Abstract:
The present invention relates to a technique for outputting threshold voltages by properly changing the threshold voltages such that the threshold voltages can protect low-voltage driving elements within an analog to digital converter when the threshold voltages of an OLED display panel are sensed and outputted to the analog to digital converter. The present invention comprises: a sampling capacitor which samples threshold voltages sensed and inputted from an organic light-emitting diode on a display panel; a charge-sharing capacitor which charges and shares the threshold voltages sampled from the sampling capacitor, or solely charges the threshold voltages to bypass the threshold voltages; and a sample-and-hold unit which has a plurality of switches for performing switching operations for the sampling operation of the sampling capacitor and the charging and the sharing of the charge-sharing capacitor, and scales the threshold voltages to threshold voltage areas having a certain value or less.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a threshold voltage sensing circuit of an organic light-emitting diode (OLED) display device, and more particularly, to a threshold voltage sensing circuit of an OLED display device, which changes the threshold voltage of an OLED to a voltage suitable for protecting a low-voltage driving element in an analog-to-digital converter, when sensing the threshold voltage of the OLED and outputting the sensed threshold voltage to the analog-to-digital converter. 
     Description of the Related Art 
     In general, a display panel of an OLED display device includes a plurality of pixels arranged in a matrix shape, and each of the pixels includes an OLED. When a signal is supplied to a gate line, each of the pixels is turned on by a data signal supplied from a data line, and emits light. The unit pixels of the display panel include OLEDs arranged therein and showing a unique color of red, green, and blue. The colors of the OLEDs may be combined to express a target color. 
     However, since the OLEDs on the display panel gradually deteriorate with time, the threshold values thereof are changed. Thus, although the same driving current is supplied to the OLEDs, the brightness of the OLEDs may be gradually changed with time. 
     Thus, the threshold voltages of the OLEDs may be sensed and stored in a memory. When a data signal is outputted to the display panel, the data signal may be compensated for according the changes of the threshold voltages based on the stored threshold voltages. Therefore, the OLEDs may maintain constant brightness at all times, regardless of the use time of the OLEDs. 
       FIG. 1  is a block diagram of a conventional threshold voltage sensing device of an OLED display device. As illustrated in  FIG. 1 , the conventional threshold voltage sensing device includes a display panel  10 , a gate driver  20 , a source driver  30 , and a threshold voltage sensing controller  40 . 
     Each of pixels arranged in the display panel  10  includes a switching transistor TFT-S which transmits a data signal to a driving transistor TFT-D through data lines DL 1  to DLn of the source driver  30 . The driving transistor TFT-D supplies a driving current corresponding to the data signal supplied through the switching transistor TFT-S to the corresponding OLED. A capacitor C coupled between one terminal and the gate of the driving transistor TFT-D and maintains the turn-on state of the driving transistor TFT-D during one frame, the corresponding OLED may maintain the light-emitting state during one frame. 
     Before the system is powered on to display an image on the display panel  10  or in a threshold voltage sensing mode, the threshold voltage sensing controller  40  sequentially outputs a control signal to threshold voltage compensation control lines CL 1  to CLn. Thus, threshold voltage sensing transistors TFT-V of a corresponding horizontal line are sequentially turned on. 
     When the control signal is supplied to the first threshold voltage compensation control line CL 1  to turn on the threshold voltage sensing transistors TFT-V, the source driver  30  transmits precharge voltages to the data lines DL 1  to DLn through buffers BUF 1  to BUFn, respectively. At this time, the precharge voltages are supplied to the anodes of the OLEDs, respectively. 
     Then, when the precharge voltages of the OLEDs are sufficiently discharged, sample and hold circuits SH 1  to SHn sample and hold the threshold voltages Vth of the OLEDs, sensed through the threshold voltage sensing transistors TFT-V and the corresponding data lines DL, respectively. The analog threshold voltages Vth sampled and held through the sample and hold circuits SH 1  to SHn are converted into digital signals through an analog-to-digital converter  31 , and stored in a memory. 
     Subsequently, the same operation is repeated on the next horizontal line. Whenever the same operation is repeated on each horizontal line, the threshold voltages of the OLEDs are converted into digital signals and stored in the memory. 
     In an image display mode, when data signals are outputted to the OLEDs, the data signals may be compensated for as much as the changes of the threshold voltages based on the threshold voltages stored in the memory. Thus, the OLEDs maintain the constant brightness regardless of the changes of the threshold voltages. 
     However, since the sample and hold circuits SH 1  to SHn and the analog-to-digital converter  31  perform a digital logical circuit operation, the sample and hold circuits SH 1  to SHn and the analog-to-digital converter  31  are typically implemented with transistors which are driven at a low voltage. Thus, when a threshold voltage is sensed and transmitted to the analog-to-digital converter  31 , the PN-junction diode of the transistor (for example, LV PMOS transistor) may be turned on in case where the threshold voltage is higher than the limit voltage (for example, VDD+Vth) which guarantees stable operations of the transistors within the analog-to-digital converter  31 . Thus, a discharge operation may occur due to leakage current in the analog-to-digital converter  31 . 
     Nevertheless, the conventional threshold voltage sensing device does not include a function of changing or limiting a sampled and held threshold voltage to the limit voltage or less, which guarantees the stable operations of the transistors within the analog-to-digital converter. Thus, a discharge operation may be caused by leakage current, and the values of the threshold voltages sensed from the OLEDs may not be normally stored in the memory. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made in an effort to solve the problems occurring in the related art, and an object of the present invention is to provide a threshold voltage sensing circuit of an OLED display device, which is capable of scaling down threshold voltages sensed from OLEDs of a display panel to threshold voltages within a predetermined range through charge sharing, when the threshold voltages are sampled and held and then transmitted to an analog-to-digital converter (ADC). 
