Driving circuit for light-emitting diodes

Details Driving circuit for light-emitting diodes Driving circuit for light-emitting diodes

1999-07-29

2001-07-24

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Plural power supplies

Plural load devices

C315S169300

Reexamination Certificate

active

06265832

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a driving circuit for light-emitting diodes which can be used to set the brightness of the light-emitting diodes. Light-emitting diodes are also used for showing information. In this case, either particular items of information are assigned particular light-emitting diodes, or else the displays are divided into a plurality of segments or matrices, each containing one or more light-emitting diodes. In this way, different items of information can be shown on a display according to whether the light-emitting diodes are switched on or off.
To drive the light-emitting diodes, it is known practice to impress the current through the light-emitting diodes using a series resistor connected between the driving voltage and the light-emitting diodes. Nevertheless, different forward voltages U
LED
of the individual light-emitting diodes (e.g. U
LED
=1.8 V to 2.5 V for red light-emitting diodes) mean different sizes of current through the light-emitting diodes and hence different brightnesses as well.
It is also known practice to drive the light-emitting diodes using a current source. In this way, forward-voltage tolerances of the light-emitting diodes and voltage-supply tolerances now have only a small influence on the brightness of the light-emitting diodes.
Finally, it is known practice to arrange a plurality of current sources in an integrated circuit. To keep the power loss in the integrated circuit to a level which prevents the integrated circuit from being destroyed as a result of thermal overstressing, known driving circuits involve the use of a series resistor for each current source, some of the superfluous voltage dropping across said series resistor and thus preventing the integrated circuits from overheating.
A disadvantage of this is that a relatively large number of series resistors are required for the known driving circuit.
SUMMARY OF THE INVENTION
Hence, the object of the invention is to provide a driving circuit for light-emitting diodes, in which the current through each light-emitting diode can be set by a respective current source, and in which a plurality of current sources are combined in an integrated circuit which requires few series resistors and is thus less expensive.
This object is achieved in that one series resistor is used for a plurality of light-emitting diodes and hence current sources as well. This makes it possible to eliminate a large portion of the unrequited voltage across the series resistor. Since each light-emitting diode is controlled by a separate current source, it is of no consequence for the light intensity of the individual switched-on light-emitting diodes how many of the light-emitting diodes supplied with current via a respective series resistor are presently switched on or off. By reducing the number of series resistors, the space requirement for the driving circuit is also reduced. Thus, with a nominal current through a light-emitting diode of 30 mA, series resistors with a permissible power loss of 250 mW and a supply voltage of 5 V for the driving circuit, three light-emitting diodes can be supplied via one series resistor.
In ordinary integrated circuits, the average possible power loss is indicated as a value at a rated temperature (e.g. 25° C.) with a derating for higher ambient temperatures. This value applies to the whole package. In this case, limits are formed by the package size (number of pins and dimensions) and the design of the so-called frame. This can mean that, for power loss reasons, larger-scale integration, i.e. a higher number of outputs (with appropriate loading), cannot be used or a corresponding number of current sources cannot be integrated.
It is particularly advantageous to make the solder pads for the series resistors on the printed circuit board such that their surface is larger than the surface required for connecting the solder pad and the connection for the series resistor. The cooling effect produced means that the series resistors can then cope with a power loss above their rated power loss.


REFERENCES:
patent: 3666960 (1972-05-01), Dawson
patent: 4100552 (1978-07-01), Muto et al.
patent: 4920283 (1990-04-01), Elmer et al.
patent: 4963798 (1990-10-01), McDermott
patent: 5034757 (1991-07-01), Godlove
patent: 5600157 (1997-02-01), Abike et al.
patent: 6031855 (2000-02-01), Watanabe
patent: 19602891 (1997-08-01), None
patent: 0377956 (1990-07-01), None
patent: 2265978 (1993-10-01), None

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