JP2016129098A - Led driving device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an LED driving device capable of preventing sounding during PWM lighting control.SOLUTION: An LED driving device comprises: a booster circuit for amplifying an input voltage to a predetermined voltage; a switch control circuit 11 which operates or stops the booster circuit correspondingly to a lighting control signal; an overvoltage protection circuit 13 which stops operating the booster circuit if a boosted voltage is an overvoltage; a monitor signal extraction line 5 connected to an anode line for supplying the boosted voltage to an LED and a reference voltage supply line; and a sounding prevention switch 2 which is provided in the monitor signal extraction line and makes a connection of the monitor signal extraction line and the anode line intermittent correspondingly to the lighting control signal. While the booster circuit is operated, the sounding prevention switch continues the connection of the monitor signal extraction line and the anode line and while the booster circuit is not operated, the sounding prevention switch releases the connection of the monitor signal extraction line and the anode line.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to an LED driving device.

  In recent years, LEDs (Light Emitting Diodes) are not limited to conventional light bulb replacements due to their low power consumption and long life, and are used in a wide range of applications such as liquid crystal display devices, in-vehicle devices, and lighting fixtures. It has become.

  The LED driving device is a circuit for controlling lighting and extinguishing of LEDs. As an LED driving method, a method of performing PWM (Pulse Width Modulation) dimming using a step-up chopper circuit to control lighting and extinguishing of a plurality of LEDs connected in series is known.

JP 2012-104367 A

  By the way, when PWM dimming is performed using the above-described LED driving device, there is a case where a sound is generated.

  An object of this invention is to provide the LED drive device which can prevent the sounding at the time of PWM light control.

An LED driving device according to an embodiment includes a booster that amplifies the input voltage to a predetermined voltage by controlling the current flowing through the LED to a predetermined value in the LED driving device that amplifies the input voltage and supplies the amplified LED to the LED. A circuit, a switch control circuit for operating or stopping the booster circuit in response to a dimming signal that is a binary rectangular wave signal, and overvoltage protection for stopping the operation of the booster circuit when the boosted voltage is overvoltage A circuit, a monitor signal extraction line connected to an anode line for supplying a boosted voltage to the LED and a reference voltage supply line, and a monitor signal extraction line; A resistance divider circuit that is taken out as a monitor voltage to be input to the overvoltage protection circuit, and a monitor signal take-out line that is provided in front of the dimming signal. It has a sounding prevention switch to interrupt the connection between the monitor signal extraction line and the anode line, wherein the sound whine prevention switch, in response to the dimming signal,
When the booster circuit is in operation, the connection between the monitor signal extraction line and the anode line is continued. When the booster circuit is not in operation, the connection between the monitor signal extraction line and the anode line is opened. LED driving device.

It is a figure which shows the structure of the LED drive device of 1st Embodiment with LED. It is a figure for demonstrating the reason which can aim at prevention of sound in the LED drive device of this Embodiment. It is a figure which shows the voltage waveform of an anode line in case the noise prevention switch is not provided in the LED drive device of this Embodiment. It is a figure for demonstrating operation | movement of the sound prevention switch when the dimming signal in an LED drive device of 1st Embodiment is an on level. It is a figure for demonstrating operation | movement of the sound prevention switch when the light control signal in the LED drive device of 1st Embodiment is an off level. It is a figure which shows the form of the variation of the LED drive device of 1st Embodiment with LED. It is a figure which shows the form of the other variation of the LED drive device of 1st Embodiment with LED. It is a figure which shows the structure of the LED drive device of 2nd Embodiment with LED. It is a figure for demonstrating operation | movement of the sound prevention switch when the dimming signal in an LED drive device of 2nd Embodiment is an ON level. It is a figure for demonstrating operation | movement of the sound prevention switch when the light control signal in the LED drive device of 2nd Embodiment is an off level. It is a figure which shows the form of the variation of the LED drive device of 2nd Embodiment with LED. It is a figure which shows the form of the other variation of the LED drive device of 2nd Embodiment with LED. It is a figure which shows the structure of the LED drive device of 3rd Embodiment with LED. It is a figure which shows an example of the voltage distribution on the monitor signal extraction line of the LED drive device of 3rd Embodiment. It is a figure which shows an example of the load curve characteristic of the load resistance on the monitor signal extraction line of the LED drive device of 3rd Embodiment, and Nch-FET. It is a figure which shows the form of the variation of the LED drive device of 3rd Embodiment with LED. It is a figure which shows the form of the other variation of the LED drive device of 3rd Embodiment with LED.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
It should be noted that the disclosure is merely an example, and those skilled in the art can easily conceive of appropriate modifications while maintaining the gist of the invention are naturally included in the scope of the present invention. In addition, the drawings may be schematically represented with respect to the width, thickness, shape, and the like of each part in comparison with actual aspects for the sake of clarity of explanation, but are merely examples, and the interpretation of the present invention is not limited. It is not limited. In addition, in the present specification and each drawing, elements similar to those described above with reference to the previous drawings are denoted by the same reference numerals, and detailed description may be omitted as appropriate.