     In order to achieve the above object, according to one aspect of the present invention, a threshold voltage sensing circuit of an OLED display device including an OLED may include: a sampling capacitor configured to sample a threshold voltage of the OLED; a charge-share capacitor configured to charge-share the voltage sampled in the sampling capacitor; and a comparator configured to compare the variation range of the threshold voltage to a reference value, wherein when the variation range of the threshold voltage is larger than the reference value, the threshold voltage is stored in the sampling capacitor and the charge-share capacitor to make the variation range of the threshold voltage smaller than the reference value. 
     According to another aspect of the present invention, a threshold voltage sensing circuit of an OLED display device including an OLED may include: a sampling capacitor configured to sample a threshold voltage of the OLED; a charge-share capacitor configured to charge-share the voltage sampled in the sampling capacitor; an amplification section configured to variably amplify the threshold voltage outputted from the charge-share capacitor; and a comparator configured to compare the variation range of the threshold voltage to a reference value, wherein when the variation of the threshold voltage is larger than the reference value, the threshold voltage is stored in the sampling capacitor and the charge-share capacitor to make the variation range of the threshold voltage smaller than the reference value, and then transmitted to the amplification section. 
     According to another aspect of the present invention, a threshold voltage sensing circuit of an OLED display device including an OLED may include: a sampling capacitor configured to sample a threshold voltage of the OLED; one or more charge-share capacitors configured to charge-share the voltage sampled in the sampling capacitor; and a comparator configured to compare the variation range of the threshold voltage to a reference value, wherein when the variation range of the threshold voltage is larger than the reference value, the threshold voltage is stored in the sampling capacitor and the charge-share capacitor to make the variation range of the threshold voltage smaller than the reference value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects, and other features and advantages of the present invention will become more apparent after a reading of the following detailed description taken in conjunction with the drawings, in which: 
         FIG. 1  is a block diagram of a conventional threshold voltage sensing device of an OLED display device; 
         FIG. 2  is the entire block diagram of a threshold voltage sensing circuit of an organic light emitting diode (OLED) display device according to a first embodiment of the present invention; 
         FIGS. 3 to 5  are detailed circuit diagrams of respective units of  FIG. 2 ; 
         FIGS. 6 and 7  are circuit diagrams for explaining the operation of a first sample and hold section of  FIG. 4 ; 
         FIG. 8  is a timing diagram of the first sample and hold section of  FIG. 4 ; 
         FIGS. 9 to 12  are diagrams for explaining the operation of the first sample and hold section of  FIG. 4 ; 
         FIG. 13  is an analog-to-digital conversion timing diagram of an analog-to-digital conversion unit of  FIG. 5 ; 
         FIG. 14  is the entire block diagram of a threshold voltage sensing circuit of an OLED display device according to a second embodiment of the present invention; 
         FIGS. 15 to 17  are detailed circuit diagrams of respective units of  FIG. 14 ; 
         FIGS. 18 to 20  are circuit diagrams for explaining the operation of a first sample and hold section of  FIG. 16 ; 
         FIGS. 21A to 21C  are diagrams showing sensing voltage ranges and input conditions in  FIGS. 18 to 20 ; and 
         FIG. 22  is a diagram illustrating a range of sensed and inputted threshold voltages in the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Through the specification, when an element is referred to as being ‘electrically coupled’, ‘coupled’, or ‘connected’ between other elements, it may indicate that the elements are directly coupled or connected to each other or indirectly coupled or connected to each other through an intermediate medium, while each of the elements maintains its property to some extent or more. Furthermore, when a signal is referred to as being ‘transmitted’ or ‘derived’, it may indicate that the signal is directly transmitted or derived or indirectly transmitted or derived through an intermediate medium, while the signal maintains its property to some extent or more. Furthermore, when a voltage or signal is referred to as being ‘applied’ or ‘inputted’, it may indicate that the signal is directly applied or inputted or indirectly applied or inputted through an intermediate medium. 
     Furthermore, plural expressions of each element may be omitted. For example, although an element includes a plurality of switches or a plurality of signal lines, the plurality of switches or signal lines may be represented as ‘switches’ or ‘signal lines’ or ‘switch’ or ‘signal line’. This is because the switches may complementarily operate or independently operate depending on cases, and when a plurality of signals having the same property, for example, data signal lines are provided as a bundle of signals lines, the signal lines do not need to be divided into singular and plural forms. Thus, throughout the specification, similar expressions may be analyzed in the same manner. 
     The advantages and purpose accomplished by embodiments of the present invention will be understood with reference to the following descriptions and the accompanying drawings. 
     Hereafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 2  is the entire block diagram of a threshold voltage sensing circuit of an organic light emitting diode (OLED) display device according to a first embodiment of the present invention. The threshold voltage sensing circuit includes a data signal and precharge voltage output unit  100 , a sample and hold unit  200 , and an analog-to-digital conversion unit  300 .  FIGS. 3 to 5  are detailed circuit diagrams of the respective units. 
     The installation positions of the data signal and precharge voltage output unit  100 , the sample and hold unit  200 , and the analog-to-digital conversion unit  300  are not limited, but may be installed within a source driver for driving a display panel  400 . 
     Referring to  FIGS. 2 to 5 , the embodiment of the present invention will be described in detail. 
     The data signal and precharge voltage output unit  100  includes first to third digital to analog converters (DAC)  111  to  113 , the first to third switch sections  121  to  123 , first to third buffers  131  to  133 , an output signal control section  141 , and a threshold voltage sensing switch  151 . 
     In an image display mode for the display panel  400 , the first to third DACs  111  to  113  output a red data signal DATA_R, a green data signal DATA_G, and a blue data signal DATA_B, respectively. 
