JP2011199220A - Light emitting element driving device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light emitting element driving device that efficiently and stably supplies an anode voltage most suitable for prescribed luminance to a light emitting element.SOLUTION: The light emitting element driving device includes a light emitting element circuit 11 including one light emitting element or a plurality of light emitting elements connected in series, a power supply circuit 20, a constant current driving circuit 30, a constant current driving control circuit 40 which outputs a pulse-width modulated signal Spwm for turning on or off a constant current supplied to the light emitting element circuit 11 on the basis of logical data representing luminance of the light emitting elements, a constant current driving ON time detecting circuit 65 which detects an ON time of the pulse-width modulated signal, a clock signal output circuit 60, and a control voltage generating circuit 50 which generates a control voltage in synchronism with a clock signal CLK according to a terminal voltage Vcath of the constant current driving circuit 30 connected to a cathode end of the light emitting element circuit 11, cycles of the clock signal CLK being changed associatively with the ON time of the pulse-width modulated signal.

Description

  The present invention relates to a driving device for a light emitting element such as a light emitting diode (LED), for example, a backlight for illuminating a liquid crystal panel from the back and other display devices using the light emitting diode.

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

  Until now, a CCFL (Cold Cathode Fluorescent Lamp) type using a fluorescent tube has been used as a backlight of a liquid crystal display device. However, from the environmental point of view, light-emitting diodes that are mercury-free compared to CCFLs that require mercury are desired, and backlights that use light-emitting diodes are also anxious from the viewpoint of life, power consumption, and thinning. Yes.

  Further, as a merit of using a backlight using a light emitting diode, it is possible to adjust the luminance of each light emitting diode individually, so that a so-called area active type backlight can be realized.

  The area active method is to change the light intensity of the backlight according to the display image of the liquid crystal display device. For example, in a place where an image close to black is displayed, the light intensity of the backlight in the area is reduced. In a place where a white or bright image is displayed, a high-contrast image can be displayed by increasing the light intensity of the backlight in that region.

  On the other hand, the light emitting diode has a large production variation and temperature dependency of the forward voltage (hereinafter referred to as Vf), and it is necessary to change the voltage applied to the anode of the light emitting diode according to the value of Vf. For this reason, in the configuration in which a certain anode voltage is always applied to the anode of the light emitting diode, the anode voltage is set higher assuming a light emitting diode having the highest Vf that should be considered in production. For this reason, if a light-emitting diode having a lower Vf than expected is used, it causes unnecessary power consumption and a significant increase in the amount of heat generated. Failure to output an anode voltage corresponding to Vf that varies with temperature leads to a decrease in luminance and an increase in heat generation.

  As a solution to the above problem, Patent Document 1 generates a control voltage (first reference voltage: Vref1) corresponding to the voltage at the cathode end of the light emitting element 10, and generates an anode voltage (Vout) corresponding to the voltage. Thus, there is disclosed a light emitting element driving device 70 that can suppress useless power consumption due to variations in Vf of light emitting elements and avoid an increase in power consumption. FIG. 14 shows a circuit configuration diagram of the light-emitting element driving device described in Patent Document 1.

  Note that when the circuit configuration of FIG. 14 is adopted for the backlight of a liquid crystal display device using a general area active method, as shown in FIG. Based on the video signal input to the image processing circuit, it is input to the constant current drive control circuit 74 as n-bit logic data, converted into a pulse width modulation (PWM) signal Spwm, and input to the constant current circuit 72. Since the control voltage generation circuit (first reference voltage generation circuit 73) generally changes the control voltage Vref1, which is an output in synchronization with the clock signal CLK, the clock signal output circuit 75 is shown in FIG. It is added.

JP 2008-283033 A

  However, in the light emitting element driving device described in Patent Document 1, since the ON time of the pulse width modulation signal input to the constant current circuit and the period of increase and decrease of the control voltage are not related to each other, When the ON time of the signal is short, that is, when the lighting time of the light emitting element is short, the control voltage cannot be increased according to the terminal voltage of the constant current driving circuit (the voltage at the cathode end of the light emitting element). A power supply circuit that supplies a voltage to the anode may not be able to output a voltage necessary for causing a current to flow through the light emitting element. As a result, the luminance of the light emitting element is lowered.

  On the other hand, when the ON time of the pulse width modulation signal is long, that is, when the lighting time of the light emitting element is long, the control voltage rises according to the voltage of the terminal of the constant current driving circuit (the voltage at the cathode end of the light emitting element). The output of the power supply circuit that supplies voltage to the anode of the light emitting element is not stable, and as a result, the luminance of the light emitting element may be unstable and the power consumption may increase. To do.

  Further, when a plurality of light emitting elements connected in parallel are connected to a plurality of constant current drive circuits, and each light emitting element performs an on / off operation with different pulse width modulation signals, It becomes difficult to output a control voltage according to Vf, and it is difficult to configure a power supply circuit that supplies an optimum voltage to the anode of each light emitting element.

  Accordingly, an object of the present invention is to stabilize the optimum anode voltage so that a predetermined luminance can be obtained efficiently with respect to one or a plurality of light emitting elements connected in series, in view of the above-described problems of the prior art. It is to provide a light emitting element driving device capable of realizing low power consumption.

  In order to solve the above problems, a light emitting element driving device according to the present invention includes a light emitting element circuit including one light emitting element or a plurality of light emitting elements connected in series, and one end of the light emitting element circuit. A power supply circuit for supplying a voltage necessary for driving the light emitting element to the light emitting element circuit, and a constant current necessary for driving the light emitting element are connected to the other end of the light emitting element circuit. Based on the constant current drive circuit, the logical data indicating the brightness of the light emitting element, the constant current drive control circuit that outputs a pulse width modulation signal for switching on and off the constant current, and on the basis of the logical data, A constant current drive on time detection circuit for detecting an on time of a pulse width modulation signal, a clock signal output circuit for generating a clock signal, and the constant current drive circuit connected to the other end of the light emitting element circuit A control voltage generation circuit that generates a control voltage in synchronization with the clock signal according to a terminal voltage, and the voltage supplied to the light emitting element circuit by the power supply circuit is the terminal voltage of the constant current drive circuit. The first feature is that the control voltage is controlled so as to be equal to or higher than a predetermined first voltage, and the period of the clock signal is controlled in conjunction with the ON time of the pulse width modulation signal.

