Patent Publication Number: US-7916119-B2

Title: Inverter for setting initial driving frequency for a liquid crystal display and driving method thereof

Description:
This application claims the benefit of Korean Patent Application No. 10-2006-0111602, filed on Nov. 13, 2006, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to liquid crystal displays and more particularly to an inverter for use in a liquid crystal display. 
     2. Discussion of the Related Art 
     A liquid crystal display device (LCD) is a display device in which a liquid crystal layer with an anisotropic dielectric constant is formed between upper and lower substrates, transparent insulation substrates. By controlling an electric field applied to the liquid crystal layer, the molecular arrangement of the liquid crystal layer material is varied to control an amount of light transmitted through the liquid crystal layer and the upper substrate display surface to display a desired image. 
     The LCD includes a liquid crystal module (LCM), a driving circuit unit that drives the LCM, and an outer case that covers and protects the LCM. 
     The LCM includes a liquid crystal panel in which liquid crystal cells are arranged in a matrix form between the two transparent insulation substrates, a backlight assembly that provide light to the liquid crystal panel, and a cover that protects the liquid crystal panel and the backlight assembly. 
     The backlight assembly is provided to emit light and uses cold cathode fluorescent lamps (CCFLs), or exterior electrode fluorescent lamps (EEFLs) or the like as a light source. 
     Backlight assembly that use CCFLs or the EEFLs typically use an inverter that converting DC power into AC power for driving the lamps. 
     The inverter includes a transformer that supplies AC power to an output terminal, and a balancer capacitor that is connected between a secondary side of the transformer the lamps and that limits and balances current supplied to each lamp and that matches the impedance between the output terminal of the inverter to the impedance of lamps. 
     However, when the backlight assembly is driven by the inverter of the related art, a partial dimness appears during an initial driving due to an unbalance between an impedance component by equivalent capacitors of the lamps and that by the balancer capacitors, resulting in degradation of display quality. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an inverter for a liquid crystal display and a driving method thereof that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An advantage of the present invention is to provide an inverter for a liquid crystal display (LCD) capable of minimizing an impedance deviation between an output terminal and a lamp, a load of the output terminal, by properly lowering a driving frequency and applying the lowered frequency during an initial driving time, and a method for driving the same. 
     Another advantage of the present invention is to provide an inverter for an LCD capable of reducing a partial dimness phenomenon caused by an impedance deviation and obtaining display quality, and a method for driving the same. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. These and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an inverter for an LCD includes: a DC/AC converting unit that converts DC power supplied from a voltage source into AC power; a transformer that converts the AC power supplied from the DC/AC converting unit into a high voltage AC; and a frequency controller that determines the frequency of an output of the DC/AC converting unit such the output of the DC/AC converting unit has an initial driving frequency during an initial driving time that beings upon initiating driving of the liquid crystal panel and such that that the output of the DC/AC converting unit has a normal driving frequency larger than the initial driving frequency after the initial driving time. 
     In another aspect, a method for driving an inverter for an LCD includes: converting DC power supplied to an input terminal into AC power; converting the AC power into a high voltage AC and outputting the same; controlling converting DC power supplied to the input terminal into AC power such that the AC power has an initial driving frequency during an initial driving; and controlling converting DC power supplied to the input terminal into AC power such that the AC power has a normal driving frequency larger than the initial driving frequency during a normal driving that follows the initial driving. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  is a view schematically illustrating the construction of a liquid crystal display (LCD) employing an inverter according to an embodiment of the present invention. 
         FIG. 2  is a view showing details of the construction of the inverter in  FIG. 1 . 