[First embodiment]
FIG. 1 is a diagram illustrating the configuration of the LED driving device according to the first embodiment together with LEDs. The LED drive device 1 shown in FIG. 1 is configured using an LED driver IC 10 in which a part of a circuit for driving an LED is integrated. However, the LED driving device according to the present invention may be configured without being integrated into an IC.

  The input voltage VIN is input to the coil L1 through the fuse F1 and the smoothing capacitor C1. The output end of the coil L1 is connected to the diode D1, and the output end of the coil L1 is connected to the boost switch Q1 through the terminal LX of the LED driver IC10. The output of the diode D1 is input to the anode line through the output capacitor C2. Here, the input voltage VIN is about 12 V for in-vehicle use, and 5 to 6 V for mobile devices.

  A monitor signal extraction line 5 for connecting the anode line and the ground (reference voltage supply line) is provided. In the monitor signal extraction line 5, a noise prevention switch 2, a resistor R1, and a resistor R2 are provided in series in this order from the anode line side and connected to the ground. The resistors R1 and R2 constitute a resistor dividing circuit, and an intermediate voltage between the resistors R1 and R2 is connected to the comparator 14 through the terminal OVP of the LED driver IC 10 as a monitor voltage.

  The sounding prevention switch 2 includes a switch Q2 of a P-channel FET, a switch Q3 of an N-channel FET, and a resistor R3. The source terminal of the switch Q2 is connected to the anode line, and the drain terminal is connected to one end of the resistor R1. One end of the resistor R3 is connected to the source terminal of the switch Q2, and the other end is connected to the gate terminal of the switch Q2. The drain terminal of the switch Q3 is connected to the gate terminal of the switch Q2, and the source terminal of the switch Q3 is connected to the ground line. A dimming signal described later is input to the gate terminal of the switch Q3. Here, the switch Q3 and the resistor R3 constitute a level shifter 3.

  The anode line is provided with a plurality of rows (in this case, two rows) of a plurality of LEDs connected in series. The anodes of the plurality of LEDs connected in series are connected to the anode line, and the cathodes of the plurality of LEDs connected in series are connected to the LED current control circuit 12 through the terminals LED1 and LED2 of the LED driver IC 10.

  The LED driver IC 10 includes a switch control circuit 11, an LED current control circuit 12, an overvoltage protection circuit 13, a comparator 14, and a boost switch Q1. The switch control circuit 11 controls the on / off of the boost switch Q1 based on the dimming signal received through the terminal PWM. The LED current control circuit 12 outputs a control signal to the switch control circuit 11 so that the LED current received via the terminals LED1 and LED2 becomes a predetermined current value, so that the anode line becomes a predetermined boosted voltage. Control. The comparator 14 compares the monitor voltage input via the terminal OVP with the standard value Vovp. The overvoltage protection circuit 13 outputs an operation stop signal to the switch control circuit 11 when the monitor voltage becomes larger than the standard value Vovp.

  The coil L1, the boost switch Q1, the diode D1, and the output capacitor C2 shown in FIG. 1 constitute a boost chopper circuit.

  Next, the operation of the LED drive device according to the first embodiment will be described.

  The dimming signal is a rectangular wave signal in which binary signals of an on level (here, 3.3 V) and an off level (here, 0 V) are repeated. The repetition frequency of the dimming signal is generally about 100 Hz to 2 kHz. When the dimming signal is on level (3.3 V), the switch control circuit 11 outputs a driving signal having a switching frequency to the gate terminal of the boost switch Q1. The switching frequency is generally about 500 kHz to 2 MHz.

  When the dimming signal is on level (3.3 V), the boost switch Q1 repeats on / off according to the switching frequency. When boost switch Q1 is on, a current flows through coil L1 to charge energy. When the boost switch Q1 is turned off, the energy stored in the coil L1 is charged through the diode D1 to the output capacitor C2. Thus, whenever the boost switch Q1 is repeatedly turned on / off, the energy stored in the coil L1 is charged to the output capacitor C2 through the diode D1, and the output capacitor C2 is charged to a voltage higher than the input voltage VIN.

  Since the anode of the LED is connected to the output capacitor C2, when the voltage of the output capacitor C2 (the voltage of the anode line) becomes higher than the threshold voltage of the LED connected in series, a current flows through the LED and the LED is Emits light. Generally, since the threshold value of the LED is about 3V, for example, when 10 straight LEDs (10 LEDs connected in series) are connected to the LED driving device 1, the voltage of the anode line of the output capacitor C2 When the voltage reaches about 30V, the 10 straight LED emits light. Even when the LED groups connected in series are connected in parallel in a plurality of columns, all the LEDs emit light when the voltage of the anode line is about 30V.

  The current flowing through the LED is input to the LED current control circuit 12 of the LED driver IC 10. The LED current control circuit 12 controls the output / stop of the drive signal of the switch control circuit 11 so that the current flowing through the LED becomes a preset current value. As a result, the boost switch Q1 is controlled and a constant current always flows through the LED, so that the LED emits light with a constant brightness.

