JP2013020929A - Led drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an LED drive circuit, constructed to be capable of direct drive by AC input though, which is composed so as to restrict flickers.SOLUTION: An LED drive circuit used to drive an LED unit including a plurality of series connected LEDs comprises a rectifier which rectifies AC input and a constant current circuit which includes an op-amp and a plurality of drive transistors connected to the output stage of the op-amp via voltage dividing resistors. The output of the rectifier is connected at one end to the input of the LED unit, and the outputs of the plurality of transistors respectively are connected at one end to the connection points of different LED stages in the LED unit. In this way, the plurality of transistors are made to selectively drive the LED unit according to the voltage of AC input.

Description

  The present invention relates to an LED drive circuit for driving an LED array.

  Lighting equipment using LEDs is attracting attention as lighting equipment that achieves both energy saving and environmental consideration. However, the current situation is that the transition from incandescent bulbs and fluorescent lighting devices to LED lighting has not yet progressed. One of the factors is that the majority of LED lighting devices convert AC to DC once, and the internal operation of the LED lighting devices is relatively complicated. As a result, the drive circuit of the LED lighting device becomes relatively complicated, which increases the cost. It is that. The following Patent Document 1 discloses an example of the configuration of such a conventional LED lighting device.

JP 2009-134945 A

  In order to promote the widespread use of LED lighting devices, it is required to review the conventional LED drive system from a basic aspect and to simplify the drive circuit configuration. In particular, a drive circuit configuration that can be directly driven by an AC input is considered to be extremely beneficial from the viewpoint of greatly promoting the spread of LED lighting devices. However, if the circuit is configured to simply drive the LED strings with AC input, especially when driving a multi-stage LED string, there are many periods when the LED strings are not lit due to fluctuations in the AC input voltage, resulting in flicker. It becomes. Therefore, it is required to realize a driving method that can suppress flicker.

  The present invention has been made in view of the above circumstances. That is, an object of the present invention is to provide an LED driving circuit configured to suppress flicker while being configured to be directly driven by an AC input.

  According to one aspect of the present invention, there is provided an LED driving circuit for driving an LED unit having a plurality of LEDs connected in series, a rectifier that rectifies an AC input, an operational amplifier, and a voltage dividing resistor at an output stage of the operational amplifier. A constant current circuit having a plurality of driving transistors connected via each other, one end on the output side of the rectifier connected to the input side of the LED unit, and one end on the output side of the plurality of transistors, respectively, Provided is an LED drive circuit characterized in that a plurality of transistors are selectively driven according to an AC input voltage by connecting to connection points of different LED stages in the unit. .

  According to the configuration of the LED driving circuit, flicker can be suppressed while the configuration can be directly driven by AC input.

  Each of the plurality of transistors may be a bipolar transistor. In this case, the collector terminals of the plurality of bipolar transistors are respectively connected to connection points having different numbers of LED stages in the LED unit.

  A configuration may be adopted in which base resistors are respectively arranged at the bases of the plurality of bipolar transistors.

  Each of the plurality of transistors may be a MOS transistor. In this case, the output stage of the operational amplifier is connected to the gates of the plurality of MOS transistors via voltage dividing resistors, and the drain terminals of the plurality of MOS transistors are respectively connected to connection points having different numbers of LED stages in the LED unit.

  The LED unit may be configured to have a plurality of LED rows. In this case, the plurality of driving transistors are arranged in parallel for each of the plurality of LED rows.

  The constant current circuit is configured such that the current value for driving the LED unit by each of the plurality of driving transistors has at least two different values, whereby the waveforms of the rising and falling portions of the current waveform for driving the LED unit are You may comprise so that it may change in steps.

  The resistance values of the driving current detection resistors connected to the emitter terminals of the plurality of driving transistors are at least two different values, whereby the waveforms of the rising and falling portions of the current waveform for driving the LED unit are You may comprise so that it may change in steps.

  According to the present invention, it is possible to provide an LED drive circuit capable of suppressing flicker while being configured to be directly driven by an AC input.