         FIGS. 3A and 3B  are views showing details of a frequency controller shown in  FIG. 2 . 
         FIGS. 4A and 4B  are views showing waveforms of a high voltage AC supplied to a lamp by the inverter in  FIG. 1 . 
         FIG. 5  is a flow chart illustrating a method for driving an inverter according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to an embodiment of the present invention, an example of which is illustrated in the accompanying drawings. 
     As illustrated in  FIGS. 1 and 2 , an inverter for a liquid crystal display (LCD) according to an embodiment of the present invention includes: a DC/AC converting unit  220  that converts DC power supplied from a voltage source into AC power; a transformer  230  that converts the AC power supplied from the DC/AC converting unit  220  into a high voltage AC; and a frequency controller  240  that controls an output of the DC/AC converting unit  220  have an initial driving frequency when a lamp for providing a light source to a liquid crystal panel  100  is initially driven, and that further controls the output of the DC/AC converting unit  220  to have a normal driving frequency larger than the initial driving frequency when the lamp is normally driven following the initial driving. 
     The frequency controller  240  may be combined with or connected to an input terminal of the DC/AC converting unit  220  and may determine the frequency of an AC voltage output from the DC/AC converting unit  220 . 
     The frequency controller may include a resistor and a capacitor for determining an initial driving time. 
     As shown in  FIG. 3B , the frequency controller  240  may include: a first resistor having one end connected to a power source voltage Vcc; a first capacitor connected between the other end of the first resistor and a ground potential source; a transistor including a base connected to a node between the first resistor and the first capacitor, and an emitter connected to a ground potential; a second resistor connected between a collector of the transistor and a first input terminal of the DC/AC converting unit; and a third resistor having one end connected to the first input terminal of the DC/AC converting unit  220  and the other end connected to the ground potential so as to be connected in parallel with the second resistor. 
     The inverter may further include: a second capacitor connected between a second input terminal of the DC/AC converting unit  220  and the ground potential, and having a parallel relationship with the third resistor. 
     Upon application of the initial driving voltage, the transistor of the frequency controller  240  may be maintained in a turned-off state for a time period determined by the first resistor and the first capacitor. While transistor is in the turned-off state, the initial driving frequency is then determined by the third resistor R 2  and the second capacitor C 2  which are connected in parallel during the initial driving time. 
     After the initial driving time, transistor is turned for normal driving and the normal driving frequency may be determined by the parallel combination of the second resistor, the third resistor, and the second capacitor formed by the turning on of the transistor. 
     The transistor Q 1  may be a bipolar junction transistor (BJT). 
     The inverter  210  may further include a feedback circuit unit  250  that detects a feedback signal from the lamp connected to a secondary side of the transformer and supplies the feedback signal to the DC/AC converting unit. 
     The feedback signal may be a tube current supplied to the lamp may alternatively be a signal representing a voltage applied across the lamp. 
     A method for driving an inverter for an LCD includes: converting DC power supplied to an input terminal into AC power; converting the AC power into a high voltage AC and outputting the same; controlling the conversion of DC power to AC power such that the AC has an initial driving frequency during an initial driving time; and controlling the conversion of DC power to AC power such that the AC has a normal driving frequency larger than the initial driving frequency during a normal driving time that follows the initial driving time. 
     The controlling such that the AC has an initial driving frequency during the initial driving and the controlling such that the AC has a normal driving frequency larger than the initial driving frequency during the normal driving that follows the initial driving may be performed by a frequency controller that includes a first resistor having one end connected with a power source voltage, a first capacitor formed between the other end of the first resistor and a ground potential source, a transistor including a base connected with a node between the first resistor and the first capacitor, and an emitter connected with a ground potential source, a second resistor connected between a collector of the transistor and a first input terminal of the DC/AC converting unit, and a third resistor having one end connected with the first input terminal of the DC/AC converting unit and the other end connected with a ground potential source, so as to be connected in parallel with the second resistor. 
