Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 10-2011-0077687 filed on Aug. 4, 2011, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a light emitting diode driving device for driving a light emitting diode and a method thereof. 
         [0004]    2. Description of the Related Art 
         [0005]    In general, a cold cathode fluorescent lamp (CCFL) used as a light source of a backlight unit of a liquid crystal display (LCD) uses mercury gas, thereby leading to disadvantages such as the possibility of environmental pollution, slow response speeds, low color reproducibility and unsuitability for a light, thin, short, and small LCD panel. 
         [0006]    Thus, recently, LEDs have been actively employed for display devices. Compared with CCFLs, the LEDs are environmentally-friendly, have a response speed in the range of a few nano-seconds to enable a high speed response and are thus effective for a video signal stream, are available for impulsive driving, have high color reproducibility, are capable of arbitrarily changing luminance, color temperature, or the like by adjusting the quantity of light emitted from red, green and blue versions thereof, and are suitable for a light, thin, short, and small LCD panel. 
         [0007]    In a backlight unit employing such LEDs, an LED driving device for supplying a current to the LEDs to drive them is necessarily employed. 
         [0008]    In order to enhance the stability of an operation of the LED driving device, in general, a capacitor may be connected to the end of the LED driving device; however, the driving of the LED driving device may be slow when a current charged in the capacitor has been discharged to a certain level. 
       SUMMARY OF THE INVENTION 
       [0009]    An aspect of the present invention provides a light emitting diode (LED) driving device capable of increasing an LED driving rate by discharging a charge stored in a capacitor during a certain period of time, and a method thereof. 
         [0010]    According to an aspect of the present invention, there is provided a light emitting diode (LED) driving device including: a driving unit detecting a current, as a voltage, the current flowing across an LED unit having at least one LED, controlling the current flowing across the LED unit according to a comparison result between the detected voltage and a reference voltage having a pre-set voltage level, and having a capacitor stabilizing an operation of the LED unit when the LED unit is driven; and a discharging unit discharging a charge stored in the capacitor during a pre-set discharge time when the LED unit is driven. 
         [0011]    The driving unit may include: a comparison unit supplying a switching signal according to the comparison result between the detected voltage and the reference voltage; a transistor turned on and turned off according to the switching signal to control a level of the current flowing across the LED unit; and a first resistor detecting the current flowing across the LED unit, wherein the capacitor is connected between an end of the LED unit and a ground. 
         [0012]    The discharging unit may discharge the charge stored in the capacitor during the pre-set discharge time when the transistor is turned on. 
         [0013]    The discharging unit may include: an inverter inverting a level of a pulse width modulation (PWM) signal from the outside; a first transistor receiving the switching signal at a gate thereof; a second transistor connected in series between a driving power terminal supplying a pre-set driving power and a drain of the first transistor, and receiving the PWM signal inverted by the inverter at a gate thereof; a third transistor connected between the end of the LED unit and the ground and having a gate connected between the drain of the first transistor and a drain of the second transistor; and a fourth transistor connected in parallel to the first transistor and receiving the PWM signal inverted by the inverter at a gate thereof. 
         [0014]    The discharging unit may further include a resistor and a first capacitor delaying the switching signal from the comparison unit for a pre-set period of time and transferring the delayed switching signal to the gate of the first transistor. 
         [0015]    When the PWM signal is a high level signal having a pre-set level, the fourth transistor may be turned off, the second transistor may be turned on, and the third transistor may be turned on, to thereby discharge the charge stored in the capacitor, and the first transistor may be turned on after being delayed for the pre-set period of time and interrupt the discharge path after the discharge time. 
         [0016]    When the PWM signal may be a low level signal having a level lower than that of the high level signal, the fourth transistor may be turned on, the second transistor may be turned off, and the third transistor may be turned off, to thereby interrupt the discharge path. 
         [0017]    According to another aspect of the present invention, there is provided method of driving a light emitting diode (LED), the method including: a driving operation of detecting a current, as a voltage, the current flowing across an LED unit having at least one LED, and controlling the current flowing across the LED unit according to a comparison result between the detected voltage and a reference voltage having a pre-set voltage level; and a discharging operation of discharging a charge stored in a capacitor stabilizing an operation of the LED unit when the LED unit is initially driven, during a pre-set discharge time. 
         [0018]    In the discharging operation, the charge stored in the capacitor may be discharged during the pre-set discharge time when a transistor for driving the LED unit is turned on. 
