Abstract:
There is provided a voltage control circuit that is applicable to a LED device, a power supply or the like. The voltage control circuit includes: a voltage dividing unit dividing a supply voltage into a first voltage and a second voltage different from each other; a shunt regulator adjusting the first voltage according to the second voltage; and an output circuit unit outputting the voltage regulated by the shunt regulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2012-0151458 filed on Dec. 21, 2012, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a voltage control circuit that may be applied to a LED device or a power supply. 
     Generally, in a LED device or a power supply, a detection voltage and a reference voltage are used in order to provide a desired driving voltage. 
     Although it is desired that such a detection voltage and a reference voltage are not affected by a change in temperature, devices used to detect the detection voltage and devices used to generate the reference voltage have characteristics that vary, depending on a change in temperature. 
     In order to provide a voltage that is constant regardless of a change in temperature, typical voltage control circuits employ a temperature compensating device such as a thermistor and a semiconductor switch such as a transistor. 
     By using such typical voltage control circuits, the reference voltage for current feedback may be varied. 
     In such typical voltage control circuits, however, the threshold voltage of the transistor used as the semiconductor switch may be varied according to a change in temperature and thus the comparison function of the transistor may be deteriorated. Therefore, it is difficult to precisely control the voltage if temperature changes. 
     Patent Document 1 referenced below relates to a temperature compensation circuit in a modulator, but does not disclose any technical feature to overcome the problem that the voltage control function deteriorates due to temperature characteristics of a transistor. 
     RELATED ART DOCUMENT 
     
