Patent Publication Number: US-7911441-B2

Title: Current-controlling apparatus for controlling current of light emitting diode string

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a current-controlling apparatus, and more particularly, to a current-controlling apparatus using a feedback control to adjust the current passing through a light emitting diode string (LED string) for adjusting the brightness of the LED string. 
     2. Description of the Related Art 
     For a backlight source implemented in LED mode of a liquid crystal display television (LCD television), a large number of LEDs are employed to make the backlight source match a cold cathode fluorescent lamp (CCFL) in terms of the brightness thereof. In order to reduce the number of the driving integrated circuits (driving IC) for the LEDs and lower the total driving current of the LEDs, the circuit of the backlight source is usually designed by employing multiple LEDs in series connection for lightening the same. Such a design not only reduces the set number of the driving ICs, but also lowers the total driving current of the LEDs and further lowers the consumption power of the driving ICs. 
     However, it is difficult to make the cut-in voltage (standing for the lowest voltage to turn on an LED) of every LED completely consistent with each other in an LED manufacturing process. Consequently, the error values for the cut-in voltage of every LED would be accumulated, which results in difference between the currents of each LED string set due to the inconsistent cut-in voltages under a constant input voltage. As a result, each of the individual LED string sets will have a different brightness. Therefore, a phenomenon of uneven brightness or uneven chrominance appears on the backlight source of a display panel. 
     To solve the above-mentioned problem, some of improvement schemes by using current mirrors were provided. In the U.S. Pat. No. 5,701,133, for example, a scheme is given by  FIG. 1 .  FIG. 1  is a conventional brightness-adjusting circuit. Referring to  FIG. 1 , the symbol VLED represents a power voltage, GND represents a grounding voltage and Vin represents an input signal. The circuit shown by  FIG. 1  is two current mirrors in series connection ( 102  and  103  in  FIG. 1 ) formed by bipolar junction transistors (BJTs, for example,  101  in  FIG. 1 ), respectively. Wherein, the current amount of the LED string  104  is controlled by taking the advantage that the current Im 1  of the current mirror  102 , the current Im 2  of the current mirror  103  and the current Ic are equal to each other. In this way, the currents of every LED string set in a circuit with multiple sets of LED strings are controlled to be consistent with each other, thus the desired even brightness is achieved. 
     Note that the above-described circuit is a control system with an open loop by nature. Therefore, once an LED string in the system is malfunctioned (for example, some of LEDs in an LED string are short circuited), or an LED string has an excessive error of the total cut-in voltage (for example, the temperature characteristic of each LED slightly different from each other results in a larger error of the total cut-in voltage), the malfunction can not be detected due to lack of a feedback control mechanism. The BJTs of the current mirror may receive a great amount of voltage and currents, resulting in an overheat risk due to a constantly rising temperature thereof. Therefore, the reliability of products based on the above-described scheme is questionable. 
     Similarly, the U.S. Pat. No. 6,556,067 and No. 6,636,104 also employ current mirrors characterizing the same open loop control mode to make the currents of all LED string sets consistent with each other to achieve the brightness evenness. Thus, the reliability of such products is also in doubt. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention is to provide a current-controlling apparatus which uses feedback control to adjust the current passing through an LED string, thereby achieving the purpose of adjusting the brightness of an LED string with high reliability. 
     Based on the above-mentioned or other objectives, the present invention provides a current-controlling apparatus suitable for controlling the current passing through a light emitting device string (LEDS). Wherein, an end of the LEDS is electrically connected to a power voltage. The current-controlling apparatus includes a current-adjusting unit and a control unit. The current-adjusting unit is electrically connected between another end of the LEDS and a grounding voltage for detecting the current of the LEDS and producing a feedback signal accordingly. According to a conductance-controlling signal and an impedance-controlling signal, the current-adjusting unit also controls the impedance value between the LEDS and the grounding voltage and further controls the current of the LEDS. The control unit is electrically connected to the current-adjusting unit for receiving a reference signal and a feedback signal, followed by comparing the two received signals with each other to produce a comparison result. Afterwards, the control unit performs a current compensation on the comparison result and converts the compensated comparison result into the conductance-controlling signal and the impedance-controlling signal. 
