KR20070046696A - Backlight unit and method of driving the same - Google Patents

Abstract

The present invention relates to an LED backlight and a driving method thereof, and more particularly, to an LED backlight for a liquid crystal display device that can compensate for a luminance difference between LED arrays due to a change in luminance according to the use temperature of the LED and individual characteristics of the LED, and The driving method is related.

Specifically, the present invention compares an LED array between an input terminal and an output terminal to which a driving current is applied, a switching unit interposed between the input terminal and the LED array, a reference current and an output current detected at the output terminal, It provides a LED backlight and a driving method including a comparator for outputting a control signal when the reference current is smaller than the reference current and a control unit for controlling the amount of the drive current by adjusting the opening and closing degree of the switching unit according to the control signal.

As a result, the luminance degradation of the LED array can be sensed and compensated in real time, and the luminance difference between the LED arrays caused by the individual characteristics of the LED can be compensated, thereby greatly improving the color uniformity of the liquid crystal display device.

Description

BACKLIGHT UNIT AND METHOD OF DRIVING THE SAME}

1 is a schematic diagram of a liquid crystal display device using a LED backlight according to the prior art.

2 is a schematic circuit diagram of a LED backlight according to the prior art.

3 is a graph showing a relationship between temperature and luminance of a typical LED.

4 is a schematic circuit diagram of an LED backlight including a feedback circuit according to the present invention.

5 is a flowchart of a process of driving an LED backlight including a feedback circuit according to the present invention.

<Description of the symbols for the main parts of the drawings>

52: input stage 56: output stage

60: LED array 62: LED

70: switching unit 80: comparison unit

82: comparator 84: PWM terminal

85,86: first and second ground terminals

90: control unit R1, R2, R3, R4, R5, R6, R7: first to seventh resistors

C: Capacitor

The present invention relates to an LED back light and a driving method thereof, and more particularly, to a feedback circuit capable of compensating a luminance difference between LED arrays due to a change in luminance according to the use temperature of the LED and individual characteristics of the LED ( The present invention relates to an LED backlight for a liquid crystal display device including a feedback circuit and a driving method thereof.

Liquid crystal display device is an image implementation principle that applies optical anisotropy and polarization properties, which are inherent in liquid crystals. Liquid crystals are well known for their thin and long molecular structure. It has an optical anisotropy with directionality and polarization property when it is placed in an electric field, depending on its size.

In general, a liquid crystal display device is an essential component of a liquid crystal panel bonded to face each other via a liquid crystal layer between two substrates having transparent field generating electrodes formed on opposite sides thereof, and an electric field between the two field generating electrodes is an essential component. Through the artificially adjust the arrangement direction of the liquid crystal molecules to generate a difference in transmittance.

However, since the liquid crystal panel does not have a self-luminous element, a separate dimming means is required to externally express the difference in transmittance. For this purpose, a backlight having a light source is provided on the back of the liquid crystal panel. do.

In general, the backlight may be classified into a side light type and a direct type according to the position of the light source that emits light. The former light metering type is a light source emitted from at least one rear side thereof with respect to the liquid crystal panel. The light is refracted by a separate light guide panel (LGP) to enter the liquid crystal panel, while the latter direct type supplies light by directly placing a plurality of light sources facing the rear of the liquid crystal panel.

Meanwhile, as a light source of a backlight for a general liquid crystal display device, a cold cathode fluorescent lamp (CCFL) or an external electrode fluorescent lamp (EEFL) has been mainly used. In order to achieve the effect of color reproduction and brightness without using (Ag), the use of LED (Light Emitting Diode) is increasing. In this way, the backlight using the LED as a light source is called an LED backlight.

1 is a schematic view of a liquid crystal display device using a general direct type LED backlight 20, including a liquid crystal panel 10 and an LED backlight 20 provided on a rear surface thereof, and a double LED backlight 20 Includes a plurality of printed circuit boards 22 arranged in a stripe at regular intervals and a plurality of LEDs 24 mounted in rows on each of them.

