JP2020202689A - Led control device, led driving device, and display apparatus - Google Patents

Abstract

To reduce a ripple of an output voltage of a chopper circuit, in an LED control device which PWM-dims an LED connected between the chopper circuit and a current driver.SOLUTION: An LED control device disclosed herein PWM-dims an LED connected between a chopper circuit and a current driver. The LED control device includes a power supply control unit and a dimming unit. The power supply control unit outputs a first PWM signal Spwm to the chopper circuit. The dimming unit controls a current IL flowing through the current driver on the basis of a second PWM signal DM1. A first cycle of the first PWM signal Spwm is shorter than a second cycle of the second PWM signal DM1. The power supply control unit synchronizes a rise timing of the first PWM signal Spwm with a rise timing of second PWM signal DM1.SELECTED DRAWING: Figure 7

Description

The present disclosure relates to an LED control device, an LED drive device, and a display device that PWM (Pulse Width Modulation) dimming an LED connected between a chopper circuit and a current driver.

Conventionally, an LED control device for PWM dimming an LED (Light Emitting Diode) connected between a chopper circuit and a current driver is known. For example, Japanese Patent Application Laid-Open No. 2018-19498 (Patent Document 1) describes an LED driver IC (Integrated Circuit) for PWM dimming of a plurality of LEDs connected in series between a chopper circuit and a constant current control circuit. The semiconductor circuit of is disclosed.

JP-A-2018-19498

Energy is stored in the coil of the chopper circuit by repeating the switching of the switching element of the chopper circuit. The current flowing through the chopper circuit reaches a steady state in which it increases and decreases within a certain range while repeating increasing and decreasing according to the switching of the switching element. Ripple occurs in the output voltage of the chopper circuit until the current flowing through the chopper circuit reaches a steady state. When the output voltage of the chopper circuit is lowered due to the ripple, the voltage of the current driver is lowered, so that the controllability for the LED may be lowered.

The present disclosure has been made to solve such a problem, and an object thereof is to obtain an output voltage of a chopper circuit in an LED control device for PWM dimming of an LED connected between a chopper circuit and a current driver. It is to reduce the ripple.

The LED control device according to the present disclosure PWM dims the LED connected between the chopper circuit and the current driver. The LED control device includes a power supply control unit and a dimming unit. The power supply control unit outputs the first PWM signal to the chopper circuit. The dimming unit controls the current flowing through the current driver based on the second PWM signal. The first cycle of the first PWM signal is shorter than the second cycle of the second PWM signal. The power supply control unit synchronizes the rising timing of the first PWM signal with the rising timing of the second PWM signal.

According to the LED control device according to the present disclosure, the power supply control unit synchronizes the rising timing of the first PWM signal with the rising timing of the second PWM signal, so that the ripple of the output voltage of the chopper circuit can be reduced.

It is a functional block diagram which shows the structure of the display device which concerns on Embodiment 1. FIG. It is a functional block diagram which also shows the specific configuration of the LED control device and the current driver of FIG. 1 when the value of a dimming signal is an H value. It is a functional block diagram which also shows the specific configuration of the LED control device and the current driver of FIG. 1 when the value of a dimming signal is an L value. It is a figure which also shows the waveform diagram of a dimming signal, the waveform diagram of a switching signal, and the waveform diagram of an LED current. It is a figure which enlarges the rising timing part of the dimming signal of FIG. 4, and also shows the waveform diagram of the coil current and the waveform diagram of the output voltage of the step-up chopper circuit. It is a figure which also shows the waveform diagram of a dimming signal and the waveform diagram of an output voltage. When the rising timing of the dimming signal and the rising timing of the switching signal are synchronized, the waveform diagram of the dimming signal, the waveform diagram of the switching signal, the waveform diagram of the coil current, the waveform diagram of the LED current, and the waveform of the output voltage. It is a figure which also shows the figure. The waveform diagram of the dimming signal, the waveform diagram of the switching signal, and the waveform diagram of the coil current when the rising timing of the dimming signal and the rising timing of the switching signal are synchronized and the initial duty ratio of the switching signal is increased. , The waveform diagram of the LED current, and the waveform diagram of the output voltage are also shown. It is a functional block diagram which shows the structure of the display device which concerns on the modification of Embodiment 1. FIG. It is a functional block diagram which shows the structure of the display device which concerns on Embodiment 2. FIG. It is a functional block diagram which also shows the specific configuration of the LED control device and the current driver of FIG. FIG. 5 is a diagram showing a waveform diagram of a dimming signal, a waveform diagram of a switching signal, a waveform diagram of an LED current, a waveform diagram of a coil current, and a waveform diagram of an output voltage according to the second embodiment.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the description thereof will not be repeated.

