CN1758175A - Led driving apparatus and method of controlling luminous power - Google Patents

Abstract

The present invention provides a light emitting diode (LED) driving device, comprising: a driving device for driving the LED to emit light; and a control device, the control device is used for controlling the driving current supplied from the driving device to the LED, if If the target value is less than a predetermined value, the control means performs optical power control by controlling the on/off ratio of the drive current, and if the target value is equal to or greater than the predetermined value, the control means performs optical power control by controlling the level of the drive current.

Description

Light-emitting diode driving device and optical power control method

Cross References to Related Applications

The present invention contains subject matter of Japanese Patent Applications JP 2004-296148 and JP 2005-227965 filed in the Japan Patent Office on Oct. 8, 2004 and Aug. 5, 2005, respectively, the entire contents of which are hereby incorporated by reference.

technical field

The present invention relates to a light emitting diode (LED) driving device and a method of controlling light power.

Background technique

A display that cannot emit light by itself (for example, a liquid crystal display) is equipped with a backlight. As such a backlight, a backlight using a cold-cathode tube as a light source has been known. In addition, in recent years, backlights using light emitting diodes (LEDs) that consume less power have also been known.

Regarding methods of controlling the luminous power of such backlights including LEDs, two methods have been known. One is a current value control method for controlling the level of driving current supplied to the LED. The other is to perform pulse width modulation (PWM) control to keep the drive current level constant.

In the current value control method, the level itself of the driving current continuously supplied to the LED is changed so that the target optical power is obtained. In the PWM control, the on/off ratio of the drive current per unit time is changed so that desired optical power is obtained.

An example of a document disclosing the related art is Japanese Patent Laid-Open No. 4-134486.

It is known that the luminous efficiency of an LED varies depending on its drive current value.

This aspect will be described with reference to FIGS. 7 to 9 .

FIG. 7 shows a graph showing the relationship between forward voltage and forward current of a certain LED. FIG. 8 shows the relationship between the forward current and the optical power of the LED. In particular, Figure 7 shows the corresponding forward voltage values obtained when certain values of forward current are supplied to the LED. Figure 8 shows the corresponding optical power values obtained when certain values of forward current are supplied to the LED.

The luminous efficiency can be obtained by dividing the optical power by the input power. Therefore, the luminous efficiency of the LED is obtained through the following steps: multiply a certain forward current value in Figure 7 by the corresponding forward voltage value to obtain the input power; after that, use the optical power corresponding to the forward current value in Figure 8 value divided by the input power obtained.

FIG. 9 is a graph showing the relationship between the luminous efficiency and the forward current value obtainable through the above steps.

As shown, for this LED, as the forward current value increases from 50mA to 100mA, the luminous efficiency increases, and as the forward current value increases above 100mA, the luminous efficiency decreases. The maximum luminous efficiency is obtained near the current value of 100mA.

It is easy to know from this figure that the LED has a characteristic that the luminous efficiency changes depending on the value of the forward current (drive current). Specifically, the luminous efficiency of an LED tends to increase as the driving current value increases until the driving current value reaches a certain current value, above which the luminous efficiency tends to decrease as the current value increases.

Since the luminous efficiency of the LED changes depending on the driving current value, the next problem arises in the LED driving device that controls the luminous efficiency of the LED using one of the above-mentioned current value control and PWM control methods.

In the current value control, if the target value of the optical power to be controlled is the same as the value corresponding to the current value providing the highest luminous efficiency shown in FIG. 9, the LED can be driven with the highest luminous efficiency. However, the target value does not necessarily correspond to this value, and thus, there is a possibility of driving the LED with low luminous efficiency.

As far as PWM control is concerned, what is controlled is the on/off of the current to keep the current value constant. Therefore, the constant current value must be a value corresponding to the maximum optical power value within its allowable range.

However, the current value at the maximum optical power within the allowable range does not necessarily correspond to the current value at the above-mentioned maximum luminous efficiency. Therefore, even in PWM control, there is a possibility that LEDs are driven with low luminous efficiency.

Driving LEDs with low luminous efficiency requires unnecessary extra power higher than originally required input power, which results in increased power consumption. In addition, if the LED is driven with low luminous efficiency, the heat generation of the LED, its driving circuit, and the power supply unit tends to increase, for example, causing a problem that measures for preventing heat generation hinder miniaturization of equipment.

