Patent Publication Number: US-11380269-B2

Title: Display device and method of driving the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. patent application Ser. No. 16/800,122 filed Feb. 25, 2020, which claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2019-0051765, filed on May 2, 2019 in the Korean Intellectual Property Office (KIPO), the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     Exemplary embodiments of the inventive concept relate to a display device and a method of driving the display device. 
     DISCUSSION OF RELATED ART 
     Recently, various flat panel display devices with reduced weight and volume, as compared to conventional cathode ray tube (CRT) display devices, have been developed. Such flat panel display devices include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP), and organic light emitting displays (OLED). 
     Generally, a display device may include a display panel and a display panel driver. Here, when an image displayed on the display panel is a still image or when the display panel operates in an always-on mode (AOD), power consumption of the display device may be reduced by decreasing a driving frequency. However, when the driving frequency is decreased, flicker may be visible. 
     SUMMARY 
     According to an exemplary embodiment of the inventive concept, a display device may include a display panel including a plurality of pixels and a display panel driver configured to drive the display panel. The display panel driver may be configured to receive input image data, to drive the display panel at a first driving frequency when the input image data corresponds to a moving image, and to select one of a plurality of flicker lookup tables based on the first driving frequency and drive the display panel at a second driving frequency based on the selected flicker lookup table when the input image data corresponds to a still image. 
     In an exemplary embodiment of the inventive concept, each of the plurality of flicker lookup tables may store flicker values respectively corresponding to grayscales of the input image data that is driven at the first driving frequency and store the second driving frequency that is changed according to the flicker values. 
     In an exemplary embodiment of the inventive concept, the first driving frequency may be higher than the second driving frequency. 
     In an exemplary embodiment of the inventive concept, the display panel driver may include an image determiner configured to receive the input image data to determine whether the input image data corresponds to the moving image or the still image, a storage unit configured to store the plurality of flicker lookup tables, a selector configured to select one of the plurality of flicker lookup tables based on the first driving frequency, and a frequency determiner configured to determine the second driving frequency based on the selected flicker lookup table. 
     In an exemplary embodiment of the inventive concept, the selector may select one of the plurality of flicker lookup tables according to the first driving frequency. 
     In an exemplary embodiment of the inventive concept, the selector may select one of the plurality of flicker lookup tables according to a frequency band in which the first driving frequency is included. 
     In an exemplary embodiment of the inventive concept, the selector may receive the first driving frequency from an external device. 
     In an exemplary embodiment of the inventive concept, the selector may calculate the first driving frequency based on the input image data. 
     In an exemplary embodiment of the inventive concept, the selector may calculate the first driving frequency by counting a reference clock signal that is input during an active period of a vertical synchronization signal. 
     In an exemplary embodiment of the inventive concept, the display panel driver may store the plurality of flicker lookup tables during a manufacturing process of the display device. 
     In an exemplary embodiment of the inventive concept, the display panel driver may store a reference flicker lookup table corresponding to a reference driving frequency during a manufacturing process of the display device and may generate the plurality of flicker lookup tables by using a conversion coefficient for converting the reference flicker lookup table. 
     According to an exemplary embodiment of the inventive concept, a method of driving a display device may include receiving input image data, determining whether the input image data corresponds to a moving image or a still image, driving a display panel at a first driving frequency when the input image data corresponds to the moving image, selecting one of a plurality of flicker lookup tables based on the first driving frequency and determining a second driving frequency based on the selected flicker lookup table when the input image data corresponds to the still image, and driving the display panel at the second driving frequency when the input image data corresponds to the still image. 
     In an exemplary embodiment of the inventive concept, each of the plurality of flicker lookup tables may store flicker values respectively corresponding to grayscales of the input image data that is driven at the first driving frequency and store the second driving frequency that is changed according to the flicker values. 
     In an exemplary embodiment of the inventive concept, the first driving frequency may be higher than the second driving frequency. 
     In an exemplary embodiment of the inventive concept, the selected flicker lookup table may be selected according to the first driving frequency. 
     In an exemplary embodiment of the inventive concept, the selected flicker lookup table may be selected according to a frequency band in which the first driving frequency is included. 
     In an exemplary embodiment of the inventive concept, the first driving frequency may be provided from an external device. 
     In an exemplary embodiment of the inventive concept, the first driving frequency may be calculated based on the input image data. 
