Patent Publication Number: US-9418623-B2

Title: Backlight unit with over-current detection and display device having the same

Description:
This application claims priority to Korean Patent Application No. 10-2013-0002033, filed on Jan. 8, 2013, and all the benefits accruing therefrom under 35 U.S. C. §119, the content of which in its entirety is herein incorporated by reference. 
     BACKGROUND 
     1. Field 
     The disclosure relates to a backlight unit and a display device including the backlight unit. 
     2. Description of the Related Art 
     A display device is typically employed in electronic devices as one of user interfaces, and a flat-panel display device is widely used as the display device for light weight, slimness, low power consumption of the electronic devices. 
     A liquid crystal display, which is one of most widely used types of the flat-panel display device, controls an amount of light provided thereto from an exterior to display an image. In the liquid crystal display, the liquid crystal display includes a separately provided light source, i.e., a backlight unit including a backlight lamp, since the liquid crystal display is not self-emissive. 
     In recent, a light emitting diode (“LED”) is widely used as the light source due to the characteristics of the LED including low power consumption, environment-friendly features and slim design, for example. 
     SUMMARY 
     The disclosure provides a backlight unit that detects an over-current flowing through a light emitting diode (“LED”) string. 
     The disclosure provides a display device including the backlight unit. 
     In an exemplary embodiment of the invention, a backlight unit includes a power converter configured to generate a light source driving voltage in response to a voltage control signal, a plurality of LED strings, where each of the LED strings receives the light source driving voltage through a first terminal thereof, a plurality of transistors corresponding to the LED strings, where each of the transistors includes: a first electrode connected to a second terminal of a corresponding LED string thereof; a second electrode; and a control electrode, and a controller connected to the control electrode and the second electrode, where the controller outputs a plurality of current control signals to control electrodes of the transistors and generate the voltage control signal, where the controller generates an over-current detection signal when any one of the current control signals has a pulse width less than a predetermined reference width. 
     In an exemplary embodiment, the controller may include an over-current detection circuit which outputs the over-current detection signal when the any one of the current control signals has the pulse width less than the predetermined reference range, and the controller controls the power converter such that the light source driving voltage is not generated when the over-current detection signal is output from the over-current detection circuit. 
     In an exemplary embodiment, the controller may stop generating the voltage control signal such that the light source driving voltage is not generated when the over-current detection signal is output from the over-current detection circuit. 
     In an exemplary embodiment, the controller may set the voltage control signal to a predetermined level such that the light source driving voltage is not generated when the over-current detection signal is output from the over-current detection circuit. 
     In an exemplary embodiment, the over-current detection circuit may include a plurality of diodes corresponding to the transistor, where each of the diodes includes a first terminal connected to the control electrode of a corresponding transistor thereof and a second terminal, a resistor connected between a first node therein, which is connected to the second terminal of the diodes, and a source voltage, a second resistor connected between the first node and a ground voltage, a third resistor connected between the first node and a second node therein, and a first comparator which receives a voltage of the second node and a first reference voltage, and outputs the over-current detection signal through an output terminal thereof. 
     In an exemplary embodiment, the over-current detection circuit may include a fourth resistor connected between the output terminal of the first comparator and the source voltage, and a fifth resistor connected between the output terminal of the first comparator and the source voltage. 
     In an exemplary embodiment, the controller may further include a voltage control signal generator which generates the voltage control signal in response to a plurality of current control signals, and a switching circuit connected between the source voltage and the voltage control signal generator, where the switching circuit operates in response to the over-current detection signal. 
     In an exemplary embodiment, the backlight unit may further include a plurality of current controllers corresponding to the LED strings, where each of the current controllers is connected to a second terminal of a corresponding LED string thereof, and generates the current control signals to control a current of the corresponding LED string thereof. 
     In an exemplary embodiment, each of the current controllers may generate the current control signal having a pulse width corresponding to a forward driving voltage of the corresponding LED string thereof. 
     In an exemplary embodiment, the backlight unit may further include a plurality of pull-down resistors corresponding to the transistors, where each of the pull-down resistors includes a first end connected to the second electrode of a corresponding transistor thereof and a second end connected to the ground voltage. 
     In an exemplary embodiment, each of the current controllers may include a resistor connected between the first end of a corresponding pull-down resistor of the pull-down resistors and a third node therein, a second comparator which receives a voltage of the third node and a second reference voltage, and outputs a voltage corresponding to a difference between the voltage of the third node and the second reference voltage to a fourth node therein, a capacitor connected between the third node and the fourth node, and a third comparator which receives a voltage of the fourth node and a third reference voltage and outputs current control signal. 
     In an exemplary embodiment, the third reference voltage may be a triangular wave or a sawtooth wave, which has a predetermined frequency. 
