Abstract:
An apparatus of driving a light source for a display device includes a switching unit for selectively transmitting a DC input voltage, an oscillator for converting the DC input voltage into an AC voltage, and a transformer including a primary coil and a secondary coil for boosting the converted AC voltage using mutual inductance and for providing the boosted voltage for the light source. The driving apparatus further includes a current sensor for sensing a current in the oscillator or the secondary coil, and a light controller for controlling the switching unit based on the sensed current by the current sensor and a dimming control signal from an external device. Therefore, the current in the light source can be detected without a separate wire for connecting the light source and the light controller.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (a) Field of the Invention  
           [0002]    The present invention relates to an apparatus of driving a light source for a display device.  
           [0003]    (b) Description of the Related Art  
           [0004]    Display devices used for monitors of computers and television sets include self-emitting displays such as light emitting diodes (LEDs), electroluminescences (ELs), vacuum fluorescent displays (VFDs), field emission displays (FEDs) and plasma panel displays (PDPs) and non-emitting displays such liquid crystal displays (LCDs) requiring light source.  
           [0005]    An LCD includes two panels provided with field-generating electrodes and a liquid crystal (LC) layer with dielectric anisotropy interposed therebetween. The field-generating electrodes supplied with electric voltages generate electric field in the liquid crystal layer, and the transmittance of light passing through the panels varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are obtained by adjusting the applied voltages.  
           [0006]    The light may be emitted from a light source equipped in the LCD or may be natural light. When using the equipped light source, the total brightness of the LCD screen is usually adjusted by regulating the ratio of on and off times of the light source or regulating the current through the light source.  
           [0007]    A lighting device for an LCD, i.e., a backlight unit includes a light source and an inverter for driving the light source.  
           [0008]    The light source typically includes a fluorescent lamp and the inverter converts an input DC (direct current) voltage into an AC (alternating current) voltage and applies the AC voltage to the lamp.  
           [0009]    The backlight unit is classified into a backlighting type and an edge lighting type depending on the packaging configuration of mounting the lamps. The backlighting type configures the lamps directly behind the panels, and normally includes several lamps.  
           [0010]    When using several lamps, the brightness of the each lamp needs to be kept uniform, and thus the current flowing through each lamp always needs to be maintained constant. For this purpose, a current sensor for detecting the current of the lamp is installed at each lamp and the amount of the current in each lamp is controlled based on the detected current.  
           [0011]    When feedback-controlling the operations of the lamps in independent manner, separate wires for connecting the respective lamps to the inverter are required. Accordingly, the productivity is reduced since the circuit design for an inverter may consider several wires, and the feedback-control of the lamps may become inexact due to the increased noise and interference between the wires.  
           [0012]    For solving these problems, it is suggested that all the lamps are connected to the inverter by a single line and the control of the lamps are based on the current in the single line.  
           [0013]    However, this method cannot separately control the lamps since the state of each lamp cannot be distinguished. For example, when one of the lamps is out of order and the current therein does not flow or decreases, it cannot be determined whether the lamp is out of order since the total current flowing through the entire lamps, which is detected to be decreased, does not tell the state of each lamp. In this case, the feedback control increases the total current in the entire lamps, and then the defected lamp, which cannot capable of performing normal lighting operations, is also supplied with current or voltage. The continuous application of the voltage to the defected lamp may result in arcs or spikes, which exerts a bad influence on the lamp and the entire backlight unit as well.  
         SUMMARY OF THE INVENTION  
         [0014]    An apparatus of driving a light source for a display device is provided, which includes: an electricity supplying unit supplying electricity to the light source; a current sensor detecting a current outputted from the electricity supplying unit; and a light controller controlling the electricity supplying unit based on a signal from the current sensor and a dimming control signal from an external device.  
           [0015]    The electricity supplying unit preferably includes a transformer including a primary coil and a secondary coil and applying a voltage induced in the secondary coil to the light source. The electricity supplying unit may further includes a switching unit switching an input voltage from an external device under the control of the light controller, and an oscillator generating an AC voltage based on the input voltage from the switching unit and supplies the generated AC voltage to the primary coil of the transformer.  
