Patent Application
20080238864
The present invention relates to a light emitting diode driving circuit capable of modulating a driving current of a light emitting diode according to a working temperature, and a liquid crystal display using the light emitting diode driving circuit.
Liquid crystal displays are commonly used as displays for compact electronic apparatuses. This is because they not only provide good quality images with little power source consumption, but also they are very thin. The liquid crystal in a liquid crystal display does not emit any light beams itself. Thus, a backlight module is generally needed for a liquid crystal display. The backlight module typically includes one or more light emitting diodes.
Referring to
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Similarly, when the light emitting diode 733 works at a certain driving current, the working temperature should be less than a predetermined threshold value, otherwise the light emitting diode 733 will be damaged. The threshold value is the working temperature limit. For example, when the light emitting diode 733 works at a driving current of 25 mA, a working temperature of the light emitting diode 733 should be less than 40° C.
Usually, a driving current of the light emitting diode 733 is constant, and a working temperature of the light emitting diode 733 rises gradually. Once the working temperature exceeds the working temperature limit corresponding to the present driving current, the light emitting diode 733 is liable to be damaged. Even if the light emitting diode 733 is not damaged, the liquid crystal display 700 may work abnormally.
What is needed, therefore, is a light emitting diode driving circuit that can overcome the above-described deficiencies. What is also needed is a liquid crystal display using such a light emitting diode driving circuit.
In one preferred embodiment, a light emitting diode driving circuit includes a light emitting diode, a temperature detector provided adjacent to the light emitting diode, a micro-processor and a constant current circuit. The constant current circuit is configured for generating a driving current to drive the light emitting diode. The micro-processor is configured for generating a plurality of pulse signals with an adjustable duty-cycle, and applying the pulse signals to the constant current circuit. The temperature detector is configured for detecting a present working temperature of the light emitting diode. The micro-processor is configured for adjusting the duty-cycle of the pulse signals according to the detected present working temperature of the light emitting diode. The constant current circuit adjusts the driving current according to the duty-cycle of the pulse signals.
Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the described embodiments. In the drawings, like reference numerals designate corresponding parts throughout various diagrams.
Reference will now be made to the drawings to describe preferred and exemplary embodiments in detail.
Referring to
The converter 135 includes an input terminal (not labeled) and an output terminal (not labeled). The input terminal is connected with the output terminal of the temperature detector 134 to receive the present working temperature signal. The output terminal is connected with the comparator 136. The converter 135 further includes a program (not shown). The program reads the present working temperature from the present working temperature signal, and outputs a maximum driving current signal accordingly. The maximum driving current signal corresponds to a maximum driving current of the present working temperature. The converter 135 applies the maximum driving current signal to the comparator 136.
The comparator 136 includes a first input terminal (not labeled), a second input terminal (not labeled), and an output terminal (not labeled). The first input terminal of the comparator 136 is connected with the output terminal of the converter 135. The second input terminal of the comparator 136 is connected with the constant current circuit 132. The output terminal of the comparator 136 is connected with the pulse signal generating circuit 137.
The pulse signal generating circuit 137 includes an input terminal (not labeled) connected with the output terminal of the comparator 136, and an output terminal (not labeled) connected with the constant current circuit 132. The pulse signal generating circuit 137 is configured for generating a plurality of pulse signals, and applying the pulse signals to the constant current circuit 132. The pulse signals have constant duty-cycles.
The constant current circuit 132 includes an input terminal (not labeled) connected with the output terminal of the pulse signal generating circuit 137, a first output terminal (not labeled) connected with the second input terminal of the comparator 136, and a second output terminal (not labeled) connected with the light emitting diode 133. The constant current circuit 132 is configured for generating a driving current according to the duty-cycles of the pulse signals. In particular, the driving current is proportional to the duty-cycles of the pulse signals. The driving current drives the light emitting diode 133 to emit light. The constant current circuit 132 further generates a feedback signal according to the present driving current, and provides the feedback signal through the second output terminal thereof to the comparator 136.
