Backlight units and current control methods thereof

A backlight unit including: at least one light emitting diode (“LED”) string having an anode, which receives a string current, and a chassis-grounded cathode; and a current source control unit which receives a driving current and outputs the string current to the at least one LED string, where the current source control unit senses the driving current and compensates for the string current based on the sensed driving current and a reference voltage.

This application claims priority to Korean Patent Application No. 10-2011-0073949, filed on Jul. 26, 2011, and all the benefits accruing therefrom under 35 U.S.C. §119, the content of which in its entirety is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Exemplary embodiments of the invention relate to a backlight unit and a current control method thereof.

Generally, liquid crystal display (“LCD”) devices include a liquid crystal panel that displays an image, and a backlight unit disposed under the liquid crystal panel to supply light to the liquid crystal panel. When light emitting diodes (“LED”s) are used as a light source of the backlight unit, the backlight unit typically includes a plurality of light source strings that are connected to each other in parallel, a direct current to direct current (“DC” to “DC”) converter for supplying a driving voltage to the light source strings, and a driver integrated circuit (“IC”) connected to the light source strings through a plurality of channels. Typically, each light source string includes a plurality of serially-connected LEDs.

BRIEF SUMMARY OF THE INVENTION

Exemplary embodiments of the invention provide a backlight unit and a current control method thereof, which effectively prevent heat generation or ignition when a light emitting diode (“LED”) string is shorted.

An exemplary embodiment of the invention provides a backlight unit including: at least one LED string having an anode, which receives a string current, and a chassis-grounded cathode; and a current source control unit which receives a driving current and outputs the string current to the at least one LED string, where the current source control unit senses the driving current and compensates for the string current based on the sensed driving current and a reference voltage.

In an exemplary embodiment, the reference voltage may correspond to luminance of light emitted from the at least one LED string.

In an exemplary embodiment, the current source control unit may include: a current feedback unit connected between a first node and a second node, and which receives a DC voltage from the first node to output a driving voltage to the second node and outputs the input driving current to the second node; a current compensator which senses the driving current flowing in the current feedback unit and compares the sensed driving current and the reference voltage to output current compensation information; and a current regulator connected between the second node and the anode, and which receives the driving voltage and the driving current to output the string current and compensates for the string current based on of the current compensation information.

In an exemplary embodiment, the current feedback unit may include a sensing resistor between the first and second nodes, and the current compensator may sense a voltage difference between a voltage of the first node and a voltage of the second node to sense the driving current flowing in the sensing resistor.

In an exemplary embodiment, the current feedback unit may include: a photodiode between the first node and the second node and which emits light; and a photocoupler including a transistor which is turned on based on the light emitted from the photodiode, where the light emitted from the photodiode corresponds to the driving current.

In an exemplary embodiment, the current source unit may include: an operational amplifier which receives the reference voltage and a voltage corresponding to the driving current to output a voltage corresponding to the current compensation information; a current compensation transistor which is turned on based on the voltage corresponding to the current compensation information; and a current regulator having a current mirror structure, where the current regulator outputs the string current in response to a current flowing in the current compensation transistor.

In an exemplary embodiment, the backlight unit may further include a voltage detector which detects a driving voltage and a string voltage of the anode to output a feedback voltage, where the driving voltage corresponds to the string voltage.

In an exemplary embodiment, a voltage difference between the driving voltage and the string voltage may be maintained to be less than a predetermined value.

In an exemplary embodiment, the driving current supplied to the at least one LED string may be blocked when a voltage difference between the driving voltage and the string voltage is equal to or greater than a predetermined value.

In an exemplary embodiment, the backlight unit may further include a DC-to-DC converter which boosts an input source voltage to output a DC voltage and controls the DC voltage based on the feedback voltage, where the DC voltage corresponds to the driving voltage.

In an exemplary embodiment, a voltage difference between the DC voltage and the driving voltage may be about 0.1 volt (V) to about 0.5 volt (V).

In an exemplary embodiment, the DC to DC converter may include an inductor booster which boosts the source voltage to the DC voltage.

In an exemplary embodiment, the current source control unit may compensate for the string current when light is emitted from the at least one LED string.

