Patent Publication Number: US-11030962-B2

Title: Backlight unit and display device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2018-0169352, filed on Dec. 26, 2018, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     Technical Field 
     Embodiments relate to a backlight unit and a display device. 
     Discussion of the Related Art 
     With the development of the information society, there has been increasing demand for a variety of image display devices. In this regard, a range of display devices, such as liquid crystal display (LCD) devices and organic light-emitting diode (OLED) display devices, have recently come into widespread use. 
     Such display devices include a display panel, in which a plurality of subpixels are arrayed. An image can be displayed by controlling levels of luminance of the plurality of subpixels. 
     In addition, to reduce power consumption, the display device may be driven in a low-speed driving mode, depending on an image displayed thereon. For example, the display device may be driven in the low-speed mode in an always on display (AoD) mode in which specific information is only disposed on a portion of the area of the display panel. 
     As the display device is driven at a low display driving frequency, the length of a data retaining period may be increased in a one-frame period. Thus, a degree by which luminance is reduced may be increased in the one-frame period. In addition, due to increases in deviations in luminance among frames, flicker may be observed. Accordingly, there may be a number of difficulties in realization of the low-speed driving mode. 
     SUMMARY 
     Accordingly, embodiments of the present disclosure are directed to a backlight unit and a display device that substantially obviate one or more of the problems due to limitations and disadvantages of the related art. 
     An aspect of the present disclosure is to provide a display device able to prevent flicker from occurring in an image displayed on a display panel while the display device is being driven in a low-speed driving mode. 
     An aspect of the present disclosure is to provide a backlight unit and a driving method thereof able to prevent flicker from occurring in a low-speed driving mode. 
     Additional features and aspects will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the inventive concepts provided herein. Other features and aspects of the inventive concepts may be realized and attained by the structure particularly pointed out in the written description, or derivable therefrom, and the claims hereof as well as the appended drawings. 
     To achieve these and other aspects of the inventive concepts, as embodied and broadly described, a display device may comprise: a display panel; a backlight unit including a plurality of light sources to supply light to the display panel; and a driver circuit driving the plurality of light sources. During a period in which the display panel is driven in a low driving mode, the driver circuit may output at least one first light source driving signal and at least one second light source driving signal in a one-frame period. At least one of a frequency, a duty, an amplitude, or combinations thereof, of the first light source driving signal, may be different from that of the second light source driving signal. 
     In another aspect, a display device may comprise: a display panel; a backlight unit supplying light to the display panel; and a plurality of light sources provided in the backlight unit. A driving frequency, a ratio of emission time, a level of emission luminance, and emission start timing of at least one light source among the plurality of light sources may be constant in a first driving mode. At least one of the driving frequency, the ratio of emission time, the level of emission luminance, and the emission start timing, or combinations thereof, may be variable in a second driving mode. 
     In another aspect, a backlight unit may comprise a plurality of light sources. A driving frequency, a ratio of emission time, a level of emission luminance, and emission start timing of at least one light source among the plurality of light sources may be constant in a first driving mode. At least one of the driving frequency, the ratio of emission time, the level of emission luminance, and the emission start timing, or combinations thereof, may be variable in a second driving mode 
     According to exemplary embodiments, a light source driving signal for driving light sources of a backlight unit is variable in a low-speed driving mode, so that a frequency component causing flicker to be observed, among frequency components of luminance appearing in a one-frame period, can be reduced. 
     Accordingly, it is possible to prevent flicker from being visible in the low-speed mode, and reduce power consumption by driving in the low-speed mode, thereby improving the efficiency of the display device. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the inventive concepts as claimed. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this application, illustrate embodiments of the disclosure and together with the description serve to explain various principles. In the drawings: 
         FIG. 1  illustrates a schematic configuration of a display device according to embodiments; 
         FIGS. 2A and 2B  illustrate examples of changes in luminance in a normal driving mode and a low-speed driving mode of the display device according to embodiments. 
