JP2005252211A - Led luminance adjustment circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the deterioration of an LED by preventing a current exceeding the allowed maximum current set beforehand in a circuit for adjusting LED luminance by adjusting a drive current for the LED according to the input voltage. <P>SOLUTION: An LED luminance adjustment circuit comprises a voltage adjustment means 70 for setting voltage for setting the current value controlled by an LED drive current control means 30 and supplying voltage to this LED drive current control means and a maximum voltage control means 74 for restricting the voltage supplied by the voltage adjustment means to the voltage corresponding to the allowed maximum current value supplied to the LED by the LED drive current control means. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an LED brightness adjustment circuit, and in particular, an LED current drive having a function of adjusting LED drive current according to an input voltage and preventing deterioration and destruction of the LED due to the LED drive current exceeding the LED rated current value. Regarding the circuit.

  In order to drive an LED (light emitting diode), a constant current circuit is required as a current driving circuit for supplying a predetermined current to the LED. Conventionally, as an example of such a constant current circuit, the LED brightness adjustment shown in FIG. A circuit is known. FIG. 11 is a circuit diagram of a conventional LED brightness adjusting circuit 10. The conventional LED brightness adjusting circuit 10 is configured by connecting an LED drive current control means 30 for supplying current to an LED between a voltage source VDD that supplies a desired current while maintaining a predetermined voltage and a ground GND. Composed. The LED drive current control means 30 shown in FIG. 11 operates to absorb current when the LED 20 is considered as a load.

  The LED drive current control means 30 is configured as follows. First, the output of the operational amplifier 60 is connected to the gate of an n-channel MOS transistor 40 that functions as an LED driving element. A negative feedback loop from the source of the n-channel MOS transistor 40 to the negative input terminal of the operational amplifier 60 is formed. The positive input terminal of the operational amplifier 60 is connected to the voltage adjusting means 70.

  The source of the n-channel MOS transistor 40 connected to the negative input terminal of the operational amplifier 60 is further connected to one terminal of the current sense resistor 50, and the other terminal of the current sense resistor 50 is grounded. Further, the drain of the n-channel MOS transistor 40 is connected to the cathode of the LED 20 that is a load.

  The LED drive current control means 30 suppresses variations in LED brightness by driving the LEDs at a constant current. Specifically, the voltage supplied from the voltage adjustment means 70 applies a predetermined fixed voltage to the current sense resistor 50 by a virtual short circuit between the positive input and the negative input of the operational amplifier 60. The current flowing through the current sense resistor 50 is constant by the applied constant voltage. On the other hand, when a current flows through the LED, there is only a voltage drop of about 0.7 V, so that the drain potential of the n-channel MOS transistor 40 is maintained as a potential substantially equal to the power supply voltage VDD.

  Here, if the voltage of the output of the operational amplifier 60 connected to the gate of the n-channel MOS transistor 40 is lower than that of the drain of the n-channel MOS transistor 40, a predetermined potential difference is generated between the source and the drain. In this state, a current flows between the drain of the n-channel MOS transistor 40 and the source of the n-channel MOS transistor 40.

  As described above, since the current value of the flowing current is a current value determined by the voltage applied to the current sense resistor 50 and the resistance value of the current sense resistor 50, it is constant according to the voltage value set by the luminance adjustment voltage source. Current can be supplied.

  Here, even when the LED drive current control means 30 for supplying a constant current is configured, it is possible to easily drive the LED element with low power consumption and high Vf, paying attention to the variation in brightness for each LED. There is an LED element driving circuit for illuminating a display device in a portable device that is small in size, has little luminance variation among products, and can suppress a change in luminance even when the voltage of a battery decreases. (See Patent Document 1)

  FIG. 12 is a circuit diagram showing another LED brightness adjustment circuit 10 of the prior art. The LED drive current control means 30 shown in FIG. 12 operates to discharge current when the LED 20 is considered as a load. The basic operation is the same as that shown in FIG.

JP 2002-359090 A

However, according to the prior art described in FIGS. 11 and 12, since the drive current of the LED 20 is proportional to the reference voltage output from the voltage adjusting means 70, the drive current exceeds the rated current of the LED 20 when the reference voltage is high. A drive current source that does not exceed the rated current has been desired.
Accordingly, an object of the present invention is to adjust the LED drive current according to the input voltage and to prevent the LED from deteriorating and breaking when the LED drive current exceeds the LED rated current value in the circuit for adjusting the LED brightness. It is to provide an LED luminance adjustment circuit to which is added.