In an alternative exemplary embodiment the invention, a backlight unit include: a plurality of LED strings having an anode, which receives a string current, and a chassis-grounded cathode; a DC-to-DC converter which boosts a source voltage to output a DC voltage; a current feedback unit which receives the DC voltage to output a plurality of driving voltages and outputs a plurality of driving currents corresponding to the LED strings, respectively; a current regulator which receives the driving voltages and the driving currents and outputs a plurality of string currents respectively flowing in the LED strings based on of current control information; and an LED driving controller which senses the driving currents flowing in the current feedback unit to output the current control information to compensate for the string currents and controls the DC voltage based on relationships between the driving voltages and the string voltages, where the string voltages are voltages at anodes of the LED strings, respectively.

In an exemplary embodiment, the LED driving controller may be configured as an integrated circuit (“IC”).

In an exemplary embodiment, the IC may include: a plurality of current source control units which senses the driving currents to output current compensation information for controlling the string currents; a maximum value circuit which detects a maximum value among the string voltages and the driving voltage; and an output voltage control unit which receives an output of the maximum value circuit to output a feedback voltage.

In an exemplary embodiment, each of the current source control units may include: a first operational amplifier which outputs a voltage corresponding to a voltage difference between the DC voltage and the driving voltage; a second operational amplifier which outputs a voltage corresponding to a voltage difference between the output value of the first operational amplifier and a reference voltage; a third operational amplifier which outputs a voltage corresponding to a voltage difference between a divided voltage corresponding to the DC voltage and the string voltage; and a current balance control unit which outputs the reference voltage in response to a pulse width modulation signal.

In an exemplary embodiment, the current feedback unit may include a plurality of sensing resistors, in which the driving currents flow.

In an exemplary embodiment, the current regulator may include a plurality of metal-oxide-semiconductor (“MOS”) transistors having a gate which receives the current control information, where the MOS transistors receive the driving currents to output the string currents.

In another exemplary embodiment of the invention, a current control method of a backlight unit include: sensing a driving current flowing in a hot side of each of a plurality of LED strings; compensating for the driving current based on of the sensed driving current and a reference voltage; and regulating a plurality of string currents respectively flowing in the LED strings based on the compensated driving current, where cathodes of the LED strings are chassis-grounded.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a block diagram illustrating an exemplary embodiment of a backlight unit according to the invention.

Referring toFIG. 1, the backlight unit10includes an light emitting diode (“LED”) driving circuit100and at least one LED string200(also referred to as an “LED array”).

The LED driving circuit100receives a source voltage VINto drive the at least one LED string200. The LED driving circuit100includes a direct-current-to-direct-current (“DC”-to-“DC”) converter110, a current feedback unit120, a current regulator130and an LED driving controller140.

The DC-to-DC converter110boosts the source voltage VINto generate a DC voltage VDC, and regulates the DC voltage VDCwith a feedback voltage VFB. In an exemplary embodiment, the feedback voltage VFBis a voltage based on a relationship between a driving voltage VLEDOUTand a plurality of string voltages VLED1to VLED4.

The current feedback unit120outputs a driving current ILEDand the driving voltage VLEDOUTcorresponding to the DC voltage Vic. In an exemplary embodiment, the driving current ILEDmay be a total current for driving the at least one LED string200. In such an embodiment, a voltage difference between the driving voltage VLEDOUTand the DC voltage VDCis substantially equal to a voltage between both ends of a sensing resistor for detecting the driving current ILEDof the current feedback unit120. In one exemplary embodiment, for example, the DC voltage VDCmay be greater than the driving voltage VLEDOUTby about 0.1 volt (V) to about 0.5 volt (V).

The current regulator130receives the driving current ILEDfrom the current feedback unit120and outputs a plurality of string currents ILED1to ILED4for driving the at least one LED string200, and maintains the string currents ILED1to ILED4based on compensation information of the driving current ILED(hereinafter referred to as “current compensation information”). In an exemplary embodiment, the current compensation information of the driving current ILEDmay be information based on a reference voltage VREF. The reference voltage VREFis a voltage corresponding to luminance of light emitted from the at least one string200.

The LED driving controller140detects the driving voltage VLEDOUTand the string voltages VLED1to VLED4to control the driving voltage VLEDOUT, and senses the driving current ILEDto compensate for the driving current ILED. The LED driving controller140includes a voltage detector142and a current compensator144.

