JP2020198244A - Led driving device and display apparatus, and control apparatus for the led driving device - Google Patents

Abstract

To provide an LED driving device capable of avoiding an LED from being unable to light when a cathode side of an LED is short-circuited.SOLUTION: A driving device 50 drives a DC/DC converter 20. A detector 80 detects a feedback voltage Vfb at a side of a cathode of an LED row 10. A resistance element 40 and an error amplifier 85 form a current detector and detect a current Iout flowing through the LED row 10. A controller 70 performs feedback voltage control based on the feedback voltage Vfb when a short-circuit failure at the side of the cathode of the LED row 10 does not occur. When the above-mentioned short-circuit failure occurs, the controller 70 performs limited current control for driving the DC/DC converter based on the current Iout.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to an LED drive device, a display device including the LED drive device, and a control device for the LED drive device.

The use of LEDs (Light Emitting Diodes) as energy-saving light sources has been expanding in recent years. To drive the LED, an LED driver for supplying a constant current to the LED is used based on the forward voltage-current characteristic and the light emission characteristic of the LED.

Japanese Patent Application Laid-Open No. 2018-19498 (Patent Document 1) discloses such an LED driver. This LED driver includes a DC / DC converter that supplies a current to the LED and a current driver (constant current control circuit) that adjusts the current flowing through the LED. In this LED driver, feedback control is performed using the voltage on the cathode side of the LED as a feedback signal (second feedback control mode), and the DC / DC converter is driven by PWM (Pulse Width Modulation) control to drive the cathode of the LED. The voltage on the side is controlled to be constant (see Patent Document 1).

JP-A-2018-19498

In the LED driver as described above, when the cathode side of the LED is short-circuited (ground fault), the DC / DC converter operates in the direction of increasing the voltage on the cathode side of the LED, and as a result, a large current can flow through the LED. For this reason, in general, when a short-circuit failure on the cathode side of an LED occurs, the DC / DC converter is shut down so that a large current flows through the LED and the LED is not damaged. However, when the DC / DC converter is shut down, the LED turns off. Such a problem has not been particularly examined in the above-mentioned Patent Document 1.

The present disclosure has been made in order to solve such a problem, and an object thereof is an LED drive device capable of avoiding the LED from turning off when the cathode side of the LED is short-circuited, and a display device including the LED drive device. , As well as providing a control device for the LED drive device.

The LED drive device of the present disclosure includes a DC / DC converter configured to supply power to the LED, a drive device configured to drive the DC / DC converter, and a voltage (feedback voltage) on the cathode side of the LED. It includes a first detector configured to detect Vfb) and a second detector configured to detect the current (current Iout) flowing through the LED. The drive device is configured to execute the first process when the abnormality that the cathode of the LED is grounded has not occurred, and to execute the second process when the above abnormality has occurred. The first process includes a process of driving the DC / DC converter based on the voltage detected by the first detector. The second process includes a process of driving the DC / DC converter based on the current detected by the second detector.

In the above LED drive device, when an abnormality in which the cathode of the LED is grounded (hereinafter referred to as "short failure" of the cathode of the LED) occurs, the DC / DC converter is driven based on the voltage on the cathode side of the LED. The second process is executed instead of the first process, and the DC / DC converter is driven based on the current flowing through the LED. As a result, it is not necessary to shut down the DC / DC converter in order to prevent a large current from flowing through the LED, and the current flowing through the LED can be adjusted to turn on the LED.

In the first process, the DC / DC converter may be driven so that the voltage detected by the first detector becomes the first voltage. Then, when the voltage detected by the first detector is lower than the second voltage lower than the first voltage, the drive device may determine that the short circuit failure of the cathode of the LED has occurred.

In the second process, the DC / DC converter may be driven so that the current detected by the second detector becomes a predetermined current. Alternatively, the second process may drive the DC / DC converter so that the current detected by the second detector does not exceed a predetermined current. The predetermined current is smaller than the current flowing through the LED when the first process is executed.

Further, the display device of the present disclosure includes any of the above-mentioned LED drive devices and LEDs driven by the LED drive devices.

