Abstract:
Disclosed herein is a device for controlling current of an LED including a constant current generator, a current mirror, and a current amplifier, the device controlling current of an LED, comprising: a selector outputting an L signal or an H signal according to an input control signal; and a current changing unit formed by connecting at least one switching unit in parallel, the switching unit including a transistor and a switch connected to the transistor in series. The device for controlling current of an LED adjusts current supplied to the current amplifier using a SEL input without an effect of a noise, thereby making it possible to change the current of the LED without separately changing a resistor.

Description:
CROSS REFERENCE(S) TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. Section [120, 119, 119(e)] of Korean Patent Application Serial No. 10-2010-0090997, entitled “Device for Controlling Current of LED” filed on Sep. 16, 2010, which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a device for controlling current of a light emitting diode (LED), and more particularly, to a device for changing current of an LED capable of changing current determining brightness of the LED without separately changing an external power supply or a resistor. 
     2. Description of the Related Art 
     An organic light emitting diode (LED) has been widely used in various fields such as illumination, a backlight unit (BLU), or the like. Recently, as a market for the LED has rapidly expanded, the related technologies have rapidly advanced correspondingly. 
     Meanwhile, the current of the LED are mainly set and controlled by a conversion dimming signal (ADIM) and a resistor (RLED) parameter. When voltage is applied from the outside, a noise included in the voltage appears in output current as it is. Therefore, a scheme of changing an external resistor has been mainly used. 
     However, even in the case of using the scheme of changing the external resistor as described above, since a resistor of each of the LED modules should be changed, it is difficult to standardize the LED modules. 
       FIG. 1  shows an LED driving circuit most widely used in the related art. 
     As shown in  FIG. 1 , a constant current generator  110  generates reference current (Iref) to LED current (ILED) using ADIM voltage and an RLED and transfers the reference current to an input end  131  of a current amplifier  130  using a current mirror  120 , and the current amplifier  130  may obtain the LED current (ILED) corresponding to N times of the reference current using the ratio (N:1) of a resistor A to a resistor B. 
     Herein, in order to change the LED current (ILED), the ADIM voltage or the RLED should be changed. However, as described above, when the ADIM voltage is input from the outside, an input noise may be reflected on an output as it is, and when the RLED is changed, it is difficult to standardize the LED module. 
     In addition, in order to change the LED current, a single wire control scheme, a single wire clock counting scheme, and the like, in which a separate digital block is added may also be used. However, in the case of using the digital block, a separate digital source for generating a complicated digital clock signal is required. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a device for changing current of an LED without separately adding a digital block or separately changing a resistor by adjusting current supplied to a current amplifier using a SEL input without an effect of a noise. 
     According to an exemplary embodiment of the present invention, there is provided a device for controlling current of an LED including a constant current generator, a current mirror, and a current amplifier, the device controlling current of an LED, including: a selector outputting an L signal or an H signal according to an input control signal; and a current changing unit including at least one switching unit connected in parallel, the switching unit including a transistor and a switch connected to the transistor in series, wherein the switch of the current changing unit is controlled by the signal output from the selector. 
     The selector may include at least one comparator and at least one resistor. 
     The selector may have non-inverting terminals receiving the control signal, inverting terminals receiving voltage divided by the resistor, and at least one amplifier comparing voltage of the inverting terminal with that of the non-inverting terminal to output the L signal or the H signal to an output terminal. 
     The selector may have inverting terminals receiving the control signal, non-inverting terminals receiving voltage divided by the resistor, and at least one amplifier comparing voltage of the inverting terminal with that of the non-inverting terminal to output the L signal or the H signal to an output terminal. 
     According to an exemplary embodiment of the present invention, there is provided a device for controlling current of an LED, including: a constant current generator; a current mirror including a first MOS transistor and a second MOS transistor; a selector outputting an L signal or an H signal according to an input signal; a MOS transistor having a source receiving driving power and a gate connected to a gate of the first MOS transistor; a current changing unit including a switch of which one end is connected to a drain of the MOS transistor; and a current amplifier having an input end connected to the other end of the switch of the current changing unit, wherein the switch is controlled by the signal output from the selector. 
     The current changing unit may include a plurality of switching units in which a switch is connected to the MOS transistor, the plurality of switching units being connected in parallel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a configuration of a device for controlling current of an LED according to the related art; 
         FIG. 2  is a circuit diagram showing a configuration according to an exemplary embodiment of the present invention; 
         FIG. 3  is a circuit diagram showing a configuration according to another exemplary embodiment of the present invention; 
         FIG. 4  is a graph showing current at a VDAIM node according to an exemplary embodiment of the present invention; and 
         FIG. 5  is a graph showing current and voltage of a final output end according to an exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, configuration and operation of the present invention will be described in more detail with reference to the accompanying drawings. 
       FIG. 2  is a circuit diagram showing a configuration according to an exemplary embodiment of the present invention. A configuration in which a selector  10  and a current changing unit  20  are further included in the LED driving circuit according to the related art including the constant current generator  110 , the current mirror  120  and the current amplifier  130  is shown in  FIG. 2 . 
     At this time, the specific configuration of the constant current generator  110 , the current mirror  120  and the current amplifier  130  is similar to that widely used in the related art. Therefore, the detailed description thereof will be omitted. 