     In controlling such that the AC has an initial driving frequency during the initial driving, the transistor may be maintained in a turned-off state during the initial driving time having a duration determined by the first resistor and the first capacitor, and the initial driving frequency may be determined by the third resistor and the second capacitor during the initial driving time. 
     In controlling such that the AC has a normal driving frequency larger than the initial driving frequency during the normal driving that follows the initial driving, the transistor may be turned on during the normal driving following the initial driving time, and the normal driving frequency may be determined by the second resistor, the third resistor, and the second capacitor that are connected in parallel. 
     The method for driving an inverter for an LCD may further include: detecting a feedback signal from a lamp that provides a light source; and controlling the AC according to the feedback signal. 
     An example embodiment of the present invention will now be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a view schematically showing the construction of a liquid crystal display (LCD) employing an inverter according to an embodiment of the current invention. 
     With reference to  FIG. 1 , the LCD according to the illustrated embodiment of the present invention includes a liquid crystal panel  100 , a gate driver  110 , a data driver  120 , a timing controller  130 , a gamma voltage generator  140 , a backlight assembly  200 , and an inverter  210 . 
     The liquid crystal panel  100  includes a plurality of pixels (P) defined by gate lines GL 1 , GL 2 , . . . , GLn and data lines DL 1 , DL 2 , . . . , DLm that cross each other. Thin film transistors (TFTs), each having a gate electrode, an active layer, a source electrode, and a drain electrode, are disposed at respective crossings of the gate lines GL 1 , GL 2 , . . . , GLn and the data lines DL 1 , DL 2 , . . . , DLm. 
     A liquid crystal material is filled in each pixel (P) to form liquid crystal cells represented in  FIG. 1  as equivalent circuits Clc, and a storage capacitor Cst is formed to uniformly maintain a pixel voltage applied to the liquid crystal cells Clc. 
     In the liquid crystal panel  100 , an image is displayed by pixels (P) according to scan signals supplied through the gate lines GL 1 , GL 2 , . . . , GLn and analog pixel signals supplied through the data lines DL 1 , DL 2 , . . . , DLm. Here, the scan signal includes a pulse in which a gate high voltage supplied during a horizontal period for each gate line and a gate low voltage supplied to the gate line during the other horizontal periods of each image frame period. 
     When the gate high voltage is supplied to the gate lines GL 1 , GL 2 , . . . , GLn, the TFTs formed at respective pixels (P) supply the analog pixel signals provided from the data lines DL 1 , DL 2 , . . . , DLm to the liquid crystal cells Clc. When the gate low voltage is supplied to the gate lines GL 1 , GL 2 , . . . , GLn, the TFTs are turned off, so the analog pixel signals filled in the liquid crystal cells Clc can be maintained for a certain time period. 
     The gate driver  110  sequentially supplies scan signals to the gate lines GL 1 , GL 2 , . . . , GLn according to the gate control signals supplied from the timing controller  130 . 
     In response to control signals supplied from the timing controller  130 , the data driver  120  converts red, green, and blue pixel data inputted from the timing controller  130  into analog pixel signals and supplies the analog pixel signals to the data lines DL 1 , DL 2 , . . . , DLm. 
     The analog pixel signals have a gamma voltage selected according to red, green, and blue pixel data (gray level) inputted from the exterior among gamma voltages supplied from the gamma voltage generator  140 . 
     The timing controller  130  receives the red, green, and blue pixel data from a source external to the liquid crystal display device, the re-processes red, green, and blue pixel data, and outputs the re-processed data to the data driver  120 . In addition, the timing controller  130  generates a gate control signal for controlling a driving timing of the gate driver  110  and a data control signal for controlling a driving timing of the data driver  120  by using vertical and horizontal synchronous signals Vsync and Hsync, and a clock CLK. 
