Abstract:
A light sensing circuit for use in auto brightness control (ABC), and a flat panel display including the light sensing circuit. The light sensing circuit includes a first photodiode; a second photodiode which is electrically connected to the first photodiode and includes a shielding film for shielding externally incident light; a first voltage fixing unit which is connected to the first photodiode and the second photodiode and maintains a voltage applied to the first photodiode at a certain value; and an analog-to-digital converter (ADC) which generates a digital value that depends on a current flowing in the first photodiode and the second photodiode. Accordingly, the light sensing circuit can precisely sense the brightness of a surrounding environment and drive a flat panel display with the most suitable brightness.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application entitled LIGHT SENSING CIRCUIT AND FLAT PANEL DISPLAY INCLUDING THE SAME earlier filed in the Korean Industrial Property Office on 17 Oct. 2008, which was duly assigned Serial No. 10-2008-0102105 by that Office. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a light sensing circuit and a flat panel display including the light sensing circuit, and more particularly, to a light sensing circuit for use in automatic brightness control and a flat panel display including the light sensing circuit. 
         [0004]    2. Description of the Related Art 
         [0005]    After the development of CRTs, various flat panel displays such as PDPs, LCDs, and OLEDs have been developed and widely used. These flat panel displays are used in various products such as TVs, computer monitors, cellular phone screens, etc. 
         [0006]    However, flat panel displays used as the screens of products, such as TVs or computer monitors, may not be properly viewed according to the brightness levels of the surrounding environment. For example, if such a product displays images with normal brightness in dark places, the screen thereof is too bright for users to open their eyes. On the other hand, if such a product displays images with normal brightness in bright places, users think that the screen of the product is too dark. 
         [0007]    To address this problem, auto brightness control is applied to flat panel displays in recent years. In auto brightness control, the brightness of a surrounding environment where a flat panel display is used is detected, and the brightness of images displayed on the flat panel display is adjusted according to the detected brightness of the surrounding environment. 
         [0008]    To accomplish such auto brightness control, a light sensing circuit is used. The light sensing circuit includes a photodiode which may generate current according to the brightness of incident light and detect the level of the brightness of the incident light according to the current. However, the photodiode generates different magnitudes of currents according to not only the brightness of the incident light but also an ambient temperature. 
         [0009]      FIG. 1  is a graph showing a current that flows in a photodiode included in an existing light sensing circuit. In  FIG. 1 , the current was measured at 25° C. and 40° C., and the brightnesses of light incident upon the photodiode at 25° C. and 40° C. were both 0 Lux. A horizontal axis of  FIG. 1  represents a voltage, and a vertical axis thereof represents a current. 
         [0010]    Referring to  FIG. 1 , when an identical voltage is applied to both ends of the photodiode and light with certain brightness is incident upon the photodiode, the higher the temperature is, the more current flows. 
         [0011]    In addition, when considering each of the cases where the current was measured at 25° C. and 40° C., if the magnitude of the voltage applied to the both ends of the photodiode changes, the magnitude of the generated current changes. More specifically, as the magnitude of the voltage applied to the both ends of the photodiode increases, the magnitude of the generated current also increases. 
         [0012]    As described above, since the current generated in the photodiode included in the existing light sensing circuit changes according to not only the brightness of incident light but also an ambient temperature, the existing light sensing circuit fails to accurately measure the brightness of a surrounding environment where a product such as a TV or a monitor is used. 
         [0013]    Moreover, when a current is generated in the photodiode, the potential of an anode electrode or a cathode electrode of the photodiode changes according to the time, and thus the magnitude of a voltage applied to the both ends of the photodiode is changed. Accordingly, even in an identical surrounding environment, the existing light sensing circuit recognizes that the brightness of the surrounding environment changes as the time elapses. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention provides a light sensing circuit capable of precisely sensing the brightness of a surrounding environment and driving a flat panel display with optimal brightness, and the flat panel display including the light sensing circuit. 
         [0015]    According to an aspect of the present invention, there is provided a light sensing circuit including: a first sensing circuit which generates a current according to incident light and an ambient temperature; and a second sensing circuit which generates a current according to the ambient temperature, wherein each of the first sensing circuit and the second sensing circuit includes a photodiode; an OP (operational) amplifier connected to the photodiode; and an analog-to-digital converter (ADC) which measures a current generated in the photodiode and converts the current into a digital value. 
         [0016]    In at least one of the first sensing circuit and the second sensing circuit, a first input terminal of the OP amplifier may be connected to the photodiode, a reference voltage may be applied to a second input terminal of the OP amplifier, and an output terminal of the OP amplifier may be connected to the ADC. 
         [0017]    The at least one of the first sensing circuit and the second sensing circuit may further include a capacitor connected between the first input terminal of the OP amplifier and the output terminal of the OP amplifier; and a switch connected between the first input terminal of the OP amplifier and the output terminal of the OP amplifier. 
         [0018]    In the at least one of the first sensing circuit and the second sensing circuit, the reference voltage may be applied to the first input terminal of the OP amplifier, and the second input terminal of the OP amplifier may be connected to the photodiode. 
         [0019]    The at least one of the first sensing circuit and the second sensing circuit may further include a transistor including a first electrode connected to the photodiode, a second electrode connected to the ADC, and a gate electrode connected to the output terminal of the OP amplifier; and a switch connected to the photodiode. 
         [0020]    The light sensing circuit may further include a calculation unit which receives digital values obtained by the ADCs of the first and second sensing circuits and measures the brightness of the incident light. 
         [0021]    According to another aspect of the present invention, there is provided a light sensing circuit including a first photodiode; a second photodiode which is electrically connected to the first photodiode and includes a shielding film for shielding externally incident light; a first voltage fixing unit which is connected to the first photodiode and the second photodiode and maintains a voltage applied to the first photodiode at a certain value; and an ADC which generates a digital value that depends on a current flowing in the first photodiode and the second photodiode. 
         [0022]    The first voltage fixing unit may include a first OP amplifier including a first input terminal to which the reference voltage is applied, a second input terminal connected to the first photodiode, and an output terminal; and a first transistor including a first electrode connected to the first photodiode, a second electrode connected to the ADC, and a gate electrode connected to the output terminal of the OP amplifier. 