Contents of the invention

In view of the above problems, it is desired to provide an LED driving device with the following structure.

Specifically, the LED driving device includes a driving device for driving the LED to emit light.

Furthermore, the LED driving device includes control means for controlling the driving current supplied to the LED from the driving means. If the target value of the optical power of the LED is smaller than a predetermined value stored in the control means, the control means performs optical power control by controlling the on/off ratio of the driving current. If the target value is equal to or greater than a predetermined value, the control means performs optical power control by controlling the level of the drive current.

In addition, according to an embodiment of the present invention, the method for controlling optical power has the following features.

Specifically, regarding the method of controlling the LED light power, if the target value of the LED light power is less than a predetermined value, the light power control is performed by controlling the on/off ratio of the drive current, and if the target value is equal to or greater than the predetermined value, by The level of the drive current is controlled to perform optical power control.

According to an embodiment of the present invention, a light emitting diode (LED) driving device includes a driving section for driving the LED to emit light, and a control section controlling a driving current supplied from the driving section to the LED. If the target value of the optical power of the LED is smaller than the predetermined value stored in the control section, the control section performs optical power control by controlling the on/off ratio of the driving current. If the target value is equal to or greater than a predetermined value, the control section performs optical power control by controlling the level of the drive current.

According to an embodiment of the present invention, the driving control method of the LED can be switched between the above-mentioned current value control and PWM control according to the target value of the optical power. Specifically, according to the above setup, if the target value is smaller than the optical power value that provides the highest luminous efficiency of the LED, the control method can be switched to PWM control. If the target value is equal to or greater than a predetermined value, the control method can switch to drive current control.

If such optical power control is possible, when the target value is smaller than a predetermined value, PWM control can be performed in which the drive current level is kept constant at a level that provides the highest luminous efficiency of the LED. Therefore, it is possible to drive the LED with the highest luminous efficiency.

In addition, if the target value is equal to or greater than a predetermined value, switching to current value control enables desired optical power to be obtained with as little power as possible. If PWM control is performed even when the target value is equal to or greater than a predetermined value, a larger drive current level is inevitably required than in the case of performing current value control. Larger current levels result in driving the LED with lower luminous efficiency, which can be easily seen in the characteristic diagram of Figure 9. Therefore, switching to current value control drives the LED most efficiently.

As described above, according to the embodiment of the present invention, the optical power control is switched between PWM control and current value control depending on the target value of optical power. Therefore, it is possible to drive the LED with the highest possible luminous efficiency.

In addition, this switching of the control method can drive the LED with the minimum power consumption, and further can minimize the heat generation of the LED, its driving circuit, and the power supply unit. Therefore, the device can be miniaturized.

Description of drawings

With reference to the following description in conjunction with the accompanying drawings, it will be seen that the above-mentioned and other objects of the present invention, in the accompanying drawings:

1 shows a block diagram of a configuration example of a liquid crystal display configured with an LED driving device according to an embodiment of the present invention;

Fig. 2 shows a block diagram according to the structure of the LED driving device of the first embodiment of the present invention;

Fig. 3 shows the circuit diagram of the internal structure of LED driving circuit;

Figure 4 shows a flowchart of processing operations for implementing the operations of the first embodiment;

Fig. 5 shows a block diagram according to the structure of the LED driving device of the second embodiment of the present invention;

FIG. 6 shows a flowchart of processing operations for realizing the operations of the second embodiment;

Fig. 7 shows the relationship diagram between the forward voltage and the forward current of a certain LED;

Figure 8 shows a graph of the relationship between the forward current and optical power of a certain LED; and

FIG. 9 is a graph showing the relationship between forward current and luminous efficiency of a certain LED.

Detailed ways

The best mode for carrying out the invention (hereinafter, the embodiment) will be described below.

First, a structural example of a liquid crystal display configured with a light emitting diode (LED) driving device according to an embodiment will be described with reference to FIG. 1 .

Referring to FIG. 1, AC power is input to the liquid crystal display from an AC power input terminal tAC.