     In an exemplary embodiment of the inventive concept, the first driving frequency may be calculated by counting a reference clock signal that is input during an active period of a vertical synchronization signal. 
     In an exemplary embodiment of the inventive concept, the plurality of flicker lookup tables may be generated by using a conversion coefficient for converting a reference flicker lookup table corresponding to a reference driving frequency. 
     According to an exemplary embodiment of the inventive concept, a method of driving a display device may include receiving input image data, determining that the input image data corresponds to a still image, looking up an intermediary second driving frequency in a reference flicker lookup table according to a first driving frequency; looking up a conversion coefficient in a conversion coefficient lookup table according to the first driving frequency; determining a second driving frequency by multiplying the intermediary second driving frequency by the conversion coefficient; and driving the display panel at the second driving frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the inventive concept will be better understood by describing in detail exemplary embodiments thereof with reference to the accompanying drawings. 
         FIG. 1  is a block diagram illustrating a display device according to an exemplary embodiment of the inventive concept. 
         FIG. 2  is a circuit diagram illustrating a pixel included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept. 
         FIG. 3  is a diagram for describing an operation of a display panel driver included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept. 
         FIG. 4  is a block diagram illustrating a display panel driver included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept. 
         FIGS. 5A to 5C  are diagrams illustrating flicker lookup tables included in the display panel driver of  FIG. 4  according to exemplary embodiments of the inventive concept. 
         FIG. 6  is a flowchart illustrating an operation of a display panel driver included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept. 
         FIG. 7  is a flowchart illustrating an operation of a display panel driver included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept. 
         FIG. 8  is a flowchart illustrating a method of driving a display device according to an exemplary embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the inventive concept provide a display device that can reduce power consumption of a display panel and can improve display quality. 
     Exemplary embodiments of the inventive concept also provide a method of driving a display device that can reduce power consumption of a display panel and can improve display quality. 
     Hereinafter, exemplary embodiments of the inventive concept will be explained in detail with reference to the accompanying drawings. Like reference numerals may refer to like elements throughout this application. 
       FIG. 1  is a block diagram illustrating a display device according to an exemplary embodiment of the inventive concept, and  FIG. 2  is a circuit diagram illustrating a pixel included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept. 
     Referring to  FIG. 1 , a display device  100  may include a display panel  110  and a display panel driver  120 . 
     The display panel  110  may include data lines DL, gate lines GL, and pixels PX. The gate lines GL may extend in a first direction D 1  and may be arranged in a second direction D 2  perpendicular to the first direction D 1 . The data lines DL may extend in the second direction D 2  and may be arranged in the first direction D 1 . The first direction D 1  may be substantially parallel to a long side of the display panel  110 , and the second direction D 2  may be substantially parallel to a short side of the display panel  110 . Each of the pixels PX may be formed in an area where the data lines DL intersect the gate lines GL. 
     Referring to  FIG. 2 , each of the pixels PX may include a first-type switching element and a second-type switching element different from the first-type switching element. For example, the first-type switching element may be a polysilicon thin film transistor. For example, the first-type switching element may be a low temperature polysilicon (LTPS) thin film transistor. For example, the second-type switching element may be an oxide thin film transistor. For example, the first-type switching element may be a P-type transistor, and the second-type switching element may be an N-type transistor. 
     For example, data write gate signals GWP and GWN may include a first data write gate signal GWP and a second data write gate signal GWN. The first data write gate signal GWP may be applied to the P-type transistor and may have a low-level activation signal at a data write timing. The second data write gate signal GWN may be applied to the N-type transistor and may have a high-level activation signal at the data write timing. 
     Each of the pixels PX may include first to seventh switching elements T 1 , T 2 , T 3 , T 4 , T 5 , T 6 , and T 7 , a storage capacitor CST, and an organic light emitting diode OLED. 
     The first switching element T 1  may include a gate electrode connected to a first node N 1 , a first electrode connected to a second node N 2 , and a second electrode connected to a third node N 3 . For example, the first switching element T 1  may be a polysilicon thin film transistor. The first switching element T 1  may be a P-type thin film transistor. The first electrode of the first switching element T 1  may be a source electrode, and the second electrode of the first switching element T 1  may be a drain electrode. 