           [0016]    The current sensor senses a current related to the primary coil of the transformer, and in particular, a current in the oscillator. Alternatively, the current sensor is connected to the secondary coil of the transformer and senses a current in the secondary coil of the transformer.  
           [0017]    The light controller controls the switching unit preferably in a pulse width modulation manner based on the dimming control signal and the signal from the current sensor.  
           [0018]    Preferably, the light controller determines an overcurrent in the light source on the basis of the signal from the current sensor and turns on/off the switching unit based on the determination of the overcurrent.  
           [0019]    According to an embodiment of the present invention, the current sensor comprises a capacitor and a diode connected in parallel between the electricity supplying unit and a predetermined voltage and a voltage divider connected to the capacitor and the diode and to and the light controller.  
           [0020]    The light source may include a fluorescent lamp. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The above and other advantages of the present invention will become more apparent by describing preferred embodiments thereof in detail with reference to the accompanying drawings in which:  
         [0022]    [0022]FIG. 1 is a block diagram of an LCD according to an embodiment of the present invention;  
         [0023]    [0023]FIG. 2 is an exploded perspective view of an LCD according to an embodiment of the present invention;  
         [0024]    [0024]FIG. 3 is an equivalent circuit diagram of a pixel of an LCD according to an embodiment of the present invention;  
         [0025]    [0025]FIG. 4 is a circuit diagram of an inverter and a lamp unit according to an embodiment of the present invention; and  
         [0026]    [0026]FIG. 5 is a circuit diagram of an inverter and a lamp unit according to another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention 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.  
         [0028]    In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numerals refer to like elements throughout. It will be understood that when an element such as a layer, film, region, substrate or panel is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.  
         [0029]    Then, apparatus of driving a light source for a liquid crystal display according to embodiments of the present invention will be described with reference to the drawings  
         [0030]    [0030]FIG. 1 is a block diagram of an LCD according to an embodiment of the present invention, FIG. 2 is an exploded perspective view of an LCD according to an embodiment of the present invention, and FIG. 3 is an equivalent circuit diagram of a pixel of an LCD according to an embodiment of the present invention.  
         [0031]    Referring to FIG. 1, an LCD according to an embodiment of the present invention includes a LC panel assembly  300 , and a gate driver  400  and a data driver  500  connected thereto, a gray voltage generator  800  connected to the data driver  500 , a lighting unit  900  for illuminating the LC panel assembly  300 , and a signal controller  600  controlling the above-described elements.  
         [0032]    In structural view, the LCD according to an embodiment of the present invention includes a LC module  350  including a display unit  330  and a backlight unit  340 , and a pair of front and rear cases  361  and  362 , a chassis  363 , and a mold frame  364  for containing and fixing the LC module  350  as shown in FIG. 2.  
         [0033]    The display unit  330  includes the LC panel assembly  300 , a plurality of gate flexible printed circuit (FPC) films  410  and a plurality of data FPC films  510  attached to the LC panel assembly  300 , and a gate printed circuit board (PCB)  450  and a data PCB  550  attached to the associated FPC films  410  and  510 , respectively.  
         [0034]    The LC panel assembly  300 , in structural view shown in FIGS. 2 and 3, includes a lower panel  100 , an upper panel  200  and a liquid crystal layer  3  interposed therebetween while it includes a plurality of display signal lines G 1 -G n  and D 1 -D m  and a plurality of pixels connected thereto and arranged substantially in a matrix in circuital view shown in FIGS. 1 and 3.  
         [0035]    The display signal lines G 1 -G n  and D 1 -D m  are provided on the lower panel  100  and include a plurality of gate lines G 1 -G n  transmitting gate signals (called scanning signals) and a plurality of data lines D 1 -D m  transmitting data signals. The gate lines G 1 -G n  extend substantially in a row direction and are substantially parallel to each other, while the data lines D 1 -D m  extend substantially in a column direction and are substantially parallel to each other.  
