Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to devices for displaying images, and more particularly, to a device and a method of driving a light source for image display devices. 
         [0003]    2. Description of the Related Art 
         [0004]    Display devices, such as computer monitors, television sets, etc., generally include self-emitting display devices employing light emitting diodes (LEDs), electroluminescences (ELs), vacuum fluorescent display (VFD), field emission display (FED) and plasma panel display (PDP), and non-emitting display devices employing liquid crystal display (LCD) which necessitates a light source. 
         [0005]    LCD devices are generally equipped with two panels each having a field-generating electrode and a liquid crystal (LC) layer with dielectric anisotropy. The LC layer is interposed between the two panels. The field-generating electrodes are each supplied with an electric voltage to generate electric fields across the liquid crystal layer. The light transmittance of the LC layer varies in association with the strength of the electric fields, which is controlled by the applied voltage. Accordingly, desired images are displayed by adjusting the applied voltage. 
         [0006]    The light for an LCD device is provided by a light source equipped within the LCD device or may be the natural light. In case of employing a light source to provide the light, the brightness on a screen of the LCD device is usually adjusted by regulating the ratio of on- and off-time of the light source or regulating the current flowing the light source. 
         [0007]    As a light source for the LCD devices, fluorescent lamps are usually used. The fluorescent lamps generally require a high AC voltage of which magnitude is typically in the range of several kilovolts and frequency in the range of dozens of kilohertz. The current flowing such fluorescent lamps has a magnitude of several milli-amperes. Since the lamps are disposed at the rear side of an LCD panel and close to the panel at a distance of several millimeters, electric fields and magnetic fields from the lamps make noise to signals in wires and thin film transistors (TFTs) of the LCD panel. In particular, since the frequency of a driving signal for the lamps and the frequency of a horizontal synchronization signal for the LCD panel are similar to each other but a slight difference, a beating occurs to cause interference which makes horizontal stripes, called waterfall, on the LCD screen. 
         [0008]    In order to coinciding the frequency of the lamp driving signal and the frequency of the horizontal synchronization signal for removing such problems, a triangular pulse width modulation (PWM) reference signal having a frequency lower than the frequency of the horizontal synchronization signal is generated and the reference signal is dropped to a bottom level at the time of synchronization by using short pulses to be initiated. 
         [0009]    However, in the display devices employing the conventional light source driving devices and method, the triangular reference wave is generated to have rising portions and falling portions, which are asymmetric to each other. As a result, are caused several problems such as reduced lifetime and unstable ignition of the lamps. 
       SUMMARY OF THE INVENTION 
       [0010]    The above mentioned and other drawbacks and deficiencies of the prior art are overcome or alleviated by a display device according to the present invention. In one embodiment, a device for driving a light source of an image display device comprises input terminals to receive a horizontal synchronization signal and a control signal externally provided, an oscillator to generate a reference signal having a frequency, a controller to modulate the reference signal in response to the control signal and output a modulated signal, and a phase difference detecting unit to receive the horizontal synchronization signal and the modulated signal and detect a phase difference between the horizontal synchronization signal and the modulated signal to generate an output signal indicating the phase difference, wherein the oscillator adjusts the frequency of the reference signal in response to the output signal of the phase difference detecting unit so that the horizontal synchronization signal and the reference signal are synchronized with each other. The control signal externally provided includes a signal to control luminance on a screen of the image display device. 
         [0011]    The phase difference detecting unit may include a phase comparator to compare phases of the horizontal synchronization signal and the modulated signal and generate an output signal of which value is determined based on the comparison, and an integrator to generate a voltage signal having a magnitude proportional to an integration of the output signal of the phase comparator. The integrator may include a voltage divider having resistors connected between a supply voltage and ground, an operational amplifier having an inverting terminal to receive the output signal of the phase comparator and a non-inverting terminal connected to the voltage divider, a capacitor connected between the inverting terminal and an output terminal of the operational amplifier, and a reset unit to initiate the integrator in response to an externally provided instruction signal by discharging the capacitor in the integrator. 