         FIGS. 3 and 4  illustrate an example of a method of reducing flicker by performing frequency component analysis on luminance appearing in a one-frame period of the low-speed driving mode in the display device according to embodiments; 
         FIGS. 5A to 5D  illustrate examples of the backlight driving signal in a normal mode and a low-speed driving mode of the display device according to embodiments; 
         FIG. 6  illustrates an example of a configuration of generating and outputting the backlight driving signal in the display device according to embodiments; 
         FIGS. 7 to 10  illustrate examples of the backlight driving signal in a case in which the backlight unit according to embodiments is an edge-lit backlight unit; and 
         FIGS. 11 and 12  illustrate an example of the backlight driving signal in a case in which the backlight unit according to embodiments is a direct-lit backlight unit. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, some embodiments of the present disclosure will be described in detail with reference to the accompanying illustrative drawings. In designating elements of the drawings by reference numerals, the same elements will be designated by the same reference numerals although they are shown in different drawings. Further, in the following description of the present disclosure, a detailed description of known functions and configurations incorporated herein will be omitted in the situation in which the subject matter of the present disclosure may be rendered rather unclear thereby. 
     In addition, terms, such as first, second, A, B, (a), (b) or the like may be used herein when describing components of the present disclosure. Each of these terminologies is not used to define an essence, order or sequence of a corresponding component but used merely to distinguish the corresponding component from other component(s). In the case that it is described that a certain structural element “is connected to”, “is coupled to”, or “is in contact with” another structural element, it should be interpreted that another structural element may “be connected to”, “be coupled to”, or “be in contact with” the structural elements as well as that the certain structural element is directly connected to or is in direct contact with another structural element. 
       FIG. 1  illustrates a schematic configuration of a display device  100  according to embodiments. 
     Referring to  FIG. 1 , the display device  100  according to embodiments may include a display panel  110  comprised of an active area A/A and a non-active area N/A, as well as components for driving the display panel  110 , such as a gate driver circuit  120 , a data driver circuit  130 , and a controller  140 . 
     In the display panel  110 , a plurality of gate lines GL and a plurality of data lines DL are disposed, and a plurality of subpixels SP are arrayed in areas in which the gate lines GL cross the data lines DL. 
     The gate driver circuit  120  is controlled by the controller  140 , and controls the driving timing of the plurality of subpixels SP by sequentially outputting a scan signal to the plurality of gate lines GL disposed in the display panel  110 . 
     The gate driver circuit  120  may include one or more gate driver integrated circuits (GDICs), and may be disposed on one or both sides of the display panel  110  depending on the driving system. 
     Each of the GDICs may be connected to a bonding pad of the display panel  110  by a tape-automated bonding (TAB) method or a chip-on-glass (COG) method, may be directly mounted on the display panel  110 , or in some cases, may be integrated with the display panel  110 . In addition, each of the GDICs may be implemented using a chip-on-film (COF) structure mounted on a film connected to the display panel  110 . 
     The data driver circuit  130  receives image data from the controller  140 , and converts the image data into an analog data voltage. In addition, the data driver circuit  130  outputs the data voltage to the data lines DL in the timing in which the scan signal is applied through the gate lines GL, so that each of the subpixels SP expresses a luminance level according to the image data. 
     The data driver circuit  130  may include one or more source driver integrated circuits (SDICs). 
     Each of the SDICs may include a shift register, a latch circuit, a digital-to-analog converter (DAC), an output buffer, etc. 
     Each of the SDICs may be connected to a bonding pad of the display panel  110  by a tape-automated bonding (TAB) method or a chip-on-glass (COG) method, may be directly mounted on the display panel  110 , or in some cases, may be integrated with the display panel  110 . In addition, each of the SDICs may be implemented using a chip-on-film (COF) structure. In this case, the SDIC may be mounted on a film connected to the display panel  110 , and may be electrically connected to the display panel  110  via electrical connections on the film. 
     The controller  140  supplies a variety of control signals to the gate driver circuit  120  and the data driver circuit  130 , and controls the operations of the gate driver circuit  120  and the data driver circuit  130 . 
     The controller  140  may be mounted on a printed circuit board (PCB), a flexible PCB, or the like, and may be electrically connected to the gate driver circuit  120  and the data driver circuit  130  via the PCB, the flexible PCB, or the like. 