[Embodiment 1]
FIG. 1 is a functional block diagram showing the configuration of the display device 1 according to the first embodiment. As shown in FIG. 1, the display device 1 includes an LED row 10 and an LED drive device 100. The LED drive device 100 includes a step-up chopper circuit 20 and a semiconductor device 40. The semiconductor device 40 includes external terminals T1, T2, T3, an LED control device 50, and a current driver 60. The semiconductor device 40 is formed as, for example, an IC (Integrated Circuit) in which an LED control device 50 and a current driver are integrated.

The LED row 10 includes a plurality of LEDs 12. The plurality of LEDs 12 are connected in series between the boost chopper circuit 20 and the current driver 60. FIG. 1 shows a case where the number of LEDs included in the LED row 10 is 4 or more. The number of LEDs 12 included in the LED row 10 may be 3 or less.

The boost chopper circuit 20 includes a power supply node 22, a coil 24, a switching element Q1, a diode 26, and a capacitor 28. The coil 24 is connected between the power supply node 22 and the switching element Q1. The switching element Q1 is connected between the coil 24 and the ground node. The gate of the switching element Q1 is connected to the external terminal T2 of the LED control device 50. FIG. 1 shows a case where the switching element Q1 is an N-type MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The switching element Q1 may be an NPN type bipolar transistor or the like.

The anode of the diode 26 is connected to the connection node between the coil 24 and the switching element Q1. The cathode of the diode 26 is connected to the anode side of the LED row 10. The diode 26 shown in FIG. 1 is an SBD (Schottky Barrier Diode). Since the SBD has a small forward voltage, it is known that the forward loss is small, the efficiency is high, and the switching speed is high. The diode 26 may be a diode other than the SBD. The capacitor 28 is connected between the cathode of the diode 26 and the ground node. The switching element Q1 is switched and controlled by the LED control device 50, and the electric power supplied from the power supply node 22 is boosted and supplied to the LED row 10.

The LED control device 50 includes hardware (electronic circuit) in which an error amplifier, a PWM circuit, and the like are formed. The PWM circuit of the LED control device 50 generates a switching signal Spwm (first PWM signal) which is a PWM (Pulse Width Modulation) signal for switching and driving the switching element Q1, and the gate of the switching element Q1 connects the switching signal Spwm. Output to the external terminal T2. A dimming signal DM1 (second PWM signal), which is a PWM signal, is input to the external terminal T3. The period of the switching signal Spwm (first period) is shorter than the period of the dimming signal DM1 (second period). For example, the frequency of the switching signal Spwm is 300 kHz, and the frequency of the dimming signal DM1 is 150 Hz. The LED control device 50 performs PWM dimming on the LED row 10 by controlling the current flowing through the current driver 60 based on the dimming signal DM1.

The function of the LED control device 50 is not limited to that realized by hardware, and may be realized by software. For example, the LED control device 50 includes a CPU (Central Processing Unit), a memory (ROM (Read Only Memory) and RAM (Random Access Memory)), an input / output buffer for inputting / outputting various signals, and the like, and the CPU is a ROM. The function of the LED control device 50 can also be realized by expanding and executing the program stored in the RAM or the like in RAM or the like. The program stored in the ROM is a program in which the processing procedure of the LED control device 50 is described. The LED control device 50 may be configured to execute various processes according to the program.

2 and 3 are functional block diagrams showing specific configurations of the LED control device 50 and the current driver 60 of FIG. As shown in FIGS. 2 and 3, the LED control device 50 includes a power supply control unit 52 and a dimming unit 54. The current driver 60 includes a switching element 62, a resistance element 64, an amplifier 66, an inverter 68, and switches SW1 to SW3.

The power supply control unit 52 includes a PWM circuit. The power supply control unit 52 generates a switching signal Spwm and outputs it to the external terminal T2. The dimming unit 54 outputs the reference voltage Vref to the non-inverting input terminal (+) of the amplifier 66. The dimming unit 54 controls the connection state of the switches SW1 to SW3 based on the dimming signal DM1. FIG. 2 shows the connection state of the current driver 60 when the value of the dimming signal DM1 is the H value. FIG. 3 shows the connection state of the current driver 60 when the value of the dimming signal DM1 is the L value. The connection states of the switches SW1 to SW3 are different between FIGS. 2 and 3.

