Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-05-14
2001-05-08
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S076000
Reexamination Certificate
active
06229266
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a display device comprising a plurality of electroluminescent diodes and means for causing one or more of the diodes to luminesce in dependence upon a voltage to be supplied between two connection points.
Electroluminescent (EL) or light-emitting display devices of this type may be used in, for example displays and level indicators for checking battery voltages, and as voltage meters in, for example shavers, portable telephones, for output power display in audio applications, etc.
Such level indicators are generally known and comprise a plurality of juxtaposed light-emitting diodes (LEDs) with the voltage supplied to the above-mentioned connection points determining the number of LEDs which luminesces. Such a circuit comprises a plurality of LEDs, each in its own housing, which are arranged on a printed circuit board so as to work together functionally. The supplied voltage is converted via an A/D converter into a digital value; the number of diodes to be energized is determined via a second IC (or via integration in the A/D converter).
The costs of placing and mounting said components are generally high. Moreover, the printed circuit requires extra space and covers a considerable part of the volume, particularly in the case of portables.
SUMMARY OF THE INVENTION
It is, inter alia, an object of the invention to obviate one or more of the above-mentioned problems.
To this end, a display device according to the invention is characterized in that at least the electroluminescent diodes are integrated in one semiconductor body.
Such an integration of the diodes reduces the costs of placement and generally reduces the number of connections to be provided.
A preferred embodiment of the display device is characterized in that the electroluminescent diodes comprise a layer of electroluminescent material with a common active layer of an organic material, which layer is present between a first and a second pattern of electrodes, at least one of the two patterns being transparent to light to be emitted by the active layer, and the first pattern comprising a material which is suitable for injecting charge carriers.
Organic materials such as, for example, semiconducting organic polymers are used for the active layer. This increases the number of possible materials for use in these types of display devices. The active layer and the two electrode layers jointly constitute a plurality of LEDs, for example, in the form of light-emitting surfaces which are arranged side by side. The operation is based on the recombinations of electron hole pairs which are injected into the semiconductor material (during use in the forward direction) from electrodes located on both sides of the active layer. Due to these recombinations, energy in the form of (visible) light is released, which is a phenomenon referred to as electroluminescence. The wavelength and hence the color of the emitted light are also determined by the band gap of the (semiconductor) material.
To cause the juxtaposed light-emitting surfaces to function as a level indicator, for example, the threshold voltage for each branch comprising a LED is different so that, with an increasing voltage, an increasing number of surfaces lights up. This discriminating power can be realized in different ways.
A first embodiment of a display device according to the invention is therefore characterized in that the electroluminescent diodes are arranged in parallel branches and are provided with mutually different series resistors in the parallel branches.
Since different diodes now have at least one connection in common, the total number of connections to be provided is reduced.
The display device preferably comprises parallel resistors arranged parallel to the electroluminescent diodes, or the electroluminescent diodes are provided with mutually different series resistors between the electroluminescent diodes and a voltage divider.
In both cases, the discriminating power of the level indicator is increased. A further improvement is obtained by using an optical filter.
Said resistors are preferably integrated in the same semiconductor body, for example, by dimensioning the connection tracks which are preferably made of the same transparent material (for example, indium-tin-oxide) as the transparent electrodes.
Instead of the above-described resistive discrimination, frequency discrimination is alternatively possible. A display device according to the invention, based on this principle, is characterized in that the display device comprises a voltage-controlled oscillator arranged between a connection point and the part of the parallel branches which is common for the series resistors.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
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“LED Display's”, Katalogus Display Electronika 1995, p. 1-67 and 9-5.
Liedenbaum Coen T. H. F.
Van Velzen Jeroen
Alemu Ephrem
U.S. Philips Corporation
Waxler Aaron
Wong Don
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