Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-03-24
2003-04-29
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S077000, C345S080000, C345S074100, C345S075200, C315S169300, C315S169400
Reexamination Certificate
active
06556176
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active type display device employing a thin film transistor (TFT) to drive an emissive element such as an organic electroluminescence (EL) element.
2. Description of Related Art
Organic EL elements are ideal for thin display configurations as they emit light and can eliminate need for a backlight as required in liquid crystal displays, and because they have unlimited viewing angles. Thus, wide application of organic EL elements is expected in the next generation of display devices.
As shown in an example in
FIG. 1
, an organic EL element
51
can be constituted by forming an anode
53
comprising a transparent electrode made of ITO (Indium Tin Oxide) or the like on a transparent glass substrate
52
, and forming, between the anode
53
and a cathode
54
comprising an MgIn alloy, a hole-transport layer
55
comprising MTDATA (4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine), an emissive layer
56
comprising TPD (N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine) and Rubrene, and an electron-transport layer
57
comprising Alq3 (8-hydroxyquinoline aluminum), which are stacked in this order. Holes injected from the anode
53
and electrons injected from the cathode
54
are recombined within the emissive layer
56
to emit light, which is radiated outward from the transparent anode side in the direction shown in the figure.
Display devices employing an organic EL element configured as described above can be classified into a passive type having a simple matrix structure and an active type using TFTS. In the active type device, a circuit structure illustrated in
FIG. 2
is employed for each pixel.
Referring to
FIG. 2
, numeral
70
indicates an organic EL element. A driver circuit corresponding to one pixel is constituted by a switching TFT
71
to be switched on and off according to a selection signal SCAN, in which a display signal DATA supplied from a data line
75
is applied to a drain and the selection signal from a selection signal line
76
is applied to a gate, a capacitor
72
connected between a source of the TFT
71
and a predetermined dc voltage Vsc, which is charged with a display signal supplied when the TFT
71
is ON and which holds a charging voltage VG when the TFT
71
is OFF, and a driving TFT
74
in which a drain is connected to a power source line
77
that supplies a drive power source voltage Vdd. A source is further connected to the anode of the organic EL element
70
, and a hold voltage VG is supplied to a gate of the TFT
74
from the capacitor
72
to drive the organic EL element
70
. In this example, the cathode of the organic EL element
70
is connected to a ground (GND) potential, and the drive power source voltage Vdd is set to a positive potential, 10V for example. Also, the voltage Vsc is of the same potential as Vdd or the ground (GND) potential.
In the above mentioned configuration, a voltage supplied from the data line
75
when the TFT
71
is ON is applied to the gate of the driving TFT
74
, and a current in accordance with this voltage flows through the EL element
70
which then emits light.
Video signals can be classified into an analog signal and a digital signal. None of active type organic EL display devices have been heretofore applicable to a digital video signal. On the other hand, a liquid crystal display device applicable to a digital video signal is conventionally configured as shown in FIG.
3
.
Specifically, digital video data D
0
to D
3
supplied from outside are captured in accordance with a shift clock from a shift register
30
into a data register
31
, which latches the received data to a latch circuit
32
. The digital data thus latched are then converted by a D/A converter
33
into an analog display signal, which is then supplied to a data line
34
. To the data line
34
, a drain of a TFT
36
which is opened and closed in accordance with a selection signal from a selection signal line
35
is connected. A pixel electrode
370
and a storage capacitor
38
are connected to a source of the TFT
36
. A common electrode (
371
) faces the pixel electrode
370
, and a liquid crystal
37
fills between both electrodes.
In the conventional liquid crystal display device to which a digital video signal is input, the driver circuit includes a D/A converter for converting a digital signal into an analog signal, as described above. For instance, when the bit number of digital data is 4, “16” stages of gray scale voltage (tone scale voltage) V
0
to V
15
are necessary for converting the digital data into an analog signal.
Therefore, in order to integrate the driver circuit within a display panel, the display panel must include terminals and wiring patterns for inputting as many as 16 stages of gray scale voltage V
0
to V
15
, thereby failing to downsize the panel. Even if the driver circuit is configured as an IC, the number of terminals for the IC increases.
The same problem is found in an active type EL display device in which each pixel has a circuit structure such as shown in FIG.
2
and the EL element can be driven by a circuit similar to the driver circuit of FIG.
3
.
SUMMARY OF THE INVENTION
The present invention was made to solve the aforementioned problem of the related art, and aims to provide an active type EL display device which, when a digital video signal is input thereto, does not need the gray scale voltage in accordance with the bit number of the digital data.
In accordance with one aspect of the present invention, there is provided an active matrix type emissive display device including, for each pixel, a thin film transistor to be opened and closed according to a selection signal and an emissive element connected to a data line via said thin film transistor for emitting light according to a supplied current, comprising n transistors for receiving, bit by bit, n bit input digital data sequentially captured for each data line and respectively outputting a different electrical current, wherein a current signal corresponding to a total current amount from said n transistors is output to a corresponding data line and is supplied to said emissive element via said thin film transistor.
In accordance with another aspect of the present invention, there is provided an active matrix type electroluminescence display device comprising an electroluminescence element connected to a data line via a thin film transistor to be opened and closed according to a selection signal, a data register for sequentially capturing input digital data for each data line, a latch circuit for holding the data received by said data register for a predetermined period, and n switches (n: an integer value greater than or equal to 2) to which n bit of digital data for each data line is input bit by bit as a control signal, each of said n switches supplying a different amount of electrical current.
In accordance with another aspect of the present invention, there is provided a driver circuit for an electroluminescence display device including, for each pixel, a thin film transistor to be opened and closed according to a selection signal and an electroluminescence element connected to a data line via said thin film transistor for emitting light according to supplied electrical current, said driver circuit comprising n transistors for capturing, bit by bit, n bit input digital data sequentially supplied for each data line and for respectively outputting a different amount of electrical current, wherein a signal corresponding to a total electrical current from said n transistors is output to a corresponding data line.
In accordance with still another aspect of the present invention, in a device or driver circuit as described above, each of said n transistors has a different size.
In accordance with still another aspect of the present invention, the emissive element referred to above is an organic electroluminescence element comprising an emissive layer containing an organic compound between
Komiya Naoaki
Okuyama Masahiro
Mengistu Amare
Nguyen Jimmy H.
Sanyo Electric Co,. Ltd.
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