Constant current driving circuit and light emitting diode backlight apparatus using the same

A constant current driving circuit includes a control integrated circuit which generates a switching signal, a switching device which switches an input power supply voltage based on the switching signal, a rectifying diode which rectifies a current of the input power supply voltage, a smoothing inductor which smoothes the current of the input power supply voltage, and a smoothing condenser which outputs an output current. The control integrated circuit includes a reference signal generator which generates a reference signal having information about a target constant current, a comparator which compares the current of the input power supply voltage with the reference signal, a flip-flop circuit which outputs a flip-flop signal having information about a time period during which a set state is maintained, and a delay circuit which outputs the switching signal to the switching device based on the flip-flop signal to control the switching device.

This application claims priority to Korean Patent Application No. 10-2011-0133000, filed on Dec. 12, 2011, and Japanese Patent Application No. 2011-131682, filed on Jun. 13, 2011, and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which in their entirety are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosure relates to a constant current driving circuit and a light emitting diode (“LED”) backlight apparatus using the constant current driving circuit.

2. Description of the Related Art

In general, a conventional constant current driving circuit is configured to detect current flowing through a load circuit in real time. For instance, the conventional constant current driving circuit is configured as shown inFIGS. 11 and 12. The conventional constant current driving circuit performs basic operations as follows.

(1) A current detector RR is connected to a light emitting diode (“LED”) string including N number of LEDs in series to measure a voltage Vsen.

(2) The constant current driving circuit controls an output voltage Vout to allow the detected voltage Vsen to become equal to a target value REF and controls an output current Iout to be constant. The output voltage Vout and the output current Iout are controlled by controlling a gate voltage applied to a switching device SI. Referring toFIG. 13, the output voltage Vout and the output current Iout are controlled by adjusting a ratio of a turn-on time period Tonto a turn-off time period Toff.

The conventional constant current driving circuit needs to detect the voltage Vsen of the current detector RR regardless of whether the switching device SI is turned on or turned off. However, since the current detector RR is connected to the load circuit in series, power loss in the current detector RR increases when the output current Tout increases.

In addition, when the current detector RR is connected to a high voltage side of the load circuit, a control integrated circuit (“IC”) is required to have a high endurance to a high voltage. On the other hand, in a case where the current detector RR is connected to a low voltage side of the load circuit, the load circuit is not grounded directly at one terminal thereof.

In addition, since a signal from the current detector RR is transmitted to the control IC, the load circuit does not needed to be provided with an input pin at an output terminal thereof.

Japanese Patent Publication No. 2010-040509 (hereinafter, “patent document”) discloses a conventional constant current driving circuit having a circuit configuration different than those as shown inFIGS. 11 and 12. The driving circuit disclosed in the patent document includes a current detecting resistor RISEN_i connected to the switching device in series and an inductance coil connected to the load circuit to perform basic operations as follows.

(1) A multiplier multiplies a reference signal REF, which is a desired value of an LED current, by an on/off control signal (e.g., a pulse width modulation (“PWM”) signal) of the switching device SI.

(2) The current detecting resistor RISEN_i detects a monitor signal ISEN_i. Since the current detecting resistor RISEN_i is connected to the switching device SI in series, the monitor signal ISEN_i is equal to the current flowing through the switching device SI. The monitor signal ISEN_i is calculated based on the following Equations 1 and 2. A differential amplifier compares a result of Equation 1 with the monitor signal ISEN_i to obtain a control signal PWM_i.
Iled=ISEN—i/PWM—i[Equation 1]
ISEN—i=REF×PWM—i[Equation 2]

FIG. 14shows a relationship between the LED current Iled, the monitor signal ISEN_i, and the PWM signal PWM_i in the conventional constant current driving circuit disclosed in the patent document.

(3) The control signal PWM_i is feedback controlled such that the result of Equation 2, i.e., “REF×PWM_i,” becomes equal to the monitor signal ISEN_i. In other words, the control signal PWM_i is feedback controlled to allow the LED current Iled to become equal to the desired value of the LED current.

The reference signal REF, the monitor signal ISEN_i, and current ILin the inductance coil have waveforms, which are respectively shown inFIG. 9of the patent document.

However, since the driving circuit disclosed in the patent document requires the multiplier, a circuit configuration of a switching balance controller becomes complex. In addition, the differential amplifier generates the control signal PWM_i based on the monitor signal ISEN_i, which is obtained by using the control signal PWM_i. That is, according to the driving circuit disclosed in the patent document, a convergence defect exists in feedback control of the control signal PWM_i.