The voltage detector142detects the driving voltage VLEDOUTfrom an input terminal of the current regulator130and the string voltages VLED1to VLED4from an input terminal of at least one LED string200, and outputs the feedback voltage VFBcorresponding to a relationship between the driving voltage VLEDOUTand the string voltages VLED1to VLED4. In an exemplary embodiment, the feedback voltage VFBmay be a voltage corresponding to a difference between the driving voltage VLEDOUTand the maximum value of the string voltages VLED1to VLED4. In an alternative exemplary embodiment, the feedback voltage VFBmay be a voltage corresponding to a difference between the driving voltage VLEDOUTand the minimum value of the string voltages VLED1to VLED4.

The current compensator144senses the driving current ILEDlowing in the current feedback unit120, and outputs the current compensation information for compensating for the driving current ILEDbased on the sensed driving current ILEDand the for the driving current ILEDwith the reference voltage VREF. In an exemplary embodiment, the current compensation information may be an analog current or a digital control signal.

Hereinafter, as illustrated inFIG. 1, the current feedback unit120, current regulator130and current compensator144are collectively referred to as a current source control unit101. The current source control unit101senses the driving current ILED, and controls/regulates/varies the string currents ILED1to ILED4flowing in the at least one LED string200, based on the sensed driving current ILEDand the reference voltage VREF. The current source control unit101allows a constant current to flow in the at least one LED string200.

In an exemplary embodiment, the current source control unit101compensates for a string current when light is emitted from at least one LED string200.

The at least one LED string200includes a plurality of serially-connected LEDs. In an exemplary embodiment, an anode of the at least one LED string200may be connected to the current regulator130, and a cathode of the at least one LED string200may be chassis-grounded. In one exemplary embodiment, for example, a first LED string220of the at least one LED string200has an anode that receives a first string voltage VLED1and first string current ILED1from the current regulator130, and a chassis-grounded cathode.

In one exemplary embodiment, as illustrated inFIG. 1, the at least one LED string200may include four LED strings, but the invention is not limited thereto. The backlight unit10may include at least one LED string, e.g., more than four LED strings or less than four LED strings.

A conventional backlight unit controls a constant current at a cathode of an LED string. A method of controlling a constant current at a cathode of an LED string has been described in U.S. Patent Application Publication No. 2011/012521, which is filed by Samsung Electronics Co., Ltd and herein incorporated by reference.

In an exemplary embodiment, the backlight unit10controls a current at the anode of the at least one LED string200, and chassis-grounds the cathode of the at least one LED string200. In such an embodiment, even when any one of the LED strings200is shorted, the backlight unit10enables the control of a constant current for the LED string200. In such an embodiment, the backlight unit10effectively prevents heat generation or ignition even when an LED string is shorted.

Exemplary embodiments of the invention that implement the current source control unit101ofFIG. 1as an analog circuit will now be described with reference toFIGS. 2 to 4. Hereinafter, for convenience of description, it is assumed that the at least one string200includes only one LED string, e.g., first LED string220.

FIG. 2is a block diagram illustrating an exemplary embodiment of a current source control unit101according to the invention. Referring toFIG. 2, the current source control unit101includes a current feedback unit120, a current regulator130and a current compensator144.

The current feedback unit120includes a sensing resistor RSconnected between first and second nodes N1and N2, an emitter resistor REconnected to the first node N1, a first collector resistor RC1connected to the third node N3, a second collector resistor RC2connected between the third node N3and a ground terminal, and a current sensing transistor TCS. In an exemplary embodiment, the current sensing transistor TCShas an emitter connected the emitter resistor RE, a collector connected to the first collector resistor RC1, and a base connected to the second node N2. The emitter resistor REmay have a low resistance value from about 0 ohm (Ω) to about 100 ohms (Ω). The emitter resistor REfunctions to render current tuning be less sensitive.

In one exemplary embodiment, for example, the current sensing transistor TCSmay be a P-channel (i.e., a P-N-P type) bipolar transistor.

The current feedback unit120senses a current in the sensing resistor RS, and outputs a pertinent sensing voltage to the third node N3.

The current regulator130includes a voltage regulation resistor RRconnected between the second node N2and a fourth node N4, a compensation current collector resistor RNCconnected to the fourth node N4, a compensation current emitter resistor RNEconnected to the ground terminal, a current regulation transistor TCRand a current compensation transistor TCC.

The current regulation transistor TCRoutputs a string current ILED1corresponding to a voltage difference between the fourth node N4and a fifth node N5. In such an embodiment, a voltage of the fourth node N4varies based on a compensation current ILEDC. Therefore, the current regulation transistor TCRmay output the string current ILED1corresponding to the compensation current ILEDC.