         
         (Patent Document 1) Japanese Patent Laid-Open Publication No. 1994-045965 
       
    
     SUMMARY 
     An aspect of the present disclosure may provide a voltage control circuit with a temperature compensation function allowing for greater precision in voltage controlling, and thus, the problem in which a voltage control function deteriorates due to temperature characteristics of a transistor may be overcome. 
     According to an aspect of the present disclosure, a voltage control circuit may include: a voltage dividing unit dividing a supply voltage into a first voltage and a second voltage different from each other; a shunt regulator adjusting the first voltage according to the second voltage; and an output circuit unit outputting the voltage regulated by the shunt regulator. 
     The voltage dividing unit may include first to third resistors connected in series between a supply voltage terminal and a ground, outputs the first voltage from a first connection node between the first resistor and the second resistor, and outputs the second voltage from a second connection node between the second resistor and the third resistor, wherein the second resistor is a NTC thermistor having an impedance characteristic inversely proportional to temperature. 
     The shunt regulator may include: a comparator connected to the second connection node to compare the second voltage with a threshold voltage and providing a comparison voltage; and a transistor connected between the second connection node and the ground and adjusting the second voltage according to the comparison voltage. 
     The output circuit unit may include a resistor and a capacitor connected in series between the supply voltage terminal and the ground, wherein a connection node between the resistor and the capacitor is connected to the first connection node and an output terminal. 
     The voltage control circuit may further include a voltage adjusting unit adjusting an output voltage from the output terminal according to a dimming voltage to provide a reference voltage. 
     The voltage adjusting unit may include: first to fifth output resistors connected in series between the supply voltage terminal and the ground; and a zener diode connected between the ground and a connection node between the second output resistor and the third output resistor, wherein the voltage adjusting unit receives the dimming voltage in the connection node between the second output resistor and the third output resistor and provides the reference voltage to a connection node between the third output resistor and the fourth output resistor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other aspects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a voltage control circuit according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is a block diagram illustrating the shunt regulator according to the exemplary embodiment of the present disclosure in detail; 
         FIG. 3  is an equivalent circuit diagram of a voltage control circuit according to the exemplary embodiment of the present disclosure; 
         FIG. 4  is another equivalent circuit diagram of a voltage control circuit according to the exemplary embodiment of the present disclosure; and 
         FIG. 5  is a block diagram of a variant of the voltage control circuit according to the exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Throughout the drawings, the same or like reference numerals will be used to designate the same or like elements. 
       FIG. 1  is a block diagram of a voltage control circuit according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 1 , the voltage control circuit according to the exemplary embodiment of the present disclosure may include a voltage dividing unit  100 , a shunt regulator  200 , and an output circuit unit  300 . 
     The voltage dividing unit  100  may divide a predetermined supply voltage V dd  into a first voltage V 1  and a second voltage V 2  different from each other. 
     As an exemplary implementation, the voltage dividing unit  100  may include a first resistor R 11 , a second resistor R 12 , and a third resistor R 13  connected in series between the supply voltage V DD  terminal and a ground. Here, the second resistor R 12  may be a NTC thermistor having impedance inversely proportional to temperature. 
     In this case, a first voltage V 1  may be output at a first connection node N 1  between the first resistor R 11  and the second resistor R 12 , and a second voltage V 2  may be output at a second connection node N 2  between the second resistor R 12  and the third resistor R 13 . 
     For example, if the supply voltage V dd  is 12 V, the first voltage V 1  and the second voltage V 2  may be 5V and 2.5V, respectively. 
     The shunt regulator  200  may adjust the first voltage V 1  according to the second voltage V 2 . Here, the shunt regulator  200  may be a TL431 IC, which has an inner threshold voltage that does not substantially vary with temperature, so that it exhibits significantly lower variation errors than the turn on voltage of an existing transistor. For example, the TL431 IC has an inner threshold voltage that ranges between 2.4825V (min) and 2.5075V (max), typically 2.495V, and has an variation error of about ±13 mV. 
     For example, if the second voltage V 2  is lower than a threshold voltage set in the shunt regulator  200 , the first voltage V 1  is provided to an output terminal as it is, and if the second voltage V 2  is higher than the threshold voltage set in the shunt regulator  200 , the first voltage V 1  may be adjusted to a lower voltage. 
     The output circuit unit  300  may output the voltage regulated by the shunt regulator  200 . 
     For example, the output circuit unit  300  may stabilize the voltage regulated by the shunt regulator  200  to output it. 
       FIG. 2  is a block diagram illustrating the shunt regulator according to the exemplary embodiment of the present disclosure in detail. 
     Referring to  FIG. 2 , the shunt regulator  200  may include a comparator OPA 1  and a transistor Q 1 . 
     The comparator OPA 1  may be connected to the second connection node N 2  and may compare the second voltage V 2  with a threshold voltage V th  to provide a comparison voltage V com . 
     The transistor Q 1  may be connected between the second connection node N 2  and the ground and may adjust the second voltage V 2  according to the comparison voltage V com . 
     Referring to  FIGS. 1 and 2 , for example, the output circuit unit  300  may include a resistor R 31  and a capacitor C 31  connected in series between the supply voltage V dd  terminal and the ground. 
     The voltage provided through the shunt regulator  200  is charged by the capacitor C 31  to be stabilized, and the stabilized voltage is output through an output terminal OUT. 
     Equivalent circuits of the voltage control circuit according to the exemplary embodiment of the present disclosure may be represented as shown in  FIGS. 3 and 4  depending on the operation of the shunt regulator  200 . 
     For example, the comparator OPA 1  may output the comparison voltage V com  of a high level if the second voltage V 2  is equal to or higher than the threshold voltage V th . For example, the ambient temperature becomes higher and accordingly, the resistance value of the second resistor R 12  becomes lower, and then the second voltage V 2  across the third resistor R 13  may be increased. As an example for the case, the threshold voltage V th  may be 2.5V and the second voltage V 2  may be 2.6V. 
     The transistor Q 1  may be turned on when the comparison voltage V com  has a high level. In this case, the voltage control circuit according to the exemplary embodiment of the present disclosure may be represented by the equivalent circuit shown in  FIG. 3 . 
     In contrast, the comparator OPA 1  may output the comparison voltage V com  of a low level if the second voltage V 2  is lower than the threshold voltage V th . As an example for the case, the threshold voltage V th  may be 2.5V and the second voltage V 2  may be 2.4V. The transistor Q 1  may be turned off when the comparison voltage V com  has a low level. In this case, the voltage control circuit according to the exemplary embodiment of the present disclosure may be represented by the equivalent circuit shown in  FIG. 4 . 
       FIG. 3  is a first equivalent circuit diagram of the voltage control circuit according to the exemplary embodiment of the present disclosure, and  FIG. 4  is a second equivalent circuit diagram of the voltage control circuit according to the exemplary embodiment of the present disclosure. 
     At first, referring to  FIG. 3 , since the transistor Q 1  is turned on, the first voltage V 1  at the first connection node N 1  may be determined by the shunt regulator  200  according to Equation 1. 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   = 
                   
                     Vth 
                     * 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             12 
                           
                           
                             R 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             13 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     For example, assuming that the supply voltage V dd  is 12V, the resistance of the second resistor R 12 , which is a NTC thermistor, is 430 KΩ at 25° C., the first and third resistors R 11  and R 13  are 75Ω and 51Ω, respectively, and the threshold voltage V th  is changed from 1.7V to 2.5V, the first voltage V 1  is initially 11.5V (=1.70792079*(1+430/75)). 
     Then, if the resistance of the second resistor R 12  is gradually decreased as temperature becomes higher, the first voltage V 1  has the values shown in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 R12 
                   
                   
                   
               
               
                 R11 
                 (NTC) 
                 R13 
                 V th   
                 V1 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 51Ω 
                 430Ω 
                 75Ω 
                 1.70792079  
                 V 
                 11.5  
                 V 
               
             
          
           
               
                 51Ω 
                 300Ω 
                 75Ω 
                 2.3  
                 V 
                 11.5 
               
             
          
           
               
                 51Ω 
                 200Ω 
                 75Ω 
                 2.5  
                 V 
                 9.17 
                 V 
               
               
                 51Ω 
                 120Ω 
                 75Ω 
                 2.5  
                 V 
                 6.5  
                 V 
               
               
                 51Ω 
                  75Ω 
                 75Ω 
                 2.5  
                 V 
                 5.0  
                 V 
               
