JP4845694B2 - LED drive circuit - Google Patents

Description

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

  Currently, LEDs are mounted on various electronic devices. For example, the LED is mounted on a mobile phone and used as a flashlight for a camera.

  The LED is driven by a switching regulator control circuit in the LED drive circuit. FIG. 4 is a diagram showing an outline of a conventional LED driving circuit and LEDs.

  The switching regulator control circuit 308 in the LED drive circuit is connected to the VIN terminal 309 of the LED drive circuit, the VOUT terminal 306 of the LED drive circuit, the FB terminal 304 of the LED drive circuit, and the ground 310 of the LED drive circuit via the coil 302. It is connected. A voltage source 301 is provided between the VIN terminal 309 and the ground 310, a high brightness white LED 305 is provided between the VOUT terminal 306 and the FB terminal 304, and a resistor 303 is provided between the FB terminal 304 and the ground 310. A capacitor 307 is provided between the VOUT terminal 306 and the ground 310 (for example, Patent Document 1).

  When a predetermined forward voltage is applied to the LED 305, the LED 305 and the resistor 303 cause a forward current to flow. The predetermined forward voltage is about 3V to 4V. Based on the forward current, a voltage is generated in the resistor 303 and output to the FB terminal 304. The switching regulator control circuit 308 controls the current of the LED 305 and the resistor 303 by controlling the voltage of the FB terminal 304 at the connection point between the LED 305 and the resistor 303, thereby controlling the luminance of the LED 305.

  Here, the voltage source of the mobile phone is mainly a lithium ion secondary battery, and varies from 3V to 4.2V. Then, the forward voltage applied to the LED 305 may become low, and the luminance of the LED 305 may not be controlled.

As a countermeasure against this, the switching regulator control circuit 308 normally controls the forward voltage applied to the LED 305 by controlling the voltage of the lithium ion secondary battery, thereby controlling the luminance of the LED 305.
Japanese Patent Laid-Open No. 11-068161

  However, when the voltage of the voltage source 301 becomes equal to or higher than the predetermined voltage, the switching regulator control circuit 308 does not need to boost the voltage of the voltage source 301 and normal boosting operation is not performed. At this time, since the voltage of the FB terminal 304 at the connection point between the LED 305 and the resistor 303 rises without being controlled normally, the currents of the LED 305 and the resistor 303 rise without being controlled properly. Therefore, the brightness of the LED 305 is not normally controlled.

  The present invention has been made in view of the above points, and provides an LED driving circuit capable of normally controlling the luminance of the LED 305 even when the voltage of the voltage source 301 becomes high.

  In the present invention, in order to solve the above problems, in an LED driving circuit for driving an LED provided between an output terminal and a feedback terminal of a switching regulator control circuit, a first voltage source and a positive voltage of the first voltage source are provided. A coil provided between a terminal and an input terminal of the switching regulator control circuit, a first resistor provided between the feedback terminal and the plus terminal, and a voltage of the first voltage source The voltage generated in the first resistor is preset by controlling the voltage of the feedback terminal by outputting the boosted voltage of the first voltage source from the output terminal and controlling the voltage of the feedback terminal. And a switching regulator control circuit that controls the voltage to be a predetermined voltage. LED drive circuit is provided that.

  In the present invention, since the circuit design is such that the voltage of the output terminal of the switching regulator control circuit is always higher than the voltage of the input terminal of the switching regulator control circuit, the voltage of the first voltage source is increased and the voltage of the input terminal is increased. However, the voltage at the output terminal also increases accordingly. Therefore, even when the voltage at the input terminal increases, the switching regulator control circuit can always normally boost the voltage at the input terminal. Therefore, the voltage of the feedback terminal of the switching regulator control circuit is normally controlled, and the current of the LED and the first resistor is normally controlled, so that the brightness of the LED is normally controlled.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First embodiment]
First, the LED drive circuit and LED of the first embodiment will be described. FIG. 1 is a diagram schematically illustrating an LED drive circuit and LEDs according to the first embodiment.

  The switching regulator control circuit 108 in the LED driving circuit is connected to the VIN terminal 109 of the LED driving circuit, the VOUT terminal 106 of the LED driving circuit, the FB terminal 104 of the LED driving circuit, and the ground 110 of the LED driving circuit via the coil 102. It is connected. A voltage source 101 is provided between the VIN terminal 109 and the ground 110, a high brightness white LED 105 is provided between the VOUT terminal 106 and the FB terminal 104, and a resistor 103 is provided between the FB terminal 104 and the VIN terminal 109. , And a capacitor 107 is provided between the VOUT terminal 106 and the ground 110.

  The voltage of the voltage source 101 is applied to the coil 102, and the coil 102 stores magnetic energy for a predetermined time based on this voltage. The stored energy is discharged to the capacitor 107 connected to the VOUT terminal 106 for a predetermined time, so that a voltage higher than the voltage of the voltage source 101 is output to the VOUT terminal 106. At this time, the switching regulator control circuit 108 controls the time for storing energy and the time for discharging energy so that a desired voltage is output to the VOUT terminal 106. Control of these times is performed by comparing the voltage of the FB terminal 104 with a preset voltage.