  Furthermore, in addition to the first feature, the light emitting element driving device according to the present invention is configured such that the period of the clock signal is controlled to be shorter than the on-time of the pulse width modulation signal. Features.

  Here, when a plurality of light emitting elements are connected in series in the light emitting element circuit, the sum of Vf of the light emitting elements is defined as Vf of the entire light emitting element circuit.

  According to the light emitting element driving device of the present invention, it is possible to output an optimum control voltage with respect to a change in the on-time of the pulse width modulation signal based on the logical data indicating the luminance of the light emitting element. The power supply circuit can supply a stable anode voltage to the light emitting element in accordance with Vf of the light emitting element circuit, and can light up the light emitting element with low power consumption and no flickering of luminance.

  Furthermore, when the period of the clock signal is made shorter than the ON time of the pulse width modulation signal, the light emitting element is driven at a low on-duty in order to reduce the luminance, that is, when the ON time of the pulse width modulation signal is short. Even in such a case, the control voltage can be increased or decreased during a period in which a constant current is flowing through the light emitting element, and a control voltage corresponding to the terminal voltage of the constant current driving circuit can be output. As a result, the power supply circuit can supply an anode voltage corresponding to Vf of the light emitting element circuit to the light emitting element, and can stably turn on the light emitting element even at a low on-duty and the duty of the input pulse width modulation signal. On the other hand, it is possible to turn on the light emitting element having linearity.

  Further, in the light emitting element driving device according to the present invention, in addition to the first or second feature, the constant current driving circuit includes an amplifier, an output terminal connected to the other end of the light emitting element circuit, and a control terminal. Has at least one constant current driving transistor connected to the output terminal of the amplifier.

  Furthermore, in the light emitting element driving device according to the present invention, in addition to the third feature, the constant current driving transistor is a bipolar transistor, and an output terminal of the amplifier of the constant current driving circuit is connected via a resistor. The fourth feature is that it is connected to the base terminal of the bipolar transistor.

  According to the light emitting element driving device of the third aspect of the present invention, the constant current driving circuit includes at least one constant current driving transistor and an amplifier that controls the constant current driving transistor, thereby simplifying the operation. A constant current drive is possible with a simple circuit.

  In particular, by using a bipolar transistor as the constant current driving transistor and having an amplifier that controls the base terminal of the bipolar transistor via a resistor, a light emitting element driving device having a lower cost can be realized.

  Further, in the light emitting element driving device according to the present invention, in addition to any of the first to fourth features, a plurality of the light emitting element circuits are arranged in parallel, and one end of each of the light emitting element circuits is common. The fifth feature is that the power supply circuit is connected.

  Furthermore, in addition to the fifth feature, the light emitting element driving device according to the present invention has a plurality of the constant current driving circuits, and the other ends of the light emitting element circuits arranged in parallel are different from each other. A sixth feature is that it is connected to the constant current drive circuit.

  Furthermore, in addition to the sixth feature, the light-emitting element driving device according to the present invention controls on and off of constant currents flowing through the plurality of constant current driving circuits, respectively, by separate pulse width modulation signals. This is a seventh feature.

  According to the light emitting element driving apparatus of the fifth feature of the present invention, a plurality of light emitting element circuits each including one light emitting element or a plurality of light emitting elements connected in series are arranged in parallel. Therefore, it becomes possible to control lighting of more light emitting elements and to provide a cheaper light emitting element driving device.

  Furthermore, according to the light emitting element driving device of the sixth aspect of the present invention, separate constant current driving circuits are connected to the respective light emitting element circuits, so that each light emitting element circuit is individually set. It becomes possible to pass a current, and for example, the above-described area active control can be performed by adjusting the luminance and chromaticity of the light emitting element in the light emitting element circuit.

  Furthermore, according to the light emitting element driving device of the seventh feature of the present invention, the constant current flowing through the constant current driving circuit can be individually controlled to be turned on and off. Can be freely set, and for example, a light-emitting element driving device optimal for a backlight system of a liquid crystal TV that performs area active control can be provided.

  Further, in the light emitting element driving device according to the present invention, in addition to any of the first to seventh features, the control voltage generating circuit includes the terminal voltage of the constant current driving circuit and the predetermined first voltage. A first voltage comparison circuit for comparing the counter, a counter circuit for increasing or decreasing a count value in synchronization with the clock signal according to an output of the first voltage comparison circuit, and converting the count value of the counter circuit to an analog voltage In addition, an eighth feature is that a D / A conversion circuit for generating the control voltage is provided.

  According to the light emitting element driving apparatus of the eighth feature of the present invention, the control voltage generation circuit including the first voltage comparison circuit, the counter circuit, and the D / A conversion circuit can be realized with an easy circuit configuration.

  Furthermore, in the light emitting element driving device according to the present invention, in addition to the eighth feature, when the first voltage comparison circuit includes a plurality of the constant current driving circuits, the light emitting element driving device includes the plurality of constant current driving circuits. A ninth feature is that a comparison result between the lowest terminal voltage of the terminal voltages and the predetermined first voltage is output to the counter circuit.

  According to the light emitting element driving apparatus of the ninth feature of the present invention, the first voltage comparison circuit compares the lowest voltage among the terminal voltages of the plurality of constant current driving circuits with the first voltage. In addition, among the plurality of light emitting element circuits, it is possible to output a control voltage corresponding to the light emitting element circuit having the highest Vf, and it is possible to provide control of a large number of light emitting elements with an inexpensive system.

