Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-10-24
2004-08-10
Clinger, James (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S204000
Reexamination Certificate
active
06774572
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a drive circuit for driving a current-driven display unit using organic electroluminescent devices (hereinafter called “EL devices”), light-emitting diodes (hereinafter called “LEDs”), etc. which respectively emit light according to the supply of currents.
This application is counterpart of Japanese patent applications, Ser. No. 328997/2001, filed Oct. 26, 2001, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 1
is a circuit diagram showing the outline of a general display unit using EL devices.
The present display unit principally comprises a display panel
1001
, a scan line drive circuit
1003
, a data line drive circuit
1005
, and a control circuit
1007
.
The display panel
1001
has a plurality of scan lines COM
1
through COMn, a plurality of data lines SEG
1
through SEGm, and a plurality of EL devices EL
11
through ELnm respectively placed at points where the scan lines and the data lines intersect one another.
The scan line drive circuit
1003
comprises a plurality of switch means SWc
1
through SWcn respectively electrically connected to the plurality of scan lines COM
1
through COMn. The switch means SWc
1
through SWcn electrically connect their corresponding scan lines COM
1
through COMn to either a ground potential GND (e.g., 0V) or a scan line source potential Vc (e.g., 20V).
The data line drive circuit
1005
principally comprises a plurality of switch means SWs
1
through SWsm respectively electrically connected to the plural data lines SEG
1
through SEGm, and a plurality of constant current devices CC
1
through CCm. The switch means SWs
1
through SWsm electrically connect their corresponding data lines SEG
1
through SEGm to the ground potential GND or the constant current devices CC
1
through CCm. The constant current devices CC
1
through CCm are respectively electrically connected to the data line source potential Vs (e.g., 20V).
The control circuit
1007
controls the operations of the switch means SWc
1
through SWcn and the switch means SWs
1
through SWsm, based on control data.
The states of the individual switch means at the time that only the EL device EL
11
is in a light emitting state, are shown in FIG.
1
. The light emitting state and non-light emitting state of the EL device will be described below in brief.
The cathode of the EL device EL
11
, i.e., the scan line COM
1
is supplied with the ground potential GND by the switch means SWc
1
of the scan line drive circuit
1003
. Incidentally, when the ground potential GND is supplied to the corresponding scan line, the scan line is defined as an active state or a selected state. On the other hand, when the scan line source potential Vc is supplied thereto, it is defined as an inactive state or a non-selected state. Accordingly, the scan line COM
1
is in an active state at present.
On the other hand, the anode of the EL device EL
11
, i.e., the data line SEG
1
is supplied with the data line source potential Vs by the switch means SWs
1
of the data line drive circuit
1005
. Since the EL device EL
11
is biased in the forward direction in this condition, a current path extending from the data line source potential Vs to the ground potential GND is formed. Thus, such a current I
1
as shown in the drawing flows through the EL device EL
11
. Owing to the flow of the current I
1
through the EL device EL
11
in this way, the EL device EL
11
is allowed to transition to the light emitting state.
The cathode of the EL device EL
21
, i.e., the scan line COM
2
is supplied with the scan line source potential Vc by the switch means SWc
2
of the scan line drive circuit
1003
. Since there is no difference in potential between the anode and cathode of the EL device EL
21
in this condition, a current path extending from the data line source potential Vs to the ground potential GND is not formed. Thus, since no current I
1
flows through the EL device EL
21
, the EL device EL
21
does not change to the light emitting state.
The cathode of the EL device EL
12
, i.e., the data line SEG
2
is supplied with the ground potential GND by the switch means SWs
2
of the data line drive circuit
1005
. Since the anode of the EL device EL
12
is not supplied with a current through the constant current device CC
2
, current I
1
does not flow through the EL device EL
12
and hence the EL device EL
12
is not caused to transition to the light emitting state.
Similarly, the cathode of the EL device EL
22
, i.e., the data line SEG
2
is supplied with the ground potential GND by the switch means SWs
2
of the data line drive circuit
1005
. Further, the cathode of the EL device EL
22
, i.e., the scan line COM
2
is supplied with the scan line source potential Vc by the switch means SWc
2
of the scan line drive circuit
1003
. Since the EL device EL
22
is biased in the reverse direction in this condition, no current I
1
flows through the EL device EL
22
and hence the EL device EL
22
is not transitioned to the light emitting state.
While each of the EL devices is caused to transition to the light emitting state by being supplied with the current as described above, the amount of light emitted therefrom (the degree of light emitted) depends on a current value. When the amount of light emitted from each EL device falls outside a predetermined set value (corresponding to a standardized value set in consideration of an error), an intended display cannot be realized. Thus, the current values supplied to the respective data lines need to be constant values equal to one another. In order to keep constant the current values supplied to the data lines, the data line drive circuit
1005
is provided with the constant current devices CC
1
through CCm. The constant current devices CC
1
through CCm are supplied with a constant voltage at their gates, for example, and comprise MOS transistors operated in their saturated regions.
Since, however, variations and errors in manufacturing exist, the characteristics of all the MOS transistors that function as the constant current devices, do not necessarily fall within the set values (standardized values set in consideration of the errors). While, for example, a threshold voltage exists as one parameter indicative of the characteristic of each MOS transistor, a current Ids flowing between the drain and source of the MOS transistor also falls outside a set value where the threshold voltage takes values different from one another every MOS transistors respectively constituting the constant current devices. Thus, the current values supplied to the respective data lines are not brought to constant values equal to one another and vary each other. As a result, a problem arises in that the amounts of light emitted from the EL devices will vary every data lines.
Accordingly, there has been a demand for the advent of an improved drive circuit hard to be affected by the variations in manufacturing.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a drive circuit that includes an input node for receiving data, an output node. The drive circuit also includes a first MOS transistor of a first conductivity type and a second MOS transistor of the first conductivity type. The first MOS transistor has a source, a drain connected to the output node, and a gate connected to the input node. The second MOS transistor has a source, a drain connected to the source of the first MOS transistor, and a gate supplied with a predetermined potential level. The drive circuit also includes resistance means connected between the source of the second MOS transistor and a source node supplied with a source potential level.
REFERENCES:
patent: 4358728 (1982-11-01), Hashimoto
patent: 5289112 (1994-02-01), Brown et al.
patent: 6445222 (2002-09-01), Hidaka et al.
patent: 6459322 (2002-10-01), Cho
patent: 0407065 (1991-01-01), None
patent: 3-45364 (1991-02-01), None
patent: 6-204564 (1994-07-01), None
patent: 11-291550 (1999-10-
Clinger James
Oki Electric Industry Co. Ltd.
Tran Thuy Vinh
Volentine & Francos, PLLC
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