AC power is supplied to the power supply circuit 6 from an AC power input terminal tAC, thereby generating a DC voltage. Then, the power supply circuit 6 supplies the generated DC voltage to the signal processing circuit 7 , the panel drive circuit 8 , the controller 2 , and the LED drive circuit 3 as the illustrated DC power supply.

Further, a video signal is input to the liquid crystal display from the video input terminal tv. The video signal is supplied to the signal processing circuit 7 . The signal processing circuit 7 performs required signal processing on the supplied video signal, thereby obtaining a signal required for driving and controlling the liquid crystal panel 5 .

The signal processing circuit 7 supplies signals necessary for controlling the driving of the panel to the panel driving circuit 8 . The panel drive circuit 8 drives the liquid crystal panel 5 based on this signal.

In addition, the signal processing circuit 7 extracts a luminance signal from the input video signal, and supplies the luminance signal to the controller 2 .

The controller 2 is, for example, a microcomputer including a central processing unit (CPU), read only memory (ROM), random access memory (RAM), etc., and performs overall control of the liquid crystal display.

The controller 2 adjusts the light power of the LED 4a provided in the backlight 4 based on information of an average picture level (APL) (average luminance) calculated based on the luminance signal supplied from the signal processing circuit 7. According to the optical power control based on APL information, when the APL is lower than, for example, a certain level, the optical power of the backlight is reduced to a predetermined value (eg, one tenth), thereby achieving high contrast.

In addition to the adjustment based on the APL, the controller 2 also adjusts the light power of the LED 4a in the backlight 4 in response to an operation by the user through the user interface (I/F) 9.

The user I/F 9 includes a command receiver for receiving a command signal from a remote controller or an operating element provided on the outer surface of the liquid crystal display case. For example, the user operates a button operating element for brightness adjustment provided on the outer surface of the casing, thereby being able to input an instruction regarding the light power of the backlight 4 to the liquid crystal display. Alternatively, the user selects a brightness adjustment option from a configuration menu displayed on the screen of the liquid crystal panel 5, thereby performing an operation input according to an instruction of the screen. Thus, an instruction regarding the optical power of the backlight 4 can be input.

If an instruction is input by the user through the user I/F 9, the controller 2 controls the light power of the backlight 4 in response to the instruction input information.

FIG. 2 shows a block diagram of the part of the structure shown in FIG. 1 related to the optical power control of the LED 4a in the backlight 4.

Referring to Fig. 2, DC power is provided to LED drive circuit 3 from power supply circuit 6 shown in Fig. 1, and this LED drive circuit is based on DC power supply, provides driving current to LED 4a in backlight 4.

The LED driving circuit 3 is provided with a current value control terminal t1 and an on/off control terminal t2. Each terminal receives a control signal from a controller 2 as shown. With a structure that will be described below, the LED drive circuit 3 changes the level of the drive current supplied to the LED 4a in response to the control signal supplied to the current value control terminal t1. In addition, in response to the control signal supplied to the on/off control terminal t2, the LED driving circuit 3 changes the on/off time of the driving current while keeping the level of the driving current constant.

Note that the controller 2 and the LED driving circuit 3 constitute the LED driving device 1 as the first embodiment shown in FIG. 2 .

The controller 2 sets the light of the LED 4a according to the APL information calculated based on the brightness signal from the signal processing circuit 7 shown in FIG. Target value for power. The controller 2 then provides a control signal to the current value control terminal t1 or the on/off control terminal t2 of the LED driving circuit 3, so as to obtain the optical power of the target value.

The controller 2 of the present embodiment stores in advance information on the forward current value (drive current level) of the LED 4a providing the highest luminous efficiency and the optical power value (predetermined value) obtained from the forward current value.

The controller 2 is based on the comparison result between the optical power value stored as a predetermined value and the set target value, between the optical power control derived from supplying the control signal to the current value control terminal t1 (current value control) and the control derived from supplying the current value. The signal is switched between on/off control terminal t2 (PWM control) for optical power control. This switching will be described in detail below.

In addition, in the first embodiment, corresponding to each target value of the optical power, the ROM of the controller 2 stores information on the driving current value and the on/off ratio of the driving current for obtaining the corresponding optical power.

Specifically, if the controller 2 performs current value control, the controller 2 obtains driving current value information based on the set target value and stored corresponding information, and provides the current value information to the current value of the LED driving circuit 3 Control terminal t1 to control the optical power. When current value control is performed, a control signal for turning on supply of the drive current is applied to the on/off control terminal t2.