     The controller  140  controls the gate driver circuit  120  to output the scan signal in timing realized in each frame, converts image data, received from an external source, into a data signal format readable by the data driver circuit  130 , and outputs the converted image data to the data driver circuit  130 . 
     The controller  140  receives a variety of timing signals, including a vertical synchronization signal VSYNC, a horizontal synchronization signal HSYNC, an input data enable signal DE, a clock signal CLK, and the like, from an external source (e.g. a host system). 
     The controller  140  can generate a variety of control signals using the variety of timing signals received from the external source, and output the variety of control signals to the gate driver circuit  120  and the data driver circuit  130 . 
     For example, the controller  140  outputs a variety of gate control signals GCS, including a gate start pulse signal GSP, a gate shift clock signal GSC, a gate output enable signal GOE, and the like, to control the gate driver circuit  120 . 
     Here, the gate start pulse signal is used to control the operation start timing of one or more GDICs of the gate driver circuit  120 . The gate shift clock signal GSC is a clock signal commonly input to the one or more GDICs to control the shift timing of the scan signal. The gate output enable signal GOE designates timing information of the one or more GDICs. 
     In addition, the controller  140  outputs a variety of data control signals DCS, including a source start pulse signal SSP, a source sampling clock signal SSC, a source output enable signal SOE, and the like, to control the data driver circuit  130 . 
     Here, the source start pulse signal SSP is used to control the data sampling start timing of one or more SDICs of the data driver circuit  130 . The source sampling clock signal SSC is a clock signal controlling the sampling timing of data in each of the SDICs. The source output enable signal SOE controls the output timing of the data driver circuit  130 . 
     The touch display device  100  may further include a power management IC supplying various forms of voltage or current to the display panel  110 , the gate driver circuit  120 , the data driver circuit  130 , and the like, or controls various forms of voltage or current to be supplied to the same. 
     The plurality of subpixels SP are defined by crossing of the gate lines GL and the data lines DL. Liquid crystal cells or light-emitting elements may be disposed in the subpixels SP, depending on the type of the touch display device  100 . 
     For example, in a case in which the display device  100  is a liquid crystal display (LCD) device, a light source device, such as a backlight unit, illuminating the display panel  110 , is provided, and liquid crystal cells are disposed in the subpixels SP thereof. In addition, the LCD display device  100  can express levels of luminance and display an image depending on image data by adjusting the alignment of liquid crystal cells using electromagnetic fields generated in response to the data voltage applied to the subpixels SP. 
     Here, the backlight unit may be disposed below the display panel  110  to supply light to the display panel  110 . The backlight unit is driven by a pulse-width modulated (PWM) signal to reduce power consumption, so that light sources provided in the backlight unit may be repeatedly turned on or off at regular intervals. 
     In addition, the display device  100  may be driven in a normal mode and a low-speed mode to reduce power consumption thereof. 
       FIGS. 2A and 2B  illustrate examples of changes in luminance in a normal driving mode and a low-speed driving mode of the display device  100  according to embodiments. 
     Referring to  FIG. 2A , the display device  100  may be driven at a display driving frequency of, for example, 60 Hz, in the normal mode (or a first driving mode). Thus, during a frame period corresponding to 60 Hz according to a synchronization signal SYNC of 60 Hz, a data voltage Vdata can be supplied to each of the subpixels SP disposed in the display panel  110 . 
     Here, a case in which the polarity of the data voltage Vdata is reversed in every frame in order to prevent liquid crystal cells from deteriorating due to offsetting of liquid crystal cells is illustrated in  FIG. 2A . 
     In addition, each of the subpixels SP expresses a luminance level corresponding to a difference between a pixel voltage Vpxl according to the data voltage Vdata and a voltage of a common electrode Vcom, so that that the display panel  110  can display an image. 
     Here, although the pixel voltage Vpxl applied to the pixel electrode may be reduced during a one-frame period, a reduction in luminance due to the reduced pixel voltage Vpxl may not be observed, due to the short frame period. 
     In contrast, in a case in which the display device  100  is driven in the low-speed driving mode, the reduction of luminance may be observed, due to the increased one-frame period. 
