Patent Publication Number: US-9905189-B2

Title: Liquid crystal display and common voltage compensation driving method thereof

Description:
This application claims the benefit of Korean Patent Application No. 10-2014-0188915, filed on Dec. 24, 2014, which is incorporated herein by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     Field 
     The present disclosure relates to a liquid crystal display and a driving method thereof. 
     Description of the Related Art 
     As information technology has advanced, the market of display devices as mediums for connecting users with information has grown. In line with this, the use of flat panel displays (FPDs) such as liquid crystal displays (LCDs), organic light emitting display devices, and plasma display panels (PDPs) has increased. Among them, LCDs, capable of implementing high resolution and both reductions and increases in size, have been widely used. 
     An LCD typically includes a liquid crystal panel and a backlight unit. The liquid crystal panel typically includes a transistor substrate in which thin film transistors (TFTs), storage capacitors, and pixel electrodes are formed, a color filter substrate in which color filters and a black matrix are formed, and a liquid crystal layer positioned between the transistor substrate and the color filter substrate. 
     The liquid crystal panel, displaying an image, is typically operated by a gate driver supplying a gate signal, a data driver supplying a data signal, and a power supply unit supplying a common voltage, or the like. In the liquid crystal layer, liquid crystal moves to correspond to an electric field generated between a pixel voltage and a common voltage. 
     In the LCD, a load may be determined according to patterns displayed on the liquid crystal panel, and power consumption varies depending on the load. For example, when the LCD displays a maximum (“max”) pattern in which an image fully transitions during one frame, the data driver may consume power twice to thrice as much as that of a case in which a normal pattern is displayed. 
     In addition to increasing the power consumption, such a max pattern displayed on the liquid crystal panel may cause heat generation and degradation of other characteristics of the device. Thus, a scheme for solving the problems arising when a max pattern is generated is proposed. 
     The proposed scheme may advantageously reduce power consumption by changing a driving algorithm, but has the tendency of causing a voltage drop in an input terminal of the power supply unit when power is turned on in a state in which the max pattern is applied. In addition, when the voltage drop increases, an under-voltage lock-out (UVLO) of the power supply unit may occur, making the device inoperative. Due to such various problems, the proposed scheme may be improved. 
     SUMMARY 
     In an aspect of the present disclosure, there is provided a liquid crystal display device including a liquid crystal panel configured to display an image, a driver configured to drive the liquid crystal panel, a timing controller configured to control the driver, and a power supply. The power supply is configured to be supplied by an input voltage, supply a common voltage to the liquid crystal panel, and temporarily vary a compensation ratio of the common voltage when a pattern causing a drop of the input voltage is displayed by the display device. 
     In another aspect, there is provided a method for driving a liquid crystal display device, the method comprising turning on power so that an external input voltage is supplied to a power supply, varying a compensation ratio of a common voltage output from the power supply during a first period of time, and returning the compensation ratio of the common voltage output from the power supply to an original compensation ratio thereof during a second period of time that is after the first period of time. 
     It is to be understood that both the foregoing general description and the following detailed description of the present disclosure are exemplary and explanatory and are intended to provide further explanation of the disclosure as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompany drawings, which are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description serve to explain the principles of the disclosure. 
         FIG. 1  is a block diagram schematically illustrating an example of a liquid crystal display (LCD) device. 
         FIG. 2  is a circuit diagram schematically illustrating an example sub-pixel as shown in  FIG. 1 . 
         FIG. 3  is a waveform view illustrating output states of a power supply unit of a proposed scheme according to the related art. 
         FIG. 4  is a waveform view illustrating output states of the power supply unit of a related art LCD device when the power supply unit of the related art LCD device performs a normal operation and an abnormal operation. 
         FIG. 5  is a waveform view illustrating a possible problem of the related art. 
         FIG. 6  is a waveform view illustrating a first example embodiment of the present disclosure. 
         FIG. 7  is a flow chart illustrating a method for driving an LCD device according to an example of the first embodiment of the present disclosure. 
         FIGS. 8A and 8B  are block diagrams illustrating a comparison between common voltage generating units according to the first example embodiment of the present disclosure and the related art. 
         FIG. 9  is a block diagram illustrating an example of the common voltage generating unit according to the first example embodiment of the present disclosure. 
         FIG. 10  is a block diagram illustrating an example of a portion of the common voltage generating unit according to a second example embodiment of the present disclosure. 
         FIG. 11  is a block diagram illustrating an example of a portion of the common voltage generating unit according to a third example embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS 
     Reference will now be made in detail embodiments of the disclosure examples of which are illustrated in the accompanying drawings. 
