Abstract:
Provided are an apparatus and method for driving LEDs. The apparatus comprise a plurality of red, green, and blue light emitting diodes connected, respectively; switching units turned on or off by an inputted pulse to turn on or off the red, green and blue light emitting diodes, respectively; and a control unit outputting respective pulses to sequentially delay a turn-on or turn-off time between the switching units.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus and method for driving a light emitting diode (LED). 
   2. Description of the Related Art 
   LEDs are widely used as a light source of a backlight unit in a liquid crystal display (LCD) device to enhance a color reproduction and a high brightness. 
   The backlight unit includes an edge type and a direct type according to a method of using a light source. In the edge type, a light source is installed on a side of a light guiding plate. The edge type is applied to a relatively small sized LCD device such as a computer monitor. The direct type is applied to a relatively large sized LCD device such as larger than a 20 inch monitor. 
   The backlight unit includes a plurality of arrays for lights of red, green, and blue LEDs. Driving pulses are supplied into red LED, green LED, and blue LED, respectively. A white light including red, green, and blue lights is used as a light source. 
   As illustrated in  FIG. 1 , the driving pulse is outputted to each LED at the same predetermined interval T. Therefore, the red, green, and blue LEDs are simultaneously turned on or off. However, all the LEDs in the backlight unit are turned off or on simultaneously. Accordingly, numerous compound harmonics occur such that wave noise in an output screen of an LCD panel of  FIG. 2  is generated. The wave noise distorts an image in the LCD panel. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to an apparatus and method for driving LEDs that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   The embodiment of the present invention provides an apparatus and method for driving LEDs for reducing wave noise, which is caused by the LEDs, by sequentially turning on or off the LEDs that emits three colors. 
   The embodiment of the invention provides an apparatus for driving a light emitting diode comprising: a plurality of red, green, and blue light emitting diodes connected respectively; switching units turned on or off by an inputted pulse to turn on or off the red, green and blue light emitting diodes, respectively; and a control unit outputting respective pulses to sequentially delay a turn-on or turn-off time between the switching units. 
   The embodiment of the invention provides a method for driving a light emitting diode, the method comprising: outputting pulses respectively to sequentially delay a turn-on or turn-off time of switching units; sequentially turning on or off the switching units to turn on or off red, green, blue light emitting diodes electrically connected to the switching units, respectively. 
   The embodiment of the invention provides a method for driving a light emitting diode, the method comprising: detecting a state of a pulse; sequentially delaying a turn-on time of switching units to sequentially turn on red, green, and blue light emitting diodes when the pulse is changed in a first state; and sequentially delaying a turn-off time of switching units to sequentially turn off red, green, and blue light emitting diodes when the pulse is changed in a second state. 
   According to the present invention, wave noise can be reduced in a combined light by sequentially turning on or off LEDs emitting three colors. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view of pulses supplied to related art red, green, and blue LEDs; 
       FIG. 2  is a view of wave noise generated by the LEDs of  FIG. 1 ; 
       FIG. 3  is a view of an LED driving unit according to an embodiment of the present invention; 
       FIG. 4  is a view of driving pulses of LEDs according to an embodiment of the present invention; and 
       FIG. 5  is a flowchart illustrating a method for driving LEDs according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 3  is a view of an LED driving unit according to an embodiment of the present invention. 
   Referring to  FIG. 3 , the LED driving unit includes a light emitting unit  110 , a switching unit  120 , a feedback unit  130 , and a control unit  140 . 
   The light emitting unit  110  includes red, green, and blue LEDs  111 ,  112 , and  113  to provide a white light. In the red LED  111 , a plurality of red LED chips are connected in serial to each other. In the green LED  112 , a plurality of green LED chips are connected in serial to each other. In the blue LED  113 , a plurality of blue LED chips are connected in serial to each other. 
   Here, one of the red LED chips, one of the green LED chips, and one of the blue LED chips constitute a cluster in a package. A plurality of clusters are disposed to form one module. Additionally, a plurality of modules are connected to form one LED bar. The backlight unit of a related art medium or large sized LED panel uses a plurality of LED bars. 
   The light emitting unit  110  is turned off or on by the switching unit  120 . 