     Referring to  FIG. 2 b   , the display device  100  may be driven at a display driving frequency in the range of, for example, 1 Hz to 30 Hz, in the low-speed driving mode (or a second driving mode). Thus, the data voltage Vdata may be supplied to each of the subpixels SP during a frame period corresponding to the frequency from 1 Hz to 30 Hz. 
     Since the one-frame period is increased in the low-speed driving mode, the degree by which the pixel voltage Vpxl is reduced may be increased in the one-frame period. 
     Accordingly, the reduction of luminance due to the reduced pixel voltage Vpxl may be increased. Due to deviations in luminance between frames, caused by the reduced luminance, flicker may be observed in the low-speed driving mode. 
     In addition, in luminance expressed by an image in which flicker is observed in the low-speed driving mode, a specific frequency component may have a significant amplitude. 
     That is, although flicker may appear to be observable in the low-speed driving mode as luminance is reduced during the one-frame period, flicker may also be observed depending on the frequency components of luminance appearing during the one-frame period. 
     Embodiments propose a method able to prevent flicker from being visible in the low-speed driving mode by frequency component analysis of luminance appearing during a one-frame period in the low-speed driving mode. 
       FIGS. 3 and 4  illustrate an example of a method of reducing flicker by performing frequency component analysis on luminance appearing in a one-frame period of the low-speed driving mode in the display device  100  according to embodiments. 
     Referring to  FIG. 3 , the display device  100  can be driven at a display driving frequency, e.g. 20 Hz, in the low-speed driving mode. Thus, the one-frame period may be longer than a frame period of the normal driving mode, thereby increasing the degree by which luminance is reduced during the one-frame period. 
     Here, when luminance appearing in the one-frame period in the low-speed driving mode is analyzed by performing frequency component analysis on an output signal measured using a photodiode, frequency components may appear in specific bandwidths, e.g. 20 Hz, 40 Hz, and 60 Hz. In particular, the frequency component appearing in 20 Hz bandwidth, i.e. a display driving frequency bandwidth, has the highest amplitude. 
     Here, when a backlight driving signal for supplying light to the display panel  110 , i.e. a signal for on-off controlling of a light source of the backlight unit, is adjusted, a frequency component of 20 Hz bandwidth, among frequency components of luminance appearing in the one-frame period, can be reduced. 
     For example, frequency components of luminance appearing in a one-frame period can be adjusted by varying at least one of a frequency, a duty (or a pulse width), an amplitude, a delay time, or combinations thereof, of the backlight driving signal during the corresponding frame period. 
     That is, it is possible to change frequency components of luminance produced by the display panel  110  in the low-speed driving mode by varying at least one of a driving frequency, a ratio of emission time, a level of emission luminance, emission start timing, or combinations thereof, of the light source of the backlight unit in the low-speed driving mode. 
     Described in detail with reference to  FIG. 4 , a modulated signal, i.e. a luminance waveform appearing in response to the backlight driving signal, can be obtained, on the basis of a sawtooth pulse or waveform similar to luminance appearing in the frame period of the low-speed driving mode and the backlight driving signal. 
     Here, the backlight driving signal may be an aperiodic signal having different duties. The backlight driving signal can be divided into signals having the same cycles. Then, on the basis of the divided backlight driving signal and the waveform similar to luminance appearing in the frame period of the low-speed driving mode, the modulated signal according to the divided backlight driving signal can be obtained. 
     Such modulated signals can be subjected to Fourier transform, thereby producing frequency components having different amplitudes and periods. 
     That is, since the driving signal of the backlight unit have different delay times D 1 , D 2 , D 3 , D 4 , and D 5  or different duties W 1 , W 2 , W 3 , W 4 , and W 5 , different frequency components of luminance can be caused by the driving signal of the backlight unit. 
     In addition, when the modulated signals having different frequency components are combined, a specific frequency component can be canceled. 
     Accordingly, it is possible to adjust frequency components of luminance appearing during a one-frame period of the low-power mode by varying the duty or the like of the backlight driving signal during the one-frame period. 