In addition, if PWM control is performed, the controller 2 obtains on/off ratio information based on the set target value and stored corresponding information, and based on this information, provides an on/off control signal to the on/off control terminal t2 , thereby controlling the optical power. When PWM control is performed, current value information for keeping the current value constant at the forward current value that provides the highest luminous efficiency of the LED 4a is supplied to the current value control terminal t1.

In this case, such on/off ratio information for PWM control is provided such that the ratio of the length of the on-time period divided by the total length of the on-time period is equal to dividing the target value of the optical power by The value obtained from the optical power value.

FIG. 3 shows the internal structure of the LED driving circuit 3 shown in FIG. 2 .

Since the DC power shown in FIG. 2 is applied to the capacitor Ci, a DC voltage is stored on the capacitor Ci. The switching element Q1, the diode D1, and the choke coil L1 constitute a down converter that uses the voltage across the capacitor Ci as an operating power source. Operation of the buck converter produces an LED drive voltage of the DC voltage across the smoothing capacitor Co. Therefore, a DC drive current is supplied to the LED 4a as shown in FIG. 2 .

As the switching element Q1, a MOS-FET is used.

The switching element Q1 is driven and controlled by the control circuit 12 .

As shown, the control circuit 12 receives an on/off control signal from the on/off control terminal t2, and turns on and off the switching element Q1 based on the on/off control signal.

In addition, a feedback signal from an error amplifier (E/A) 11 is also input to the control circuit 12 .

The current detecting resistor R1 incorporated in the output line of the LED driving voltage detects the level of the driving current supplied to the LED 4a, and inputs the level to the error signal amplifier 11. In addition to the measured current level, the input to the error amplifier 11 is also a current value control signal input to the current value control terminal t1 as shown in FIG. 2 and converted into an analog signal by the D/A converter 10 . The error signal amplifier 11 outputs a signal corresponding to the difference between the input driving current level and the level of the current value control signal.

The control circuit 12 controls the operation of the switching element Q1 based on the output signal from the error signal amplifier 11, thereby performing control such that the level of the driving current supplied to the LED 4a is at a level corresponding to the current value control signal supplied to the current value control terminal t1. value remains constant.

Here, as described above, the luminous efficiency of the LED changes depending on the level of the supplied drive current (forward current). Specifically, as shown in Fig. 9, the luminous efficiency tends to increase with the forward current value until a certain forward current value is reached, and above this value, the luminous efficiency tends to increase with the forward current value increase and decrease.

Since the luminous efficiency changes depending on the forward current value, the above-described current value control and PWM control include the possibility of driving the LED 4a with low luminous efficiency.

Therefore, in this embodiment, referring to the experimental results (similar to the characteristic diagram in FIG. 9 ) regarding the relationship between the forward current value and luminous efficiency of the LED 4a, the forward current value ( optimal current value) and the optical power (predetermined value) obtained from this forward current value. When the LED 4a is driven to emit light with an optical power less than the predetermined value of the optical power, PWM control is used to control the optical power so as to keep the driving current value at an optimum current value. Current value control is performed when the LED 4a is driven to emit light at an optical power equal to or greater than a predetermined value.

Therefore, when the LED 4a is driven to emit light with an optical power less than a predetermined value, the LED 4a can be driven with a constant optimum current value. Therefore, it is possible to drive the LED 4a with the highest luminous efficiency.

In addition, also in the case of driving the LED 4a to emit light with at least a predetermined value of optical power, it is possible to drive the LED 4a with as high a luminous efficiency as possible. Specifically, if PWM control is maintained even for an optical power value equal to or greater than a predetermined value, it is unavoidable to set the constant drive current level of the PWM control higher than that used when performing current value control of the optical power value. level of current. It is apparent from the characteristic diagram of Fig. 9 that an increase in the drive current level reduces the optical power. Therefore, when the LED 4a is driven to emit light at least with a predetermined value of light power, the current value control can drive the LED 4a with higher luminous efficiency.