     The second switching element T 2  may include a gate electrode to which the first data write gate signal GWP is applied, a first electrode to which a data voltage Vd is applied, and a second electrode connected to the second node N 2 . For example, the second switching element T 2  may be a polysilicon thin film transistor. The second switching element T 2  may be a P-type thin film transistor. The first electrode of the second switching element T 2  may be a source electrode, and the second electrode of the second switching element T 2  may be a drain electrode. 
     The third switching element T 3  may include a gate electrode to which the second data write gate signal GWN is applied, a first electrode connected to the first node N 1 , and a second electrode connected to the third node N 3 . For example, the third switching element T 3  may be an oxide thin film transistor. The third switching element T 3  may be an N-type thin film transistor. The first electrode of the third switching element T 3  may be a source electrode, and the second electrode of the third switching element T 3  may be a drain electrode. 
     The fourth switching element T 4  may include a gate electrode to which a data initialization gate signal GI is applied, a first electrode to which an initialization voltage VI is applied, and a second electrode connected to the first node N 1 . For example, the fourth switching element T 4  may be an oxide thin film transistor. The fourth switching element T 4  may be an N-type thin film transistor. The first electrode of the fourth switching element T 4  may be a source electrode, and the second electrode of the fourth switching element T 4  may be a drain electrode. 
     The fifth switching element T 5  may include a gate electrode to which an emission control signal EM is applied, a first electrode to which a high power supply voltage ELVDD is applied, and a second electrode connected to the second node N 2 . For example, the fifth switching element T 5  may be a polysilicon thin film transistor. The fifth switching element T 5  may be a P-type thin film transistor. The first electrode of the fifth switching element T 5  may be a source electrode, and the second electrode of the fifth switching element T 5  may be a drain electrode. 
     The sixth switching element T 6  may include a gate electrode to which the emission control signal EM is applied, a first electrode connected to the third node N 3 , and a second electrode connected to an anode of the organic light emitting diode OLED. For example, the sixth switching element T 6  may be a polysilicon thin film transistor. The sixth switching element T 6  may be a P-type thin film transistor. The first electrode of the sixth switching element T 6  may be a source electrode, and the second electrode of the sixth switching element T 6  may be a drain electrode. 
     The seventh switching element T 7  may include a gate electrode to which an organic light emitting diode initialization gate signal GB is applied, a first electrode to which the initialization voltage VI is applied, and a second electrode connected to the anode of the organic light emitting diode OLED. For example, the seventh switching element T 7  may be an oxide thin film transistor. The seventh switching element T 7  may be an N-type thin film transistor. The first electrode of the seventh switching element T 7  may be a source electrode, and the second electrode of the seventh switching element T 7  may be a drain electrode. 
     The storage capacitor CST may include a first electrode to which the high power supply voltage ELVDD is applied and a second electrode connected to the first node N 1 . 
     The organic light emitting diode OLED may include the anode and a cathode to which a low power supply voltage ELVSS is applied. 
     Although the pixel PX including the first-type switching element and the second-type switching element is described with reference to  FIG. 2 , the pixel PX included in the display panel  110  of  FIG. 1  is not limited thereto. For example, the pixel PX included in the display panel  110  of  FIG. 1  may include first to seventh first-type switching elements and capacitors or may include first to seventh second-type switching elements and capacitors. 
     The display panel driver  120  may generate a signal for driving the display panel  110  to supply the generated signal to the display panel  110 . The display panel driver  120  may receive input image data IMG, may drive the display panel  110  at a first driving frequency when the input image data IMG corresponds to (or is for) a moving image, may select one of a plurality of flicker lookup tables based on the first driving frequency when the input image data IMG corresponds to (or is for) a still image, and may drive the display panel  110  at a second driving frequency based on the selected flicker lookup table when the input image data IMG corresponds to the still image. In this case, the first driving frequency may be a high frequency, and the second driving frequency may be a low frequency. In other words, the first driving frequency may be higher than the second driving frequency. In detail, the display panel driver  120  may include a driving controller  130 , a gate driver  140 , and a data driver  150 . 
     The driving controller  130  may receive the input image data IMG and an input control signal CON from an external device. For example, the input image data IMG may include red image data, green image data, and blue image data. For example, the input image data IMG may include white image data. For example, the input image data IMG may include magenta image data, yellow image data, and cyan image data. The input control signal CON may include a master clock signal and a data enable signal. The input control signal CON may further include a vertical synchronization signal and a horizontal synchronization signal. 