     Based on the above-mentioned or other objectives, the present invention provides a current-controlling apparatus suitable for controlling the currents of multiple LEDSes. Wherein, each of an end of the above-mentioned multiple LEDSes is electrically connected to a power voltage. The current-controlling apparatus includes a current-adjusting unit set and a control unit. The current-adjusting unit set is electrically connected between another end of the above-mentioned multiple LEDSes and a grounding voltage for detecting the current of every the LEDS and producing multiple feedback signals accordingly. The current-adjusting unit set also receives multiple conductance-controlling signals and multiple impedance-controlling signals and controls the impedance value between one of the above-mentioned LEDSes and the grounding voltage according to one of the above-mentioned conductance-controlling signal and one of the above-mentioned impedance-controlling signal, and further controls the current passing though the LEDS. 
     The control unit is electrically connected to the current-adjusting unit set for receiving a reference signal and the above-mentioned multiple feedback signals, followed by comparing every feedback signal with the reference signal to produce multiple comparison results. Afterwards, the control unit performs a current compensation on every comparison result and converts the compensated comparison results into the above-mentioned multiple conductance-controlling signals and the multiple impedance-controlling signals. 
     According to an embodiment of the present invention, the above-mentioned control unit includes an error amplifier, a current compensator, an impedance controller and a driving buffer. Wherein, the error amplifier is electrically connected to the current-adjusting unit for receiving a reference signal and a feedback signal and comparing the received signals with each other to produce a comparison result accordingly. The current compensator is electrically connected to the error amplifier for receiving the comparison result, performing a current compensation on the comparison result and outputting the compensated comparison result. The impedance controller is electrically connected to the current compensator for receiving the output from the current compensator and converting the output from the current compensator into a conductance-controlling signal and an impedance-controlling signal. The driving buffer is electrically connected to the impedance controller for receiving the conductance-controlling signal, buffering the conductance-controlling signal and outputting the buffered conductance-controlling signal. 
     According to an embodiment of the present invention, the above-mentioned current-adjusting unit includes a metal-oxide semiconductor transistor (MOS transistor), a variable impedance device, a feedback unit, a first resistor, a first capacitor, a second capacitor and a diode. Wherein, a source/drain of the MOS transistor is electrically connected to another end of the LEDS and the MOS transistor works in the linear zone thereof. The first resistor is electrically connected between another end of the LEDS and the first capacitor. The first capacitor is electrically connected between the first resistor and the gate of the MOS transistor. The second capacitor is electrically connected between the gate of the MOS transistor and the grounding voltage. 
     The variable impedance device is electrically connected between the control unit and the gate of the MOS transistor for delivering the conductance-controlling signal to the gate of the MOS transistor and dynamically adjusting the resistance of the variable impedance device according to the impedance-controlling signal, so as to make the MOS transistor shift the on/off status thereof according to the conductance-controlling signal and the resistance of the variable impedance device and further to adjust the impedance of the MOS transistor in on status. The anode of the diode is electrically connected to the gate of the MOS transistor, while the cathode thereof is electrically connected to the conductance-controlling signal. The feedback unit is electrically connected between another source/drain of the MOS transistor and the grounding voltage for detecting the current of the LEDS and producing a feedback signal accordingly. 