Therefore, the optical power control of the present embodiment can drive the LED 4a under the condition of constantly providing the highest possible luminous efficiency. Therefore, power consumption due to driving the LED 4a can be minimized. In addition, if the LED 4a can be driven to emit light under the condition of providing as constant high luminous efficiency as possible, the LED 4a itself, the LED drive circuit 3, and the power supply circuit 6 (refer to FIG. 1 ) that provide DC power to the LED drive circuit 3 can be made ) has the least calorific value. Therefore, increasing the size of the device to solve the problem of heat generation is avoided.

FIG. 4 shows a flowchart of processing operations for realizing the operations of the first embodiment.

For example, the processing operations shown in FIG. 4 are executed based on a program stored in a ROM or the like built in the controller 2 .

Referring to FIG. 4 , in step S101 , a process of comparing the set target value with the aforementioned predetermined value is performed.

Next, in step S102, based on the result of the comparison process in step S101, a process of determining whether the target value is smaller than a predetermined value is performed.

If an affirmative determination result that the target value is smaller than the predetermined value is obtained in step S102, the processing sequence proceeds to step S103, where a process of setting PWM control is performed. Specifically, in order to perform on/off control of the drive current as PWM control to keep the drive current value constant at the above-mentioned optimum current value, supply of a current value control signal representing the optimum current value to the LED drive circuit 3 may be started. The current value of the control terminal t1.

Next, in step S104, an on/off ratio is set according to the optical power as a target value. Specifically, on/off ratio information associated with the input target value is taken out from corresponding information stored in a ROM or the like. Then, an on/off control signal based on the on-off ratio information is provided to the on/off control terminal t2, thereby controlling the optical power through PWM control.

In addition, if a negative determination result is obtained in step S102 that the target value is not smaller than the predetermined value (the target value is equal to or greater than the predetermined value), processing of setting current value control is performed in step S105. Specifically, in order to perform current value control to continuously supply the drive current, supply of the ON control signal to the ON/OFF control terminal t2 is started.

Next, in step S106, a current value is set according to the optical power as a target value. Specifically, the current value information associated with the input target value is extracted from the corresponding information. Then, a current value control signal based on the current value information is supplied to the current value control terminal t1, thereby controlling the optical power through the current value control.

Through the above processing operations, optical power control can be performed by PWM control when the target optical power is less than a predetermined value, and can be performed by current value control when the target optical power is equal to or greater than a predetermined value.

FIG. 5 shows the structure of an LED driving device 20 as a second embodiment of the present invention.

The LED driving device 20 of the second embodiment also performs switching of the optical power control method performed in the first embodiment. In addition, the LED driving device 20 includes an optical power sensor 21 in addition to the structure of the LED driving device 1 shown in FIG. 2 . The optical power sensor 21 is arranged at a certain position in the backlight 4, so as to detect the optical power of the LED 4a (the optical power sensor 21 is represented by a dotted line in FIG. 1 ).

Optical power information detected by the optical power sensor 21 is input to the controller 2 .

Based on the optical power value detected and input by the optical power sensor 21, the controller 2 sets the target value of the optical power according to the luminance signal from the signal processing circuit 7 and the instruction input from the user I/F 9 as described above. As a basis, control the light power of LED 4a.

Specifically, the optical power is controlled by changing the current value control signal supplied to the current value control terminal t1 or changing the on/off control signal supplied to the on/off control terminal t2, so that the light detected and input by the optical power sensor 21 The value of the power is equal to the target value.

Since the optical power is controlled on the basis of the actual measured value of the optical power of the LED 4a, even if there is an optical power variation between the LED 4a for the same forward current value, the optical power as the target value can be achieved more accurately. .

This optical power control can effectively prevent variations in the optical power of the LED 4a between devices.

FIG. 6 shows a flowchart of processing operations for realizing the operation of the LED driving device 20 of the second embodiment.

The processing operations shown in FIG. 6 are also executed based on a program stored in a ROM or the like built in the controller 2 .

In steps S201 and S202, as in steps S101 and S102 shown in FIG. 4, a process of comparing the target value with a predetermined value and a process of determining whether the target value is smaller than the predetermined value from the comparison result are performed.

Next, if the target value is smaller than the predetermined value and an affirmative determination result is obtained, as in step S103, the process of setting PWM control in step S203 is performed. Alternatively, if a negative determination result that the target value is not smaller than the predetermined value is obtained, processing of setting current value control is performed in step S205 as in step S105.