BRIEF SUMMARY OF THE INVENTION

Exemplary embodiments of the invention provide a constant current driving circuit capable of controlling a constant current using a converter.

Exemplary embodiments of the invention provide a light emitting diode (“LED”) backlight apparatus having the constant current driving circuit.

According to an exemplary embodiment, a constant current driving circuit includes a control integrated circuit which generates a switching signal, a switching device, a rectifying diode, a smoothing inductor, and a smoothing condenser. The switching device includes an input terminal to which an input power supply voltage is applied and switches the input power supply voltage based on the switching signal. The rectifying diode rectifies a current of the input power supply voltage switched by the switching device, the smoothing inductor smoothes the current of the input power supply voltage, and the smoothing condenser outputs an output current. The control integrated circuit includes a reference signal generator which generates a reference signal having information about a target constant current, a comparator which compares the current of the input power supply voltage with the reference signal to output a reset signal, a flip-flop circuit which outputs a flip-flop signal having information about a time period during which a set state is maintained based on an external clock received as a set signal and the reset signal, and a delay circuit which outputs the switching signal to the switching device based on the flip-flop signal to control the switching device.

According to an exemplary embodiment, a constant current provided to a load circuit may be controlled to have a desired level without using a multiplier, and thus a circuit configuration of the constant current driving circuit may be simplified.

According to an exemplary embodiment, the constant current driving circuit may include a load circuit driven at a constant current, a constant power supply voltage connected to a high voltage terminal of the load circuit, a converter connected to a low voltage side of the load circuit, and a control integrated circuit connected to the converter to generate a switching signal may be included. The converter may include a switching device, a rectifying diode, a smoothing inductor, a smoothing condenser, and a resistor. The switching device may include an input terminal to which an input power supply voltage is applied and switch the input power supply voltage based on the switching signal. The rectifying diode may rectify a current of the input power supply voltage switched by the switching device, the smoothing inductor smoothes the current of the input power supply voltage, and the smoothing condenser outputs an output current. The resistor may include a first terminal connected to a low voltage terminal of the switching device and a second terminal grounded.

In an exemplary embodiment, the control integrated circuit may include a reference signal generator which generates a reference signal having information about a target constant current, a comparator which compares the current of the switching device with the reference signal to output a reset signal, a flip-flop circuit which outputs a flip-flop signal having information about a time period during which a set state is maintained based on an external clock received as a set signal and the reset signal, and a delay circuit which outputs the switching signal to the switching device based on the flip-flop signal to control the switching device. The current of the switching device may be measured based on a current flowing through the resistor.

According to an exemplary embodiment of the constant current driving circuit, since a time duration required for the current flowing through the switching device to reach a desired current value is controlled, the constant current may be controlled by setting the desired current value without inductance and voltage of the inductor.

In one exemplary embodiment, the constant current driving circuit may be configured to include a current detector which detects a current provided to the switching device and a slope compensation circuit connected between the current detector and the comparator. Thus, even when a duty cycle is equal to or larger than 50%, the current flowing through the inductor may be stabilized.

In addition, in an exemplary embodiment, an inductance of the smoothing inductor, a capacitance of the smoothing condenser, and a period of the set signal may be determined such that the current flowing through the smoothing inductor has a value larger than zero when the switching device is in a turned-on state or a turned-off state. According to an exemplary embodiment of the constant current driving circuit, the current of the smoothing inductor has a value larger than zero (0) when the switching device SI is in the turned-off state.

In addition, in an exemplary embodiment, the constant current driving circuit may be configured to a control circuit which applies an OFF signal to the switching device independent from the delay circuit and compulsively transits the state of the switching device to the turned-off state. Thus, the start and stop of the operation of the load circuit may be freely controlled.

According to an exemplary embodiment, a light emitting apparatus may include a plurality of light emitting devices connected to one another in parallel and a constant current driving circuit having the above-mentioned configuration, which allows the light emitting devices to be driven at a constant current. According to an exemplary embodiment, cathodes of an LED string are commonly grounded, and thus wires for the LED string may be easily designed.

According to an exemplary embodiment, a liquid crystal display may include a liquid crystal panel and a light emitting apparatus having the above-mentioned configuration, which is prepared as a backlight for the liquid crystal panel. Since the liquid crystal display includes the constant current driving circuit, power consumption in the liquid crystal display may be reduced.