  Further, in the light emitting element driving device according to the present invention, in addition to the eighth feature, the control voltage generating circuit includes a predetermined second voltage higher than the terminal voltage of the constant current driving circuit and the predetermined first voltage. A second voltage comparison circuit for comparing voltages, wherein the counter circuit increases or decreases the count value according to outputs of the first voltage comparison circuit and the second voltage comparison circuit, and the constant current drive circuit; A tenth feature is that the count value is not increased or decreased when the terminal voltage is higher than the predetermined first voltage and lower than the predetermined second voltage.

  According to the light emitting element driving device of the tenth aspect of the present invention, the control voltage generation circuit further includes the second voltage comparison circuit, so that the terminal voltage of the constant current driving circuit is equal to the first voltage and the second voltage. The power supply circuit can be controlled so as to be a voltage between the voltages, and the power supply circuit can supply a more stable anode voltage to the light emitting element, thereby realizing stable lighting of the light emitting element.

  Furthermore, in addition to the tenth feature, the light-emitting element driving device according to the present invention includes a plurality of the constant current driving circuits, and the first voltage comparison circuit includes the plurality of constant current driving circuits. A comparison result between the lowest terminal voltage of the terminal voltages and the predetermined first voltage is output to the counter circuit, and the second voltage comparison circuit outputs the terminal voltage of the plurality of constant current drive circuits. Of these, an eleventh feature is that a comparison result between the lowest terminal voltage and the predetermined second voltage is output to the counter circuit.

  According to the light emitting element driving apparatus of the eleventh feature of the present invention, the first voltage comparison circuit compares the lowest voltage among the terminal voltages of the plurality of constant current driving circuits with the first voltage, The voltage comparison circuit of 2 corresponds to the light emitting element circuit having the highest Vf among the plurality of light emitting element circuits by comparing the lowest voltage of the terminal voltages of the plurality of constant current driving circuits with the second voltage. Thus, it is possible to output a control voltage, and it is possible to provide control of a large number of light emitting elements with an inexpensive system.

  Furthermore, in the light-emitting element driving device according to the present invention, in addition to any of the third to seventh features, the control voltage generation circuit sets a voltage at an output terminal of the amplifier of the constant current driving circuit to a predetermined value. A third voltage comparison circuit for comparing with the third voltage; a counter circuit for increasing or decreasing a count value in synchronization with the clock signal according to an output of the third voltage comparison circuit; and an analog value for the count value of the counter circuit. A twelfth feature includes a D / A conversion circuit that converts the voltage into a voltage and generates the control voltage.

  According to the light emitting element driving device of the twelfth aspect of the present invention, the terminal voltage of the constant current driving circuit is obtained by providing the third voltage comparison circuit that compares the voltage at the output terminal of the amplifier with the third voltage. There is no need for direct monitoring, and an output of the power supply circuit corresponding to Vf of the light emitting element circuit can be obtained with a cheaper system.

  Furthermore, in addition to the twelfth feature, in the light emitting element driving device according to the present invention, when the third voltage comparison circuit includes a plurality of constant current driving circuits, the plurality of constant current driving circuits include the plurality of constant current driving circuits. A thirteenth feature is that a comparison result between the highest voltage among the voltages at the output terminals of the amplifier and the predetermined third voltage is output to the counter circuit.

  According to the light emitting element driving device of the thirteenth aspect of the present invention, the third voltage comparison circuit includes the outputs of the amplifiers that control the constant current driving transistors provided in the plurality of constant current driving circuits. By comparing the highest voltage with the third voltage, it becomes possible to output a control voltage according to the light emitting element circuit having the highest Vf among the plurality of light emitting element circuits, and control of many light emitting elements is inexpensive. It can be provided by the system.

  Furthermore, the light emitting element driving device according to the present invention, in addition to any of the first to thirteenth features, includes the power supply circuit, the constant current driving circuit, the constant current driving control circuit, and the constant current driving on time. A fourteenth feature is that at least two of the detection circuit, the clock signal output circuit, and the control voltage generation circuit are provided on the same chip.

  According to the light emitting element driving device of the fourteenth aspect of the present invention, at least two or more circuits among the circuits constituting the light emitting element driving device are provided on the same chip, so that it is cheaper. Thus, a light emitting element driving device with a small number of parts can be realized.

  Therefore, according to the light emitting element driving apparatus of the present invention, the optimum anode voltage is stably applied to one light emitting element or a plurality of light emitting elements connected in series efficiently so as to obtain a predetermined luminance. A light-emitting element driving device that can be supplied and can realize low power consumption can be provided.

The circuit block diagram of the light emitting element drive device which concerns on 1st Embodiment of this invention. FIG. 2 is a diagram showing an example of a circuit configuration of the constant current drive circuit shown in FIG. 1. The figure which shows another example of the circuit structure of the constant current drive circuit shown in FIG. FIG. 2 is a diagram illustrating an example of a circuit configuration of a control voltage generation circuit illustrated in FIG. 1. FIG. 4 is a diagram showing another example of the circuit configuration of the control voltage generation circuit shown in FIG. 1. FIG. 4 is a diagram showing another example of the circuit configuration of the control voltage generation circuit shown in FIG. 1. The figure which shows an example of the circuit structure of the light emitting element drive device which concerns on 2nd Embodiment of this invention. The figure which shows another example of the circuit structure of the light emitting element drive device which concerns on 2nd Embodiment of this invention. The figure which shows another example of the circuit structure of the light emitting element drive device which concerns on 2nd Embodiment of this invention. The figure which shows the structural example of the control voltage generation circuit which receives the input of a some cathode voltage. The figure which shows the structural example of the control voltage generation circuit which receives the input of a some cathode voltage. The figure which shows the structural example of the control voltage generation circuit which receives the input of a some cathode voltage. The figure which shows the structural example of the control voltage generation circuit which receives the input of a some cathode voltage. The circuit block diagram of the light emitting element drive device which concerns on a prior art. The circuit block diagram of the light emitting element drive device which concerns on a prior art.