Further, the control device of the present disclosure is a control device of an LED drive device. The LED drive includes a DC / DC converter configured to power the LEDs. The control device has a first detector configured to detect the voltage on the cathode side of the LED (feedback voltage Vfb) and a second detector configured to detect the current (current Iout) flowing through the LED. It is equipped with a vessel and a controller. The controller is configured to execute the first process when the abnormality that the cathode of the LED is grounded has not occurred, and to execute the second process when the abnormality has occurred. The first process includes a process of driving the DC / DC converter based on the voltage detected by the first detector. The second process includes a process of driving the DC / DC converter based on the current detected by the second detector.

According to the LED drive device and the display device of the present disclosure, and the control device of the LED drive device, it is possible to prevent the LED from being turned off when a short-circuit failure on the cathode side of the LED occurs.

It is a figure which shows the whole structure of the display device which used the LED driver according to the embodiment of this disclosure. It is a timing chart which shows operation example of the LED driver shown in FIG. 1 schematically. It is a flowchart which shows an example of the procedure of the process executed by a controller.

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.

<LED driver configuration>
FIG. 1 is a diagram showing an overall configuration of a display device using an LED driver according to an embodiment of the present disclosure. With reference to FIG. 1, the display device 1 includes an LED row 10, a DC / DC converter 20, a resistance element 40, and a drive device 50. The DC / DC converter 20, the driving device 50, and the resistance element 40 constitute an LED driver that drives the LED row 10.

The LED row 10 includes a plurality of LEDs 12 connected in series. In this example, one row of LED rows 10 including a plurality of LED 12s more than four is shown, but the number of LED rows may be plural, and the number of LEDs 12 included in each LED row is four. It may be less than one.

The DC / DC converter 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 T1 of the drive device 50. In this example, the switching element Q1 is an N-type MOSFET (Metal Oxide Semiconductor Field Effect Transistor), but may be an NPN-type bipolar transistor or the like.

In the diode 26, the anode is connected to the connection point between the coil 24 and the switching element Q1, and the cathode is connected to the anode side of the LED row 10. In this example, the diode 26 is a Schottky barrier diode (hereinafter referred to as "SBD (Schottky Barrier Diode)"). Since the SBD has a small forward voltage, the forward loss is small, the efficiency is high, and the switching speed is high. It is not essential that the diode 26 is an SBD, and a diode other than the SBD can be adopted. The capacitor 28 is connected between the power line to which the cathode of the SBD 26 is connected and the ground node.

A so-called asynchronous rectification type step-up chopper circuit is formed by the power supply node 22, the coil 24, the switching element Q1, the SBD 26, and the capacitor 28. The switching element Q1 is switched and controlled by the drive device 50, and the electric power supplied from the power supply node 22 is boosted and supplied to the LED row 10.

The resistance element 40 is provided on the power line between the DC / DC converter 20 and the LED row 10. One end of the resistance element 40 is connected to the external terminal T2 of the drive device 50, and the other end of the resistance element 40 is connected to the external terminal T3 of the drive device 50. The resistance element 40 is a sense resistor for detecting the current Iout flowing from the DC / DC converter 20 to the LED row 10.

The drive device 50 includes a current driver 60, a controller 70, a detector 80, and an error amplifier 85. The drive device 50 is configured as, for example, an IC (Integrated Circuit) in which each of these elements is integrated. The drive device 50 further includes external terminals T1 to T4 for electrically connecting to an external element.

The current driver 60 includes a switching element 62, a resistance element 64, and an error amplifier 66. The switching element 62 is connected between the external terminal T4 to which the cathode side of the LED row 10 is connected and the resistance element 64. In this example, the switching element 62 is an N-type MOSFET, but 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 connection point between the switching element 62 and the resistance element 64 is connected to the inverting input end (−) of the error amplifier 66. A reference voltage Vref is applied to the non-inverting input end (+) of the error amplifier 66. The output end of the error amplifier 66 is connected to the gate of the switching element 62.