     According to an embodiment of the present invention, the above-mentioned control unit includes an error amplifier, a current compensator, an impedance controller and a driving buffer. Wherein, the error amplifier is electrically connected to the current-adjusting unit set for receiving the above-mentioned reference signal and the above-mentioned multiple feedback signals and comparing every feedback signal with the above-mentioned reference signal to produce the above-mentioned multiple comparison results. The current compensator is electrically connected to the error amplifier for receiving the above-mentioned multiple comparison results, performing a current compensation on every comparison result and respectively outputting the compensated comparison results. The impedance controller is electrically connected to the current compensator for receiving the outputs from the current compensator and converting the outputs from the current compensator into multiple conductance-controlling signals and multiple impedance-controlling signals. The driving buffer is electrically connected to the impedance controller for receiving the above-mentioned multiple conductance-controlling signals, buffering the conductance-controlling signals and respectively outputting the buffered conductance-controlling signals. 
     According to an embodiment of the present invention, the above-mentioned current-adjusting unit set includes multiple current-adjusting units and each current-adjusting unit includes a MOS transistor, a variable impedance device, a feedback unit, a first resistor, a first capacitor, a second capacitor and a diode. Wherein, a source/drain of the MOS transistor is electrically connected to another end of one of the above-mentioned multiple LEDSes and the MOS transistor works in the linear zone thereof. The first resistor is electrically connected between another end of the LEDS and the first capacitor. The first capacitor is electrically connected between the first resistor and the gate of the MOS transistor. The second capacitor is electrically connected between the gate of the MOS transistor and the grounding voltage. 
     The variable impedance device is electrically connected between the control unit and the gate of the MOS transistor for delivering one of the above-mentioned multiple conductance-controlling signals to the gate of the MOS transistor and dynamically adjusting the resistance of the variable impedance device according to one of the above-mentioned multiple impedance-controlling signals, so as to make the MOS transistor shift the on/off status thereof according to the conductance-controlling signal and the resistance of the variable impedance device and further to adjust the impedance of the MOS transistor in on status. The anode of the diode is electrically connected to the gate of the MOS transistor, while the cathode thereof is electrically connected to the conductance-controlling signal. The feedback unit is electrically connected between another source/drain of the MOS transistor and the grounding voltage for detecting the current of one of the LEDSes and producing one of the above-mentioned multiple feedback signals accordingly. 
     The present invention uses the current of the LEDS as a feedback control, performs a current compensation on the current of the LEDS and converts the compensated current into two signals to control the impedance of the MOS transistor in on status (i.e. to control the channel size of the MOS transistor in on status). In this way, i.e. adjusting the current passing through the LEDS by changing the impedance of the MOS transistor in on status, the goal of adjusting the brightness of the LEDS is achieved. Therefore, compared with the conventional brightness-adjusting circuit where current mirrors are used to realize an open loop control mode, the present invention has a better reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve for explaining the principles of the invention. 
         FIG. 1  is a conventional brightness-adjusting circuit. 
         FIG. 2  is a current-controlling apparatus according to an embodiment of the present invention. 
         FIG. 3  is the schematic drawing of the partial circuit of  FIG. 2 . 
         FIG. 4  is a characteristic chart of a MOS transistor. 
         FIG. 5  is a current-controlling apparatus according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
       FIG. 2  is a current-controlling apparatus according to an embodiment of the present invention. Referring to  FIG. 2 , the current-controlling apparatus is suitable for controlling the current In passing through the LEDS  210 . In the embodiment, the LEDS  210  is formed by LEDs  211 ,  212 ˜N and an end of the LEDS  210  is electrically connected to a power voltage VLED (i.e. a first voltage level). The present invention, however, does not limit the LEDS  210  to be formed by LEDs only. 