         [0023]    The first voltage fixing unit may further include a first switch which is connected to the first photodiode and applies the reference voltage to the first photodiode. 
         [0024]    The first voltage fixing unit may include a second OP amplifier including a first input terminal connected to the first photodiode, a second input terminal to which the reference voltage is applied, and an output terminal; and a first capacitor including a first electrode connected to the first input terminal of the second OP amplifier and a second electrode connected to the output terminal of the second OP amplifier. 
         [0025]    The first voltage fixing unit may further include a second switch which is connected between the first electrode and the second electrode of the capacitor and short-circuits the capacitor; and a third switch which is connected to the first photodiode and applies the reference voltage to the first photodiode. 
         [0026]    The light sensing circuit may further include a second voltage fixing unit which constantly maintains a voltage applied to the second photodiode. 
         [0027]    The second voltage fixing unit may include a third OP amplifier including a first input terminal connected to the second photodiode, a second input terminal to which the reference voltage is applied, and an output terminal; and a second transistor including a first electrode connected to the second photodiode, a second electrode connected to the first photodiode, and a gate electrode connected to the output terminal of the third OP amplifier. 
         [0028]    The second voltage fixing unit may further include a fourth switch connected to the second photodiode. 
         [0029]    According to another aspect of the present invention, there is provided a flat panel display including a plurality of pixels; a light sensing circuit which senses the brightness of externally incident light; a plurality of driving units which drive the plurality of pixels; and a controller which controls the driving units and controls the brightness of data displayed on the plurality of pixels according to the light brightness sensed by the light sensing circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
           [0031]      FIG. 1  is a graph showing a current that flows in a photodiode included in an existing light sensing circuit; 
           [0032]      FIG. 2  is a block diagram of a light sensing circuit according to an embodiment of the present invention; 
           [0033]      FIG. 3  is a circuit diagram of the light sensing circuit of  FIG. 2 , according to an embodiment of the present invention; 
           [0034]      FIG. 4  is a circuit diagram of the light sensing circuit of  FIG. 2 , according to another embodiment of the present invention; 
           [0035]      FIG. 5  is a block diagram of a light sensing circuit according to another embodiment of the present invention; 
           [0036]      FIG. 6  is a circuit diagram of the light sensing circuit of  FIG. 5 , according to an embodiment of the present invention; 
           [0037]      FIG. 7  is a circuit diagram of the light sensing circuit of  FIG. 5 , according to another embodiment of the present invention; 
           [0038]      FIG. 8  is a block diagram of a light sensing circuit according to another embodiment of the present invention; 
           [0039]      FIG. 9  is a circuit diagram of the light sensing circuit of  FIG. 8 , according to an embodiment of the present invention; and 
           [0040]      FIG. 10  is a block diagram of a flat panel display including a light sensing circuit, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. 
         [0042]      FIG. 2  is a block diagram of a light sensing circuit  100  according to an embodiment of the present invention. 
         [0043]    Referring to  FIG. 2 , the light sensing circuit  100  includes a first sensing circuit and a second sensing circuit. The first sensing circuit and the second sensing circuit may include photodiodes  110  and  110 - 1 , respectively, voltage fixing units  120  and  120 - 1 , respectively, and analog-to-digital converters (ADCs)  130  and  130 - 1 , respectively, and share a calculation unit  140 . 
         [0044]    The photodiode  110  generates a current according to a brightness of externally incident light and an ambient temperature. The photodiode  110 - 1  further includes a shielding film  111  in order to shield the externally incident light. Accordingly, the photodiode  110 - 1  generates a current which depends on only the ambient temperature. 
         [0045]    The voltage fixing units  120  and  120 - 1  fix voltages applied to the photodiodes  110  and  110 - 1  to certain values. The voltage fixing units  120  and  120 - 1  include voltage comparing units  121  and  121 - 1 , respectively, and voltage adjusting units  122  and  122 - 1 , respectively. 
         [0046]    The voltage comparing units  121  and  121 - 1  receive a reference voltage V ref  and voltages of anode electrodes of the photodiodes  110  and  110 - 1 , respectively, and compare the reference voltage V ref  with the voltage of the anode electrode of the photodiode  110  and the reference voltage V ref  with the voltage of the anode electrode of the photodiode  110 - 1 , respectively. Output voltages corresponding to results of the comparisons are applied to the voltage adjusting units  122  and  122 - 1 , respectively. 
         [0047]    The voltage adjusting units  122  and  122 - 1  receive the output voltages of the voltage comparing units  121  and  121 - 1 , respectively, and adjust the received voltages so that the voltages of the anode electrodes of the photodiodes  110  and  110 - 1  are equal to the reference voltage V ref . 
         [0048]    The ADCs  130  and  130 - 1  receive voltage or current values depending on the currents generated in the photodiodes  110  and  110 - 1 , respectively, and convert the received voltage or current values into digital values. The ADCs  130  and  130 - 1  are connected to the voltage fixing units  120  and  120 - 1 , respectively. 
         [0049]    The calculation unit  140  receives the digital value output from the ADC  130  and the digital value output from the ADC  130 - 1  and calculates the brightness of the externally incident light. 
         [0050]    Embodiments of the light sensing circuit  100  of  FIG. 2  will now be described with reference to  FIGS. 3 and 4 . 
         [0051]      FIG. 3  is a circuit diagram of the light sensing circuit  100  of  FIG. 2 , according to an embodiment of the present invention. The light sensing circuit of  FIG. 3  may be one of the first sensing circuit and the second sensing circuit. Alternatively, the light sensing circuit of  FIG. 3  may be applied to both the first sensing circuit and the second sensing circuit. 
         [0052]    The light sensing circuit of  FIG. 3  may include the photodiode  110 , an OP (operational) amplifier  121   a , a capacitor C 3 , a switch SW 3 , and the ADC  130 . 
         [0053]    The photodiode  110  generates a current according to the brightness of externally incident light and an ambient temperature. The reference voltage V ref  is applied to an anode electrode of the photodiode  110 , and a first voltage V REV  is applied to a cathode electrode of the photodiode  110 . The first voltage V REV  is greater than the reference voltage V ref . Accordingly, a reverse bias voltage is applied to the photodiode  110 . The reference voltage V ref  may be a ground voltage. 