It is a circuit diagram of the LED drive circuit by the 1st Embodiment of this invention. It is a figure showing the signal waveform for demonstrating operation | movement of the LED drive circuit of FIG. It is a circuit diagram showing the structure of the LED drive circuit in the case of driving a some LED row | line | column. It is a figure showing the current waveform in the LED drive circuit of FIG. It is a circuit diagram of the negative feedback circuit in an LED drive circuit as the 2nd Embodiment of this invention. FIG. 6 is a diagram illustrating a current waveform in the negative feedback circuit of FIG. 5. It is a circuit diagram showing another example of the negative feedback circuit in a LED drive circuit as the 2nd Embodiment of this invention. It is a figure showing the current waveform in the negative feedback circuit of FIG. It is a circuit diagram showing another example of the negative feedback circuit in a LED drive circuit as the 2nd Embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

First Embodiment FIG. 1 is a circuit diagram of an LED drive circuit 10 as a first embodiment of the present invention. As will be described in detail below, the LED drive circuit 10 is configured to be able to light the LED array 15 directly with AC input.

  First, the configuration of the LED drive circuit 10 will be described. As shown in FIG. 1, the LED drive circuit 10 has a full-wave rectifier FR1, and the AC input is input to one end of the LED array 15 via the full-wave rectifier FR1. The operational amplifier OP1, the transistors TR1 to TR3, and the resistors R11, R12, and R13 constitute a constant current driving circuit that drives the LED array 15 with a constant current. A feedback circuit is formed from the emitters of the constant current driving transistors TR1 to TR3 to the operational amplifier OP1 via the resistors R12 and R13. The reference voltage Vref is input to the other input terminal (+) of the operational amplifier OP1.

  As described above, the LED drive circuit 10 has a plurality of transistors (three in this embodiment) for driving the LED array 15. In the example of FIG. 1, the LED string 15 has a total of eight stages of LEDs, and the collector of the transistor TR3 is connected to one end of the LED string 15 on the output side. On the other hand, the collectors of the transistors TR1 and TR2 are respectively connected to terminals taken from a predetermined number of positions in the LED array 15. In the example of FIG. 1, the collector of the transistor TR <b> 1 is connected to the second connection point of the LED string 15, and the collector of the transistor TR <b> 2 is connected to the fifth connection point of the LED string 15.

  The output of the operational amplifier OP1 is divided by resistors R1, R2, and R3, and the constant current driving transistors TR1 to TR3 are configured to be driven by the voltage divided by the resistors R1, R2, and R3. Specifically, the output voltage of the operational amplifier OP1 is input to the base of the transistor TR3 via the resistor R23, and the voltage between the resistors R1 and R2 is input to the base of the transistor TR2 via the resistor R22. The voltage across R3 is input to the base of transistor TR3 via resistor R21.

  Next, the operation of the LED drive circuit 10 will be described. The LED drive circuit 10 operates to turn on the LEDs (D1 to D8) of all the stages of the LED array 15 when the AC input is a sufficiently high voltage that can drive the LED array 15, and the voltage of the AC input. Even if the voltage becomes lower than a voltage sufficient to drive the entire number of stages of the LED string 15, the transistors TR1 or TR2 operate to light the LEDs in the LED string 15 with a small number of stages. With this configuration, the lighting state of the LED array can be maintained even when the AC input voltage is low. That is, since the period during which the LED row 15 is extinguished can be shortened as much as possible, flicker felt by human eyes can be eliminated.

  FIG. 2 is a diagram showing the operation of the LED drive circuit 10 as a signal waveform. In FIG. 2, the top signal waveform represents the AC input Vin, and reference numeral 52 represents the voltage at the output terminal of the operational amplifier OP1. The reference numerals 53, 54, and 55 represent the on / off states between the base and emitter of the transistors TR1, TR2, and TR3. A shaded portion indicated by reference numeral 56 in FIG. 2 represents the light emission period of the LED array 15.

As shown in FIG. 2, when the AC input Vin is in a sufficiently high state (Vin ≧ V ₃ ) (state S ₁ ), the voltage that can drive all the LEDs (D1 to D8) of the LED array 15 is the transistor TR3. Between the collector and emitter. Therefore, in this state, the transistor TR3 drives the LED string 15. At this time, a constant current circuit is formed by the operational amplifier OP1, the resistor R23, the transistor TR3, the resistor R11, and the negative feedback circuit, and the LED row 15 is driven. In this case, the output voltage of the operational amplifier OP1 is divided by the resistors R1 to R3, so that the base voltages of the transistors TR1 and TR2 are set so that the bases and emitters of the transistors TR1 and TR2 cannot be switched. . That is, in this state S1, only the transistor TR3 is in a state where the LED is driven.