     In an exemplary embodiment, a display device including a display panel which includes a plurality of pixels, a driving circuit which controls the display panel to display an image on the display panel, and a backlight unit which provides light to the display panel. In such an embodiment, the backlight unit includes a power converter which generates a light source driving voltage in response to a voltage control signal, a plurality of LED strings, where each of the LED strings receives the light source driving voltage through a first terminal thereof, a plurality of transistors corresponding to the LED strings, where each of the transistors includes a first electrode connected to a second terminal of a corresponding LED string, a second electrode, and a control electrode, a plurality of pull-down resistors corresponding to the transistors, where each of the pull-down resistors includes a first end connected to the second electrode of a corresponding transistor thereof and a second end connected to a ground voltage, and a controller connected to the control electrode and the second electrode to output a plurality of current control signals to the control electrode of each transistor and generate the voltage control signal. In such an embodiment, the controller outputs an over-current detection signal when any one of the current control signals has a pulse width greater than a predetermined reference width. 
     In an exemplary embodiment, the controller may control the power converter such that the light source driving voltage is not generated when the over-current detection signal is output from the over-current detection circuit. 
     In an exemplary embodiment, the controller may stop generating the voltage control signal such that the light source driving voltage is not generated when the over-current detection signal is output from the over-current detection circuit. 
     In an exemplary embodiment, the controller may set the voltage control signal to a predetermined level such that the light source driving voltage is not generated when the over-current detection signal is output from the over-current detection circuit. 
     In an exemplary embodiment, the over-current detection circuit may include a plurality of diodes corresponding to the transistors, where each of the diodes includes a first terminal connected to the control electrode of a corresponding transistor thereof and a second terminal, a resistor connected between a first node therein, which is connected to the second terminal of the diodes, and a source voltage, a second resistor connected between the first node and a ground voltage, a third resistor connected between the first node and a second node therein, and a first comparator which receives a voltage of the second node and a first reference voltage, and outputs the over-current detection signal through an output terminal thereof. 
     In an exemplary embodiment, the controller may further include a voltage control signal generator which generates the voltage control signal in response to the current control signals and a switching circuit connected between the source voltage and the voltage control signal generator, where the switching circuit operates in response to the over-current detection signal. 
     In an exemplary embodiment, the display device may further include a plurality of current controllers corresponding to the LED strings, where each of the current controllers is connected to a second terminal of a corresponding LED string thereof, and generates a current control signal of the current control signals to control a current of the corresponding LED string thereof. 
     In an exemplary embodiment, each of the current controllers may generate the current control signal having a pulse width corresponding to a forward driving voltage of the corresponding LED string thereof. 
     According to exemplary embodiments, the backlight unit detects the over-current flowing through the LED strings, such that the LED strings is effectively prevented from being damaged due to the over-current flowing through the LED strings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram showing an exemplary embodiment of a backlight unit according to the invention; 
         FIG. 2  is a graph of current (milliampere: mA) versus voltage (volt: V) showing a current-voltage characteristic of an exemplary embodiment of a light emitting diode (“LED”) string shown in  FIG. 1 ; 
         FIG. 3  is a view showing a variation of power consumption based on the current-voltage characteristic of the LED string shown in  FIG. 2 ; 
         FIG. 4  is a circuit diagram showing an exemplary embodiment of a controller shown in  FIG. 1 ; 
         FIG. 5  is a circuit diagram showing an exemplary embodiment of a current controller shown in  FIG. 4 ; 
         FIG. 6  is a waveform diagram showing signals generated by the current controller shown in  FIG. 4 ; 
         FIG. 7  is a circuit diagram showing an alternative exemplary embodiment of a controller shown in  FIG. 1  according to the invention; 
         FIG. 8  is a view showing a signal at a node of an over-current detector based on first, second, and third feedback signals shown in  FIG. 7 ; 
         FIG. 9  is a circuit diagram showing another alternative exemplary embodiment of a controller shown in  FIG. 1  according to the invention; and 
         FIG. 10  is a view showing an exemplary embodiment of a display device including a backlight unit according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will be described more fully hereinafter with reference to the accompanying drawings, in which various embodiments are shown. This invention may, however, be embodied in many different forms, and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. 
     It will be understood that when an element or layer is referred to as being “on”, “connected to” or “coupled to” another element or layer, it can be directly on, connected or coupled to the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the invention. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms, “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Exemplary embodiments are described herein with reference to cross section illustrations that are schematic illustrations of idealized embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments described herein should not be construed as limited to the particular shapes of regions as illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region illustrated or described as flat may, typically, have rough and/or nonlinear features. Moreover, sharp angles that are illustrated may be rounded. 
     Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region and are not intended to limit the scope of the claims set forth herein. 
     All methods described herein can be performed in a suitable order unless other wise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”), is intended merely to better illustrate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention as used herein. 
     Hereinafter, exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a circuit diagram showing an exemplary embodiment of a backlight unit according to the invention. 
     Referring to  FIG. 1 , a backlight unit  100  includes a light source  110 , a power converter  120 , a controller  130 , a plurality of resistors, e.g., a first resistor R 1 , a second resistor R 2  and a third resistor R 3 , and a plurality of transistors, e.g., a first transistor T 1 , a second transistor T 2  and a third transistor T 3 . In an exemplary embodiment, the resistors T 1 , T 2  and T 3  may be pull-down resistors. The backlight unit  100  may be a light source of a display panel of a liquid crystal display. Hereinafter, the backlight unit  100  employed in the display panel will be described, but not be limited thereto. In an alternative exemplary embodiment, the backlight unit  100  may be employed in various devices, e.g., an illumination device, a commercial image board, etc. 
     The light source  110  includes a plurality of light emitting diode (“LED”) strings  111 ,  112  and  113 . In an exemplary embodiment, as shown in  FIG. 1 , the light source  110  includes three LED strings, e.g., a first LED string  111 , a second LED string  112  and a third LED string  113 , but the number of the LED strings should not be limited to three. 
     Each of the LED strings  111 ,  112  and  113  includes a plurality of LEDs connected to each other in series. Each LED includes a white LED that emits white light, a red LED that emits red light, a blue LED that emits blue light, and a green LED that emits green light. The white, red, blue and green LEDs have different light emitting characteristics from each other, e.g., forward driving voltages (Vf) of the LEDs. As the forward driving voltage of the LEDs decreases, power consumption of the LEDs decreases. In an exemplary embodiment, when a deviation of the forward driving voltage (Vf) small, uniformity of brightness may be effectively secured. In an exemplary embodiment, as shown in  FIG. 1 , the light source  110  includes the LED strings  111 ,  112  and  113 , each including the LEDs, but not being limited thereto. In an alternative exemplary embodiment, with the light source  110  may include laser diodes or carbon nano tubes, for example. 
     An end, e.g., a first end, of each of the LED strings  111 ,  112  and  113  is connected to a light source driving voltage VLED from the power converter  120 . The other end, e.g., a second end, of each of the LED strings  111 ,  112  and  113  is connected to a corresponding transistor of the transistors, e.g., the first transistor T 1 , the second transistor T 2  or the third transistor T 3 . The first transistor T 1  is connected between the other end of the first LED string  111  and an end, e.g., a first end, of the first resistor R 1  and includes a gate terminal controlled by a first current control signal PWM 1 . The first transistor T 2  is connected between the other end of the second LED string  112  and an end, e.g., a first end, of the second resistor R 2  and includes a gate terminal controlled by a second current control signal PWM 2 . The third transistor T 3  is connected between the other end of the third LED string  113  and an end, e.g., a first end, of the third resistor R 3  and includes a gate terminal controlled by a third current control signal PWM 3 . The other end, e.g., a second end, of each of the resistors R 1 , R 2  and R 3  is grounded. 
     The power converter  120  converts a source voltage EVDD from an external device to the light source driving voltage VLED. The light source driving voltage VLED has a voltage level, which may be higher than a predetermined voltage to drive the LEDs of the LED strings  111 ,  112  and  113 . 
     The power converter  120  includes an inductor  121 , an n-type metal-oxide-semiconductor (“NMOS”) transistor  122 , a diode  123  and a capacitor  124 . The inductor  121  is connected between the source voltage EVDD and a first node Q 1  in the power converter  120 . The NMOS transistor  122  is connected between the first node Q 1  and a ground voltage. The NMOS transistor  122  includes a gate electrode which receives a voltage control signal CTRLV from the controller  130 . The diode  123  is connected between the first node Q 1  and a second node Q 2  in the power converter. In an exemplary embodiment, the diode  123  may be a Schottky diode. The capacitor  124  is connected between the second node Q 2  and the ground voltage. The light source driving voltage VLED at the second node Q 2  is applied to the first end of each of the LED strings  111 ,  112  and  113 . 
     In such an embodiment, the power converter  120  converts the source voltage EVDD to the light source driving voltage VLED. In an exemplary embodiment, the NMOS transistor  122  is turned on or off in response to the voltage control signal CTRLV applied to the gate electrode of the NMOS transistor  122 , and thus the voltage level of the light source driving voltage VLED is controlled. 