         [0012]    In another embodiment, the light source drive device may also include a frequency divider to divide a frequency of the modulated signal provided from the controller to generate a frequency-divided signal. The light source drive device may further include a low pass filter connected between the phase comparator and the integrator, in which the low pass filter filters out high frequency components of the output signal of the phase comparator. 
         [0013]    In another embodiment, the light source drive device may also include a switch circuit to receive the modulated signal from the controller and generate a switch signal having on and off levels by switching a supply voltage in accordance with the modulated signal, and a transformer to receive the switch signal from the switch circuit and generate a sinusoidal signal which is applied to the light source. 
         [0014]    In another embodiment, there is provided a method of driving a light source in an image display device, comprising the steps of generating a reference signal having a frequency, detecting a phase difference between a horizontal synchronization signal for the image display device and the reference signal to generate a detect signal, adjusting the frequency of the reference signal in response to the detect signal, and providing a driving signal to the light source in response to the adjusted reference signal. 
         [0015]    The detecting step may include comparing the horizontal synchronization signal and the reference signal, integrating a result signal obtained from the comparing step to generate an integrated voltage signal as the detect signal, and resetting the integrating step such that the integrated voltage signal returns to an initial status. The method may further include performing pulse width modulation with respect to the reference signal to generate a modulated signal, and dividing a frequency of the modulated signal to generate a frequency-divided signal, in which the detect signal is obtained by detecting the phase difference between the horizontal synchronization signal and the frequency-divided signal. 
         [0016]    These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    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: 
           [0018]      FIG. 1  is an exploded perspective view of an image display device according to an embodiment of the present invention; 
           [0019]      FIG. 2  is a block diagram illustrating a part of the image display device of the present invention; 
           [0020]      FIG. 3  is an equivalent circuit diagram of a pixel of the image display device according to an embodiment of the present invention; 
           [0021]      FIG. 4  is a circuit diagram of the inverter in  FIG. 2  according to an embodiment of the present invention; and 
           [0022]      FIG. 5  shows waveforms of output voltages of the parts in  FIG. 4  and a lamp current provided to the lamp unit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Detailed illustrative embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing exemplary embodiments of the present invention. 
         [0024]    In the drawings, the thickness of layers and regions are exaggerated for clarity, and the like numerals refer to like elements. 
         [0025]      FIG. 1  is an exploded perspective view of an image display device according to an embodiment of the present invention,  FIG. 2  is a block diagram illustrating a part of the image display device of the present invention, and  FIG. 3  is an equivalent circuit diagram of a pixel of the image display device according to an embodiment of the present invention. 
         [0026]    Referring to  FIG. 1 , the image display device, such as a liquid crystal display (LCD) device, of the present invention includes a display module  350  including a display unit  330 , a backlight unit  340 , a pair of front and rear cases  361  and  362 , a chassis  363 , and a mold frame  364  containing and fixing the display module  350 . 
         [0027]    The display unit  330  includes a display panel assembly  300 , gate tape carrier packages (TCPs) or chip-on-film (COF) type packages  510  mounting gate driving ICs and data TCPs  410  attached to the display panel assembly  300 , and a gate printed circuit board (PCB)  550  and a data PCB  450  attached to the gate and data TCPs  510  and  410 , respectively. 
         [0028]    The backlight unit  340  includes lamps  341  disposed behind the display panel assembly  300 , a spread plate  342  and optical sheets  343  disposed between the panel assembly  300  and the lamps  341 . The spread plate  342  guides and diffuses light from the lamps  341  to the panel assembly  300 . The backlight unit also includes a reflector  344  disposed under the lamps  341  and reflecting the light from the lamps  341  toward the panel assembly  300 . 
         [0029]    The lamps  341  are, for example, fluorescent lamps such as CCFL (cold cathode fluorescent lamp) and EEFL (external electrode fluorescent lamp) or LED lamps. 
         [0030]    Referring to  FIG. 2 , the display device of the present invention also includes a gate driver  400  and a data driver  500  which are connected to the display panel assembly  300 , a gray voltage generator  800  connected to the data driver  500 , a lighting unit  900  for illuminating the panel assembly  300 , and a signal controller  600  controlling the above elements. 