     First Example Embodiment 
       FIG. 1  is a block diagram schematically illustrating an example of a liquid crystal display (LCD) device, and  FIG. 2  is a circuit diagram schematically illustrating an example of a sub-pixel (SP) as illustrated in  FIG. 1 . 
     As illustrated in  FIG. 1 , the LCD device includes an image supply unit  120 , a timing controller  130 , a gate driver  140 , a data driver  150 , a liquid crystal panel  160 , a backlight unit  170 , and a power supply unit  180 . 
     The image supply unit  120  processes a data signal and outputs the data signal together with a vertical synchronization signal, a horizontal synchronization signal, a data enable signal, and a clock signal. The image supply unit  120  supplies the vertical synchronization signal, the horizontal synchronization signal, the data enable signal, the clock signal, and the data signal to the timing controller  130 . 
     The timing controller  130  generates a gate timing control signal GDC for controlling an operation timing of the gate driver  140  and a data timing control signal DDC for controlling an operation timing of the data driver  150  on the basis of various signals supplied from the image supply unit  120 , and outputs the generated gate timing control signal GDC and the data timing control signal DDC. The timing controller  130  supplies a data signal (or a data voltage), supplied from the image processing unit  110  to the data driver  150 , together with the data timing control signal DDC. 
     In response to the gate timing control signal GDC, the gate driver  150  outputs a gate signal while shifting a level of a gate voltage. The gate driver  140  supplies a gate signal to subpixels SP included in the liquid crystal panel  160  through gate lines GL (see  FIG. 2 ). The gate driver  140  may be formed as an integrated circuit (IC) or in a gate-in-panel manner in the liquid crystal panel  160 . 
     In response to the data timing control signal DDC supplied from the timing controller  130 , the data driver  150  samples, latches, and converts a data signal DATA into a gamma reference voltage, and outputs the same. The data driver  150  supplies the data signal DATA to the subpixels SP included in the liquid crystal panel  160  through data lines DL (see  FIG. 2 ). The data driver  150  is formed as an IC. 
     The liquid crystal panel  160  displays an image in response to a gate signal output from drivers, including the gate driver  140  and the data driver  150 , and a common voltage that may be output from the power supply unit  180 . The liquid crystal panel  160  includes subpixels SP controlling light provided from the backlight unit  170 . 
     With reference to  FIG. 2 , a single subpixel includes a switching transistor SW, a storage capacitor Cst, and a liquid crystal layer Clc. A gate electrode of the switching transistor SW is connected to a gate line GL 1 , and a source electrode thereof is connected to a data line DL 1 . The storage capacitor Cst is connected to a drain electrode of the switching transistor SW at one end thereof and is connected to a common voltage line Vcom at the other end thereof. The liquid crystal layer Clc is formed between a pixel electrode  1  connected to the drain electrode of the switching transistor SW and a common electrode  2  connected to the common voltage line Vcom. 
     The liquid crystal panel  160  may be implemented in a twisted nematic (TN) mode, a vertical alignment (VA) mode, an in-plane switching (IPS) mode, a fringe field switching (FFS) mode, or an electrically controlled birefringence (ECB) mode, according to the structures of the pixel electrode  1  and the common electrode  2 . 
     The backlight unit  170  provides light to the liquid crystal panel  160  using a light source that outputs light. The backlight unit  170  may include a light emitting diode (LED), an LED driver driving the LED, an LED board on which the LED is mounted, a light guide plate converting light output from the LED into a surface light source, a reflective plate reflecting light from below the light guide plate, and optical sheets collecting and diffusing light output from the light guide plate. 
     The power supply unit  180  generates various types of power on the basis of an input voltage Vin supplied from the outside, and outputs the same. The power supply unit  180  generates a first source voltage VDD, a second source voltage VCC, a gate high voltage VGH, a common voltage VCOM, and a low potential voltage GND. The first source voltage VDD may be supplied to the data driver  150 , the second source voltage VCC may be supplied to the timing controller  130 , the gate high voltage VGH may be supplied to the gate driver  140 , and the common voltage VCOM may be supplied to the liquid crystal panel  160 . In the present disclosure, for example, the power supply unit  180  may generate all of the voltages described above. However, this is merely illustrative, and the power supply unit  180  may be configured differently according to a configuration of the display device or voltage levels. 
     The LCD device described above may display an image through the liquid crystal panel  160  by interworking without the gate driver  160  supplying a gate signal, the data driver  150  supplying a data signal DATA, and the power supply unit  180  supplying the common voltage VCOM, or the like. 