According to an exemplary embodiment, the constant current driving circuit may control the constant current only by using the converter even though no detector is installed at an output side thereof, and the constant current driving circuit may control the constant current only by detecting the ON state of the switching device.

In addition, since the time duration required for the current flowing through the switching device to reach a desired current value is controlled, the target constant current may be set to a constant value without depending on the current of the smoothing inductor.

In addition, since the detector is not installed at the output side of the constant current driving circuit, power consumption caused by detecting the current of the switching device may be reduced.

In addition, the cathodes of the LED string are directly grounded. That is, cathodes of the LED string are commonly grounded, and thus wires for the LED string may be easily designed.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the invention will be described in further detail with reference to the accompanying drawings.

FIG. 1is a block diagram showing an exemplary embodiment of a constant current driving circuit according to the invention. In detail,FIG. 1shows a light emitting diode (“LED”) driving circuit that drives an LED string500configured to include N number of LEDs, LED1, . . . , LEDn, (n>1). The constant current driving circuit in the LED driving circuit is a voltage-drop type.

According to the exemplary embodiment of the constant current driving circuit, a current value of a target constant current Itargetis i predetermined, and the LED string500is driven at the target current value Itarget.

The LED driving circuit includes a switching device SI, a control integrated circuit (“IC”)100, a rectifying diode D1, a smoothing inductor L1, a smoothing condenser C1, and the LED string500including N number of LEDs LED1, . . . LEDn(N>1).

One terminal of the switching device SI is directly connected to a terminal of an input power supply voltage VIN.

The switching device SI is gate-controlled by the control IC100. The control IC100receives a clock CLK from an outside as a set signal RS_S, which is shown inFIG. 2A. The switching device SI is turned on or turned off by a switching signal Delay_O generated by the control IC100based on the set signal RS_S. An output current ISOof the switching device SI is smoothed by the rectifying diode D1, the smoothing inductor L1, and the smoothing condenser C1, and thus a current Iout is provided to the LED string500.

The control IC100includes a reference signal generator REF, a comparator CMP, a flip-flop circuit FF, and a delay circuit DLY.

A reference signal generator REF generates a reference signal ref_s including information about the target constant current Itarget. The reference signal generator REF applies the reference signal ref_s to the comparator CMP.

The comparator CMP compares the target constant current Itargetwith a current Is(hereinafter, referred to as current of the switching device SI) of the input power supply voltage VIN, which is directly provided to the switching device SI. When the switching device SI is turned on, the current ISof the switching device SI and a current ILof the smoothing inductor L1are as shown asFIGS. 2C and 2D.

That is, the current ISof the switching device SI linearly increases as the waveform of the current ILof the smoothing inductor L1shown inFIG. 3Aduring the turned-on period. The waveform of the current Isof the switching device SI is detected by a current detector Di.

However, when an ON-duty ratio of the gate control signal Delay_O of the switching device SI shown inFIG. 2Gis equal to or greater than 50%, the current ISof the switching device SI may be oscillated. Accordingly, the constant current driving circuit according to this exemplary embodiment further includes a slope compensation circuit SLOPE so as to compensate for the oscillation. Hereinafter, the current ISof the switching device SI refers to a current obtained by slope-compensating the current ISof the switching device SI, which is detected by the current detector Di.

The current ISof the switching device SI has a waveform as shown inFIGS. 2D and 3B.

The comparator CMP detects a time point Point1at which the current ISof the switching device SI reaches the target constant current Itargetand outputs a reset signal RS_R shown inFIG. 2E.

The flip-flop circuit FF includes an input terminal S, a reset terminal R, and an output terminal Q.

The clock CLK is input to the input terminal S of the flip-flop circuit FF as the set signal RS_S as shown inFIG. 2A. When the set signal RS_S is input to the input terminal S, the flip-flop circuit FF is placed in a set state. Then, the reset signal RS_R as shown inFIG. 2Eis input to the reset terminal R at the time point Point1. The flip-flop circuit FF is maintained in the set state during a time period Ta1from a time point at which the set signal RS_S is input to the time point Point1at which the reset signal RS_R is input. The flip-flop circuit FF outputs a flip-flop signal RS_Q through the output terminal Q, and the flip-flop signal RS_Q is input to the delay circuit DLY. The flip-flop signal RS_Q is a pulse signal as shown inFIG. 2Fand includes information about the time period Ta1.