     The current-controlling apparatus includes a current-adjusting unit  220  and a control unit  230 . The current-adjusting unit  220  is used for detecting the current In of the LEDS  210 , producing a feedback signal FS hereby and controlling the impedance between the LEDS  210  and the grounding voltage GND (i.e. the second voltage level) according to a conductance-controlling signal CCS and an impedance-controlling signal ICS, and further controlling the current In of the LEDS  210 . The control unit  230  is used for receiving a reference signal Vref and a feedback signal FS, followed by comparing the two received signals with each other to produce a comparison result CS. Afterwards, the control unit  230  performs a current compensation on the comparison result CS and converts the compensated comparison result CS into the conductance-controlling signal CCS and the impedance-controlling signal ICS. 
     The control unit  230  includes an error amplifier  231 , a current compensator  232 , an impedance controller  233  and a driving buffer  234 . Wherein, the error amplifier  231  is used for receiving the reference signal Vref and the feedback signal FS, comparing the feedback signal FS with the reference signal Vref to produce the comparison result CS. The current compensator  232  is used for receiving the comparison result CS output from the error amplifier  231 , performing a current compensation on the comparison result CS and outputting the compensated comparison result. The impedance controller  233  is used for receiving the output from the current compensator  232  and converting the received output into the digitalized conductance-controlling signal CCS and impedance-controlling signal ICS. The driving buffer  234  is used for receiving the conductance-controlling signal CCS, buffering the received signal and outputting the buffered conductance-controlling signal CCS. 
     The above-mentioned driving buffer  234  is employed mainly for buffering and amplifying the conductance-controlling signal CCS output from the impedance controller  233 . Thus, a user can decide whether or not to employ the driving buffer  234  in the control unit  230  according to the real need. 
     The current-adjusting unit  220  includes a MOS transistor  221 , a variable impedance device  222 , a feedback unit  223 , a first resistor  224 , a first capacitor  225 , a second capacitor  226  and a diode  227 . In the embodiment, the MOS transistor  221  is implemented by an NMOS transistor and assumed to be operated in the linear zone thereof. In addition, the feedback unit  223  is implemented by a second resistor  228 , which detects the current from the MOS transistor  221  to the grounding voltage GND and converts the current into a voltage signal, i.e. the above-mentioned feedback signal FS. 
     The variable impedance device  222  delivers the conductance-controlling signal CCS output from the driving buffer  234  to the gate of the MOS transistor  221  and dynamically adjusts the resistance of the variable impedance device  222  according to the impedance-controlling signal ICS output from the impedance controller  233 , so as to make the MOS transistor  221  shift on/off status in response to the conductance-controlling signal CCS and the resistance of the variable impedance device  222  and further to adjust the impedance of the MOS transistor  221  in on status, i.e. to adjust the channel size of the MOS transistor  221 . In other words, the current In of the LEDS  210  is able to be controlled by adjusting the channel size of the MOS transistor  221 , so that the brightness of the LEDS  210  is adjusted. 
       FIG. 3  is the schematic drawing of the partial circuit of  FIG. 2 .  FIG. 4  is a characteristic chart of a MOS transistor. In  FIGS. 3 and 4 , how the conductance-controlling signal CCS and the impedance-controlling signal ICS are used to control the current-adjusting unit  220  is illustrated. Referring to  FIG. 3  first, Rg in the current-adjusting unit  220  represents the resistance of the variable impedance device  222 , Ig represents the current passing through the variable impedance device  222 , Vg represents the voltage at the electrical node between the variable impedance device  222  and the driving buffer  234 , Vplt represents the voltage at the electrical node between the variable impedance device  222  and the MOS transistor  221 , Cgd and Cgs respectively represent the capacitance of the first capacitor  225  and the capacitance of the second capacitor  226  in  FIG. 2 , Rgd represents the resistance of the first resistor  224  in  FIG. 2 , Icgd represents the current passing through the first resistor  224 , Vds represents the voltage difference between the drain and the source of the MOS transistor  221  and Vled 1 , Vled 2 ˜VledN respectively represent the voltages of the LED  211 ,  212 ˜N in  FIG. 2 . According to  FIG. 3 , there are the following six equations to depict the relationships among the above-mentioned parameters: 
     