<First Embodiment>
FIG. 1 shows a configuration example of a light-emitting element driving device 1 according to an embodiment of the present invention (hereinafter referred to as “the present invention device 1” as appropriate). Needless to say, the present invention is not limited to the following embodiments, and can be arbitrarily changed without departing from the scope described in the claims. The same applies to the embodiments described below.

  As shown in the circuit configuration diagram of FIG. 1, the device 1 of the present invention is connected to a light emitting element circuit 11 in which a plurality of light emitting elements (LEDs) are connected in series and an anode end of the light emitting element circuit 11. A power supply circuit 20 for supplying a voltage (anode voltage) necessary for driving the light emitting element to the circuit 11 and a cathode terminal of the light emitting element circuit 11 are connected, and a constant current necessary for driving each light emitting element of the light emitting element circuit 11 is supplied. Based on the constant current drive circuit 30 that flows, the constant current drive control circuit 40 that outputs the pulse width modulation signal Spwm for switching on and off the constant current based on the logical data indicating the luminance of the light emitting element, and on the basis of the logical data The constant current drive on time detection circuit 65 for detecting the on time of the pulse width modulation signal Spwm, the clock signal output circuit 60 for generating the clock signal CLK, and the cathode of the light emitting element circuit 11 Depending on the constant-current terminal voltage (cathode voltage) of the drive circuit 30 Vcath be connected to the de-end, and a control voltage generating circuit 50 which generates a control voltage in synchronism with the clock signal CLK.

  When logical data indicating the luminance of the light emitting element is output from, for example, an image processing circuit of the liquid crystal display device, the logical data is input to the constant current drive control circuit 40. The constant current drive control circuit 40 generates a pulse width modulation signal Spwm based on the logical data and inputs it to the constant current drive circuit 30. By controlling on / off of the constant current supplied by the constant current drive circuit 30 by the pulse width modulation signal Spwm, lighting and extinguishing of the light emitting element are controlled, and a light emitting operation corresponding to the video signal can be performed.

  The power supply circuit 20 controls the output voltage Vout supplied to the anode terminal of the light emitting element circuit 11 with reference to the control voltage Vref output from the control voltage generation circuit 50. For example, when the control voltage Vref increases, the output voltage Vout is increased accordingly, and the anode voltage of the light emitting element circuit 11 is increased. On the other hand, when the control voltage Vref falls, the output voltage Vout is lowered accordingly, and the anode voltage of the light emitting element circuit 11 is lowered. As a result, it is possible to output an anode voltage corresponding to the total value of Vf of each light emitting element of the light emitting element circuit 11, and a low power consumption operation is possible. As the circuit configuration of the power supply circuit 20, for example, the configuration described in FIG. 3 of Patent Document 1 can be used, but the present invention is not limited to this configuration. Other known configurations can be used as the circuit configuration of the power supply circuit 20.

  The control voltage generation circuit 50 detects the cathode voltage Vcath of the light emitting element circuit 11, compares it with a predetermined first voltage, and changes the control voltage Vref according to the period of the clock signal CLK based on the comparison result. For example, when the cathode voltage Vcath is lower than the first voltage, the control voltage Vref is increased, and when the cathode voltage Vcath is higher than the first voltage, the control voltage Vref is decreased. As a result, the cathode voltage Vcath is controlled to be equal to or higher than the first voltage, and the cathode voltage Vcath maintains a voltage necessary for the constant current operation of the constant current drive circuit 30.

  In the present invention, from the value of the logical data input to the constant current drive control circuit 40, the constant current drive ON time detection circuit 65 turns on the ON time of the pulse width modulation signal Spwm, that is, the lighting of each light emitting element of the light emitting element circuit 11. By detecting the period and controlling the cycle of the clock signal CLK generated by the clock signal output circuit 60 in conjunction with the ON time, it is possible to increase and decrease the control voltage according to the lighting period of the light emitting element. The output voltage of the circuit 20 is stabilized, and further low power consumption operation and stable light emitting operation of the light emitting element are possible.

  That is, when a value for increasing the lighting period of each light emitting element of the light emitting element circuit 11 is input to the constant current drive control circuit 40 as logical data indicating the luminance of the light emitting element, the constant current drive on time detection circuit 65 is used. Increases the period of the clock signal CLK, and increases the period of increase and decrease of the control voltage Vref by the control voltage generation circuit 50. As a result, the number of fluctuations of the control voltage Vref is reduced, and accordingly, the number of fluctuations of the output Vout of the power supply circuit 20 is also reduced, and a stable output voltage Vout can be supplied to the light emitting element circuit 11. As a result, useless fluctuations in the output voltage Vout can be suppressed, and low power consumption operation and stable light emitting operation of the light emitting element are possible.

  On the other hand, when a value that shortens the lighting period of each light emitting element of the light emitting element circuit 11 is input to the constant current drive control circuit 40 as logic data indicating the luminance of the light emitting element, the constant current drive on time detection circuit 65 is used. Shortens the cycle of the clock signal CLK and shortens the cycle of increase and decrease of the control voltage Vref by the control voltage generation circuit 50. Here, by making the cycle of the clock signal CLK shorter than the lighting period of the light emitting element (the ON period of the pulse width modulation signal Spwm), the control voltage generation circuit 50 lights the light emitting element and a constant current flows. During the period, it is possible to output a control voltage according to the cathode voltage Vcath. Therefore, even when the lighting period of each light emitting element of the light emitting element circuit 11 is short, a desired luminance can be obtained and light can be emitted with a luminance that maintains linearity with respect to the value of the logical data. Note that the lighting period of the light emitting element (the ON period of the pulse width modulation signal Spwm) is preferably several times the cycle of the clock signal CLK, but is not particularly limited as long as desired characteristics can be obtained.