Next, when the AC input Vin gradually decreases and becomes lower than the voltage V ₃ that can drive all the LEDs of the LED array 15 (state S ₂ ), the voltage between the collector and the emitter of the transistor TR 3 decreases, thereby causing the transistor TR 3. No current is driven between the collector and the emitter. In this case, since the operational amplifier OP1 tries to drive a constant current, the output voltage of the operational amplifier OP1 rises, the base-emitter of the transistor TR2 is turned on, and the transistor TR2 starts to drive the LED array 15. The transistor TR2 is driven by LEDs for five stages D1 to D5. In this state S _2, by the output voltage of the operational amplifier OP1 is divided by dividing resistors R1-R3, the base voltage of the transistor TR1, the base-emitter of the transistor TR1 is set to a state that can not be switched . That is, in this state S2, only the transistor TR2 is in a state of driving the LED.

Then, further AC input Vin decreases, becomes lower than the voltage _{V 2} which can drive the five stages of the LED D1~D5 the LED array 15 (state _S 3), the voltage drop between the collector and emitter of the transistor TR2 As a result, no current is driven between the collector and the emitter of the transistor TR2. In this case, since the operational amplifier OP1 tries to drive a constant current, the output voltage of the operational amplifier OP1 further rises, the base-emitter of the transistor TR1 is turned on, and the transistor TR1 starts to drive the LED array 15. Note that the transistor TR1 is driven by two stages of LEDs D1 to D2. That is, in the state _{S 3,} a state in which only the transistor TR1 is performing driving the LED.

When the AC voltage Vin becomes lower than the voltage V ₁ that can drive the LEDs of two stages D1 and D2 of the LED string 15 (state S ₄ ), the transistor TR1 also cannot drive the LED string 15, The LED row 15 is turned off. In this state S4, when power is supplied, the output of the operational amplifier OP1 is the maximum voltage that can be output. Note that the voltages V ₁ , V ₂ , and V ₃ are values that roughly correspond to the number of LED stages, and the voltage V ₁ is (the voltage required for the operation of the two LED stages + the constant current circuit output). _2, (LED 5 stages + constant current circuit voltage necessary for the output to work), and the voltage V ₃ is a (voltage required to LED8 stages + constant current circuit output is operated).

When the AC input Vin rises (the same applies when the absolute value of the voltage increases in the minus direction), the transistors TR1 to TR3 operate in the reverse order of the above states S1 to S3. That is, when the AC input Vin becomes _{V 1} or more, first two stages of the LED between the collector and the emitter of the transistor TR1 (D1, D2) is driven further, the AC input Vin becomes _{V 2} or more, the transistor TR2 Five stages of LEDs (D1 to D5) begin to be driven between the collector and the emitter. Then, further AC input Vin increases and the _{V 3} or more, LED total number between the collector and the emitter of the transistor TR3 (D1 to D8) starts to be driven. Hereinafter, such an operation is repeated due to a change in voltage of the AC input Vin.

  As described above, the LED drive circuit 10 can drive the LED array 15 with fewer LED stages than the LED array 15 even when the AC input drops to a voltage that cannot drive all the LED array stages. . Therefore, according to the LED drive circuit 10, even when the LED array is driven by AC input, the period during which the LED array is in the extinguished state can be shortened as much as possible. As a result, the flicker can be substantially prevented from occurring as the LED light source. Since the lighting period can be lengthened, the luminance as the LED lighting device can be increased. The LED drive circuit 10 can directly drive the LED after rectifying the AC. However, the pulsating current after the AC rectification is stepped down to a voltage necessary for the operation of the LED drive circuit to use the dedicated power source for the control circuit. There is a big merit that it can be made unnecessary.

  In FIG. 1, the number of LED stages in the LED array 15 is 8 as an example, but the number of LED stages can be set to various stages depending on the AC input. For example, as a rough guide, when the AC input is 100V, the number of LED rows may be about 40, and when the AC input is 200V, the number of LED rows may be about 80.

  The above is the description of the first embodiment of the present invention. Various modifications can be made to the above-described embodiment within the scope of the present invention. For example, as the number of LED stages of the LED array 15 shown in FIG. 1, various stages can be used depending on the magnitude of the AC input. Various stages can be considered for the number of LED stages connected to the collectors of the transistors TR1 and TR2. The LED drive circuit 10 of FIG. 1 is configured to drive the three connection points of the LED array using a total of three drive transistors (TR1 to TR3), but the number of drive transistors (That is, the number of connection points to the LED array) can be realized in two, four, or other various configurations.