  In order to solve this problem, a first LED brightness adjustment circuit according to the present invention is an LED brightness adjustment circuit that adjusts the brightness of an LED according to a drive current, and drives the LED according to a supplied voltage. LED driving current control means for controlling current, voltage adjusting means for setting a voltage for setting a current value controlled by the LED driving current control means, and supplying voltage to the LED driving current control means, and voltage adjusting means Maximum voltage control means for limiting the voltage supplied by the LED drive current control means to a voltage corresponding to the maximum allowable current value supplied to the LED by the LED drive current control means.

  The second LED brightness adjusting circuit according to the present invention is an LED brightness adjusting circuit that adjusts the brightness of the LED according to the drive current, and controls the LED drive current according to the supplied voltage. Current control means, first brightness voltage adjusting means for adjusting a first brightness voltage value for supplying a desired current for driving the LED, and outputting the adjusted first brightness voltage; and LED A second luminance voltage adjusting means for setting a second luminance voltage value for supplying a maximum allowable current set for driving and outputting the set second luminance voltage; and a first luminance Compared with the luminance voltage comparison means and the luminance voltage comparison means, which is connected to the voltage adjustment means and the second luminance voltage adjustment means and compares the voltage value of the first luminance voltage and the voltage value of the second luminance voltage. The result is either small or large The luminance voltage supply means for supplying the first luminance voltage to the LED driving current control means when it is in the state and for supplying the second luminance voltage to the LED driving current control means when it is in the other state of the magnitude Including. In this case, the luminance voltage comparison means may be a comparator with hysteresis.

(Embodiment 1)
The LED brightness adjusting circuit according to the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of a first LED brightness adjustment circuit 10 according to the present invention. FIG. 1 discloses an LED brightness adjustment circuit 10 that adjusts the brightness of the LED 20 according to the drive current. The LED drive current control means 30 controls the drive current of the LED 20 according to the supplied voltage, and the LED. A voltage for setting a current value controlled by the drive current control means 30 is set, a voltage adjusting means 70 for supplying a voltage to the LED drive current control means 30, and a voltage supplied by the voltage adjustment means 70 for LED driving. It includes a maximum voltage control means 74 that limits the voltage corresponding to the maximum allowable current value supplied to the LED 20 by the current control means 30.

  FIG. 1 discloses an LED brightness adjustment circuit 10 that adjusts the brightness of the LED 20 in accordance with the drive current. This circuit is a current drive circuit that functions as a constant current source.

  The LED drive current control means 30 controls the drive current of the LED 20 according to the supplied voltage. In this embodiment, a circuit that functions as a constant current source configured to absorb current from VDD that is a power supply voltage is described. However, a circuit that functions as a constant current source configured to discharge current may be used. The voltage adjusting unit 70 sets a voltage for setting the current value controlled by the LED drive current control unit 30 and supplies the voltage to the LED drive current control unit 30.

  Reference is now made to FIG. FIG. 2 is a circuit diagram showing a specific configuration of the voltage adjusting means 70. FIG. 2 shows a voltage adjusting means 70, for example, a voltage stabilizing circuit 72 provided therein, and an input voltage adjusting resistor for adjusting the output voltage of the voltage stabilizing circuit 72. 202 is comprised. As shown in FIG. 2, the voltage stabilization circuit 72 is configured by, for example, a voltage follower, and an input terminal to a circuit having a relatively high input impedance by passing a voltage determined by the adjustment resistor 202 through the voltage follower. To facilitate the connection.

  The voltage adjusting means 70 has been shown as an example configured using the adjusting resistor 202 and a voltage follower functioning as a voltage stabilizing circuit 72 included therein. However, the present invention is not limited to this. In short, the output voltage can be easily varied, and all circuits configured to stably supply a constant voltage are included.

  Moreover, although the voltage adjustment means 70 showed the example which contains the voltage stabilization circuit 72 inside, it is needless to say that it does not necessarily need to be inside.

  Returning to FIG. The maximum voltage control unit 74 limits the voltage supplied by the voltage adjustment unit 70 to a voltage corresponding to the maximum allowable current value supplied to the LED 20 by the LED drive current control unit 30.