The switching element 62 is connected between the external terminal T1 to which the cathode side of the LED row 10 is connected and the resistance element 64. FIG. 2 shows a case where the switching element 62 is an N-type MOSFET. The switching element 62 may be an NPN type bipolar transistor or the like. The resistance element 64 is connected between the switching element 62 and the ground node.

The switch SW1 is connected between the output end of the amplifier 66 and the gate of the switching element 62. The switch SW2 is connected between the connection node between the switching element 62 and the resistance element 64 and the inverting input end (−) of the amplifier 66. The switch SW3 is connected between the gate of the switching element 62 and the ground node. The inverter 68 is connected between the dimming unit 54 and the switch SW3.

A reference voltage Vref is applied to the non-inverting input end (+) of the amplifier 66 from the dimming unit 54. A detection signal (voltage) obtained by converting the LED current IL flowing through the switching element 62 (that is, the current flowing through the LED row 10) into a voltage by the resistance element 64 is input to the inverting input end (−) of the amplifier 66. The reference voltage Vref input to the non-inverting input terminal (+) of the amplifier 66 is a signal obtained by converting the target value of the current flowing through the LED row 10 into a voltage by the resistance value of the resistance element 64.

The amplifier 66 drives the switching element 62 by outputting a signal obtained by amplifying the difference between the reference voltage Vref and the detection signal (voltage) to the gate of the switching element 62. The LED current IL flowing through the switching element 62 is adjusted to a target value corresponding to the reference voltage Vref. The LED control device 50 and the current driver 60 may be formed as separate ICs from each other.

The connection state of the switches SW1 and SW2 is switched between the conductive state and the non-conducting state based on the dimming signal DM1. The connection state of the switch SW3 is switched between the conduction state and the non-conduction state based on the signal in which the dimming signal DM1 is logically inverted by the inverter 68.

When the dimming signal DM1 has an H value, the switch SW1 and SW2 are brought into a conductive state, and the switch SW3 is brought into a non-conducting state, so that the LED current IL flows through the LED row 10. When the dimming signal DM1 has an L value, the LED current IL is stopped by setting the switches SW1 and SW2 in the non-conducting state and the switch SW3 in the conducting state. By alternately repeating the H value and the L value in the dimming signal DM1, the connection state shown in FIG. 2 and the connection state shown in FIG. 3 are alternately repeated.

FIG. 4 is a diagram showing a waveform diagram of the dimming signal DM1, a waveform diagram of the switching signal Spwm, and a waveform diagram of the LED current IL flowing through the LED row 10. As shown in FIG. 4, the power supply control unit outputs the switching signal Spwm in the time zone when the dimming signal DM1 has an H value. The dimming unit 54 starts flowing the LED current IL in synchronization with the rising timing of the dimming signal DM1, and stops the LED current IL at the falling timing of the dimming signal DM1.

FIG. 5 is a diagram showing an enlarged portion of time tm1 to tm2 in FIG. 4 and a waveform diagram of the coil current Id flowing through the coil 24 in FIG. 1 and a waveform diagram of the output voltage Vout of the boost chopper circuit 20. .. Note that FIG. 5 shows a case where the rising timing of the switching signal Spwm and the rising timing of the dimming signal DM1 are not synchronized as a comparative example of the first embodiment.

As shown in FIG. 5, energy is stored in the coil 24 of the boost chopper circuit 20 by repeating switching of the switching element Q1 of the boost chopper circuit 20 based on the switching signal Spwm. The switching element Q1 is in a conductive state in a time zone when the switching signal Spwm is an H value, and is in a non-conducting state in a time zone when the switching signal Spwm is an L value. The coil current Id flowing through the boost chopper circuit 20 reaches a steady state in which the coil current Id increases and decreases within a certain range while repeating increasing and decreasing according to the switching of the switching element Q1. In FIG. 5, the coil current Id reaches a steady state at time tm3 (tm1 <tm3 <tm2). In the steady state, the coil current Id reaches the current Is, then decreases, and then increases again to the current Is, repeating the process of change. The dimming unit 54 starts to flow the LED current IL in synchronization with the rising timing of the dimming signal DM1.