At this time, the plurality of LEDs 24 arranges RGB LEDs emitting R (red), G (green), and B (blue) color light, respectively, according to a predetermined rule, and then lights them all at once to realize white light by color mixing. In a large liquid crystal display, for example, two to ten LEDs are repeatedly arranged on each printed circuit board 22 in units of one array due to power consumption or the like.

In addition, these LED arrays are driven by respective driving circuits. FIG. 2 is a view showing a general LED backlight driving circuit.

As shown, a typical LED backlight driving circuit includes an LED array 30 located between an input terminal 32 and an output terminal 34, a switch unit 40 interposed between the input terminal 32 and the LED array 30, and the switch. It includes a control unit 50 for controlling the operation of the unit 40.

At this time, at least two LEDs are connected in series to the LED array 30, the input voltage Vin is input to the input terminal 32, and the output voltage Vout is output to the output terminal 34. In addition, a kind of transistor such as a field effect transistor (FET) may be used as the switch 40 that selectively transfers the driving voltage Vin of the input terminal 32 to the LED array 30. The control unit 50 controls the on / off of the switch unit 40 through an external signal.

On the other hand, in general LEDs, a sudden change in luminance occurs with temperature. As the light emission time of the LED increases, the internal temperature increases, and the luminance rapidly decreases when the temperature rises excessively. That is, Figure 3 is a graph showing the relationship between the use temperature and the brightness of a typical LED, it can be seen that the output brightness is lower as the temperature of the LED increases, for example, the brightness is 80% when the temperature of the LED is 80 ℃ or more When it falls below and it becomes more than 120 degreeC, LED will no longer drive.

Due to the change in luminance according to the use temperature of the LED, the LED backlight is also accompanied by a sharp decrease in luminance when the use time is long, and thus directly leads to a deterioration in image quality.

In particular, considering that the characteristics of each of the LEDs included in the LED array, for example, turn-on voltage for light emission are different, the luminance of each of the LED array according to the use time even if the same driving circuit is used. The difference between the LEDs and the inherent characteristics of the LEDs is that a sudden decrease in luminance according to the use temperature and a luminance difference between the LED arrays are added, resulting in a significant decrease in the color uniformity of the image displayed externally through the liquid crystal panel.

However, the general LED backlight driving circuit shown in FIG. 2 shows only the on / off control of the driving voltage input to the LED array 30. Currently, the luminance change and the LED array according to the use temperature of the LED backlight are shown. There is no way to overcome the luminance difference between the two.

Accordingly, the present invention has been made to solve the above problems, LED backlight that can prevent the brightness decrease according to the use temperature of the LED, and at least two or more LEDs are compensated for the difference in brightness of each LED array connected in series; The purpose is to provide the driving method.

The backlight according to the present invention includes an input terminal, an output terminal, and an array positioned between the input terminal and the output terminal. A drive signal can be provided at the input. A switching unit may be connected between the input terminal and the array to adjust an amount of the driving signal supplied to the array. Based on the output of the array, the comparator may provide feedback to the controller. The control unit may provide a signal to the switching unit that can change the amount of drive input signal to the array. The method of driving the backlight may include receiving an output current from an array including a plurality of LEDs, comparing the output current with a reference current, and changing a driving current input to the array including the plurality of LEDs. Include.

In order to achieve the above object, in the first aspect of the present invention, an LED array between an input terminal and an output terminal to which a driving current is applied, a switching unit interposed between the input terminal and the LED array, and an output detected by the reference current and the output terminal. Comparing the current, LED comprising a comparison unit for outputting a control signal when the output current is less than the reference current and a control unit for controlling the amount of the drive current by adjusting the opening and closing degree of the switching unit according to the control signal Provide backlight.

In this case, the switching unit comprises a field effect transistor (FET), wherein the FET is spaced apart from the drain electrode and the drain electrode connected to the input terminal, the source electrode to be grounded and the gate electrode connected to the control unit Characterized in that it comprises a. At this time, the control unit is characterized in that for adjusting the gate voltage applied to the gate electrode of the FET in accordance with the control signal.

The comparator may include a comparator, wherein the comparator includes the inverting terminal into which the output current is input, the non-inverting terminal into which the reference current is input, and the control signal by comparing the output current with the reference current. It characterized in that it comprises an output terminal is output.

The LED array includes at least two LEDs connected in series.