The delay circuit DLY applies the switching signal Delay_O as shown inFIG. 2Gto the switching device SI. The switching device SI is maintained in the turned-on state in response to the switching signal Delay_O during a time period Tonthat is two times longer than the time period Ta1. Thus, the switching device SI may be gate-controlled as shown inFIG. 2B.

The switching device SI is controlled at a period TSWas the set signal RS_S shown inFIG. 2A. As shown inFIG. 2B, the switching device SI is in the turned-on state during the time period Tonand is in the turned-off state during a time period Toff, which is (TSW-Ton). That is, the switching device SI is transited to the turned-off state after the time period Tonelapses from a start of the turned-on state.

The constant current driving circuit controls an operation frequency and an operation period to be constant and controls the turned-on and turned-off time periods of the switching device SI using the control IC100by means of a pulse width modulation (“PWM”) scheme.

The constant current driving circuit does not limit the period TSW, which is the operation period. As shown inFIGS. 2C and 3A, the current ILof the smoothing inductor L1has a value larger than zero (0) when the switching device SI is in the turned-off state. In the exemplary embodiment, an inductance of the smoothing inductor L1, a capacitance of the smoothing condenser C1, and the period TSWof the set signal RS_S, which is the clock CLK, are preferably determined such that the current ILflowing through the smoothing inductor L1has the value larger than zero when the switching device SI is in the turned-on or turned-off state.

As described above, in the constant current driving circuit according to the exemplary embodiment, the current ILof the smoothing inductor L1is linearly increased within the time period Ton. Accordingly, a current component S1below the target constant current Itargetin a first half of the time period Tonis equal to a current component S2exceeding the target constant current Itargetin a later half of the time period Ton, as expressed in the following Equation 3.
S1=½×(Itarget−h1)×Ta1=S2=½×h2×Ta1[Equation 3]

In Equation 3, h1denotes a value of the current ILof the smoothing inductor L1measured at a beginning of the turned-on state of the switching device SI, and h2denotes a difference between the current value h1and a value of the current ILof the smoothing inductor L1measured at a beginning of the turned-off state of the switching device SI.

As the current value h2and the difference between the current value h1and the target constant current Itargetdecrease, the variation of the current ILof the smoothing inductor L1becomes smaller during the period TSW, and the LED string500is continuously driven by current closer to the target constant current Itargetregardless of the turned-on or turned-off state of the switching device SI.

In this exemplary embodiment, a time duration Tonof the turned-on state of the switching device SI in Equation 3 is two times longer than a time duration Ta1during which the current ISof the switching device SI reaches the target constant current Itargetafter the start of the turned-on state of the switching device SI.

On the other hand, when the switching device SI is in the turned-on state, charging of the smoothing inductor L1may not be affected by performance of the smoothing inductor L1or a level of a voltage applied to the smoothing inductor L1. Therefore, when a size of the smoothing inductor L1increases, a charging time of the smoothing inductor L1and a measurement time period T are increased.

In addition, by using only the measurement time period T during which the current ILof the switching device SI reaches a certain current value I before reaching the target constant current Itarget, the time period Tonof the turned-on state of the switching device SI may be determined. In this case, the time period Tonis determined by the following Equation 4.
Ton=2×Itarget/I×T[Equation 4]

The constant current driving circuit according to this exemplary embodiment is described as a voltage-drop type. However, it should be noted that, in an alternative embodiment, the constant current driving circuit may be a voltage-boosting type.FIG. 4shows another exemplary embodiment of a constant current driving circuit of the voltage-boosting type according to the invention. InFIG. 4, like reference numerals denote like elements inFIG. 1, and thus repetitive explanation will be omitted.

FIG. 5is a block diagram showing yet another exemplary embodiment of a constant current driving circuit according to the invention. In this exemplary embodiment, a high voltage side of an LED string500is connected to a constant source voltage Vconstand a low voltage side of the LED string500is connected to a converter600.

The converter600includes a switching device SI, a rectifying diode D1, a smoothing inductor L1, a smoothing condenser C1, and a resistor R1. The converter600is driven by a control IC200to allow the LED string500to be driven at the target constant current Itarget.

The control IC200has the same structure and function as those of the control IC according to the exemplary embodiments ofFIGS. 1 and 4except that the control IC200includes a control circuit CC connected between the switching device SI and the delay circuit DLY.