         [0019]    The discharging operation may include: inverting a level of a pulse width modulation (PWM) signal from the outside; and when the PWM signal is a high level signal having a pre-set level, discharging the charge stored in the capacitor by turning off a fourth transistor, turning on a second transistor, and turning on a third transistor, and interrupting a discharge path after the pre-set discharge time by turning on the first transistor after a delay thereof for a pre-set period of time, among the first transistor receiving a switching signal for driving the transistor at a gate thereof, the second transistor connected in series between a driving power terminal supplying a pre-set driving power and the first transistor, and receiving the inverted PWM signal at a gate thereof, the third transistor connected between an end of the LED unit and a ground and having a gate connected to drains of the first and second transistors, and the fourth transistor connected in parallel to the first transistor and receiving the inverted PWM signal at a gate thereof. 
         [0020]    In the interrupting of the discharge path, when the PWM signal is a low level signal having a level lower than that of the high level signal, the fourth transistor may be turned on, the second transistor may be turned off, and the third transistor may be turned off to thereby interrupt the discharge path. 
         [0021]    In the discharging operation, the switching signal may be delayed for the pre-set period of time and then transferred to the gate of the first transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0023]      FIG. 1  is a schematic configuration view of a light emitting diode (LED) driving device according to an embodiment of the present invention; 
           [0024]      FIGS. 2A and 2B  are graphs showing electrical characteristics of the LED driving device according to the embodiment of the present invention and a general LED driving device; and 
           [0025]      FIGS. 3A and 3B  are operational flow charts illustrating a method of driving an LED driving device according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
         [0027]      FIG. 1  is a schematic configuration view of a light emitting diode (LED) driving device according to an embodiment of the present invention. 
         [0028]    With reference to  FIG. 1 , an LED driving device  100  according to an embodiment of the present invention may include a driving unit  110  and a discharging unit  120 . 
         [0029]    The driving unit  110  may control a current flowing across an LED unit L having at least one LED to thereby drive the LED unit L. 
         [0030]    In detail, the LED unit L may include a single LED, an LED array having a plurality of LEDs connected in series, an LED group having a plurality of LEDs connected in parallel, or an LED group having a plurality of LED arrays connected in parallel, which receives a pre-set driving power V DC . 
         [0031]    Namely, the driving unit  110  may detect the current flowing across the LED unit L as a voltage, compare the level of the detected voltage with the level of a pre-set reference voltage, and control the current flowing across the LED unit L according to the comparison result to control a brightness of the LED unit L. 
         [0032]    To this end, the driving unit  110  may include a comparison unit  111 , a transistor Q 1 , a first resistor R 1 , and a capacitor Cs. 
         [0033]    The comparison unit  111  may compare the level of the detected voltage with the level of the pre-set reference voltage, the transistor Q 1  may be connected between the other end, opposite to one end, of the LED unit L to which the driving power V DC  is inputted, and a ground, and may be turned on or turned off according to a switching signal V B  having a comparison result from the comparison unit  111  to control the current flowing across the LED unit L. The first resistor R 1  may be connected between the transistor Q 1  and the ground and controlled by the transistor Q 1  to thereby detect the current flowing across the LED unit Las a voltage value. The capacitor Cs removes a ripple of the current flowing across the LED unit L to allow the LEDs to be stably driven. 
         [0034]    Meanwhile, when the transistor Q 1  is turned on after being turned-off, the turning on of the transistor Q 1  may be delayed because a charge stored in the capacitor Cs may not have been sufficiently discharged. 
         [0035]    When the driving unit  110  is driven, the discharging unit  120  may provide a discharge path for the charge stored in the capacitor Cs during a pre-set discharge time. Namely, the discharging unit  120  may provide a discharge path for the charge stored in the capacitor Cs during the discharge time when the transistor Q 1  is turned on. 
         [0036]    To this end, the discharging unit  120  may include an inverter Iv, first to fourth transistors M 1 , M 2 , M 3 , and M 4 , a resistor R T , and a first capacitor C 1 . 
         [0037]    The inverter Iv may invert the level of a pulse width modulation (PWM) signal supplied to the comparison unit  111  of the driving unit  110  and transfer the inverted PWM signal to gates of the second and fourth transistors M 2  and M 4 . 
         [0038]    The second transistor M 2  and the first transistor M 1  may be connected in series between a driving power terminal supplying a driving power V DD , and the ground. 