     The controller  130  receives a source voltage VCC. The controller  130  receives a current flowing through a node, at which the first transistor T 1  and the first resistor R 1  are connected to each other, as a first feedback signal FB 1  and outputs the first current control signal PWM 1  to the gate terminal of the first transistor T 1 . The controller  130  receives a current flowing through a node, at which the second transistor T 2  and the second resistor R 2  are connected to each other, as a second feedback signal FB 2  and outputs the second current control signal PWM 2  to the gate terminal of the second transistor T 2 . The controller  130  receives a current flowing through anode, at which the third transistor T 3  and the third resistor R 3  are connected to each other, as a third feedback signal FB 3  and outputs the third current control signal PWM 3  to the gate terminal of the third transistor T 3 . 
     The first transistor T 1  is turned on or off in response to the first current control signal PWM 1 . The current flowing through the first LED string  111  is controlled by the turning on and off of the first transistor T 1 . The second transistor T 2  is turned on or off in response to the second current control signal PWM 2 . The current flowing through the second LED string  112  is controlled by the turning on and off of the second transistor T 2 . The third transistor T 3  is turned on or off in response to the third current control signal PWM 3 . The current flowing through the third LED string  113  is controlled by the turning on and off of the third transistor T 3 . 
     In an exemplary embodiment, the resistors R 1 , R 2  and R 3  compensate non-uniform voltage distribution between the LED strings  111 ,  112  and  113 . In such an embodiment, a resistor of the resistors R 1 , R 2  and R 3 , which has relatively low resistance, is connected to an LED string of the LED strings  111 ,  112  and  113 , which may receive relatively high forward driving voltage Vf, and another resistor of the resistors R 1 , R 2  and R 3 , which has relatively high resistance, is connected to an LED string of the LED strings  111 ,  112  and  113 , which may receive relatively low forward driving voltage Vf. In such an embodiment, a total power consumed in the LED strings  111 ,  112  and  113  and the resistors R 1 , R 2  and R 3  may be substantially uniform. 
     The controller  130  outputs the voltage control signal CTRLV based on the first, second and third current control signals PWM 1 , PWM 2 , and PWM 3  generated by the first, second and third feedback signals FB 1 , FB 2  and FB 3 , such that the voltage level of the light source driving voltage VLED is effectively controlled. 
       FIG. 2  is a graph of current (milliampere: mA) versus voltage (volt: V) showing a current-voltage characteristic of an LED string shown in  FIG. 1 , and  FIG. 3  is a view showing a variation of power consumption based on the current-voltage characteristic of the LED string shown in  FIG. 2 . 
     Referring to  FIGS. 1 and 2 , when the forward driving voltage Vf of the first LED string  111  is about 100 V, a first current IL 1  flowing through the first LED string  111  is about 100 mA, and when the forward driving voltage Vf of the first LED string  111  is about 110 V, the first current IL 1  flowing through the first LED string  111  is about 110 mA. 
     Referring to  FIGS. 1 and 3 , in an exemplary embodiment, where the forward driving voltages Vf of the first, second and third LED strings  111 ,  112  and  113  are different from each other, the currents flowing through the first, second and third LED strings  111 ,  112  and  113 , e.g., the first current IL 1 , a second current IL 2  and a third current IL 3 , respectively, are controlled such that the first, second and third LED strings  111 ,  112 , and  113  have substantially the same brightness as each other. In such an embodiment, the current flowing through the first, second and third LED strings  111 ,  112  and  113  may be controlled by the turning on and off of the first, second and third transistors T 1 , T 2  and T 3 , respectively. 
     In one exemplary embodiment, for example, the first current IL 1  of about 100 mA may flow through the first LED string  111  during a predetermined time period t 1  when the forward driving voltage Vf of the first LED string  111  is about 100 V. In such an embodiment, the first current IL 1  of about 110 mA may flow through the first LED string  111  during a predetermined time period t 2  when the forward driving voltage Vf of the LED string  111  is about 110 V to maintain substantially uniform brightness. In such an embodiment, the time period t 1  is greater than the time period t 2  (t 1 &gt;t 2 ). In one exemplary embodiment, for example, when t 1  is 1, t 2  may be t 1 ×0.909. 
     When the forward driving voltage Vf is about 100 V, power consumption P 1  is represented by the following Equation 1.
 
 P 1=100 V×100 mA×1.0=10 W  Equation 1
 
     When the forward driving voltage Vf is about 110 V, power consumption P 2  is represented by the following Equation 2.
 
 P 2=110 V×110 mA×0.909=10.99 W  Equation 2
 
     In such an embodiment, as shown in  FIG. 3 , a pulse width of the current control signal PWM 1  applied to the gate electrode of the first transistor T 1  is narrower when the forward driving voltage Vf is about 110 V than when the forward driving voltage Vf is about 100 V (e.g., PW 1 &gt;PW 2 ). In such an embodiment, the power consumption of the LED strings  111 ,  112  and  113  is greater when the forward driving voltage Vf is about 110 V than that when the forward driving voltage Vf is about 100 V (e.g., P 1 &lt;P 2 ). 