The current regulation transistor TCRhas an emitter connected to the second node N2, a collector connected to the fifth node N5, and a base connected to the fourth node N4. In such an embodiment, the fifth node N5corresponds to the anode of the LED string200, and the string voltage VLED1is output through the fifth node N5. In one exemplary embodiment, for example, the current regulation transistor TCRmay a P-channel bipolar transistor.

The current compensation transistor TCCoutputs the compensation current ILEDCbased on the current compensation information.

The current compensation transistor TCChas a collector connected to the compensation current collector resistor RNC, an emitter connected to the compensation current emitter resistor RNE, and a base that receives the current compensation information.

The current compensator144compares the reference voltage VREFand the sensing voltage from the current feedback unit120(i.e., the voltage of the third node N3) to output the current compensation information. The current compensator144includes an operational amplifier OP. The operational amplifier OP includes a positive input terminal (+) that receives the reference voltage VREF, a negative input terminal (−) that receives the voltage of the third node N3, and an output terminal connected to the base of the current compensation transistor TCC. The operational amplifier OP may output a voltage corresponding to a difference between the reference voltage VREFand the sensing voltage.

Controlling of the string current ILED1based on the reference voltage VREFin the current source control unit101will now be described in greater detail. Hereinafter, for convenience of description, it is assumed that a resistance value of the emitter resistor REis 0 and a resistance value of the voltage regulation resistor RRis infinite. Therefore, a current LED flowing in the sensing resistor RSis the same as the string current ILED1. The string current ILED1satisfies Equation I below.

In Equation I, VBEis a voltage between the base and emitter of the current sensing transistor TCS, ICis a current flowing in the collector of the current sensing transistor TCS, ISis a reverse saturation current of the current sensing transistor TCS, and VTis a thermal voltage that has a constant voltage at a room temperature (for example, about 300 kelvin [K]) of the current sensing transistor TCS, and RCis the sum of RC1and RC2.

As seen in Equation (1), the string current ILED1is proportional to the reference voltage VREF.

Accordingly, the current source control unit101may regulate/control/vary the string current ILED1with the reference voltage VREF.

InFIG. 2, the current feedback unit120of the current source control unit101senses a driving current ILEDflowing in the sensing resistor RSto compensate for the string current ILED1. In an exemplary embodiment, the current feedback unit120may sense the driving current ILEDwith a photocoupler.

FIG. 3is a block diagram illustrating an alternative exemplary embodiment of a current source control unit according to the invention. Referring toFIG. 3, a current source control unit101_1includes a current feedback unit121, a current regulator130and a current compensator144. The current source control unit101_1shown inFIG. 3includes a current feedback unit121having a configuration different from the configuration of the current source control unit100shown inFIG. 2.

The current feedback unit121includes a photocoupler122, and an emitter resistor REhaving one end connected to a ground terminal The photocoupler122emits light corresponding to a driving current ILED, and outputs a sensing voltage of a third node N3_1by allowing a current corresponding to the emitted light to flow. The photocoupler122includes a diode that receives a driving voltage VACfrom a first node N1, outputs the driving current ILEDto a second node N2, and emits the light corresponding to the driving current emitted from the diode. In an exemplary embodiment, the current sensing transistor TCShas a collector connected to a current compensation voltage VCC, an emitter connected to the other end of an emitter resistor RE, and a base that receives the light emitted from the diode. The current flowing in the current sensing transistor TCSis substantially proportional to the quantity of internal light emitted from the diode. The quantity of the internal light emitted from the diode is substantially proportional to the driving current LED.

In such an embodiment, the current source control unit101_1may regulate/control/vary the string current ILED1with the reference voltage VREF.

In an exemplary embodiment, the current source control unit101_1may be realized in a current mirror structure.

FIG. 4is a block diagram illustrating another alternative exemplary embodiment of a current source control unit according to the invention. Referring toFIG. 4, a current source control unit1012includes a current feedback unit123having a current mirror structure, a current regulator131and a current compensator144_1.

The current feedback unit123includes a voltage regulation resistor RRhaving one end connected to a first node N1, a current compensation collector resistor RNChaving one end connected to a fourth node N4, a sensing resistor RSconnected between a third node N3_2and a ground terminal, first and second current mirror transistors TMR1and TMR2, and a current compensation transistor TCC.

