Patent Publication Number: US-7903081-B2

Title: Backlight driver, display apparatus having the same and method of driving backlight

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority to Korean Patent Application No. 2006-82382, filed on Aug. 29, 2006, the contents of which are herein incorporated by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure relates to a backlight driver, a display apparatus having the same and a method of driving the backlight. More particularly, the present disclosure relates to a backlight driver capable of improving the response speed of the backlight and stabilizing the brightness, a display apparatus having the backlight driver and a method of driving the backlight. 
     2. Discussion of Related Art 
     In general, a liquid crystal display (LCD) includes an LCD panel that is illuminated by a backlight. A cold cathode fluorescent lamp (CCFL) is commonly employed as the LCD backlight. 
     Many applications require dimming capabilities, for example, to improve the contrast ratio of the display device and increase battery life. A dimming method has been utilized that controls the brightness of the backlight to increase a contrast ratio of the LCD or to decrease electric power consumption by the backlight. The dimming method can be classified into a pulse width modulation (PWM) control method and a current control method. 
     In dimming by a PWM control method, the CCFL is repeatedly turned on and off in accordance with the duty ratio of the PWM signal, thereby causing the CCFL brightness to vary. That is, the PWM control method involves switching the CCFL on and off at a regular interval to adjust the brightness of the backlight by changing the duty ratio. 
     Tube current is the current flowing through the CCFL tube after the tube has been lit. The CCFL brightness is directly proportional to the tube current. The current control method controls a voltage level applied to the CCFL to vary the tube current of the CCFL, thereby adjusting the brightness of the backlight. However, since the current control method is not designed to control the brightness in a low current range, the PWM control method has been mainly used for illumination control of the CCFL of the LCD backlight. 
     In the PWM control method, however, the response speed of the backlight decreases in the range of a low duty ratio of the PWM signal while the backlight is being dimmed, and a brightness blurring phenomenon can occur, thereby deteriorating the image display quality on the LCD. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the present invention, a backlight driver includes an inverter controller, a dimming signal compensator and an inverter. The inverter controller generates a pulse width modulation signal in response to a control signal and outputs a first dimming signal that represents a duty ratio of the pulse width modulation signal and a second dimming signal that represents an amplitude of the pulse width modulation signal. The dimming signal compensator receives the first and second dimming signals and compares the first dimming signal with a predetermined reference duty ratio. The dimming signal compensator compensates the second dimming signal in accordance with the compared result to generate a third dimming signal. The inverter outputs a driving voltage to drive a backlight and varies a voltage level of the driving voltage in response to the first and third dimming signals to control a brightness of the backlight. 
     In an exemplary embodiment of the present invention, a display apparatus includes an inverter controller, a dimming signal compensator, an inverter a backlight and a display unit. The inverter controller generates a pulse width modulation signal in response to a control signal and outputs a first dimming signal that represents a duty ratio of the pulse width modulation signal and a second dimming signal that represents an amplitude of the pulse width modulation signal. The dimming signal compensator receives the first and second dimming signals and compares the first dimming signal with a predetermined reference duty ratio. The dimming signal compensator compensates the second dimming signal in accordance with the compared result to generate a third dimming signal. The inverter receives a power voltage and changes the power voltage into a driving voltage and varies a voltage level of the driving voltage in response to the first dimming signal and the third dimming signal. The backlight receives the driving voltage to generate light and the display unit displays an image using the light. 
     In an exemplary embodiment of the present invention, a method of driving a backlight includes receiving a first dimming signal that represents a duty ratio of a pulse width modulation signal and a second dimming signal that represents an amplitude of the pulse width modulation signal, comparing first dimming signal with a predetermined reference duty ratio, compensating the second dimming signal in accordance with the compared result to generate a third dimming signal, and receiving a power voltage to change the power voltage into a driving voltage for the backlight and varying a voltage level of the driving voltage in response to the first dimming signal and the third dimming signal to control a brightness of the backlight. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become readily apparent to those of ordinary skill in the art when descriptions of exemplary embodiments thereof are read with reference to the accompanying drawings. 
         FIG. 1  is a block diagram showing a backlight driver according to an exemplary embodiment of the present invention. 
         FIG. 2  is a block diagram of the dimming signal compensator of  FIG. 1 , according to an exemplary embodiment of the present invention. 