     In an exemplary embodiment, when any one of the LEDs of the LED string  111  is damaged, the amount of the current flowing through the LED string  111  increases, and the increase of the amount of the current flowing through the LED string  111  may cause damage on the LEDs. In an exemplary embodiment, the amount of the current flowing through the LED string  111  is detected. 
     In such an embodiment, where the display panel including the light source is a three-dimensional image display device, the LED strings  111 ,  112  and  113  may be periodically turned on and off, and the voltage level of the light source driving voltage VLED may be boosted, thereby uniformly maintaining the brightness of the display panel. As described above, in such an embodiment, when the voltage level of the light source driving voltage VLED becomes substantially high, the LED strings  111 ,  112  and  113  may be damaged by the increase of the amount of the current flowing through the LED string  111 . 
     In an exemplary embodiment, as shown in  FIG. 3 , when the amount of the current flowing through the LED string  111  is increased, a pulse width of the first current control signal PWM 1  is reduced. In such an embodiment, when at least one pulse width of the first current control signal PWM 1 , the second current control signal PWM 2  and the third current control signal PWM 3  is narrower than a predetermined pulse width, an over-current detection signal is output. 
       FIG. 4  is a circuit diagram showing an exemplary embodiment of a controller shown in  FIG. 1 . 
     Referring to  FIG. 4 , the controller  130  includes an over-current detector  132 , a voltage control signal generator  134 , a switching circuit  136  and current controllers, e.g., a first current controller  138   a , a second current controller  138   b  and a third current controller  138   c.    
     The over-current detector  132  receives the first, second and third current control signals PWM 1 , PWM 2  and PWM 3 , and activates the over-current detection signal DET when the narrowest pulse width of the pulse widths of the first, second and third current control signals PWM 1 , PWM 2  and PWM 3  is narrower than a reference pulse width. 
     The voltage control signal generator  134  generates the voltage control signal CTRLV corresponding to the current control signal of the first, second and third current control signals PWM 1 , PWM 2  and PWM 3 , which has the widest pulse width. In such an embodiment, as described with reference to Equations 1 and 2, the light source driving voltage VLED allows the pulse width of the first, second and third current control signals PWM 1 , PWM 2  and PWM 3  to be maximized, the power consumption of the backlight unit  100  may be reduced. 
     The switching circuit  136  applies the source voltage VCC to the voltage control signal generator  134  in response to the over-current detection signal DET. 
     The first, second and third current controllers  138   a ,  138   b  and  138   c  correspond to the first, second and third LED strings  111 ,  112 , and  113 , respectively. The first current controller  138   a  receives a first feedback signal FB 1 , and generates the first current control signal PWM 1 . The second current controller  138   b  receives a second feedback signal FB 2 , and generates the second current control signal PWM 2 . The third current controller  138   c  receives a third feedback signal FB 3 , and generates the third current control signal PWM 3 . 
     Hereinafter, the over-current detector  132  will be described in detail. 
     The over-current detector  132  includes diodes, e.g., a first diode D 11 , a second diode D 12  and a third diode D 13 , resistors, e.g., first to fifth resistors R 11  to R 15 , a capacitor C 11  and a comparator C 1 . The first, second and third diodes D 11 , D 12  and D 13  correspond to the first, second and third LED strings  111 ,  112  and  113 , respectively. An anode terminal of each of the diodes D 11 , D 12  and D 13  is connected to a correspond current control signal of the first, second and third current control signals PWM 1 , PWM 2  and PWM 3  output from the current controllers  138   a ,  138   b  and  138   c . A cathode terminal of each of the diodes D 11 , D 12  and D 13  is connected to a first node N 1 . The first resistor R 11  is connected between the source voltage VCC and the first node N 1  and functions as a pull-up resistor. The second resistor R 12  is connected between the first node N 1  and the ground voltage, and functions as a pull-up resistor. 
     The third resistor R 13  is connected between the source voltage VCC and a second node N 2 . The capacitor C 11  is connected between the second node N 2  and the ground voltage. The comparator C 1  receives a voltage of the second node N 2  through an inverting terminal thereof and a first reference voltage VREF 1  through a non-inverting terminal thereof. The fourth resistor R 14  is connected between the source voltage VCC and a third node N 3 . The fifth resistor R 15  is connected between the third node N 3  and the ground voltage. The voltage of the third node N 3  is output as the over-current detection signal DET. 