     The gate control signal includes a gate start pulse (GSP), a gate shift clock (GSC), a gate output enable (GOE) signal, and the data control signal includes a source start pulse (SSP), a source shift clock (SSC), a source output enable (SOE), a polarity (POL) signal. 
     The gamma voltage generator  140  generates gamma voltages required for digital/analog conversion of the data driver  120  by gray level within a gray scale range and supplies the gamma voltages to the data driver  120 . 
     The backlight assembly  200  includes a plurality of lamps such as the cold cathode fluorescent lamps (CCFLs) or the exterior electrode fluorescent lamps (EEFLs), etc., and provides light toward the liquid crystal panel  100 . 
     The inverter  210  converts DC power inputted from the exterior into AC power having a certain frequency and a voltage level suitable for the lamps of the backlight assembly  200  to drive the lamps. 
     In order to drive the CCFLs or the EEFLs used for the backlight assembly  200 , the inverter  210  should be stably driven at a high frequency of 20 kHz to 200 kHz. According to the properties of the lamps, the inverter  210  supplies a high voltage required for turning on the lamps to the lamps at an initial stage of lighting, and after the lamps are turned on, the inverter  210  serves to maintain a uniform brightness of the lamps by controlling a tube current of the lamps. 
     The advantage of high driving the lamps using a high frequency supply is that the inverter  210  can be made small, the luminous efficacy of the lamps can be increased, and the life span of the lamps can be lengthened. 
       FIG. 2  is a view showing details of the construction of the inverter  210  shown in  FIG. 1 . 
     With reference to  FIG. 2 , the inverter  210  includes a DC/AC converting unit  220 , a transformer  230 , a frequency controller  240 , a feedback circuit unit  250 , etc. 
     The DC/AC converting unit  220  includes two or more switching elements that are alternately switched on or off and a constant current circuit that uniformly maintains the brightness of the lamps after the lamps are turned on, and converts DC power Vin inputted through an input terminal from an external voltage source into AC power and provides the AC power to a primary coil of the transformer  230 . 
     The transformer  230  includes a primary coil connected to the DC/AC converting unit  220  and a secondary coil connected to the lamps via balancer capacitors Cb, and drives the lamps by converting an AC voltage supplied from the DC/AC converting unit  220  into a high voltage AC. 
     The transformer  230  steps up a voltage supplied to the primary coil according to a turn ratio of the primary coil and the secondary coil, and outputs the stepped up voltage from the secondary coil. The stepped up voltage output from the secondary coil is supplied to the lamps via electrodes formed at each of the two ends of the lamps to energize the lamps to emit light. 
     The balancer capacitors Cb are connected between an output terminal of the inverter  210  and each ends of the lamps. The balance capacitors maintain a balance of current to the lamps, and match impedance components of the output terminal of the inverter  210  to the impedances if lamps. 
     The frequency controller  240  is combined with the input terminal of the DC/AC converting unit  220  to control the DC/AC conversion so that upon initial driving, an output of the DC/AC converting unit  220  has an initial driving frequency during an initial driving time, and has a normal driving frequency larger than the initial driving frequency subsequent to the initial driving time. 
     The feedback circuit unit  250  may be used to detect a feedback signal from the lamps connected with the secondary coil of the transformer  230  and to supplies the feedback signal to the DC/AC converting unit  220  to allow the DC/AC converting unit  220  to maintain a uniform brightness for the lighted lamps. The feedback signal may represent the tube current flowing through either end of the lamps or may represent voltage applied across the ends of the lamps. 
       FIGS. 3A and 3B  are views showing details of a frequency controller  240  shown in  FIG. 2 . 
       FIG. 3A  is a view illustrating operation without the frequency controller  240 . As shown, in the absence of the frequency controller  240 , a resistor (R) and a capacitor (C) connected in parallel with a first input terminal (RT) and a second input terminal (CT) of the DC/AC converting unit  220  determine the driving frequency Fop of the DC/AC converting unit  220 . 
     For the circuit illustrated in  FIG. 3A , it may be assumed that the driving frequency Fop of an AC waveform output from the DC/AC converting unit  220 , the resistor (R) and the capacitor (C) of the input terminal satisfy equation (1) shown below: 
     
