JP2017103010A - Light-emitting element drive circuit and circuit device having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light-emitting element drive circuit which is suppressed in generation of noise.SOLUTION: A DC drive voltage Vo generated by a voltage regulator circuit 13 is supplied to an LED 2. The drive voltage Vo is adjusted in accordance with a control signal Vc having a voltage according to a PWM signal Vp for making light emission intensity of the LED 2 variable. For that reason, generation of noise can be remarkably suppressed as compared with a conventional LED drive circuit in which an LED current is intermittent in accordance with the PWM signal.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a light emitting element driving circuit for driving a light emitting element such as a light emitting diode and a circuit device having the light emitting element driving circuit.

  PWM dimming is generally known as a method for adjusting the light quantity of a light emitting diode (LED) (see Patent Document 1 below). FIG. 4 is a diagram illustrating a general configuration of a PWM dimming type LED driving circuit. In the LED drive circuit shown in FIG. 4, a current limiting resistor R5 and a switch SW1 such as a transistor are provided in series with the LED 3, and a power supply voltage VDD is applied to the series circuit. The switch SW1 is turned on / off according to the PWM signal Sc generated by the control circuit 50. When the switch SW1 is turned on, the light emitting diode D2 emits light. The light quantity of the light emitting diode D2 changes according to the ratio (duty ratio) of the ON period to the pulse period of the PWM signal Sc.

JP 2008-24225 A

  In the LED drive circuit shown in FIG. 4, since the switch SW1 intermittently switches a relatively large current flowing through the LED 3 in accordance with the PWM signal Sc, noise due to a sharp change in current is generated. This noise may affect the operation of other circuits. For example, a touch sensor equipped with a capacitance detection circuit detects a minute change in capacitance due to the proximity of a finger or the like to the operation unit. easy.

  Therefore, a method of reducing the influence of noise by separating the grounds of the LED drive circuit that generates noise and the ground of other circuits (capacitance detection circuit or the like) and separating the ground from each other is also conceivable. However, separating the ground complicates the configuration of the substrate and the device, and increasing the distance between the circuits increases the size of the device.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a light emitting element driving circuit that suppresses the generation of noise, and a small circuit including such a light emitting element driving circuit and a capacitance detection circuit. To provide an apparatus.

  A light emitting element driving circuit according to a first aspect of the present invention includes a control circuit that generates a pulse width modulation signal for varying the light emission intensity of the light emitting element, and a control signal having a signal level corresponding to the pulse width modulation signal. And a voltage regulator circuit for adjusting a DC driving voltage supplied to the light emitting element according to the control signal.

  According to the above configuration, a DC driving voltage is supplied to the light emitting element by the voltage regulator circuit. The drive voltage is adjusted according to the control signal having a signal level corresponding to the pulse width modulation signal for changing the light emission intensity of the light emitting element. Therefore, the generation of noise can be suppressed as compared with the case where the current of the light emitting element is intermittent according to the pulse width modulation signal.

  Preferably, the control signal output circuit may include a smoothing circuit that smoothes the pulse width modulation signal, and a voltage buffer circuit that outputs a DC voltage corresponding to the output voltage of the smoothing circuit as the control signal.

  According to the above configuration, the output voltage of the smoothing circuit obtained by smoothing the pulse width modulation signal is input to the voltage buffer circuit, and a DC voltage corresponding to the output voltage is output to the voltage regulator circuit as the control signal. Is done. Therefore, the configuration is simpler than the case where the control signal is generated using the DA converter, and the circuit size is reduced.

  Preferably, the control circuit may compare the drive voltage with a target value and adjust the duty ratio of the pulse width modulation signal so that the drive voltage approaches the target value according to a result of the comparison.

  According to the above configuration, the duty ratio of the pulse width modulation signal is adjusted so that the drive voltage approaches the target value according to the result of comparing the drive voltage with the target value. Thereby, the light emission intensity is adjusted to a desired light emission intensity with high accuracy.