         [0054]    The OP amplifier  121   a , which is an embodiment of a voltage comparing unit, includes an inverting input terminal, a non-inverting input terminal, and an output terminal. The inverting input terminal of the OP amplifier  121   a  is connected to the anode electrode of the photodiode  110 , and the output terminal thereof is connected to the ADC  130 . The reference voltage V ref  is applied to the non-inverting input terminal of the OP amplifier  121   a.    
         [0055]    The OP amplifier  121   a  compares the reference voltage V ref  with the voltage of the anode electrode of the photodiode  110 , and outputs as an output voltage a value corresponding to a difference between the two voltages. 
         [0056]    The capacitor C 3 , which is an embodiment of a voltage adjusting unit, includes a first electrode and a second electrode. The first electrode of the capacitor C 3  is connected to the inverting input terminal of the OP amplifier  121   a , and the second electrode of the capacitor C 3  is connected to the output terminal of the OP amplifier  121   a.    
         [0057]    As the output voltage of the OP amplifier  121   a , which is output via the output terminal, changes, the capacitor C 3  shifts a voltage of the second electrode in order to constantly maintain a voltage between the first electrode and the second electrode of the capacitor C 3 . 
         [0058]    Switch SW 3  is connected between the first and second electrodes of the capacitor C 3 . When the switch SW 3  is turned on by an initiation signal INIT, the capacitor C 3  is short-circuited. 
         [0059]    The ADC  130  generates a digital value corresponding to the current generated in the photodiode  110 . More specifically, the ADC  130  is connected to the output terminal of the OP amplifier  121   a  and accordingly receives the output voltage from the OP amplifier  121   a  and generates the digital value into which the output voltage is converted from an analog value. 
         [0060]    In an operation of the light sensing circuit of  FIG. 3 , when light is externally incident upon the photodiode  110 , a current is generated in the photodiode  110 . The current flows from the cathode electrode of the photodiode  110  to the anode electrode thereof. 
         [0061]    The current increases the voltage of the anode electrode of the photodiode  110 . Due to the increase in the voltage of the anode electrode, a voltage applied to the non-inverting input terminal of the OP amplifier  121   a  may be greater than that applied to the inverting input terminal thereof. Thus, the output voltage of the output terminal of the OP amplifier  121   a  is decreased, and the capacitor C 3  shifts the voltage of the first electrode thereof in order to maintain the voltage between the inverting input terminal and the output terminal of the OP amplifier  121   a.    
         [0062]    Accordingly, the cathode electrode of the photodiode  110  is maintained to have the first voltage V REV , and the anode electrode thereof is maintained to have the reference voltage V ref . 
         [0063]    When the light sensing circuit detects the brightness of periodically incident light, the initiation signal INIT is applied to the switch SW 3  to turn on the switch SW 3 . Thus, not only the voltage of the anode electrode of the photodiode  110  but also the voltage of the output terminal of the OP amplifier  121   a  may be initiated to the reference voltage V ref . 
         [0064]    In the light sensing circuit of  FIG. 3 , the reference voltage V ref  is applied to the anode electrode of the photodiode  110 , and the first voltage V REV  is applied to the cathode electrode thereof. However, the present invention is not limited to this embodiment. In other words, the structure of the photodiode  110  may be changed as long as a reverse bias voltage can be applied to the photodiode  110 . 
         [0065]    For example, a second voltage −V REV  may be applied to the anode electrode of the photodiode  110 , and the cathode electrode may be connected to the inverting input terminal of the OP amplifier  121   a.    
         [0066]      FIG. 4  is a circuit diagram of the light sensing circuit  100  of  FIG. 2 , according to another embodiment of the present invention. The light sensing circuit of  FIG. 4  may be one of the first sensing circuit and the second sensing circuit. Alternatively, the light sensing circuit of  FIG. 4  may be applied to both the first sensing circuit and the second sensing circuit. The light sensing circuit of  FIG. 4  will now be described by focusing on differences from the light sensing circuit of  FIG. 3 , and a description of the same elements as those of  FIG. 3  will be omitted. 
         [0067]    The light sensing circuit according to the current embodiment of  FIG. 4  may include the photodiode  110 , an OP amplifier  121   a , a transistor Tr 4 , a switch SW 4 , and the ADC  130 . 
         [0068]    The OP amplifier  121   a , which is an embodiment of a voltage comparing unit, includes an inverting input terminal, a non-inverting input terminal, and an output terminal. The non-inverting input terminal of the OP amplifier  121   a  is connected to the anode electrode of the photodiode  110 , and the output terminal thereof is connected to the gate electrode of the transistor Tr 4 . The reference voltage V ref  is applied to the inverting input terminal of the OP amplifier  121   a.    
         [0069]    The transistor Tr 4 , which is an embodiment of a voltage adjusting unit, includes a first electrode, a second electrode, and a gate electrode. The first electrode of the transistor Tr 4  is connected to the anode electrode of the photodiode  110 , the second electrode thereof is connected to the ADC  130 , and the gate electrode thereof is connected to the output terminal of the OP amplifier  121   a.    
         [0070]    The transistor Tr 4  is turned on or off according to an output voltage of the OP amplifier  121   a , which is received via the gate electrode of the transistor Tr 4 , and thus controls the voltage of the anode electrode of the photodiode  110  to have a constant level. 
         [0071]    Switch SW 4  is connected between the anode electrode of the photodiode  110  and a source of the reference voltage V ref . When the switch SW 4  is turned on by the initiation signal INIT, voltages applied to the anode electrode of the photodiode  110  and the non-inverting input terminal of the OP amplifier  121   a  are initiated. Although in the present embodiment the switch SW 4  is connected to the anode electrode of the photodiode  110  and the source of the reference voltage V ref , the present invention is not limited thereto. The light sensing circuit may have another structure as long as it allows the voltage applied to the photodiode  110  to be initialized. For example, the light sensing circuit may have a structure in which the switch SW 4  is connected between the first electrode and the second electrode of the transistor Tr 4 . 
         [0072]    In an operation of the light sensing circuit of  FIG. 4 , when external light is incident upon the photodiode  110 , a current is generated in the photodiode  110 . The generated current flows from the cathode electrode of the photodiode  110  to the anode electrode thereof. 