     The first to third switch sections  121  to  123  include a plurality of switches SP_ 11 , SR_ 11 , and SG_ 11 , a plurality of switches SP_ 12 , SR_ 12 , and SG_ 12 , and a plurality of switches SP_ 13 , SR_ 13 , and SG_ 13 , respectively. The first switch section  121  selects and outputs the red data signal DATA_R through the first-first red switch SR_ 11  or selects and outputs the green data signal DATA_G through the first-first green switch SG_ 11  in the image display mode, and selects and outputs a threshold voltage detection precharge voltage V PRE0  through the first-first output switch SP_ 11  in a threshold voltage sensing mode. 
     The second switch section  122  selects the red data signal DATA_R through the first-second red switch SR_ 12  or selects and outputs the blue data signal DATA_B through the first-second blue switch SB_ 12  in the image display mode, and selects and outputs the threshold voltage detection precharge voltage V PRE0  through the first-second output switch SP_ 12  in the threshold voltage sensing mode. 
     The third switch section  123  selects and output the red data signal DATA_G through the first-third green switch SG_ 13  or selects and outputs the blue data signal DATA_B through the first-third blue switch SB_ 13  in the image display mode, and selects and outputs the threshold voltage detection precharge voltage V PRE0  through the first-third output switch SP_ 13  in the threshold voltage sensing mode. 
     The first to third buffers  131  to  133  buffer a corresponding output signal among output signals of the first to third switch sections  121  to  123 . 
     The output signal control section  141  includes the first to third output signal control switches P 1 _ 1  to P 1 _ 3  for controlling signals which are outputted to data lines DL 1  to DL 3  from the first to third buffers  131  to  133 . 
     The threshold voltage sensing switch  151  selectively receives threshold voltages sensed from a corresponding pixel, after the threshold voltage detection precharge voltage V PRE0  is supplied to the OLED of the pixel. For this operation, the threshold voltage sensing switch  151  includes threshold voltage sensing switches SVT_ 11 , SVT_ 12 , SVT_ 21 , and SVT_ 22 . The first-first threshold voltage sensing switch SVT_ 11  selects and outputs a threshold voltage sensed from an arbitrary red OLED or green OLED coupled to the data line DL 1 . The first-second threshold voltage sensing switch SVT_ 12  and the second-first threshold voltage sensing switch SVT_ 21  select and output a threshold voltage sensed from an arbitrary blue OLED or red OLED coupled to the data line DL 2 . The second-second threshold voltage sensing switch SVT_ 22  selects and outputs a threshold voltage sensed from an arbitrary green OLED or blue OLED coupled to the data line DL 3 . 
     The method of selecting threshold voltages sensed from the OLEDs arranged in each horizontal line on the display panel and transmitting the selected threshold voltages to the sample and hold unit  200  may be implemented in various manners, but the present invention is not limited to a specific method. In the first embodiment of the present invention, a pair of threshold voltages are selected through the first-first to second-second threshold voltage sensing switches SVT_ 11 , SVT_ 12 , SVT_ 21 , and SVT_ 22 , and then transmitted to the sample and hold unit  200 . 
     For example, when the first-first threshold voltage sensing switch SVT_ 11  selects and outputs a threshold voltage sensed from an arbitrary red OLED coupled to the first data line DL 1 , the second-first threshold voltage sensing switch SVT_ 21  selects and outputs a threshold voltage sensed from an arbitrary red OLED coupled to the second data line DL 2 . 
     When the first-first threshold voltage sensing switch SVT_ 11  selects and outputs a threshold voltage sensed from an arbitrary green OLED coupled to the first data line DL 1 , the second-second threshold voltage sensing switch SVT_ 22  selects and outputs a threshold voltage sensed from an arbitrary green OLED coupled to the third data line DL 3 . 
     When the first-second threshold voltage sensing switch SVT_ 12  selects and outputs a threshold voltage sensed from an arbitrary blue OLED coupled to the second data line DL 2 , the second-second threshold voltage sensing switch SVT_ 22  selects and outputs a threshold voltage sensed from an arbitrary blue OLED coupled to the third data line DL 3 . 
     For reference, on the display panel  400 , a MOS transistor M_R for red serves to transmit the threshold voltage sensed from the red OLED to the corresponding data line. A MOS transistor M_G for green and a MOS transistor M_B for blue perform the same operation. 
     The sample and hold unit  200  includes first and second sample and hold sections  210  and  220  corresponding to a pair of threshold voltages inputted from the data signal and precharge voltage output unit  100 . The second sample and hold section  220  serves to provide a differential input to the sample and hold unit  200 , and has the same configuration as the first sample and hold section  210 . Thus, the following descriptions will be focused on the first sample and hold section  210 , for convenience of description. 
     The first sample and hold section  210  includes a sensing switch SVT_SEN, a sampling capacitor C S , a charge-share switch SVT_CS, a bypass switch SVT_BY, a charge-share capacitor C CS , a reset switch SVT_RST, a MOS transistor S_CA 1 , and a reference voltage source VREF. 
     The sensing switch SVT_SEN is coupled between a sensing voltage input terminal SVT_IN and one terminal of the sampling capacitor C S , and transmits a threshold voltage sensed from a corresponding OLED on the display panel  400  to the sampling capacitor C S . The sampling capacitor C S  is coupled between the other terminal of the sensing switch SVT_SEN and the reference voltage source VREF, and samples a threshold voltage inputted through the sensing switch SVT_SEN. 
     The charge-share switch SVT_CS is coupled between the one terminal of the sampling capacitor C S  and one terminal of the charge-share capacitor C CS , and transmits the sampled threshold voltage to the charge-share capacitor C CS . 
     The bypass switch SVT_BY is coupled between a sensing voltage input terminal SVT_IN and the one terminal of the charge-share capacitor C CS , and transmits the sensed threshold voltage to the charge-share capacitor C CS . 
     The charge-share capacitor C CS  is coupled between the reference voltage source VREF and the other terminals of the charge-share switch SVT_CS and the bypass switch SVT_BY, and charge-shares the threshold voltage stored in the sampling capacitor C S  or temporarily stores the threshold voltage inputted through the bypass switch SVT_BY and bypasses the threshold voltage. 