  An example of the circuit configuration of the constant current drive circuit 30 is shown in FIG. A constant current drive circuit 30a shown in FIG. 2 includes an N-channel MOSFET (constant current drive transistor) 31, a current detection resistor 32, a control amplifier 33, a control amplifier reference voltage source 34, and the pulse width modulation signal described above. It comprises a switch 35 that receives Spwm and switches a voltage applied to the gate terminal of the MOSFET 31. The drain terminal of the MOSFET 31 is connected to the cathode terminal of the light emitting element circuit 11, and the gate terminal of the MOSFET 31 is connected to the output terminal of the control amplifier 33 via the switch 35. In a state where the constant current drive circuit 30a is passing a constant current, that is, in a state where the light emitting element is lit, the output voltage of the control amplifier reference voltage source 34 and the inversion of the control amplifier 33 connected to the current detection resistor 32 are obtained. The control amplifier 33 controls the gate voltage of the MOSFET 31 via the switch 35 so that the voltages at the input terminals are the same, so that the output voltage of the control amplifier reference voltage source 34 and the resistance value of the current detection resistor 32 are determined. In this configuration, a constant current is supplied to the light emitting element circuit 11 via the MOSFET 31 to enable a constant current operation. On the other hand, by connecting the gate terminal of the MOSFET 31 to a predetermined fixed potential (here, ground potential) by the switch 35, the constant current is cut off and the light emitting element is turned off.

  The configuration of the constant current driving circuit 30 is not particularly limited as long as it includes at least one constant current driving transistor and an amplifier that controls the constant current driving transistor. For example, the above-described N-channel MOSFET 31 may be composed of two transistors, a withstand voltage protection MOSFET and a current control MOSFET, or may be composed of a bipolar transistor.

  Another example of the circuit configuration of the constant current driving circuit is shown in FIG. A constant current drive circuit 30b shown in FIG. 3 receives an NPN bipolar transistor 31b, a current detection resistor 32, a control amplifier 33, a control amplifier reference voltage source 34, and the pulse width modulation signal Spwm. The switch 35 switches the voltage applied to the base terminal, and the resistor 36 is connected to the base terminal of the bipolar transistor 31b. The collector terminal of the bipolar transistor 31 b is connected to the cathode terminal of the light emitting element circuit 11, and the base terminal of the bipolar transistor 31 b is connected to the output terminal of the control amplifier 33 via the resistor 36 and the switch 35. In a state where the constant current driving circuit 30 b is passing a constant current, that is, in a state where the light emitting element is lit, the output voltage of the reference voltage source 34 for the control amplifier and the inversion of the control amplifier 33 connected to the current detection resistor 32. The control amplifier 33 controls the base voltage of the bipolar transistor 31b via the switch 35 and the resistor 36 so that the voltages at the input terminals are the same, so that the output voltage of the control amplifier reference voltage source 34 and the current detection resistor 32 are controlled. The constant current determined by the resistance value is supplied to the light-emitting element circuit 11 via the bipolar transistor 31b, thereby enabling a constant current operation. On the other hand, by connecting the base terminal of the bipolar transistor 31b to a predetermined fixed potential (here, ground potential) by the switch 35, the constant current is cut off and the light emitting element is turned off.

  In the constant current driving circuit 30b, an inexpensive light emitting element driving device can be provided by using a bipolar transistor that is inexpensive and has high current driving capability. The circuit configuration of the constant current drive circuit 30b is shown as using one bipolar transistor 31b. For example, in order to improve the current amplification factor of the bipolar transistor, two bipolar transistors, so-called Darlington connection, are used. The configuration is not particularly limited as long as desired characteristics can be obtained.

  Next, an example of the circuit configuration of the control voltage generation circuit 50 is shown in FIG. The control voltage generation circuit 50a shown in FIG. 4 compares the terminal voltage (cathode voltage) Vcath of the constant current drive circuit 30 (30a, 30b) connected to the cathode terminal of the light emitting element circuit 11 with a predetermined first voltage V1. A first voltage comparison circuit including a comparator 51 and a reference voltage source 52 that supplies the first voltage V1, and a counter that increases or decreases a count value in synchronization with the clock signal CLK according to an output result of the comparator 51 The circuit 53 includes a D / A conversion circuit 54 that converts a digital count value output from the counter circuit 53 into an analog voltage and generates a control voltage Vref.

  When the comparator 51 compares the cathode voltage Vcath with the first voltage V1, the counter circuit 53 reduces the count value based on the comparison result, for example, when the cathode voltage Vcath is higher than the first voltage V1, and the cathode voltage Vcath. Is lower than the first voltage V1, a signal for increasing the count value is output together with the clock signal CLK. The D / A conversion circuit 54 converts the digital output of the count value of the counter circuit 53 into an analog voltage, and outputs a control voltage Vref. By the operation as described above, the control voltage Vref is controlled so that the cathode voltage Vcath matches the first voltage V1, and the control voltage Vref corresponding to the cathode voltage of the light emitting element circuit 11 can be output. As a result, the power supply circuit 20 can supply the anode voltage corresponding to Vf of the light emitting element circuit 11 to the light emitting element circuit 11, and a light emitting element driving device capable of low power consumption operation can be provided.

  In the above description, the count circuit 53 is a signal that decreases the count value when the cathode voltage Vcath is higher than the first voltage V1, and increases the count value when the cathode voltage Vcath is lower than the first voltage V1. However, when the cathode voltage Vcath is higher than the first voltage V1, the anode voltage decreases and the cathode voltage Vcath decreases. As long as the control voltage Vref is controlled so that the anode voltage increases when the voltage is lower than the first voltage V1, the configuration is not particularly limited.