Herein, the first current mirror transistor TMR1has an emitter connected to the other end of the voltage regulation resistor RR, and a collector and base commonly connected to the fourth node N4. The second current mirror transistor TMR2has an emitter connected to the first node N1, a collector connected to a fifth node N5and a base connected to the fourth node N4. In the embodiment, each of the first and second current mirror transistors TMR1and TMR2may be a p-channel bipolar transistor.

Moreover, the current compensation transistor TCCincludes a collector connected to the other end of the current compensation collector resistor RNC, an emitter connected to the third node N3_2, and a base receiving the current compensation information.

The current regulator131, as illustrated inFIG. 4, is provided in the current feedback unit123and outputs a compensation current ILEDCbased on the current compensation information.

The current source control unit101_2ofFIG. 4may have a current mirror structure, and thus the compensation current ILEDCand the string current ILED1may have the same level. Therefore, the string current ILED1satisfies Equation II below.

In Equation II, a is a constant greater than 1 and predetermined based on the voltage regulation resistor RR.

Accordingly, the current source control unit1012may regulate/control/vary the string current ILED1with the reference voltage VREF.

In an exemplary embodiment, the at least one LED string200ofFIG. 1may have the shape of a bar.

FIG. 5is a block diagram illustrating an exemplary embodiment of an LED bar according to the invention. Referring toFIG. 5, an LED bar201includes an LED string202and a printed circuit board (“PCB”)204. A cathode of the LED string202is connected to the PCB204, which is connected to a chassis. In an exemplary embodiment, the PCB204may be directly connected to the chassis. In an exemplary embodiment, the PCB204may be connected to the chassis with a screw.

FIG. 6is a block diagram illustrating an alternative exemplary embodiment of an LED bar according to the invention. Referring toFIG. 6, the LED bar211may include first and second LED strings212and213, and a PCB214. A cathode of each of the first and second LED strings212and213is connected to the PCB214, which is connected to a chassis.

In an exemplary embodiment, as shown inFIG. 6, the LED bar211may include two LED strings, e.g., the first and second Led strings211and213, but the invention is not limited thereto. In an alternative exemplary embodiment, the LED bar211may include three or more LED strings.

A conventional LED bar has a structure where both an anode and a cathode are connected to an LED driving circuit.

In an exemplary embodiment of an LED bar according to the invention, for example, in the LED bars201and211inFIGS. 5 and 6, a cathode of an LED string may be chassis-grounded, and thus, only an anode may be connected to an LED driving circuit (for example, the LED driving circuit100inFIG. 1). In an exemplary embodiment where the LED bar includes a plurality of LED strings, the number of connected pins in the LED bar is substantially reduced, and the LED bar is substantially efficiently connected with the LED driving circuit100. In an exemplary embodiment, the number of connected pins may correspond to the number of anodes in the LED strings.

In an exemplary embodiment, the connection between the LED bar and the LED driving circuit100may be implemented in a socket type.

In an exemplary embodiment, an LED bar may be connected to the LED driving circuit100disposed, e.g., mounted, on a substrate of a source driver (not shown) via cable.

FIG. 7is a block diagram illustrating an exemplary embodiment of a backlight unit. Referring toFIG. 7, the backlight unit20includes a plurality of LED strings200, e.g., four LED strings, and an LED driving circuit300that controls the LED strings200.

The LED driving circuit300includes a DC-to-DC converter310, a current feedback unit320, a current regulator330and an LED driving controller340.

The DC-to-DC converter310boosts the input source voltage VINwith an inductor L. In an exemplary embodiment, the source voltage VINmay be in a range from about 22 V to about 26 V. In an exemplary embodiment, the DC-to-DC converter310may be implemented as a coupled inductor boost converter.

The DC-to-DC converter310includes an input capacitor CIN, an output capacitor CDC, an inductor L, a boosting control transistor MT, a diode D, a plurality of dividing resistors RDC1and RDC2, and a boost controller312.

When the boosting control transistor MT is turned off, a voltage is stored in a first inductor L1with the input voltage VIN. When the boosting control transistor MT is turned on, a reverse bias is applied to the diode D, and thus, the voltage stored in the first inductor L1is applied to a second inductor L2.

The boost controller312outputs a boosting control signal to a gate of the boosting control transistor MT, and controls a duty cycle of the boosting control signal based on first and second feedback voltages FB and VFB. In an exemplary embodiment, the first feedback voltage FB is a divided voltage corresponding to a DC voltage VDCof a first node N1(for example, VDC×RDC1/(RDC1+RDC2)), and the second feedback voltage VFBis a voltage corresponding to a relationship between a driving voltage VLEDOUTand a plurality of string voltages LED1to LED4(for example, VLEDOUT−VLEDMAX).