A detection signal (voltage) obtained by converting the current 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 terminal (−) of the error amplifier 66. The reference voltage Vref input to the non-inverting input end (+) of the error 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 error 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. As a result, the current flowing through the switching element 62, that is, the current flowing through the LED row 10, is adjusted to the current target value corresponding to the reference voltage Vref. The current driver 60 may be provided outside the IC.

The detector 80 is electrically connected to the external terminal T4 to which the cathode side of the LED row 10 is connected, and detects the voltage Vfb on the cathode side of the LED row 10. The voltage Vfb detected by the detector 80 (hereinafter, may be referred to as “feedback voltage Vfb”) is output to the controller 70.

The controller 70 drives the DC / DC converter 20 based on the feedback voltage Vfb detected by the detector 80. Specifically, the controller 70 includes an error amplifier and a PWM (Pulse Width Modulation) circuit (neither of them is shown). The error amplifier amplifies and outputs the difference between the feedback voltage Vfb and the predetermined value so that the feedback voltage Vfb becomes a predetermined value (for example, 0.5 V). The PWM circuit generates a PWM signal based on the output of the error amplifier, and outputs the generated PWM signal to the external terminal T1 to which the gate of the switching element Q1 is connected.

The controller 70 is not limited to the one executed by hardware (electronic circuit), and may be configured by one executed by software. Specifically, the controller 70 is configured to include 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. You may. The CPU expands the program stored in the ROM into a RAM or the like and executes it. The program stored in the ROM is a program in which the processing procedure of the controller 70 is described. Then, the controller 70 may be configured to execute various processes according to this program.

In this LED driver, the current driver 60 adjusts the current flowing through the LED row 10 to the current target value corresponding to the reference voltage Vref. Further, the DC / DC converter 20 is driven by the controller 70 based on the feedback voltage Vfb so that the feedback voltage Vfb becomes a predetermined value. As a result, the current flowing through the LED row 10 is controlled as a target.

<Short failure on the cathode side of LED row 10>
In the above configuration, consider the case where the cathode side of the LED row 10 is short-circuited (ground fault). In such a situation, for example, when the connector of the flat cable extending from the cathode side of the LED row 10 is connected to the connector at the external terminal T4, a short circuit occurs at the connection portion, or the external terminal T4 is provided for the purpose of noise removal. This can occur if the capacitors (not shown) are short-circuited.

When the cathode of the LED row 10 is short-circuited, the feedback voltage Vfb drops to the ground level, so that the DC / DC converter 20 operates in the direction of increasing the feedback voltage Vfb, and as a result, a large current can flow in the LED row 10. In such cases, it is generally possible to shut down the DC / DC converter so that the LEDs do not break. However, when the DC / DC converter 20 shuts down, the LED row 10 is turned off.

Therefore, in the LED driver according to the present embodiment, when the current Iout flowing in the LED row 10 is detected and a short-circuit failure on the cathode side of the LED row 10 occurs, the LED row 10 can be turned on based on the current Iout. The DC / DC converter 20 is driven while limiting the current Iout.

Therefore, the drive device 50 further includes an error amplifier 85 for detecting the current Iout. The non-inverting input end (+) of the error amplifier 85 is connected to the external terminal T2 to which one end of the resistance element 40 is connected, and the inverting input end (-) of the error amplifier 85 is connected to the other end of the resistance element 40. It is connected to the external terminal T3. Then, the error amplifier 85 outputs a signal obtained by amplifying the difference between the voltages input from the external terminals T2 and T3, that is, a signal corresponding to the voltage generated in the resistance element 40 to the controller 70. Since the voltage generated in the resistance element 40 is obtained by converting the current Iout flowing through the resistance element 40 into a voltage by the resistance element 40, the resistance element 40 and the error amplifier 85 constitute a current detector for detecting the current Iout.

