Abstract:
An exemplary backlight control circuit includes a transformer, a control circuit, a lamp. The control circuit and the transformer form an inverter circuit to providing an alternating current (AC) voltage for driving the lamp. When the backlight control circuit works in a startup mode, the backlight control circuit defines a first current path including the lamp and the first current path forms a first resonant circuit. When the backlight control circuit works in an operation mode, the backlight control circuit defines a second current path including the lamp and the second current path forms a second resonant circuit. The first and second resonant circuits have different resonant frequencies from each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to, and claims the benefit of, a foreign priority application filed in China as Ser. No. 200710077000.1 on Sep. 14, 2007. The related application is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present disclosure relates to a backlight control circuit which can for example be employed in a liquid crystal display (LCD), and more particularly to a backlight control circuit defining two different resonant circuits and a method for driving the backlight control circuit. 
     GENERAL BACKGROUND 
     LCDs have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), and video cameras, due to advantages such as portability, low power consumption, and low radiation. LCDs are poised to completely replace cathode ray tube monitors and televisions in some markets. A typical LCD includes an LCD panel, a backlight for illuminating the LCD panel, and a backlight control circuit for controlling the backlight. When a cold cathode fluorescent lamp (CCFL) is employed as the backlight, a high frequency alternating current (AC) voltage is generated by the backlight control circuit for driving the CCFL. 
     Referring to  FIG. 4 , one such backlight control circuit  100  includes a control circuit  110 , a transformer  120 , a lamp  130 , and a capacitor  140 . 
     The transformer  120  includes a primary winding  122  and a secondary winding  124 . Two terminals of the primary winding  122  are electrically coupled to the control circuit  110 . One terminal of the secondary winding  124  is grounded via the lamp  130 , and the other terminal of the secondary winding  124  is grounded via the capacitor  140 . The lamp  130  is a CCFL. 
     The control circuit  110  and the transformer  120  constitute an inverter circuit configured for providing an AC voltage to driving the lamp  130 . Normally, because the AC voltage outputted from the secondary winding  124  is not a sine wave, the capacitor  140  and the secondary winding  124  need to form an resistor inductor capacitor (RLC) resonant circuit in order to provide an AC voltage with a desired sine wave for driving the lamp  130 . 
     The RLC resonant circuit includes a fixed resonant frequency f 0 . When the resonant frequency f 0  is equal to or close to a driving frequency of the AC voltage, an efficiency of the backlight control circuit  100  is high and energy waste is low. Thus an important quality factor of the backlight control circuit  100  is high. 
     The AC voltage includes a normal operation frequency f 1 , and a startup frequency f 2  for lighting up the lamp  130  when the backlight control circuit  100  starts to work. Because the startup frequency f 2  is higher than the normal operation frequency f 1 , the fixed resonant frequency f 0  of the RLC resonant circuit can only correspond to one of the normal operation frequency f 1  and the startup frequency f 2 . If the fixed resonant frequency f 0  corresponds to the startup frequency f 1 , the fixed resonant frequency f 0  is higher than the normal operation frequency f 1 . Thus the efficiency of the backlight control circuit  100  is low and energy waste is high. If the fixed resonant frequency f 0  corresponds to the normal operation frequency f 1 , the fixed resonant frequency f 0  is lower than the startup frequency f 1 , Thus each time the lamp  130  is lighted up, flicker is generated in the lamp  130 , and the working lifetime of the lamp  130  is reduced by a decrement. 
     It is desired to provide a new backlight control circuit which can overcome the above-described deficiencies 
     SUMMARY 
     A backlight control circuit includes a transformer, a control circuit, a lamp. The control circuit and the transformer form an inverter circuit to providing an AC voltage for driving the lamp. When the backlight control circuit works in a startup mode, the backlight control circuit defines a first current path including the lamp and the first current path forms a first resonant circuit. When the backlight control circuit works in an operation mode, the backlight control circuit defines a second current path including the lamp and the second current path forms a second resonant circuit. The first and second resonant circuits have different resonant frequencies from each other. 
     Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a diagram of a first embodiment of a backlight control circuit. 