         [0036]    Each pixel includes a switching element Q connected to the display signal lines G 1 -G n  and D 1 -D m , and an LC capacitor C LC  and a storage capacitor C ST  that are connected to the switching element Q. The storage capacitor C ST  may be omitted if unnecessary.  
         [0037]    The switching element Q such as a TFT is provided on the lower panel  100  and has three terminals: a control terminal connected to one of the gate lines G 1 -G n ; an input terminal connected to one of the data lines D I -D m ; and an output terminal connected to the LC capacitor C LC  and the storage capacitor C ST .  
         [0038]    The LC capacitor C LC  includes a pixel electrode  190  on the lower panel  100 , a common electrode  270  on the upper panel  200 , and the LC layer  3  as a dielectric between the electrodes  190  and  270 . The pixel electrode  190  is connected to the switching element Q, and the common electrode  270  covers the entire surface of the upper panel  100  and is supplied with a common voltage Vcom. Alternatively; both the pixel electrode  190  and the common electrode  270 , which have shapes of bars or stripes, are provided on the lower panel  100 .  
         [0039]    The storage capacitor C ST  is an auxiliary capacitor for the LC capacitor C LC . The storage capacitor C ST  includes the pixel electrode  190  and a separate signal line (not shown), which is provided on the lower panel  100 , overlaps the pixel electrode  190  via an insulator, and is supplied with a predetermined voltage such as the common voltage Vcom. Alternatively, the storage capacitor C ST  includes the pixel electrode  190  and an adjacent gate line called a previous gate line, which overlaps the pixel electrode  190  via an insulator.  
         [0040]    For color display, each pixel uniquely represents one of three primary colors such as red, green and blue colors or sequentially represents the three primary colors in time, thereby obtaining a desired color. FIG. 3 shows an example that each pixel includes a color filter  230  representing one of the three primary colors in an area of the upper panel  200  facing its pixel electrode  190 . Alternatively, the color filter  230  is provided on or under the pixel electrode  190  on the lower panel  100 .  
         [0041]    Referring to FIG. 2, the backlight unit  340  includes a plurality of lamps  341  disposed behind the LC panel assembly  300 , a light guide  342  and a plurality of optical sheets  343  disposed between the panel assembly  300  and the lamps  341  and guiding and diffusing light from the lamps  341  to the panel assembly  300 , and a reflector  344  disposed under the lamps  341  and reflecting the light from the lamps  341  toward the panel assembly  300 .  
         [0042]    The lamps  341  preferably include fluorescent lamps such as CCFL (cold cathode fluorescent lamp) and EEFL (external electrode fluorescent lamp). An LED is another example of the lamp  341 .  
         [0043]    Referring to FIG. 1, the lighting unit  900  includes a plurality of lamp units  910  corresponding to the lamps  341  shown in FIG. 2 and a plurality of inverters  920  connected to the respective lamp units  910 . The inverter  920  initiates the lamp units  910  and dimming the lamp units  910  by controlling the duty ratio of the on time and the off time of the lamp units  910 . The inverters  920  are provided on either separate PCBs called inverter PCBs, or the gate PCB  450  or the data PCB  550 . The detailed configuration of the inverters  920  are described later in detail.  
         [0044]    A pair of polarizers (not shown) polarizing the light from the lamps  341  are attached on the outer surfaces of the panels  100  and  200  of the panel assembly  300 .  
         [0045]    Referring to FIGS. 1 and 2, the gray voltage generator  800  generates two sets of a plurality of gray voltages related to the transmittance of the pixels and is provided on the data PCB  550 . The gray voltages in one set have a positive polarity with respect to the common voltage Vcom, while those in the other set have a negative polarity with respect to the common voltage Vcom.  