  Another example of the circuit configuration of the control voltage generation circuit is shown in FIG. In addition to the configuration of FIG. 4, the control voltage generation circuit 50b shown in FIG. 5 further includes a terminal voltage (cathode voltage) Vcath of the constant current drive circuit 30 connected to the cathode terminal of the light emitting element circuit 11 and a first voltage V1. The counter circuit 53 includes two comparators 51, each of which includes a comparator 56 that compares the second predetermined voltage V 2 with a higher voltage and a reference voltage source 57 that supplies the second voltage V 2. And 56, the count value is increased or decreased in synchronization with the clock signal CLK.

  The comparator 51 compares the cathode voltage Vcath and the first voltage V1 as in FIG. 4. For example, when the cathode voltage Vcath is higher than the first voltage V1, the comparator 51 outputs a low level signal, and the cathode voltage Vcath exceeds the first voltage V1. When it is low, a high level signal is output. The comparator 56 compares the cathode voltage Vcath with the second voltage V2. For example, when the cathode voltage Vcath is higher than the second voltage V2, a high level signal is output. When the cathode voltage Vcath is lower than the second voltage V2, the comparator 56 compares the cathode voltage Vcath with the second voltage V2. The signal is output. When the cathode voltage Vcath is lower than the first voltage V1 according to the output results of the comparators 51 and 56, the counter circuit 53 receives the High level signal from the comparator 51 and increases the count value, and the cathode voltage Vcath. Is higher than the second voltage V2, the high level signal from the comparator 56 is received and the count value is decreased. On the other hand, when the cathode voltage Vcath is higher than the first voltage V1 but lower than the second voltage V2, the counter circuit 53 does not increase or decrease the count value. The D / A conversion circuit 54 converts the digital output of the count value of the counter circuit 53 into an analog voltage, and outputs a control voltage Vref.

  By the operation as described above, the control voltage Vref is controlled so that the cathode voltage Vcath becomes a voltage between the first voltage V1 and the second voltage V2, and the optimum anode voltage is supplied according to the cathode voltage of the light emitting element circuit 11. Is possible. In other words, the anode voltage does not follow minute fluctuations in the cathode voltage including noise. For example, even when there is a fluctuation in Vf of the light emitting element due to temperature or the like, a stable anode voltage can be supplied. Accordingly, it is possible to provide a light-emitting element driving device with low power consumption.

  FIG. 6 shows still another example of the circuit configuration of the control voltage generation circuit. In the control voltage generation circuit 50c shown in FIG. 6, the comparator 58 does not compare the cathode voltage Vcath and the first voltage V1, but outputs the voltage Vam at the output terminal of the control amplifier 33 of the constant current drive circuit 30c and the reference voltage source. The third voltage V3 supplied from 59 is compared. The counter circuit 53 increases or decreases the count value in synchronization with the clock signal CLK according to the output result of the comparator 58. The D / A conversion circuit 54 converts the digital output of the count value of the counter circuit 53 into an analog voltage, and outputs a control voltage Vref.

  With such a configuration, since the voltage input to the comparator 58 is equal to or lower than the maximum voltage of the output of the control amplifier 33, the comparator 58 has the cathode voltage Vcath of the light emitting element circuit 11 directly at the inverting input terminal. An element having a lower withstand voltage can be used as compared with the case of inputting, and it is possible to realize a light-emitting element driving device that has low power consumption and is inexpensive.

Second Embodiment
The above-described inventive device 1 has one light emitting element circuit 11, one end (anode end) of the light emitting element circuit 11 is connected to the power supply circuit, and the other end (cathode end) is connected to the constant current drive circuit. Although the example in the case of having carried out was demonstrated, this invention is not limited to the said structure. FIG. 7 shows a configuration example of a light-emitting element driving device 2 according to an embodiment of the present invention (hereinafter referred to as “the present invention device 2” as appropriate).

  In the device 2 of the present invention shown in FIG. 7, a plurality (n) of light emitting element circuits L1 to Ln are arranged in parallel, and the anode ends of the respective light emitting element circuits L1 to Ln are connected to a common power supply circuit 20. Yes. The cathode ends of the light emitting element circuits L1 to Ln are connected to a common constant current driving circuit 30. Power supply circuit 20, constant current drive circuit 30 (30a, 30b, 30c), constant current drive control circuit 40, control voltage generation circuit 50 (50a, 50b, 50c), clock signal output circuit 60, and constant current drive on time Each configuration of the detection circuit 65 is the same as that of the above-described device 1 of the present invention.

  The device 2 of the present invention can drive a large number of light emitting elements with a single light emitting element driving device, and can realize a light emitting element driving device with high brightness with an inexpensive system.

<Third Embodiment>
FIG. 8 shows a configuration example of the light emitting element driving device 3 according to an embodiment of the present invention (hereinafter, referred to as “the present invention device 3” as appropriate). The inventive device 3 shown in FIG. 8 has the same configuration as the inventive device 2 except that the cathode ends of the light emitting element circuits L1 to Ln are connected to the constant current drive circuits D1 to Dn, respectively. It is. The constant current drive circuits D1 to Dn have the same configuration as the constant current drive circuit 30 (30a, 30b, 30c) in the device 1 of the present invention.

  With such a configuration, the plurality of light emitting element circuits L1 to Ln are driven by constant current by different constant current driving circuits D1 to Dn, respectively, and have different luminance in one light emitting element driving device. Light emitting elements can be arranged.

  Further, like the light emitting element driving device 4 shown in FIG. 9, the constant current driving circuits D1 to Dn are respectively supplied with constant currents supplied to the light emitting element circuits L1 to Ln based on different pulse width modulation signals Spwm1 to Spwmn. Can be configured to be controlled on and off separately. With such a configuration, for example, a backlight system such as an area active method can be realized at low cost.