         FIG. 3  is a waveform diagram showing the tube current of a cold cathode fluorescent lamp according to a pulse width modulation signal. 
         FIG. 4  is a graph showing the difference value between the tube current and the peak tube current in relation to the duty ratio. 
         FIG. 5  is a graph showing the tube current in relation to the duty ratio. 
         FIG. 6  is a block diagram showing a backlight driver according to an exemplary embodiment of the present invention. 
         FIG. 7  is a block diagram showing a display apparatus having the backlight driver of  FIG. 1 , according to an exemplary embodiment of the present invention. 
         FIG. 8  is a flowchart illustrating a method of driving the backlight driver of  FIG. 1 , according to an exemplary embodiment of the present invention. 
         FIG. 9  is a flowchart illustrating the step  320  of  FIG. 8 , according to an exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals may refer to similar or identical elements throughout the description of the figures. 
       FIG. 1  is a block diagram showing a backlight driver according to an exemplary embodiment of the present invention.  FIG. 2  is a block diagram of the dimming signal compensator of  FIG. 1 , according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , a backlight driver  100  drives a backlight  140  disposed at the rear of a display unit (not shown) for backlighting of the display unit. The backlight driver  100  includes an inverter controller  110 , a dimming signal compensator  120  and an inverter  130 . 
     The inverter controller  110  generates a pulse width modulation (PWM) signal in response to a control signal CON from an external source. The PWM signal may be generated to adjust the brightness of the backlight  140  during a power saving mode or to enhance a contrast ratio C/R of the display unit. For example, the control signal CON may be generated according to a user&#39;s operation or based on an image data for the display. 
     The inverter controller  110  outputs a first dimming signal Dim 1  representing the duty ratio of the PWM signal and a second dimming signal Dim 2  representing the amplitude of the PWM signal, based on the PWM signal. 
     The dimming signal compensator  120  receives the first dimming signal Dim 1  and the second dimming signal Dim 2  outputted from the inverter controller  110 . The dimming signal compensator  120  compares the first dimming signal Dim 1  with a predetermined reference duty ratio Ref and compensates the second dimming signal Dim 2  in accordance with the compared result to generate a third dimming signal Dim 3 . 
     Referring to  FIG. 2 , the dimming signal compensator  120  includes a comparing part  121  and an adding part  122 . The comparing part  121  includes a low pass filter  121   a , a reference voltage generator  121   b  and a differential amplifier  121   c . The adding part  122  includes an adder  122   a.    
     The comparing part  121  outputs a difference value that is the difference between the first dimming signal Dim 1  and the reference duty ratio Ref when the first dimming signal Dim 1  is smaller than the reference duty ratio Ref, and outputs the difference value equal to zero when the first dimming signal Dim 1  is equal to or larger than the reference duty ratio Ref. For example, the reference duty ratio Ref may be less than 30%. 
     The low pass filter  121   a  converts the first dimming signal Dim 1  into a first voltage V 1 . The reference voltage generator  121   b  outputs a reference voltage Vr corresponding to the reference duty ratio Ref. The differential amplifier  121   c  outputs a second voltage V 2  corresponding to the difference between the first voltage V 1  and the reference voltage Vr. 
     The adder  122   a  adds the second voltage V 2  and the second dimming signal Dim 2  to generate the third dimming signal Dim 3 . For example, the adder  122   a  may include an operational amplifier. 
     As shown in  FIG. 1 , the inverter  130  receives an inverter voltage Vint that is a direct current voltage from an external source and changes the inverter voltage Vint into a driving voltage Vd that is an alternating current voltage to provide the backlight  140  with the driving voltage Vd. Thus, the backlight  140  generates light in response to the driving voltage Vd. Although not shown in  FIGS. 1 and 2 , the backlight  140  includes at least one lamp, such as for example, a cold cathode fluorescent lamp (CCFL), which receives the driving voltage Vd to generate the light. 
     The inverter  130  intermittently provides the backlight  140  with the driving voltage Vd in response to the first dimming signal Dim 1  and the third dimming signal Dim 3 , such that the lamp of the backlight  140  is repeatedly turned on and off and the overall brightness of the backlight  140  may be controlled. 
       FIG. 3  is a waveform diagram showing the tube current of the CCFL according to the PWM signal. 