         [0031]    The display panel assembly  300  includes a lower panel  100 , an upper panel  200  and a liquid crystal (LC) layer  3  interposed therebetween (referring to  FIG. 3 ). The display panel assembly  300  includes display signal lines G 1 -G n  and D 1 -D m  and pixels which are connected to the display signal lines G 1 -G n  and D 1 -D m  and arranged in a matrix form. 
         [0032]    The display signal lines G 1 -G n  and D 1 -D m  are disposed on the lower panel  100  and include gate lines G 1 -G n  transmitting gate signals (called scanning signals) and data lines D 1 -D m  transmitting data signals. The gate lines G 1 -G n  are arranged in a row direction and substantially parallel to each other, and the data lines D 1 -D m  are arranged in a column direction and substantially parallel to each other. 
         [0033]    Each pixel of the display device includes a switching element Q connected to the display signal lines G 1 -G n  and D 1 -D m , and capacitors C LC  and C ST  that are connected to the switching element Q. Capacitor C LC  is, for example, a liquid crystal (LC) capacitor formed between the lower and upper panels  100  and  200 . The storage capacitor C ST  may be omitted. 
         [0034]    The switching element Q is implemented with, for example, a thin film transistor and disposed on the lower panel  100 . The switching element Q 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 1 -D m , and an output terminal connected to the LC capacitor C LC  and the storage capacitor C ST . 
         [0035]    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 disposed on the lower panel  100 . 
         [0036]    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) disposed on the lower panel  100 . An insulator (not shown) is disposed between the separate signal line and the pixel electrode  190 , and the separate signal line is supplied with a predetermined voltage such as the common voltage Vcom. It is noted that the storage capacitor C ST  may include in another embodiment the pixel electrode  190  and an adjacent gate line (or a previous gate line), in which an insulator is disposed between the adjacent gate line and the pixel electrode  190 . 
         [0037]    For color display, each pixel uniquely represents one of three primary colors (i.e., spatial division) or each pixel represents three primary colors in turn (i.e., time division) such that spatial or temporal sum of the three primary colors are recognized as a desired color.  FIG. 3  shows an example of the spatial division that each pixel is provided with a color filter  230 , one of red, green and blue color filters, in an area of the upper panel  200  facing the pixel electrode  190 . Alternatively, the color filter  230  is provided on or under the pixel electrode  190  on the lower panel  100 . 
         [0038]    Referring again to  FIG. 2 , the lighting unit  900  includes a lamp unit  910  having the lamps  341  shown in  FIG. 1  and an inverter  920  connected to the lamp unit  910 . The inverter  920  turns on and off the lamp unit  910  and controls the timing of on-time and off-time of the lamp unit to adjust luminance of a display screen. The inverter  920  may be mounted on a stand-alone inverter PCB (not shown) or mounted on the gate PCB  550  or the data PCB  450 . A detailed configuration of the inverter  920  will be described. 
         [0039]    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 . 
         [0040]    The gray voltage generator  800  is disposed on the data PCB  450 . The gray voltage generator  800  generates two sets of gray voltages related to the transmittance of the pixels. The gray voltages in one set have a positive polarity with respect to the common voltage Vcom, and those in the other set have a negative polarity with respect to the common voltage Vcom. 
         [0041]    The gate driver  400  includes integrated circuit (IC) chips mounted on the respective gate TCPs  510 . 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 an external device to generate gate signals for application to the gate lines G 1 -G n . The data driver  500  includes IC chips mounted on the respective data TCPs  410 . 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 . 
         [0042]    For example, in another embodiment the IC chips of the gate driver  400  and/or the data driver  500  are mounted on the lower panel  100 . In further another embodiment, one or both of the drivers  400  and  500  are incorporated along with other elements into the lower panel  100 . The gate PCB  550  and/or the gate TCPs  510  may be omitted in such embodiments. 
         [0043]    The signal controller  600  controlling the drivers  400  and  500 , etc. is disposed on the data PCB  450  or the gate PCB  550 . 
         [0044]    Now, the overall operation of the image display device will be described in detail. Referring to  FIG. 2 , 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 graphics 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 and the input image signals R, G and B, 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 . 
         [0045]    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 duration 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. 
         [0046]    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 analog 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 . 