     In an exemplary embodiment, a current path is formed through the source voltage VCC, the first resistor R 11 , and the diodes D 1 , D 2  and D 3 , based on a low-to-high or high-to-low transition of the first, second and third current control signals PWM 1 , PWM 2  and PWM 3 . 
     In an exemplary embodiment, due to the characteristics of the diodes D 1 , D 2  and D 3 , the current path is formed through the diode connected to the current control signal having the relatively narrow pulse width among the first, second and third current control signals PWM 1 , PWM 2  and PWM 3 . Therefore, the voltage of the first node N 1  corresponds to the current control signal having the relatively narrow pulse width among the first, second and third current control signals PWM 1 , PWM 2  and PWM 3 . In such an embodiment, the voltage of the first node N 1  is varied based on the relative narrow pulse width of the current control signal of the first, second and third current control signals PWM 1 , PWM 2  and PWM 3 . In such an embodiment, since the time period to form the current path through the diode becomes longer as the pulse width of the current control signal having the relatively narrow pulse width among the first, second and third current control signals PWM 1 , PWM 2  and PWM 3  becomes narrower, the voltage level of the second node N 2  is increased. 
     In an exemplary embodiment, the voltage of the second node N 2  rectified by the third resistor R 13  and the capacitor C 11  is input to the non-inverting terminal of the comparator C 1 . When the voltage corresponding to the current control signal having the relatively narrow pulse width among the first, second and third current control signals PWM 1 , PWM 2  and PWM 3  is lower than the first reference voltage VREF 1 , the over-current detection signal DET has the high level. In such an embodiment, the voltage corresponding to the current control signal having the relatively narrow pulse width among the first, second and third current control signals PWM 1 , PWM 2  and PWM 3  is higher than the first reference voltage VREF 1 , the over-current detection signal DET has the low level. 
     Therefore, when the pulse width of the first, second and third current control signals PWM 1 , PWM 2  and PWM 3  is narrower than of the reference pulse width corresponding to the first reference voltage VREF 1 , the over-current detection signal DET is transited to the low level. When the over-current detection signal DET is in the low level, the switching circuit  136  is turned off, and thus the source voltage VCC is not applied to the voltage current signal generator  134 . The voltage control signal generator  134  receives the source voltage VCC as an enable signal EN. When the source voltage VCC is not applied to the voltage control signal generator  134 , the voltage control signal generator  134  transits the voltage control signal CTRLV to the high level. When the voltage control signal CTRLV is maintained at the high level, the light source driving voltage VLED is not generated since the transistor  122  shown in  FIG. 1  is in the turn-on state. 
     As described above, when the over-current flows through at least one of the LED strings  111  to  113 , the generation of the light source driving voltage VLED stops, thereby effectively preventing the LED strings  111  to  113  from being damaged. 
       FIG. 5  is a circuit diagram showing an exemplary embodiment of a current controller shown in  FIG. 4 . In an exemplary embodiment, the first, second and third current controllers  138   a ,  138   b  and  138   c  have substantially the same circuit configuration and function, and thus, for the convenience of description, only one current controller, e.g., the first current controller  138   a , will be described in detail with reference to  FIG. 5 . 
     Referring to  FIG. 5 , the first current controller  138   a  includes a resistor R, a capacitor C, and comparators, e.g., a first comparator CP 11  and a second comparator CP 12 . The resistor R of the first current controller  138   a  is connected between the first resistor R 1  of the backlight unit  100  and a fourth node N 4 . The capacitor C is connected between the fourth node N 4  and a fifth node N 5 . The first comparator CP 11  receives a voltage of the fourth node N 4  and a second reference voltage VREF 2 , and outputs a feedback voltage, e.g., a first feedback voltage FV 1 . The second comparator C 12  receives the feedback voltage FV 1  of the fifth node N 5  and a third reference voltage VREF 3 , and outputs the first current control signal PWM 1 . 
       FIG. 6  is a waveform diagram showing signals generated by the current controller shown in  FIG. 4 . 
     Referring to  FIGS. 5 and 6 , the first feedback signal FB 1  from the first resistor R 1  of the light source unit  100  is rectified by the capacitor C and the resistor R the first current controller  138   a  to give a direct current voltage FBV 1 . The first comparator CP 11  compares the second reference voltage VREF 2  and the direct current voltage FBV 1  to output the feedback voltage FV 1 . The second comparator CP 12  compares the feedback voltage FV 1  and the third reference voltage VREF 3  to output the first current control signal PWM 1 . In an exemplary embodiment, the third reference voltage VREF 3  is a triangular wave or a sawtooth wave, for example. 