     Referring to  FIGS. 1 and 3 , the PWM signal PWM is divided into a first period t 1 , in which the lamp of the backlight  140  is turned on, and a second period t 2  where the lamp of the backlight  140  is turned off. A ratio of the first period t 1  to one period T of the PWM signal PWM is defined as the duty ratio t 1 /T. In an exemplary embodiment of the present invention, the first dimming signal Dim 1  and the second dimming signal Dim 2  are determined by the duty ratio t 1 /T and the amplitude of the PWM signal PWM, respectively. 
     The third dimming signal Dim 3  is generated by adding the difference value between the first dimming signal Dim 1  and the reference duty ratio Ref to the second dimming signal Dim 2 . Thus, when the first dimming signal Dim 1  is smaller than the reference duty ratio Ref, the PWM signal PWM having an amplitude corresponding to the third dimming signal Dim 3  that is larger than the second dimming signal Dim 2  is applied to the inverter  130 . 
     In an exemplary embodiment of the present invention, the inverter  130  applies the driving voltage Vd corresponding to the third dimming signal Dim 3  to the backlight  140  during the first period t 1 . 
     As shown in  FIG. 3 , the tube current of the lamp increases to a peak tube current Ipeak when the driving voltage corresponding to the third dimming signal Dim 3  is applied to the backlight  140 , in comparison with when the driving voltage corresponding to the second dimming signal Dim 2  is applied to the backlight  140 , and the response speed of the backlight  140  may be improved. 
       FIG. 4  is a graph showing the difference value between the tube current and the peak tube current in relation to the duty ratio.  FIG. 5  is a graph showing the tube current in relation to the duty ratio. 
     Referring to  FIGS. 4 and 5 , assuming that the reference duty ratio Ref is, for example, about 30%, the difference value increases as the first dimming signal Dim 1  decreases where the duty ratio is less than 30%. 
     Meanwhile, the tube current of the lamp may be increased in proportion to the difference value when the duty ratio is less than 30%. The tube current of the lamp may be substantially equal to the peak tube current Ipeak that is an instantaneous current when the duty ratio is 100%. 
     As shown in  FIGS. 3 and 5 , the period during which the voltage level of the driving voltage reaches to a target driving voltage is shortened even though the voltage level of the driving voltage increases in proportion to the difference value when the duty ratio decreases during the period where the duty ratio is less than 30%, so that the tube current of the lamp is maintained at the peak tube current Ipeak. In an exemplary embodiment of the present invention, the target driving voltage increases when the duty ratio decreases during the period where the duty ratio is less than about 30%, and the time in which the tube current of the lamp reaches the peak tube current Ipeak is shortened during the period where the duty ratio is less than about 30%, and the response speed of the backlight  140  may be improved. 
       FIG. 6  is a block diagram showing a backlight driver according to an exemplary embodiment of the present invention. In  FIG. 6 , the same reference numerals denote the same elements in  FIG. 1 , and thus further descriptions of the common elements will be omitted in the interests of brevity. 
     Referring to  FIG. 6 , a backlight driver  105  includes an inverter controller  110 , a dimming signal compensator  125  and an inverter  130 . 
     The dimming signal compensator  125  includes an analog-to-digital (A/D) converter  126  and a microcomputer  127 . The A/D converter  126  receives a second dimming signal Dim 2  outputted from the inverter controller  110  to output a fourth dimming signal Dim 4 . The AND converter  126  converts the second dimming signal Dim 2  in an analog form to the fourth dimming signal Dim 4  in a digital form. A first dimming signal Dim 1  from the inverter controller  110  and the fourth dimming signal Dim 4  are applied to the microcomputer  127 . 
     The microcomputer  127  is programmed to compare the first dimming signal Dim 1  with a predetermined reference duty ratio and add a difference value between the first dimming signal Dim 1  and the reference duty ratio to the fourth dimming signal Dim 4  to generate a third dimming signal Dim 3 . For example, the microcomputer  127  outputs the third dimming signal Dim 3  that is equal to a sum of the fourth dimming signal Dim 4  and the difference value when the first dimming signal Dim 1  is smaller than the reference duty ratio, and outputs the fourth dimming signal Dim 4  as the third dimming signal Dim 3  when the first dimming signal Dim 1  is equal to or larger than the reference duty ratio. 
     Although not shown as such in  FIG. 6 , the A/D converter  126  may be built in the microcomputer  127 . The backlight driver  105  may include a control board  105  on which the inverter controller  110  is mounted. The dimming signal compensator  125  may be mounted on the control board  105 . 