         [0073]    A voltage of the anode electrode of the photodiode  110  is increased by the current. Due to the increase in the voltage of the anode electrode, a voltage applied to the non-inverting input terminal of the OP amplifier  121   a  is greater than that applied to the inverting input terminal thereof. Thus, an output voltage of the OP amplifier  121   a , which is output through the output terminal, is increased. Since the output voltage is applied to the gate electrode of the transistor Tr 4 , the transistor Tr 4  is turned on. 
         [0074]    As the transistor Tr 4  is turned on, an electric charge moving toward the anode electrode of the photodiode  110  is supplemented, that is, the current generated in the photodiode  110  is flowed to the ADC  130 . Thus, the voltage of the anode electrode of the photodiode  110  is maintained constantly, that is, at the reference voltage V ref . 
         [0075]    When the light sensing circuit senses the brightness of periodically incident light, not only the voltage of the anode electrode of the photodiode  110  but also the voltage of the output terminal of the OP amplifier  121   a  may be initialized by applying the initiation signal INIT to the switch SW 4  and thus turning on the switch SW 4 . 
         [0076]    Similar to the light sensing circuit of  FIG. 3 , it is illustrated in the light sensing circuit of  FIG. 4  that the reference voltage V ref  is applied to the anode electrode of the photodiode  110  and the first voltage V REV  is applied to the cathode electrode thereof. However, the present invention is not limited to this embodiment. In other words, the structure of the photodiode  110  may be changed as long as the reverse bias voltage can be applied to the photodiode  110 . 
         [0077]    For example, the second voltage −V REV  may be applied to the anode electrode of the photodiode  110 , and the cathode electrode of the photodiode  110  may be connected to the inverting input terminal of the OP amplifier  121   a . In this case, the reference voltage V ref  may be applied to the non-inverting input terminal of the OP amplifier  121   a.    
         [0078]    Although not shown in  FIGS. 3 and 4 , when the light sensing circuit of  FIG. 3  or  4  is applied to the second sensing circuit of the light sensing circuit  100  of  FIG. 2 , the light sensing circuit of  FIG. 3  or  4  further includes a shielding film  111  which shields light from being incident upon the photodiode  110 . 
         [0079]    As described above, a light sensing circuit according to the present invention can more precisely sense the brightness of surrounding environment by calculating the brightness in consideration of a current that only depends on an ambient temperature. In addition, a current is generated while a voltage applied between both ends of a photodiode is maintained constantly, is whereby a sensing operation of the light sensing circuit is reliable. 
         [0080]      FIG. 5  is a block diagram of a light sensing circuit  200  according to another embodiment of the present invention. Referring to  FIG. 5 , the light sensing circuit  200  includes two photodiodes  210  and  210 - 1 , a first voltage fixing unit  220 , and an ADC  230 . 
         [0081]    Among the two photodiodes  210  and  210 - 1 , the photodiode  210  generates a current according to the brightness of externally incident light and an ambient temperature. On the other hand, since the photodiode  210 - 1  further includes a shielding film  211  for shielding externally incident light, the photodiode  210 - 1  generates a current that depends upon only the ambient temperature. For convenience sake, the photodiode  210  having no shielding films is referred to as a first photodiode  210 , and the photodiode  210 - 1  having the shielding film  211  is referred to as a second photodiode  210 - 1 . 
         [0082]    The first voltage fixing unit  220  fixes a voltage of an anode electrode of the first photodiode  210  and a voltage of a cathode electrode of the second photodiode  210 - 1  to certain voltage values. The first voltage fixing unit  220  includes a first voltage comparing unit  221  and a first voltage adjusting unit  222 . 
         [0083]    The first voltage comparing unit  221  receives a reference voltage V ref  and the voltage of the anode electrode of the first photodiode  210  and compares the two voltages with each other. An output voltage corresponding to a result of the comparison is applied to the first voltage adjusting unit  222 . 
         [0084]    The first voltage adjusting unit  222  receives the output voltage from the first voltage comparing unit  221  and adjusts the voltage of the anode electrode of the first photodiode  210  so as to be equal to the reference voltage V ref . 
         [0085]    The ADC  230  receives voltage or current values depending on currents generated in the first photodiode  210  and the second photodiode  210 - 1 , and converts the voltage or current values into digital values. The ADC  230  is connected to the first voltage fixing unit  220 . 
         [0086]    Embodiments of the light sensing circuit  200  of  FIG. 5  will now be described with reference to  FIGS. 6 and 7 . 
         [0087]      FIG. 6  is a circuit diagram of the light sensing circuit  200  of  FIG. 5 , according to an embodiment of the present invention. 
         [0088]    The light sensing circuit of  FIG. 6  may include a first photodiode  210 , a second photodiode  210 - 1 , a shielding film  211 , an OP amplifier  221   a , a transistor Tr 6 , a switch SW 6 , and an ADC  230 . 
         [0089]    The first photodiode  210  generates a current according to the brightness of externally incident light and an ambient temperature. The reference voltage V ref  is applied to the anode electrode of the first photodiode  210 , and the first voltage V REV  is applied to the cathode electrode thereof. Since the first voltage V REV  is greater than the reference voltage V ref , a reverse bias voltage is applied to the first photodiode  210 . 
         [0090]    The second photodiode  210 - 1 , shielded from external incident light by shielding film  211 , generates a current that depends upon only the ambient temperature. The second voltage −V REV  is applied to the anode electrode of the second photodiode  210 - 1 , and the reference voltage V ref  is applied to the cathode electrode thereof. The second voltage −V REV  has the same magnitude as the first voltage V REV  and a polarity opposite to that of the first voltage V REV . Therefore, a reverse bias voltage having the same magnitude as that applied to the first photodiode  210  is applied to the second photodiode  210 - 1 . 
         [0091]    The OP amplifier  221   a , which is an embodiment of the first voltage comparing unit  221 , includes an inverting input terminal, a non-inverting input terminal, and an output terminal. The non-inverting input terminal of the OP amplifier  221   a  is connected to the anode electrode of the first photodiode  210 , and the output terminal thereof is connected to a gate electrode of the transistor Tr 6 . The reference voltage V ref  is applied to the inverting input terminal of the OP amplifier  221   a.    