Ripple occurs in the output voltage Vout of the boost chopper circuit 20 until the coil current Id flowing through the boost chopper circuit 20 reaches a steady state. In FIG. 5, the output voltage Vout, which is the voltage V1 at the time tm1, has dropped to the voltage V3 (<V1). When the output voltage of the step-up chopper circuit 20 is lowered due to the ripple, the drain voltage of the switching element 62 of the current driver 60 is lowered, so that the controllability for the LED 12 may be lowered.

FIG. 6 is a diagram showing a waveform diagram of the dimming signal DM1 and a waveform diagram of the output voltage Vout together. As shown in FIG. 6, ripples are periodically generated at the output voltage Vout at each rising timing of the dimming signal DM1. Ripple is periodically generated at the output voltage, which may cause noise in the capacitor 28 of the step-up chopper circuit 20.

Therefore, the power supply control unit 52 synchronizes the rising timing of the switching signal Spwm with the rising timing of the dimming signal DM1. Since the coil current Id can be reliably increased only during the time period when the switching signal Spwm is the H value in one cycle of the switching signal Spwm from the rising timing of the dimming signal DM1, the time until the coil current Id becomes a steady state. Can be shortened. As a result, the ripple of the output voltage Vout can be suppressed.

FIG. 7 shows a waveform diagram of the dimming signal DM1, a waveform diagram of the switching signal Spwm, a waveform diagram of the coil current Id, and an LED current when the rising timing of the dimming signal DM1 and the rising timing of the switching signal Spwm are synchronized. It is a figure which also shows the waveform diagram of IL and the waveform diagram of output voltage Vout. As shown in FIG. 7, the coil current Id reaches a steady state at a time tm4 earlier than the time tm3. The output voltage Vout, which is the voltage V1 at the time tm1, is reduced only to the voltage V4 (> V3), and the amount of decrease from the voltage V1 is reduced as compared with the case shown in FIG.

By synchronizing the rising timing of the dimming signal DM1 and the rising timing of the switching signal Spwm, and by lengthening the time zone in which the switching signal Spwm is the H value in one cycle of the switching signal Spwm from the rising timing of the dimming signal DM1. , The time until the coil current Id reaches the steady state can be further shortened. That is, the duty ratio (first duet ratio) of the switching signal Spwm during one cycle of the switching signal Spwm from the rising timing of the dimming signal DM1 is changed from the rising timing of the dimming signal DM1 to the switching of the switching signal Spwm after one cycle. It is desirable to increase the duty ratio of the signal Spwm. The duty ratio is a ratio to one cycle of the time zone which is the H value in one cycle of the PWM signal.

FIG. 8 shows a waveform diagram of the dimming signal DM1 and a switching signal Spwm when the rising timing of the dimming signal DM1 and the rising timing of the switching signal Spwm are synchronized and the initial duty ratio of the switching signal Spwm is increased. The waveform diagram of the above, the waveform diagram of the coil current Id, the waveform diagram of the LED current IL, and the waveform diagram of the output voltage Vout are also shown. As shown in FIG. 8, the coil current Id reaches a steady state at a time tm5 earlier than the time tm4. The output voltage Vout, which is the voltage V1 at the time tm1, is reduced only to the voltage V5 (> V4), and the amount of decrease from the voltage V1 is reduced as compared with the case shown in FIG.

In FIG. 1, a case where the number of LED rows 10 is one has been described. The number of LED rows 10 included in the display device according to the embodiment may be plural. FIG. 9 is a functional block diagram showing the configuration of the display device 1A according to the modified example of the first embodiment. The display device 1A has a plurality of LED rows 10 in FIG. 1, and the semiconductor device 40 in FIG. 1 is replaced with the semiconductor device 40A. The semiconductor device 40A has a plurality of external terminals T1 and current drivers 60 so as to correspond to the plurality of LED rows 10, and the LED control device 50 in FIG. 1 is replaced with the LED control device 50A. Is. Other than these, the same applies, so the description will not be repeated.

As described above, according to the display device according to the first embodiment and the modified example, the ripple of the output voltage of the chopper circuit is reduced in the LED control device that PWM dimmes the LED connected between the chopper circuit and the current driver. be able to.