  As described above, since the LED driver IC 10 generates a voltage higher than the input voltage VIN by the switching operation, it is necessary to prevent accidents such as heat generation, smoke generation, and ignition of the LED drive device 1 due to malfunction. Therefore, the LED driver IC 10 is provided with an overvoltage protection circuit 13.

  When the dimming signal is on level (3.3 V), an on-level potential is applied to the gate terminal of the switch Q3 of the level shifter 3 of the sound generation prevention switch 2, so that the switch Q3 is in the on state (source terminal and drain). The terminals become conductive. Accordingly, since the GND (ground) potential is applied to the gate terminal of the switch Q2, the switch Q2 is turned on. As a result, the voltage of the anode line is divided by the resistance dividing circuit by R1 and R2, and the intermediate voltage of R1 and R2 is taken out as the monitor voltage.

  The monitor voltage is input to the comparator 14 via the terminal OVP of the LED driver IC 10. The comparator 14 compares the monitor voltage with the standard value Vovp. When the monitor voltage becomes larger than the standard value Vovp, the overvoltage protection circuit 13 controls the switch control circuit 11 to stop the switching operation of the boost switch Q1.

  The voltage Va of the anode line at which the overvoltage protection circuit operates is calculated by Va = (1 + R1 / R2) * Vovp. For example, when R1 = 900 kΩ, R2 = 30 kΩ, and Vovp = 1.2V, the voltage Va of the anode line at which the overvoltage protection circuit operates is Va = 37.2V. Since the maximum value of the threshold value of the LED is about 3.5V, the voltage of the anode line can be only about 35V at the maximum even with 10 series LEDs. Therefore, if the overvoltage protection circuit operates at 37.2V, there is no problem. The standard value Vovp is preferably about 1 to 2V. This is because the LED driver IC 10 can easily generate the standard value Vovp with the band gap voltage.

  On the other hand, when the dimming signal is off (0 V), the LED current control circuit 12 forcibly cuts off the LED current. As a result, the LED is turned off. In this way, the brightness of the LED is adjusted by changing the light emission intensity of the LED by turning on / off the LED in accordance with the on / off of the dimming signal.

  When the dimming signal is off (0 V), the LED current is forcibly cut off, and the switch control circuit 11 forcibly turns off the boost switch Q1 (the source terminal and the drain terminal are not conducting). The boosting operation is stopped.

  When the dimming signal is off (0 V), an off-level potential is applied to the gate terminal of the switch Q3 of the level shifter 3 of the sound prevention switch 2, so that the switch Q3 is in the off state (between the source terminal and the drain terminal). Non-conducting state). Therefore, since the potential of the anode line is applied to the gate terminal of the switch Q2, the switch Q2 is turned off. Even if the switch Q2 is turned off, the GND potential is applied to the comparator 14 via the terminal OVP, so that the operation of the overvoltage protection circuit 13 is not adversely affected.

  Next, the reason why the sound generation prevention switch 2 can prevent sound generation will be described.

FIG. 2 is a diagram for explaining the reason why it is possible to prevent noise generation in the LED driving device of the present embodiment. In FIG. 2, the noise prevention switch 2 shown in FIG. 1 is not provided, but the other parts are the same as those in FIG.
Since the resistors R1 and R2 for generating the monitor voltage for overvoltage protection are connected to the anode line as described above, the resistors R1 and R2 are turned off when the dimming signal is turned off and the boosting operation is stopped. As a result, the leakage current is discharged to GND, and the voltage of the anode line drops. Since the boosting operation is resumed when the dimming signal is turned on, the voltage of the anode line is recovered until the set LED current is reached again. The above operation is repeated according to ON / OFF of the dimming signal.

  By the way, it is possible to reduce the voltage drop of the anode line as much as possible by increasing the resistances R1 and R2 as much as possible. However, if the resistors R1 and R2 are too large, malfunction may occur due to an increase in the offset voltage of the comparator 14 or a decrease in follow-up due to the influence of the input impedance of the operational amplifier used for the comparator 14 of the LED driver IC 10. Can be considered. In order to prevent the overvoltage protection circuit 13 from malfunctioning, it is desirable that the resistances R1 and R2 are 1 MΩ or less. However, in this case, the leakage current cannot be sufficiently suppressed, and noise may be caused by a voltage drop. is there.

  FIG. 3 is a diagram showing the voltage waveform of the anode line when the sounding prevention switch is not provided in the LED driving device of the present embodiment.

  The potential of the output capacitor C2 connected to the anode line fluctuates in a waveform similar to the sawtooth shape shown in FIG. 3 in synchronization with the dimming signal. That is, when the dimming signal is 3.3 V (on), the LED drive device 1 performs a boosting operation, and when it is 0 V (off), the boosting operation is stopped. When the boosting operation is stopped, the boosted voltage of the anode line is held by the charge accumulated in the output capacitor C2, but the charge accumulated in the output capacitor C2 leaks by the resistors R1 and R2, and the voltage of the anode line is lowered. For this reason, the expansion and contraction of the output capacitor C2 due to the above-described voltage fluctuation propagates to the substrate on which the LED driving device is mounted. The vibration of the substrate or the expansion / contraction of the output capacitor C2 itself causes the generation of sound. FIG. 3 shows the case where the threshold voltage of the 10 series LEDs is 3V and there is a 0.5V voltage drop. The voltage drop values are the conditions (resistances R1, R2, dimming signal frequency, dimming rate). ) Will increase or decrease. In particular, when the dimming rate (ON / OFF ratio) is about 1%, the voltage drop is large, and the anode voltage drops about 0.5V to 1V. Of course, the larger the voltage drop value, the louder the output capacitor C2.