     In the LCD device, a load may be determined according to patterns displayed on the liquid crystal panel  160 , and power consumption of the device is varied by the load. For example, when the LCD device displays a max pattern in which an image full-transitions during one frame, the data driver  150  may consume power twice to thrice greater as compared to a case in which a normal pattern is displayed. 
     In addition to increasing power consumption, such a max pattern displayed on the liquid crystal panel  160  may cause heat generation and degradation of other characteristics of the device. Thus, a scheme for solving the problems arising when the max pattern is generated is proposed. 
       FIG. 3  is a waveform view illustrating output states of a power supply unit to briefly explain a proposed scheme of related art, and  FIG. 4  is a waveform view illustrating output states of the power supply unit of a related art LCD device when its power supply unit performs a normal operation and an abnormal operation. 
     As illustrated in  FIGS. 3 and 4 , in the related art, in order to solve a problem of a voltage drop due to a max pattern (real max pattern section) when the LCD device is initially driven, a scheme of changing a driving algorithm of a timing controller Tcon is proposed. For example, the max pattern appears after an initial black pattern is displayed for a predetermined period of time. However, an increase in power consumption or heat generation and degradation of other characteristics of the device are resolved, to a degree, by the driving algorithm of the timing controller Tcon. 
     In  FIGS. 3 and 4 , Vin denotes an input voltage input to the power supply unit, VDD denotes a first source voltage, VGH denotes a gate high voltage, and Iin denotes an input current input to the power supply unit. 
     In this manner, the proposed scheme of the related art is advantageous in that it reduces power consumption. However, as illustrated in plot (b) of  FIG. 4 , when power is turned on in a state in which the max pattern is applied, a voltage drop (e.g., when an amount of drop during a normal operation is V 1  (plot (a)), an amount of drop during an abnormal operation is as severe as V 2  (plot (b)) tends to occur in an input terminal of the power supply unit. Also, when the voltage drop increases, under-voltage lock out (UVLO) of the power supply unit may operate to put the device into a state in which the device cannot normally operate. Thus, the proposed scheme of the related art could use improvement. 
       FIG. 5  is a waveform view of voltages which illustrate a problem of the related art,  FIG. 6  is a waveform view of voltages which illustrates a first example embodiment of the present disclosure to improve the problem of the related art, and  FIG. 7  is a flow chart illustrating a method for driving an LCD device according to the first example embodiment of the present disclosure. 
     The proposed scheme of the related art may improve the problems of the increase in power consumption, the heat generation, and the degradation of other characteristics of the device due to the max pattern, to a degree. However, when power is turned on in a state in which the max pattern is applied, a voltage drop may occur in the input terminal of the power supply unit, and when the voltage drop increases, the under-voltage lock-out (UVLO) of the power supply unit may operate, thereby making it impossible for the device to normally operate. Analysis results reveal that causes of the voltage drop were related to a compensation problem of a common voltage used in the LCD device. 
     As illustrated in  FIG. 5 , in the proposed scheme of the related art, when the input voltage Vin is supplied to the power supply unit, the power supply unit compensates for the common voltage Vcom with a predetermined compensation ratio, and outputs the same. That is, in such a proposed scheme, an increase in current according to the compensation operation of a common voltage amplifying unit (e.g., an operational amplifier for Vcom) included in the power supply unit may be the main cause of the increase in power consumption for the max pattern. A degree of the increase in current may be varied according to compensation ratios of the common voltage amplifying unit. For this reason, when power is turned on in a state in which the max pattern is applied, the voltage drop may occur in connection with the common voltage compensation operation of the power supply unit. 
     As illustrated in  FIG. 6 , in order to improve the problem arising in the proposed scheme of the related art, in a first example embodiment of the present disclosure, when the input voltage Vin is supplied to the power supply unit, a common voltage compensation ratio to be applied is lowered for a predetermined period of time. After the predetermined period of time has lapsed, a previously set value of the common voltage compensation ratio is normally applied such that the common voltage Vcom may be output at a predetermined compensation ratio from the power supply unit. 
     For example, in the proposed scheme of the related art, in order to improve crosstalk, a common voltage compensation ratio of twenty times or greater, compared with that of normal driving, may be applied. However, in the first example embodiment of the present disclosure, the common voltage compensation ratio applied may be less than half of twenty times that of normal driving. For example, in the first example embodiment of the present disclosure, the common voltage compensation ratio of M times (where M is 1 to 10 times) may be applied. 