     The driving controller  130  may generate a gate control signal CTL_G, a data control signal CTL_D, and an input data signal IDATA based on the input image data IMG and the input control signal CON. The driving controller  130  may generate the gate control signal CTL_G for controlling an operation of the gate driver  140  based on the input control signal CON and output the generated gate control signal CTL_G to the gate driver  140 . The gate control signal CTL_G may include a vertical start signal and a gate clock signal. The driving controller  130  may generate the data control signal CTL_D for controlling an operation of the data driver  150  based on the input control signal CON. The data control signal CTL_D may include a horizontal start signal and a load signal. The driving controller  130  may generate the input data signal IDATA based on the input image data IMG. The driving controller  130  may output the input data signal IDATA to the data driver  150 . 
     The gate driver  140  may generate gate signals GATE in response to the gate control signal CTL_G received from the driving controller  130 . The gate driver  140  may output the gate signals GATE to the pixels PX connected to the gate lines GL. 
     The data driver  150  may generate an analog data voltage Vdata based on the data control signal CTL_D and the input data signal IDATA received from the driving controller  130 . The data driver  150  may output the analog data voltage Vdata to the pixels PX connected to the data lines DL. 
       FIG. 3  is a diagram for describing an operation of a display panel driver included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept,  FIG. 4  is a block diagram illustrating a display panel driver included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept, and  FIGS. 5A to 5C  are diagrams illustrating flicker lookup tables included in the display panel driver of  FIG. 4  according to exemplary embodiments of the inventive concept. 
     Referring to  FIG. 3 , the display panel driver may drive the display panel at the first driving frequency when the input image data corresponds to a moving image and may drive the display panel at the second driving frequency when the input image data corresponds to a still image. In this case, the second driving frequency may be determined based on the flicker lookup table selected based on the first driving frequency. 
     Referring to  FIG. 4 , a display panel driver  200  may include an image determiner  210 , a storage unit  220 , a selector  230 , and a frequency determiner  240 . The display panel driver  200  of  FIG. 4  may correspond to the display panel driver  120  of  FIG. 1 . For example, the image determiner  210 , the storage unit  220 , the selector  230 , and the frequency determiner  240  may be included in the display panel driver  120  of  FIG. 1 . 
     The image determiner  210  may receive the input image data IMG and may determine whether the input image data IMG corresponds to a moving image or a still image. When the input image data IMG corresponds to the still image, the image determiner  210  may output a still image determination signal SD. 
     The storage unit  220  may store a plurality of flicker lookup tables FLUT. Each of the flicker lookup tables FLUT may store respective flicker values corresponding to grayscales (or gray-levels) of the input image data IMG driven at the first driving frequency and store the second driving frequency changed according to the flicker value. The flicker value may indicate a degree of flicker occurring in each of the grayscales, and the second driving frequency may be a lowest frequency at which the flicker is not viewed or visible. The flicker value and the second driving frequency corresponding to the flicker value may be determined by an external evaluation, and the flicker lookup tables FLUT may be stored in the storage unit  220  during a manufacturing process of the display device. 
     Referring to  FIG. 5A , the storage unit  220  may include a first flicker lookup table  221  and a second flicker lookup table  222 . Each of the first flicker lookup table  221  and the second flicker lookup table  222  may store respective flicker values FLICKER VALUE corresponding to grayscales GRAYSCALE of the input image data IMG driven at different first driving frequencies and a second driving frequency 2ND DF changed according to the flicker value. For example, the first flicker lookup table  221  may store flicker values respectively corresponding to the grayscales of the input image data IMG at the first driving frequency of 60 Hz and the second driving frequency 2ND DF, and the second flicker lookup table  222  may store flicker values respectively corresponding to the grayscales of the input image data IMG at the first driving frequency of 120 Hz and the second driving frequency 2ND DF. 