     In addition, since it is possible to vary the backlight driving signal so that a specific frequency component is reduced, the specific component can be avoided in luminance appearing in the low-speed driving mode. Accordingly, flicker according to the specific frequency component can be prevented from being visible. 
     That is, it is possible to adjust frequency components of luminance appearing in the one-frame period of the low-speed driving mode by varying at least one of the frequency, duty, amplitude, delay time, or combinations thereof, of the driving signal supplied to the light source of the backlight frame during the one-frame period of the low-speed driving mode. 
       FIGS. 5A to 5D  illustrate examples of the backlight driving signal in a normal mode and a low-speed driving mode of the display device  100  according to embodiments. 
     Referring to  FIG. 5A , in the normal driving mode, all of the frequency W, duty W, amplitude A, and delay time D of the backlight driving signal can be constant or uniform. In the normal driving mode, the frequency of the backlight driving signal may be in a kHz bandwidth. 
     That is, in the normal driving mode, the light source of the backlight unit can be turned on and off in the same cycles during a one-frame period. In addition, in each of the cycles, the ratio of emission time can be constant, and the level of emission luminance can be constant. In addition, emission start timing in each of frame periods can be constant. 
     As described above, in the normal driving mode, the backlight driving signal is applied uniformly, so that the light source of the backlight unit can be periodically driven to express a constant level of luminance. That is, in the normal driving mode, the duty of the backlight driving signal can be determined to be constant in order to reduce power consumption or adjust overall levels of luminance. 
     In addition, referring to  FIG. 5B , in the low-speed driving mode, at least one of a frequency, a duty, an amplitude, or combinations thereof, of the backlight driving signal, can be varied in a one-frame period. 
     For example, in a first subframe (or section) period of the one-frame period, the backlight driving signal may have a frequency F 1 , a pulse width W 1 , and an amplitude A 1 . 
     In addition, in a second subframe (or section) period, the backlight driving signal may have a frequency F 2 , a pulse width W 2 , and an amplitude A 2 . The driving signal, supplied in the second subframe period, may differ from the driving signal supplied in the first subframe period. 
     In addition, a frequency F 3 , a pulse width W 3 , and an amplitude A 3  of the driving signal, supplied in a third subframe period, may differ from those of the driving signal supplied in either the first subframe period or the second subframe period. 
     In addition, as illustrated in  FIG. 5B , the delay time in each of the frame periods before the supply of the driving signal can be varied. 
     That is, the delay time D 1  in the first frame period may differ from the delay time D 2  in the second frame period. 
     Alternatively, as illustrated in  FIG. 5C , a plurality of driving signal having the same frequency, duty, and amplitude can be applied in some of the subframe periods of the one-frame period, as required. 
     Accordingly, the driving signal may be supplied different number of times according to the subframe periods. 
     Here, in the low-speed driving mode, the frequency of the backlight driving signal may be in a Hz bandwidth. That is, the frequency of the backlight driving signal in the low-speed driving mode may be lower than the frequency of the backlight driving signal in the normal driving mode. 
     In the low-speed driving mode, the backlight driving signal may be varied to be supplied in every subframe period of the one-frame period by a variety of methods in addition to the above-described examples. Accordingly, the driving frequency, ratio of emission time, level of emission luminance, and the like, of the light source of the backlight unit may be different according to the subframe periods. In addition, emission start timing may be different according to the frame periods. 
     Since the duty of the driving signal is variable whenever supplied in the subframe periods, luminance can have different frequency components according to the first subframe period, the second subframe period, and the third subframe period, in which the luminance appears. In addition, the driving signal can be supplied such that a frequency component causing flicker to be observed, among frequency components in luminance appearing in each of the subframe periods, is canceled. 
     As described above, the frequency component analysis, performed on luminance appearing during the one-frame period in the low-speed driving mode, can vary the backlight driving signal so that a frequency component causing flicker to be visible can be reduced. Accordingly, this can prevent or reduce flicker from being visible in the low-speed driving mode. 
     In addition, since flicker is not visible in the low-speed driving mode, it is possible to reduce power consumption using the low-speed driving mode and drive the display device  100  at a lower display driving frequency, thereby improving the efficiency of the display device  100 . 