Then, when a short-circuit failure on the cathode side of the LED row 10 occurs, the controller 70 replaces the control for driving the DC / DC converter 20 based on the feedback voltage Vfb (hereinafter referred to as “feedback voltage control”) with a current. Based on the Iout, the control for driving the DC / DC converter 20 (hereinafter referred to as "current limit control") is executed while limiting the current Iout to the extent that the LED row 10 can be lit. As a result, it is not necessary to shut down the DC / DC converter 20 in order to prevent a large current from flowing through the LED row 10, and the LED row 10 can be turned on while adjusting the current Iout flowing through the LED row 10.

The short circuit failure on the cathode side of the LED row 10 is determined based on the feedback voltage Vfb detected by the detector 80. In the feedback voltage control, the controller 70 drives the DC / DC converter 20 so that the feedback voltage Vfb becomes a predetermined value (for example, 0.5V), so that the feedback voltage Vfb is lower than the above-mentioned predetermined value (for example). For example, when it falls below 0.2 V), it is determined that a short-circuit failure on the cathode side of the LED row 10 has occurred.

FIG. 2 is a timing chart schematically showing an operation example of the LED driver shown in FIG. With reference to FIG. 2, it is assumed that a short-circuit failure on the cathode side of the LED row 10 has occurred at time t1.

Before the occurrence of a short-circuit failure, the feedback voltage Vfb is adjusted to a predetermined value V1 (for example, 0.5V) by appropriately adjusting the duty of the switching element Q1 based on the feedback voltage Vfb (feedback voltage control).

At time t1, when the feedback voltage Vfb drops to V2, which is lower than the threshold value Vth (Vth <V1), and it is determined that a short-circuit failure on the cathode side of the LED row 10 has occurred, the LED protection flag is turned on. become. This LED protection flag indicates that if the feedback voltage control is continued, a large current may flow in the LED row 10 and the LED row 10 may be damaged. Therefore, the control for protecting the LED row 10 is performed. It is a flag to indicate.

When the LED protection flag is turned on, the LED driver operates in protection mode. That is, the controller 70 drives the DC / DC converter 20 based on the current Iout flowing in the LED row 10 while limiting the current Iout to the extent that the LED row 10 can be lit (current limit control). The current limit control is continued until the LED protection flag is turned off, that is, until the short circuit failure on the cathode side of the LED row 10 is resolved by repair.

During the current limit control, the controller 70 appropriately adjusts the duty of the switching element Q1 based on the current Iout so that the current Iout becomes a predetermined current smaller than the current Iout under the feedback voltage control. Alternatively, the controller 70 may appropriately adjust the duty of the switching element Q1 so that the current Iout does not exceed the above-mentioned predetermined current during the current limit control. The predetermined current is appropriately determined based on the characteristics of each LED 12 constituting the LED row 10.

FIG. 3 is a flowchart showing an example of a processing procedure executed by the controller 70. The series of processes shown in this flowchart are repeatedly executed at predetermined intervals.

With reference to FIG. 3, the controller 70 determines whether or not the LED protection flag indicating whether or not a short-circuit failure on the cathode side of the LED row 10 has occurred is on (step S10). When the LED protection flag is on (YES in step S10), the controller 70 shifts the process to step S50 (described later).

When it is determined in step S10 that the LED protection flag is off (NO in step S10), the controller 70 determines whether the feedback voltage Vfb, which is the voltage on the cathode side of the LED row 10, is lower than the threshold value Vth. Is determined (step S20). The feedback voltage Vfb is detected by the detector 80 (FIG. 1). Further, the threshold value Vth is a value lower than the adjustment value (predetermined value V1) of the feedback voltage Vfb in the feedback voltage control based on the feedback voltage Vfb.

When it is determined in step S20 that the feedback voltage Vfb is equal to or higher than the threshold value Vth (NO in step S20), a short-circuit failure on the cathode side of the LED row 10 has not occurred, and the controller 70 uses the feedback voltage Vfb. The feedback voltage control for driving the DC / DC converter 20 is executed based on the above (step S30).

On the other hand, if it is determined in step S20 that the feedback voltage Vfb is lower than the threshold value Vth (YES in step S20), the controller 70 determines that a short-circuit failure on the cathode side of the LED row 10 has occurred. The LED protection flag is turned on (step S40).