  Preferably, the light emitting element driving circuit may include a current detection circuit for detecting a driving current flowing through the light emitting element. In this case, the control circuit compares the drive current detected by the current detection circuit with a target value, and the pulse width modulation signal of the pulse width modulation signal is adjusted so that the drive current approaches the target value according to a result of the comparison. The duty ratio may be adjusted.

  According to the above configuration, the duty ratio of the pulse width modulation signal is adjusted so that the drive current approaches the target value according to a result of comparing the drive current detected by the current detection circuit with the target value. Is adjusted. Thereby, the light emission intensity is adjusted to a desired light emission intensity with high accuracy.

  A circuit device according to a second aspect of the present invention is a light emitting element driving circuit according to the first aspect that supplies a DC driving voltage to the light emitting element, and detects a change in capacitance according to the proximity of an object. The light emitting element drive circuit and the capacitance detection circuit are connected to a common ground electrode.

  According to the above configuration, since the light emitting element driving circuit and the capacitance detection circuit are connected to a common ground electrode, the configuration is simplified and the circuit size is reduced. In addition, since noise generated in the light emitting element driving circuit is reduced, detection accuracy of the capacitance in the capacitance detection circuit is improved, and erroneous detection due to noise is less likely to occur.

  According to the present invention, it is possible to adjust the light emission intensity of the light emitting element while suppressing the generation of noise. In addition, the size of the circuit device including the capacitance detection circuit and the light emitting element driving circuit can be reduced.

It is a figure which shows an example of a structure of the light emitting element drive circuit which concerns on 1st Embodiment. It is a figure which shows an example of a structure of the light emitting element drive circuit which concerns on 2nd Embodiment. It is a figure which shows an example of a structure of the circuit apparatus which concerns on 3rd Embodiment. It is a figure which shows the general structure of the LED drive circuit of a PWM light control system.

<First Embodiment>
FIG. 1 is a diagram illustrating an example of the configuration of a light-emitting element driving circuit 10 according to the first embodiment of the present invention. The light emitting element driving circuit 10 is a circuit that supplies a direct current driving voltage Vo to the LED 2 as a light emitting element, and adjusts the light emission intensity of the LED 2 by changing the driving voltage Vo.

  The light emitting element driving circuit 10 shown in FIG. 1 includes a control circuit 11, a control signal output circuit 12, and a voltage regulator circuit 13.

  The control circuit 11 is a circuit that generates a pulse width modulation signal (PWM signal) Vp for changing the light emission intensity of the LED 2, and in order to change the level of the drive voltage Vo, the duty ratio (with respect to the pulse period) of the PWM signal Vp. Adjust the pulse width ratio.

  The control circuit 11 compares the drive voltage Vo with a predetermined target voltage, and adjusts the duty ratio of the PWM signal Vp so that the drive voltage Vo approaches the target value according to the comparison result. For example, the control circuit 11 includes an analog-digital converter (AD converter) that converts the drive voltage Vo into a digital value, and the PWM signal is reduced so that the difference between the digital value of the drive voltage Vo and a predetermined target digital value becomes small. A process of changing the duty ratio of Vp is performed.

  For example, the control circuit 11 is configured to include a microcontroller unit (MCU) having an arithmetic processing function by the CPU, a function of generating a PWM signal, and a function of an AD converter.

  The control signal output circuit 12 outputs a control signal Vc having a signal level corresponding to the PWM signal Vp generated by the control circuit 11.

In the example of FIG. 1, the control signal output circuit 12 includes a smoothing circuit 121 and a voltage buffer circuit 122.
The smoothing circuit 121 is a low-pass filter that smoothes the PWM signal Vp and outputs a low-frequency component. In the example of FIG. 1, a ladder using two resistors (R1, R2) and two capacitors (C1, C2). Configured as a type filter. The resistor R1 has one end receiving the PWM signal Vp and the other end connected to the ground GND via the capacitor C1. One end of the resistor R2 is connected to the midpoint of connection between the resistor R1 and the capacitor C1, and the other end is connected to the ground GND via the capacitor C2. The voltage generated in the capacitor C2 becomes the output voltage Vpa of the smoothing circuit 121.