       
         
           
             
               
                 
                   
                     Ig 
                     × 
                     Rg 
                   
                   = 
                   
                     Vg 
                     - 
                     Vplt 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   Icgd 
                   ≅ 
                   Ig 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   Icgd 
                   = 
                   
                     Cgd 
                     ⁢ 
                     
                       
                         
                           ⅆ 
                           V 
                         
                         ⁢ 
                         
                           ⅆ 
                           s 
                         
                       
                       
                         ⅆ 
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         ⅆ 
                         V 
                       
                       ⁢ 
                       
                         ⅆ 
                         s 
                       
                     
                     
                       ⅆ 
                       t 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         Vg 
                         - 
                         Vplt 
                       
                       ) 
                     
                     
                       Rg 
                       × 
                       Cgd 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Vds 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           Vg 
                           - 
                           Vplt 
                         
                         ) 
                       
                       
                         Rg 
                         × 
                         Cgd 
                       
                     
                     × 
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     t 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   VLED 
                   = 
                   
                     
                       ( 
                       
                         
                           Vled 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         + 
                         
                           Vled 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         + 
                         … 
                         + 
                         
                           Vled 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           N 
                         
                       
                       ) 
                     
                     + 
                     Vds 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     From equation (5) it can be seen, ΔVds can be determined by the given Rg and Δt, where Δt represents a temperature variation and ΔVds represents the Vds variation corresponding to Δt. Referring to  FIG. 4 , after the MOS transistor falls in the linear zone, the voltage Vds varies linearly with the temperature, while the current In keeps constant. Referring to  FIG. 3  again, during the MOS transistor  221  is working in the linear zone, the conductance-controlling signal CCS and the impedance-controlling signal ICS are used to respectively modulate the Δt parameter and the Rg parameter, so that the impedance of the MOS transistor  221  in on status is able to be varied. In other words, the voltage Vds is controlled by changing the channel size of the MOS transistor, and the obtained ΔVds is used to compensate the variation of the sum (Vled 1 +Vled 2 + . . . +VledN) caused by an accidental LED short circuit or the inconsistent temperature characteristics among the LEDs, so as to further control the current In of the LEDS  210 . 
     Anyone skilled in the art can further implement a control on the currents of multiple LEDSes according to the spirit of the present invention and the above-described instructions of the embodiment.  FIG. 5  is one of the examples. 
       FIG. 5  is a current-controlling apparatus according to another embodiment of the present invention. Wherein, the current-controlling apparatus is suitable for controlling the currents I 1 , I 2  and I 3  respectively passing through the LEDS  510 , LEDS  520  and LEDS  530 . The symbol I in  FIG. 5  represents the current sum of I 1 , I 2  and I 3 . i.e. the total driving current of the LEDSes  510 ,  520  and  530 . In the embodiment, all of the LEDSes  510 ,  520  and  530  are respectively formed by LEDs and an end of every of the LEDSes is electrically connected to the power voltage VLED (i.e. the first voltage level). However, the present invention does not limit the LEDSes  510 ,  520  and  530  to be formed by LEDs only. 
     The current-controlling apparatus includes a current-adjusting unit set  540  and a control unit  550 . The current-adjusting unit set  540  is used for detecting the currents of the LEDSes  510 ,  520  and  530  and respectively producing feedback signals FS 1 , FS 2  and FS 3  accordingly. The current-adjusting unit set  540  receives three conductance-controlling signals CCS 1 , CCS 2  and CCS 3  and three impedance-controlling signals ICS 1 , ICS 2  and ICS 3 . 
     The current-adjusting unit set  540  controls the impedance between the LEDS  510  and the grounding voltage GND (i.e. the second voltage level) according to the conductance-controlling signal CCS 1  and the impedance-controlling signal ICS 1 , controls the impedance between the LEDS  520  and the grounding voltage GND according to the conductance-controlling signal CCS 2  and the impedance-controlling signal ICS 2  and controls the impedance between the LEDS  530  and the grounding voltage GND according to the conductance-controlling signal CCS 3  and the impedance-controlling signal ICS 3 . In this way, the current-adjusting unit set  540  is able to respectively control the currents passing through the LEDSes  510 ,  520  and  530 . 
     The control unit  550  is used for receiving a reference signal Vref and feedback signals FS 1 , FS 2  and FS 3 , followed by comparing every received feedback signal with the reference signal to respectively produce comparison results CS 1 , CS 2  and CS 3 . Afterwards, the control unit  550  performs a current compensation on every the comparison result CS and respectively converts the compensated comparison results CS 1 , CS 2  and CS 3  into the conductance-controlling signals CCS 1 , CCS 2  and CCS 3  and the impedance-controlling signals ICS 1 , ICS 2  and ICS 3 . 