     The reset switch SVT_RST is coupled in parallel to both terminals of the charge-share capacitor C CS , and resets the voltage stored in the charge-share capacitor C CS . 
     The MOS transistor S_CA 1  is coupled between one terminal of the charge-share capacitor C CS  and the analog-to-digital conversion unit  300 , and transmits the threshold voltage stored in the charge-share capacitor C CS  to the analog-to-digital converter  300 . 
     The reference voltage source VREF is coupled between the ground terminal and the other terminals of the sampling capacitor C S  and the charge-share capacitor C CS , and supplies a predetermined reference voltage to the other terminals of the sampling capacitor C S  and the charge-share capacitor C CS . 
     When the first sample and hold section  210  samples and holds the sensed threshold voltages inputted through the data signal and precharge voltage output unit  100  and outputs the sampled and held threshold voltages to the analog-to-digital conversion unit  300  at the next stage, the first sample and hold section  210  may scale down the threshold voltages to threshold voltages having a variation within a predetermined range through charge sharing. 
     For example, when the variation ranges of the threshold voltages inputted to the first sample and hold section  210  correspond to Δ4V, Δ2.7V, Δ1.5V, and Δ1V, respectively, the first sample and hold section  210  scales down the threshold voltages of Δ4V and Δ2.7V using a scale factor of 0.375, and outputs threshold voltages of Δ1.5 and Δ1V, respectively. Furthermore, the first sample and hold section  210  bypasses the threshold voltages of Δ1.5V and Δ1V without scaling. Here, ‘Δ’ represents the variation range of a voltage. For example, ‘Δ4V’ may indicate that the corresponding voltage has a variation range of 4V. In the following descriptions, ‘Δ’ will be used as the same meaning. 
     The second sample and hold section  220  serves to supply a differential input to the analog-to-digital conversion unit  300 , and performs the same operation as the first sample and hold section  210 . Thus, the detailed descriptions thereof are omitted therein. As a result, the first sample and hold section  210  may output threshold voltages having a variation range of Δ1.5V to Δ1V, even though threshold voltages having various variation ranges are inputted. Such a process will be described with reference to  FIGS. 6 to 12 . 
     First, as illustrated in  FIG. 8 , precharge and sensing operations are performed on the OLEDs arranged on the display panel  400  of  FIG. 2  according to a precharge signal PRE and a sensing signal SEN. In  FIG. 8 , a channel select signal OES is used to determine whether to select unit pixels belonging to an odd channel on the display panel  400  or unit pixels belonging to an even channel on the display panel  400 . While the precharge signal PRE is activated, the precharge operation is performed. When the precharge operation is ended, the sensing switch SVT_SEN, the charge-share switch SVT_CS, and the reset switch SVT_RST are sequentially turned on. The first to 345th switching signals CA_ 1  to CA_ 345  indicate that 345 sample and hold operations are sequentially performed on the analog-to-digital conversion unit  300 . 
     At this time, when a threshold voltage having a variation of 4V (Δ4V) is transmitted to the sensing voltage input terminal SVT_IN of the first sample and hold section  210  through the first-first threshold voltage sensing switch SVT_ 11  or the first-second threshold voltage sensing switch SVT_ 12  from the threshold voltage sensing switch  151  of the data signal and precharge voltage output unit  100 , the first sample and hold section  210  is set in the scale mode by a controller (not illustrated), because the variation range of Δ4V is larger than the variation range of a threshold voltage to be outputted through the first sample and hold section  210 , that is, the variation range of Δ1.5V to Δ1.0V. Then, the first sample and hold section  210  performs a scaling operation as illustrated in  FIG. 9 . The controller includes a comparator (not illustrated) configured to compare the variation range of the threshold voltage to a reference value. According to the comparison result of the comparator, the controller performs the scale mode when the variation range of the threshold voltage is larger than the reference value, and performs the bypass mode when the variation range of the threshold voltage is smaller than the reference value. The reference value may be set in the range of 1.2V to 2.2V as in the embodiment of the present invention. 
     In the scale mode, since the sensing switch SVT_SEN is turned on as illustrated in  FIG. 6 , the threshold voltage of Δ4V, transmitted to the sensing voltage input terminal SVT_IN, is sampled into the sampling capacitor C S  through the sensing switch SVT_SEN. At this time, a voltage ranging from 1.2V to 1.7V is supplied to the reference voltage source VREF. In the present embodiment, the case in which a voltage of 1.5V is supplied to the reference voltage source VREF will be taken as an example for description. 
     After the charge voltage of the charge-share capacitor C CS  is reset by a turn-on operation of the reset switch SVT_RST, the charge-share switch SVT_CS is then turned on. Thus, the threshold voltage sampled in the sampling capacitor C S  is scaled (divided) by the charge-share capacitor C CS . At this time, in order to change the threshold voltage of Δ4V, sampled in the sampling capacitor C S , into a threshold voltage of Δ1.5V, the threshold voltage of Δ4V needs to be scaled down through a scale factor of 0.375. The operation of scaling down the threshold voltage through the scale factor of 0.375 may be accomplished by properly setting the capacitance values of the sampling capacitor C S  and the charge-share capacitor C CS . 
     The threshold voltage of Δ1.5V, scaled down through the above-described process, is outputted to the analog-to-digital conversion unit  300  through the MOS transistor S_CA 1 . 
     When a threshold voltage of Δ2.7V is transmitted to the sensing voltage input terminal SVT_IN as illustrated in  FIG. 10 , the operation mode is set to the scale mode, because Δ2.7V is larger than the variation range of Δ1.5V to Δ1.0V. Thus, the following scaling operation is performed. 