Reference is now made to FIG. FIG. 3 is a circuit diagram showing a specific configuration of the maximum voltage control means 74. FIG. 3 shows an example of a circuit that realizes the maximum voltage control means 74. Specifically, as shown in FIG. 3, a Zener diode 302 is connected to the input terminal via a current limiting resistor 306 in the reverse direction, and further connected to a voltage follower 304.
The maximum voltage control means 74 shown in FIG. 3 is regulated by the reverse voltage of the Zener diode when a predetermined voltage is applied to the input terminal and a current of a predetermined current value or more flows in the reverse direction of the Zener diode 302. The output impedance is lowered by 304, and connection to the subsequent stage is facilitated. Thereby, it is possible to limit to a constant voltage indicated by the reverse current-voltage characteristic of the Zener diode 302.

  When the output from the voltage adjusting means 70 is input to the maximum voltage control means 74, if the output from the voltage adjusting means 70 is larger than the constant voltage indicated by the reverse voltage characteristic of the Zener diode 302, the Zener diode 302 The output voltage of the maximum voltage control unit 74 is limited to the constant voltage of the Zener diode 302 as the maximum value, and the output voltage of the maximum voltage control unit 74 is supplied to the LED drive current control unit 30. Is done.

    FIG. 4 is a circuit diagram showing a specific configuration of the LED drive current control means 30 of the LED brightness adjustment circuit according to the present invention. FIG. 4 is a constant current circuit that is basically the same as the LED brightness adjustment circuit described in the background art, and the same elements are denoted by the same reference numerals as those described in the background art, and thus description thereof is omitted. .

  Here, the maximum allowable current is set so that the current flowing in the LED drive current control means 30 becomes maximum when the maximum value of the voltage supplied from the maximum voltage control means 74 to the LED drive current control means 30 is applied. be able to. Specifically, assuming that the maximum allowable current flowing in the LED is 100 mA and the maximum voltage value supplied by the maximum voltage control means 74 is 2.5 V, the voltage is set by setting the current sense resistor in FIG. 4 to 25Ω. Even if the voltage supplied by the adjusting means 70 becomes larger than 2.5 V, the maximum voltage supplied to control the LED drive current control means 30 is regulated by the maximum voltage control means 74. The current flowing through the LED 20 does not exceed 100 mA.

    As described above, the maximum voltage control unit 74 can regulate a current greater than or equal to the maximum allowable current flowing in the LED 20 by regulating the input voltage of the LED drive current control unit 30 at the upper limit. As a result, even if the output voltage from the voltage adjusting means 70 increases for some reason, the maximum voltage control means 74 limits the input voltage of the LED drive current control means 30 to a predetermined value, so that a current equal to or greater than the maximum allowable current. Does not flow to the LED, and the performance of the LED is not deteriorated.

(Embodiment 2)
FIG. 5 is another circuit diagram showing a specific configuration of the LED brightness adjustment circuit 500 according to the present invention. Elements having the same functions as those in FIGS. 1 to 4 are denoted by the same reference numerals. FIG. 5 discloses an LED brightness adjustment circuit 500 that adjusts the brightness of the other LEDs 20 according to the drive current, and LED drive current control means 30 that controls the LED drive current according to the supplied voltage. The first luminance voltage adjusting unit 502 that adjusts the first luminance voltage value for outputting a desired current for driving the LED 20 and outputs the adjusted first luminance voltage, and drives the LED A second luminance voltage adjusting means 504 for setting a second luminance voltage value for supplying the set maximum allowable current and supplying the set second luminance voltage; and a first luminance voltage adjustment A luminance voltage comparison unit 506 that is connected to the unit 502 and the second luminance voltage adjustment unit 504 and compares the voltage value of the first luminance voltage and the voltage value of the second luminance voltage; Was judged When the comparison result is any one of the large and small states, the first luminance voltage is supplied to the LED drive current control means 30, and when the comparison result is the other state, the second luminance is obtained. And a luminance voltage supply means 514 for supplying a voltage to the LED drive current control means 30.

  FIG. 5 discloses an LED brightness adjustment circuit 500 that adjusts the brightness of the LED 20 in accordance with the drive current. This circuit is also a current driving circuit that functions as a constant current source when driving the LED.

  The LED drive current control means 30 controls the drive current of the LED 20 according to the supplied voltage. In this embodiment, as described with reference to FIGS. 1 and 4, a circuit that functions as a constant current source configured to absorb a current from VDD that is a power supply voltage will be described. It may be a circuit that functions as a source.