         [0092]    The OP amplifier  221   a  compares the reference voltage V ref  with the voltage of the anode electrode of the first photodiode  210  and outputs as an output voltage a value corresponding to a difference between the two voltages. 
         [0093]    The transistor Tr 6 , which is an embodiment of the first voltage adjusting unit  222 , includes a first electrode, a second electrode, and a gate electrode. The first electrode of the transistor Tr 6  is connected to the anode electrode of the first photodiode  210 , the second electrode thereof is connected to the ADC  230 , and the gate electrode thereof is connected to the output terminal of the OP amplifier  221   a.    
         [0094]    The transistor Tr 6  is turned on or off according to the output voltage of the OP amplifier  221   a , which is received via the gate electrode, and thus adjusts each of the voltages of the anode electrode of the first photodiode  210  and the cathode electrode of the second photodiode  210 - 1  to be constant. 
         [0095]    The switch SW 6  is connected between a source of the reference voltage V ref  and the anode electrode of the first photodiode  210 , the cathode electrode of the second photodiode  210 - 1  and the non-inverting input terminal of the OP amplifier  221   a . When the switch SW 6  is turned on by the initiation signal INIT, the voltages applied to the anode electrode of the first photodiode  210 , the cathode electrode of the second photodiode  210 - 1 , and the non-inverting input terminal of the OP amplifier  221   a  are initialized. 
         [0096]    Although it is illustrated in the present embodiment that the switch SW 6  is connected between the anode electrode of the first photodiode  210  and the source of the reference voltage V ref , the present invention is not limited thereto. The light sensing circuit may have another structure as long as it allows the voltage applied to the photodiode  210  to be initialized. For example, the light sensing circuit may have a structure in which the switch SW 6  is connected between the first electrode and the second electrode of the transistor Tr 6 . 
         [0097]    The ADC  230  generates a digital value corresponding to a current generated in the first photodiode  210  and the second photodiode  210 - 1 . More specifically, the ADC  230  is connected to the second electrode of the transistor Tr 6  and thus a current flows from the anode electrode of the first photodiode to the ADC  230  when the transistor Tr 6  is turned on. The ADC  230  generates a digital value into which the current is converted from an analog value. 
         [0098]    In an operation of the light sensing circuit of  FIG. 6 , when external light is incident upon the first photodiode  210 , a first current which depends on the brightness of the incident light and an ambient temperature is generated in the first photodiode  210 . The first current flows from the cathode electrode of the first photodiode  210  to the anode electrode thereof. 
         [0099]    At the same time, a second current that depends on only the ambient temperature is generated in the second photodiode  210 - 1 . The second current flows from the cathode electrode of the second photodiode  210 - 1  to the anode electrode thereof. Since the first current is generated according to the incident light and the ambient temperature, the magnitude of the first current is greater than that of the second current. 
         [0100]    A current corresponding to the second current from among the first current flows toward the second photodiode  210 - 1 . Accordingly, the voltage of the anode electrode of the first photodiode  210  is increased by a current with a magnitude obtained by subtracting the second current from the first current. 
         [0101]    Due to the increase in the voltage of the anode electrode of the first photodiode  210 , a voltage applied to the non-inverting input terminal of the OP amplifier  221   a  is greater than that applied to the inverting input terminal thereof. Accordingly, the output voltage output through the output terminal of the OP amplifier  221   a  is increased. Since the output voltage is applied to the gate electrode of the transistor Tr 6 , the transistor Tr 6  is turned on. 
         [0102]    As the transistor Tr 6  is turned on, an electric charge moving toward the anode electrode of the first photodiode  210  is supplemented, that is, the current of the magnitude obtained by subtracting the second current from the first current is flowed to the ADC  130 . Thus, the voltage of the anode electrode of the first photodiode  210  is maintained constantly, that is, at the reference voltage V ref . 
         [0103]    When the light sensing circuit senses the brightness of periodically incident light, not only voltage of the anode electrode of the first photodiode  210  but also the voltages of the cathode electrode of the second photodiode  210 - 1  and the output terminal of the OP amplifier  221   a  may be initialized by applying the initiation signal INIT to the switch SW 6  and thus turning on the switch SW 6 . 
         [0104]    It is illustrated in the light sensing circuit of  FIG. 6  that the reference voltage V ref  is applied to the anode electrode of the first photodiode  210  and the first voltage V REV  is applied to the cathode electrode thereof and that the second voltage −V REV  is applied to the anode electrode of the second photodiode  210 - 1  and the reference voltage V ref  is applied to the cathode electrode thereof. However, the present invention is not limited to this embodiment. In other words, the structures of the first and second photodiodes  210  and  210 - 1  may be changed as long as reverse bias voltages can be applied to the first and second photodiodes  210  and  210 - 1 . 
         [0105]    For example, the second voltage −V REV  may be applied to the anode electrode of the first photodiode  210 , and the cathode electrode of the first photodiode  210  may be connected to the inverting input terminal of the OP amplifier  221   a . In this case, the reference voltage V ref  may be applied to the non-inverting input terminal of the OP amplifier  221   a.    
         [0106]    When the structure of the first photodiode  210  is changed as described above, the first voltage V REV  may be applied to the cathode electrode of the second photodiode  210 - 1 , and the anode electrode of the second photodiode  210 - 1  may be connected to the first photodiode  210 . 
         [0107]      FIG. 7  is a circuit diagram of the light sensing circuit  200  of  FIG. 5 , according to another embodiment of the present invention. The light sensing circuit of  FIG. 7  will now be described by focusing on differences from the light sensing circuit of  FIG. 6 , and a description of the same elements as those in  FIG. 6  will be omitted. 
         [0108]    The light sensing circuit of  FIG. 7  may include a first photodiode  210 , a second photodiode  210 - 1 , a shielding film  211 , an OP amplifier  221   a , a capacitor C 7 , a first switch SW 7 - 1 , a second switch SW 7 - 2 , and an ADC  230 . 
         [0109]    The first photodiode  210 , the second photodiode  210 - 1 , and the shielding film  211  have the same structures as those in the light sensing circuit of  FIG. 6 , so a detailed description of thereof will be omitted. 