               
                 51Ω 
                  30Ω 
                 75Ω 
                 2.5  
                 V 
                 3.5  
                 V 
               
               
                 51Ω 
                  10Ω 
                 75Ω 
                 2.5  
                 V 
                 2.83  
                 V 
               
             
          
           
               
                 51Ω 
                  1Ω 
                 75Ω 
                 2.5  
                 V 
                 2.53 
               
             
          
           
               
                 51Ω 
                  0Ω 
                 75Ω 
                 2.5  
                 V 
                 2.5  
                 V 
               
               
                   
               
             
          
         
       
     
     Referring to Table 1, the first voltage V 1  is gradually decreased, so does an output current. 
     Referring to  FIG. 4 , since the transistor Q 1  is turned off, the first voltage V 1  at the first connection node N 1  may be determined by the ratio of the resistance between the sum of resistance values of the second resistor R 12  and the third resistor R 13  and the first resistance R 11  according to Equation 2. 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     = 
                     
                       Vdd 
                       * 
                       
                         ( 
                         
                           Ra 
                           
                             
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               11 
                             
                             + 
                             
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               12 
                             
                             + 
                             
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               13 
                             
                           
                         
                         ) 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     Ra 
                     = 
                     
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         12 
                       
                       + 
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         12 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     Rt 
                     = 
                     
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         11 
                       
                       + 
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         12 
                       
                       + 
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         13 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     Then, if the resistance of the second resistor R 12  is gradually decreased as temperature becomes higher, the first voltage V 1  has the values shown in Table 2. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 R12 
                   
                 Ra (= R12 +  
                 Rt (= R11 +  
                   
               
               
                 R11 
                 (NTC) 
                 R13 
                 R13) 
                 R12 + R13) 
                 V1 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 51Ω 
                 430Ω 
                 75Ω 
                 505 
                 556 
                 10.89  
                 V 
               
             
          
           
               
                 51Ω 
                 300Ω 
                 75Ω 
                 375 
                 426 
                 11.5 
               
             
          
           
               
                 51Ω 
                 200Ω 
                 75Ω 
                 275 
                 326 
                 9.17  
                 V 
               
               
                 51Ω 
                 120Ω 
                 75Ω 
                 195 
                 246 
                 6.5  
                 V 
               
               
                 51Ω 
                  75Ω 
                 75Ω 
                 150 
                 201 
                 5.0  
                 V 
               
               
                 51Ω 
                  30Ω 
                 75Ω 
                 105 
                 156 
                 3.5  
                 V 
               
               
                 51Ω 
                  10Ω 
                 75Ω 
                 85 
                 136 
                 2.83  
                 V 
               
             
          
           
               
                 51Ω 
                  1Ω 
                 75Ω 
                 76 
                 127 
                 2.53 
               
             
          
           
               
                 51Ω 
                  0Ω 
                 75Ω 
                 75 
                 126 
                 2.5  
                 V 
               
               
                   
               
             
          
         
       
     
     Referring to Table 2, the first voltage V 1  is gradually decreased, so does an output current. 
     According to the foregoing description, the voltage control circuit according to the exemplary embodiment may decrease the first voltage V 1  if the second voltage V 2  corresponding to temperature change is equal to or higher than the reference voltage Vref of the shunt regulator  200  to thereby decrease the current by the first voltage V 1 . 
     Further, the voltage control circuit according to the present disclosure may further include a voltage adjusting unit  400 . The voltage adjusting unit  400  may adjust the output voltage V out  from the output terminal OUT according to a dimming voltage V dim  to provide a reference voltage Vref. 
       FIG. 5  is a block diagram of a variant of the voltage control circuit according to the exemplary embodiment of the present disclosure. Referring to  FIG. 5 , the voltage adjusting unit  400  may include first to fifth output resisters R 41  to R 45  connected in series between the supply voltage V dd  terminal and the ground, and a zener diode ZD 41  connected between the connection node, which is between the second output resistor R 42  and the third output resistor R 43 , and the ground. 
     Here, at the connection node between the second output resistor R 42  and the third output resistor R 43 , the dimming voltage V dim  may be received, and at the connection node between the third output resistor R 43  and the fourth output resistor R 44 , the reference voltage Vref may be provided. 
     When the dimming voltage V dim  is adjusted, the voltage at the connection node between the second output resistor R 42  and the third output resistor R 43  at which the dimming voltage V dim  is received in the ground is forcibly varied, and the reference voltage at Vref provided at the connection node between the third output resistor R 43  and the fourth output resistor R 44  may be varied. 
     The zener diode ZD 41  is operated if the output voltage V out  of the output terminal OUT is abnormally increased beyond a predetermined voltage or a very high voltage is applied from the outside, so that it may prevent the output voltage Vout becomes overvoltage and may protect internal circuits. 
     As set forth above, according to exemplary embodiments of the present disclosure, the problem that the voltage control function deteriorates due to temperature characteristics of a transistor is overcome, so that a voltage can be controlled more precisely while the temperature compensation function is still performed. 
     While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the spirit and scope of the present disclosure as defined by the appended claims.