       FIG. 7  is a block diagram showing a display apparatus having the backlight driver of  FIG. 1 , according to an exemplary embodiment of the present invention. In  FIG. 7 , the same reference numerals denote the same elements in  FIG. 1 , and thus further descriptions of the common elements will be omitted. 
     Referring to  FIG. 7 , a display apparatus  200  includes a display unit  150 , a timing controller  160  and a DC/DC converter  170 , a backlight  140  and a backlight driver  100 . 
     The backlight  140  is arranged in the lower portion of the display unit  150  and includes at least one CCFL  141 . The CCFL  141 , which is connected to the inverter  130  of the backlight driver  100  to receive the driving voltage, generates light to provide the display unit  150  with the light. The inverter  130  controls the tube current of the CCFL  141 , for example, so that brightness of the backlight  140  is controlled. 
     The display unit  150  displays an image using the light emitted from the backlight  140 . 
     The timing controller  160  receives various control signals OC and an image data Idata from an external device. The timing controller  160  changes the various control signals OC into a data control signal DC and a gate control signal GC to output the data control signal DC and the gate control signal GC and outputs the image data Idata at an appropriate time. 
     The display unit  150  includes a liquid crystal display (LCD) panel displaying the image, a gate driving circuit and a data driving circuit driving the LCD panel. The gate driving circuit outputs a gate signal in response to the gate control signal GC, and the data driving circuit changes the image data Idata into a pixel voltage to output the pixel voltage in response to the data control signal DC. Thus, the LCD panel controls an arrangement of a liquid crystal layer in response to the gate signal and the pixel voltage to control a transmittance of the light provided from the backlight  140 , so that the LCD panel may display a desired image thereon. 
     The DC/DC converter  170  receives a power voltage Vp from an exterior and changes the power voltage Vp into a driving voltage (i.e., a gate-on voltage Von or a gate-off voltage Voff, which are applied to the gate driving circuit) for the display unit  150 . The DC/DC converter  170  changes the power voltage Vp into the inverter voltage Vint that is applied to the inverter  130 . 
       FIG. 8  is a flowchart illustrating a method of driving the backlight driver shown in  FIG. 1 , according to an exemplary embodiment of the present invention.  FIG. 9  is a flowchart illustrating the step  320  of  FIG. 8 , according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , a first dimming signal representing a duty ratio of a PWM signal and a second dimming signal representing an amplitude of the PWM signal are received from an external source (S 310 ). The first dimming signal is compared with a predetermined reference value (a reference duty ratio). According to the compared result, the second dimming signal is compensated to generate a third dimming signal (S 320 ). 
     When a power voltage is input from an external source, the power voltage is changed into a driving voltage for a backlight, and a voltage level of the driving voltage is changed in response to the first and third dimming signals to control brightness of the backlight (S 330 ). 
     As shown in  FIG. 9 , in the comparing of the first dimming signal and the predetermined reference value (S 320 ), the first dimming signal is compared with the predetermined reference value to determine whether or not the predetermined reference value is smaller than the first dimming signal (S 321 ). 
     When the first dimming signal is smaller than the reference value, a difference value between the first dimming signal and the reference value is output (S 322 ). The difference value equal to zero is output when the first dimming signal is equal to or larger than the reference value (S 323 ). 
     Next, the third dimming signal is generated by adding the difference value and the second dimming signal (S 323 ). 
     According to an exemplary embodiment of the present invention, when the first dimming signal representing the duty ratio of the PWM signal is smaller than the predetermined reference duty ratio, the second dimming signal representing the amplitude of the PWM signal increases by the difference value, and the response speed may be improved when controlling the brightness of the backlight and the brightness blurring phenomenon may be prevented. 
     According to an exemplary embodiment of the present invention, the backlight driver may prevent deterioration of the response speed of the backlight even when the duty ratio is small. 
     Although the exemplary embodiments of the present invention have been described in detail with reference to the accompanying drawings for the purpose of illustration, it is to be understood that the inventive processes and apparatus should not be construed as limited thereby. It will be apparent to those of ordinary skill in the art that various changes and modifications to the foregoing exemplary embodiments can be made without departing from the scope of the present invention as defined by the appended claims, with equivalents of the claims to be included therein.