     In the scale mode, since the sensing switch SVT_SEN is turned on, the threshold voltage of Δ2.7V, transmitted to the sensing voltage input terminal SVT_IN, is sampled into the sampling capacitor C S  through the sensing switch SVT_SEN. At this time, a voltage ranging from 1.2V to 2.2V is supplied to the reference voltage source VREF. In the present embodiment, the case in which a voltage of 2V is supplied to the reference voltage source VREF will be taken as an example for description. 
     After the charge voltage of the charge-share capacitor C CS  is reset by a turn-on operation of the reset switch SVT_RST, the charge-share switch SVT_CS is turned on. Thus, the threshold voltage sampled in the sampling capacitor C S  is scaled down by the charge-share capacitor C CS . At this time, in order to change the threshold voltage of Δ2.7V, sampled in the sampling capacitor C S , to a threshold voltage of Δ1V, the threshold voltage of Δ2.7V needs to be scaled down through a scale factor of 0.375. The operation of scaling down the threshold voltage through the scale factor of 0.375 may be accomplished by properly setting the capacitance values of the sampling capacitor C S  and the charge-share capacitor C CS . 
     The threshold voltage of Δ1V, scaled down through the above-described process, is outputted to the analog-to-digital conversion unit  300  through the MOS transistor S_CA 1 . 
     However, when the threshold voltage of Δ1.5V is transmitted to the sensing voltage input terminal SVT_IN, no scaling operation is required because Δ1.5V falls within the variation range of a threshold voltage to be outputted by the first sample and hold section  210 . Thus, the operation mode is set in the bypass mode (1:1 mode) to perform the following operation. 
     In the bypass mode, the charge voltage of the charge-share capacitor C CS  is reset by the turn-on operation of the reset switch SVT_RST. Then, as illustrated in  FIG. 7 , the bypass switch SVT_BY is turned on to bypass the threshold voltage transmitted to the sensing voltage input terminal SVT_IN to the charge-share capacitor C CS  through the bypass switch SVT_BY. 
     At this time, a voltage ranging from 1.2V to 1.7V is supplied to the reference voltage source VREF. In the present embodiment, the case in which a voltage of 1.7V is supplied to the reference voltage source VREF will be taken as an example for description. The threshold voltage of Δ1.5V, bypassed through the above-described process, is outputted to the analog-to-digital conversion unit  300  through the MOS transistor S_CA 1 . 
     Furthermore, when a threshold voltage of Δ1V is transmitted to the sensing voltage input terminal SVT_IN, the operation mode is set to the bypass mode, because Δ1V falls within the variation range of a threshold voltage to be outputted by the first sample and hold section  210 . Then, the following operation is performed. 
     In the bypass mode, the charge voltage of the charge-share capacitor C CS  is reset by a turn-on operation of the reset switch SVT_RST. Then, the bypass switch SVT_BY is turned on to bypass the threshold voltage of Δ1V, transmitted to the sensing voltage input terminal SVT_IN, to the charge-share capacitor C CS  through the bypass switch SVT_BY. 
     At this time, a voltage ranging from 1.2V to 2.2V is supplied to the reference voltage source VREF. In the present invention, the case in which a voltage of 2.2V is supplied to the reference voltage source VREF will be taken as an example for description. 
     The threshold voltage of Δ1V, bypassed through the above-described process, is outputted to the analog-to-digital conversion unit  300  through the MOS transistor S_CA 1 . 
     The analog-to-digital conversion unit  300  converts the threshold voltage scaled down or bypassed through the sample and hold unit  200  into a digital signal, and outputs the digital signal. For this operation, the analog-to-digital conversion unit  300  includes an amplification section  310 , an analog-to-digital converter (ADC)  320 , a latch  330 , and a data driver  340  as illustrated in  FIG. 5 . 
     The amplification section  310  includes input switches P 1 _ 4  to P 1 _ 6  and input switches P 3 _ 1  and P 3 _ 2  for inputting the threshold voltages sampled and held through the first and second sample and hold sections  210  and  220 , a capacitor C CSP , a MOS transistor P 2 , an amplifier  311  for amplifying the input threshold voltages, capacitors C 85  to C 88  for adjusting the amplification factor of the amplifier  311 , and feedback switches P 4 _ 1  and P 4 _ 2 . The amplifier  311  includes two input terminals and two output terminals, in order to amplify the threshold voltages outputted from the first and second sample and hold sections  210  and  220 . 
     As described above, the amplification section  310  amplifies and outputs the threshold voltages outputted from the first and second sample and hold units  210  and  220 . However, the following descriptions will be focused on the case in which the amplification section  310  amplifies and outputs the threshold voltage outputted from the first sample and hold section  210 . 
     In the scale mode or bypass mode, when a threshold voltage of Δ1.5V is sampled and held by the first sample and hold section  210 , the fourth-first feedback switch P 4 _ 1  is turned on. Thus, the first and second capacitors C S5  and C S6  are coupled in parallel to each other between input and output terminals at one side of the amplifier  311 . Therefore, the amplifier  311  amplifies the threshold voltage of Δ1.5V, inputted from the first sample and hold section  210  through the switch P 3 _ 1 , at an amplification factor of 4/3 using the first and second capacitors C S5  and C S6  coupled in parallel to each other, and outputs the changed threshold voltage of Δ2V to the ADC  320  (refer to  FIGS. 9 and 11 ). 
     In the scale mode or bypass mode, when a threshold voltage of Δ1V is sampled and held by the first sample and hold section  210 , the fourth-first feedback switch P 4 _ 1  is turned off. Thus, the first capacitor C S5  is solely coupled between the input and output terminals at one side of the amplifier  311 . Therefore, the amplifier  311  amplifies the threshold voltage of Δ1V, inputted from the first sample and hold section  210  through the third-first input switch P 3 _ 1 , at an amplification factor of 2 using the capacitor C S5 , and outputs the changed threshold voltage of Δ2V to the analog-to-digital conversion unit  320  (refer to  FIGS. 10 and 12 ). 