  At this time, the constant current drive on time detection circuit 65 may detect the on time of the individual pulse width modulation signals Spwm1 to Spwmn, that is, the lighting period of the light emitting elements of the individual light emitting element circuits L1 to Ln. Of these, only the shortest on-time may be detected, and a signal for determining the cycle of the clock signal CLK generated by the clock signal output circuit 60 in conjunction with the on-time may be output to the clock signal output circuit 60. However, the present invention is not necessarily limited thereto, and is not particularly limited as long as desired light emitting diode characteristics can be obtained.

  In the light emitting element driving device shown in FIGS. 8 and 9, the control voltage generation circuit includes a plurality of terminal voltages of the constant current driving circuits D1 to Dn connected to the cathode ends of the light emitting element circuits L1 to Ln, respectively. Vcath1 to Vcatn are input. In this case, the first voltage comparison circuit compares the lowest voltage among the plurality of cathode voltages with the first voltage, and the counter circuit 53 may increase or decrease the count value based on the comparison result. By doing so, it becomes possible to output a control voltage corresponding to the light emitting element circuit having the highest Vf among the plurality of light emitting element circuits, and it is possible to provide control of a large number of light emitting elements with an inexpensive system. It becomes.

  An example of the circuit configuration of the control voltage generation circuit 50d (50e) that receives a plurality of cathode voltages Vcath1 to Vcatn as inputs is shown in FIGS. As shown in FIG. 10, the first voltage comparison circuit 48 uses n comparators 51 to compare individual cathode voltages Vcath1 to Vcatn with the first voltage V1, respectively. On the basis of this, the selector 49 selects the one having the lowest potential difference from the first voltage and may input it to the counter circuit 53. As shown in FIG. 11, one comparator 51 has the cathode voltages Vcath1 to Vcathn. After selecting the one having the lowest voltage, any one of the selected cathode voltages Vcath1 to Vcatn may be compared with the first voltage V1. Further, the control voltage generation circuits 50d and 50e shown in FIGS. 10 and 11 are configured to receive a plurality of different cathode voltages in the control voltage generation circuit 50a shown in FIG. The control voltage generation circuit 50b or 50c shown in FIG. 5 or 6 may receive a plurality of different cathode voltages, and the configuration is not particularly limited. For example, FIG. 12 shows an example of a control voltage generation circuit 50f that receives a plurality of different cathode voltages in the control voltage generation circuit 50b of FIG. 5, and FIG. An example of the control voltage generation circuit 50g that receives the input is shown in FIG.

  In the control voltage generation circuit 50f of FIG. 12, for example, the comparator 51 selects one of the cathode voltages Vcath1 to Vcatn that has the lowest voltage, and then selects one of the selected cathode voltages Vcath1 to Vcatn and the first voltage V1. In comparison, when the cathode voltage having the lowest voltage is lower than the first voltage V <b> 1, a high level signal is output to the counter circuit 53. The comparator 56 selects one of the cathode voltages Vcath1 to Vcatn having the lowest voltage, then compares any one of the selected cathode voltages Vcath1 to Vcatn with the second voltage V2, and the cathode voltage having the lowest voltage is the second voltage. When the voltage is higher than the voltage V <b> 2, a high level signal is output to the counter circuit 53.

  In the control voltage generation circuit 50g of FIG. 13, the lower the cathode voltage Vcath, the higher the output voltage of the control amplifier 33. Therefore, the comparator 58 outputs the output voltages Vam1 to Vamn of the control amplifiers of the constant current drive circuits D1 to Dn. After selecting the one having the highest voltage, the selected voltage Vam1 to Vamn is compared with the third voltage V3.

  The light emitting element driving devices 1 to 4 according to the present invention described above are efficient for each light emitting element of the light emitting element circuit 11 or each light emitting element in the plurality of light emitting element circuits L1 to Ln arranged in parallel. It is possible to provide a light emitting element driving device capable of stably supplying an optimum anode voltage so as to obtain a predetermined luminance and realizing low power consumption.

  Another embodiment will be described below.

  <1> In the above-described embodiment, the light emitting element circuit 11 exemplifies the case where a plurality of light emitting elements are connected in series, but the anode end of one light emitting element is a power supply circuit, and the cathode end is a constant current drive circuit. Needless to say, the present invention is not limited to the number of light emitting elements connected in series. Further, the emission color of each light emitting element connected in series is not particularly limited.

  <2> In the above-described second and third embodiments, the cathode terminals of the plurality of light emitting element circuits L1 to Ln are connected to the common constant current drive circuit 30 (FIG. 7), and the plurality of light emitting element circuits L1. Although the configuration (FIG. 8) in which the cathode ends of .about.Ln are separately connected to the constant current drive circuits D1 to Dn has been described, a common constant for each of a predetermined number of light emitting element circuits among the plurality of light emitting element circuits L1 to Ln. A current driving circuit may be provided, and the cathode end may be connected to the same constant current driving circuit for each of the predetermined number of light emitting element circuits.

  <3> In the above-described embodiment, at least two of the power supply circuit, the constant current drive circuit, the constant current drive control circuit, the control voltage generation circuit, the constant current drive on-time detection circuit, and the clock signal output circuit. It is a desirable mode that the above circuits are provided on the same chip because the light emitting element can be turned on and off with a simple system at a lower cost.

  INDUSTRIAL APPLICABILITY The present invention can be used for a light emitting element driving device that uses a plurality of LEDs as light sources.