In this processing operation, in step S204 after the PWM control has been set, processing of controlling the on/off ratio so that the detection value becomes equal to the target value is performed. Specifically, the ratio of the on/off control signal supplied to the on/off control terminal t2 of the LED drive circuit 3 is controlled so that the set target value is equal to the optical power value from the optical power sensor 21 .

Furthermore, in step S206 after the current value control has been set, a process of controlling the current value so that the detected value becomes equal to the target value is performed. Specifically, control the current value control signal provided to the current value control terminal t1 of the LED driving circuit 3 so that the set target value is equal to the optical power value from the optical power sensor 21 .

Through the processing operations described above, when the present embodiment is implemented, the optical power can be controlled more accurately based on actual measurement values while switching between PWM control and current value control.

It should be noted that the present invention is not limited to the structures of the above-described embodiments.

For example, in the above embodiments, it is the LEDs in the backlight of the liquid crystal display that are driven to emit light. However, the present invention can also be widely applied to optical power control of other LEDs.

In addition, for the convenience of description, this embodiment has a structure of driving one LED to emit light. However, it is clear that similar optical power control for multiple LEDs can achieve similar advantages.

Furthermore, the present embodiment sets the target value of optical power based on the APL information based on the luminance signal extracted from the video signal and the user's operation. However, elements for setting the optical power target value are not limited to these elements. Target values can be set based on other elements.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (8)

1. light emitting diode drive apparatus comprises:

Drive unit is used for the driven for emitting lights led lighting; And

Control device, be used to control the drive current that offers described light emitting diode from described drive unit, if the desired value of the luminous power of described light emitting diode is less than predetermined value, then described control device is recently carried out luminous power control by the on/off of controlling described drive current, if described desired value is equal to or greater than described predetermined value, then described control device is carried out luminous power control by the level of controlling described drive current.

2. light emitting diode drive apparatus according to claim 1, wherein, the luminous power that obtains according to the driving current value by the high-luminous-efficiency that described light emitting diode is provided is provided with described predetermined value.

3. light emitting diode drive apparatus according to claim 1, wherein, in the described luminous power control of recently being carried out by the described on/off of controlling described drive current by described control device, the level of described drive current is held constant at the driving current value of the high-luminous-efficiency that described light emitting diode is provided.

4. light emitting diode drive apparatus according to claim 1, wherein, the storage of described control device relate to described desired value and described on/off than and relate to the corresponding information of described desired value and described driving current value, described control device is carried out described luminous power and is controlled by described on/off ratio relevant with described desired value in described corresponding information or described driving current value are set.

5. light emitting diode drive apparatus according to claim 1 further comprises: optical power detection apparatus, be used to detect the luminous power of described light emitting diode,

Wherein, described control device by control described on/off than or described driving current value carry out described luminous power control, make and detect and the value of the described luminous power of input equals described desired value by described optical power detection apparatus.

6. the light power control method of a light emitting diode, wherein, if the desired value of the luminous power of light emitting diode is less than predetermined value, then recently carry out luminous power control by the on/off of controlling and driving electric current, if and described desired value is equal to or greater than described predetermined value, then carry out luminous power control by the level of controlling described drive current.

7. light emitting diode drive apparatus comprises:

Drive division is used for the driven for emitting lights led lighting; And

Control part, be used to control the drive current that offers described light emitting diode from described drive division, if the desired value of the luminous power of described light emitting diode is less than predetermined value, then described control part is recently carried out luminous power control by the on/off of controlling described drive current, if described desired value is equal to or greater than described predetermined value, then described control part is carried out luminous power control by the level of controlling described drive current.

8. liquid crystal display comprises:

Display panels;

Panel driving portion drives described display panels based on image input signal;

Light emitting diode is as the light source that is used for described display panels;

Drive division is used for the driven for emitting lights led lighting; And

Control part, be used to control the drive current that offers described light emitting diode from described drive division, if the desired value of the luminous power of described light emitting diode is less than predetermined value, then described control part is recently carried out luminous power control by the on/off of controlling described drive current, if described desired value is equal to or greater than described predetermined value, then described control part is carried out luminous power control by the level of controlling described drive current.

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