In an exemplary embodiment, pulse width modulation (“PWM”) or pulse frequency modulation (“PFM”) may be used in controlling the duty cycle. Hereinafter, for convenience of description, it is assumed that PWM is used in controlling the duty cycle.

The current feedback unit320outputs a power corresponding to a DC voltage VDCoutput from the DC-to-DC converter310and the driving current IFED. In such an embodiment, the output power may correspond to the driving voltage VLEDOUTand the driving current ILED. The driving voltage VLEDOUTis a voltage obtained by subtracting a voltage between both ends of a sensing resistor RSfrom the DC voltage VDC. The current feedback unit320includes the sensing resistor RSconnected between first and second nodes N1and N2. The driving current IFEDflows in the sensing resistor RS.

The current regulator330receives the driving voltage VLEDOUTand the driving current ILEDto output the string voltages LED1to LED4to the LED strings200, respectively, in a current mirror scheme, and compensates for the string voltages LED1to LED4based on the current compensation information. The current regulator330includes a voltage regulation resistor RR, a current compensation collector resistor RNC, a plurality of current regulating transistors TCR1to ICR4, and a current compensation transistor TCC. A string current supplying method or string current compensating method of the current regulator330is substantially similar to the methods described above with reference toFIGS. 2 to 4, and thus any repetitive detailed description thereof will hereinafter be omitted.

The LED driving controller330controls the driving voltage VLEDOUTand the driving current ILEDby outputting a feedback voltage VFBcorresponding to a relationship between the driving voltage VLEDOUTand the string voltages VLED1to VLED4. The LED driving controller330senses the driving current ILEDto output the current compensation information, thereby compensating for the string voltages VLED1to VLED4.

The LED driving controller340includes a voltage detector342and a current compensator344. The voltage detector342includes a maximum voltage detector342_1and a feedback voltage generator342_2. The maximum voltage detector342_1outputs a string voltage, having the highest level among the string voltages VLED1to VLED4, as a maximum string voltage VLEDMAX.

In an exemplary embodiment, when a voltage deviation (a difference between a minimum string voltage and a maximum string voltage) of the voltage detector342is greater than a predetermined value (for example, when some LED strings are shorted), the LED driving controller340may be configured to protect the LED strings200.

The feedback voltage generator342_2outputs the feedback voltage VFBcorresponding to a difference between the driving voltage VLEDOUTand the maximum string voltage VLEDMAX.

In an exemplary embodiment, the LED driving controller340may control the driving voltage VLEDOUTsuch that a voltage difference (a difference between the driving voltage VLEDOUTand the maximum string voltage VLEDMAX) of the feedback voltage generator342_2maintains a predetermined value (for example, about 1 V).

In such an embodiment, when the voltage difference of the feedback voltage generator342_2is equal to or less than a predetermined value (for example, about 0.5 V) (for example, when some LED strings are shorted), the LED driving controller340may be configured to protect the LED strings200.

The current compensator344includes a current sensing unit344_1, a holder344_2and an operational amplifier345.

The current sensing unit344_1senses a sensing current ILEDby sensing voltages between both ends of the sensing resistor RS.

The holder344_2maintains a voltage, corresponding to the driving current ILEDsensed by the current sensing unit344_1, based on a PWM signal PWM.

The operational amplifier345compares a voltage output from the holder344_2and the reference voltage VREFto output the current compensation information.

The backlight unit20senses the driving current ILEDat a hot side (corresponding to an anode), and compensates for the string currents ILED1to ILED4based on the sensed driving current ILED.

InFIGS. 1 to 8, each of the current feedback units120and320outputs one driving current ILEDand one driving voltage VLEDOUT. However, the invention is not limited thereto. In an alternative exemplary embodiment, the current feedback unit may output a plurality of driving currents and driving voltages respectively corresponding to a plurality of LED strings.

FIG. 8is a block diagram illustrating an alternative exemplary embodiment of a backlight unit according to the invention. Referring toFIG. 8, a backlight unit30includes an LED driving circuit400and a plurality of LED strings500. The LED driving circuit400inFIG. 8is substantially the same as the LED driving circuit100ofFIG. 1except that a current feedback unit420outputs a plurality of driving currents (not shown) and driving voltages (not shown) respectively corresponding to the LED strings500.

