Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-10-06
2002-02-26
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169200, C315S169300, C315S169400, C345S214000, C345S055000, C345S077000
Reexamination Certificate
active
06351076
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a unit for driving a luminescent display panel using a capacitive luminescent element, such as an organic electro-luminescent element.
As a display which attains low power dissipation, high-quality display, and lower profile, an electro-luminescent display, in which a plurality of organic electro-luminescent elements are arranged in a matrix pattern, has attracted attention. As shown in
FIG. 1
, the organic electro-luminescent element is formed by means of stacking, on a transparent substrate
100
such as a glass plate on which a transparent electrode
101
is formed, at least one organic function layer
102
which is made up of an ion transport layer, a light-emitting layer; and a positive hole transport layer, and a metal electrode
103
. A positive voltage is applied to the anode of the transparent electrode
101
, and a negative voltage is applied to the cathode of the metal electrode
103
. A d.c. current is applied across the transparent electrode
101
and the metal electrode
103
, wherewith the organic function layer
102
illuminates. Use of an organic compound which can be expected to exhibit a superior luminous characteristic embodies a practicable electro-luminescent display.
The organic electro-luminescent element (hereinafter referred to simply as an “EL element”) can be electrically expressed as an equivalent circuit shown in FIG.
2
. As can be seen from the drawing, the EL element can be replaced with a capacitive component C and a diode component E which is connected in shunt with the capacitive component and has a diode characteristic. For this reason, the organic electro-luminescent element is considered to be a capacitive luminescent element. When a light-emitting d.c. drive voltage is applied across electrodes of the organic electro-luminescent element, electric charge is stored in the capacitive component C. When the light-emitting d.c. drive voltage exceeds a barrier voltage or threshold illumination voltage unique to the EL element, an electric current starts flowing from the electrode (i.e., the anode of the diode component E) to the organic function layer, which also acts as a light-emitting layer, whereupon the organic electro-luminescent element illuminates at an intensity proportional to the electric current.
As shown in
FIG. 3
, the characteristic of the EL element concerning a voltage V, a current I, and luminance L is analogous to that of a diode. The current I is considerably small at a voltage lower than the threshold illumination voltage V
th
and abruptly increases at a voltage higher than the threshold illumination voltage V
th
. The electric current I is substantially proportional to the luminance L. When a drive voltage exceeding the threshold illumination voltage V
th
is applied to the EL element, the EL element illuminates at an intensity proportion to the electric current corresponding to the drive voltage. If the drive voltage to be applied to the EL element is below the threshold illumination voltage V
th
no drive current flows through the EL element, and hence the luminous intensity of the EL element remains substantially zero.
A passive matrix drive method has hitherto been known as a method of driving a luminescent display panel using a plurality of EL elements.
FIG. 4
shows an example structure of a driver device of passive matrix drive type for driving a luminescent display panel. In a luminescent display panel, “n” cathode lines (i.e., metal electrodes) B
1
to B
n
are arranged in parallel with each other so as to extend in the lateral direction, and “m” anode lines (i.e., transparent electrodes) A
1
to A
m
are arranged in parallel with each other so as to extend in the longitudinal direction. In respective intersections (a total number of “n×m”) between the cathode lines and the anode lines, light-emission layers of EL elements E
1
to E
m
are sandwiched. The EL elements E
1
to E
m
, which serve as pixels, are arranged in a matrix pattern and are positioned in respective intersections between the anode lines A to A
h
and the cathode lines B
1
to B
n
. One end of the EL element (i.e., the anode of the diode component E of the equivalent circuit) is connected to the anode line, and the other end of the EL element (i.e., the cathode of the diode component E of the equivalent circuit) is connected to the cathode line. The cathode line is connected to and activated by a cathode line scanning circuit
1
, and the anode line is connected to and activated by an anode line drive circuit
2
.
The cathode line scanning circuit
1
has scan switches
5
1
to
5
n
assigned to respective cathode lines B
1
to B
n
for determining respective electric potentials thereof. Each of the scanning switches
51
to
5
n
connects to a corresponding cathode line either a reverse bias voltage (e.g., 10 volts) produced from a supply voltage, or a ground potential (e.g., 0 volt).
The anode drive circuit
2
has current sources
2
1
to
2
m
(e.g., constant-current sources) for supplying a drive current to respective EL elements, and drive switches
6
1
to
6
m
, which are assigned to the anode lines A
1
to A
m
. The drive switches
6
1
to
6
m
supply a current to the respective anode lines A
1
to A
n
by means of switching operations. A voltage source, such as a constant-voltage source, can be used as a drive source. The previously-described current-luminance characteristic is stable against temperature variations, whereas a voltage-luminance characteristic is unstable against temperature variations. For this reason, a current source (a source circuit which is to be controlled such that the amount of supply current assumes a desired value) is commonly used. The amount of current supplied from current sources
2
1
to
2
m
is the amount of current required for sustaining a state in which an EL element illuminates at desired instantaneous luminance (this state will hereinafter be referred to as a “steady luminous state”). When the EL element is in a steady luminous state, electric charge corresponding to the amount of supply current is charged into the capacitive component C of the EL element. The voltage across the EL element assumes a specified value Ve corresponding to instantaneous luminance (hereinafter referred to as a “specified illumination voltage”).
The anode lines A
1
to A
m
are connected to an anode line reset circuit
3
. The anode line reset circuit
3
has shunt switches
7
1
to
7
m
assigned to respective anode lines A
1
to A
m
. The anode lines A
1
to A
m
are brought into ground potential by means of selection of the shunt switches
7
1
to
7
m
. The cathode line scanning circuit
1
, the anode line drive circuit
2
, and the anode line reset circuit
3
are connected to an illumination control circuit
4
.
The illumination control circuit
4
controls the cathode line scanning circuit
1
, the anode line drive circuit
2
, and the anode line reset circuit
3
, to thereby display a video in accordance with a video signal supplied from an unillustrated video signal generation system. The illumination control circuit
4
sends a scanning line selection control signal to the cathode line scanning circuit
1
, to thereby perform operations for selecting a cathode line corresponding to a horizontal scanning period of a video signal and setting the thus-selected cathode line to ground potential. The scanning switches
5
1
to
5
n
are switched so as to apply a reverse bias voltage Vcc to the remaining cathode lines. The reverse bias voltage Vcc is applied from the constant-voltage line connected to the cathode line, in order to prevent illumination of EL elements connected to intersections between the anode line through which a drive current is flowing and cathode lines which are not selected for scanning, which would otherwise be caused by crosstalk. Here, the reverse bias voltage Vcc is usually set equal to the specified illumination voltage Ve. During each horizontal scanning period, the scanning switches
5
1
to
5
n
are sequentially switch
Satake Yoichi
Yoshida Takayoshi
Tohoku Pioneer Corporation
Vu Jimmy T.
Wong Don
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