In addition, according to the second aspect of the present invention, a method of driving an LED backlight for compensating a luminance change according to a use temperature of an LED array, the method comprising: comparing a reference current with an output current of the LED array and the output current is smaller than the reference current; In this case, it provides a method of driving an LED backlight comprising the step of increasing the amount of driving current input to the LED array.

According to a third aspect of the present invention, an array unit including an input terminal, an output terminal, and an array connected to the input terminal and the output terminal and including a plurality of light emitting diodes (LEDs), the input terminal, a switching unit connected to the array, and the output terminal And a comparison unit connected to a control unit and capable of comparing the output current and the reference current of the output terminal, wherein the control unit controls the switching unit.

The comparison unit may generate a control signal when the output current of the array is different from the reference current, and the array driving current is changed according to the control signal.

The switching unit may be a field effect transistor (FET). The field effect transistor (FET) may include a drain electrode connected to the input terminal and a source electrode spaced apart from the drain electrode and a ground electrode connected to the control unit. It is characterized by including.

The control unit may adjust a gate voltage applied to the gate electrode, and the comparator may include a comparator, and the comparator may include an operational-amplifier (OP-AMP). It is done.

The comparator may further include a second output terminal configured to output a control signal by comparing the inverting terminal receiving the output current, the non-inverting terminal receiving the reference current, and the output current and the reference current.

The array is composed of red, green, blue LED or red, green, blue LED arranged in sequence.

The lower portion of the array is characterized in that it further comprises a printed circuit board (PCB).

The backlight unit may be connected to a liquid crystal panel including first and second substrates facing each other and a liquid crystal layer interposed between the first and second substrates.

In a fourth aspect of the present invention, the method includes receiving an output current from an array of a plurality of LEDs, comparing the output current with a reference current, and changing a driving input current applied to the array of the plurality of LEDs. It is characterized by including.

The changing of the driving input current may include changing when the output current is different from the reference current, and changing the driving input current may include changing the driving input current in real-time. Varying the current.

Changing the drive input current comprises supplying a regulated drive input current to the array comprising a plurality of LEDs.

The reference current is applied to each comparison unit.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

4 is a circuit diagram of the LED backlight driving circuit according to the present invention, wherein the LED array 60 between the input terminal 52 and the output terminal 56 to which a driving current of a predetermined width is applied, and the input terminal 52 and A switching unit 70 interposed between the LED arrays 60, a comparison unit 80 for comparing a preset reference current with an output current output from the LED array 60 and outputting a predetermined control signal P; The controller 90 controls the width of the driving current applied to the LED array 60 by controlling the operation of the switching unit 70 based on the control signal P.

More specifically, first, the LED array 60 may include at least two series-connected LEDs 62, for example, in the case of using RGB LEDs that emit RGB color light, respectively, the LED array 60 is an RGBGR LED. It may be made of. In addition, the driving voltage Vin of the LED array 60 is applied through the input terminal 52. The driving voltage Vin is equal to or higher than a turn-on voltage of the LED array 60 and is constant within a range that does not damage the LED 62. Can be represented. And output stage 56 may be grounded.

Next, the switching unit 70 interposed between the input terminal 52 and the LED array 60 controls the driving voltage Vin input to the LED array 60 on / off and at the same time, is applied to the LED array 60. The width of the current can be adjusted, for which a transistor can be used. In this case, assuming that the transistor of the switching unit 70 is n-type, the gate electrode G connected to the controller 90, the drain electrode D connected to the input terminal 52, and the drain electrode D A source electrode S spaced apart from and grounded, and selectively drains the drain electrode D and the source electrode S to the LED array 60 by a gate voltage Vg transmitted from the controller 90. The input driving voltage Vin may be controlled on / off and the width of the driving current applied to the LED array 60 may be adjusted according to the magnitude of the gate voltage Vg.

A comparator 80 is provided between the output terminal 56 of the LED array 60 and the controller 90, which is a reference current by a pulse width modulation (PWM) voltage and the output terminal 56 of the LED array 60. Compares the output current detected by and outputs a predetermined control signal P to the controller 90 when the output current is equal to or less than the reference current. To this end, an operational amplifier (OP-AMP) may be used as the comparator 82 in the comparator 80. An inverting terminal into which the output current of the LED array 60 is input, and a non-inverting in which the reference current is input. Terminal and an output terminal for outputting a control signal P when the reference current is greater than the output current through comparison of these output currents and the reference current.

