JP2014203524A - Light-emitting element drive device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light-emitting element drive device capable of saving power even in a case where there are variations in productions in forward voltage of a plurality of light-emitting element rows.SOLUTION: A light-emitting element drive device configured by one light-emitting element or a plurality of series-connected light-emitting elements to drive a plurality of light-emitting element rows 101-103 connected in parallel, comprises: a constant current drive circuit 200 applying a constant current to each of the light-emitting element rows; and a constant current value selection circuit 500 determining a current value of the constant current to be applied to each of the light-emitting element rows on the basis of a difference in forward voltage of the light-emitting element row.

Description

  The present invention relates to a light emitting element driving device.

  In recent years, as represented by liquid crystal display devices and the like, displays have been made thinner. For example, in a liquid crystal display device, the liquid crystal panel itself does not emit light, and a light source device, a so-called backlight is required. .

  In the past, most of the backlights of liquid crystal display devices used the CCFL (Cold Cathode Fluorescent Lamp) type using a fluorescent tube. However, from the viewpoints of life, power consumption, and thinning, a backlight using a light emitting diode is currently used. Light is becoming mainstream.

  Particularly in portable devices such as mobile phones and mobile computers, it is essential to make the display thin, and backlights using light emitting diodes are widely used.

  Furthermore, the above-mentioned portable devices are premised on battery driving, and in order to realize long-term use, the power consumption of a backlight that consumes relatively large power among the power consumption of the portable devices is reduced. It is important to do.

  Therefore, in the display of the current portable device, the brightness of the backlight is adjusted according to the ambient brightness to save power (for example, see Patent Document 1).

  Further, as a method for saving power of a lighting device using light emitting diodes, for example, in the light emitting element driving device disclosed in Patent Document 2 (FIG. 4), power is supplied to two light emitting diode rows and the light emitting diode rows. A step-up circuit that performs a constant current flow through the light-emitting diode array, a withstand voltage protection transistor of the constant current source, and each voltage input to the constant current source is compared with a reference voltage Vref. And an error amplifier for amplifying the signal.

  Then, the minimum voltage among the voltages input to the constant current source is detected, and the output voltage Vout (= light emission) of the booster circuit is set so that the minimum voltage matches the reference voltage Vref that changes according to the load current. The anode voltage of the diode) is controlled. Thereby, it is supposed that the power loss which arises in each constant current source is suppressed, and power saving is realized.

JP 2010-224075 A JP 2010-63332 A (FIG. 4)

  However, when a light-emitting element driving device that controls a plurality of light-emitting diode rows is configured as in Patent Document 2, even if all the light-emitting diode rows have the same temperature and the same load current value, due to production variations The forward voltage of the light-emitting diode array varies relatively greatly. Thereby, in the said patent document 2, there exists a problem that the power consumption by a light emitting diode row | line | column increases.

  Particularly in portable devices, there is a great demand for power saving due to the battery drive as described above.

  In view of the above problems, an object of the present invention is to provide a light-emitting element driving device that enables power saving even when there is a production variation in the forward voltage of a plurality of light-emitting element arrays. And

In order to achieve the above object, one embodiment of the present invention is a light-emitting element driving device configured to include one light-emitting element or a plurality of light-emitting elements connected in series, and drive a plurality of light-emitting element arrays connected in parallel. There,
A constant current driving circuit for supplying a constant current to each of the light emitting element rows;
A constant current value selection circuit that determines a current value of a constant current that flows to each of the light emitting element rows based on a difference in forward voltage of the light emitting element rows;
It is set as the structure provided with.

  According to such a configuration, even when forward voltages of a plurality of light emitting element arrays vary due to production variations, an increase in power consumption in the light emitting element arrays can be suppressed and power saving can be achieved.

  The above configuration may further include a forward voltage difference detection circuit that detects a difference in forward voltage of the light emitting element array based on a parameter of the constant current drive circuit.

  In the above configuration, the parameter of the constant current driving circuit may be a current sink terminal voltage.

In any of the above configurations, a reference voltage generation circuit that generates one or more reference voltages based on a current value determination result of the constant current value selection circuit;
One or more comparison circuits for comparing a current sink terminal voltage of the constant current drive circuit and the generated reference voltage;
And a power supply circuit that controls a voltage supplied to the light emitting element array based on a comparison result by the comparison circuit.

  According to such a configuration, the current sink terminal voltage can be controlled to a voltage suitable for flowing a constant current through the light emitting element array, and power consumption in the constant current drive circuit can be suppressed.