     Meanwhile, the selector  10  generates an output signal according to a signal (SEL signal) input to a control signal input terminal  13  to control the current changing unit  20 . The current changing unit  20  changes current supplied to an input end (VADIM node)  131  of a current amplifier  130  according to an output signal from the selector  10 . 
       FIG. 3  is a circuit diagram showing a configuration according to another exemplary embodiment of the present invention. The configurations of the selector  10  and the current changing unit  20  as described above are shown in detail in  FIG. 3 . 
     The selector  10  generates an output signal of L or H according to an input signal. As shown in  FIG. 3 , the selector  10  can include at least one comparator  11  whose non-inverting terminals receive the control signal, inverting terminals receive voltage divided by the resistor  12 , and compare voltage of the inverting terminal with that of the non-inverting terminal. 
     According to the configuration as described above, reference voltage (Vref) is divided by resistors (R 1  to Rn) to be applied to the inverting terminals of the amplifiers (Amp 1  to Ampn), SEL voltage is applied to the non-inverting terminals, and the amplifiers compare the voltage of the inverting terminal with the voltage of the non-inverting terminal to output an L signal or an H signal. Therefore, the SEL voltage is adjusted, thereby making it possible to adjust the number of the L signal or the H signal output from the selector  10 . 
     Meanwhile, the current changing unit  20  includes a plurality of switching units  23  connected in parallel, wherein a switch  22  is connected to a transistor in series in the switching unit. 
     At this time, although the transistor is shown as being a MOS transistor  21  and the switch  22  is shown as being a junction transistor in  FIG. 3 , it is obvious that other kinds of transistors than the configuration shown in  FIG. 3  may be used and a scope of the present invention is not limited by the accompanying drawings. 
     In addition, the kinds of voltages input to the inverting terminal and the non-inverting terminal of the amplifier may be different according to the kinds of switches  22 . 
     In addition, an output end of the selector  10  is connected to the switches  22  to transfer the L signal or the H signal to each of the switches  22 . 
     According to another exemplary embodiment of the present invention, the transistor configuring the current changing unit  20  is the MOS transistor  21 . The MOS transistor  21  has a source receiving a driving power and a drain connected to one end of the switch  22  to configure the switching unit  23 . At this time, a gate of the MOS transistor  21  is connected to a gate of a first MOS transistor  121  (M 1 ) configuring the current mirror  120 . 
     The plurality of switching units  23  as described above are connected in parallel and the other end of the switch  22  is connected to an input end (VADIM node) ( 131 ) of the current amplifier  130  to configure the current changing unit  20 . 
     Hereinafter, an operation principle according to an exemplary embodiment of the present invention will be described with reference to  FIGS. 1 and 3 . 
     In the device for controlling current of an LED according to the related art in  FIG. 1 , the reference current (Iref) is generated by the ADIM voltage and the RLED in the constant current generator  110  and is applied to the VDAIM node by the current mirror  120  circuit. 
     A device according to an exemplary embodiment of the present invention shown in  FIG. 3  is basically operated in the same scheme as that of the device for controlling current of an LED described above. 
     However, when a portion of the switches  22  of the current changing units  20  are turned-on by the output signal generated in the selector  10 , each of the MOS transistors  21  connected to one end of the turned-on switch  22  forms the current mirror  120  together with the first MOS transistor  121 , such that the reference current (Iref) is applied to the VDAIM node. 
     Accordingly, assuming that the sum of the turned-on switches  22  among the switches  22  (S 1 , S 2 , S 3  . . . Sn) is C, the current applied to the VADIM becomes Iref×(1+C). 
     At this time, the C may not be an integer according to the ratio of the current mirror  120 . 
     Meanwhile, since the turn-on/off of the switches  22  may be controlled by the selector  10 , the magnitude of the control signal (SEL voltage) is adjusted, thereby making it possible to change the C. As a result, it is possible to change the current of the LED only through the adjustment of the control signal. 
     Results shown in  FIGS. 4 and 5  have been obtained by simulating the configuration according to the present invention using the method as described above. 
       FIG. 4  shows current at a VDAIM node according to a change in SEL voltage, and  FIG. 5  shows current and voltage of a final output end according to a change in SEL voltage. 
     Although the case in which the current at VDAIM is increased according to the increase of the SEL voltage has been shown in  FIG. 4 , it will be easily appreciated that the current at VADIM may be reduced according to the increase of the SEL voltage by inverting the polarity of the reference voltage (Vref) divided by the resistor  12 . 
     As set forth above, the present invention adjusts the SEL input to control the current of the LED, thereby not generating a problem due to a noise as in an input voltage control scheme according to the related art. In addition, it is not required to replace the resistor or configure the resistor as a variable resistor. Therefore, the LED driving module may be standardized, thereby making it possible to reduce cost of the LED driving module. 
     The present invention has been described in connection with what is presently considered to be practical exemplary embodiments. Although the exemplary embodiments of the present invention have been described, the present invention may be also used in various other combinations, modifications and environments. In other words, the present invention may be changed or modified within the range of concept of the invention disclosed in the specification, the range equivalent to the disclosure and/or the range of the technology or knowledge in the field to which the present invention pertains. The exemplary embodiments described above have been provided to explain the best state in carrying out the present invention. Therefore, they may be carried out in other states known to the field to which the present invention pertains in using other inventions such as the present invention and also be modified in various forms required in specific application fields and usages of the invention. Therefore, it is to be understood that the invention is not limited to the disclosed embodiments. It is to be understood that other embodiments are also included within the spirit and scope of the appended claims.