  The first luminance voltage adjusting unit 502 adjusts a first luminance voltage value for outputting a desired current for driving the LED 20, and supplies the adjusted first luminance voltage. The first luminance voltage adjusting means 502 is basically the same as the voltage adjusting means 70 in FIGS. 1 and 2, and the main point is that the output voltage can be easily changed and a constant voltage can be stably supplied. All circuits configured to be supplied are included.

  FIG. 6 is a further example of the first luminance voltage adjusting means 502. The first luminance voltage adjusting means 502 shown in FIG. 6 includes a voltage stabilizing circuit 72 inside, and includes an input voltage adjusting resistor 202 for adjusting the output voltage of the voltage stabilizing circuit 72. In addition, trimming resistors 602 and 604 are provided at the output of the voltage stabilization circuit 72 so that fine adjustment can be performed, and the voltage distributed by the voltage dividing specific resistance of the trimming resistors 602 and 604 is output. The trimming resistor is adjusted by adjusting the offset of the operational amplifier, the variation of the current sense resistor 50 in the LED drive current control means 30 in the actual design such as when such a circuit is provided in the IC, for example. Therefore, laser trimming may be performed.

  The voltage stabilization circuit 72 is configured by, for example, a voltage follower, and facilitates connection with a circuit having a relatively high input impedance by passing a voltage determined by the adjustment resistor 202 through the voltage follower. Is the same as the description of the voltage adjusting means 70 disclosed in FIG. Of course, the trimming resistors 602 and 604 used in the first luminance voltage adjustment unit 502 can be used in the voltage adjustment unit 70 described in FIG.

  Returning to FIG. The second luminance voltage adjusting means 504 sets a second luminance voltage value for outputting the maximum allowable current set for driving the LED, and outputs the set second luminance voltage 50. Since the second luminance voltage adjusting means 504 sets the second luminance voltage value for outputting the maximum allowable current, the circuit disclosed in FIG. 6 can be used as a specific circuit. It is not limited to.

  The luminance voltage comparison unit 506 is connected to the first luminance voltage cliff adjustment unit 502 and the second luminance voltage adjustment unit 504, and the voltage value of the first luminance voltage and the voltage value of the second luminance voltage are calculated. Compare large and small. As the luminance voltage comparison means 506, for example, a comparator can be used, but it is not limited to this.

In the embodiment disclosed in FIG. 5, the luminance voltage comparison means 506 will be described as a comparator. The luminance voltage comparison unit 506 (comparator) compares the voltage value output from the first luminance voltage adjustment unit 502 with the voltage value output from the second luminance voltage adjustment unit 504, and compares the first luminance voltage. When the voltage value output by the adjusting means 502 is larger, an H (high) level signal is output, and when the voltage value is lower, an L (low) level signal is output. If the input terminal of the luminance voltage comparison means 506 (comparator) is reversed, of course, the level of the output signal is reversed.

  The luminance voltage supply unit 514 supplies the first luminance voltage when the comparison result determined by the luminance voltage comparison unit 506 (comparator) is in one of the large and small states, In this state, the second luminance voltage is supplied. For example, the luminance voltage supply unit 514 includes switches 508 and 512, and turns off the switch 508 when the output of the luminance voltage comparison unit 506 (comparator) is L. On the other hand, when the output of the luminance voltage comparison means 506 (comparator) is H, the switch 508 is turned on.

  Now, when the output of the luminance voltage comparison unit 506 (comparator) is at L level, that is, when the output voltage of the first luminance voltage adjustment unit 502 is smaller than the output voltage of the second luminance voltage adjustment unit 504. The luminance voltage supply means 514 outputs the output voltage of the first luminance voltage adjustment means 502 and supplies the voltage to the LED drive current control means 30.

  On the other hand, when the output of the luminance voltage comparison unit 506 (comparator) is at the H level, that is, when the output voltage of the first luminance voltage adjustment unit 502 is larger than the output voltage of the second luminance voltage adjustment unit 504. The luminance voltage supply means 514 outputs the output voltage of the second luminance voltage adjustment means 504 and supplies the voltage to the LED drive current control procedure 30. Here, the output voltage of the second luminance voltage adjusting means 504 is set so as to define the maximum allowable current flowing in the LED drive current control means 30.

  The setting of the output voltage of the second luminance voltage adjusting means 504 is performed as follows. For example, assuming that the maximum allowable current flowing in the LED is 100 mA and the maximum voltage value output by the second luminance voltage adjusting means 504 is 2.5 V, the first current sense resistor in FIG. Even if the voltage output from the brightness voltage adjusting means 502 becomes larger than 2.5 V, the second brightness voltage adjusting means 504 supplies the LED driving current control means 30 to control the LED driving current control means 30. Since the maximum value of the applied voltage is regulated, the current flowing through the LED 20 does not exceed 100 mA.