In addition, the control circuit CC applies an ON signal or an OFF signal to the switching device SI independently from the delay circuit DLY to place the switching device SI in the turned-on or turned-off state. Thus, the switching device SI according to this exemplary embodiment is transited to the turned-on or turned-off state by logically multiplying the switching signal Delay_O output from the delay circuit DLY by the ON/OFF signal from the control circuit CC. That is, the control circuit CC controls the ON/OFF of the LED string500. The constant current driving circuit may be repeatedly turned on and off at a high frequency to control brightness by means of light pulse width modulation. In addition, when the ON/OFF control of the LED string500is not needed, the switching signal Delay_O output from the delay circuit DLY is directly input to the switching device SI, and thus the switching device SI may be gate-controlled.

In addition, a low voltage side of the switching device SI is connected to an end of the resistor R1of which another end is connected to a ground, and the current ISof the switching device SI is measured from the current flowing through the resistor R1. The current ISof the switching device SI is detected by the current detector Di and the current detector Di applies a detection signal CS having information about the detected current ISto a comparator CMP. The detection signal CS may be a low voltage signal. Accordingly, power consumption in the constant current driving circuit may be reduced in case of controlling plural LED strings.

FIG. 6is a circuit diagram used to simulate an operation of the constant current driving circuit shown inFIG. 5, andFIGS. 7A to 7Eare views showing signals used in the circuit diagram shown inFIG. 6.

A voltage signal V(Ifb) shown inFIG. 7Ais an output signal obtained by amplifying the detection signal CS, which is detected by the current detector Di, using an amplifier10. In addition, the waveform of the voltage signal V(Ifb) shown inFIG. 7Acorresponds to a waveform of the current ISflowing through a switching device M5and a resistor R4that is grounded. When comparing the waveform shown inFIG. 7Awith the waveform shown inFIG. 2D, it is shown that the waveform of the voltage signal V(Ifb) corresponds to the waveform of the current IS.

The LED string500configured to include light emitting diodes D1to D12is directly affected by the switching device M5. The waveform of the current ILflowing through the smoothing inductor L1is represented by the waveform of the current IS. Accordingly, the current ILflowing through the smoothing inductor L1may be monitored by measuring the voltage signal V(Ifb). A current fled flowing through the LED string500and the current ILflowing through the smoothing inductor L1are smoothed by the capacitor C1.

The voltage signal V(Ifb) is slope-compensated by the slope compensation circuit SLOPE, input to the comparator, and compared with a voltage signal corresponding to the target constant current Itarget. In addition, the constant current Itargetand the voltage signal corresponding to the target constant current Itargetare set by the reference signal generator REF as described above.

The comparator detects a timing point Point1at which the voltage signal corresponding to the current ISis matched with the voltage corresponding to the target constant current Itargetand outputs a reset signal RS_R corresponding to the waveform shown inFIG. 2E.

The flip-flop circuit FF is placed in a set state by a set signal RS_S (refer toFIGS. 7B and 2A) input to an input terminal S thereof based on an external clock V6and receives the reset signal RS_R through a reset terminal R thereof at the time point Point1. The flip-flop circuit FF is maintained in the set state during a time period Ta1from an input of the clock CLK (or the set signal RS_S) to the time point Point1at which the reset signal RS_R is input. The pulse signal (refer toFIG. 2F) having a period of Ta1output from the output terminal Q of the flip-flop circuit FF is input to the delay circuit DLY.

The delay circuit generates a pulse signal V(Co) shown inFIG. 7Bto be output to control the switching device M5during a time period Tontwo times longer than the time period Ta1to allow the switching device M5to maintain the turned-on state.

Referring toFIG. 6, the delay circuit includes a control circuit CC. The control circuit CC applies the ON/OFF signal to the switching device M5to compulsively transit the switching device M5to the turned-off state. In this case, a unit of measure a horizontal axis inFIGS. 7A and 7Bis 1/20 times smaller than that of a horizontal axis inFIGS. 7C to 7E.

The switching device M5is transited to the turned-on state or the turned-off state based on a signal obtained by logically multiplying the pulse signal V(Co) by the ON/OFF signal from the control circuit CC.