         [0046]    The gate driver  400  preferably includes a plurality of integrated circuit (IC) chips mounted on the respective gate FPC films  410 . The gate driver  400  is connected to the gate lines G 1 -G n  of the panel assembly  300  and synthesizes the gate-on voltage Von and the gate off voltage Voff from the driving voltage generator to generate gate signals for application to the gate lines G 1 -G n .  
         [0047]    The data driver  500  preferably includes a plurality of IC chips mounted on the respective data FPC films  510 . The data driver  500  is connected to the data lines D 1 -D m  of the panel assembly  300  and applies data voltages selected from the gray voltages supplied from the gray voltage generator  800  to the data lines D 1 -D m .  
         [0048]    According to another embodiment of the present invention, the IC chips of the gate driver  400  and/or the data driver  500  are mounted on the lower panel  100 , while one or both of the drivers  400  and  500  are incorporated along with other elements into the lower panel  100  according to still another embodiment. The gate PCB  450  and/or the gate FPC films  410  may be omitted in both cases.  
         [0049]    The signal controller  600  controlling the drivers  400  and  500 , etc. is provided on the data PCB  550  or the gate PCB  450 .  
         [0050]    Now, the operation of the LCD will be described in detail.  
         [0051]    The signal controller  600  is supplied with RGB image signals R, G and B and input control signals controlling the display thereof such as a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a main clock MCLK, and a data enable signal DE, from an external graphic controller (not shown). After generating gate control signals CONT 1  and data control signals CONT 2  and processing the image signals R, G and B suitable for the operation of the panel assembly  300  on the basis of the input control signals, the signal controller  600  provides the gate control signals CONT 1  for the gate driver  400 , and the processed image signals R′, G′ and B′ and the data control signals CONT 2  for the data driver  500 . In addition, the signal controller  600  generates and provides a dimming control signal DIM for the inverters  920 .  
         [0052]    The gate control signals CONT 1  include a vertical synchronization start signal STV for informing of start of a frame, a gate clock signal CPV for controlling the output time of the gate-on voltage Von, and an output enable signal OE for defining the width of the gate-on voltage Von. The data control signals CONT 2  include a horizontal synchronization start signal STH for informing of start of a horizontal period, a load signal LOAD or TP for instructing to apply the appropriate data voltages to the data lines D 1 -D m , an inversion control signal RVS for reversing the polarity of the data voltages (with respect to the common voltage Vcom) and a data clock signal HCLK.  
         [0053]    The data driver  500  receives a packet of the image data R′, G′ and B′ for a pixel row from the signal controller  600  and converts the image data R′, G′ and B′ into the analogue data voltages selected from the gray voltages supplied from the gray voltage generator  800  in response to the data control signals CONT 2  from the signal controller  600 .  
         [0054]    Responsive to the gate control signals CONT 1  from the signals controller  600 , the gate driver  400  applies the gate-on voltage Von to the gate line G 1 -G n , thereby turning on the switching elements Q connected thereto.  
         [0055]    The data driver  500  applies the data voltages to the corresponding data lines D 1 -D m  for a turn-on time of the switching elements Q (which is called “one horizontal period” or “1H” and equals to one periods of the horizontal synchronization signal Hsync, the data enable signal DE, and the gate clock signal CPV). Then, the data voltages in turn are supplied to the corresponding pixels via the turned-on switching elements Q.  
         [0056]    The difference between the data voltage and the common voltage Vcom applied to a pixel is expressed as a charged voltage of the LC capacitor C LC , i.e., a pixel voltage. The liquid crystal molecules have orientations depending on the magnitude of the pixel voltage and the orientations determine the polarization of light passing through the LC capacitor C LC . The polarizers convert the light polarization into the light transmittance.  
         [0057]    By repeating this procedure, all gate lines G 1 -G n  are sequentially supplied with the gate-on voltage Von during a frame, thereby applying the data voltages to all pixels. When the next frame starts after finishing one frame, the inversion control signal RVS applied to the data driver  500  is controlled such that the polarity of the data voltages is reversed (which is called “frame inversion”). The inversion control signal RVS may be also controlled such that the polarity of the data voltages flowing in a data line in one frame are reversed (which is called “line inversion”), or the polarity of the data voltages in one packet are reversed (which is called “dot inversion”).  