         FIG. 2  is a diagram of a second embodiment of a backlight control circuit. 
         FIG. 3  is a diagram of a third embodiment of a backlight control circuit. 
         FIG. 4  is a diagram of a typical backlight control circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the drawings to describe various embodiments in detail. 
     Referring to  FIG. 1 , a first embodiment of a backlight control circuit  200  includes a control circuit  210 , a transformer  220 , a lamp  230 , a first capacitor  240 , a reactance element  250 , and a switching element  260 . 
     The lamp  230  is typically a cold cathode fluorescent lamp. The control circuit  210  and the transformer  220  constitute an inverter circuit to providing an AC voltage for driving the lamp  230 . The transformer  220  includes a primary winding  222  and a secondary winding  224 . Two terminals of the primary winding  222  are electrically coupled to the control circuit  210 . A first terminal of the secondary winding  224  is grounded via the lamp  230 . A second terminal of the secondary winding  224  is grounded via the first capacitor  240 . 
     The switching element  260  is a metal-oxide-semiconductor field-effect transistor (MOSFET)  261 , which includes a gate electrode “G”, a source electrode “S”, and a drain electrode “D”. The reactance element  250  includes a second capacitor  251 . The gate electrode “G” of the MOSFET  261  is connected to the control circuit  210 . The drain electrode “D” of the MOSFET  261  is connected to the second terminal of the secondary winding  224 . The source electrode “S” of the MOSFET  261  is grounded via the second capacitor  251 . 
     When the backlight control circuit  200  works in a startup mode for initially lighting up the lamp  230 , the inverter circuit formed by the control circuit  210  and the transformer  220  outputs a startup AC voltage with a first frequency f 1  to light up the lamp  230 . The control circuit  210  outputs a low level voltage to the gate electrode “G” of the MOSFET  261  in order to switch off the MOSFET  261 . Thus the lamp  230 , the secondary winding  224 , and the first capacitor  240  form a first resonant circuit which has a resonant frequency f 01  corresponding to or equal to the first frequency f 1 . 
     When the backlight control circuit  200  works in an operation mode for driving the lamp  230  to radiate light according to desired normal operation, the inverter circuit formed by the control circuit  210  and the transformer  220  outputs an operation AC voltage with a second frequency f 2  to drive the lamp  230 . The control circuit  210  outputs a high level voltage to the gate electrode “G” of the MOSFET  261  in order to switch on the MOSFET  261 . Thus the lamp  230 , the secondary winding  224 , the first capacitor  240 , the on-state MOSFET  261 , and the second capacitor  251  form a second resonant circuit which has a second resonant frequency f 02  corresponding to or equal to the second frequency f 2 . 
     Each of the first resonant frequency f 01  and the second resonant frequency f 02  can be calculated according to the following formula (1): 
                   f   =       1     2   ⁢   π   ⁢     LC         .             (   1   )               
In formula (1), “f” denotes a resonant frequency of a resonant circuit. “L” denotes a sum of inductances of the resonant circuit. “C” denotes a sum of capacitances of the resonant circuit. Because the second resonant circuit further includes the second capacitor  251  connected in parallel with the first capacitor  240 , the sum of capacitances of the second resonant circuit is larger than that of the first resonant circuit. Thus the second resonant frequency f 02  is less than the first resonant frequency f 01 . The second resonant frequency f 02  can be set to be the second frequency f 2  of the operation AC voltage by setting an appropriate capacitance of the second capacitor  251 .
 
     Because the backlight control circuit  200  respectively defines the first resonant circuit in the startup mode and the second resonant circuit in the operation mode, the first resonant frequency f 01  of the first resonant circuit corresponds to the first frequency f 1  of the startup AC voltage, and the second resonant frequency f 02  of the second resonant circuit corresponds to the second frequency f 2  of the operation AC voltage. Accordingly, any flicker of the lamp  230  that might otherwise occur is eliminated or depressed, and the efficiency of the backlight control circuit  200  is high. 