     Meanwhile, in the first example embodiment of the present disclosure, when the device was initially driven, the common voltage compensation ratio was, at one time, in a state in which a special pattern such as the max pattern was displayed, but an abnormal screen problem of the display panel did not occur. Thus, in the first example embodiment of the present disclosure, when the device is initially driven, the common voltage amplifying unit of the power supply unit may compensate for the common voltage with a first compensation ratio, and thereafter, the common voltage may be compensated with a second compensation ratio. Here, the first compensation ratio is lower than the second compensation ratio by M times (M is 1 to 10 times). 
     In this manner, in the first example embodiment of the present disclosure, when power of the LCD device is turned on, the common voltage compensation ratio performed by the common voltage amplifying unit of the power supply unit may be temporarily lowered to improve a voltage margin and resolve the voltage drop, thus preventing a problem in which the UVLO of the power supply unit operates. 
     To this end, as illustrated in  FIG. 3 , the common voltage compensation ratio may be applied by M times (for example, one time) in a first section (or an initial section) in which initial black data and a real max pattern are generated. Thereafter, in the second section (after the section in which the initial black data and the real max pattern are generated is terminated), the common voltage compensation may be applied as a normal compensation ratio (e.g., a preset compensation ratio or the original compensation ratio). 
     The timing controller may generate a signal capable of controlling the common voltage compensation ratio and outputting the same to the power supply unit, or may vary the signal supplied to the power supply unit. For example, in a case in which the timing controller and the power supply unit are connected as a communication interface of I2C protocol, and the common voltage compensation ratio of the power supply unit is set to one time, the timing controller may output a control signal through I2C after the lapse of a predetermined delay time. 
     As illustrated in  FIG. 7 , the LCD device according to the first example embodiment of the present disclosure may operate as follows. 
     Power is turned on so that an input voltage, generated by an external source, is supplied to the power supply unit (S 110 ). When power is turned on (Y), it means (for example) that a user turned on power of the LCD device, and when power is not turned on (N), it means (for example) that the user did not turn on power of the LCD device. 
     When the input voltage generated by an external source is supplied to the power supply unit, the power supply unit lowers a compensation ratio of a common voltage and applies the lowered compensation ratio (S 120 ). 
     The power supply unit lowers the common voltage compensation ratio during an ‘N’ amount of time (a first section or a first time) (S 130 ). For example, under the control of the timing controller, the power supply unit lowers the common voltage compensation ratio during N time (N time corresponds to the sum of sections in which initial black data and the max pattern are generated). Here, when the N time has not lapsed (N), the common voltage compensation ratio is lowered to be applied until the N time lapses. 
     When the N time has lapsed (Y) (a second section or a second time positioned after the first section), the power supply unit normally applies the compensation ratio of the common voltage. When the compensation ratio is normally applied, it may mean that a preset compensation ratio or the original compensation ratio is restored. 
     Through the foregoing operation, the problem that may arise during an initial operation in relation to the compensation ratio for the common voltage output from the power supply unit may be solved. Thus, the LCD device may perform a normal operation, such as displaying an image on the display panel in response to a data signal, a gate signal, or a common voltage (S 150 ). 
     Hereinafter, a common voltage generation unit of the related art as described above, and that of a first example embodiment of the present disclosure will both be described. 
       FIGS. 8A and 8B  are a block diagrams illustrating a comparison between common voltage generating units of the related art and the first example embodiment of the present disclosure, and  FIG. 9  is a block diagram specifically illustrating the common voltage generating unit according to the first example embodiment of the present disclosure. 
     As illustrated in  FIG. 8A , the common voltage generation unit  180 _V according to the related art includes a common voltage amplifying unit  186  amplifying a common voltage Vcom and outputting the amplified common voltage. The common voltage amplifying unit  186  amplifies the common voltage Vcom on the basis of a first source voltage VDD and a low potential voltage GND. 
     The common voltage generation unit  180 _V according to the related art may vary a compensation ratio of the common voltage in response to a voltage or a signal supplied to a non-inverting terminal (+) and an inverting terminal (−) of the common voltage amplifying unit  186 . 
     As illustrated in  FIG. 8B , a common voltage generation unit  180 _V according to the first example embodiment of the present disclosure may include an interface unit  182 , a voltage adjuster  184 , and a common voltage amplifying unit  186 . 
     The interface unit  182  may exchange data with an external circuit unit (hereinafter referred to as a “timing controller”) according to a communication interface (IF) scheme. For example, the interface unit  182  may receive a power control signal through a communication interface (IF) with the timing controller, and deliver the received power control signal to the voltage adjusting unit  184 . 