     When the amount of the current flowing through the first LED string  111  is increased, the voltage of the fourth node N 4  is increased. As a result, the feedback voltage FV 1  output from the comparator CP 11  is lowered, and the pulse width pa of the first current control signal PWM 1  becomes shorter. Since the turn-on time of the first transistor T 1  of the light source unit  100  becomes shorter when the pulse width pa of the first current control signal PWM 1  becomes narrower, the amount of the current flowing through the first LED string  111  may be decreased. 
     When the amount of the current flowing through the LED string  111  is decreased, the voltage of the fourth node N 4  is decreased. As a result, the feedback voltage FV 1  output from the comparator CP 11  become higher, the pulse width pa of the first current control signal PWM 1  becomes longer. Since the turn-on time of the transistor T 1  becomes longer when the pulse width pa of the first current control signal PWM 1  becomes wider, the amount of the current flowing through the LED string  111  may be increased. 
     As described above, the amount of the current flowing through the first LED string  111  are controlled by adjusting the pulse width pa of the first current control signal PWM 1 , and the brightness of the LED string  111  is thereby controlled. 
       FIG. 7  is a circuit diagram showing an alternative exemplary embodiment of a controller shown in  FIG. 1  according to the invention. 
     Referring to  FIG. 7 , a controller  330  includes an over-current detector  332 , a voltage control signal generator  334 , a switching circuit  336  and current controllers  338   a ,  338   b  and  338   c.    
     The over-current detector  332 , the voltage control signal generator  334 , the switching circuit  336  and the current controllers  338   a ,  338   b  and  338   c  of the controller  330  shown in  FIG. 7  are substantially the same as the over-current detector  132 , the voltage control signal generator  134 , the switching circuit  136  and the current controllers  138   a ,  138   b  and  138   c  of the controller  130  shown in  FIG. 4  except that anode terminals of diodes D 21 , D 22  and D 23  are respectively connected to source electrodes of transistors T 1 , T 2  and T 3  of the light source unit  100 , i.e., first, second and third feedback signals FB 1 , FB 2  and FB 3 . 
       FIG. 8  is a view showing a signal at a node of the over-current detector based on the first, second and third feedback signals shown in  FIG. 7 . 
     Referring to  FIG. 8 , a first feedback signal FB 1  has a first pulse width P 1 , a second feedback signal FB 2  has a second pulse width P 2 , and a third feedback signal FB 3  has a third pulse width P 3 . A signal of a first node N 21  of the over-current detector  332  has a pulse width Pd corresponding to a low-level period of the first feedback signal FB 1 . As a pulse width of any one of the first, second and third feed back signals FB 1 , FB 2  and FB 3  becomes narrower, the pulse width Pd of the signal of the node N 21  becomes wider. Therefore, when the pulse width Pd of the signal of the node N 21  is wider than a predetermined width, the voltage level of the first reference voltage VREF 1  is set to allow the over-current detection signal DET to be transited. 
       FIG. 9  is a circuit diagram showing another alternative exemplary embodiment of a controller shown in  FIG. 1  according to the invention. 
     Referring to  FIG. 9 , a controller  430  includes an over-current detector  432 , a voltage control signal generator  434 , a switching circuit  436  and current controllers  438   a ,  438   b  and  438   c.    
     The voltage control signal generator  434 , the switching circuit  436 , and the current controllers  438   a ,  438   b  and  438   c  of the controller  430  shown in  FIG. 9  are substantially the same as the voltage control signal generator  334 , the switching circuit  336 , and the current controllers  338   a ,  338   b  and  338   c  of the controller  330  shown in  FIG. 7 . In an exemplary embodiment, as shown in  FIG. 9 , the first, second and third LED strings  111 ,  112  and  113  are connected to different light source driving voltages, e.g., a first light source driving voltage VLED 1 , a second light source driving voltage VLED 2  and a third light source driving voltage VLED 3 , respectively. 
     In an exemplary embodiment, the over-current detector  432  includes a first detection circuit  441 , a second detection circuit  442 , a third detection circuit  443  and an AND gate  444 . The first detection circuit  441  receives the first feedback signal FB 1  and outputs a first detection signal DET 1 , the second detection circuit  442  receives the second feedback signal FB 2  and outputs a second detection signal DET 2 , and the third detection circuit  443  receives the third feedback signal FB 3  and outputs a third detection signal DET 3 . Each of the first, second and third detection circuits  441 ,  442  and  443  has a circuit configuration substantially the same as the circuit configuration of the over-current detector  332  shown in  FIG. 7 , except that each of the first, second and third detection circuits  441 ,  442  and  443  includes one diode connected to a corresponding feedback signal of the first, second and third feedback signals FB 1 , FB 2  and FB 3 . 