         [0047]    Responsive to the gate control signals CONT 1  from the signals controller  600 , the gate driver  400  applies the gate-on voltage Von to selected one(s) of the gate lines G 1 -G n , thereby turning on the switching elements Q connected thereto. 
         [0048]    The data driver  500  applies the data voltages to the corresponding data lines D 1 -D m  for an on-time of the switching elements Q (which is called “one horizontal period” or “1H” and equals to one period 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. 
         [0049]    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. 
         [0050]    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”). 
         [0051]    The inverter  920  drives the lamp unit  910  based on a luminance control signal Vdim, the horizontal synchronization signal Hsync, and an instruction signal EN for turning on and off the lamp unit  910 . 
         [0052]    Referring to  FIG. 4 , the inverter  920  according to an embodiment of the present invention includes a transformer (TRANS)  921 , a switch circuit (SW)  922 , a controller (CTN)  930 , an oscillator (OSC)  940 , and a phase difference detecting circuit  950 , which are connected in series from the lamp unit  910 . 
         [0053]    The phase difference detecting circuit  950  includes a phase comparator  951 , a low pass filter (LPF)  952 , a proportional integrator  953 , a reset unit  954 , and a frequency divider  955 . The phase comparator  951  receives the horizontal synchronization signal Hsync and an output from the frequency divider  955  and outputs logic ‘0’ when the inputs have different logic values and outputs logic ‘1’ the inputs have a same logic value. In this embodiment, the phase comparator  951  is implemented with an XNOR gate. The XNOR gate can be substituted with an XOR gate. 
         [0054]    The low pass filter  952  includes two resistors RI and a capacitor C 1  connected in series between the phase comparator  951  and the ground, and passes low frequency components of an input signal by filtering out high frequency components of the input signal. 
         [0055]    The proportional integrator  953  includes an operational amplifier OP having a negative feedback through an integration capacitor C 2  and a resistor R 5  connected in series and receiving an output of the low pass filter  952  at its inverting terminal (−). The operational amplifier OP has a non-inverting terminal (+) connected to a voltage divider including a pair of resistors R 3  and R 4  connected in series between a supply voltage VDDA and the ground. The operational amplifier OP is biased with the supply voltage VDDA and the ground. The proportional integrator  953  outputs a voltage having a magnitude proportional to a temporal integration of the output of the low pass filter  952 . 
         [0056]    The reset unit  954  includes a switching element Q 1  connected with the proportional integrator  953  and a differentiation circuit including a resistor R 6  and a capacitor C 3  connected in series between a control terminal of the switching element Q 1  and an input terminal receiving the instruction signal EN. The reset unit  954  initiates the proportional integrator  953  by discharging the charges stored in the integration capacitor C 2 . Although the switching element Q 1  is implemented with an NPN bipolar transistor in this embodiment, a PNP bipolar transistor or a MOS transistor is also used as the switching element Q 1 . It is apparent to those skill in the art that some design modifications such as inversion of the value of the instruction signal EN are required when using the PNP transistor or a P-channel MOS transistor. 
         [0057]    The frequency divider  955  divides the frequency of the output signal of the controller  930  and outputs the frequency-divided signal to the phase comparator  951 . For example, the frequency divider  955  employs a T-flipflop that makes the frequency of a signal inputted into a clock terminal become half. The frequency divider may be omitted when the frequency is maintained same. 
         [0058]    Now, the operation of the inverter is described in detail with reference to  FIGS. 4 and 5 .  FIG. 5  shows waveforms of output voltages of the parts in  FIG. 4  and a lamp current provided to the lamp unit. 
         [0059]    When the dimming control signal Vdim and the instruction signal EN are received, the oscillator  940  generates a reference signal OSC having a triangular waveform or a saw-toothed waveform for pulse width modulation (PWM). The controller  930  pulse-width-modulates the reference signal OSC by using a predetermined reference voltage and supplies a PWM signal to the switch circuit  922 . An exemplary frequency of the reference signal OSC is twice the frequency of the horizontal synchronization signal Hsync. 