Finally, the controller 90 controls the on / off of the switch unit 70 through various signals input from the outside, such as a liquid crystal display driving system. In particular, the control unit 80 controls the control signal ( When P) is delivered, the switching current of the switching unit 70 is controlled through the gate voltage Vg to increase the driving current input to the LED array 60.

The operating principle of the LED backlight driving circuit according to the present invention described above is characterized by compensating for luminance by using the output current of the LED array 60. The temperature of the LED array 60 is increased due to prolonged use. In this case, the luminance is lowered, and accordingly, the magnitude of the output current detected by the output stage 56 is also lowered.

Therefore, the present invention compensates the brightness of the LED array 60 by applying the relationship between the temperature and the output current of the LED array 60, the output current of the LED array 60 through the comparator 80 of a certain size As a result of comparison with the reference current, if the output current of the LED array 60 is higher than the reference current, it means that the brightness of the LED did not appear, but vice versa, that is, the output current of the LED array 60 Lower than the reference current means that the brightness of the LED is reduced.

Therefore, in the latter case, the comparator 80 transmits the control signal P to the controller 90, and the controller 90 increases the gate voltage Vg of the transistor of the switching unit 70 to increase the LED array. As the width of the driving current delivered to 60 is increased, the brightness of the LED array 60 is increased again.

This process is performed in real time during the driving of the LED array 60, thereby keeping the brightness of the LED array 60 constant.

As a result of using the LED backlight driving circuit according to the present invention, it was possible to reduce the amount of change in the output current, which was changed by more than 25% according to the operating temperature of the LED array 60 to less than 3%, especially 20% or more. It has been found to reduce the output current variation between different LED arrays 60 to less than 5%.

On the other hand, the preferred configuration of the comparator described briefly above in more detail, the first resistor (R1) interposed between the output terminal 56 of the LED array 60 and the inverting terminal of the comparator 82, and the first A capacitor C interposed between the resistor R1 and the output terminal of the comparator 82, a second resistor R2 interposed between the PWM terminal 84 to which the PWM voltage is applied and the non-inverting terminal of the comparator 82; And a third resistor R3 interposed between the non-inverting terminal of the comparator 82 and the first ground terminal 85, and a fourth resistor R4 interposed between the output terminal of the comparator 82 and the controller 90. The second ground terminal 86 drawn between the fourth resistor R4 and the controller 90 may include a fifth resistor R5 interposed therebetween.

In addition, a sixth resistor R6 interposed between the input terminal and the switching unit, and a seventh resistor R7 interposed at the output ground terminal which grounds the output terminal 56 after the connection point with the first resistor R1. As can be seen, they each support the smooth operation of the LED backlight driving circuit according to the present invention.

5 is a flowchart of a process of driving a backlight unit including a feedback circuit according to the present invention.

In a first step 500, the feedback circuit receives an output signal from an array comprising a plurality of LEDs. The output signal may be an output charge. The output charge may vary in relation to the temperature of one or more LEDs included in the array.

In a second step 502, the feedback circuit compares a given output signal with a reference signal. The reference signal may be a reference charge. The reference charge may be generated by the pulse width of the PWM circuit. Comparators such as OP-AMP can be used to compare the output signal with the reference signal. The comparison between the output signal and the reference signal may be made in real time. The comparison determines that when the amplitude / magnitude of the output signal is smaller than the amplitude / magnitude of the reference signal, the feedback circuit generates a drive signal supplied to the array to be changed. Pass a control signal.

In a third step 504, the switch receives a signal from the controller and changes the amount of drive signal supplied to the array. For example, the array portion may be increased, and the driving charge may increase driving charge supplied to the array.

In the present invention, by applying the same reference current for each array, it is possible to supply the same constant current regardless of the inter-array characteristics. In other words, in the present invention, reference currents for multiple arrays may be used with one reference current to prevent deviation between arrays.