       
         
           
             
               
                 
                   Fop 
                   = 
                   
                     
                       59 
                       × 
                       
                         10 
                         4 
                       
                     
                     
                       
                         C 
                         CT 
                       
                       × 
                       
                         R 
                         RT 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, the C CT  is a capacitance value in picofarads [pF] of the capacitor (C), R RT  is a resistance value in kilohms [kΩ], and Fop is the driving frequency measured in kilohertz [kHz], and 59×104 is a constant that is determined according to the details of the structure and integration type of the DC/AC converting unit  220 . 
     For example, assuming that the driving frequency Fop of the AC waveform supplied from the DC/AC converting unit  220  satisfies equation (1), and that the resistor (R) is 41 kΩ, and the capacitor (C) is 220 pF, then the driving frequency Fop can be expressed by equation (2) shown below: 
     
       
         
           
             
               
                 
                   Fop 
                   = 
                   
                     
                       
                         59 
                         × 
                         
                           10 
                           4 
                         
                       
                       
                         220 
                         × 
                         41 
                       
                     
                     = 
                     
                       65.4 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       kHz 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     In this case, the DC/AC converting unit  220  continuously receives a fixed driving frequency Fop during the initial driving operation of the lamps and during the normal driving operation of the lamps following the initial driving operation. 
     When only the resistor (R) and the capacitor (C) are simply connected with the first input terminal RT and the second input terminal CT of the DC/AC converting unit  220  as shown in  FIG. 3A , the DC/AC converting unit  220  outputs only an AC waveform of the normal driving frequency Fop. 
     When a single frequency is supplied by the DC/AC converting unit, the impedance deviation between the balancer capacitors Cb and the lamps formed at the rear stage of the DC/AC converting unit  220  cannot be reduced, inevitably causing the partial dimness phenomenon. 
     In addition, the balancer capacitors Cb should obtain a uniform capacitance value in order to compensate for impedance deviations in the equivalent capacitors of the lamps to thus maintain a uniform luminance during changes in the surroundings of the lamps such as temperature (high temperature or low temperature) or humidity changes. 
     In this respect, however, when the capacitance value of the balancer capacitors Cb is lowered, the compensation effect is reduced so that compensating the impedance change by changing the capacitance value of the balancer capacitors Cb limited. 
       FIG. 3B  shows the construction of the frequency controller  240  that may address the problems described above. The frequency controller  240  is connected with the input terminal of the DC/AC converting unit  220  to determine the frequency of the AC waveform output from the DC/AC converting unit  220 . 
     With reference to  FIG. 3A , the frequency controller  240  includes a first resistor R 1  having one end connected with a power source terminal Vcc, a first capacitor C 1  formed between the other end of the first resistor R 1  and a ground terminal, a transistor Q 1  having a base connected to a node between the first resistor R 1  and the first capacitor C 1  and an emitter connected to ground, a second resistor R 2  formed between the first input terminal RT of the DC/AC converting unit  220  and a ground terminal, and a third resistor R 3  formed between a collector of the transistor Q 1  and the second resistor R 2  and connected with the second resistor R 2  in parallel, etc. The transistor Q 1  may be a bipolar junction transistor (BJT) or may be another switching device that controls voltage or current in an output circuit by varying a control current or voltage may be used. 
     The frequency controller  240  further includes a second capacitor C 2  formed between the second input terminal CT of the DC/AC converting unit  220  and a ground terminal, and connected with the elements connected with the first input terminal RT in parallel. 
     During the initial driving operation, when a driving voltage is initially applied to the power source terminal Vcc, the transistor Q 1  is turned off during for an initial driving time (τ) having a duration determined by the first resistor R 1  and the first capacitor C 1 . The initial driving frequency output by the DC/AC converter Fig is determined by the second resistor R 2  and the second capacitor C 2 . 
     During the normal driving operation that follows the initial driving time (τ), the transistor Q 1  is turned on and the driving frequency Fop is determined by a parallel resistance value (R 3 ∥R 2 ) of the third resistor R 3  and the second resistor R 2  and the capacitance value of the second capacitor C 2 . 
     This will be described in detail as follows. 
     During the initial driving operation, after a driving voltage (e.g., 5V) is applied to the power source terminal Vcc, the transistor Q 1  is maintained in the OFF state during a certain time period (τ), namely, during the initial driving time (τ), determined by the time required to charge the first capacitor C 1  through the first resistor R 1  connected to the power source Vcc. Accordingly, the DC/AC converting unit  220  is driven using the initial frequency Fig. The initial frequency Fig is determined by the second resistor R 2  connected with the first input terminal RT of the DC/AC converting unit  220  and the second capacitor C 2  connected with the second input terminal CT of the DC/AC converting unit  220 . 
     After the initial driving time (τ) in which the DC/AC converting unit  220  is driven by the initial frequency Fig, the voltage across the capacitor increases by charging through R 1  sufficiently towards the driving voltage (e.g. 5V) to turn on the transistor Q 1 . Upon turning on the transistor Q 1 , the driving frequency Fop is determined by the parallel resistance value (R 3 ∥R 2 ) of the second resistor R 2  and the third resistor R 3  and the capacitance value of the second capacitor C 2 , and the DC/AC converting unit  220  is driven using the determined driving frequency Fop. 
     For example, assuming that the resistance value of the second resistor R 2  is 44 kΩ, the resistance value of the third resistor R 3  is 600 kΩ, and the capacitance value of the second capacitor C 2  is 220 pF, when equation (1) is applied to the input/output terminal of the DC/AC converting unit  220 , the initial frequency (Fig) during the initial driving operation and the driving frequency Fop during the normal driving operation can be determined by equation (3) and equation (4) respectively as shown below: 
     