         [0058]    Meanwhile, the inverters  920  control the lamp units  910  in response to the dimming control signal DIM from the signal controller  550 .  
         [0059]    A detailed configuration of the inverter  920  will be described more in detail with reference to FIG. 4.  
         [0060]    [0060]FIG. 4 is a circuit diagram of an inverter and a lamp unit according to an embodiment of the present invention.  
         [0061]    As shown in FIG. 4, a lamp unit  910  includes a lamp  911  and a capacitor C 1  connected in series between a ground and an inverter  920  and the capacitor C 1  is a ballast stabilizer.  
         [0062]    The inverter  920  includes a transformer  921 , an oscillator  922 , a current sensor  923 , a light controller  930 , a buffer  924 , a switching element Q 1 , a diode D 1  for preventing reverse voltage, and an inductor L 1 .  
         [0063]    The transformer  921  is connected to the lamp unit  910  to supply a voltage for lighting the lamp  911 . The transformer  921  includes two primary coils L 11  and L 12  and a secondary coil L 13  for generating sufficiently large voltage by mutual induction, and one end of the secondary coil L 13  is connected to the capacitor C 1  while the other end thereof is connected to the ground.  
         [0064]    The oscillator  922  is connected to the primary coils L 11  and L 12  of the transformer  921  and supplies an AC voltage to the primary coils L 11  and L 12 . The oscillator  922  includes a pair of transistors Q 21  and Q 22 , a capacitor C 21 , and a pair of resistors R 21  and R 22 .  
         [0065]    Both ends of one primary coil L 11  of the transformer  921  are connected to collectors of the transistors Q 21  and Q 22 , respectively, and the capacitor C 21  is connected in parallel to the primary coil L 11 . In addition, both ends of the other primary coil L 12  are connected to bases of the transistors Q 21  and Q 22 , respectively, and emitters of the transistors Q 21  and Q 22  are commonly connected to the current sensor  923 . One ends of the resistors R 21  and R 22  are connected to the bases of the transistors Q 21  and Q 22 , respectively, and the other ends thereof is commonly connected to an intermediate tap of the primary coil L 11  of the transformer  921 .  
         [0066]    The current sensor  923  senses the current flowing through the oscillator  922  or the current flowing through the primary coil L 11  and L 12  of the transformer  921 , and includes a filtering capacitor C 31 , a voltage divider including two resistors R 31  and R 32 , and a rectifying diode D 31 . The capacitors C 31 , the resistor R 31  and the diode D 31  are connected in parallel between the common emitter of the transistors Q 21  and Q 22  and the ground. The resistor R 32  is connected between the common emitter of the transistors Q 21  and Q 22  and the light controller  930 .  
         [0067]    The light controller  930  includes a feedback controller  931 , an overload protection unit  932 , and an on/off controller  933 . The feedback controller  931  is connected to the current sensor  923  and is provided with the dimming control signal DIM. The overload protection unit  932  is connected to the current sensor  923 . The on/off controller  933  is connected to an output terminal of the feedback controller  931  and the overload protection unit  932  and is provided with on/off voltages On/Off. The feedback controller  931  and the overload protection unit  932  may include comparators.  
         [0068]    The switching MOS transistor Q 1  and the inductor L 1  are serially connected between an input voltage Vin to be inputted to the oscillator  922  and the oscillator  922 , and has a gate connected to the on/off controller  933  via the buffer  924 .  
         [0069]    The diode D 1  is connected in forward direction from the ground to a node the transistor Q 1  and the inductor L 1 .  
         [0070]    The operation of the inverter  920  having this configuration will be described in detail.  
         [0071]    The transistor Q 1  is turned on or off in response to a control signal from the buffer  924  to selectively transmit the DC input voltage Vin. The output of the transistor Q 1  is supplied to the oscillator  922  via the inductor L 1 .  
