused by the deviation of the driving voltage V
OLED
and the driving circuit cooperated with the TFT is described hereinafter.
When the organic light emitting device
104
is activated, the TFT driving current I
D
has a relationship as shown in formula (1)-(4):
I
D
=½
k
(
V
Ss
−V
th
)
2
(1)
I
D
=½
k
(
V
G
−V
S
−V
th
)
2
(2)
V
S
=V
OLED
+V
SS
(3)
I
D
=½
k
(
V
G
−V
OLED
−V
SS
−V
th
)
2
(4)
where k is a TFT characteristic constant, V
G
=Vdata, and V
OLED
is the driving voltage of the light emitting device
104
. As shown in the formula (1) (4) above, when the driving voltage V
OLED
increases, since it is activated for a long time, the driving current I
D
flowing through the organic light emitting device
104
reduces accordingly, thus impacts the light emitting condition of the organic light emitting device
104
, and the brightness is also reduced accordingly. The life of the organic light emitting device
104
depends on its light emitting capability. Therefore, the variance of the driving voltage V
OLED
greatly impacts the organic light emitting device
104
.
In addition, similarly, when the transistor
102
is activated for a long time, its threshold voltage Vth increases accordingly. The threshold voltage Vth is the same as the driving voltage V
OLED
, the current flowing through the light emitting device
102
reduces when the threshold voltage Vth increases. Therefore, the threshold voltage Vth further deteriorates the light emitting quality.
SUMMARY OF INVENTION
Therefore, the present invention provides a driving circuit for the light emitting device, able to avoid the deviation of the driving voltage V
OLED
of the light emitting device, and at least maintaining the driving voltage V
OLED
on a stable value even under a long time operation of displaying image, so as to efficiently improve the display product quality. Furthermore, the threshold voltage Vth can also maintain a stable value without any deviation.
The present invention provides a driving circui
Au Optronics Corporation
Jiang Chyun IP Office
Lee Wilson
LandOfFree
Driving circuit and method for light emitting device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Driving circuit and method for light emitting device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Driving circuit and method for light emitting device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3208676