       FIG. 6  illustrates an example of a configuration of generating and outputting the backlight driving signal in the display device according to embodiments. 
     Referring to  FIG. 6 , a calculation circuit  300  receives image data Image (Pattern) from a host system  200 . 
     The calculation circuit  300  generates light source control data PWM Data to control the driving of the backlight unit, on the basis of at least one of the image data received from the host system  200 , a display driving frequency of the low-speed driving mode, or a combination thereof. 
     The light source control data PWM Data may include, for example, a frequency (or a subframe period), a duty (or a pulse width), an amplitude, and a delay time of the backlight driving signal. 
     For example, the calculation circuit  300  can generate and output the light source control data PWM Data by referring to a look-up table in which the driving frequency of the low-speed driving mode or the light source control data PWM Data according to the image data is stored. 
     The calculation circuit  300  may be the controller  140  of the display device  100 , may be provided in the form of a chip within the controller  140 , or may be a micro controller unit MCU provided separately from the controller  140 , by way of example. 
     The light source control data PWM Data, generated by the calculation circuit  300 , is transmitted to a driver circuit  400 . 
     The driver circuit  400  outputs a light source driving signal PWM to drive the light source  500  of the backlight unit, on the basis of the light source control data PWM Data received from the calculation circuit  300 . 
     The light source driving signal PWM may be a signal, the frequency, duty, amplitude, and delay time of which are determined according to the light source control data PWM Data. 
     That is, a light source driving signal PWM, of which at least one of the frequency, the duty, the amplitude, or combinations thereof, is variable, can be output by the driver circuit  400  in the one-frame period. In addition, the light source driving signal PWM having different delay times according to the frame periods may be output. 
     The driver circuit  400  may be a light source integrated circuit (IC, e.g. LED IC) for driving the light source  500 , a power management integrated circuit (PMIC), or a pulse-width modulation integrated circuit (PWM IC). In addition, the driver circuit  400  may be connected to a printed circuit of the backlight unit or may be disposed in a printed circuit connected to the display panel  110 . 
     As described above, due to the calculation circuit  300  and the driver circuit  400 , the light source driving signal PWM for driving the light source  500  of the backlight unit in the low-speed driving mode can be applied in a varied state during a one-frame period. 
     Accordingly, in the low-speed driving mode, the driving frequency, the ratio of emission time, the level of emission luminance, the emission start timing, and the like, of the light source  500  can be varied, so that a frequency component causing flicker to be observed can be avoided from frequency components of luminance appearing in the low-speed driving mode. Accordingly, this can prevent or reduce flicker in the low-speed driving mode. 
       FIGS. 7 to 10  illustrate examples of the backlight driving signal in a case in which the backlight unit according to embodiments is an edge-lit backlight unit. 
     Referring to  FIGS. 7 and 8 , a number of light sources  500  may be disposed on at least one side surface of the backlight unit.  FIG. 7  illustrates an example in which the light sources  500  are arranged in a direction the same as a direction of scanning of the display panel  110 , while  FIG. 8  illustrates an example in which the light sources  500  are arranged in a direction intersecting the direction of scanning of the display panel  110 . 
     The light sources  500  may be grouped into light source arrays  610 ,  620 , and  630 , each of which is comprised of a plurality of light sources  500 . 
     For example, a first light source  510 , a second light source  520 , a third light source  530 , and a fourth light source  540  may be disposed in each of the light source arrays  610 ,  620 , and  630 . 
     In addition, the first light source  510  can be driven by the light source driving signal PWM supplied through a first channel, while the second light source  520 , the third light source  530 , and the fourth light source  540  can be driven by the light source driving signal PWM supplied through a second channel, a third channel, and a fourth channel, respectively. 
     In the light source driving signal PWM supplied through each of the channels in the low-speed driving mode, at least one of a frequency, a duty, an amplitude, a delay time, or combinations thereof, can be varied. 
     That is, in the one-frame period, the length of the subframe in which the light source driving signal PWM is applied, the ratio of the period in the subframe, in which the light source  500  is turned on, or the voltage level, at which the light source  500  is turned on, can be varied. In addition, a point in time in the frame period, at which the light source  500  starts to emit light, can be varied. 