The light emitting diode 133 is an AELVVU-D type. Referring also to
wherein T denotes the present working temperature of the light emitting diode 133, and I denotes the maximum driving current of the light emitting diode 133.
Operation of the light emitting diode driving circuit 130 is as follows. The pulse signal generating circuit 137 generates a plurality of pulse signals with a constant duty-cycle. The constant current circuit 132 generates a driving current according to the duty-cycle of the pulse signals. The driving current drives the light emitting diode 133 to emit light. The light emitting diode 133 works normally. For example, when the light emitting diode 133 works under a present working temperature of 20° C. and a driving current of 26 mA, the temperature detector 134 detects the present working temperature of the light emitting diode 133, generates a present working temperature signal, and transmits the present working temperature signal to the converter 135. The program in the converter 135 reads the present working temperature, and outputs a maximum driving current signal to the comparator 136. The maximum driving current signal corresponds to a maximum driving current of the present working temperature of 20° C., that is, 27 mA.
The constant current circuit 132 generates a feedback signal according to the present driving current of 26 mA, and provides the feedback signal to the comparator 136. The converter 135 outputs the maximum driving current signal corresponding to the 27 mA driving current to the comparator 136. The comparator 136 compares the two driving signals. Since the present driving current is less than the maximum driving current, the comparator 136 does not act. The light emitting diode driving circuit 130 continues to work normally.
In a further example, the present working temperature of the light emitting diode 133 rises to 40° C., and the present driving current of the light emitting diode 133 remains at 26 mA. The temperature detector 134 detects the present working temperature of 40° C., generates a present working temperature signal accordingly, and provides the present working temperature signal to the converter 135. The program in the converter 135 reads the present working temperature of 40° C., and outputs a maximum driving current signal to the comparator 136. The maximum driving current signal corresponds to a maximum driving current for the present working temperature of 40° C., that is, 21 mA.
The constant current circuit 132 generates a feedback signal according to the present driving current of 26 mA, and provides the feedback signal to the comparator 136. The converter 135 outputs the maximum driving current signal corresponding to 21 mA to the comparator 136. The comparator 136 compares the two driving signals. Since the present driving current is greater than the maximum driving current, the comparator 136 generates a control signal, and applies the control signal to the pulse signal generating circuit 137. The pulse signal generating circuit 137 reduces the duty-cycle of the pulse signals. As a result, the constant current circuit 132 reduces the present driving current to be equal to or less than 21 mA. Therefore, the light emitting diode driving circuit 130 continues to work normally.
Thus, when the present working temperature of the light emitting diode 133 rises from 20° C. to 40° C., the present driving current of the light emitting diode 133 is modulated to be substantially equal to or less than a corresponding maximum driving current. Thereby, the light emitting diode 133 can work normally all the time, even when its working temperature changes.
The converter 135 of the light emitting diode driving circuit 130 can include a look-up table instead of the program. The look-up table stores a plurality of maximum driving current values corresponding to a plurality of working temperatures. The working temperatures and the maximum driving current values follow the relationship shown by the line “b” in
Compared with a conventional light emitting diode driving circuit, the light emitting diode driving circuit 130 includes the temperature detector 134, the comparator 135, the pulse signal generating circuit 137, and the constant current circuit 132. The comparator 136 compares the present driving current and the maximum driving current according to the present working temperature. When the present driving current exceeds the maximum driving current, the comparator 136 generates a control signal, and applies the control signal to the pulse signal generating circuit 137 to reduce the duty-cycle of the pulse signals. As a result, the constant current circuit 132 reduces the driving current of the light emitting diode 133 to be equal to or less than the corresponding maximum driving current. Thus, the light emitting diode 133 can work normally regardless of variations in the working temperature thereof.
Referring to
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Further or alternative embodiments may include the following. In one example, a light emitting diode driving circuit can include one or more light emitting diodes of another type instead of the AELWU-D type. In such case, a relationship between a present working temperature and a present driving current can be different from the relationship shown as the line “b” in
It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200710073757.3 | Mar 2007 | CN | national |