  The voltage buffer circuit 122 outputs a DC voltage corresponding to the output voltage Vpa of the smoothing circuit 121 as the control signal Vc. In the example of FIG. 1, the voltage buffer circuit 122 includes a transistor Q1 that is a pnp bipolar transistor and a resistor R3. The collector of the transistor Q1 is connected to the power supply voltage VDD, the output voltage Vpa of the smoothing circuit 121 is input to its base, and its emitter is connected to the ground GND via the resistor R3. The control signal Vc is lower than the output voltage Vpa of the smoothing circuit 121 by the base-emitter voltage Vbe of the transistor Q1.

  The voltage regulator circuit 13 is a circuit that supplies a direct current drive voltage Vo to the LED 2 and has a function of adjusting the drive voltage Vo according to the control signal Vc. For example, the voltage regulator circuit 13 is a linear regulator circuit in which a variable resistance element such as a transistor is provided in a current path between the input terminal IN of the power supply voltage VDD and the output terminal OUT of the drive voltage Vo. The voltage regulator circuit 13 performs negative feedback control of the impedance of the variable resistance element so that the drive voltage Vo output at the output terminal OUT approaches a voltage corresponding to the control signal Vc input to the control terminal CNT.

  The LED 2 is connected between the output terminal OUT of the drive voltage Vo of the voltage regulator circuit 13 and the ground GND. A current limiting resistor R4 is inserted in series in the path of the current flowing through the LED2.

  The operation of the light emitting element driving circuit 10 having the above-described configuration will be described.

  The control circuit 11 generates a PWM signal Vp having a duty ratio set so that a desired drive voltage Vo is supplied to the LED 2. The PWM signal Vp becomes a voltage Vpa smoothed by the smoothing circuit 121 and is input to the voltage buffer circuit 122. The voltage buffer circuit 122 outputs a control signal Vc corresponding to the voltage Vpa to the voltage regulator circuit 13. The voltage regulator circuit 13 supplies the LED 2 with the driving voltage Vo adjusted according to the control signal Vc.

  The control circuit 11 converts the drive voltage Vo into a digital value using an AD converter, and compares this digital value with a predetermined target digital value. The control circuit 11 changes the duty ratio of the PWM signal Vp so that the difference between the digital value of the drive voltage Vo and the target digital value becomes small. As a result, the drive voltage Vo approaches a certain target voltage corresponding to the target digital value.

  As described above, according to the light emitting element drive circuit 10 according to the present embodiment, the DC drive voltage Vo generated by the voltage regulator circuit 13 is supplied to the LED 2. The drive voltage Vo is adjusted according to a control signal Vc having a voltage corresponding to the PWM signal Vp for changing the light emission intensity of the LED 2. Therefore, compared with the case where the LED current is intermittently generated according to the PWM signal as in the LED driving circuit shown in FIG.

  Further, according to the light emitting element driving circuit 10 according to the present embodiment, the output voltage Vpa of the smoothing circuit 121 obtained by smoothing the PWM signal Vp is input to the voltage buffer circuit 122, and a direct current voltage corresponding to the output voltage Vpa is a control signal. It is output to the voltage regulator circuit 13 as Vc. Therefore, the circuit configuration can be simplified and the circuit size can be reduced as compared with the case where an analog control signal is generated from a digital signal using a DA converter.