  As described above, the voltage output from the second luminance voltage adjusting unit 504 is input to the LED drive current control unit 30 by the luminance voltage adjusting unit 514 in accordance with the output of the luminance voltage comparing unit 506 (comparator). Then, the input voltage is regulated at the upper limit. Thereby, the electric current more than the maximum permissible current which flows into LED20 can be controlled. Thereby, even if the output voltage from the first luminance voltage adjusting unit 502 becomes large for some reason, the second luminance voltage adjusting unit 504, the luminance voltage comparing unit 506 (comparator), and the luminance voltage adjusting unit 514 Since the input voltage of the LED drive current control means 30 can be limited to a predetermined value, the current exceeding the maximum allowable current does not flow through the LED, and the performance of the LED is not deteriorated.

  The LED drive current control means 30 has been described as a current sink type constant current circuit, but the present invention can be implemented in a current discharge type. In that case, as for the input to the LED drive current control means 30, the maximum allowable current is regulated by the lower limit voltage. In that case, the input of the comparator is replaced or the characteristics of the switch are reversed. You can do it. Further, the luminance voltage comparison unit 506 may be a comparator with hysteresis.

  Please refer to FIG. FIG. 7 is a circuit diagram showing an example of a comparator with hysteresis. FIG. 7 discloses a diagram in which the hysteresis resistors 702 and 704 are connected in the form of voltage division at the plus side input terminal with respect to the comparator that functions as the luminance voltage comparison means 506 in FIG. It is not limited to this.

The operating characteristics in that case are shown in FIGS. FIG. 8 is a graph showing the output voltage of the first luminance voltage adjusting unit 502 with respect to the time axis. This is the input voltage. This is an input to the luminance voltage comparison means. FIG. 9 is a diagram showing the value of current flowing inside the LED drive current control means 30 with respect to the time axis. And the actual operating current with respect to the input voltage of FIG. 8 is shown.

The timings A and B shown in FIGS. 8 and 9 indicate the same time point. When the voltage value output from the first luminance voltage adjustment unit 502 is larger than the voltage value output from the second luminance voltage adjustment unit 504, the luminance voltage supply unit 514 performs the second luminance voltage adjustment. As a result of outputting the voltage of the means 504, it is shown that the current inside the LED drive current control means 30 is the maximum allowable current value. Even if the voltage output from the first luminance voltage adjusting unit 502 starts to decrease and becomes smaller than the voltage value output from the original second luminance voltage adjusting unit 504, the hysteresis-equipped comparator operates. The state in which the comparator is not switched on / off (immediately before B in the figure) is shown.

  Thus, according to the comparator with hysteresis, it has a characteristic having hysteresis with respect to the voltage that regulates the maximum allowable current from the second luminance voltage adjusting means 504 connected to the negative terminal, and the first luminance voltage Chattering can be prevented when the voltage value output from the adjusting unit 502 is around the voltage set by the second luminance voltage adjusting unit 504.

(Embodiment 3)
FIG. 10 is a diagram showing an application example of the LED brightness adjustment circuit according to the present invention. FIG. 10 basically discloses the same components as in FIG. 5, but includes a voltage stabilization circuit 70 in the first luminance voltage adjusting means 502, and connects the output terminal and the voltage follower 1004. A trimming resistor 1002 is connected to the output of the voltage follower 1004. In addition, the same code | symbol is attached | subjected to the same element as the previous figure. The second luminance voltage adjusting unit 504 also includes a trimming resistor 1006 for adjusting the voltage.

  Considering the application to a multi-channel LED brightness adjustment circuit for a plurality of LEDs, based on this circuit configuration. Since each operational amplifier has characteristic variations, when considering a multi-channel LED luminance adjustment circuit, the only way to adjust the luminance is to finely adjust the trimming resistors 1002 and 1006. In this case, a plurality of first luminance voltage adjusting means 502 may be provided so as to correspond to the plurality of LED drive current control means 30, respectively, and trimming resistors may be provided, respectively, or a first for setting the maximum allowable current. The trimming resistors 1006 of the second luminance voltage adjusting means 504 may be provided in accordance with the number of channels.