         [0110]    The OP amplifier  221   a , which is another embodiment of the first voltage comparing unit  221 , includes an inverting input terminal, a non-inverting input terminal, and an output terminal. The inverting input terminal of the OP amplifier  221   a  is connected to the anode electrode of the first photodiode  210 , and the output terminal thereof is connected to the ADC  230 . The reference voltage V ref  is applied to the non-inverting input terminal of the OP amplifier  221   a.    
         [0111]    The OP amplifier  221   a  compares the reference voltage V ref  with the voltage of the anode electrode of the first photodiode  210 , and outputs as an output voltage a value corresponding to a difference between the two voltages. 
         [0112]    The capacitor C 7 , which is another embodiment of the first voltage adjusting unit  222 , includes a first electrode and a second electrode. The first electrode of the capacitor C 7  is connected to the inverting input terminal of the OP amplifier  221   a , and the second electrode thereof is connected to the output terminal of the OP amplifier  221   a.    
         [0113]    As the output voltage of the output terminal of the OP amplifier  221   a  varies, the capacitor C 7  shifts the voltage of the second electrode in order to constantly maintain a voltage between the first and second electrodes of the capacitor C 7 . 
         [0114]    The first switch SW 7 - 1  is connected between the first electrode and the second electrode of the capacitor C 7 . When the first switch SW 7 - 1  is turned on by the initiation signal INIT, the capacitor C 7  is short-circuited. 
         [0115]    The second switch SW 7 - 2  is connected between a source of the reference voltage V ref  and the anode electrode of the first photodiode  210 , the cathode electrode of the second photodiode  210 - 1  and the non-inverting input terminal of the OP amplifier  221   a . When the second switch SW 7 - 2  is turned on by the initiation signal INIT, voltages applied to the anode electrode of the first photodiode  210  and the cathode electrode of the second photodiode  210 - 1  are initialized. 
         [0116]    The ADC  230  generates a digital value corresponding to a current generated in the first photodiode  210  and the second photodiode  210 - 1 . More specifically, the ADC  230  is connected to the output terminal of the OP amplifier  221   a  and thus receives the output voltage from the OP amplifier  221   a  and generates a digital value into which the output voltage is converted from an analog value. 
         [0117]    In an operation of the light sensing circuit of  FIG. 7 , similar to the light sensing circuit of  FIG. 6 , a first current that depends on externally incident light and an ambient temperature is generated in the first photodiode  210 , and a second current that depends on only the ambient temperature is generated in the second photodiode  210 - 1 . A voltage of the anode electrode of the first photodiode  210  is increased by a current of a magnitude corresponding to a value obtained by subtracting the second current from the first current. 
         [0118]    Due to the increase in the voltage of the anode electrode of the first photodiode  210 , a voltage applied to the inverting input terminal of the OP amplifier  221   a  is greater than that applied to the non-inverting input terminal thereof. Thus, an output voltage of the OP amplifier  221   a , which is output through the output terminal, is decreased, and the capacitor C 7 , which is connected between the inverting input terminal and the output terminal of the OP amplifier  221   a , shifts a voltage of the first electrode in order to constantly maintain the voltage between the first and second electrodes of the capacitor C 7 . 
         [0119]    Thus, the cathode electrode of the first photodiode  210  is maintained to constantly have the first voltage V REV , and the anode electrode thereof is maintained to constantly have the reference voltage V ref . The anode electrode of the second photodiode  210 - 1  is maintained to constantly have the second voltage −V REV , and the cathode electrode thereof is maintained to constantly have the reference voltage V ref . 
         [0120]    When the light sensing circuit senses the brightness of periodically incident light, not only the voltage of the anode electrode of the first photodiode  210  but also the voltage of the output terminal of the OP amplifier  221   a  may be initialized by applying the initiation signal INIT to the first switch SW 7 - 1  and thus turning on the switch SW 7 - 1 . 
         [0121]    Not only the anode electrode of the first photodiode  210  but also the cathode electrode of the second photodiode  210 - 1  may be initialized by applying the initiation signal INIT to the second switch SW 7 - 2  and thus turning on the switch SW 7 - 2 . 
         [0122]    Similar to the light sensing circuit of  FIG. 6 , it is illustrated in the light sensing circuit of  FIG. 7  that the reference voltage V ref  is applied to the anode electrode of the first photodiode  210  and the first voltage V REV  is applied to the cathode electrode thereof and that the second voltage −V REV  is applied to the anode electrode of the second photodiode  210 - 1  and the reference voltage V ref  is applied to the cathode electrode thereof. However, the present invention is not limited to this embodiment. In other words, the structures of the first and second photodiodes  210  and  210 - 1  may be changed as long as inverse bias voltages can be applied to the first and second photodiodes  210  and  210 - 1 . 
         [0123]    For example, the second voltage −V REV  may be applied to the anode electrode of the first photodiode  210 , and the cathode electrode of the first photodiode  210  may be connected to the inverting input terminal of the OP amplifier  221   a.    
         [0124]    In this case, the first voltage V REV  may be applied to the cathode electrode of the second photodiode  210 - 1 , and the anode electrode of the second photodiode  210 - 1  may be connected to the first photodiode  210 . 
         [0125]    As described above, a light sensing circuit according to the present invention can more precisely sense the brightness of surrounding environment by calculating the brightness in consideration of a current that only depends on an ambient temperature. In addition, a current is generated while a voltage applied between both ends of a photodiode is maintained constantly, whereby a sensing operation of the light sensing circuit is reliable. 
         [0126]      FIG. 8  is a block diagram of a light sensing circuit  300  according to another embodiment of the present invention. Referring to  FIG. 8 , the light sensing circuit  300  includes a first photodiode  310 , a second photodiode  310 - 1 , a shielding film  311 , a first voltage fixing unit  320 , a second voltage fixing unit  330 , and an ADC  340 . 
         [0127]    The first photodiode  310 , the second photodiode  310 - 1 , and the shielding film  311  may have the same structures as those in the light sensing circuit of  FIG. 5 , so a detailed description thereof will be omitted. 
         [0128]    The first voltage fixing unit  320  fixes a voltage of an anode electrode of the first photodiode  310  to a certain voltage value. The first voltage fixing unit  320  includes a first voltage comparing unit  321  and a first voltage adjusting unit  322 . 