  Next, the configuration and operation of the sounding prevention switch 2 in the LED drive device of the first embodiment will be described in detail.

  The squeal prevention switch 2 is provided to prevent the output capacitor C2 connected to the anode line from squeezing by suppressing the voltage fluctuation of the anode line due to the on / off of the dimming signal. A switch Q2 provided in the noise prevention switch 2 shown in FIG. 1 is a Pch-FET provided between the anode line and the resistors R1 and R2. When there are a plurality of LEDs, the voltage of the anode line is generally about 10 to 40 V, and therefore the switch Q2 cannot be turned on / off with a low voltage of the dimming signal (generally about 3.3 V). Therefore, the level shifter circuit composed of the switch Q3 and the resistor R3 turns on / off the switch Q2 in synchronization with the on / off of the dimming signal. The resistance R3 is preferably about 1 MΩ.

  FIG. 4 is a diagram for explaining the operation of the sounding prevention switch when the dimming signal is on-level in the LED drive device of the first embodiment.

  When the dimming signal is on (3.3 V), as shown in FIG. 4, a high level (3.3 V) is applied to the gate terminal of the switch Q3 (Nch-FET), and the switch Q3 is turned on. . As a result, a low level is applied to the gate terminal of the switch Q2 (Pch-FET), and the switch Q2 is turned on. Therefore, the voltage of the anode line is applied to the resistors R1 and R2, and the monitor voltage is output from the intermediate point between the resistors R1 and R2. The monitor voltage is compared with the standard value Vovp by the comparator 14 through the terminal OPV of the LED driver IC 10. If the monitor voltage is larger than the standard value Vovp, the overvoltage protection circuit 13 controls the switch control circuit 11 to stop the boosting operation. If the monitor voltage is smaller than the standard value Vovp, the boost switch Q1 continues the on / off operation as usual, and the boost operation continues.

  FIG. 5 is a diagram for explaining the operation of the anti-sound switch when the dimming signal is off level in the LED drive device of the first embodiment.

  When the dimming signal is off (0 V), a low level is applied to the gate of the switch Q3 (Nch-FET) as shown in FIG. 5, and the switch Q3 is turned off. As a result, a high level (the same potential as the anode line) is applied to the gate terminal of the switch Q2 (Pch-FET), and the switch Q2 is turned off. Therefore, since the voltage of the anode line is disconnected from the resistors R1 and R2, the leakage current from the output capacitor C2 due to the resistors R1 and R2 does not flow even when the dimming signal is off and the booster circuit is stopped, and the voltage drop is almost no. No. Even if the resistors R1 and R2 are disconnected from the anode line, since the resistor R2 is connected to the GND, the GND level is input to the terminal OVP of the LED driver IC 10, so that the comparator 14 erroneously detects an overvoltage. Absent.

  As described above, even when the dimming signal is repeatedly turned on / off, the voltage of the anode line hardly changes, and thus it is possible to prevent the generation of sound from the output capacitor C2 connected to the anode line. Become. Further, since the Pch-FET is used as the switch Q2, when the switch Q2 is on (when the dimming signal is on), a high voltage (10 to 40 V) of the anode line is applied between the gate terminal and the source terminal. Therefore, the ON resistance of the switch Q2 can be almost ignored (the drain-source voltage is almost 0). For this reason, it is not necessary to consider the characteristics of the switch Q2 in the design of the resistors R1 and R2. Of course, the withstand voltage between the drain and source of the Pch-FET must be a high withstand voltage product that is equal to or higher than the voltage of the anode line. Moreover, the breakdown voltage between the source and gate of the Pch-FET must be a high breakdown voltage product that is equal to or higher than the voltage of the anode line. By providing a voltage limiting circuit such as a diode between the source and gate, the breakdown voltage between the source and gate can be reduced. It is also possible to use a low Pch-FET.

(Variation 1 of the first embodiment)
FIG. 6 is a diagram showing a variation of the LED driving device of the first embodiment together with LEDs.

  In the variation, the sounding prevention switch 2 provided outside the LED driver IC 10 in the LED driving device of the first embodiment, that is, the switch Q2, and the switch Q3 and the resistor R3 constituting the level shifter 3 are LED. Built in the driver IC 10. Since the sounding prevention switch 2 is built in the LED driver IC 10, the LED driver IC 10 is provided with an input terminal AIN for the anode line and an output terminal AOUT for the switch Q2.