         [0039]    Namely, a source of the second transistor M 2  may receive the driving power V DD , a gate thereof may receive the inverted PWM signal, and a drain thereof may be connected to a drain of the first transistor M 1  and a gate of the third transistor M 3 . A gate of the first transistor M 1  may receive a switching signal V B  applied to the transistor Q 1 , and a source thereof may be connected to the ground. Here, the switching signal V B  received to the gate of the first transistor M 1  may be a switching signal which has been RC delayed by the resistor R T  and the first capacitor C 1  and transferred to the gate of the first transistor M 1 . Accordingly, the resistor R T  may be connected in series between a gate of the transistor Q 1  and the gate of the first transistor M 1 , and the first capacitor C 1  may be connected in series between the gate of the first transistor M 1  and the ground. 
         [0040]    A drain of the third transistor M 3  may be connected to the end of the LED unit L, and a source thereof may be connected to the ground. A drain of the fourth transistor M 4  may be connected to a drain of the second transistor M 2 , and a source thereof may be connected to the ground. 
         [0041]      FIGS. 3A and 3B  are operational flow charts illustrating a method of driving an LED driving device according to an embodiment of the present invention. 
         [0042]    A discharging operation of the foregoing discharging unit  120  will now be described with reference to  FIGS. 3A and 3B , together with  FIG. 1 . 
         [0043]    First, when the level of the PWM signal is a low level lower than a pre-set level (S 1 ), the switching signal V B  applied to the transistor Q 1  may be a low level signal (S 2 ). Accordingly, the fourth transistor M 4  is turned on, the second transistor M 2  is turned off, and the level of a voltage applied to the gate of the third transistor M 3  is low level, such that the third transistor M 3  is turned off (S 3  and S 4 ). In addition, the first transistor M 1  is also turned off (S 5 ). Namely, the discharge path is opened and a charge equal to a level of driving power V DC  may be stored in the capacitor Cs. 
         [0044]    Next, when the level of the PWM signal is a high level higher than a pre-set level (S 6 ), the level of the switching signal V B  rises, the fourth transistor M 4  is turned off, the second transistor M 2  is turned on, and thus the third transistor M 3  may be also turned on (S 7  and S 8 ). Accordingly, the discharge path is formed and a charge stored in the capacitor Cs may be discharged (S 9 ). When the third transistor M 3  is continuously turned on, the current flowing across the LED unit L also flows through the third transistor M 3 , to thereby result in degradation in power efficiency. 
         [0045]    Thus, when the level of the PWM signal is a high level, the level of the switching signal V B  rises (S 10 ), and a gate voltage of the first transistor M 1  is slowly increased by an RC delay circuit including the resistor R T  and the first capacitor C 1 . Namely, after the second transistor M 2  is turned on and the charge stored in the capacitor Cs is discharged during a discharge time by the foregoing RC delay circuit (S 11 , S 12 ), the first transistor M 1  may be turned on (S 13 ). 
         [0046]    When the first transistor M 1  is turned on, the third transistor M 3  is turned off, such that the formed discharge path may be opened (S 14 ). In a section in which the first transistor M 1  and the second transistor M 2  are simultaneously turned on, the third transistor M 3  needs to be turned off, such that a ratio (W/L) of a width to a length of the first transistor M 1  may be considerably higher than that of the second transistor M 2 . 
         [0047]    Namely, when the level of the PWM signal is a high level, the charge stored in the capacitor Cs may be discharged during the discharge time. 
         [0048]      FIGS. 2A and 2B  are graphs showing electrical characteristics of the LED driving device according to the embodiment of the present invention and a general LED driving device. 
         [0049]    With reference to  FIGS. 2A and 2B , together with  FIG. 1 , in the general LED driving device employing only the capacitor Cs, it can be seen that the voltage of the switching signal applied to the transistor Q 1  is slowly increased (B), such that the level of the current flowing across the LED unit L slowly rises (D). 
         [0050]    On the other hand, in the LED driving device  100  according to the embodiment of the present invention, it can be seen that when the level of the PWM signal is a high level, the discharge path for the charge stored in the capacitor Cs is formed during the discharge time to allow the voltage of the switching signal applied to the transistor Q 1  to be quickly increased (A), such that the level of the current flowing across the LED unit L rapidly rises (C). 
         [0051]    As set forth above, according to embodiments of the invention, in driving an LED, a charge stored in a capacitor is discharged during a pre-set period of time, whereby the driving of the LED may be stably performed using the capacitor while increasing an LED driving speed. 
         [0052]    While the present invention has been shown and described in connection with the embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Technology Category: 5