   The switching unit  120  includes a plurality of switching elements Q 1 , Q 2 , and Q 3  formed of a metal oxide semiconductor field effect transistor (MOSFET). A drain terminal of each of the switching elements Q 1 , Q 2 , and Q 3  is commonly connected to a current power source terminal Vdc. A gate terminal is connected to pulse output terminals P 1 , P 2 , and P 3  of the control unit  140 . Additionally, a source terminal of each of the switching elements Q 1 , Q 2 , and Q 3  is connected to anode terminals of the first red LED, the first green LED, and the first blue LED. Resistors R 1 , R 2 , and R 3  of the feedback unit  130  are connected to cathode terminals of the last LEDs. 
   The feedback unit  130  includes resistors R 1 , R 2 , and R 3  connected to the cathode of the last LEDs, respectively. A voltage applied to the resistors R 1 , R 2 , and R 3  is applied to feedback terminals FB 1 , FB 2 , and FB 3  of the control unit  140 . 
   The control unit  140  outputs pulse signals through pulse output terminals P 1 , P 2 , and P 3  to turn on or off the switching elements Q 1 , Q 2 , and Q 3  of the switching unit  120 , and thus the LEDs  111 ,  112 , and  113  can be turned on or off by turning on or off the switching elements Q 1 , Q 2 , and Q 3 . 
   At this point, the control unit  140  adjusts a duty ratio of a pulse to control a turn-on or turn-off period. This normally maintains a high color reproduction and a brightness uniformity, and also adjusts an entire brightness through overall dimming of RGB. 
   The control unit  140  delays a pulse in each of the switching elements Q 1 , Q 2 , and Q 3  by a predetermined interval. For example, the second switching element Q 2  is delayed by a predetermined interval compared to the first switching element Q 1 . The third switching element Q 3  is delayed by a predetermined interval compared to the second switching element Q 2 . 
   At this point, since the pulses are delayed by a predetermined interval and then inputted, the three switching elements Q 1 , Q 2 , and Q 3  are not simultaneously turned on or off, and are turned on or off in a delayed interval. Moreover, the red LED  111 , green LED  112 , and blue LED  113  connected to switching elements Q 1 , Q 2 , and Q 3  are not simultaneously turned on or off, and are turned on or off in a delayed interval. Because of a delayed operation in each of the red LED  111 , the green LED  112 , and the blue LED  113 , wave noise due to an impulse noise between colors can be prevented. 
   Here, the pulses can be inputted into the three switching elements Q 1 , Q 2 , and Q 3  at the same interval. For example, the second switching element Q 2  receives a pulse that is delayed by 140 μs and the third switching element Q 3  receives a pulse delayed by 140*2 μs by using a starting point of the first switching element Q 1  as a reference. At this point, the pulse delaying time is in a sequential order of the first switching element Q 1  through the third switching element Q 3 . The sequential order can be in a reverse order. Moreover, the pulse inputted into the third switching element Q 3  is delayed using a starting point of the second switching element Q 2  as a reference. 
   The control unit  140  receives feedbacks from a feedback unit  130 . The feedbacks include operation times of the red LED  111 , the green LED  112 , the blue LED  113  in the light emitting unit  110 , respectively. Then, the control unit  140  stores the feedbacks. After a predetermined delay time, the control unit  140  controls an operation of the next switching element. 
   Here, the control unit  140  outputs a driving pulse into the next switching element. The driving pulse is constantly delayed from a starting point of a previously operating switching element or a changing point of an LED operation. 
   The control unit  140  controls an ascending or descending edge of a pulse as an operation starting point, and delays another pulse by a predetermined period based on the turning on or off of the three color LEDs  111 ,  112 , and  113  as a reference. The synchronization time of the three color LEDs is reduced through a periodic delay of the three color LEDs. Consequently, wave noise can be prevented. 
     FIG. 4  is a view of driving pulses of LEDs according to an embodiment of the present invention. 
   Referring to  FIGS. 3 and 4 , each of the switching elements Q 1 , Q 2 , and Q 3  is turned off at a descending edge of a pulse, and is turned on at an ascending edge of a pulse. 
   The first switching element Q 1  performs a periodic operation by a period T including a turn-on time T 1  and a turn-off time T 2  in an R pulse. 