According to the simulation circuit shown inFIG. 6, an electric potential of V(fls) of the ground condenser C1, the current ILof the inductor L1, and current I(D8) of a diode D8are represented as shown inFIGS. 7C,7D, and7E, respectively. In addition, the electric potential V(fls) shown inFIG. 7Ccorresponds to the signal obtained by logically multiplying the pulse signal V(Co) by the ON/OFF signal from the control circuit CC, and the current ILand the current I(D8) becomes substantially zero ampere (A) in a case where the electric potential of V(fls) is equal to or higher than that of an anode of the diode D20.

Referring toFIGS. 7D and 7E, the current ILflowing through the inductor L1and the current I(D8) flowing through the diode D8are substantially similar to each other. As described above, according to the constant current driving circuit of the exemplary embodiment, the LED string500may be driven at the target constant current Itargetby measuring the current ISflowing through the resistor R4that is grounded.

That is, the constant current driving circuit controls the time period Ta1required for the current ISof the switching device SI to reach the desired current value. Accordingly, the desired constant current value may be obtained without depending on reactance or voltage of the inductor. In addition, according to this exemplary embodiment, the current detector Di is provided at an output terminal of the LED string500so that the output current Iled or the output voltage of the LED string500do not need to be detected. Thus, power loss in the constant current driving circuit may be effectively prevented. In addition, according to the constant current driving circuit of the exemplary embodiment, only the current ISflowing through the grounded resistor R4when the switching device SI is in the turned-on state is detected. Thus, the current flowing through the load circuit driven under the constant current does not need to be detected constantly. Thus, the power loss in the constant current driving circuit may be effectively prevented.

FIG. 8is a block diagram showing yet another exemplary embodiment of a constant current driving circuit according to the invention. InFIG. 8, the constant current driving circuit includes M number of LED strings700(M>2), wherein each LED string CH1to CHm has substantially the same structure and function as those of the LED string shown inFIG. 5. That is, the M number of LED strings700are connected to the converter and the control IC shown inFIG. 5and independently controlled.

In addition, high voltage output terminals of the M number of LED strings700are connected to a constant source voltage Vconst in parallel. An i-th LED string CHi of the M number of LED strings700is connected to a terminal DLi (not shown) of the control IC corresponding to the i-th LED string CHi, and thus one LED string may be connected to one corresponding terminal.

In addition, in this exemplary embodiment, the load circuit driven under the constant current is the M number of LED strings700, however, it should be noted that the invention is not be limited thereto. In an alternative exemplary embodiment, the load circuit may be one of various load circuits other than the LED string, such as, for example, a gas sensor, a stepping motor, or a pulse motor.

FIG. 9is a block diagram showing an exemplary embodiment of a liquid crystal display according to the invention.

Referring toFIG. 9, the liquid crystal display900includes an AC/DC power supply910and an LCD module920. The LCD module920includes a backlight unit930and the backlight unit930includes a backlight driver931and a backlight932.

The AC/DC power supply910includes a plug911, an AC/DC rectifier912, and a DC/DC converter913. The AC/DC power supply910converts an alternating current (“AC”) voltage, e.g., 100 volts or 240 volts, into a direct current (“DC”) voltage and provides the direct current voltage to the LCD module920.

The LCD module920includes a DC/DC converter921, a common electrode voltage generator (or Vcom generator)922, a gamma voltage generator923, an LCD panel part924, and the backlight unit930. The LCD module920receives an image data from an external graphic controller (not shown) and displays an image based on the received image data.

The LCD panel part924includes a thin film transistor substrate (not shown), a color filter substrate (not shown) facing the thin film transistor substrate, and a liquid crystal layer (not shown) interposed between the thin film transistor substrate and the color filter substrate. The thin film transistor substrate includes a display area and a non-display area, and a gate driver and a data driver are arranged in the non-display area. The display area includes a plurality of gate lines extended from the gate driver and a plurality of data lines extended from the data driver and insulated from and crossing the gate lines. The gate lines and the data lines define a plurality of pixel areas.

Although not shown inFIG. 9, each pixel area includes a thin film transistor and a pixel electrode, and the pixel electrode faces a common electrode disposed on the color filter substrate while interposing the liquid crystal layer therebetween. Accordingly, a transmittance of a light passing through the liquid crystal layer is controlled by an intensity of an electric field generated between the pixel electrode and the common electrode, thereby displaying an image having a desired gray scale through the LCD panel part924.