     When the capacitance of the capacitor for one-time amplification in the amplifier  311  is set to C A , the capacitance of the capacitor for two-times amplification may be set to ½*C A , and the capacitance of the capacitor for 4/3-time amplification may be set to ¼*C A . 
     The analog threshold voltage of Δ2V, outputted from the amplification section  310 , is converted into a predetermined-bit digital signal (for example, 10-bit digital signal) by the ADC  320 , and latched in the latch  330 . 
     Furthermore, the digital signal latched in the latch  330  is outputted through the data driver  340 . 
     Therefore, when a threshold voltage of Δ4V or 2.7V is inputted to the sample and hold unit  200 , the threshold voltage may be scaled down as described above, and when a threshold voltage of Δ1.5 or Δ1V is inputted, the threshold voltage may be bypassed as described above. Then, the threshold voltage may be amplified through the amplification section  310 . Thus, even when four kinds of threshold voltages having different variation ranges are inputted as illustrated in  FIGS. 9 to 12 , an analog threshold voltage having a variation range of 2V may be inputted to the analog-to-digital conversion unit  320 . 
       FIG. 13  is a timing diagram of the analog-to-digital conversion unit  300 . In  FIG. 13 , CA_ 1  to CA_K represent the output timings of threshold voltages supplied to the ADC  320  from a predetermined number of sample and hold units (for example, 240 sample and hold units), P 1  represents the reset timing of the amplifier  311 , and P 2  represents the timing of the reference voltage supplied to the amplifier  311 . As illustrated in  FIG. 13 , the reference voltage may be supplied in synchronization with the output timings of the threshold voltages. 
       FIG. 14  is a circuit diagram of a threshold voltage sensing circuit of an OLED display device according to a second embodiment of the present invention. As illustrated in  FIG. 14 , the threshold voltage sensing circuit includes a data signal and precharge voltage output unit  500 , a sample and hold unit  600 , and an analog-to-digital conversion unit  700 . 
     The installation positions of the data signal and precharge voltage output unit  500 , the sample and hold unit  600 , and the analog-to-digital conversion unit  700  are not limited, but may be installed within a source driver. 
     The data signal and precharge voltage output unit  500  includes first to sixth DACs  511  to  516 , first to sixth buffers  521  to  526 , first to sixth switch sections  531  to  536 , and a threshold voltage sensing switch section  541 . 
     In the image display mode for a display panel, the first DAC  511  and the fourth DAC  514  output a red data signal DATA_R, the second DAC  512  and the fifth DAC  515  output a green data signal DATA_G, and the third DAC  513  and the sixth DAC  516  output a blue data signal DATA_B. 
     Each of the first to sixth buffers  521  to  526  buffers and outputs the corresponding data signal among the red, green, and blue data signals DATA_R, DATA_G, and DATA_B outputted from the first to sixth DACs  511  to  516 . 
     The first to sixth switch sections  531  to  536  include switches SP_ 21  and SR_ 21 , switches SP_ 22  and SG_ 21 , switches SP_ 23  and SB_ 21 , switches SP_ 24  and SR_ 22 , switches SP_ 25  and SG_ 22 , and switches SP_ 26  and SB_ 22 , respectively. The first switch section  531  selects and outputs the red data signal DATA_R through the second-first red switch SR_ 21  in the image display mode, and selects and outputs a threshold voltage detection precharge voltage V PRE0  through the second-first output switch SP_ 21  in the threshold voltage sensing mode. The second switch section  532  selects and outputs the green data signal DATA_G through the second-first green switch SG_ 21  in the image display mode, and selects and outputs the threshold voltage detection precharge voltage V PRE0  through the second-second output switch SP_ 22  in the threshold voltage sensing mode. The third switch section  533  selects and outputs the blue data signal DATA_B through the second-first blue switch SB_ 21  in the image display mode, and selects and outputs the threshold voltage detection precharge voltage V PRE0  through the second-third output switch SP_ 23  in the threshold voltage sensing mode. The fourth switch section  534  selects and outputs the red data signal DATA_R through the second-second blue switch SR_ 22  in the image display mode, and selects and outputs the threshold voltage detection precharge voltage V PRE0  through the second-fourth output switch SP_ 24  in the threshold voltage sensing mode. The fifth switch section  535  selects and outputs the green data signal DATA_G through the second-second green switch SG_ 22  in the image display mode, and selects and outputs the threshold voltage detection precharge voltage V PRE0  through the second-fifth output switch SP_ 25  in the threshold voltage sensing mode. The sixth switch section  536  selects and outputs the blue data signal DATA_B through the second-second blue switch SB_ 22  in the image display mode, and selects and outputs the threshold voltage detection precharge voltage V PRE0  through the second-sixth output switch SP_ 26  in the threshold voltage sensing mode. 
     The threshold voltage sensing switch section  541  includes a plurality of threshold voltage sensing switches SVT_ 31  to SVT_ 33  and SVT_ 41  to SVT_ 43 . The third-first threshold voltage sensing switch SVT_ 31  selects and outputs a threshold voltage sensed from an arbitrary red OLED coupled to a first data line DL 1 . The third-second threshold voltage sensing switch SVT_ 32  selects and outputs a threshold voltage sensed from an arbitrary green OLED coupled to a second data line DL 2 . The third-third threshold voltage sensing switch SVT_ 33  selects and outputs a threshold voltage sensed from an arbitrary blue OLED coupled to a third data line DL 3 . The fourth-first threshold voltage sensing switch SVT_ 41  selects and outputs a threshold voltage sensed from an arbitrary red OLED coupled to a fourth data line DL 4 . The fourth-second threshold voltage sensing switch SVT_ 42  selects and outputs a threshold voltage sensed from an arbitrary green OLED coupled to a fifth data line DL 5 . The fourth-third threshold voltage sensing switch SVT_ 43  selects and outputs a threshold voltage sensed from an arbitrary blue OLED coupled to a sixth data line DL 6 . 