   The second switching element Q 2  performs a turn-on or a turn-off periodically by a period including a turn-on time and a turn-off time in a G pulse. The turn-off point is delayed by a delay time ΔT compared to the turn-off point of the R pulse. 
   The third switching element Q 3  performs a turn-on or a turn-off periodically by a period including a turn-on time and a turn-off time in a B pulse. The turn-off point is delayed by a delay time ΔT compared to the turn-off point of the G pulse inputted into the second switching element Q 2 . The turn-on point is delayed by a delay time ΔT compared to the turn-on point of the G pulse inputted into the second switching element Q 2 . 
   The three switching elements Q 1 , Q 2 , and Q 3  are not turned on or off simultaneously, and turned on or off by a predetermined delay time ΔT. The turn-on or turn-off starting point of the red, green, and blue LEDs  111 ,  112 , and  113  is delayed by the delay time ΔT through the turn-on or turn-off operation in each of the switching elements Q 1 , Q 2 , and Q 3 . 
     FIG. 5  is a flowchart illustrating a method for driving LEDs according to an embodiment of the present invention. The present invention exemplifies the sequentially delayed RGB LEDs, but is not limited to the sequential order. 
   Referring to  FIG. 5 , states of the LEDs are stored in operation S 101 , and it is confirmed whether an event of a pulse signal corresponding to a red LED occurs or not in operation S 103 . 
   When the pulse for the red LED is an event of a falling edge, the red LED is turned off and the turn-off time is stored in a register in operation S 105 . When a clock counted from the turn-off time of the red LED is identical to a predetermined time (Count=ΔT) in operation S 107 , the green LED is turned off, and the turn-off time is stored in a register in operation S 109 . 
   Here, a predetermined delay time is calculated by counting a clock from the turn-off time of the red LED. 
   When the clock counted from the turn-off time of the green LED is identical to the predetermined delay time (Count=ΔT), the blue LED is tuned off. Then, the turn-off time of the blue LED is stored in a register in operation S 113 . At this point, the turn-off time of the blue LED (i.e., the last LED) may not be stored. 
   Moreover, when the event of operation S 103  is for an ascending edge of a pulse, the red LED is turned on and the turn-on time is stored in a register in operation S 115 . A clock is counted from the turn-on time of the red LED. When the clock counted from the turn-on time of the red LED is identical to a predetermined time (Count=ΔT) in operation S 117 , the green LED is turned on, and the turn-on time is stored in a register in operation S 119 . 
   A clock is counted from the turn-on time of the green LED. When the clock counted from the turn-on time of the green LED is identical to a predetermined time (Count=ΔT) in operation S 121 , the blue LED is turned on, and the turn-on time is stored in a register in operation S 123 . Here, the turn-on time of the blue LED, that is, the turn-on time of the last LED may not be stored. 
   After the blue LED is turned on or off, it is confirmed whether an ascending edge or descending edge event of a pulse occurs or not in operation S 103 . This will repeat periodically. 
   Likewise, a predetermined time is delayed by using the shift point as a reference, where the shift point is from the turn-on to the turn-off or from the turn-off to the turn-on in the first switching element. Then, the state of the second switching element is changed. Moreover, since according to a shift in each of the switching elements, the turn-on or off starting operation of a corresponding LED is delayed, wave noise can be minimized by the delay between the three color LEDs. 
   The LED driving device of the present invention sequentially controls the LEDs in the backlight unit of a large sized liquid crystal panel, and minimizes wave noise in the liquid crystal panel. 
   Additionally, when the LEDs are mounted on the direct type backlight unit, a plurality of LED bars can be disposed. The same delay time (e.g., 148 μs (microsecond)) is used to control the LED bars. That is, the delay of the LED bars connected in parallel besides the delay of the three color LEDs can be performed simultaneously. For example, when there are seven LED bars, total 27 delays (three color LEDs and seven LED bars) can be performed. 
   According to the present invention, since three LEDs are sequentially turned on or off, a synchronization time of three LEDs is reduced to minimize wave noise. 
   Moreover, the reliability of a backlight unit and a liquid crystal display device for a large sized panel can be enhanced. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.