  Here, if the switching regulator control circuit 108 controls the voltage of the FB terminal 104 to be 0.5 V higher than the voltage of the VIN terminal 109 and the resistance value of the resistor 103 is 500Ω, the LED 105 and the resistor 103 are set to 0. A forward current of 5 V / 500Ω = 100 mA is passed. If the forward voltage of the LED 105 at this time is 3.5 V, the voltage at the VOUT terminal 106 is 0.5 V + 3.5 V = 4 V higher than the voltage at the VIN terminal 109. The voltage at the VOUT terminal 106 is always higher than the voltage at the VIN terminal 109.

In addition, the switching regulator control circuit 108 controls the voltage at the FB terminal 104 (the voltage generated at the resistor 103 due to the forward current of the LED 105) so that the voltage generated at the resistor 103 is constant regardless of the voltage of the voltage source 101. The sum of the voltage and the voltage of the voltage source 101 is controlled. For example, if the voltage generated at the resistor 103 is set to 0.5V and the voltage at the VIN terminal 109 is 4V, the switching regulator control circuit 108 sets the voltage at the FB terminal 104 to 0.5V + 4V = 4.5V. I have control. If the voltage at the VIN terminal 109 is 5V, the switching regulator control circuit 108 controls the voltage at the FB terminal 104 to 0.5V + 5V = 5.5V. Therefore, since the voltage generated in the resistor 103 is constant, the current of the resistor 103 and the LED 105 is also constant. When the forward current of the LED 105 is IF, the voltage of the FB terminal 104 is V _FB , the voltage of the VIN terminal 109 is V _IN, and the resistance value of the resistor 103 is R, IF is
IF = (V _FB −V _IN ) / R
Is calculated by

  Next, the switching regulator control circuit 108 will be described. FIG. 2 is a diagram showing an outline of the switching regulator control circuit.

  The switching regulator control circuit 108 includes a VIN terminal 108m connected to the VIN terminal 109 via the coil 102, a VOUT terminal 108l connected to the VOUT terminal 106, an FB terminal 108k connected to the FB terminal 104, and a ground 110. It has a connected ground 108n.

  The voltage at the VOUT terminal 108l drops by the LED 105 and is input to the FB terminal 108k. The voltage of the FB terminal 108k is divided by the resistors 108a, 108b, and the voltage source 108c, and input to the inverting input terminal of the error amplifier 108e. At this time, the voltage of the voltage source 108d (the voltage of the voltage source 101) is also input to the non-inverting input terminal of the error amplifier 108e. The voltages input to these error amplifiers 108e are compared and amplified by the error amplifier 108e. The amplification result here is input to the non-inverting input terminal of the comparator 108g. At this time, the triangular wave or sawtooth wave generated by the circuit 108f is also input to the inverting input terminal of the error amplifier 108e. The amplification result of the error amplifier 108e and the triangular wave or sawtooth wave are compared by the comparator 108g. The comparison result here is input to the gate electrode of the transistor 108i through the buffer 108h. Based on the voltage of the gate electrode at this time, the voltage of the VIN terminal 108m is switched. Thereafter, the voltage at the VIN terminal 108m is output to the VOUT terminal 108l via the diode 108j.

  Here, it is assumed that the switching regulator control circuit 108 generates a reference voltage VREF in the resistor 103 and passes a current calculated by dividing the reference voltage VREF by the resistance value of the resistor 103 to the LED 105 and the resistor 103. Further, it is assumed that the resistance value of the resistor 108a is equal to the resistance value of the resistor 108b.

  Then, the sum of the voltage VDD of the voltage source 101 and the reference voltage VREF is applied to the FB terminal 104 and the FB terminal 108k. Since the voltage VDD of the voltage source 101 is applied to the non-inverting input terminal of the error amplifier 108e, the voltage of the inverting input terminal of the error amplifier 108e is also the voltage VDD. Therefore, the voltage generated in the resistor 108a is the reference voltage VREF, and based on this, the voltage generated in the resistor 108b is also the reference voltage VREF. The voltage at the inverting input terminal of the error amplifier 108e is the voltage VDD, and the voltage drops by the reference voltage VREF by the resistor 108b. Therefore, the voltage of the voltage source 108c is calculated by subtracting the reference voltage VREF from the voltage VDD. Therefore, by adjusting the voltage VDD-VREF of the voltage source 108c, the reference voltage VREF is set in advance, and the currents of the LED 105 and the resistor 105 are set based on the reference voltage VREF and the resistance value of the resistor 103. become. The reference voltage VREF is designed so as not to fluctuate even if the voltage VDD of the voltage source 101 fluctuates.

  Note that although the switching regulator control circuit 108 includes the transistor 108i, the transistor 108i may be provided outside.