     Referring to  FIG. 5B , the storage unit  220  may include a first flicker lookup table  223  and a second flicker lookup table  224 . Each of the first flicker lookup table  223  and the second flicker lookup table  224  may store respective flicker values corresponding to the grayscales of the input image data IMG driven at first driving frequencies within different frequency bands and the second driving frequency 2ND DF changed according to the flicker value. For example, the first flicker lookup table  223  may store respective flicker values corresponding to the grayscales of the input image data IMG at the first driving frequency which is greater than or equal to 60 Hz and less than 90 Hz and the second driving frequency 2ND DF, and the second flicker lookup table  224  may store respective flicker values corresponding to the grayscales of the input image data IMG at the first driving frequency which is greater than or equal to 90 Hz and less than 120 Hz and the second driving frequency 2ND DF. 
     Although the storage unit  220  configured to store the first flicker lookup tables  221  and  223  and the second flicker lookup tables  222  and  224  is described with reference to  FIGS. 5A and 5B , the storage unit  220  may further store flicker lookup tables in addition to the first flicker lookup tables  221  and  223  and the second flicker lookup tables  222  and  224 . 
     Referring to  FIG. 5C , the storage unit  220  may include a reference flicker lookup table  225  and a conversion coefficient lookup table  226 . The reference flicker lookup table  225  may store respective flicker values corresponding to the grayscales of the input image data IMG driven at a reference frequency and the second driving frequency 2ND DF changed according to the flicker value. For example, the reference flicker lookup table  225  may store respective flicker values corresponding to the grayscales of the input image data IMG at the reference frequency of 60 Hz and the second driving frequency 2ND DF. The conversion coefficient lookup table  226  may include conversion coefficients CC for converting the reference flicker lookup table  225  according to a first driving frequency 1ST DF. The storage unit  220  may generate a plurality of flicker lookup tables based on the reference flicker lookup table  225  and the conversion coefficient lookup table  226 . 
     For example, the storage unit  220  may generate a second flicker lookup table  227  by multiplying the second driving frequency 2ND DF of the reference flicker lookup table  225  by the conversion coefficient CC. For example, when the reference flicker lookup table  225  stores the flicker values corresponding to the grayscales of the input image data IMG at the reference frequency of 60 Hz and the second driving frequency 2ND DF and when the first driving frequency 1ST DF of the input image data IMG is 120 Hz, the flicker value of the second flicker lookup table  227  may be two times the reference flicker lookup table  225 , and the second driving frequency 2ND DF may be generated according to the flicker value. However, in this case, the second driving frequency 2ND DF may be set too high, so that a power consumption reduction effect obtained by low-frequency driving may be reduced. 
     Therefore, the second flicker lookup table  227  that stores the second driving frequencies 2ND DF respectively corresponding to the grayscales of the input image data IMG at 120 Hz may be generated by multiplying the second driving frequency 2ND DF (e.g., an intermediary second driving frequency), which is increased by double according to the flicker value, by the conversion coefficient CC of 0.5 included in the conversion coefficient lookup table  226 . 
     Referring back to  FIG. 4 , the selector  230  may select one of the flicker lookup tables FLUT based on the first driving frequency. According to an exemplary embodiment of the inventive concept, the selector  230  may receive the first driving frequency of the input image data IMG from the external device. According to an exemplary embodiment of the inventive concept, the selector  230  may calculate the first driving frequency based on the input image data IMG. In this case, the selector  230  may calculate the first driving frequency by counting a reference clock signal which is input during an active period of the vertical synchronization signal. The selector  230  may select one of the flicker lookup tables FLUT stored in the storage unit  220  according to the first driving frequency. For example, the selector  230  may select one of the first flicker lookup tables  221  and  223  or the reference flicker lookup table  225  shown in  FIGS. 5A to 5C  when the first driving frequency is 60 Hz, and the selector  230  may select one of the second flicker lookup tables  222 ,  224 , and  227  shown in  FIGS. 5A to 5C  when the first driving frequency is 120 Hz. The selector  230  may output a flicker lookup table SFLUT, selected based on the first driving frequency, to the frequency determiner  240 . 
     The frequency determiner  240  may determine the second driving frequency 2ND DF based on the flicker lookup table SFLUT selected by the selector  230 . For example, when the first driving frequency is 60 Hz, the selector  230  may select one of the first flicker lookup tables  221  and  223  or the reference flicker lookup table  225  shown in  FIGS. 5A to 5C  to supply the selected flicker lookup table to the frequency determiner  240 . 