         [0072]    The oscillator  922  converts the DC input voltage Vin into an AC sinusoidal voltage by the alternate turn on and off of the two transistors Q 21  and Q 22  and supplies the generated AC voltage to the primary coils L 11  and L 12  of the transformer  921 .  
         [0073]    A boosted AC voltage generated in the secondary coil L 13  of the transformer  921 , which is induced by the current flowing in the primary coils L 11  and L 12 , is applied to the lamp  911  via the capacitor C 1  to turn on the lamp  911 . The current flowing through the secondary coil L 13  varies depending on the change of the current flowing in the lamp  911  due to the failure of the lamp  911 , and the variation of the current in the secondary coil L 13  in turn changes the currents flowing in the primary coils L 11  and L 12 .  
         [0074]    The sinusoidal signal generated by the oscillator  922  is also supplied to the current sensor  923 . The input sinusoidal signal is filtered by the capacitor C 31 , half-wave rectified by the diode D 31 , and voltage-divided by the voltage divider R 21  and R 22  to be supplied to the light controller  930  as a DC current detecting signal CDS.  
         [0075]    The feedback controller  931  and the overload protection unit  932  of the light controller  930  respectively perform their operations responsive to the current detecting signal CDS from the current sensor  923 .  
         [0076]    The feedback controller  931  supplies a control signal TCS for regulating the duty ratio of the on time and the off time of the transistor Q 1  to the on/off controller  933  based on the dimming control signal DIM, and varies the control signal TCS in response to the current detecting signal CDS. The on/off controller  933  outputs the on/off voltages On/Off to the buffer  924  with the duty ratio determined by the control signal TCS from the light controller  930 , thereby controlling turn-on/off of the transistor Q 1 .  
         [0077]    The overload protection unit  932  enables to cut off the voltage provided for the lamp  911  to stop the lamp  911  when the lamp  911  is in an abnormal state such as current overflow. That is, the overload protection unit  932  compares the current detecting signal CDS from the current sensor  923  with a predetermined value and generates and supplies a overload control signal OPS to the on/off controller  933 .  
         [0078]    The overload control signal OPS has a predetermined level such as a low level when the current detecting signal CDS is equal to or less than the predetermined value, which indicates a normal condition, and then, the on/off controller  933  performs on/off control of the transistor Q 1  based only on the control signal TCS of the feedback controller  931 .  
         [0079]    However, the overload control signal OPS becomes to have another predetermined level such as a high level when the current detecting signal CDS is higher than the predetermined value, which indicates an abnormal condition. Then, the on/off controller  933  supplies the off voltage Off to the buffer  924  to turn off the transistor Q 1 , thereby cut off the input voltage Vin to disable the transformer  921 , thereby putting out the lamp  911  regardless of the control signal TCS from the feedback controller  931 .  
         [0080]    [0080]FIG. 5 is a circuit diagram of an inverter and a lamp unit according to another embodiment of the present invention.  
         [0081]    The configuration of an inverter  920  shown in FIG. 5 is similar to that shown in FIG. 4 except that a current sensor  923  is connected to a secondary coil L 13  of the transformer  921 .  
         [0082]    In detail, as shown in FIG. 5, the current sensor  923  according to this embodiment is connected to the secondary coil L 13  to feed the current in the secondary coil L 13  to a light controller  930 . It is noted that emitters of two transistors Q 21  and Q 22  of an oscillator  922  are commonly grounded.  
         [0083]    As described above, the state of a lamp is determined by detecting the current in the lamp without separately equipping a wire connecting the lamp and an inverter. As a result, the lifetime of the lamp can be elongated by appropriate control of the lamp and the stability of the lamp is increased by blocking the voltage applied to the lamp to turn off the lamp when the lamp is in abnormal state.  
         [0084]    While the present invention has been described in detail with reference to the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the sprit and scope of the appended claims.