Furthermore, according to the light emitting element drive circuit 10 according to the present embodiment, the duty ratio of the PWM signal Vp is adjusted so that the drive voltage Vo approaches the target value according to the result of comparing the drive voltage Vo with the target value. The As a result, even when the power supply voltage VDD fluctuates, for example, the drive voltage Vo is controlled to a voltage close to the target value, so that the LED 2 emits light as compared with the case where the duty ratio of the PWM signal Vp is kept constant. The intensity can be adjusted to a desired value with high accuracy.
<Second Embodiment>
Next, a second embodiment of the present invention will be described.
In the light emitting element drive circuit 10 according to the first embodiment described above, the duty ratio of the PWM signal Vp is adjusted so that the drive voltage Vo approaches the target value, but the light emitting element according to the second embodiment described below. In the drive circuit 10A, the duty ratio of the PWM signal Vp is adjusted so that the drive current Io flowing through the LED 2 approaches the target value.

  FIG. 2 is a diagram illustrating an example of a configuration of a light emitting element driving circuit 10A according to the second embodiment. The light emitting element driving circuit 10A shown in FIG. 2 includes a resistor Rs, a control circuit 11A, a control signal output circuit 12, and a voltage regulator circuit 13. The control signal output circuit 12 and the voltage regulator circuit 13 are the same as the components having the same reference numerals already described in FIG.

  The resistor Rs is provided on the current path of the drive current Io of the LED 2 and constitutes a circuit (current detection circuit) for detecting the drive current Io. The resistor Rs generates a voltage Vs corresponding to the drive current Io.

  The control circuit 11A is a circuit that generates a PWM signal Vp for varying the light emission intensity of the LED 2 as with the control circuit 11 described above, and includes, for example, an MCU.

  The control circuit 11 compares the drive current Io indicated by the voltage Vs generated at the resistor Rs with a predetermined target value, and adjusts the duty ratio of the PWM signal Vp so that the drive current Io approaches the target value according to the comparison result. To do. For example, the control circuit 11 includes an AD converter that converts the voltage Vs into a digital value, and changes the duty ratio of the PWM signal Vp so that the difference between the digital value of the voltage Vs and a predetermined target digital value becomes small. Process.

  According to the light emitting element driving circuit 10A having the above-described configuration, the duty of the PWM signal Vp is adjusted so that the driving current Io approaches the target value according to the result of comparing the driving current Io detected by the resistor Rs with the target value. The ratio is adjusted. As a result, the drive current Io is controlled to a current close to the target value without being affected by fluctuations in the power supply voltage VDD or the like, so that the light emission intensity of the LED 2 can be accurately adjusted to a desired value.

  Further, since the light emission intensity of the LED is substantially proportional to the magnitude of the current, in the light emitting element drive circuit 10A that controls the drive current Io to be close to the target value, the drive current Io changes according to the variation in the forward voltage of the LED2. Compared with the light emitting element drive circuit 10 of FIG. 1, the light emission intensity of the LED 2 can be adjusted to a desired level with higher accuracy.

<Third Embodiment>
Next, a third embodiment of the present invention will be described.
FIG. 3 is a diagram illustrating an example of a configuration of a circuit device according to the third embodiment. The circuit device shown in FIG. 3 includes an LED 2, a resistor R 4, a light emitting element drive circuit 10, and a capacitance detection circuit 20. The LED 2, the resistor R4, and the light emitting element driving circuit 10 are the same as the components having the same reference numerals already described in FIG.

  The capacitance detection circuit 20 is a circuit that detects a change in capacitance according to the proximity of an object. In the example of FIG. 3, the capacitance detection circuit 20 includes a sensor electrode unit 21, a drive unit 22, a capacitance-voltage conversion unit (CV conversion unit) 23, a processing unit 24, and an interface unit 25. .

  The sensor electrode unit 21 includes a plurality of electrodes formed in a predetermined pattern on the substrate. The drive unit 22 supplies a predetermined level of drive voltage to each electrode of the sensor electrode unit 21. When the capacitor formed on each electrode is charged by the driving voltage of the driving unit 22, the CV conversion unit 23 converts the electric charge charged in each capacitor into a voltage, and detects the capacitance of the capacitor. Get as a value. The processing unit 24 calculates the proximity position of the object in the sensor electrode unit 21 based on the detected capacitance value obtained for the capacitor of each electrode of the sensor electrode unit 21. The interface unit 25 communicates with a host device (not shown), and outputs information such as the proximity position of the object calculated by the processing unit 24 to the host device.