     The method of selecting a threshold voltage sensed from an OLED arranged in each horizontal line on the display panel and transmitting the selected threshold voltage to the sample and hold unit  600  may be implemented in various manners, and the present invention is not limited to a specific method. In the second embodiment of the present invention, a pair of threshold voltages among the threshold voltages for red, green, and blue may be selected through the threshold voltage sensing switches SVT_ 31  to SVT_ 33  and SVT_ 41  to SVT_ 43 , and then transmitted to the sample and hold unit  600 . 
     For example, when the third-first threshold voltage sensing switch SVT_ 31  selects and outputs a threshold voltage sensed from an arbitrary red OLED coupled to the first data line DL 1 , the fourth-first threshold voltage sensing switch SVT_ 41  may select and output a threshold voltage sensed from an arbitrary red OLED coupled to the fourth data line DL 4 . 
     The sample and hold unit  600  includes first and second sample and hold sections  610  and  620  having the same configuration, in response to a pair of threshold voltages inputted from the data signal and precharge voltage output unit  500 . In the present embodiment, the first sample and hold section  610  will be taken as an example for description. 
     The first sample and hold section  610  includes a sensing switch SMP, a second reference voltage switch SVR 2 , a sampling capacitor C S , a first charge-share switch S_CS 1 , a first reference voltage switch SVR 1 , a first charge-sharing operation switch SCAP 1 , a first charge-share capacitor C CS1 , a second charge-sharing operation switch SCAP 2 , a second charge-share capacitor C CS2 , a reset switch RST 1 , a second charge-share switch S_CS 2 , a second reference voltage source VREF 2 , and a first reference voltage source VREF 1 . 
     The sensing switch SMP is coupled between a sensing voltage input terminal SVT_IN and one terminal of the sampling capacitor C S , and transmits a threshold voltage sensed from an OLED of the display panel to the sampling capacitor C S . The second reference voltage switch SVR 2  is coupled between the second reference voltage source VREF 2  and the other terminal of the sampling capacitor C S , and transmits the voltage of the second reference voltage source VREF 2  to the other terminal of the sampling capacitor C S . The sampling capacitor C S  is coupled between the other terminal of the sensing switch SMP and the other terminal of the second reference voltage switch SVR 2 , and samples the threshold voltage inputted through the sensing switch SMP. The first charge-share switch S_CS 1  is coupled to one terminal of the sampling capacitor C S . The first reference voltage switch SVR 1  is coupled between the other terminal of the second reference voltage switch SVR 2  and the other terminal of the first charge-share capacitor C CS1 , and transmits the voltage of the second reference voltage source VREF 2  to the first and second charge-share capacitors C CS1  and C CS2 . The first charge-sharing operation switch S_CAP 1  is coupled between the other terminal of the first charge-share switch S_CS 1  and one terminal of the first charge-share capacitor C CS1 , and determines whether to enable the charge-sharing operation of the first charge-share capacitor C CS1 . The first charge-share capacitor C CS1  is coupled between the other terminal of the first charge-sharing operation switch S_CAP 1  and the other terminal of the first reference voltage switch SVR 1 , and charge-shares the threshold voltage sampled in the sampling capacitor C S . The second charge-sharing operation switch S_CAP 2  is coupled between the other terminal of the first charge-share switch S_CS 1  and one terminal of the second charge-share capacitor C CS2 , and determines whether to enable the charge-sharing operation of the second charge-share capacitor C CS2 . The second charge-share capacitor C CS2  is coupled between the other terminal of the second charge-sharing operation switch S_CAP 2  and the other terminal of the first reference voltage switch SVR 1 , and charge-shares the threshold voltage sampled in the sampling capacitor C S . The reset switch RST 1  is coupled between the other terminal of the first charge-share switch S_CS 1  and the other terminal of the first reference voltage switch SVR 1 , and resets the threshold voltages stored in the first and second charge-share capacitors C CS1  and C CS2 . The second charge-share switch S_CS 2  is coupled between the other terminal of the first charge-share switch S_CS 1  and an input terminal of the analog-to-digital conversion unit  700 , and transmits the threshold voltages stored in the first and second charge-share capacitors C CS1  and C CS2  to the input terminal. When the first sample and hold section  610  samples and holds threshold voltages sensed and inputted from arbitrary OLEDs on the display panel through the data signal and precharge voltage output unit  500  and outputs the sampled and held threshold voltages to the analog-to-digital conversion unit  700  at the next stage, the first sample and hold section  610  may scale down the threshold voltages having a range of a reference value or more (for example, 2 or more) into threshold voltages having a range of a predetermined value or less (for example, the minimum integer 1 or less). 
     For example, when a threshold voltage having a variation range of 3V (Δ3V) or 2V (Δ2V) is inputted to the first sample and hold section  610 , the first sample and hold section  610  may scale downs the threshold voltage to a threshold voltage of Δ1V through charge sharing. When a threshold voltage of Δ1V is inputted, the first sample and hold section  610  may not perform the charge-sharing operation, but bypass the threshold voltage. Such a process will be described below with reference to  FIGS. 18 to 22 . 
     First, a precharge and sensing operation is performed on the OLEDs of the display panel. 
     At this time, when a threshold voltage having a variation range of 3V (Δ3V), for example, one of a threshold voltage ranging from 2V to 5V, a threshold voltage ranging from 3V to 6V, a threshold voltage ranging from 4V to 7V, and a threshold voltage ranging from 5V to 8V as illustrated in  FIG. 21A  is transmitted to the sensing voltage input terminal SVT_IN of the first sample and hold section  610  through any one of the threshold voltage sensing switches SVT_ 31  to SVT_ 33  in the threshold voltage sensing switch section  541  of the data signal and precharge output unit  500 , the threshold voltage may be scaled down to a threshold voltage having a variation range of Δ1V, that is, one of a threshold voltage ranging from 2V to 3V, a threshold voltage ranging from 3V to 4V, a threshold voltage ranging from 4V to 5V, and a threshold voltage ranging from 5V to 6V by a controller (not illustrated) through the following process. The scaling process will be described with reference to  FIG. 18 . 