  Since the voltage of the anode line becomes a high voltage (about 40 V) when the number of LEDs in series increases, the switch Q2 and the switch Q3 built in the LED driver IC 10 need to be manufactured by a high withstand voltage process. Further, the resistance R3 is preferably about 1 MΩ, but accuracy is not required so much, so that it is desirable in terms of area to manufacture with a diffusion resistance instead of a polysilicon resistance.

  The basic operation principle of the LED driving device according to this variation is the same as that of the first embodiment. By incorporating the audible noise prevention switch 2 in the LED driver IC 10, it is possible to simplify the configuration of the LED drive device and prevent the output capacitor from audible.

(Variation 2 of the first embodiment)
FIG. 7 is a view showing another variation of the LED driving device according to the first embodiment together with LEDs.

  In another variation, the LED driver IC 10 includes the sounding prevention switch 2 and resistors R1 and R2 provided outside the LED driver IC 10 in the LED driving device of the first embodiment. For this reason, there is a merit that the configuration of the LED driving device can be further simplified to prevent the output capacitor from sounding.

[Second Embodiment]
In the second embodiment, the configuration of the level shifter is different from that of the first embodiment. Parts that are the same as or similar to those in the first embodiment are given the same reference numerals, and detailed descriptions thereof are omitted.
In the level shifter 3 using the switch Q3 and the resistor R3 used in the first embodiment, when the dimming signal is on (3.3 V), the switch Q3 is turned on to cause the resistor R3 (about 1 MΩ) from the anode line. Since leakage current flows through the LED, the efficiency of the LED is impaired.

FIG. 8 is a diagram illustrating the configuration of the LED drive device according to the second embodiment together with the LEDs.
In the LED driving device according to the second embodiment, by using the staking type level shifter 31, leakage current in the level shifter 31 is prevented, and generation of noise from the output capacitor C <b> 2 is prevented without impairing efficiency. Can do.

  The sounding prevention switch 2 shown in FIG. 8 includes a switch Q2 of a P-channel FET and a level shifter 31 of a tapping type. The staking type level shifter 31 includes a P-channel FET switch Q3, an N-channel FET switch Q4, a P-channel FET switch Q5, an N-channel FET switch Q6, and an inverter INV.

  A circuit configuration of the level shifter 31 will be described. The gate terminal and drain terminal of the switch Q3 of the P-channel FET and the switch Q5 of the P-channel FET are connected to each other. The drain terminal of the switch Q3 and the drain terminal of the switch Q4 with the source grounded are connected, and the drain terminal of the switch Q5 and the drain terminal of the switch Q6 with the source grounded are connected to serve as the output section of the level shifter 31.

The dimming signal that is an input signal to the level shifter 31 is branched into two, and one of the branched input signals is inverted by the inverter INV and connected to the gate terminal of the switch Q4. The other of the branched input signals is connected to the gate terminal of switch Q6. Then, an output signal is taken out from the drain terminal of the switch Q6.
The source terminals of the switches Q3 and Q5 of the level shifter 31 are connected to the anode line, and the output part of the level shifter 31 is connected to the gate terminal of the switch Q2.

  FIG. 9 is a diagram for explaining the operation of the sounding prevention switch when the dimming signal is on-level in the LED driving device of the second embodiment.

  When the dimming signal is on (3.3 V), the low level (0 V) is applied to the gate terminal of the switch Q4 (Nch-FET) as shown in FIG. 9, so that the switch Q4 is turned off. Since the high level (3.3 V) is applied to the gate terminal of the switch Q6 (Nch-FET), the switch Q6 is turned on. Since the switch Q6 is in the on state, the low level (0 V) is applied to the gate terminal of the switch Q3 (Pch-FET), and the switch Q3 is in the on state. Since the switch Q3 is in an on state, a high level (anode line voltage) is applied to the gate terminal of the switch Q5 (Pch-FET), and the switch Q5 is in an off state. Accordingly, the switch Q5 is turned off, the switch Q6 is turned on, the low level (0 V) is applied to the gate terminal of the switch Q2 (Pch-FET), and the switch Q2 is turned on.

  As a result, since the switch Q2 is in the ON state, the voltage of the anode line is applied to the resistors R1 and R2, and the monitor voltage is output from the intermediate point between the resistors R1 and R2. The monitor voltage is compared with the standard value Vovp by the comparator 14 through the terminal OPV of the LED driver IC 10. If the monitor voltage is larger than the standard value Vovp, the overvoltage protection circuit 13 controls the switch control circuit 11 to stop the boosting operation. If the monitor voltage is smaller than the standard value Vovp, the boost switch Q1 continues the on / off operation as usual, and the boost operation continues.

  When the dimming signal is on, the switch Q3 is on, the switch Q4 is off, the switch Q5 is off, and the switch Q6 is on so that there is a path through which current flows. Disappear. That is, when the dimming signal is on, the switch Q2 can be turned on without leakage current flowing from the anode line to the level shifter 31.

  FIG. 10 is a diagram for explaining the operation of the anti-sound switch when the dimming signal is off level in the LED drive device of the second embodiment.