A plurality of voltages FB1to FB4inFIG. 8are the driving voltages output from the current feedback unit420, respectively. Also, a plurality of voltages VLED1to VLED4inFIG. 8are voltages into which string voltages of respective anodes of the LED strings500are divided. Hereinafter, the voltages VLED1to VLED4are referred to as divided string voltages.

An LED driving controller440includes a voltage detector442and a current compensator444. In an exemplary embodiment, the voltage detector442generates the feedback voltage VFBcorresponding to a relationship between the driving voltages FB1to FB4and the divided string voltages VLED1to VLED4. In an exemplary embodiment, the current compensator444senses the driving currents, which are respectively corresponding to the LED strings500, and outputs the current compensator information based on the reference voltage VREF.

The backlight unit30may individually control (for example, regulate or compensate for) the string currents ILED1to ILED4flowing in the LED strings500, respectively.

In an exemplary embodiment, the LED driving circuit may be implemented as an integrated circuit (“IC”).

FIG. 9is a block diagram illustrating an exemplary embodiment of an LED driving IC630according to the invention. Hereinafter, for convenience of description, it is assumed that the LED driving IC630controls four LED strings. Referring toFIG. 9, the LED driving IC630includes first to fourth current source control units631to634, a maximum value circuit636and an LED output voltage control unit637.

The first to fourth current source control units631to634output current control signals CTL1to CTL4corresponding to current control information based on a reference voltage VREFand voltages (for example, voltage differences between a DC voltage VDCand driving voltages FB1to FB4) corresponding to driving currents which pertain to a plurality of LED strings (not shown), respectively. In an exemplary embodiment, the first to fourth current source control units631to634output driving voltage control information (or a feedback voltage) based on corresponding voltages between the DC voltage VDCand string voltages, respectively (for example, voltage differences between a divided DC voltage VOSENSEand the divided string voltages LED1to LED4).

Hereinafter, a configuration of a first current source control unit631will be described. The first current source control unit631, as illustrated inFIG. 9, includes first to third operational amplifiers OP1to OP3and a current balance control unit635.

The first operational amplifier OP1outputs a voltage corresponding to a voltage difference between the DC voltage VDCand the first driving voltage FB1. The first operational amplifier OP1includes a positive input terminal (+) that receives the DC voltage VDCand a negative input terminal (−) that receives the first driving voltage FB1.

The second operational amplifier OP2outputs a voltage, corresponding to a difference between the reference voltage VREFand the output voltage of the first operational amplifier OP1, as the first current control signal CTL1. The second operational amplifier OP2includes a positive input terminal (+) that receives the reference voltage VREFand a negative input terminal (−) that receives the output voltage of the first operational amplifier OP1.

The third operational amplifier OP3outputs a voltage corresponding to a difference between the divided DC voltage VOSENSEand the first divided string voltage LED1. The divided DC voltage VOSENSEis a voltage into which the DC voltage VDCis divided at a predetermined ratio. The third operational amplifier OP3includes a positive input terminal (+) that receives the divided DC voltage VOSENSEand a negative input terminal (−) that receives the first divided string voltage LED1.

The current balance control unit635generates the reference voltage VREFin response to the PWM signal. In an exemplary embodiment, the reference voltage VREFis a voltage corresponding to luminance of each of the LED strings.

The second to fourth current source control units632to634may have structures substantially identical to the structure of the first current source control unit631.

The maximum value circuit (MAX circuit)636generates a voltage corresponding to the divided DC voltage VOSENSEand the highest voltage among the output voltages of the first to fourth current source control units631to634.

The LED output control unit637outputs the driving voltage control information for maintaining the output voltage of the maximum value circuit636as a predetermined value. In an exemplary embodiment, the LED output control unit637may output the driving voltage control information such that a voltage difference between the driving voltage and the maximum string voltage is maintained as a voltage in range from about 0.3 V to about 1.5 V.

FIG. 10is a block diagram illustrating an exemplary embodiment of an LED driving circuit600using the LED driving IC630ofFIG. 9. Referring toFIG. 10, the LED driving circuit600includes a DC-to-DC converter610, a current feedback unit620, a current regulator640, an LED driving IC630, and a plurality of resistors RVDC1, RVDC2, RLED11to RLED41, and RLED12to RLED42.