In the above configuration, the reference voltage generation circuit generates a plurality of reference voltages, and the plurality of comparison circuits provided are comparators.
A counter circuit for counting based on the output of the comparison circuit;
The power supply circuit may control a voltage supplied to the light emitting element array in accordance with an output of the counter circuit.

  According to such a configuration, it is possible to stabilize the output voltage of the power supply circuit.

  In any of the above-described configurations, the current value of at least one of the light emitting element arrays may be set to zero.

  According to such a configuration, it is possible to further reduce power consumption particularly in a situation where the luminance of the light emitting element array may be low.

  In any one of the configurations described above, the constant current value selection circuit is configured to generate the light emission based on a difference between forward voltages of the light emitting element arrays and a detection result by an illuminance detection circuit that detects brightness of an ambient environment. It is also possible to determine a current value of a constant current that flows to each of the element arrays.

  According to such a configuration, it is possible to automatically adjust the luminance of the light emitting element array in accordance with the brightness of the surrounding environment, thereby enabling further low power consumption operation.

  An illumination device according to one embodiment of the present invention includes the light-emitting element driving device having any one of the above structures and a light-emitting element array driven by the light-emitting element driving device.

  According to the present invention, even when there is a production variation in the forward voltage of a plurality of light emitting element arrays, it is possible to save power.

1 is a configuration diagram of a light emitting element driving apparatus according to a first embodiment of the present invention. It is a block diagram of the light emitting element drive device which concerns on 2nd Embodiment of this invention. It is a block diagram of the light emitting element drive device which concerns on 3rd Embodiment of this invention.

<First Embodiment>
An embodiment of the present invention will be described below with reference to the drawings. The light emitting element driving device according to the embodiments described below can be used for a backlight of an image display device using a liquid crystal panel as an example.

  FIG. 1 shows the configuration of the light emitting element driving apparatus according to the first embodiment of the present invention. The light emitting element driving device shown in FIG. 1 is a device that drives the light emitting unit 100, and includes a constant current driving circuit 200, a power supply circuit 300, a Vf difference detection circuit 400, a constant current value selection circuit 500, and a reference voltage generation. A circuit 600 and an error amplifier 700 are provided.

  The light emitting unit 100 is configured by connecting in parallel light emitting diode rows 101, 102, and 103 in which a plurality of light emitting diodes are connected in series. Note that the light-emitting diode array may be composed of one light-emitting diode. In FIG. 1, the number of light emitting diode rows is three, but the number is not limited to this as long as it is a plurality of rows.

  The power supply circuit 300 supplies a voltage to the light emitting unit 100 and includes, for example, a booster circuit.

  The constant current drive circuit 200 is a circuit that supplies a constant current to each of the light emitting diode arrays 101 to 103.

  The Vf difference detection circuit 400 is based on the voltage (current sink terminal voltage) of each current sink terminal of the constant current drive circuit 200 connected to the cathodes of the light emitting diode rows 101 to 103, and the forward voltage of the light emitting diode rows 101 to 103 is determined. The difference of is detected.

  Based on the detection result of the Vf difference detection circuit 400, the constant current value selection circuit 500 determines a current value of a constant current that flows to each of the light emitting diode arrays 101 to 103.

  The reference voltage generation circuit 600 generates the reference voltage Vref according to the current value determination result by the constant current value selection circuit 500.

  The error amplifier 700 is a circuit that compares the current sink terminal voltage of the constant current drive circuit 200 with the reference voltage Vref generated by the reference voltage generation circuit 600 and amplifies the error.

  The light emitting diode arrays 101 to 103 are supplied with the output voltage Vout from the power supply circuit 300 to the anode, and perform a constant current driving operation by the constant current driving circuit 200. The output voltage Vout generated by the power supply circuit 300 is variable depending on the output result of the error amplifier 700.

  The error amplifier 700 compares the lowest voltage and the reference voltage Vref among the cathode voltages (= current sink terminal voltages) of the light emitting diode arrays 101 to 103, and amplifies the error, thereby the lowest voltage and the reference voltage. The output voltage Vout of the power supply circuit 300 is controlled so that there is no difference from Vref.

  The reference voltage generation circuit 600 generates the reference voltage Vref based on the largest current value among the constant current values to be passed through the light emitting diode arrays 101 to 103 determined by the constant current value selection circuit 500. As a result, each current sink terminal voltage of the constant current drive circuit 200 is controlled to a voltage necessary and sufficient to flow the current having the largest current value. Thus, since each current sink terminal voltage is appropriately controlled, it is possible to suppress useless power consumption in the constant current drive circuit 200.