     For example, as in Case 1 and Case 2, the duty of the light source driving signal PWM in a first subframe SF 1  may be A %. In addition, in a second subframe SF 2  and a third subframe SF 3 , the duties of the light source driving signal PWM may be B % and Co, respectively. 
     In addition, the light source driving signal PWM may have different amplitudes in the subframes, respectively. 
     Although each of the subframes is illustrated as having the fixed length by way of example, the length of the subframe may be varied. In addition, the delay time before the supply of the light source driving signal PWM may be different according to the subframe periods. 
     In addition, in a specific subframe period, the light source driving signal PWM having the same duty and the same amplitude may be supplied a plurality of times. 
     In the edge-lit backlight unit, the channels may simultaneously supply the light source driving signal PWM as in Case 1, or may sequentially supply the light source driving signal PWM as in Case 2. 
     As described above, the duty, amplitude, and the like, of the light source driving signal PWM, supplied in the subframe periods of the one-frame period in the low-speed driving mode, are adjusted. This can cancel the frequency component causing flicker to occur, so that no flicker may be visible in the low-speed driving mode. 
     In addition, although the light sources  500  driven through the respective channels may be arranged in the light source arrays  610 ,  620 , and  630 , respectively, as in the foregoing example, the light sources  500  arranged in the same light source array  610 ,  620 , or  630  may be driven through the same channel. 
     Referring to  FIGS. 9 and 10 , the first light source  510 , the second light source  520 , the third light source  530 , and the fourth light source  540 , arranged in the light source array  610 , can be driven by the light source driving signal PWM supplied through the first channel. In addition, the light sources  500  arranged in the light source arrays  620  and  630  can be driven by the light source driving signal PWM supplied through the second channel and the third channel, respectively. 
     In addition, the channels may supply the light source driving signal PWM in the same timing or may sequentially supply the light source driving signal PWM in shifted timing. 
     That is, although flicker may be created depending on the display driving frequency of the low-speed driving mode, flicker may be visible depending on the image displayed by the display panel  110 . 
     Accordingly, the light sources  500  emitting light to a first area Area 1  can be driven through the first channel while the light sources  500  emitting light to second and third areas Area 2  and Area 3  can be driven through the second and third channels, respectively, so that the light source driving signal PWM can be easily adjusted according to the input image. Here, the first area Area 1 , the second area Area 2 , and the third area Area 3 , to which light is emitted from one or more light sources  500 , may be configured to have different numbers, sizes, and shapes, according to information regarding the input image. For example, in a case in which light having an input image divided into five areas should be controlled according to the respective areas, the area to which light is emitted from one or more light sources  500  can be set to be five areas having different sizes. In addition, the areas, such as the first area Area 1 , the second area Area 2 , and the third area Area 3 , to which light is emitted from one or more light sources  500 , may be set to be fixed or different in every frame or predetermined frames of the display. For example, in a case in which the input image is an image without a change for 30 frames, the set area can be set to be fixed without a change for 30 frames. In addition, in a case in which the input image is an image that changes by at least a predetermined reference level in every 10 frames, the set area can be changed for every 10th frame. 
     The light source driving signal PWM supplied through the respective channels can be independently varied. For example, as illustrated in  FIG. 10 , the duty of the light source driving signal PWM supplied through the first channel can be varied to be A %, B %, and Co, and the duty of the light source driving signal PWM supplied through the second channel can be varied to be D %, E %, and F %. In addition, the duty of the light source driving signal PWM supplied through the third channel can be varied to be G %, H %, and I %. 
     In addition, the light source driving signal PWM supplied through the respective channels may have different amplitudes according to the corresponding subframe periods. The corresponding subframe periods may have different lengths. 
     As described above, the adjacent light sources  500  can be driven through the same channel, and the light source driving signal PWM can have different duties, amplitudes, and the like, according to the channels, through which the light source driving signal PWM is supplied. Accordingly, it is possible to improve the ability to prevent or reduce flicker by adjusting the light source driving signal PWM in the edge-lit backlight unit. Accordingly, it is possible to prevent or reduce flicker in the edge-lit backlight unit by adjusting the light source driving signal PWM. 