     The voltage adjuster  184  may vary a first source voltage VDD and output the same. In response to a power control signal transferred through the interface unit  182 , the voltage adjuster  184  may vary the first source voltage VDD and output the varied voltage. For example, in response to the power control signal transferred through the interface unit  182 , the voltage adjuster  184  may divide the first source voltage VDD and deliver the divided voltage to the common voltage amplifying unit  186 . The voltage adjuster  184  serves to limit the first source voltage VDD supplied to the common voltage amplifying unit  186  (or lowers a level of the first source voltage and outputs the same). 
     The common voltage amplifying unit  186  may amplify the common voltage Vcom on the basis of the first source voltage VDD, delivered from the voltage adjuster  184 , and a low potential voltage GND, and output the amplified voltage. The first source voltage VDD, delivered from the voltage adjuster  184 , is supplied to a first bias terminal Vs+, and the low potential voltage GND is supplied to a second bias terminal Vs−. For example, in response to a varied level of the first source voltage VDD delivered from the voltage adjuster  184 , the common voltage amplifying unit  186  may vary a compensation ratio (or an amplification ratio) of the common voltage Vcom, and output the varied compensation ratio through a common voltage line. 
     As can be seen from  FIGS. 8A-8B , in the first example embodiment of the present disclosure, the compensation ratio of the common voltage may be varied in response to the power control signal supplied from an external source, in contrast to the related art. An example of the voltage adjusting unit illustrated in  FIG. 8B  and described above will be further described as follows. 
     As illustrated in  FIG. 9 , the example voltage adjusting unit  184  includes a decoder unit  184 D, a resistor string unit  184 R, and a transistor unit  184 T. The decoder unit  184 D generates an output in response to a power control signal. The resistor string unit  184 R includes a plurality of resistors arranged between the first source voltage VDD and the low potential voltage GND. In response to a signal output from the decoder unit  184 D, the transistor unit  184 T controls the resistor string unit  184 R and varies the first source voltage VDD, and outputs the same. 
     The voltage adjusting unit  184  includes the transistor unit  184 T capable of controlling the resistor string unit  184 R positioned between the first source voltage VDD and the low potential voltage GND in response to the power control signal delivered to the decoder unit  184 D. The voltage adjusting unit  184  may control a circuit configured with the decoder unit  184 D, the resistor string unit  184 R, and the transistor unit  184 T, in response to a power control signal, and may vary the first source voltage VDD in such a manner that a resistance value between the first source voltage VDD and the low potential voltage GND is varied, and output the same. However, the above description is merely illustrative, and the present disclosure is not limited thereto. 
     As described above, the first example embodiment of the present disclosure may include a common voltage generation unit  180 _V capable of varying a compensation ratio (or an amplification ratio) of the common voltage Vcom. 
     Therefore, in a case in which the common voltage generation unit  180 _V according to the first example embodiment is used, when power of the LCD device is turned on, a common voltage compensation ratio carried out in the common voltage amplifying unit  186  of the power supply unit (PMIC Vcom Block) may be temporarily lowered to improve a voltage margin and resolve a voltage drop, thus preventing a problem in which UVLO is applied to the power supply unit. 
     Meanwhile, the first example embodiment of the present disclosure may improve a problem caused by a special pattern such as a max pattern, or the like, when the LCD device is initially driven. However, the special pattern such as the max pattern may also be generated even after the LCD device is initially driven. In order to cope with such a case, the present disclosure proposes a scheme of changing a compensation ratio of a common voltage even while the LCD device is being driven. 
     Second Example Embodiment 
       FIG. 10  is a block diagram illustrating a portion of an example common voltage generating unit according to a second example embodiment of the present disclosure. 
     As illustrated in  FIG. 10 , the common voltage generation unit  180 _V includes a first circuit unit  180 _Va (PMIC Vin Detector) detecting an input voltage, and a second circuit unit  180 _Vb (PMIC Vcom Block) generating a common voltage Vcom. 
     The first circuit unit  180 _Va outputs a control signal CS for controlling a compensation ratio of the common voltage Vcom output from the second circuit unit  180 _Vb on the basis of a signal supplied from an external circuit unit and an input voltage Vin supplied from the outside. 
     The first circuit unit  180 _Va includes an interface unit  182 , a voltage adjusting unit  184 , and a voltage comparing unit  185 . The interface unit  182  exchanges data with an external circuit unit (hereinafter, referred to as a “timing controller”) according to a communication interface (IF) scheme. For example, the interface unit  182  receives a power control signal through a communication interface (IF) with the timing controller, and delivers the received power control signal to the voltage adjusting unit  184 . 