However, the present invention is not limited to the above embodiments and can be practiced in various ways without departing from the spirit of the present invention.

According to the LED backlight and the driving method thereof according to the present invention described above, has an effect capable of detecting and compensating the luminance degradation of the LED array in real time, and the luminance degradation and the image quality deterioration according to the use temperature of the LED array There is an advantage to prevent.

In particular, according to the LED backlight and the driving method according to the present invention, it is determined that the lowering of the brightness of the LED array as the output current of the LED array, and thus there is an unnecessary advantage, such as a separate sensor for detecting the brightness of the LED array. In this regard, it is possible to reliably detect and compensate luminance through a simpler configuration and lower cost, and the LED backlight according to the present invention can compensate for the luminance difference between the LED arrays. The effect is greatly improved.

Claims (26)

LED array between input terminal and output terminal to which driving current is applied A switching unit interposed between the input terminal and the LED array; A comparison unit comparing a reference current with an output current detected at the output terminal and outputting a control signal when the output current is smaller than the reference current; Control unit for controlling the amount of the drive current by adjusting the opening and closing degree of the switching unit in accordance with the control signal LED backlight comprising a. The method of claim 1, The switching unit includes a field effect transistor (FET). The method of claim 2, The FET is, A drain electrode connected to the input terminal; A source electrode spaced apart from the drain electrode and grounded; A gate electrode connected to the control unit LED backlight comprising a. The method of claim 3, wherein The control unit LED backlight for adjusting the gate voltage applied to the gate electrode of the FET according to the control signal. The method of claim 1, The comparator comprises a comparator LED backlight. The method of claim 5, The comparator, An inverting terminal to which the output current is input A non-inverting terminal to which the reference current is input An output terminal through which the control signal is output by comparing the output current with the reference current LED backlight comprising a. The method of claim 1, And the LED array comprises at least two LEDs connected in series. As a method of driving an LED backlight to compensate for a change in luminance according to the temperature of use of the LED array Comparing a reference current with an output current of the LED array; When the output current is less than the reference current, increasing the amount of driving current input to the LED array; Driving method of the LED backlight comprising a. An array unit including an input terminal, an output terminal, and an array connected to the input terminal and the output terminal and including a plurality of light emitting diodes (LEDs); A switching unit connected to the input terminal and the array; A comparator connected to the output terminal and the control unit and capable of comparing an output current and a reference current of the output terminal And the control unit controls the switching unit. The method of claim 9, And the comparing unit is capable of generating a control signal when the output current of the array is different from the reference current. The method of claim 10, And an array driving current is changed according to the control signal. The method of claim 9, And the switching unit is a field effect transistor (FET). The method of claim 12, The field effect transistor (FET), A drain electrode connected to the input terminal; A source electrode spaced apart from the drain electrode and grounded; A gate electrode connected to the control unit Backlight unit comprising a. The method of claim 13, And the control unit adjusts a gate voltage applied to the gate electrode. The method of claim 9, And the comparator comprises a comparator. The method of claim 15, The comparator comprises an operational-amplifier (OP-AMP). The method of claim 15, The comparator, An inverting terminal receiving the output current A non-inverting terminal receiving the reference current A second output stage for outputting a control signal by comparing the output current with a reference current; The backlight unit further comprises. The method of claim 9, The array is a backlight unit, characterized in that consisting of red, green, blue LED. The method of claim 9, And the array comprises red, green, and blue LEDs arranged in order. The method of claim 9, And a printed circuit board (PCB) below the array. The method of claim 9, The backlight unit may be connected to a liquid crystal panel including first and second substrates facing each other and a liquid crystal layer interposed between the first and second substrates. Receiving output current from an array of multiple LEDs Comparing the output current with a reference current Varying a driving input current applied to the array of the plurality of LEDs Method of driving a backlight unit comprising a. The method of claim 22, The changing of the driving input current includes changing the output current when the output current is different from the reference current. The method of claim 23, The changing of the driving input current comprises changing the driving input current in real-time. The method of claim 23, The changing of the driving input current includes supplying a regulated driving input current to the array including a plurality of LEDs. The method of claim 22, And the reference current is applied to each comparison unit.

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