         [0129]    The first voltage comparing unit  321  receives the reference voltage V ref  and the voltage of the anode electrode of the first photodiode  310  and compares the two voltages with each other. An output voltage corresponding to a result of the comparison is applied to the first voltage adjusting unit  322 . 
         [0130]    The first voltage adjusting unit  322  receives the output voltage from the first voltage comparing unit  321  and adjusts the voltage of the anode electrode of the first photodiode  310  so as to be equal to the reference voltage V ref . 
         [0131]    The second voltage fixing unit  330  fixes a voltage of a cathode electrode of the second photodiode  310 - 1  to a certain voltage value. The second voltage fixing unit  330  includes a second voltage comparing unit  332  and a second voltage adjusting unit  331 . 
         [0132]    The second voltage comparing unit  332  receives the reference voltage V ref  and the voltage of the cathode electrode of the second photodiode  310 - 1  and compares the two voltages with each other. An output voltage corresponding to a result of the comparison is applied to the second voltage adjusting unit  331 . 
         [0133]    The second voltage adjusting unit  331  receives the output voltage from the second voltage comparing unit  332  and adjusts the voltage of the cathode electrode of the second photodiode  310 - 1  so as to be equal to the reference voltage V ref . 
         [0134]    The ADC  340  has the same structure and function as that in the light sensing circuit  200  of  FIG. 5 , so a detailed description thereof will be omitted. 
         [0135]    Embodiments of the light sensing circuit  300  of  FIG. 8  will be described with reference to  FIG. 9 . 
         [0136]      FIG. 9  is a circuit diagram of the light sensing circuit  300  of  FIG. 8 , according to an embodiment of the present invention. 
         [0137]    Since the light sensing circuit of  FIG. 9  is similar to that of  FIG. 7 , the light sensing circuit of  FIG. 9  will be described by focusing on differences from the light sensing circuit of  FIG. 7 , and a description of the same elements as those in the light sensing circuit of  FIG. 7  will be omitted. 
         [0138]    The light sensing circuit of  FIG. 9  may include a first photodiode  310 , a second photodiode  310 - 1 , a shielding film  311 , two OP amplifiers  321   a  and  332   a , a capacitor C 9 , a transistor Tr 9 , first, second, and third switches SW 9 - 1 , SW 9 - 2 , and SW 9 - 3 , and an ADC  340 . 
         [0139]    Among the two OP amplifiers  321   a  and  332   a , the OP amplifier  321   a  connected to the first photodiode  310  is referred to as a first OP amplifier  321   a , and the OP amplifier  332   a  connected to the second photodiode  310 - 1  is referred to as a second OP amplifier  332   a.    
         [0140]    The second OP amplifier  332   a , which is an embodiment of the second voltage comparing unit  332 , includes an inverting input terminal, a non-inverting input terminal, and an output terminal. The non-inverting input terminal of the second OP amplifier  332   a  is connected to a cathode electrode of the second photodiode  310 - 1 , and the output terminal thereof is connected to a gate electrode of the transistor Tr 9 . The reference voltage V ref  is applied to the inverting input terminal of the second OP amplifier  332   a.    
         [0141]    The second OP amplifier  332   a  compares the reference voltage V ref  with the voltage of the cathode electrode of the second photodiode  310 - 1  and outputs as an output voltage a value corresponding to a difference between the two voltages. 
         [0142]    The transistor Tr 9 , which is an embodiment of the second voltage adjusting unit  331 , includes a first electrode, a second electrode, and a gate electrode. The first electrode of the transistor Tr 9  is connected to the cathode electrode of the second photodiode  310 - 1 , the second electrode thereof is connected to the second switch SW 9 - 2  and the anode of the first photodiode  210 , and the gate electrode thereof is connected to the output terminal of the second OP amplifier  332   a.    
         [0143]    The second switch SW 9 - 2  is further connected between the anode of the first photodiode  210  and a source of the reference voltage V ref , and is turned on by the initiation signal INIT. 
         [0144]    The transistor Tr 9  is turned on or off according to the output voltage of the second OP amplifier  332   a , which is received via the gate electrode, and thus adjusts the voltage of the cathode electrode of the second photodiode  310 - 1  to be constant. 
         [0145]    The third switch SW 9 - 3  is connected between the cathode electrode of the second photodiode  310 - 1  and a source of the reference voltage V ref . When the third switch SW 9 - 3  is turned on by the initiation signal INIT, the voltages applied to the cathode electrode of the second photodiode  310 - 1  and the non-inverting input terminal of the second OP amplifier  332   a  are initialized. 
         [0146]    Although it is illustrated in the present embodiment that the second and third switches SW 9 - 2  and SW 9 - 3  are separately installed, the present invention is not limited thereto. In other words, the second and third switches SW 9 - 2  and SW 9 - 3  may be integrated and connected between the first and second electrodes of the transistor Tr 9 . In this case, a voltage applied to the second photodiode  310 - 1  may be initialized due to a short-circuit between the first electrode and the second electrode of the transistor Tr 9 . 
         [0147]    In an operation of the light sensing circuit of  FIG. 9 , similar to the light sensing circuit of  FIG. 7 , a first current that depends on externally incident light and an ambient temperature is generated in the first photodiode  310 , and a second current that depends on only the ambient temperature is generated in the second photodiode  310 - 1  shielded from incident light by shielding film  211 . 
         [0148]    The first current flows from the anode electrode of the first photodiode  310  to the cathode electrode thereof. The voltage of the anode electrode of the first photodiode  310  is increased by the first current. Due to the increase in the voltage of the anode electrode of the first photodiode  310 , the output voltage of the first OP amplifier  321   a  is decreased, and the capacitor C 9 , which is connected between the inverting input terminal and the output terminal of the first OP amplifier  321   a , shifts a voltage of the first electrode in order to constantly maintain a voltage between the first and second electrodes of the capacitor C 9 . 
         [0149]    Thus, the cathode electrode of the first photodiode  310  is maintained to constantly have the first voltage V REV , and the anode electrode thereof is maintained to constantly have the reference voltage V ref . 