       
         
           
             
               
                 
                   Fig 
                   = 
                   
                     
                       
                         59 
                         × 
                         
                           10 
                           4 
                         
                       
                       
                         220 
                         × 
                         44 
                       
                     
                     = 
                     
                       60.9 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       kHz 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
             
               
                 
                   Fop 
                   = 
                   
                     
                       
                         59 
                         × 
                         
                           10 
                           4 
                         
                       
                       
                         220 
                         × 
                         44 
                         ⁢ 
                         
                            
                           600 
                         
                       
                     
                     = 
                     
                       65.4 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       kHz 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     In this manner, by using the circuit including the resistor, the capacitor, and the transistor of the DC/AC converting unit  220  as the frequency controller  240  as shown in  FIG. 3B , when the backlight assembly  200  is initially turned on, the driving frequency is lowered from normal operating value Fop to the initial frequency (Fig), and after a predetermined time period lapses the driving frequency is raised to the normal operating value Fop. 
     Without the frequency controller  240 , the unbalance between the impedance component due to the equivalent capacitance of the lamps and the impedance of the balancer capacitors Cb may cause the partial dimness of the backlight assembly  200  during the initial driving of the backlight assembly  200 . 
     Accordingly, during the initial driving period of operation of the lamps of the backlight assembly  200 , the driving frequency Fop is shifted to the lower initial frequency Fig, for a certain time period (driving time) which results in an increase of the impedance of the balancer capacitors Cb. 
     In other words, in comparison with the case in which the backlight assembly  200  is driven continuously with driving frequency Fop, the supply of an initial frequency Fig to the backlight assembly during the initial period results in the impedance of the balancer capacitors Cb being increased to minimize the impedance deviation due to the equivalent capacitors of the lamps, and accordingly, the partial dimness phenomenon of the backlight assembly  200  can be reduced or eliminated. 
       FIGS. 4A and 4B  are views illustrating waveforms of a high voltage AC supplied to a lamp by the inverter in  FIG. 1 . 
     The frequency controller  240  controls the DC/AC converting unit  220  to output such an AC waveform as shown in  FIG. 4A  or  FIG. 4B . 
     The DC/AC converting unit  220  outputs the AC waveform of the initial frequency Fig during the initial driving time Tig of the initial driving operation, and outputs the AC waveform of the driving frequency Fop higher than the initial frequency Fig, during the normal driving time Top that follows the initial driving time Tig. 
     The inverter  210  that includes the frequency controller  240  and the DC/AC converting unit  220  supplies a high voltage AC waveform to the lamps according to a driving method of the lamps. 
     The driving method of the lamps may be either a continuous mode driving method in which the high voltage AC waveform is continuously supplied as shown in  FIG. 4A , or a burst mode driving method in which the high voltage waveform has certain periods as shown in  FIG. 4B . 