     The frequency determiner  240  may determine the second driving frequencies 2ND DF respectively corresponding to the grayscales of the input image data IMG based on one of the first flicker lookup tables  221  and  223  or the reference flicker lookup table  225  shown in  FIGS. 5A to 5C . For example, when the input image data IMG has fifteen grayscales, the frequency determiner  240  may determine the second driving frequency 2ND DF as 30 Hz based on one of the first flicker lookup tables  221  and  223  or the reference flicker lookup table  225  shown in  FIGS. 5A to 5C . 
     For example, when the first driving frequency is 120 Hz, the selector  230  may select one of the second flicker lookup tables  222 ,  224 , and  227  shown in  FIGS. 5A to 5C  to supply the selected flicker lookup table to the frequency determiner  240 . The frequency determiner  240  may determine the second driving frequencies 2ND DF respectively corresponding to the grayscales of the input image data IMG based on one of the second flicker lookup tables  222 ,  224 , and  227  shown in  FIGS. 5A to 5C . For example, when the input image data IMG has the fifteen grayscales, the frequency determiner  240  may determine the second driving frequency 2ND as 30 Hz based on one of the second flicker lookup tables  222 ,  224 , and  227  shown in  FIGS. 5A to 5C . 
     As described above, the display panel driver  200  of the display device may store the flicker lookup tables FLUT, may select one of the flicker lookup tables FLUT according to the first driving frequency of the input image data IMG, and may determine the second driving frequency 2ND DF based on the selected flicker lookup table SFLUT, so that the still image can be displayed at an optimal low frequency. Therefore, power consumption of the display device can be reduced, and display quality can be improved. 
       FIG. 6  is a flowchart illustrating an operation of a display panel driver included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept. 
     Referring to  FIG. 6 , the display panel driver may receive input image data (S 100 ). The display panel driver may determine whether the input image data corresponds to a moving image or a still image (S 110 ). When the input image data corresponds to the moving image (e.g., does not correspond to the still image), the display panel driver may drive the display panel at the first driving frequency (S 120 ). When the input image data corresponds to the still image, the display panel driver may determine whether the first driving frequency of the input image data is the same as a first frequency (S 130 ). 
     When the first driving frequency of the input image data is the same as the first frequency, the display panel driver may select the first flicker lookup table (S 140 ). When the first driving frequency of the input image data is not the same as the first frequency, the display panel driver may determine whether the first driving frequency of the input image data is the same as a second frequency (S 150 ). When the first driving frequency of the input image data is the same as the second frequency, the display panel driver may select the second flicker lookup table (S 160 ). Although a case where the display panel driver includes the first flicker lookup table and the second flicker lookup table is described with reference to  FIG. 6 , when the display panel driver includes more flicker lookup tables, the display panel driver may compare the first driving frequency of the input image data with frequencies of the flicker lookup tables to select the flicker lookup table having a frequency equal to the first driving frequency. 
       FIG. 7  is a flowchart illustrating an operation of a display panel driver included in the display device of  FIG. 1  according to an exemplary embodiment of the inventive concept. 
     Referring to  FIG. 7 , the display panel driver may receive input image data (S 200 ). The display panel driver may determine whether the input image data corresponds to a moving image or a still image (S 210 ). When the input image data corresponds to the moving image (e.g., does not correspond to the still image), the display panel driver may drive the display panel at the first driving frequency (S 220 ). When the input image data corresponds to the still image, the display panel driver may determine whether the first driving frequency of the input image data is less than or equal to the first frequency (S 230 ). 
     When the first driving frequency of the input image data is less than or equal to the first frequency, the display panel driver may select the first flicker lookup table (S 240 ). When the first driving frequency of the input image data is greater than the first frequency, the display panel driver may determine whether the first driving frequency of the input image data is less than or equal to the second frequency (S 250 ). When the first driving frequency of the input image data is greater than the first frequency and less than or equal to the second frequency, the display panel driver may select the second flicker lookup table (S 260 ). Although a case where the display panel driver includes the first flicker lookup table and the second flicker lookup table is described with reference to  FIG. 7 , when the display panel driver includes more flicker lookup tables, the display panel driver may compare the first driving frequency of the input image data with frequency bands of the flicker lookup tables to select the flicker lookup table having a frequency band in which the first driving frequency is included. 
       FIG. 8  is a flowchart illustrating a method of driving a display device according to an exemplary embodiment of the inventive concept. 