The DC-to-DC converter610boosts an input source voltage VINto output a DC voltage VDCand a driving current, and controls the DC voltage VDCbased on driving voltage control information. The driving voltage control information is inputted through a gate pin GATE of the LED driving IC630.

The current feedback unit620includes a plurality of sensing resistors RS1to Rs4that sense driving currents corresponding to string currents ILED1to ILED4flowing in the LED strings710to740, respectively. To sense the driving currents, nodes N21to N24connected to respective ends of the sensing resistors RS1to RS4are connected to pins that receives driving voltages FB1to FB4of the LED driving IC630, respectively, and a voltage VOSENSE, into which the DC voltage VDCis resistor-divided, is connected to a pin receiving the divided DC voltage VOSENSEof the LED driving IC630. The divided DC voltage VOSENSEis generated by dividing the DC voltage VDCby a predetermined value (which is RVDC1/(RVDC1+RVDC2)).

The current regulator640includes a plurality of metal-oxide-semiconductor (“MOS”) transistors MCR1to MCR4that output the string currents ILED1to ILED4to the LED strings710to740in response to a plurality of current control signals CTL1to CTL4, respectively. In an exemplary embodiment, gates of the MOS transistors MCR1to MCR4are connected to pins for outputting the current control signals CTL1to CTL4of the LED driving IC630, respectively.

Voltages LED1to LED4, into which the string voltages of the LED strings710to740are respectively divided, are connected to pins that receive the divided string voltages LED1to LED4of the LED driving IC630, respectively.

In an exemplary embodiment, the LED driving circuit600may be configured as a digital circuit, and may digitally sense and compensate for the driving currents flowing in the LED strings, respectively, at respective hot sides.

FIG. 11is a block diagram illustrating an exemplary embodiment of an LCD device1000according to the invention. Referring toFIG. 11, the LCD device1000includes a pixel array1100, a timing controller1200, a gamma voltage generator1300, a data driver1400, a gate driver1500, a power supply1600, at least one LED bar1700and an LED driver1800.

The pixel array1100, timing controller1200, gamma voltage generator1300, data driver1400, gate driver1500and power supply1600have been specifically described in U.S. Patent Application Publication No. 2010/0315325, filed by Samsung Electronics Co., Ltd. and herein incorporated by reference, and thus, the detailed description thereof will hereinafter be omitted.

The at least one LED bar1700inFIG. 11is substantially the same as the at least one LED bar200ofFIG. 1.

In an exemplary embodiment, the LED driver1800outputs a driving current to an anode of the at least one LED bar1700, and senses and compensates for the driving current flowing in the anode. The LED driver1800includes a current compensator1820and a current regulator1840. In such an embodiment, the current compensator1820senses the driving current output to the anode of the at least one LED bar1700and outputs current compensation information. The current regulator1840outputs the driving current to the anode based on the current compensation information. The LED driving circuit1800inFIG. 11may be substantially the same as the LED driving circuit100ofFIG. 1.

FIG. 12is a flowchart illustrating an exemplary embodiment of a current control method of an LED driving circuit according to the invention. Hereinafter, the current control method of the LED driving circuit will be described referring toFIGS. 1 and 12.

In an exemplary embodiment, the current compensator144senses a driving current ILEDat a hot side (or an anode) of each of the LED strings200(S110). The current compensator144senses the driving current ILEDby sensing a voltage difference of a sensing resistor RS. In such an embodiment, a cold side (or a cathode) of each of the LED strings200is chassis-grounded.

In such an embodiment, the current compensator144outputs the current compensation information for compensating for the driving current ILED, based on a voltage corresponding to the sensed driving current ILEDand the reference voltage VREF, and the current regulator130compensates for the driving current ILEDbased on the current compensation information (S120).

In such an embodiment, the current regulator130regulates string currents respectively flowing in the LED strings200according to the compensated driving current ILED(S130). A cold side (or a cathode) of each LED string200is chassis-grounded.

In an exemplary embodiment of the current control method of the LED driving circuit, a driving current at a hot side is sensed and compensated such that a constant current are effectively controlled even when at least one of the LED strings is shorted.

In an exemplary embodiment of the backlight unit and current control method thereof, a cathode is chassis-grounded and a driving current flowing in an anode is sensed to compensate for the driving current such that a constant current is supplied even when an LED string is shorted. Accordingly, heat generation or ignition is effectively prevented even when an LED string is shorted.