     The voltage adjusting unit  184  varies a reference voltage Vin ref of the input voltage and outputs the varied reference voltage. Here, in response to the power control signal delivered through the interface unit  182 , the voltage adjusting unit  184  varies the reference voltage Vin ref of the input voltage and outputs the varied reference voltage. For example, in response to the power control signal delivered through the interface unit  182 , the voltage adjusting unit  184  divides the input voltage Vin and delivers the divided input voltage Vin to the voltage comparing unit  185 . The voltage adjusting unit  184  serves to limit the reference voltage Vin ref of the input voltage supplied to the voltage comparing unit  185  (or lowers a level of a first source voltage and outputs the same). 
     The voltage adjusting unit  184  includes a decoder unit  184 D, a resistor string unit  184 R, and a transistor unit  184 T. The voltage adjusting unit  184  includes the transistor unit  184 T, which is capable of controlling the resistor string unit  184 R positioned between the input voltage Vin and a low potential voltage GND in response to the power control signal delivered to the decoder unit  184 D. 
     The voltage adjusting unit  184  may control a circuit including the decoder unit  184 D, the resistor string unit  184 R, and the transistor unit  184 T, in response to the power control signal, and may vary the reference voltage Vin ref of the input voltage in such a manner that a resistance value between the input voltage Vin and the low potential voltage GND is varied, and output the same. The voltage adjusting unit  184  may vary (or limit) the reference voltage Vin ref of the input voltage by models of liquid crystal panels in response to the power control signal. However, this is merely illustrative and the present disclosure is not limited thereto. 
     The voltage comparing unit  185  compares the reference voltage Vin ref of the input voltage delivered from the voltage adjusting unit  184  with the input voltage Vin supplied from the outside, and outputs a control signal CS according to the comparison result. The reference voltage Vin ref of the input voltage delivered from the voltage adjusting unit  184  is supplied to an inverting terminal (−) of the voltage comparing unit  185 , the input voltage Vin is supplied to a non-inverting terminal (+) of the voltage comparing unit  185 , the first potential voltage VDD is supplied to a first bias terminal (Vs+), and the low potential voltage is supplied to a second bias terminal (Vs−). 
     When a drop occurs in the input voltage Vin due to a special pattern such as a max pattern, the voltage comparing unit  185  outputs a control signal CS according to a preset voltage. For example, when a level of the input voltage Vin is higher than that of the reference voltage Vin ref of the input voltage, the voltage comparing unit  185  outputs a control signal CS corresponding to a logic low signal Low. Meanwhile, when a level of the input voltage Vin is lower than that of the reference voltage Vin ref of the input voltage, the voltage comparing unit  185  outputs a control signal CS corresponding to a logic high signal High. 
     The second circuit unit  180 _Vb controls a compensation ratio of the common voltage Vcom in response to the control signal CS supplied from the first circuit unit  180 _Va. The second circuit unit  180 _Vb includes a common voltage amplifying unit  186  amplifying a common voltage and outputting the amplified common voltage and a switch unit FET controlling a compensation ratio of the common voltage in response to the control signal CS. 
     The common voltage amplifying unit  186  controls a compensation ratio of the common voltage on the basis of a compensation reference common voltage output from the common voltage compensation unit PVCOM_Ref and a common voltage fed back from a common voltage feedback circuit unit Vcom_FB. The compensation reference common voltage is supplied to a non-inverting terminal (+) of the common voltage amplifying unit  186 , the feedback common voltage is supplied to an inverting terminal (−) of the common voltage amplifying unit  186 , the first source voltage VDD is supplied to the first bias terminal Vs+, and the low potential voltage GND is supplied to the second bias terminal Vs−. 
     A gate electrode of the switch unit FET is connected to a control signal line to which the control signal is transferred, a first electrode thereof is connected to an output terminal of the common voltage amplifying unit  186 , and a second electrode thereof is connected to the inverting terminal (−) of the common voltage amplifying unit  186 . The switch unit FET is turned on or turned off according to a logic state of the control signal CS. 
     The common voltage feedback circuit unit Vcom_FB is used to compensate for the common voltage. The common voltage feedback circuit Vcom_FB, a circuit positioned outside of the power supply unit, serves to feed back the common voltage, returned through the liquid crystal panel  160  after being output from the power supply unit, to the second circuit unit  180 _Vb of the common voltage generation unit  180 _V. 