         [0060]    The switch circuit  922  generates a signal SW having on and off levels by switching the DC supply voltage according to the PWM signal as shown in  FIG. 5 . The transformer  921  generates a sinusoidal signal based on the on/off signal SW and transforms the sinusoidal signal to have a high voltage. The sinusoidal signal generated from the transformer  921  is provided to the lamp unit  910  as the lamp current LDS which turns on the lamps of the lamp unit  910 . As shown in  FIG. 5 , the sinusoidal signal output from the transformer  921  has amplitudes ‘a’ and ‘b’ in positive and negative polarities, respectively, which have a substantially same value. 
         [0061]    The differentiator C 3  and R 6  of the reset unit  954  flows a temporary current upon the input of the instruction signal EN to turn on the switching element Q 1  for a few microseconds. Then, the charge stored in the integration capacitor C 2  of the proportional integrator  953  is discharged and the proportional integrator  953  is initiated. 
         [0062]    The PWM signal of the controller  930  is input to the frequency divider  955 , where the frequency of the PWM signal is divided. The frequency-divided signal is then input to the phase comparator  951 . 
         [0063]    The phase comparator  951  outputs logic ‘1’ when the value of the horizontal synchronization signal Hsync is equal to the output signal of the frequency divider  955 . The phase comparator  951  also outputs logic ‘0’ when the input signals have different values. Therefore, the output of the phase comparator  951  has a longer duration of logic ‘1’ as the phases of the two input signals coincide, and, on the contrary, it has a longer duration of logic ‘0’ as the phases of the two signals are in discord. As a result, the output of the phase comparator  951  indicates the identity and/or the difference between the phases of the two input signals as function of time. 
         [0064]    The output signal of the phase comparator  951  passes through the low pass filter  952  where the high frequency components of the signal are removed, and is converted into an analog voltage, which is charged into the integration capacitor C 2  of the proportional integrator  953 . Since the output voltage of the proportional integrator  953  is proportional to a temporal integration of the output of the phase comparator  951 , it indicates the degree of the phase difference between the two input signals of the phase comparator  951 . Since appropriate resistance ratio of the voltage divider R 3  and R 4  enables to integrate the difference from a desired value, the output voltage of the proportional integrator  953  indicates the difference between the phase difference of the two input signals and the desired value. 
         [0065]    The oscillator  940  changes an oscillating frequency of the reference signal OSC based on the output voltage of the proportional integrator  953 . That is, the oscillator  940  increases a low frequency of the reference signal OSC, while it reduces a high frequency of the reference signal OSC. The controller  930  pulse-width-modulates and outputs the PWM signal having the changed frequency, and the output signal of the controller  930  is double frequency-divided and is returned to the phase comparator  951 . 
         [0066]    The horizontal synchronization signal Hsync and the output signal of the frequency divider  955  becomes synchronized by performing the above operation through the feedback loop. In other words, the phases of the horizontal synchronization signal Hsync and the output signal of the frequency divider  955  become coincident. As a result, the frequency of the reference signal OSC of the oscillator  940  becomes twice the frequency of the horizontal synchronization signal Hsync in case of employing the divider  955 . 
         [0067]    Referring to  FIG. 5 , the reference signal OSC of the oscillator  940  has a frequency twice that of the horizontal synchronization signal Hsync, and thus the lamp current LDS to be provided to the lamp unit  910  has a symmetrical waveform. Accordingly, the lifetime reduction or unstable ignition of the lamp unit  910  due to asymmetrical current therein can be prevented. 
         [0068]    Meanwhile, the luminance of an LCD screen can be controlled by adjusting the ratio of the on-time and the off-time of the lamp unit  910  based on the dimming control signal Vdim, which is inputted from a separate input device adjustable by a user or from the signal controller  600 . The controller  930  turns on or off the lamp unit  910  in response to the instruction signal EN. The controller  930  receives a voltage having a magnitude in proportion to the current in the lamp unit  910  and performs feedback control for the lamp unit  910 . 
         [0069]    According to the present invention, the life time of the lamps is elongated and stable ignition of the lamps is obtained since the positive portions and the negative portions of current waves in the lamps are substantially equal. 
         [0070]    Having described the exemplary embodiments of the image display device according to the present invention, modifications and variations can be readily made by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the present invention can be practiced in a manner other than as specifically described herein.

Technology Category: 3