  Further, the light emitting diode arrays 101 to 103 have the same number of light emitting diodes in series, use parts having the same specifications, and even when current of the same current value is caused to flow by the constant current driving circuit 200, each of the light emitting diode arrays 101 to 103 may vary depending on production variations. The forward voltage may be different. In this case, each current sink terminal voltage of the constant current drive circuit 200 corresponding to the light emitting diode arrays 101 to 103 differs depending on each light emitting diode array.

  In order to suppress an increase in power consumption due to variations in the forward voltage due to this production variation, the Vf difference detection circuit 400 uses the forward direction of the light emitting diode rows 101 to 103 based on the current sink terminal voltages of the constant current drive circuit 200. The voltage difference is detected, and a current value corresponding to the detection result is set by the constant current value selection circuit 500 to enable low power consumption driving.

  More specifically, when the light emitting element driving device of the present embodiment is activated, all the light emitting diode rows 101 to 103 are set to a current value Iref that provides a luminance required as an image display device, and a constant current is set. The drive circuit 200 performs control so that a constant current having a current value Iref flows through all the light emitting diode arrays 101 to 103. In this state, the Vf difference detection circuit 400 detects the difference between the forward voltages of the light emitting diode arrays 101 to 103 based on the current sink terminal voltages of the constant current drive circuit 200.

  Thereafter, the current value of each of the light emitting diode arrays 101 to 103 is shifted to a normal operation state according to the difference, and the constant current value selection circuit 500 emits light with a high current sink terminal voltage, that is, a low forward voltage. The current value of the diode array is increased, and the current value is determined so as to decrease the current value of the light emitting diode array having a low current sink terminal voltage, that is, a high forward voltage.

  With such a setting, the current value of the light emitting diode array having a low forward voltage, that is, the light emitting diode array capable of passing a specified current value with relatively low power consumption, is increased, and the light emitting diode having a high forward voltage. Since the current value of the light-emitting diode array that requires a relatively high power consumption for flowing the specified current is reduced, the power consumption by the light-emitting diode arrays 101 to 103 can be reduced.

  At this time, controlling the total current value of the light-emitting diode arrays 101 to 103 to be 3 × Iref is a more preferable embodiment in that the luminance of the image display device is not changed.

  With the above-described configuration, even when the forward voltages of the light-emitting diode arrays 101 to 103 vary due to production variations, a light-emitting element driving device that can operate with low power consumption and can save power. Can be provided.

  In the above embodiment, the difference in the forward voltage of each light-emitting diode array is detected at the time of starting the light-emitting element driving device. However, it is not always necessary to detect at that timing, and the detection of the difference in forward voltage is arbitrary. It does not matter at the timing.

  Further, the difference between the forward voltages of the respective light-emitting diode arrays is detected in advance at the time of shipment from the factory, and the result is stored in the storage unit included in the light-emitting element driving device. The current value may be determined based on the detection result stored in the storage unit. In this case, the Vf difference detection circuit 400 that detects the difference in the forward voltage of each light emitting diode is not necessary.

  Further, in the above embodiment, when the difference between the forward voltages of the respective light emitting diode arrays is detected, the current value Iref is obtained so as to obtain the luminance necessary for the image display device. The current value may be any value.

  Further, in a situation where the image display device has relatively low brightness, that is, the current of the light-emitting diode array may be relatively small, among the light-emitting diode arrays, the current value setting is set to zero for the column having a large forward voltage. Further power saving operation is possible.

Second Embodiment
Next, FIG. 2 shows a configuration of a light emitting element driving apparatus according to the second embodiment of the present invention. The light-emitting element driving device shown in FIG. 2 is different from the light-emitting element driving device (first embodiment) shown in FIG. 1 except that an illuminance detection circuit 800 that detects the illuminance of the external ambient environment is added. Are the same. The constant current value selection circuit 500 determines a current value of a constant current that flows through the light emitting diode arrays 101 to 103 based on the detection result by the Vf difference detection circuit 400 and the detection result by the illuminance detection circuit 800.

  With the configuration as in the present embodiment, the brightness of the image display device can be automatically adjusted according to the brightness of the external ambient environment, and each power consumption is set appropriately so that power consumption is appropriate at each brightness setting. By setting the current of the light emitting diode array and setting the reference voltage Vref according to the current value, it is possible to provide a light emitting element driving device capable of saving power.

  Note that a method for adjusting the brightness of the image display device according to the brightness of the external environment is not particularly limited. For example, the brightness of the external environment is detected as digital data by a digital illuminance sensor as the illuminance detection circuit 800, and is fixed. The current value selection circuit 500 selects a reference current value of the light emitting diode array according to the digital data of the brightness of the external environment according to a preset table, and the light emitting diode array is selected with respect to the reference current value. Every time, the voltage is converted into a current value corresponding to the difference in the forward voltage detected by the Vf difference detection circuit 400. Thereby, the light emitting element drive device which made electric power consumption suitable is realizable.