  Moreover, although the LED drive circuit 10 of FIG. 1 is configured to drive a single LED row, a circuit configuration for driving a plurality of LED rows can also be realized. In the case of driving a plurality of LED strings, the driving transistor is connected in parallel to the output of the operational amplifier OP1. FIG. 3 is a circuit diagram showing a configuration example of the LED drive circuit in a case where the two LED rows 15 and 25 are driven. In the LED drive circuit 30 shown in FIG. 3, a drive unit composed of a transistor base resistor R33 and a transistor TR13 is connected in parallel to an output of the operational amplifier OP1 to a drive unit composed of a base resistor R23 and a transistor TR3. In addition, a drive unit including the transistor base resistor R32 and the transistor TR12 is connected in parallel to a drive unit including the base resistor R22 and the transistor TR2 at a connection point between the resistors R1 and R2 obtained by dividing the output of the operational amplifier OP1. In addition, a driving unit including the transistor base resistor R31 and the transistor TR11 is connected in parallel to a driving unit including the base resistor R21 and the transistor TR1 at a connection point between the resistors R2 and R3 obtained by dividing the output of the operational amplifier OP1. The collector of the transistor TR13 is connected to the output side end of the LED array 25, the collector of the transistor TR12 is connected to the position of the LED stage 25 in the LED stage 25, and the collector of the transistor TR11 is connected to the LED array 25. The number of LED stages is connected to the second stage position. The emitters of the transistors TR11 to TR13 are commonly connected to the resistor R41. With this configuration, a plurality of LED arrays can be operated as shown in FIG.

  In the LED drive circuit 10 of FIG. 1, a battery for the reference voltage of the operational amplifier OP1 is provided. However, a configuration in which a reference voltage is generated from an AC input rectifier circuit may be used. In this case, there is an advantage that it is not necessary to provide a reference voltage battery.

  Although the LED drive circuit 10 of FIG. 1 is configured to use a bipolar transistor, a MOS transistor may be used instead of the bipolar transistor. When MOS transistors are used in the LED drive circuit of FIG. 1, the resistors R21 to R23 may be omitted. In the above-described embodiment, the negative feedback amplifier circuit constituting the LED drive circuit is described as using a positive phase amplifier circuit. However, the negative feedback amplifier circuit may be an inverting amplifier circuit or a differential amplifier circuit. The same effect can be obtained even if is used.

  Next, a second embodiment of the present invention will be described. The second embodiment relates to an improvement in a current waveform for driving an LED string (LED string 15 in the first embodiment). In the LED drive circuit 10 shown in FIG. 1, the current value for constant current drive is determined by the resistor R <b> 11 regardless of which of the transistors TR <b> 1 to TR <b> 3 drives the LED string 15. Becomes a constant value, and the LED array 15 is driven with a rectangular waveform. FIG. 4 shows a current waveform in this case together with a pulsating waveform (corresponding to an input voltage waveform after rectification) which is a waveform of the input voltage. That is, in the case of the LED drive circuit 10 of FIG. 1, the current waveform for each pulsating flow is rectangular.

  Here, when the LED light source (LED array) is driven with a rectangular wave-like current waveform as shown in FIG. 4, the drive signal, that is, the current flowing through the input includes harmonic components, so that the harmonic flowing through the input is reduced as much as possible. In that sense, there is room for improvement. The second embodiment aims to reduce the harmonic current and reduce unnecessary radiation noise by bringing the current waveform closer to a sine wave.

  FIG. 5 is a circuit diagram showing the configuration of the negative feedback circuit 101 according to the second embodiment. The negative feedback circuit 101 corresponds to a portion surrounded by a dotted-line square in FIG. FIG. 6 is a diagram showing a drive current waveform when the negative feedback circuit 101 of FIG. 5 is employed, together with a pulsating waveform as an input voltage. 5 and 6 (and the drawings described below), the same elements as those in the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. As shown in FIG. 5, in the negative feedback circuit 101, a resistor R102 is provided at the emitter of the transistor TR1 in addition to the current detection resistor R101 used in common for the three transistors TR1, TR2, and TR3. Therefore, when the transistor TR3 (drives all the LEDs D1 to D8 in the LED array 15) is driven and when the transistor TR2 (drives D1 to D5 of the LED array 15) is driven, Only the resistor R101 functions as a resistor for detecting a drive current, and a relatively large drive current can flow. On the other hand, when the transistor TR1 (driving D1 to D2 in the LED array 15) is driven, the resistance obtained by adding the resistors R101 and R102 is a resistance for detecting the driving current, and therefore the driving current is relatively small. Value.