1-4: Light-emitting element driving device 10 according to the present invention: Light-emitting element 11, L1-Ln: Light-emitting element circuit 20: Power supply circuit 30, 30a-30c, D1-Dn: Constant current drive circuit 31: MOSFET (constant current drive) Transistor)
31b: Bipolar transistor (Constant current drive transistor)
32: current detection resistor 33: control amplifier 34: control amplifier reference voltage source 35: switch 36: resistors 40, 74: constant current drive control circuit 48: first voltage comparison circuit 49: selectors 50, 50a to 50g: control Voltage generation circuits 51, 56, 58: Comparators 52, 57, 59: Reference voltage source 53: Counter circuit 54: D / A conversion circuit 60, 75: Clock signal output circuit 65: Constant current drive ON time detection circuit 70: Conventional Light-emitting element driving device 71 according to technology: Constant voltage circuit 72: Constant current circuit 73: First reference voltage generation circuit CLK: Clock signal V1: First voltage V2: Second voltage V3: Third voltages Vam, Vam1 to Vamn: Output voltages Vcath, Vcath1 to Vcatn of the control amplifier: Cathode voltage Vout: Output voltage of the power supply circuit (anode power) )
Vref, Vref1: power supply circuit control voltages Spwm, Spwm1 to Spwmn: pulse width modulation signals

Claims (14)

A light emitting element circuit comprising one light emitting element or a plurality of light emitting elements connected in series;
A power supply circuit connected to one end of the light emitting element circuit and supplying a voltage necessary for driving the light emitting element to the light emitting element circuit;
A constant current driving circuit that is connected to the other end of the light emitting element circuit and causes a constant current necessary for driving the light emitting element to flow through the light emitting element circuit;
A constant current drive control circuit that outputs a pulse width modulation signal for switching on and off the constant current based on logical data indicating luminance of the light emitting element;
A constant current drive on time detection circuit for detecting an on time of the pulse width modulation signal based on the logic data;
A clock signal output circuit for generating a clock signal;
A control voltage generation circuit that generates a control voltage in synchronization with the clock signal according to a terminal voltage of the constant current drive circuit connected to the other end of the light emitting element circuit,
The voltage supplied to the light emitting element circuit by the power supply circuit is controlled by the control voltage so that the terminal voltage of the constant current driving circuit is equal to or higher than a predetermined first voltage,
The light-emitting element driving device, wherein a cycle of the clock signal is controlled in conjunction with an ON time of the pulse width modulation signal.   The light emitting element driving device according to claim 1, wherein the cycle of the clock signal is controlled to be shorter than an ON time of the pulse width modulation signal.   The constant current driving circuit includes an amplifier and at least one constant current driving transistor whose output terminal is connected to the other end of the light emitting element circuit and whose control terminal is connected to the output end of the amplifier. The light emitting element driving device according to claim 1. The constant current driving transistor is a bipolar transistor,
4. The light emitting element driving apparatus according to claim 3, wherein an output terminal of the amplifier of the constant current driving circuit is connected to a base terminal of the bipolar transistor through a resistor.   5. The plurality of light emitting element circuits are arranged in parallel, and one end of each of the light emitting element circuits is connected to the common power supply circuit. 6. Light emitting element driving device. A plurality of the constant current drive circuits;
6. The light emitting element driving device according to claim 5, wherein the other ends of the light emitting element circuits arranged in parallel are connected to different constant current driving circuits, respectively.   7. The light emitting element driving device according to claim 6, wherein on and off of the constant currents flowing through the plurality of constant current driving circuits are individually controlled by the different pulse width modulation signals. The control voltage generation circuit includes:
A first voltage comparison circuit for comparing the terminal voltage of the constant current drive circuit with the predetermined first voltage;
A counter circuit that increases or decreases a count value in synchronization with the clock signal in accordance with an output of the first voltage comparison circuit;
The light emitting element drive device according to claim 1, further comprising a D / A conversion circuit that converts a count value of the counter circuit into an analog voltage and generates the control voltage. The first voltage comparison circuit includes:
When a plurality of the constant current drive circuits are provided, a comparison result between the lowest terminal voltage and the predetermined first voltage among the terminal voltages of the plurality of constant current drive circuits is output to the counter circuit. The light-emitting element driving device according to claim 8. The control voltage generation circuit includes:
A second voltage comparison circuit for comparing the terminal voltage of the constant current drive circuit with a predetermined second voltage higher than the predetermined first voltage;
The counter circuit is
Increase or decrease the count value according to the outputs of the first voltage comparison circuit and the second voltage comparison circuit,
9. The light emitting element driving device according to claim 8, wherein when the terminal voltage of the constant current driving circuit is higher than the predetermined first voltage and lower than the predetermined second voltage, the count value is not increased or decreased. . A plurality of the constant current drive circuits;
The first voltage comparison circuit outputs a comparison result between the lowest terminal voltage and the predetermined first voltage among the terminal voltages of the plurality of constant current drive circuits to the counter circuit,
The second voltage comparison circuit outputs a comparison result between the lowest terminal voltage among the terminal voltages of the plurality of constant current drive circuits and the predetermined second voltage to the counter circuit. The light emitting element drive device according to claim 10. The control voltage generation circuit includes:
A third voltage comparison circuit for comparing the voltage at the output terminal of the amplifier of the constant current drive circuit with a predetermined third voltage;
A counter circuit that increases or decreases a count value in synchronization with the clock signal in accordance with an output of the third voltage comparison circuit;
The light emitting element driving device according to claim 3, further comprising a D / A conversion circuit that converts a count value of the counter circuit into an analog voltage and generates the control voltage. The third voltage comparison circuit includes:
In the case of having a plurality of the constant current drive circuits, a comparison result between the highest voltage among the voltages at the output terminals of the amplifiers of the plurality of constant current drive circuits and the predetermined third voltage is output to the counter circuit. The light-emitting element driving device according to claim 12.   At least two of the power supply circuit, the constant current drive circuit, the constant current drive control circuit, the constant current drive on time detection circuit, the clock signal output circuit, and the control voltage generation circuit are the same. The light emitting element driving device according to claim 1, wherein the light emitting element driving device is provided on a chip.

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