Constant-current output driver with reduced over-shoot

Details Constant-current output driver with reduced over-shoot Constant-current output driver with reduced over-shoot

2000-01-28

2002-10-22

Tso, Edward H. (Department: 2838)

Electrical transmission or interconnection systems

Switching systems

Condition responsive

Reexamination Certificate

active

06469405

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a constant current output driver, for example, a constant current output driver for emitting a light from a light emitting element such as an organic EL (electro luminescent) panel.
A light emitting diode (LED) and an organic EL (also called “organic LED”) has widely been spread as an image source in a display unit such as a direct-view display unit or a virtual image display unit since a high visibility is obtained because the light emitting diode and the organic EL per se emit a relatively large amount of high-luminance light, and a display speed is higher than that of a TFT (thin film transistor) liquid crystal or the like to make it difficult to produce a latent image. Since the display panel using those LED or organic EL is of the current driven type, a constant current output driver is employed in the drive device.
FIG. 22
shows the structure of a conventional display device using the organic EL of this type.
As shown in
FIG. 22
, the organic EL display unit includes an organic EL panel
1
, a scanning circuit
2
, a drive circuit
3
and a not-shown light emission control circuit that controls the switching operation of the scanning circuit
2
and the drive circuit
3
.
The organic EL panel
1
includes anode lines A
1
to Am and cathode lines B
1
to Bn which are arranged in a simple matrix (lattice), and organic EL elements E
11
to Emn connected to the respective intersecting positions of the anode lines and the cathode lines which are arranged in the form of a lattice. The cathode lines B
1
to Bn are connected to a scanning circuit
51
, and the anode lines A
1
to Am are connected to the drive circuit
3
.
The scanning circuit
2
includes switches S
21
to S
2
n
and conducts scanning operation by sequentially setting the cathode lines B
1
to Bn to the earth potential (0 V) while those switches S
21
to S
2
n
sequentially change over to the earth terminal side at given time intervals.
The drive circuit
3
includes switches S
31
to S
3
m
and constant current elements C
31
to C
2
m
connected to a power supply VDD. The drive circuit
3
is so designed as to connect the anode lines A
1
to Am to the constant current elements C
31
to C
3
m
while controlling the on/off operation of the respective switches S
31
to S
3
m
in synchronism with the scanning operation of the cathode line scanning circuit
2
and to supply a drive current to the organic EL at a desired intersecting position, to thereby emit a light.
A case in which the cathode lines are scanned and the anode lines are driven is shown in FIG.
22
. The same effect is obtained in the structure in which the anode lines are scanned and the cathode lines are driven.
Now, it is assumed that the switch S
22
in the scanning circuit
2
is connected to the earth side to scan a row of the cathode line B
2
. At this timing, when the switches S
31
and S
33
in the drive circuit
3
are changed over to the constant current elements C
31
and C
33
side (turn on), currents I
12
and I
13
flow in the light emitting elements E
12
and E
32
to emit a light.
An inverse bias voltage VCC identical in potential with the supply voltage is applied to other cathode lines B
1
and B
3
to Bn other than the cathode line B
2
which is being scanned, to thereby prevent an error in light emission.
The organic EL is made to emit a light at an arbitrary position by repeating the scanning of the cathode lines B
1
to Bn and the driving of the anode lines A
1
to Am at a high speed, and display is made so that the respective organic ELs emit a light on the entire screen at the same time.
In this situation, in order to prevent the amount of light emission from being made ununiform due to a difference between the current values that flow in the respective organic ELs which is caused by a difference in the wiring distance between the organic ELs, the respective anode lines A
1
to Am are connected to the constant current elements C
31
to C
3
m
that function as the constant current source through the switches S
31
to S
3
m.
FIG. 23
shows the unknown structure proposed in a case where the conventional constant current output driver made up of the constant current elements of the above type and the switches that connect and disconnect the constant current elements and the anode lines is formed of the enhancement MOSFET (metal oxide field effect transistor).
As shown in
FIG. 23
, the constant current output driver is designed such that a p-channel MOSFET
4
that functions as the constant current element, and a p-channel MOSFET
5
and an n-channel MOSFET
6
which function as the switching elements are connected in series. The supply voltage VDD is applied to the drain of the MOSFET
4
, and a voltage VGC which is always constant is applied to the gate of the MOSFET
4
in order to output a constant current.
An input terminal IN is connected to both the gates of the MOSFETs
5
and
6
, an output terminal OUT is connected to the source of the MOSFET
5
and the drain of the MOSFET
6
, and the anode lines of the organic EL panel are connected to the output terminal OUT.
In the constant current output driver made up of the MOSFETs as described above, when a switching signal which is supplied to the input terminal changes from on (high level) to off (low level), the MOSFET
6
changes from on to off and the MOSFET
5
changes from off to on. As a result, the output terminal OUT which has been connected to the ground is connected to the MOSFET
4
which functions as the constant current element through the MOSFET
5
to output a constant current.
FIG. 25
shows the structure of a display unit using organic ELS for color display.
As shown in
FIG. 25
, in an organic EL panel
1
for color display, three times as many anode lines A
1
to As (s=3m) as in a monochrome display are disposed, and anode terminals Al to As for connection to the drive circuit are disposed.
In the organic EL panel
1
for color display, organic ELs for red (R) are connected at the intersecting positions of anode lines A
1
, A
4
, . . . As−2 and the respective cathode lines B
1
to Bn, organic ELs for green (G) are connected at the intersecting positions of anode lines A
2
, A
5
, . . . As−1 and the respective cathode lines B
1
to Bn, and organic ELs for red (R) are connected at the intersecting positions of anode lines A
3
, A
6
, . . . As and the respective cathode lines B
1
to Bn, as in
FIG. 22
although being not shown.
Those light emitting elements for R, G and B are different in optimum constant current value, respectively, when emitting the light. For that reason, as shown in
FIG. 25
, in the conventional drive circuit that drives the organic EL panel
1
for color display, there are used a drive circuit
3
R for R, a drive circuit
3
G for G and a drive circuit
3
B for B. Each of the drive circuits
3
R,
3
G and
3
B includes m (s/3) constant current output sections and m constant current output terminals O
1
to Om.
As shown in
FIG. 25
, in order to drive the organic ELs for R which are disposed at every three organic ELs, the respective output terminals O
1
to Om of the drive circuit
3
R for R are connected with the anode terminals A
1
, A
4
, . . . As−2 of the organic EL panel
1
for color display. Likewise, the respective output terminals O
1
to Om of the drive circuit
3
GR for G are connected with the anode terminals A
2
, A
5
, . . . As−12 of the organic EL panel
1
for color display, and the respective output terminals O
1
to Om of the drive circuit
3
B for B are connected with the anode terminals A
3
, A
6
, . . . As.
As described above, in the case where the conventional constant current output driver is formed of MOSFETs as. shown in
FIG. 23
, the current flowing in the respective organic EL elements can be made substantially constant by use of the constant current element.
However, the conventional constant current output driver suffers from the following problems.
A first problem is that over-shoot occurs in the current outputted

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