  The state of the drive current as described above is shown in FIG. That is, as shown in FIG. 6, at the rising portion of the input pulsating waveform, first, the LED array 15 is driven only by the transistor TR1 (location 111 in FIG. 6), so the drive current value is compared. Small value. When the input pulsating voltage further rises and the LED array 15 is driven by the transistor TR2 (location 112 in FIG. 6), the drive current increases. Even when the input pulsating voltage further rises and the LED array 15 is driven by the transistor TR3, the drive current value is the same as when the transistor TR2 is driven. That is, the drive current waveform for the LED array 15 is a waveform that changes stepwise at the rising and falling portions as shown in FIG. As a result, the waveform of the drive current changes from a single rectangular wave to a staircase waveform that is closer to a sine wave, and the harmonic component of the current flowing to the input is reduced and unnecessary radiation noise is also reduced.

  FIG. 7 shows another example of the negative feedback circuit according to the second embodiment. The negative feedback circuit 151 shown in FIG. 7 has a configuration in which a driving unit 131 including a transistor TR31 and resistors R111 and R112 is added to the negative feedback circuit 101 shown in FIG. In this configuration, when the input pulsating voltage is sufficiently high and the transistor TR3 is driven, the drive unit 131 including the transistor TR31 also drives the LED string 15, and thus the drive current further increases. Become.

  FIG. 8 shows the drive current waveform in this case together with the pulsating flow waveform as the input voltage. As shown in FIG. 8, at the rising portion of the input pulsating waveform, first, the LED array 15 is driven only by the transistor TR1 (location 111 in FIG. 8), so the drive current value is relatively small. Value. When the input pulsating voltage further rises and the LED array 15 is driven by the transistor TR2 (location 112 in FIG. 8), the drive current increases. When the input pulsating voltage further rises and the LED string 15 is driven by the transistor TR3, the LED string 15 is driven not only by the transistor TR3 but also by the transistor TR31. 8 and numeral 113). That is, the drive current waveform for the LED array 15 is a waveform that changes in three stages as shown in FIG. In this case, the rising / falling portion of the drive current waveform becomes closer to a sine wave, so that the harmonics contained in the current flowing to the input are further reduced, and accordingly, unnecessary radiation noise is further reduced.

  FIG. 9 shows still another example of the negative feedback circuit according to the second embodiment. The negative feedback circuit 171 shown in FIG. 9 can be regarded as a modified version of the negative feedback circuit 151 shown in FIG. From the viewpoint of the drive current waveform, the negative feedback circuit 171 in FIG. 9 also operates to generate a drive current waveform similar to the negative feedback circuit 151 in FIG. As shown in FIG. 9, in the negative feedback circuit 171, the emitter of the transistor TR3 is connected to the ground level via the resistor Rs3, and the emitter of the transistor TR2 is connected to one end of the resistor Rs3 via the resistor Rs2 (that is, the transistor TR3). Connected to the emitter). The emitter of the transistor TR1 is connected to one end of the resistor Rs2 (that is, the emitter of the transistor TR2) via the resistor Rs1. Note that the value of the resistor (Rs1 + Rs2 + Rs3) is set to a sufficiently small value with respect to the negative feedback resistors (R12, R13).

  According to the above configuration, regarding the current value flowing through the transistor TR1 (constant current drive current value), a voltage value of resistance value (Rs1 + Rs2 + Rs3) × current value is detected as a voltage value for constant current control. Regarding the current value flowing through the transistor TR2 (current value of constant current driving), a voltage value of resistance value (Rs2 + Rs3) × current value is detected as a voltage value for constant current control. As for the current value flowing through the transistor TR3 (current value for constant current driving), a voltage value of resistance value (Rs3) × current value is detected as a voltage value for constant current control. With this configuration, the current flowing through each transistor has a relationship of TR3> TR2> TR1, and a current waveform as shown in FIG. 8 can be easily realized.