  When the dimming signal is off (0 V), the high level (3.3 v) is applied to the gate terminal of the switch Q4 (Nch-FET) as shown in FIG. 10, so that the switch Q4 is turned on. Since the low level (0 V) is applied to the gate terminal of the switch Q6 (Nch-FET), the switch Q6 is turned off. Since the switch Q4 is in the on state, the low level (0 V) is applied to the gate terminal of the switch Q5 (Pch-FET), and the switch Q5 is turned on. Since the switch Q5 is in the on state, a high level (anode line voltage) is applied to the gate terminal of the switch Q3 (Pch-FET), and the switch Q3 is turned off. Accordingly, since the switch Q5 is on and the switch Q6 is off, a high level (anode line voltage) is applied to the gate terminal of the switch Q2 (Pch-FET) and the switch Q2 is turned off.

  As a result, since the switch Q2 is in the OFF state, the voltage of the anode line is disconnected from the resistors R1 and R2, and even if the dimming signal is OFF and the boosting operation is stopped, there is no leakage current and there is almost no voltage drop. Even if the resistors R1 and R2 are disconnected from the anode line, since the resistor R2 is connected to the GND, the GND level is input to the terminal OVP of the LED driver IC 10, so that the comparator 14 erroneously detects an overvoltage. Absent.

  When the dimming signal is off, there is no path through which current flows because the switch Q3 is off, the switch Q4 is on, the switch Q5 is on, and the switch Q6 is off when the dimming level shifter 31 is off. That is, when the dimming signal is off, the switch Q2 can be turned off without leakage current flowing from the anode line in the level shifter 31.

  With the above operation, even if the dimming signal is repeatedly turned on / off, no leakage current flows in the level shifter 31 of the staking method, and the voltage of the anode line hardly changes, so that the efficiency of the LED is not lowered. It is possible to prevent the generation of sound from the output capacitor C2 connected to the anode line.

(Variation 1 of the second embodiment)
FIG. 11 is a diagram illustrating a variation of the LED driving device according to the second embodiment together with LEDs.

  In the variation, the sounding prevention switch 2, that is, the switch Q2 and the level shifter 31 provided outside the LED driver IC 10 in the LED driving device of the second embodiment described above are incorporated in the LED driver IC 10. Since the sounding prevention switch 2 is built in the LED driver IC 10, the LED driver IC 10 is provided with an input terminal AIN for the anode line and an output terminal AOUT for the switch Q2.

  Since the voltage of the anode line becomes high voltage (about 40V) when the number of LEDs in series increases, the switch Q2, switch Q3, switch Q4, switch Q5 and switch Q6 built in the LED driver IC 10 are manufactured by a high withstand voltage process. There is a need to. However, since the inverter INV only inverts the dimming signal (about 3.3 V), it is manufactured by a low withstand voltage process.

  The basic operation principle of the LED drive device according to this variation is the same as that of the second embodiment. By incorporating the audible noise prevention switch 2 in the LED driver IC 10, it is possible to simplify the configuration of the LED drive device and prevent the output capacitor from audible.

(Variation 2 of the second embodiment)
FIG. 12 is a diagram illustrating another variation of the LED driving device according to the second embodiment together with LEDs.

  In another variation, the sounding prevention switch 2 and resistors R1 and R2 provided outside the LED driver IC 10 in the LED driving device of the second embodiment described above are incorporated in the LED driver IC 10. For this reason, there is a merit that the configuration of the LED driving device can be further simplified to prevent the output capacitor from sounding.

[Third embodiment]
In the third embodiment, the configuration of the noise prevention switch is different from that of the first embodiment. Parts that are the same as or similar to those in the first embodiment are given the same reference numerals, and detailed descriptions thereof are omitted.

FIG. 13 is a diagram illustrating the configuration of the LED drive device according to the third embodiment together with LEDs.
In the LED driving device of the third embodiment, the noise prevention switch 2 is provided between the resistor R1 and the resistor R2. The potential at the intermediate point between the resistor R1 and the resistor R2 is, for example, 1 to 2 V in order to extract the monitor voltage to the LED driver IC 10 as described above. A dimming signal is supplied to the gate of the sound prevention switch 2. By using an Nch-FET having a threshold voltage Vth that can be turned on when the dimming signal is 3.3 V, the level can be converted to a high potential as used in the first and second embodiments. There is no need. For this reason, the sounding prevention switch 2 of the third embodiment is not provided with a level shifter, and is configured only by an Nch-FET switch QN2.