     In addition, in some cases, the channels through which the light sources  500  of the edge-lit backlight unit are controlled can be independently driven, so that the light source driving signal PWM for preventing or reducing flicker can be more easily adjusted. 
       FIGS. 11 and 12  illustrate an example of the backlight driving signal in a case in which the backlight unit according to embodiments is a direct-lit backlight unit. 
     Referring to  FIG. 11 , a number of light sources  500  may be disposed on one surface of the backlight unit facing the display panel  110 . That is, the light sources  500  can supply light to the display panel  110  in a direction perpendicular to the display panel  110 . 
     While the display device  100  is being driven in the low-speed driving mode, the light source driving signal PWM, of which at least one of the frequency, the duty, the amplitude, or combinations thereof, is variable, can be output during the one-frame period. 
     For example, as illustrated in  FIG. 11 , the light source driving signal PWM having a duty A % can be supplied in the first subframe SF 1 , while the light source driving signal PWM having a duty B % and the light source driving signal PWM having a duty Co can be supplied in the second subframe SF 2  and the third subframe SF 3 , respectively. 
     In addition, the amplitude of the light source driving signal PWM, supplied in each of the subframe periods, can be varied. 
     In some cases, the length of the subframe period can be varied, or the delay time before the supply of the light source driving signal PWM, in each of the frame periods, can be varied. 
     Here, since the light sources  500  of the direct-lit backlight unit are disposed on the surface facing the display panel  110 , some of the light sources  500 , disposed in a specific area, can be driven by the same light source driving signal PWM, depending on the image pattern. 
     For example, as illustrated in  FIG. 12 , light sources  500  disposed in a fourth area Area 4  can be driven by the light source driving signal PWM having duties A %, B %, and Co in respective subframes. In addition, light sources  500  disposed in a fifth area Area 5  can be driven by the light source driving signal PWM having duties D %, E %, and F % in respective subframes. Here, areas, such as the fourth area Area 4  and the fifth area Area 5 , to which light is emitted from one or more light sources  500 , may be configured to have different numbers, sizes, and shapes, according to information regarding an input image. For example, in a case in which light having an input image divided into five areas should be controlled according to the respective areas, the area to which light is emitted from one or more light sources  500  can be set to be five areas having different sizes. In addition, the areas, such as the fourth area Area 4  and the fifth area Area 5 , to which light is emitted from one or more light sources  500 , may be set to be fixed or different in every frame or predetermined frames of the display. For example, in a case in which the input image is an image without a change for 30 frames, the set area can be set to be fixed without a change for 30 frames. In addition, in a case in which the input image is an image that changes by at least a predetermined reference level in every 10 frames, the set area can be changed for every 10th frame. 
     As set forth above, the light source driving signal PWM for driving the light sources  500  to emit in respective areas can be independently controlled. Accordingly, it is possible to easily adjust the light source driving signal PWM in order to cancel a frequency component causing flicker to be visible, according to the image pattern displayed in the low-speed driving mode. 
     In addition, since the respective light sources  500  in the direct-lit backlight unit can be driven independently, the control operation for avoiding a frequency component causing flicker to be visible can be easily performed by adjusting at least one of the frequency, duty, amplitude, delay time, or combinations thereof, of the light source driving signal (PWM). 
     According to embodiments of the present disclosure as set forth above, it is possible to reduce a frequency component that causes flicker, among frequency components of luminance occurring during a frame period, by varying and supplying the backlight driving signal in the low-speed mode of the display device  100 . 
     The frequency component causing flicker to be visible can be reduced, thereby preventing flicker from being visible in the low-speed mode. 
     Accordingly, the display device  100  can be driven in the low-speed driving mode to reduce power consumption. 
     In addition, since flicker may be prevented by frequency component analysis, the display device  100  can be driven at a lower display driving frequency, the frame length of which is longer, so that the driving efficiency of the display device  100  can be improved. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the backlight unit and the display device of the present disclosure without departing from the technical idea or scope of the disclosure. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.