[Embodiment 2]
In the first embodiment, a case where the ripple of the output voltage is reduced by synchronizing the rising timing of the dimming signal and the rising timing of the switching signal has been described. In the second embodiment, a case where the ripple of the output voltage is reduced by synchronizing the rising timing of the dimming signal and the rising timing of the switching signal and delaying the LED current until the coil current reaches the threshold value will be described. ..

FIG. 10 is a functional block diagram showing the configuration of the display device 2 according to the second embodiment. The configuration of the display device 2 is such that the semiconductor device 40 of FIG. 1 is replaced with the semiconductor device 40B. The configuration of the semiconductor device 40B is such that the external terminal T4 is added to the semiconductor device 40 of FIG. 1 and the LED control device 50 is replaced with the LED control device 50B. Other than these, the same applies, so the description will not be repeated. As shown in FIG. 10, the LED control device 50B receives the coil current Id from the external terminal T4.

FIG. 11 is a functional block diagram showing a specific configuration of the LED control device 50B and the current driver 60 of FIG. The configuration of the LED control device 50B shown in FIG. 11 is such that the dimming unit 54 of FIG. 2 is replaced with the dimming unit 54B. The dimming unit 54B controls the switches SW1 to SW3 based on the dimming signal DM1 received from the external terminal T3 and the coil current Id received from the external terminal T4.

FIG. 12 is a diagram showing a waveform diagram of the dimming signal DM1, a waveform diagram of the switching signal Spwm, a waveform diagram of the LED current IL, a waveform diagram of the coil current Id, and a waveform diagram of the output voltage Vout according to the second embodiment. Is. In FIG. 12, the rising timing of the dimming signal DM1 and the rising timing of the switching signal Spwm are synchronized, and the initial duty ratio of the switching signal Spwm is increased. As shown in FIG. 12, the dimming unit 54B delays the LED current IL until the time tm6 when the coil current Id reaches the threshold value Is. The coil current Id reaches a steady state at a time tm7 earlier than the time tm3. Since the LED current IL starts to flow after a certain amount of energy is stored in the coil 24, the time until the coil current Id reaches a steady state is shortened. Further, the output voltage Vout, which is the voltage V1 at the time tm1, is reduced only to the voltage V6 (> V4), and the amount of decrease from the voltage V1 is reduced as compared with the case shown in FIG.

As described above, according to the display device according to the second embodiment, in the LED control device that PWM dimmes the LED connected between the chopper circuit and the current driver, the ripple of the output voltage of the chopper circuit can be reduced. ..

The display device according to the embodiment and the modification can be applied to, for example, a light source of a liquid crystal display (LCD), and in particular, a light source of an in-vehicle display that displays a vehicle driving situation or the like to a driver. It is suitable for.

It is also planned that the embodiments disclosed this time will be appropriately combined and implemented within a consistent range. The embodiments disclosed this time should be considered to be exemplary in all respects and not restrictive. The scope of the present invention is shown by the scope of claims rather than the description of the embodiment described above, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

1,1A, 2 Display device, 10 LED train, 20 boost chopper circuit, 24 coils, 26 diodes, 28 capacitors, 40, 40A, 40B semiconductor device, 50, 50A, 50B control device, 52 power supply control unit, 54, 54B Dimming unit, 60 current driver, 62, Q1 switching element, 64 resistance element, 66 amplifier, 68 inverter, 100 LED drive, DM1 dimming signal, SW1 to SW3 switch, T1 to T4 external terminal.

Claims (5)

It is an LED control device that PWM dimmes the LED connected between the chopper circuit and the current driver.
A power supply control unit that outputs a first PWM signal to the chopper circuit,
It is provided with a dimming unit that controls the current flowing through the current driver based on the second PWM signal.
The first cycle of the first PWM signal is shorter than the second cycle of the second PWM signal.
The power supply control unit is an LED control device that synchronizes the rising timing of the first PWM signal with the rising timing of the second PWM signal. The power supply control unit sets the duty ratio of the first PWM signal between the rising timing of the second PWM signal and the first cycle, and the duty of the first PWM signal after the first cycle from the rising timing of the second PWM signal. The LED control device according to claim 1, wherein the LED control device is increased by a ratio. The LED control device according to claim 1 or 2, wherein the dimming unit causes a current to flow through the current driver from a timing when the current flowing through the chopper circuit exceeds a threshold value. The LED control device according to any one of claims 1 to 3.
With the chopper circuit
An LED drive device including the current driver. The LED drive device according to claim 4 and
A display device including the LED.

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