     In the continuous mode driving method, a high voltage AC waveform is continuously supplied to the lamps as shown in  FIG. 4A  and the lamps are turned on continuously. On the other hand, in the burst mode driving method, the high voltage AC waveform has a duty cycle including an ON state (Ton) in which the high voltage is supplied and an OFF state (Toff) during which no high voltage is supplied to the lamps as shown in  FIG. 4B . In the burst driving modes, the lamps are alternately turned on and off at during the corresponding Ton and Toff periods. 
       FIG. 5  is a flow chart illustrating a method for driving the inverter according to an embodiment of the present invention. 
     First, in step S 100 , the DC/AC converting unit  220  converts DC power supplied to its input terminal into AC power. 
     Next, in step S 120 , the transformer  230  converts the AC waveform into AC power of high voltage. 
     In step S 120 , the frequency controller  240  controls the converted high voltage AC power such that the AC waveform from the DC/AC converting unit  220  has the initial frequency during the initial driving time of the initial driving operation. 
     Thereafter, in step S 130 , the frequency controller  240  controls the converted high voltage AC power such that the AC waveform has the driving frequency larger than the initial frequency after the initial driving time. 
     The frequency controller  240 , which performs the steps S 120  and S 130 , may include the first resistor R 1  having one end combined with the power source terminal Vcc, the first capacitor C 1  formed between the other end of the first resistor R 1  and the ground terminal, the transistor Q 1  having the base connected with the node between the first resistor R 1  and the first capacitor and the grounded emitter, the second resistor R 2  formed between the first input terminal RT of the DC/AC converting unit  220  and the ground terminal, and the third resistor R 3  formed between the collector of the transistor Q 1  and the second resistor R 2  and connected in parallel with the second resistor R 2 . 
     During the initial driving operation in step S 120 , after the frequency controller  240  applies the driving voltage to the power source terminal Vcc, the frequency controller  240  maintains the transistor Q 1  in the turned-off state during the initial driving time determined by the first resistor R 1  and the first capacitor C 1 . 
     During the initial driving time, the initial frequency is determined by the second resistor R 2  and the second capacitor C 2 . 
     In step S 130 , during the normal driving operation that follows the initial driving time, the transistor Q 1  is turned on and the driving frequency is determined by the third resistor R 3 , the second resistor R 2 , and the second capacitor C 2  that are connected in parallel. 
     In steps S 140  and S 150 , the feedback circuit unit  250  detects the feedback signal of the lamps connected with its output terminal and supplies the feedback signal to the DC/AC converting unit  220 . The DC/AC converting unit  220  maintains the tube current of the lamps or the voltage applied across the ends of the lamps at a certain uniform value to thus obtain uniform brightness of the lamps. 
     As described above, the inverter for the LCD and its driving method according to embodiments of the present invention have such advantages that the impedance deviation between the output terminal and the lamps, the load of the output terminal, can be minimized by applying suitably lowered driving frequency during the initial driving operation. 
     Accordingly, the partial dimness phenomenon that may be caused by the impedance deviation between the output terminal and the load can be reduced or eliminated, and thus an improvement in display quality can be accomplished. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to the other types of apparatuses. It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.