Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-12-13
2004-08-10
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S076000, C345S211000
Reexamination Certificate
active
06774577
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a flat panel display device for compensating a threshold voltage of a panel; and, more particularly, to a flat panel display device capable of simplifying a circuit and a driving method, improving an opening ratio of a panel and enhancing an image quality of a display unit by compensating a panel threshold voltage through a driving circuit of the flat panel display device.
DESCRIPTION OF RELATED ARTS
Generally, electroluminescence (hereinafter referred as to EL) is a phenomenon that a fluorescent substance luminesces as a current passes through it. An EL panel is typically used for illuminating light at the back of a liquid crystal display (hereinafter referred as to LCD) of a portable computer such as a notebook computer. However, since the EL panel is recently enabled with a function of self-luminescence, an additional backlight is not required compared to a conventional LCD. Based on this advantage and studies on such methods for acquiring a high definition image and a longer lifetime of the EL, the EL panel, in today, is employed for a high definition display unit including a LCD for a mobile telecommunication terminal. Furthermore, the EL panel will have broad applications in a near future. The EL panel includes an organic or inorganic self-luminescent body being placed in between two thin electrodes. Indeed, one of the two thin electrodes is transparent. This luminescence is caused by the energy released when excited electrons of a particular impurity at a central luminescence of a luminescent substance return to their ground states. Herein, free electrons accelerated by the EL excite the electrons of the particular impurity, which is also called an activator. Intensity of the luminescence increases in proportion to exp(−c/{square root over (&ngr;)}), and frequency also increases proportionally up to a certain point.
The luminescence phenomenon due to an organic substance is discovered by Anthracene in 1960s. Thereafter, Eastman Kodak Company developed an ultra thin film double layer stacking type organic EL device in 1987, and Pioneer Corporation commercialized a single color organic EL display device by the end of 1997. A 5.5 inched natural color organic EL display device developed by Sanyo-Kodak is further demonstrated at the Society for Information Display(SID) in 2000.
The organic EL device has about 10 V of a driving voltage, which is lower than driving voltages of other display devices such as a thin film transistor-liquid crystal display (TFT-LCD), a plasma display panel (PDP), a field emission display (FED) and so forth. Also, the organic EL device has an advanced perceptibility due to self-luminescence. Furthermore, it is possible to make a thickness of the organic EL device thinner because it does not need a backlight unlike the TFT-LCD. Compared to currently used LCD, the organic EL device also has a rapid responsiveness and a wide angular field, and thus, it is expected to be a next generation display device.
FIG. 1
is a circuit diagram showing an organic EL display unit according to a prior art. The conventional organic EL display unit
100
includes a first TFT
101
having a first (source) terminal that receives a data signal from a source line and a second (gate) terminal that receives a gate enable signal from a gate enable line (GE), a second TFT
102
having a first (source) terminal supplied with power from a power line and a second (gate) terminal connected to a third (drain) terminal of the first TFT
101
, a power maintenance capacitor
103
that charges a driving voltage of the second TFT
102
through which a first terminal is connected to the first terminal of the second TFT
102
and a second terminal to the third terminal of the first TFT
101
, and an organic EL device
104
having a first terminal connected to a third (drain) terminal of the second TFT
102
and a second terminal is coupled to a ground terminal luminesces in case that currents are flowing.
The following will describe operations of the organic EL display unit
100
in accordance with the prior art.
Firstly, a gate enable signal provided from the gate enable line (GE) is activated, and the first TFT
101
is turned on. At this time, display data are transmitted to the second terminal of the second TFT
102
through the source line and the first TFT
101
. This voltage is transmitted to the second TFT
102
, which is a driving transistor, and the power maintenance capacitor
103
of the power line. Once the power maintenance capacitor
103
is charged with the driving voltage, the organic EL device
104
luminesces since currents can flow from the power line to the organic EL device
104
. Even if the gate enable signal from the gate enable line (GE) is inactivated, the power maintenance capacitor
103
is still able to luminesce because the driving voltage for making the organic EL device
104
luminesce is still remained causes currents to flow from the power line to the organic EL device
104
.
However, in case of driving the organic EL device
104
based on the above scheme, the second TFT
102
of each display unit cell has a different threshold voltage (V
th
), and thus, an amount of currents supplied to the organic EL device
104
in each cell is different. Herein, the second TFT
102
is a driving transistor for the organic EL device
104
. That is, there occur problems of a non-uniform screen and a decreased image quality because intensity of luminescent light of the organic EL device
104
changes inconsistently.
FIG. 2
is a circuit diagram showing a typical organic EL display unit
200
for coping with the inconsistent V
th
according to another prior art. The typical organic EL display unit includes a first TFT
201
having a first (source) terminal that receives a data signal from a source line and a second (gate) terminal that receives a gate enable signal from a gate enable line (GE), a first capacitor
202
that charges a driving voltage of a second TFT
203
by being connected to a third (drain) terminal of the first TFT
201
, the second TFT
203
having a first (source) terminal supplied with power from a power line and a second (gate) terminal connected to a second terminal of the first capacitor
202
, a second capacitor
204
that charges a threshold voltage of the second TFT
203
through which a first terminal is connected to the first terminal of the second TFT
203
and a second terminal to the second terminal of the first capacitor
202
, a third TET
205
having a first (source) terminal connected to the second terminal of the second TFT
203
, a second (gate) terminal receiving a first switch control signal AZ and a third (drain) terminal connected to a third (drain) terminal of the second TFT
203
, a fourth TFT
206
having a first (source) terminal connected to the third terminal of the third TFT
205
and a second (gate) terminal receiving a second switch control signal AZB and an organic EL device
207
that luminesces when currents are flowing through which a first terminal is connected to a third (drain) terminal of the fourth TFT
206
and a second terminal coupled to a ground terminal.
FIG. 3
is a diagram showing procedural timing for operating the organic EL display unit
200
according to still another prior art. With reference to the operational timing, the following will describe operations of the organic EL display unit
200
in accordance with the prior art.
Firstly, once a gate enable signal from the gate enable line GE is activated in a state that a second switch control signal AZB is precedently activated, the first switch control signal AZ is activated to primarily turn the second TFT
203
on, thereby charging a threshold voltage of the second TFT
203
connected to the second capacitor
204
. Afterwards, the first switch control signal AZ is inactivated, and a driving voltage, i.e., DATA, of the second TFT
203
is transmitted from the source line to the first capacitor
202
. Herein, the second TFT
203
is a driving transistor. Once the first capacitor
202
is
Birch & Stewart Kolasch & Birch, LLP
Hynix / Semiconductor Inc.
Tran Thuy Vinh
Wong Don
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