     Referring to  FIG. 2 , a backlight control circuit  300  of a second embodiment is shown. The backlight control circuit  300  may be substantially similar to the backlight control circuit  200 , except that the backlight control circuit  300  includes a first MOSFET  361 , a second MOSFET  362 , a first capacitor  351 , and a reactance element such as a second capacitor  352 . Gate electrodes “G” of the first and second MOSFETs  361 ,  362  are connected to the control circuit  210 . The second terminal of the secondary winding  224  is connected to drain electrodes “D” of the first and second MOSFETs  361 ,  362 . A source electrode “S” of the first MOSFET  361  is connected to ground via the first capacitor  351 . A source electrode “S” of the second MOSFET  362  is connected to ground via the second capacitor  352 . A capacitance of the first capacitor  351  is less than that of the second capacitor  352 . 
     When the backlight control circuit  300  works in a startup mode for initially lighting up the lamp  230 , the inverter circuit formed by the control circuit  210  and the transformer  220  outputs a startup AC voltage with the first frequency f 1  to light up the lamp  230 . The control circuit  210  switches on the first MOSFET  361  and switches off the second MOSFET  362 . Thus the lamp  230 , the secondary winding  224 , the on-state first MOSFET  361 , and the first capacitor  351  form a first resonant circuit, which has a resonant frequency f 01  corresponding to or equal to the first frequency f 1 . 
     When the backlight control circuit  300  works in an operation mode for driving the lamp  230  to radiate light according to desired normal operation, the inverter circuit formed by the control circuit  210  and the transformer  220  outputs an operation AC voltage with the second frequency f 2  to drive the lamp  230 . The control circuit  210  switches off the first MOSFET  361  and switches on the second MOSFET  362 . Thus the lamp  230 , the secondary winding  224 , the on-state second MOSFET  362 , and the second capacitor  352  form a second resonant circuit, which has a second resonant frequency f 02  corresponding to or equal to the second frequency f 2 . 
     Referring to  FIG. 3 , a backlight control circuit  400  of a third embodiment is shown. The backlight control circuit  400  may be substantially similar to the backlight control circuit  200  of  FIG. 1 , except that the backlight control circuit  400  includes a first MOSFET  461 , a second MOSFET  462 , a capacitor  440 , and a reactance element such as an inductor  451 . Gate electrodes “G” of the first and second MOSFETs  461 ,  462  are connected to the control circuit  210 . The second terminal of the secondary winding  224  is connected to drain electrodes “D” of the first and second MOSFETs  461 ,  462 . A source electrode “S” of the first MOSFET  461  is connected to ground via the capacitor  440 . A source electrode “S” of the second MOSFET  462  is connected to ground via the inductor  451  and the capacitor  440  in series. 
     When the backlight control circuit  400  works in a startup mode for initially lighting up the lamp  230 , the inverter circuit formed by the control circuit  210  and the transformer  220  outputs a startup AC voltage with the first frequency f 1  to light up the lamp  230 . The control circuit  210  switches on the first MOSFET  461  and switches off the second MOSFET  462 . Thus the lamp  230 , the secondary winding  224 , the on-state first MOSFET  461 , and the first capacitor  440  form a first resonant circuit, which has a resonant frequency f 01  corresponding to or equal to the first frequency f 1 . 
     When the backlight control circuit  400  works in an operation mode for driving the lamp  230  to radiate light according to desired normal operation, the inverter circuit formed by the control circuit  210  and the transformer  220  outputs an operation AC voltage with the second frequency f 2  to drive the lamp  230 . The control circuit  210  switches off the first MOSFET  461  and switches on the second MOSFET  462 . Thus the lamp  230 , the secondary winding  224 , the on-state second MOSFET  462 , the inductor  451 , and the capacitor  440  form a second resonant circuit, which has a second resonant frequency f 02  corresponding to or equal to the second frequency f 2 . 
     In an alternative embodiment, the inductor  451  can be replaced by a capacitor. In other alternative embodiments, the capacitors  251 ,  351  can be replaced by inductors. 
     It is to be further understood that even though numerous characteristics and advantages of the present disclosure have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail, especially in matters of arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.