  In the circuit device shown in FIG. 3, the light emitting element drive circuit 10 and the capacitance detection circuit 20 are connected to a common ground GND. As already described, since the light emitting element driving circuit 10 has significantly reduced noise compared to the conventional LED driving circuit, the capacitance detection circuit 20 is connected to the same ground GND as the light emitting element driving circuit 10. Even so, the influence of noise on the capacitance detection operation in the capacitance detection circuit 20 is very small.

  According to the circuit device according to the present embodiment, since the light emitting element driving circuit 10 and the capacitance detection circuit 20 are connected to the common ground GND, the configuration of the device is simplified compared to the case where the grounds are separated from each other. In addition, the circuit size can be reduced. In addition, since noise generated in the light emitting element driving circuit 10 is small, it is possible to improve the detection accuracy of the capacitance in the capacitance detection circuit 20 compared to the case of using the conventional LED driving circuit. Detection can be made difficult to occur.

  As mentioned above, although various embodiment of this invention was described, this invention is not limited to embodiment mentioned above. That is, those skilled in the art may make various modifications, combinations, and alternatives with respect to the configuration of the above-described embodiment within the technical scope of the present invention or an equivalent scope thereof.

  For example, the configurations of the smoothing circuit 121 and the voltage buffer circuit 122 in FIGS. 1 and 2 are examples, and can be changed to circuits having other configurations having similar functions.

  In the light emitting element drive circuit 10A according to the second embodiment, the drive current Io is detected using the voltage drop of the resistor Rs, but the current detection method is not limited to this example. For example, a magnetoelectric conversion element such as a Hall element may be used for current detection.

DESCRIPTION OF SYMBOLS 2 ... LED, 10 ... Light emitting element drive circuit, 11 ... Control circuit, 12 ... Control signal output circuit, 121 ... Smoothing circuit, 122 ... Voltage buffer circuit, 13 ... Voltage regulator circuit, 20 ... Electrostatic capacitance detection circuit, 21 ... Sensor electrode unit 22 ... Drive unit 23 ... Capacitance-voltage conversion unit 24 ... Processing unit 25 ... Interface unit Q1 ... Transistor, Vp ... PWM signal, Vc ... Control signal, Vo ... Drive voltage, Io ... Drive current.

Claims (5)

A control circuit for generating a pulse width modulation signal for varying the light emission intensity of the light emitting element;
A control signal output circuit for outputting a control signal having a signal level corresponding to the pulse width modulation signal;
A voltage regulator circuit that adjusts a DC driving voltage supplied to the light emitting element according to the control signal. The control signal output circuit is
A smoothing circuit for smoothing the pulse width modulation signal;
The light-emitting element drive circuit according to claim 2, further comprising: a voltage buffer circuit that outputs a direct-current voltage corresponding to the output voltage of the smoothing circuit as the control signal. The control circuit compares the drive voltage with a target value, and adjusts the duty ratio of the pulse width modulation signal so that the drive voltage approaches the target value according to a result of the comparison. Item 3. The light-emitting element driving circuit according to Item 1 or 2. A current detection circuit for detecting a drive current flowing in the light emitting element;
The control circuit compares the drive current detected by the current detection circuit with a target value, and sets a duty ratio of the pulse width modulation signal so that the drive current approaches the target value according to a result of the comparison. The light-emitting element driving circuit according to claim 1, wherein the light-emitting element driving circuit is adjusted. A light emitting element;
A light emitting element driving circuit for supplying a DC driving voltage to the light emitting element;
A capacitance detection circuit that detects a change in capacitance according to the proximity of an object,
The light emitting element driving circuit is the light emitting element driving circuit according to any one of claims 1 to 4,
The circuit device, wherein the light emitting element driving circuit and the capacitance detecting circuit are connected to a common ground electrode.

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