     The control unit  550  includes an error amplifier  551 , a current compensator  552 , an impedance controller  553  and a driving buffer  554 . In the embodiment, each of the error amplifier  551 , the current compensator  552 , the impedance controller  553  and the driving buffer  554  has at least three input terminals and three output terminals for simultaneously processing at least three signals and respectively outputs the processed results. In particular, the error amplifier  551  requires at least four input terminals to receive an extra reference signal Vref in addition to the other three signals. However, it is noted that the present invention does not limit the numbers of the input terminals and the output terminals of the error amplifier  551 , the current compensator  552 , the impedance controller  553  and the driving buffer  554  to the above-mentioned numbers, and a user can choose the altered numbers to meet the real need. 
     The error amplifier  551  in the control unit  550  is used for receiving the reference signal Vref and the feedback signals FS 1 , FS 2  and FS 3 , comparing every feedback signal with the reference signal Vref to produce the above-mentioned comparison results CS 1 , CS 2  and CS 3 . The current compensator  552  is used for receiving the comparison results CS 1 , CS 2  and CS 3  and, after performing a current compensation on every comparison result, respectively outputting the compensated comparison results. The impedance controller  553  is used for receiving the outputs from the current compensator  552  and respectively converting the received outputs into the conductance-controlling signals CCS 1 , CCS 2  and CCS 3  and the impedance-controlling signals ICS 1 , ICS 2  and ICS 3 . The driving buffer  554  is used for receiving the conductance-controlling signals CCS 1 , CCS 2  and CCS 3 , buffering the received signals and outputting the buffered conductance-controlling signals. 
     Similar to the embodiment shown by  FIG. 2 , the above-mentioned driving buffer  554  is also used for taking the conductance-controlling signals CCS 1 , CCS 2  and CCS 3  output from the impedance controller  553  to respectively buffer and amplify the signals. Therefore, a user can decide whether or not to employ the driving buffer  554  in the control unit  550  to meet the real need. 
     The above-described current-adjusting unit set  540  includes three current-adjusting units  541 ,  542  and  543 . Every current-adjusting unit has the same design architecture as the current-adjusting unit  220  shown in  FIG. 2  and the designs and the operations of the current-adjusting units  541 ,  542  and  543  are omitted to describe for simplicity herein. 
     The current-adjusting unit  541  is used for detecting the current I 1  of the LEDS  510 , producing a feedback signal FS 1  hereby and receiving the conductance-controlling signal CCS 1  and the impedance-controlling signal ICS 1  output from the control unit  550  to adjust the impedance between the LEDS  510  and the grounding voltage GND. Similarly, the current-adjusting unit  542  is used for detecting the current I 2  of the LEDS  520 , producing a feedback signal FS 2  hereby and receiving the conductance-controlling signal CCS 2  and the impedance-controlling signal ICS 2  output from the control unit  550  to adjust the impedance between the LEDS  520  and the grounding voltage GND. In addition, the current-adjusting unit  543  is used for detecting the current I 3  of the LEDS  530 , producing a feedback signal FS 3  hereby and receiving the conductance-controlling signal CCS 3  and the impedance-controlling signal ICS 3  output from the control unit  550  to adjust the impedance between the LEDS  530  and the grounding voltage GND. 
     In this way, it is implemented to control the currents I 1 , I 2  and I 3  of the LEDSes  510 ,  520  and  530  are respectively controlled to achieve the goal of adjusting the brightness of the above-mentioned LEDSes, so as to further make the brightness of the LEDSes  510 ,  520  and  530  even. However, the current-controlling apparatus is not limited to adjust the currents of the above-described three LEDSes only. In fact, anyone skilled in the art is able to determine a reasonable number of the current-adjusting units in a current-adjusting unit set  540  depending on the number of the LEDSes, and correspondingly adjust the numbers of the input terminals and the output terminals of the error amplifier  551 , the current compensator  552 , the impedance controller  553  and the driving buffer  554 . 
     Note that although a feasible design mode of the circuit inside a current-adjusting unit is given by the above-described embodiments, it is well-known for anyone skilled in the art that each manufacturer has a different design of the current-adjusting unit. Therefore, the present invention does not limit any feasible design mode in a real application. In other words, any modified design of a current-adjusting unit is considered to be within the spirit of the invention if the current of an LEDS is regulated by adjusting the channel size of a transistor according to the input signal of the current-adjusting unit, where the transistor can be, for example, a MOS transistor, a BJT or an insulated gate bipolar transistor (IGBT), the channel size of the transistor is variable and the transistor works in the linear zone thereof. 
     In summary, the present invention uses the current of an LEDS to conduct a feedback control, performs a current compensation on the current of the LED string, and after the current compensation, converts the result into two signals which control the impedance of a MOS transistor in on status, so as to adjust the impedance of the MOS transistor in on status and thereby change the current passing through the LED string, thus achieving the goal of adjusting the LED brightness. Compared with the conventional brightness-adjusting circuit, where current mirrors are used to realize an open loop control mode, the present invention has a better reliability. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.