         [0150]    The second current flows from the cathode electrode of the second photodiode  310 - 1  to the anode electrode thereof. The voltage of the cathode electrode of the second photodiode  310 - 1  is decreased by the second current. Due to the decrease in the voltage of the cathode electrode of the second photodiode  310 - 1 , a voltage applied to the non-inverting terminal of the second OP amplifier  332   a  is greater than that applied to the inverting input terminal thereof. Therefore, the output voltage of the second OP amplifier  332   a , which is output via the output terminal, is increased. Since the output voltage is applied to the gate electrode of the transistor Tr 9 , the transistor Tr 9  is turned on. As the transistor Tr 9  is turned on, the voltage of the cathode electrode of the second photodiode  310 - 1  is maintained at the reference voltage V ref . 
         [0151]    The voltages of the anode electrode of the first photodiode  310 , the cathode electrode of the second photodiode  310 - 1 , and the output terminals of the first and second OP amplifiers  321   a  may be initialized by applying the initiation signal INIT to the first, second, and third switches SW 9 - 1 , SW 9 - 2 , and SW 9 - 3  and thus turning on the first, second, and third switches SW 9 - 1 , SW 9 - 2 , and SW 9 - 3 . 
         [0152]    Similar to the light sensing circuits of  FIGS. 6 and 7 , it is illustrated in the light sensing circuit of  FIG. 9  that the reference voltage V ref  is applied to the anode electrode of the first photodiode  310  and the first voltage V REV  is applied to the cathode electrode thereof, and that the second voltage −V REV  is applied to the anode electrode of the second photodiode  310 - 1  and the reference voltage V ref  is applied to the cathode electrode thereof. However, the present invention is not limited to this embodiment. In other words, the structures of the first and second photodiodes  310  and  310 - 1  may be changed as long as reverse bias voltages can be applied to the first and second photodiodes  310  and  310 - 1 . 
         [0153]    For example, the second voltage −V REV  may be applied to the anode electrode of the first photodiode  310 , and the cathode electrode of the first photodiode  310  may be connected to the inverting input terminal of the first OP amplifier  321   a.    
         [0154]    In this case, the first voltage V REV  may be applied to the cathode electrode of the second photodiode  310 - 1 , and the anode electrode of the second photodiode  310 - 1  may be connected to the non-inverting input terminal of the second OP amplifier  332   a . In addition, the reference voltage V ref  may be applied to the inverting input terminal of the second OP amplifier  332   a.    
         [0155]    As described above, a light sensing circuit according to the present invention can more precisely sense the brightness of surrounding environment by calculating the brightness in consideration of a current that only depends on an ambient temperature. In addition, a current is generated while a voltage applied between both ends of a photodiode is maintained constantly, whereby a sensing operation of the light sensing circuit is reliable. 
         [0156]    Although not shown in the drawings, each of the light sensing circuits of  FIGS. 5 through 9  may further include a calculation unit for calculating the brightness of a surrounding environment by using the digital value obtained by the ADC. Alternatively, the calculation unit may be included outside the light sensing circuits of  FIGS. 5 through 9 . A detailed description of the calculation unit will now be described with reference to  FIG. 10 . 
         [0157]      FIG. 10  is a block diagram of a flat panel display  1000  including a light sensing circuit, according to an embodiment of the present invention. Referring to  FIG. 10 , the flat panel display  1000  may include a plurality of pixels, a light sensing circuit  1400 , driving units  1200  and  1300 , and a controller  1100 . 
         [0158]    The controller  1100  controls the driving units  1200  and  1300  so that data is displayed. The controller  1100  also controls the brightness of data pieces displayed by the pixels, according to the brightness of light sensed by the light sensing circuit  1400 . 
         [0159]    The controller  1100  may be an embodiment of the calculation unit. In other words, the controller  1100  may receive a digital value generated by the light sensing circuit  1400  and calculate the brightness of a surrounding environment. A method of calculating the brightness of the surrounding environment may be implemented according to various algorithms. For example, a voltage value obtained by an ADC included in the light sensing circuit  1400  may be converted into brightness information according to a look-up table. Alternatively, a period of time required for the voltage value obtained by the ADC included in the light sensing circuit  1400  to increase or decrease to a specific voltage value may be measured, and the measured time period may be converted into the brightness information according to the look-up table. These methods of calculating the brightness of the surrounding environment are just examples, and thus the present invention is not limited thereto. In other words, various other methods may be used. 
         [0160]    The driving units  1200  and  1300  receive a control signal and a data signal from the controller  1100  and apply corresponding signals to a plurality of scan lines S[ 1 ], S[ 2 ], . . . , and S[n] and a plurality of data lines D[ 1 ], D[ 2 ], . . . , and D[m], respectively. Due to the applications of the signals, data may be displayed on the pixels. Although it is illustrated in  FIG. 10  that the driving units  1200  and  1300  are a scan driving unit  1200  and a data driving unit  1300 , the present invention is not limited thereto. In other words, although the flat panel display  1000  is an organic light emission display device in  FIG. 10 , the flat panel display  1000  may be a PDP, an LCD, or the like. As such, it will be easily understood by one of ordinary skill in the art that the scan driving unit  1200  and the data driving unit  1300  of  FIG. 10  may be replaced by driving units required by a PDP or an LCD. 
         [0161]    The pixels are formed at regions where the scan lines S[ 1 ], S[ 2 ], . . . , and S[n] intersect the data lines D[ 1 ], D[ 2 ], . . . , and D[m]. Each of the pixels displays data according to a scan signal, a data signal, and the like. The displayed data may be data whose brightness has been controlled by the controller  1100 . 
         [0162]    The light sensing circuit  1400  may be installed at one surface of a panel on which the pixels are formed. The light sensing circuit  1400  may be one of the light sensing circuits of  FIGS. 2 through 9 . 
         [0163]    Although it is illustrated in  FIG. 10  that the entire body of the light sensing circuit  1400  is formed on the panel, the present invention is not limited thereto. For example, only a photodiode included in the light sensing circuit  1400  may be formed on the panel. In this case, the remaining portion of the light sensing circuit  1400  may be formed separately from the panel and installed to outside the panel. 
         [0164]    As described above, a flat panel display according to the present invention may is able to more properly perform automatic brightness control by applying a light sensing circuit that precisely senses the brightness of a surrounding environment. 
         [0165]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.