The common electrode voltage generator922generates a common electrode voltage Vcom based on the direct current voltage of which level is varied by the DC/DC converter921and provides the common electrode voltage Vcom to the LCD panel part924.

The gamma voltage generator923generates a gamma voltage Vdd based on the direct current voltage of which level is varied by the DC/DC converter921and provides the gamma voltage Vdd to the LCD panel part924. InFIG. 9, the common electrode voltage generator922and the gamma voltage generator923are shown as being separated from the LCD panel part924, however, the invention should not be construed as being limited thereto. That is, the common electrode voltage generator922and the gamma voltage generator923may be included in the LCD panel part924.

The backlight unit930includes the backlight driver931and the backlight932. The backlight driver931includes the control IC of the constant current driving circuit according to the exemplary embodiment shown inFIG. 8. The backlight932includes the M number of LED strings700(M>2) shown inFIG. 8. When the image is displayed through the LCD panel part924, the backlight932controls the turned-on time period of the M number of LED strings700, thereby controlling brightness of the image.

InFIG. 9, the AC/DC power supply910is described as being separated from the LCD module920, however, the invention should not be construed as being limited thereto. In alternative embodiments, the AC/DC power supply910may be included in the LCD module920.

Since the liquid crystal display900includes the constant current driving circuit according to fourth exemplary embodiment, the backlight unit930may reduce the power consumption thereof.

FIG. 10is an exploded perspective view showing an exemplary embodiment of a liquid crystal display according to the invention.

Referring toFIG. 10, a liquid crystal display1000includes a backlight assembly1010, a display unit1070, and a receiving container1080.

The display unit1070includes a liquid crystal display panel1071, and a data printed circuit board1072and a gate printed circuit board1073, which each output driving signals to drive the liquid crystal display panel1071. The data printed circuit board1072is electrically connected to the liquid crystal display panel1071through a data tape carrier package1074. The gate printed circuit board1073is electrically connected to the liquid crystal display panel1071through a gate tape carrier package1075.

The liquid crystal display panel1071includes a thin film transistor (“TFT”) substrate1076, a color filter substrate1077coupled with the TFT substrate1076, and a liquid crystal layer1078interposed between the TFT substrate1076and the color filter substrate1077.

Although not shown inFIG. 10, the TFT substrate1076includes a plurality of pixels arranged thereon in a matrix form, and each pixel includes a TFT (not shown) as a switching device. The TFT substrate1076may be formed of a transparent material (e.g., a glass). The TFT includes a gate electrode connected to a gate line, a source electrode connected to a data line, and a drain electrode connected to a pixel electrode formed of a transparent conductive material.

The color filter substrate1077includes a color filter layer (not shown) and a common electrode (not shown). The color filter layer includes red (R), green (G), and blue (B) color pixels corresponding to the pixels, respectively. The common electrode may be formed of a transparent conductive material.

The receiving container1080provides a receiving space1081therein. The backlight assembly1010and the liquid crystal display panel1071are accommodated in the receiving space1081and fixed to the receiving container1080.

In order to accommodate the backlight assembly1010, the receiving space1081has a shape corresponding to that of the backlight assembly1010when viewed in plan.

In an exemplary embodiment, the receiving space1081and the backlight assembly1010may have a rectangular shape in plan, as shown inFIG. 10.

The liquid crystal display1000further includes a backlight driver1060and a top chassis1090.

The backlight driver1060is accommodated in the receiving space1081of the receiving container1080and generates a direct current to drive the backlight assembly1010. The direct current generated by the backlight driver1060is applied to the backlight assembly1010through a first power supply voltage applying line1063and a second power supply voltage applying line1064. The first power supply voltage applying line1063may be directly connected to an anode1040aof an LED string (not shown) disposed at a side portion of the backlight assembly1010or connected to the anode1040athrough a separate member (not shown). The second power supply voltage applying line1064may be directly connected to a cathode1040bof the LED string disposed at another side portion of the backlight assembly1010or connected to the cathode1040bthrough a separate member.

The top chassis1090is coupled with the receiving container1080to cover an edge portion of the liquid crystal display panel1071. The top chassis1090effectively prevents the liquid crystal display panel1071from being damaged by an external impact and from being separated from the receiving container1080.

The liquid crystal display1000may further include at least one optical sheet1095to improve optical properties of a light emitting from the backlight assembly1010. The optical sheet1095may include a diffusion sheet to diffuse the light or a prism sheet to condense the light.

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.