     The common voltage feedback circuit unit Vcom_FB further includes a first feedback resistor RF 1  and a second feedback resistor RF 2 . The first feedback resistor RF 1  is connected to an output terminal of the common voltage feedback circuit unit Vcom_FB at one end thereof, and is connected to the inverting terminal (−) of the common voltage amplifying unit  186  at the other end thereof. The second feedback resistor RF 2  is connected to an output terminal of the common voltage generating unit  180 _V at one end thereof, and is connected to the inverting terminal (−) of the common voltage amplifying unit  186  at the other end thereof. 
     As described above, in the second example embodiment of the present disclosure, the compensation ratio of the common voltage Vcom may be varied according to a change in a level of the input voltage, even while the LCD device is being driven, through interworking between the first circuit unit  180 _Va and the second circuit unit  180 _Vb. 
     For example, when a voltage level of the input voltage Vin is 2.5V or higher, the common voltage generation unit  180 _V may compensate for the common voltage Vcom with a second compensation ratio, which is a normal compensation ratio, and output the same. Meanwhile, when a voltage level of the input voltage Vin is lower than 2.5V, the common voltage generation unit  180 _V may compensate for the common voltage Vcom with a first compensation ratio, which is a lowered compensation ratio, and output the same. Here, the first compensation ratio may be M times (M is 1 to 10 times) lower than the second compensation ratio. 
     For example, when the common voltage generation unit  180 _V compensates for the common voltage Vcom with the second compensation ratio, the compensation ratio may be expressed as “COMP RATIO=−RF 1 /RF 2 ”. Here, the common voltage is compensated with the normal compensation ratio which is to be applied to each model of a liquid crystal panel. 
     Alternatively, when the common voltage generation unit  180 _V performs compensation with a third compensation ratio, the compensation ratio may be expressed as “COMP RATIO=0 (FET Ron value)/RF 1 ”. Here, the common voltage is not compensated. That is, the compensation ratio is 0, and the common voltage amplifying unit  186  operates as an operational amplifier buffer. 
     Meanwhile, the common voltage generation unit  180 _V may perform a compensation operation with the third compensation ratio (e.g., where common voltage compensation is temporarily stopped), which does not compensate for the common voltage according to a voltage level of the input voltage Vin. In this manner, the compensation ratio of the common voltage may be varied, or compensation may be selectively performed according to a result obtained by comparing the input voltage returning the compensation ratio of the common voltage to the original compensation ratio and the reference voltage of the input voltage provided in the power supply unit. 
     As described above, the second example embodiment of the present disclosure includes the common voltage generation unit  180 _V for varying the compensation ratio (or amplification ratio) of the common voltage Vcom or for not performing compensation. 
     Therefore, when the common voltage generation unit  180 _V according to the second example embodiment is used, the compensation ratio of the common voltage may be varied according to a state (or a level) of the input voltage Vin, or compensation may be temporarily stopped, whereby a voltage margin may be improved and a voltage drop may be resolved, preventing a problem caused by the UVLO being applied to the power supply unit. 
     In this manner, since the input voltage supplied to the power supply unit or the common voltage generation unit is sensed, a problem related to generation of a special pattern, such as a max pattern, when the LCD device is initially driven or even while the LCD device is normally driven thereafter, may be improved. 
     Third Example Embodiment 
       FIG. 11  is a block diagram illustrating a portion of the common voltage generating unit according to a third example embodiment of the present disclosure. 
     As illustrated in  FIG. 11 , the example common voltage generation unit  180 _V includes a first circuit unit  180 _Va (PMIC Vin Detector) detecting an input voltage and a second circuit unit  180 _Vb (PMIC Vcom Block) generating a common voltage Vcom. 
     The first circuit unit  180 _Va outputs a control signal CS for controlling a compensation ratio of the common voltage Vcom output from the second circuit unit  180 _Vb on the basis of a signal supplied from an external circuit unit and an input voltage Vin supplied from the outside. 
     The second circuit unit  180 _Vb controls a compensation ratio of the common voltage Vcom in response to the control signal CS supplied from the first circuit unit  180 _Va. The second circuit unit  180 _Vb includes a common voltage amplifying unit  186  amplifying a common voltage and outputting the amplified common voltage, and a switch unit FET controlling a compensation ratio of the common voltage in response to the control signal CS. 
     The example common voltage generation unit according to the third example embodiment of the present disclosure may be the same as that of the second example embodiment, except for a third feedback resistor RF 3  included in the second circuit unit  180 _Va. For brevity, only the third feedback resistor RF 3  may be described. 