  That is, at the stage where the transistor TR1 is driven at the rising edge of the input pulsating voltage, the resistance value for detecting the driving current is (Rs1 + Rs2 + Rs3), and thus the driving current has a low value (in the current waveform of FIG. 8). 111). Next, when the input pulsating voltage rises and the transistor TR2 is driven, the resistance value for driving current detection decreases to (Rs2 + Rs3), and thus the driving current increases (FIG. 8). 112 in the current waveform). When the input pulsating voltage further rises and the transistor TR3 is driven, the resistance value for detecting the driving current decreases to Rs1, and the driving current further increases (FIG. 8). Reference numeral 113 in the current waveform).

  In the example shown in FIG. 9, it has been described that the current value is driven in three stages, but the LED for driving the current waveform into a plurality of staircase waveforms by dividing the resistor for detecting the current value into a plurality of stages. In the above system, as long as each resistance value is kept within a practical range, the number of steps is not limited. That is, it is possible to realize drive waveforms having four or more steps.

  As described above, according to the configuration of the negative feedback circuit according to the second embodiment, the drive current waveform of the LED light source (LED array) can be approximated to a sine wave, and thus the harmonics included in the drive current. Components can be reduced. Therefore, the harmonic current contained in the input current of the LED lighting device can be reduced, and unnecessary radiation noise associated with driving the LED light source can be reduced. According to the configuration of the second embodiment described above, it is possible to easily cope with the reduction of the harmonic current included in the input current of the LED lighting device.

  Needless to say, the second embodiment described above can be modified from various viewpoints described with respect to the first embodiment.

  In addition, according to the structure of 2nd Embodiment, the following effects are also implement | achieved. If the current flowing through the LED array is constant, the average luminance during AC lighting is a value corresponding to (average number of LEDs lit) × (LED luminance) ÷ (pulse current cycle). It is conceivable that it is lower than a certain (number of LEDs) × (LED brightness). In this regard, according to the configuration of the second embodiment, the current flowing through the LED array is increased when the number of LED lighting is large, decreased when the number of LED lighting is small, and the average current is the same as that of DC lighting. By doing so, it is possible to make the luminance equal to that in the case of direct current lighting. Such a configuration can be realized, for example, by appropriately selecting the values of resistors for detecting the current value (Rs1, Rs2, Rs3 in the configuration of FIG. 9). That is, according to the configuration of the second embodiment, it is possible to achieve the effect of increasing the luminance when the LED array is AC driven.

DESCRIPTION OF SYMBOLS 10 LED drive circuit FR1 Full wave rectifier 15 LED row TR1-TR3, TR31 Transistor R1-R3 Resistor R11-R13 Resistor R21-R23 Resistor OP1 Operational amplifier Vref Reference voltage R101, R102 Resistor 101 Negative feedback circuit 151 Negative feedback circuit 171 Negative feedback circuit

Claims (7)

An LED driving circuit for driving an LED unit having a plurality of LEDs connected in series,
A rectifier that rectifies the AC input;
A constant current circuit having an operational amplifier and a plurality of driving transistors connected to the output stage of the operational amplifier via a voltage dividing resistor,
By connecting one end of the output side of the rectifier to the input side of the LED unit, and connecting one end of the output side of the plurality of transistors to a connection point of a different number of LED stages in the LED unit, An LED driving circuit, wherein the plurality of transistors selectively drive the LED unit according to a voltage.   The each of the plurality of transistors is a bipolar transistor, and collector terminals of the plurality of bipolar transistors are respectively connected to connection points of different LED stages in the LED unit. LED drive circuit.   The LED driving circuit according to claim 2, wherein a base resistor is disposed at each of the bases of the plurality of bipolar transistors.   Each of the plurality of transistors is a MOS transistor, the output stage of the operational amplifier is connected to the gates of the plurality of MOS transistors through the voltage dividing resistor, and the drain terminals of the plurality of MOS transistors are respectively connected to the LED The LED driving circuit according to claim 1, wherein the LED driving circuit is connected to a connection point having a different number of LED stages in the unit. The LED unit has a plurality of LED rows,
5. The LED driving circuit according to claim 1, wherein the plurality of driving transistors are arranged in parallel for each of the plurality of LED rows. 6.   The constant current circuit is configured such that a current value for driving the LED unit by each of the plurality of driving transistors has at least two different values, thereby rising and falling of a current waveform for driving the LED unit. 6. The LED driving circuit according to claim 1, wherein the waveform of the portion changes stepwise. The resistance values of the driving current detection resistors connected to the emitter terminals of the plurality of driving transistors are at least two different values, so that the rising and falling portions of the current waveform for driving the LED unit The LED driving circuit according to claim 6, wherein the waveform changes stepwise.

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