Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-06-05
2002-08-06
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S164000, C345S204000
Reexamination Certificate
active
06429600
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting display device having an indicator that emits a light by supplying a d.c. current thereto and a drive circuit for the indicator, and more particularly to a light emitting indicator drive circuit that conducts a correcting process for reducing a difference in the luminance of emitted light of an indicator having a plurality of display elements each having a different area.
2. Description of the Related Art
FIGS. 8 and 9
show a schematic diagram of a light emitting indicator drive circuit in a conventional light emitting display device and the element shape of a conventional light emitting indicator, respectively. As shown in
FIG. 8
, display elements
801
to
820
are connected to each other in the form of a matrix. Also, an output of a constant-current source
821
having a switching function and allowing a current to flow out is connected to the plus electrode sides of each of the display elements
801
,
806
,
811
and
816
. Likewise, an output of a constant-current source
822
is connected to the display elements
802
,
807
,
812
and
817
, an output of a constant-current source
823
is connected to the display elements
803
,
808
,
813
and
818
, an output of a constant-current source
824
is connected to the display elements
804
,
809
,
814
and
819
, and an output of a constant-current source
825
is connected to the display elements
805
,
810
,
815
and
820
, respectively. On the other hand, a switch
826
that allows a current to flow into the ground is connected to the minus electrode sides of the display elements
801
,
802
,
803
,
804
and
805
, respectively. Likewise, a switch
827
is connected to the display elements
806
,
807
,
808
,
809
and
810
, a switch
828
is connected to the display elements
811
,
812
,
813
,
814
and
815
, and a switch
829
is connected to the display elements
816
,
817
,
818
,
819
and
820
, respectively.
As shown in
FIG. 9
, in the conventional example, the areas of the display elements
801
to
820
disposed within the light emitting indicator
901
are identical with each other, and the constant current sources
821
to
825
supply the same current. For that reason, the current densities of all the display elements become equal to each other, to thereby obtain substantially the same luminance.
In order to turn on/off the respective display elements, each of the constant current sources
821
to
825
has a switching function so that the respective constant current sources can be turned on/off. On the other hand, the switches
826
to
829
are sequentially turned on one by one in a time sharing manner, and there is no case where two or more switches are turned on at the same time. In the case of turning on the display element
801
, the constant current source
821
and the switch
826
are turned on. Similarly, all the display elements can be selectively turned on by the combination of the constant current sources
821
to
825
with the switches
826
to
829
.
With the above structure, a very large number of light emitting elements are arranged to conduct dot matrix display, thereby being capable of executing various display.
If the dot matrix display is applied to an indicator such as an arm watch which requires a small size and a low power, the power consumption of the driver and the chip size become large, and also if no fine dot matrix display is used, fine character display cannot be made. In this case, segment display is advantageous, but it is difficult to make the areas of the respective segments identical with each other, as a result of which if the segments are driven by a constant current source having the same current value, the density of current depends on the segments to produce a difference in luminance between the respective segments.
in particular, even a difference in luminance of several percent between the adjacent segments is conspicuous. For that reason, if the luminance between the respective segments is not corrected, the display quality is remarkably degraded so that the dot matrix display cannot be applied to a fashionable wristwatch or the like.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above drawbacks, and therefore an object of the present invention is to provide a light emitting indicator drive circuit which is capable of correcting a difference in luminance between the respective segments to a level where there occurs no problem from the visual viewpoint.
In order to achieve the above object, according to the present invention, there is provided a light emitting indicator drive circuit in which a current value is made variable by a gate voltage of a MOS-FET which is a constant-current source that drives display segments, and the gate voltage is controlled by using segment area information, light emitting characteristic degradation information or the like, or the current is controlled by combination of the on/off states of a plurality of FETs.
According to one aspect of the present invention, there is provided a light emitting display device, comprising a light emitting indicator having a plurality of light emitting elements different in light emitting area, a constant current source that supplies a current to the plurality of light emitting elements, respectively, and a current control circuit that controls the constant current source to supply constant currents to the respective elements corresponding to the areas of the plurality of light emitting elements. With this structure, the currents which are supplied to the respective light emitting elements can be corrected, individually, without the need to increase or reduce a generated current value using a resistor or impedance element so that the light emitting luminances of the plurality of light emitting elements become substantially equal to each other, thereby being capable of reducing a variation in the luminance between the respective display elements.
Also, according to another aspect of the present invention, there is provided a light emitting display device in which a constant current FET is used as the constant current source, and the current control circuit controls the gate voltage of the constant current FET to supply the current corresponding to the areas of the plurality of light emitting elements to the respective light emitting elements. For that reason, a variation in the luminance between the respective display elements can be reduced. By utilizing a plurality of constant current FETs of different driving capabilities, a more fine control of the current is possible.
Further, according to still another aspect of the present invention, there is provided a light emitting display device in which the area information of the plurality of light emitting elements are installed in the interior of the light emitting indicator. With this structure, the current control circuit can read the area information if necessary to correct a current which is supplied to the respective light emitting elements so that the light emitting luminances of the plurality of light emitting elements become substantially constant. In the case where a constant current FET is employed as the constant current source, the current control circuit can read the area information if necessary to correct the gate voltage of the constant current FET so that the light emitting luminances of the plurality of light emitting elements become substantially constant, to thereby control a current which is supplied to the respective light emitting elements. For that reason, a variation in the luminance between the respective display elements can be reduced with high accuracy.
Still further, according to still another aspect of the present invention, there is provided a light emitting display device comprising: a deterioration detecting circuit that detects the deterioration of a light emitting characteristic; and a signal processing circuit that calculates deterioration information outputted from the de
Akase Tokuya
Fujita Susumu
Hoshino Masafumi
Odagiri Hiroshi
Sakumoto Kazumi
Adams & Wilks
Seiko Instruments Inc.
Vu David
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