     The second circuit unit  180 _Vb controls a compensation ratio of the common voltage Vcom in response to the control signal CS supplied from the first circuit unit  180 _Va. The second circuit unit  180 _Vb includes the common voltage amplifying unit  186  amplifying a common voltage and outputting the amplified common voltage, the switch unit FET controlling a compensation ratio of the common voltage in response to the control signal CS, and the third feedback resistor RF 3 . 
     Together with the first and second feedback resistors RF 1  and RF 2 , the third feedback resistor RF 3  may serve to determine a compensation ratio of the common voltage. The third feedback resistor RF 3  may be positioned between the switch unit FET and the inverting terminal (−) of the common voltage amplifying unit  186 . The third feedback resistor RF 3  may be connected to the second electrode of the switch unit FET at one end, and connected to the inverting terminal (−) of the common voltage amplifying unit  186  at the other end. 
     As described above, in the third example embodiment of the present disclosure, the compensation ratio of the common voltage Vcom may be varied according to a change in a level of the input voltage, even while the LCD device is being driven, through interworking between the first circuit unit  180 _Va and the second circuit unit  180 _Vb. 
     For example, when a voltage level of the input voltage Vin is 2.5V or higher, the common voltage generation unit  180 _V may compensate for the common voltage Vcom with a second compensation ratio, (e.g., a normal compensation ratio), and output the same. Meanwhile, when a voltage level of the input voltage Vin is lower than 2.5V, the common voltage generation unit  180 _V may compensate for the common voltage Vcom with a first compensation ratio, (e.g., a lowered compensation ratio), and output the same. Here, the first compensation ratio may be M times (M is 1 to 10 times) lower than the second compensation ratio. 
     For example, when the common voltage generation unit  180 _V compensates for the common voltage Vcom with the second compensation ratio, the compensation ratio may be expressed as “COMP RATIO=−RF 1 /RF 2 ”. Here, the common voltage is compensated with the normal compensation ratio which is to be applied to each model of a liquid crystal panel. 
     In contrast, when the common voltage generation unit  180 _V compensates for the common voltage Vcom with the second compensation ratio, the compensation ratio may be expressed as “COMP RATIO=−RF 3 /RF 1 ”. In this case, the common voltage is compensated with a lowered compensation ratio which is to be applied to each model requiring a lower compensation ratio. 
     As described above, the third example embodiment of the present disclosure includes the common voltage generation unit  180 _V for varying the compensation ratio (or amplification ratio) of the common voltage Vcom. In particular, in the third example embodiment, in order to improve a voltage margin, different compensation ratios may be expressed for each input voltage, and also, a drop amount of an input voltage may be adjusted. 
     Therefore, when the common voltage generation unit  180 _V according to the third example embodiment is used, the compensation ratio of the common voltage may be varied according to a state (or a level) of the input voltage Vin, a voltage margin may be improved, and a voltage drop may be resolved, thereby preventing a problem in which UVLO is applied to the power supply unit. Accordingly, reliability and stability of the device may be enhanced. 
     In this manner, because the input voltage supplied to the power supply unit or the common voltage generation unit is sensed, a problem related to generation of a special pattern such as a max pattern when the LCD device is initially driven, or even while the LCD device is being normally driven thereafter (e.g., in a middle stage of driving), may be improved. 
     As described above, in example embodiments of the present disclosure, in order to reduce drop of an input voltage when power is turned on at an initial stage, to mainly aim at enhancing a voltage margin when a special pattern such as a max pattern is generated (or expressed), 1) a source voltage of the common voltage amplifying unit is limited, 2) a compensation ratio of the common voltage is lowered or a compensation operation time is delayed when power is turned on, 3) the common voltage amplifying unit is implemented as a compensation circuit or a buffer circuit of a common voltage, and 4) a drop amount of an input voltage is adjusted by differentiating a compensation ratio of the common voltage according to an input voltage. 
     As described above, in example embodiments, when a special pattern is generated (or expressed), a voltage margin is enhanced and a voltage drop at the input terminal of the power supply unit is prevented, thereby enhancing reliability and stability of the device. Also, even when the special pattern is generated at an initial stage of driving, or while the device is being normally driven thereafter (e.g., at a middle stage of driving), a voltage may be stably output. In addition, display quality may be enhanced by differentiating a common voltage compensation ratio according to a state of the power supply unit and a model of a liquid crystal panel. 
     It will be apparent to those skilled in the art that various modifications and variations may be made in the display device of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.