     Referring to  FIG. 8 , operations of the method of  FIG. 8  may receive input image data (S 300 ), may determine whether the input image data corresponds to a moving image or a still image (S 310 ), may drive a display panel at a first driving frequency when the input image data corresponds to the moving image (S 320 ), may select a flicker lookup table and determine a second driving frequency based on the flicker lookup table when the input image data corresponds to the still image (S 330 ), and may drive the display panel at the second driving frequency when the input image data corresponds to the still image (S 340 ). 
     For example, in operation S 300 , the display panel driver of the display device may receive the input image data from the external device. 
     In operation S 310 , the display panel driver of the display device may determine whether the input image data corresponds to the moving image or the still image to output a still image determination signal when the input image data corresponds to the still image. 
     In operation S 320 , the display panel driver of the display device may drive the display panel at the first driving frequency when the input image data corresponds to the moving image. In this case, the first driving frequency may be a high frequency. 
     In operation S 330 , the display panel driver of the display device may select one of a plurality of flicker lookup tables based on the first driving frequency and determine the second driving frequency based on the selected flicker lookup table when the input image data corresponds to the still image. The display panel driver of the display device may store the flicker lookup tables. Each of the flicker lookup tables may store the flicker values corresponding to the grayscales of the input image data driven at the first driving frequency and the second driving frequency changed according to the flicker value. The flicker value may indicate the degree of flicker occurring in each of the grayscales, and the second driving frequency may be the lowest frequency at which flicker is not viewed. The flicker value and the second driving frequency corresponding to the flicker value may be determined by an external evaluation, and the flicker lookup tables may be stored in the storage unit during the manufacturing process of the display device. 
     In an exemplary embodiment of the inventive concept, the display panel driver of the display device may store a plurality of flicker lookup tables corresponding to the first driving frequency. In an exemplary embodiment of the inventive concept, the display panel driver of the display device may store a plurality of flicker lookup tables corresponding to a frequency band in which the first driving frequency is included. In an exemplary embodiment of the inventive concept, the display panel driver of the display device may include the reference flicker lookup table and the conversion coefficient lookup table and may generate a plurality of flicker lookup tables by selecting a conversion coefficient according to the first driving frequency and performing a calculation by using the reference flicker lookup table and the conversion coefficient. 
     The display panel driver of the display device may select one of the flicker lookup tables based on the first driving frequency. In an exemplary embodiment of the inventive concept, the first driving frequency may be input from the external device. In an exemplary embodiment of the inventive concept, the first driving frequency may be calculated based on the input image data. For example, the first driving frequency may be calculated by counting the reference clock signal which is input during the active period of the vertical synchronization signal. The display panel driver of the display device may select one of the flicker lookup tables stored in the storage unit according to the first driving frequency. The display panel driver may determine the second driving frequencies respectively corresponding to the grayscales of the input image data based on the selected flicker lookup table. 
     In operation S 340 , the display panel driver of the display device may drive the display panel at the second driving frequency when the input image data corresponds to the still image. In this case, the second driving frequency may be a low frequency. 
     As described above, according to the method of  FIG. 8 , a plurality of flicker lookup tables are stored, one of the flicker lookup tables is selected according to the first driving frequency of the input image data, and the second driving frequency is determined based on the selected flicker lookup table, so that the still image can be displayed at an optimal low frequency. Therefore, power consumption of the display device can be reduced, and display quality can be improved. 
     The inventive concept may be applied to any electronic device including a display device. For example, the inventive concept may be applied to a television, a computer monitor, a laptop, a digital camera, a cellular phone, a smart phone, a smart pad, a tablet personal computer (PC), a personal digital assistant (PDA), a portable multimedia player (PMP), an MP 3  player, a car navigation system, a video phone, a head mounted display (HMD) device, etc. 
     As described above, a display device and a method of driving a display device according to exemplary embodiments of the inventive concept may display a still image at an optimal low frequency by storing a plurality of flicker lookup tables, by selecting one of the flicker lookup tables according to a first driving frequency of input image data, and by determining a second driving frequency based on the selected flicker lookup table. Thus, power consumption of the display device may be reduced, and display quality may be improved. 
     While the inventive concept has been shown and described with reference to exemplary embodiments thereof, it will be apparent to those of ordinary skill in the art that various modifications in form and details may be made thereto without departing from the spirit and scope of the inventive concept as set forth by the appended claims.