     First, the first and second charge-sharing operation switches S_CAP 1  and S_CAAP 2  and the reset switch RST 1  are turned on. Thus, voltages remaining in the first and second charge-share capacitors C CS1  and C CS2  are discharged by the reset switch RST 1 . At this time, the second reference voltage switch SVR 2  is turned on to supply the voltage of the second reference voltage source VREF 2  to the other terminal of the sampling capacitor C S  through the second reference voltage switch SVR 2 . 
     Subsequently, the sensing switch SMP is turned on to sample a threshold voltage of Δ3V, inputted through the sensing voltage input terminal SVT_IN, into the sampling capacitor C S . Thus, the threshold voltage sampled in the sampling capacitor C S  may have a potential obtained by adding the threshold voltage of Δ3V to the voltage of the second reference voltage source VREF 2 . 
     According to a user&#39;s request, a voltage range to be sensed may be set to a packet, and a threshold voltage may be sensed through the above-described process. Then, the voltage of the second reference voltage source EVREF 2  may be set to a proper value ranging from 2V to 5V, for example, such that the sensed threshold voltage falls within the range of a target threshold voltage. 
     Then, the second reference voltage switch SVR 2  and the sensing switch SMP are turned off, and the first reference voltage switch SVR 1  and the first charge-share switch S_SC 1  are turned on. Thus, the sampling capacitor C S  and the first and second charge-share capacitors C CS1  and C CS2  are coupled in parallel to each other. Therefore, the voltage sampled in the sampling capacitor C S  is charge-shared by the first and second charge-share capacitors C CS1  and C CS2 , and reduced to ⅓. That is, the threshold voltage of Δ3V is scaled down to a threshold voltage of Δ1V. At this time, in order to convert the sensed high-voltage level into a low-voltage level of the amplifier  711  of the analog-to-digital conversion unit  700 , the voltage of the first reference voltage source VREF 1  is supplied to the sampling capacitor C S  and the first and second charge-share capacitors C CS1  and C CS2 . 
     The threshold voltage of Δ1V, reduced to ⅓ as described above, is transmitted to the analog-to-digital conversion unit  700  at the next stage through the second charge-share switch S_CS 2 . The second charge-share switch S_CS 2  illustrated in  FIGS. 18 to 20  may be implemented with various types of switching elements, and  FIG. 16  illustrates an example in which the second charge-share switch S_CS 2  is implemented with a MOS transistor. 
     When a threshold voltage of Δ2V, for example, one of a threshold voltage ranging from 2V to 4V, a threshold voltage ranging from 3V to 5V, a threshold voltage ranging from 4V to 6V, and a threshold voltage ranging from 5V to 7V as illustrated in  FIG. 21B  is transmitted to the sensing voltage input terminal SVT_IN of the first sample and hold section  610 , the threshold voltage is scaled down to a threshold voltage of Δ1V, for example, one of a threshold voltage ranging from 2V to 3V, a threshold voltage ranging from 3V to 4V, a threshold voltage ranging from 4V to 5V, and a threshold voltage ranging from 5V to 6V is scaled down, and then outputted. The scaling process will be described with reference to  FIG. 19 . 
     The process of scaling down the threshold voltage of Δ2V to the threshold voltage of Δ1V is similar to the process of scaling down the threshold voltage of Δ3V to the threshold voltage of Δ1V. However, the process of scaling down the threshold voltage of Δ2V to the threshold voltage of Δ1V is different from the process of scaling down the threshold voltage of Δ3V to the threshold voltage of Δ1V in that the second reference voltage source VREF 2  is set in the range of 2V to 6V, one of the first and second charge-sharing operation switches S_CAP 1  and S_CAP 2 , for example, the first charge-sharing operation switch S_CAP 1  is turned on, the second charge-sharing operation switch S_CAP 2  is turned off, and the voltage sampled in the sampling capacitor C S  is scaled down to ½ by the first charge-sharing operation switch S_CAP 1 . 
     When a threshold voltage having a variation range of 1V (Δ1V), for example, one of a threshold voltage ranging from 2V to 3V, a threshold voltage ranging from 3V to 4V, a threshold voltage ranging from 4V to 5V, a threshold voltage ranging from 5V to 6V, and a threshold voltage ranging from 7V to 8V as illustrated in  FIG. 21C  is transmitted to the sensing voltage input terminal SVT_IN of the first sample and hold section  610 , the above-described scaling process is not performed, and the threshold voltage is bypassed. This process will be described with reference to  FIG. 20 . 
     The largest difference between the process of bypassing the threshold voltage of Δ1V and the process of scaling down the threshold voltage of Δ3V to the threshold voltage of Δ1V is that both of the first and second charge-sharing operation switches S_CAP 1  and S_CAP 2  are turned off and no scaling operation is performed. Furthermore, the voltage of the second reference voltage source VREF 2  is set in the range of 2V to 7V. 
     Then, the analog-to-digital conversion unit  700  processes the threshold voltage of Δ1V, scaled down or bypassed by the sample and hold unit  600  through the above-described process, in the same manner as the analog-to-digital conversion unit  300  of  FIG. 2 , and outputs the corresponding digital signal. 
     According to the embodiments of the present invention, when the threshold voltage of the OLED display panel is sensed and transmitted to the ADC, the threshold voltage may be scaled down to threshold voltages within a predetermined range through charge sharing. Thus, the low-voltage driving elements within the ADC may be protected, and the OLEDs may maintain constant brightness. 
     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and the spirit of the invention as disclosed in the accompanying claims.