  Further, the switching regulator control circuit 108 includes the diode 108j, but may include a diode 108j or a Schottky diode outside.

  The switching regulator control circuit 108 is usually realized as a single IC.

  According to the LED driving circuit as described above, the error amplifier 108e causes the voltage VDD of the voltage source 108d (the voltage VDD of the voltage source 101), the voltage VDD-VREF of the voltage source 108c, and the reference voltage VREF generated in the resistor 108b. Since the sum is equal, the preset reference voltage VREF and the reference voltage VREF actually generated in the resistor 103, the resistor 108a, and the resistor 108b are equal to each other, and the resistor 103, the resistor 108a, and the resistor 108b The generated reference voltage VREF is always controlled to be constant. Therefore, since the current of the resistor 103 and the LED 105 is controlled to be constant, the luminance of the LED 105 is normally controlled.

  In addition, since the circuit design is such that the voltage at the VOUT terminal 106 is always higher than the voltage at the VIN terminal 109, even if the voltage at the voltage source 101 becomes higher and the voltage at the VIN terminal 109 becomes higher, the VOUT The voltage at the terminal 106 also increases. Therefore, even if the voltage at the VIN terminal 109 increases, the switching regulator control circuit 108 can always boost the voltage at the VIN terminal 109 normally. Therefore, the voltage of the FB terminal 104 is normally controlled and the current of the LED 105 and the resistor 103 is normally controlled, so that the luminance of the LED 105 is normally controlled.

  Further, since the reference voltage VREF generated in the resistor 103 is set irrespective of the voltage of the voltage source 101, the degree of freedom is high. Therefore, an efficient switching regulator control circuit 108 is realized.

[Second Embodiment]
Next, the LED drive circuit and LED of the second embodiment will be described. FIG. 3 is a diagram schematically illustrating the LED drive circuit and the LED according to the second embodiment.

  In the LED driving circuit and the LED of the second embodiment, in the first embodiment, the capacitor 107 is provided between the VOUT terminal 106 and the ground 110. However, in the second embodiment, the capacitor 107 is the VOUT. It is provided between the terminal 106 and the VIN terminal 109.

  According to the LED driving circuit as described above, the capacitor 107 provided between the VOUT terminal 106 and the VIN terminal 109 functions as a coupling capacitor, so that the voltage at the VIN terminal 109 is quickly transmitted to the VOUT terminal 106. Therefore, since the voltage at the VIN terminal 109 is higher than the voltage at the VOUT terminal 106, current is less likely to flow from the VIN terminal 109 to the VOUT terminal 106.

It is a figure which shows the outline of LED drive circuit and LED of 1st embodiment. It is a figure which shows the outline of a switching regulator control circuit. It is a figure which shows the outline of LED drive circuit and LED of 2nd embodiment. It is a figure which shows the outline of the conventional LED drive circuit and LED.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Voltage source 102 Coil 103 Resistance 104 FB terminal 105 LED 106 VOUT terminal 107 Capacitance 108 Switching regulator control circuit 109 VIN terminal 110 Ground

Claims (6)

In the LED driving circuit for driving the LED provided between the output terminal and the feedback terminal of the switching regulator control circuit,
A first voltage source;
A coil provided between the positive terminal of the first voltage source and the input terminal of the switching regulator control circuit;
A first resistor provided between the feedback terminal and the plus terminal;
The voltage of the first voltage source is input to the input terminal via the coil, the boosted voltage of the first voltage source is output from the output terminal, and the voltage of the feedback terminal is controlled, thereby The switching regulator control circuit for controlling the voltage generated in the first resistor to be a predetermined voltage set in advance;
An LED driving circuit comprising: The switching regulator control circuit includes:
A second resistor, a third resistor, and a second voltage source that is lower than the voltage of the first voltage source by the predetermined voltage, and a voltage dividing circuit that divides the voltage of the feedback terminal;
An error amplifier that compares the divided voltage of the feedback terminal with the voltage of the first voltage source and operates so that the voltage generated in the first resistor is equal to the predetermined voltage;
The LED driving circuit according to claim 1, further comprising: A capacitor provided between the output terminal and the ground;
The LED driving circuit according to claim 1, further comprising: A capacitor provided between the output terminal and the input terminal;
The LED driving circuit according to claim 1, further comprising: The voltage of the external first voltage source is input to the input terminal via the coil, the boosted voltage of the first voltage source is output from the output terminal, and the voltage of the feedback terminal to which the first resistor is connected is controlled. In the switching regulator control circuit,
A voltage dividing circuit having a second resistor, a third resistor, and a second voltage source that is lower than a voltage of the first voltage source by a predetermined voltage, and dividing the voltage of the feedback terminal;
An error amplifier that compares the divided voltage of the feedback terminal with the voltage of the first voltage source and controls the voltage generated in the first resistor to be the predetermined voltage;
A switching regulator control circuit comprising:   6. The switching regulator control circuit according to claim 5, wherein the switching regulator control circuit is realized as a single semiconductor device.

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