When the LED protection flag is turned on, the controller 70 controls the current limit to drive the DC / DC converter 20 while limiting the current Iout to the extent that the LED row 10 can be lit, based on the current Iout flowing through the LED row 10. Execute (step S50). The current Iout is detected by using the resistance element 40 and the error amplifier 85 (FIG. 1). As a result, when a short-circuit failure on the cathode side of the LED row 10 occurs, the LED row 10 can be turned on without shutting down the DC / DC converter 20.

As described above, in this embodiment, when a short circuit failure on the cathode side of the LED row 10 occurs, the LED row 10 is replaced with the feedback voltage control based on the feedback voltage Vfb, which is the voltage on the cathode side of the LED row 10. The current limit control based on the current Iout flowing through the is executed. In the current limit control, the DC / DC converter 20 is driven based on the current Iout so that the current Iout becomes a predetermined current smaller than the current Iout during the feedback voltage control or does not exceed the predetermined current. Will be done. As a result, it is not necessary to shut down the DC / DC converter 20 in order to prevent a large current from flowing through the LED row 10, and the LED row 10 can be turned on while adjusting the current Iout flowing through the LED row 10.

In the above embodiment, the DC / DC converter 20 is an asynchronous rectifying type step-up chopper circuit, but the configuration of the DC / DC converter 20 is not limited to this. For example, the DC / DC converter 20 may be an asynchronous rectification type step-down chopper circuit or a synchronous rectification type DC / DC converter.

The display device 1 can be applied to, for example, a light source of a liquid crystal display (LCD), and is particularly suitable as a light source of an in-vehicle display that displays a vehicle driving situation or the like to a driver. According to the above-mentioned display device 1, even if a short-circuit failure on the cathode side of the LED row 10 occurs, it is possible to prevent the in-vehicle display from being completely turned off and perform limp home driving.

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 Display, 10 LED train, 20 DC / DC converter, 22 power supply node, 24 coil, 26 SBD, 28 capacitor, 40,64 resistor element, 50 drive unit, 60 current driver, 62, Q1 switching element, 66,85 Error amplifier, 70 controller, 80 detector, T1 to T4 external terminals.

Claims (6)

A DC / DC converter configured to supply power to the LEDs,
A drive device configured to drive the DC / DC converter and
A first detector configured to detect the voltage on the cathode side of the LED,
It includes a second detector configured to detect the current flowing through the LED.
The drive device is configured to execute the first process when the abnormality that the cathode of the LED is grounded has not occurred, and to execute the second process when the abnormality has occurred.
The first process includes a process of driving the DC / DC converter based on the voltage detected by the first detector.
The second process is an LED drive device including a process of driving the DC / DC converter based on a current detected by the second detector. In the first process, the DC / DC converter is driven so that the voltage detected by the first detector becomes the first voltage.
The driving device according to claim 1, wherein when the voltage detected by the first detector is lower than the second voltage lower than the first voltage, the driving device determines that the abnormality has occurred. LED drive device. In the second process, the DC / DC converter is driven so that the current detected by the second detector becomes a predetermined current.
The LED drive device according to claim 1 or 2, wherein the predetermined current is smaller than the current flowing through the LED when the first process is executed. In the second process, the DC / DC converter is driven so that the current detected by the second detector does not exceed a predetermined current.
The LED drive device according to claim 1 or 2, wherein the predetermined current is smaller than the current flowing through the LED when the first process is executed. The LED drive device according to any one of claims 1 to 4.
A display device including an LED driven by the LED drive device. It is a control device for LED drive devices.
The LED drive includes a DC / DC converter configured to power the LEDs.
The control device
A first detector configured to detect the voltage on the cathode side of the LED,
A second detector configured to detect the current flowing through the LED,
A controller configured to execute the first process when the abnormality that the cathode of the LED is grounded has not occurred and to execute the second process when the abnormality has occurred is provided.
The first process includes a process of driving the DC / DC converter based on the voltage detected by the first detector.
The second process is a control device for an LED drive device, which comprises a process of driving the DC / DC converter based on a current detected by the second detector.

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