For example, when a plurality of LED brightness adjustment circuits are provided in the same IC, either of the external trimming resistors 1002 and 1006 corresponds to the LED drive current control means 30 according to the embodiment of the present invention. include.

This trimming can be adjusted by, for example, laser trimming. By performing the trimming, it is possible to adjust the luminance of the LED including adjusting characteristic variations of circuit elements such as an offset voltage of an operational amplifier. Even when the LED driving current control means 30 having a plurality of channels is provided, the luminance voltage comparison means 506 (comparator) may of course have hysteresis.

  Although the present specification has been described as a so-called current sink type circuit, it is needless to say that the present invention can also be implemented by a current driving circuit that functions as a so-called current discharge type constant current source.

The invention's effect

  According to the present invention, the current exceeding the maximum allowable current is not passed through the LED, so that the deterioration of the LED due to the current exceeding the preset maximum allowable current can be prevented.

It is a block diagram of the 1st LED brightness adjustment circuit which concerns on this invention. It is a circuit diagram which shows the specific structure of the voltage adjustment means in the LED brightness adjustment circuit which concerns on this invention. It is a circuit diagram which shows the specific structure of the maximum voltage control means in the LED brightness adjustment circuit which concerns on this invention. It is a circuit diagram which shows the concrete structure of the LED drive current control means based on this invention. It is another circuit diagram which shows the specific structure of the LED brightness adjustment circuit which concerns on this invention. It is a circuit diagram which shows a further example of a 1st luminance voltage adjustment means. It is a circuit diagram which shows an example of a comparator with a hysteresis. It is a graph which shows the output voltage with respect to the time-axis of the 1st brightness | luminance voltage adjustment means of the LED brightness adjustment circuit which concerns on this invention. It is a figure which shows the electric current value which flows into the inside of the LED drive current control means with respect to the time axis of the LED brightness adjustment circuit which concerns on this invention. It is a circuit diagram which shows the application example which concerns on this invention. It is a circuit diagram which shows the conventional LED brightness adjustment circuit. It is another circuit diagram which shows the conventional LED brightness adjustment circuit.

Explanation of symbols

10 LED brightness adjustment circuit 20 LED
30 LED drive current control means 40 n-channel MOS transistor 50 current sense resistor 60 operational amplifier 70 voltage adjustment means 72 voltage stabilization circuit 74 maximum voltage control means 202 adjustment resistor 302 zener diode 304 voltage follower 306 current limit resistance 208, 212 switch (luminance) Voltage supply means)
312 CONTROL / LIMIT switch 314 amplifier 318 adjustment resistor 322 adjustment resistor 500 luminance adjustment circuit 502 first luminance voltage adjustment unit 504 second luminance voltage adjustment unit 506 comparator (luminance voltage comparison unit)
508, 512 Switch 514 Luminance voltage supply means 602 Trimming resistor 604 Trimming resistor 702 Hysteresis resistor 704 Hysteresis resistors 1002, 1006 Trimming resistor 1004 Voltage follower

Claims (3)

An LED brightness adjustment circuit for adjusting the brightness of an LED according to a drive current,
LED driving current control means for controlling the LED driving current according to the supplied voltage;
Voltage adjusting means for setting a voltage for setting a current value controlled by the LED drive current control means, and supplying the voltage to the LED drive current control means;
An LED brightness adjustment circuit including: a maximum voltage control unit configured to limit the voltage supplied by the voltage adjustment unit to a voltage corresponding to a maximum allowable current value supplied to the LED by the LED drive current control unit; An LED brightness adjustment circuit for adjusting the brightness of an LED according to a drive current,
LED driving current control means for controlling the LED driving current according to the supplied voltage;
First luminance voltage adjusting means for adjusting a first luminance voltage value for supplying a desired current for driving the LED and outputting the adjusted first luminance voltage;
A second luminance voltage adjusting means for setting a second luminance voltage value for supplying a maximum allowable current set for driving the LED and outputting the set second luminance voltage;
Luminance voltage comparison means connected to the first luminance voltage adjustment means and the second luminance voltage adjustment means, for comparing the voltage value of the first luminance voltage and the voltage value of the second luminance voltage. When,
If the result of comparison with the luminance voltage comparison means is in any one of the large and small states, the first luminance voltage is supplied to the LED drive current control means, In some cases, an LED brightness adjustment circuit including brightness voltage supply means for supplying the second brightness voltage to the LED drive current control means. 3. The LED brightness adjustment circuit according to claim 2, wherein the brightness voltage comparison means is a comparator with hysteresis.

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