     Therefore, the first detection circuit  441  outputs the first detection signal DET 1  at a low level when the pulse width of the first feedback signal FB 1  is narrower than a reference width. The second detection circuit  442  outputs the second detection signal DET 2  at a low level when the pulse width of the second feedback signal FB 2  is narrower than the reference width. The third detection circuit  443  outputs the third detection signal DET 3  at a low level when the pulse width of the third feedback signal FB 3  is narrower than the reference width. 
     The AND gate  444  outputs the over-current detection signal DET when any one of the first, second, and third detection signals DET 1 , DET 2  and DET 3  has the low level. 
     In an exemplary embodiment, the LED strings  111 ,  112  and  113  are connected to different light source driving voltages VLED 1 , VLED 2  and VLED 3 , respectively, and the LED strings  111 ,  112  and  113  thereby independently detect the over-current. 
       FIG. 10  is a view showing an exemplary embodiment of a display device including the backlight unit according to the invention. Hereinafter, an exemplary embodiment where the display device is a liquid crystal display, but the display device is not limited to the liquid crystal display. 
     Referring to  FIG. 10 , a display device  500  includes a display panel  510 , a timing controller  520 , a gate driver  530 , a data driver  540  and a backlight unit  550 . 
     The display panel  510  includes a plurality of data lines D 1  to Dm, a plurality of gate lines G 1  to Gn crossing the data lines D 1  to Dm, and a plurality of pixels PX arranged in pixel areas. In one exemplary embodiment, for example, the pixel areas may be defined by the data lines D 1  to Dm and the gate lines G 1  to Gn. The data lines D 1  to Dm are insulated from the gate lines G 1  to Gn. 
     Each pixel PX includes a switching transistor TR connected to a corresponding data line of the data lines D 1  to Dm and a corresponding gate line of the gate lines G 1  to Gn, a liquid crystal capacitor CLC connected to the switching transistor TR, and a storage capacitor CST connected to the switching transistor TR. 
     The timing controller  520 , the gate driver  530  and the data driver  540  collectively operate as a driving circuit to control the display panel  510 , and thus the image is displayed on the display panel  510 . 
     The timing controller  520  receives image signals RGB and control signals CTRL that controls the image signals RGB, such as a vertical synchronization signal, a horizontal synchronization signal, a main clock signal and a data enable signal, for example, from an external device (not shown). The timing controller  520  processes the image signals RGB based on an operation condition of the display panel  510  using the control signals CTRL to output an image data signal DATA. The timing controller  520  applies the image data signal DATA and a first control signal CTRL 1  to the data driver  540  and applies a second control signal CTRL 2  to the gate driver  530 . The first control signal CTRL 1  includes a start pulse signal, a clock signal, a polarity inverting signal and a line latch signal, and the second control signal CTRL 2  includes vertical synchronization start signal, an output enable signal and a gate pulse signal. 
     The gate driver  530  drives the gate lines G 1  to Gn in response to the second control signal CTRL 2  from the timing controller  520 . The gate driver  530  may be configured in a gate driver integrated circuit or in a circuit using oxide semiconductor, amorphous semiconductor, crystalline semiconductor, or polycrystalline semiconductor, for example. 
     The data driver  540  outputs gray-scale voltages in response to the image data signal DATA and the first control signal CTRL 1  from the timing controller  520  to drive the data lines D 1  to Dm. 
     When a gate-on voltage is applied to one gate line by the gate driver  530 , switching transistors arranged in a same row and connected to the one gate line are turned on. When the gate-on voltage is applied, the data driver  540  provides the gray-scale voltages corresponding to the image data signal DATA to the data lines D 1  to Dm. The gray-scale voltages applied to the data lines D 1  to Dm are applied to corresponding liquid crystal capacitors and corresponding storage capacitors through the turned-on switching transistors. 
     The backlight unit  550  provides light to the display panel  510 . The display panel  510  displays the image using the light from the backlight unit  550 . 
     The backlight unit  550  operates in response to a backlight control signal BLC from the timing controller  520 . In one exemplary embodiment, for example, the backlight unit  550  controls the brightness in response to the backlight control signal BLC from the timing controller  520  and changes on and off periods thereof in response to the backlight control signal BLC from the timing controller  520 . The backlight unit  550  may be the backlight unit  100  shown in  FIG. 1 , but not being limited thereto. 
     The backlight unit  550  included in the display device  500  includes the LED strings. The backlight unit  550  detects the over-current flowing through the LED strings and stops the application of the light source driving voltage to the LED strings when the over-current is detected. Therefore, the backlight unit  550  effectively prevents the LED strings from being damaged due to the over-current flowing through the LED strings. 
     Although the exemplary embodiments of the invention have been described, it is understood that the invention should not be limited to these exemplary embodiments but various changes and modifications can be made by one ordinary skilled in the art within the spirit and scope of the invention as hereinafter claimed.