<Third Embodiment>
Next, FIG. 3 shows a configuration of a light emitting element driving apparatus according to the third embodiment of the present invention. 3 is different from the configuration shown in FIG. 1 (first embodiment) in that the reference voltage generation circuit 600 generates two reference voltages Vref1 and Vref2, and comparators (comparators) 701, 702, The difference is that a counter circuit 750 and a D / A conversion circuit 770 are provided.

  The reference voltage generation circuit 600 generates reference voltages Vref1 and Vref2 corresponding to the largest current value among the current values set in the light emitting diode arrays 101 to 103 by the constant current value selection circuit 500. Here, as an example, Vref1 <Vref2.

  Of the cathode voltages (= current sink terminal voltages) of the light emitting diode arrays 101 to 103, the comparator 701 compares the lowest voltage with the reference voltage Vref1, and the comparator 702 compares the lowest voltage with the reference voltage Vref2.

  If the cathode voltage is lower than the reference voltage Vref1 as a result of comparison by the comparator 701, the counter circuit 750 counts up. If the cathode voltage is higher than the reference voltage Vref2 by the comparison by the comparator 702, the counter circuit 750 counts down. When the cathode voltage is higher than the reference voltage Vref1 by comparison with the comparator 701 and the cathode voltage is lower than the reference voltage Vref2 by comparison with the comparator 702, the counter circuit 750 does not count up or count down.

  In this way, the count value of the counter circuit 750 is controlled, and the count value is converted into an analog signal by the D / A conversion circuit 770 and fed back to the power supply circuit 300. The power supply circuit 300 controls the output voltage Vout according to the analog signal.

  With this configuration, the output voltage Vout of the power supply circuit 300 is controlled so that the lowest voltage among the current sink terminal voltages is between the reference voltages Vref1 and Vref2, and the output voltage Vout is stabilized. Then, each current sink terminal voltage is controlled to a voltage necessary and sufficient for flowing the largest current among the currents set in the constant current driving circuit 200.

  In the configuration of FIG. 3, an illuminance detection circuit as shown in FIG. 2 may be added.

  The embodiment of the present invention has been described above, but the embodiment can be variously modified within the scope of the gist of the present invention.

  For example, in the above embodiment, the difference in the forward voltage is detected based on the current sink terminal voltage of the constant current drive circuit 200. However, a current may be used as a parameter of the constant current drive circuit 200. In that case, only when detecting the forward voltage difference of the LED array, the current is detected by using the constant current drive circuit temporarily as a current detection circuit rather than as a constant current sink, and the difference in current is detected. To the difference in forward voltage of the light-emitting diode array.

DESCRIPTION OF SYMBOLS 100 Light emission part 101-103 Light emitting diode row | line | column 200 Constant current drive circuit 300 Power supply circuit 400 Vf difference detection circuit 500 Constant current value selection circuit 600 Reference voltage generation circuit 700 Error amplifier 701, 702 Comparator 750 Counter circuit 770 D / A conversion circuit 800 Illuminance detection circuit

Claims (5)

A light-emitting element driving device configured to drive one light-emitting element or a plurality of light-emitting element arrays connected in parallel, the light-emitting element driving apparatus comprising:
A constant current driving circuit for supplying a constant current to each of the light emitting element rows;
A constant current value selection circuit that determines a current value of a constant current that flows to each of the light emitting element rows based on a difference in forward voltage of the light emitting element rows;
A light-emitting element driving device comprising:   The light emitting element driving apparatus according to claim 1, further comprising a forward voltage difference detection circuit that detects a difference in forward voltage of the light emitting element array based on a parameter of the constant current driving circuit. A reference voltage generation circuit that generates one or more reference voltages based on a current value determination result of the constant current value selection circuit;
One or more comparison circuits for comparing a current sink terminal voltage of the constant current drive circuit and the generated reference voltage;
The light emitting element drive device according to claim 1, further comprising: a power supply circuit that controls a voltage supplied to the light emitting element array based on a comparison result by the comparison circuit.   4. The light emitting element driving device according to claim 1, wherein a current value of at least one of the light emitting element arrays is set to zero. 5.   The constant current value selection circuit is configured to output a constant current to be supplied to each of the light emitting element arrays based on a difference between forward voltages of the light emitting element arrays and a detection result by an illuminance detection circuit that detects brightness of an ambient environment. The light emitting element driving device according to claim 1, wherein a current value is determined.

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