FIG. 14A is a diagram illustrating an example of a voltage distribution on a monitor signal extraction line of the LED drive device according to the third embodiment.
In FIG. 14A, when the standard value Vovp is formed by a band gap circuit, the voltage of the standard value Vovp is about 1.2V. At this time, the monitor voltage at which the output of the comparator 14 is inverted is, for example, 1.2V, and the dimming signal is 3.3V. In this case, the potential difference Δ between the source and the gate of the switch QN2 constituting the sound prevention switch 2 is 2.1 (= 3.3-1.2) V. This switch QN2 has a threshold voltage Vth of 2.1V or less. By using the switch QN2 having a threshold voltage Vth of 2.1V or less, the switch QN2 can be turned on even near the switching of the comparator 14 (monitor voltage is near 1.2V). That is, when the voltage of the anode line is a low voltage at which the overvoltage protection circuit 13 does not operate and the voltage of the dimming signal is 3.3 V, the source-gate voltage of the switch QN2 is 2.1 V or more, The switch QN2 is turned on. When the ON resistance of the switch QN2 is negligibly small compared to the values of the resistors R1 and R2, the monitor voltage is a value obtained by dividing the voltage of the anode line by the resistors R1 and R2. The voltage of the anode line rises due to malfunction or the like, and when the monitor voltage becomes 1.2V or higher, the boost switch Q1 is turned off by the overvoltage protection circuit 13, and the anode line can no longer rise in voltage, and the monitor voltage Does not exceed 1.2V. As a result, the source-gate voltage of the switch QN2 is maintained at about 2.1 V or higher, and the switch QN2 is kept on. When the dimming signal is 0V, the switch QN2 is turned off, and no current flows from the anode line through the resistors R1 and R2, and the voltage held by the output capacitor C2 is maintained. Note that the drain breakdown voltage of the switch QN2 needs to be equal to or higher than the maximum voltage of the anode line, but the source-gate voltage does not exceed 3.3 V, which is the ON voltage of the dimming signal. As long as the maximum voltage value of the dimming signal can be satisfied, a low breakdown voltage type Nch-MOS transistor of about several volts can be used. Therefore, it is possible to use an Nch-MOS transistor having a small on-resistance that can be ignored with respect to the values of the resistors R1 and R2. In the above description, the monitor voltage is 1.2V. However, when the monitor voltage is 2V, a threshold voltage = 3.3V-2V = 1.3V or less can be used as the switch QN2. Good.
Further, when an Nch-FET whose on-resistance cannot be ignored as the switch QN2, the values of the resistors R1 and R2 are set using a characteristic curve as shown in FIG. 14B. For example, when the monitor voltage is 1.2 V, Vgs of the switch QN2 has a curve as shown in FIG. 14B. Here, since the currents Id1 flowing through the resistors R1 and R2 and the switch QN2 are equal, Id1 = 1.2 V / R2. At this time, the values of the resistors R1 and R2 may be determined so that the current at the point where the curve of the switch QN2 and the load curve of the resistor R1 intersect is Id1 = 1.2 V / R2.

  As described above, an Nch-FET switch QN2 is inserted as the noise prevention switch 2 between the resistor R1 and the resistor R2, so that the level shifter can be omitted. However, since there is an Nch-MOS transistor in which the ON resistance of the Nch-FET cannot be ignored, it is necessary to design the circuit so that the overvoltage protection circuit 13 does not malfunction by fully considering the characteristic curve and variation of the transistor.

(Variation 1 of the third embodiment)
FIG. 15 is a diagram illustrating a variation of the LED driving device according to the third embodiment together with LEDs.

  In the variation, the sounding prevention switch 2 provided outside the LED driver IC 10 in the LED driving device of the above-described third embodiment, that is, the switch QN2 is incorporated in the LED driver IC 10. Since the sounding prevention switch 2 is built in the LED driver IC 10, the LED driver IC 10 is provided with an input terminal MIN of the monitor signal extraction line and an output terminal MOUT of the switch QN2.

  When it is difficult to obtain a small current type Nch-MOS transistor, it is possible to build a small current type Nch-MOS transistor in the IC by incorporating the Nch-MOS transistor in the IC.

  The basic operation principle of the LED driving device of this variation is the same as that of the third embodiment. By incorporating the audible noise prevention switch 2 in the LED driver IC 10, it is possible to simplify the configuration of the LED drive device and prevent the output capacitor from audible.

(Variation 2 of the third embodiment)
FIG. 16 is a diagram illustrating another variation of the LED driving device according to the third embodiment together with LEDs.

  In another variation, the sounding prevention switch 2 and resistors R1, R2 provided outside the LED driver IC 10 in the LED driving device of the third embodiment described above are incorporated in the LED driver IC 10. For this reason, there is a merit that the configuration of the LED driving device can be further simplified to prevent the output capacitor from sounding.

  As described above, the monitor voltage extraction line 5 is provided with the noise prevention switch 2, and the noise prevention switch 2 is controlled by the dimming signal, so that the OVP (overvoltage protection) setting resistors R1 and R2 in the monitor voltage extraction line 5 are controlled. Leakage current can be suppressed and voltage fluctuation of the output capacitor C2 can be suppressed, so that the generation of noise can be prevented. The noise prevention switch 2 can be provided between the anode line and the OVP setting resistors R1 and R2, or between the two OVP setting resistors R1 and R2. When the noise prevention switch 2 is provided between the anode line and the OVP setting resistors R1 and R2, a Pch-FET switch is used and a level shifter is required. When a sound prevention switch is provided between the two OVP setting resistors R1 and R2, an Nch-FET switch is used and a level shifter is not required.

  In any case, by incorporating the noise prevention switch 2 in the LED driver IC 10, it is possible to eliminate the generation of noise from the output capacitor C2 by eliminating an extra external circuit, and the practical effect is great. Furthermore, by using a built-in level shifter, the leakage current flowing in the level shifter can be suppressed, so that it is possible to prevent the output capacitor C2 from generating noise without reducing the efficiency of the LED, and its practical effect. Is big. In the present invention, in addition to the level shifter, the OVP setting resistors R1 and R2 may be built in the LED driver IC 10.

  Any display device that can be implemented by a person skilled in the art based on the above-described display device as an embodiment of the present invention is also included in the scope of the present invention as long as it includes the gist of the present invention.

  In the scope of the idea of the present invention, those skilled in the art can conceive various changes and modifications, and it is understood that these changes and modifications also belong to the scope of the present invention. For example, those in which the person skilled in the art appropriately added, deleted, or changed the design of the above-described embodiments, or those in which the process was added, omitted, or changed the conditions are also included in the gist of the present invention. As long as it is provided, it is included in the scope of the present invention.

  In addition, other functions and effects brought about by the aspects described in the present embodiment, which are apparent from the description of the present specification, or can be appropriately conceived by those skilled in the art, are naturally understood to be brought about by the present invention. .

  Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

  L1 ... coil, D1 ... diode, Q1 ... boost switch, C2 ... output capacitor, R1 ... resistor, R2 ... resistor, Q2 ... switch, Q3 ... switch, R3 ... resistor, Vovp ... standard value, Q4 ... switch, Q5 ... switch , Q6 ... switch, INV ... inverter, QN2 ... switch, 1 ... LED driving device, 2 ... sound prevention switch, 3 ... level shifter, 5 ... monitor voltage extraction line, 10 ... LED driver IC, 11 ... switch control circuit, 12 ... LED current control circuit, 13 ... overvoltage protection circuit, 14 ... comparator, 31 ... level shifter.

Claims (10)

In the LED drive device that amplifies the input voltage and supplies it to the LED,
A step-up circuit that amplifies the input voltage to a predetermined voltage by controlling a current flowing through the LED to a predetermined value;
A switch control circuit for operating or stopping the booster circuit in response to a dimming signal which is a binary rectangular wave signal;
An overvoltage protection circuit that stops the operation of the booster circuit when the boosted voltage is an overvoltage;
A monitor signal extraction line connected to an anode line for supplying the boosted voltage to the LED and a reference voltage supply line;
A resistance divider circuit provided in the monitor signal extraction line, for extracting a divided voltage of the boosted voltage as a monitor voltage to be input to the overvoltage protection circuit;
A sounding prevention switch that is provided in the monitor signal extraction line and intermittently connects the monitor signal extraction line and the anode line in response to the dimming signal;
The sounding prevention switch corresponds to the dimming signal,
When the booster circuit is in operation, the connection between the monitor signal extraction line and the anode line is continued,
An LED driving device that opens a connection between the monitor signal extraction line and the anode line when the booster circuit is not in operation.   The LED driving device according to claim 1, wherein the sounding prevention switch is provided on the anode line side of the resistance dividing circuit of the monitor signal extraction line.   The sound generation prevention switch includes a level shifter that amplifies the level of the dimming signal, and a second switch of a P-channel FET that receives an output signal of the level shifter at a gate terminal and interrupts the monitor signal extraction line. 2. The LED driving device according to 2. The level shifter is
A resistor having one end connected to the anode line and the other end connected to the gate terminal of the second switch;
A third switch of an N-channel FET having a drain terminal connected to the other end of the resistor, a source terminal connected to a reference voltage supply line, and the dimming signal input to a gate terminal;
The LED driving device according to claim 3, comprising: The level shifter is
A third switch and a fifth switch of a P-channel FET, the gate terminal and the drain terminal of which are connected to each other;
A fourth switch of an N-channel FET having a source terminal connected to a reference voltage supply line and a drain terminal connected to the drain terminal of the third switch;
A sixth switch of an N-channel FET having a source terminal connected to a reference voltage supply line and a drain terminal connected to the drain terminal of the fifth switch;
Have
A signal obtained by inverting the dimming signal is input to the gate terminal of the fourth switch,
The dimming signal is input to the gate terminal of the sixth switch,
Source terminals of the third and fifth switches are connected to the anode line;
A drain terminal of the sixth switch is connected to a gate terminal of the second switch;
The LED driving device according to claim 3.   The sound generation prevention switch is provided at a position that provides a voltage signal of the same level as the monitor signal in the resistance dividing circuit, and is provided at a position closer to the anode line than a position from which the monitor signal is extracted. The LED drive device as described in.   The LED driving device according to claim 6, wherein the sounding prevention switch includes an N-channel FET switch that receives the dimming signal at a gate terminal and interrupts the monitor signal extraction line. The LED driving device has an LED driver IC and a circuit provided outside the LED driver IC,
The LED driver IC according to claim 2 or 6, wherein the LED driver IC includes the switch control circuit, the boost switch, the overvoltage protection circuit, and the noise prevention switch.   The LED driving device according to claim 8, wherein the LED driver IC further incorporates the resistance dividing circuit.   The LED driving device according to claim 8, wherein the sound generation prevention switch included in the LED driver IC is manufactured by a high withstand voltage process.

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