Patent Publication Number: US-9411351-B2

Title: DC-to-DC converter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-118911, filed Jun. 9, 2014, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment relates to a direct current (DC) to direct current (DC) conversion. 
     BACKGROUND 
     DC-to-DC converters convert an DC input voltage to a DC output voltage of a different amount. The DC output voltage is compared with a reference voltage to perform switching control based on the comparison results. This stabilizes the DC output voltage at a desired level. In general, a reference voltage is generated by a reference voltage source. The reference voltage source is typically driven by the DC output voltage. 
     For example, the technology can apply to charging a capacitor, a secondary battery or the like by applying to a DC-to-DC converter a minute voltage generated by a solar battery or thermo-element as a DC input voltage, and converting the DC input voltage to a DC output voltage by the DC-to-DC converter. However, the DC-to-DC converters may fail to apply a driving voltage with a level sufficient for the reference voltage source to initiate an operation. In this case, the operation of DC-to-DC converters may be unstable since they cannot use a required reference voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a DC-to-DC converter according to the first embodiment. 
         FIG. 2  is a graph illustrating a voltage of each node shown in  FIG. 1 . 
         FIG. 3  is a block diagram illustrating a DC-to-DC converter according to the second embodiment. 
         FIG. 4  is a block diagram illustrating a DC-to-DC converter according to the third embodiment. 
         FIG. 5  is a block diagram illustrating a DC-to-DC converter according to the fourth embodiment. 
         FIG. 6  is a graph illustrating a voltage of each node shown in  FIG. 5 . 
         FIG. 7  is a graph illustrating a voltage of each node shown in  FIG. 5 . 
         FIG. 8  is a time-chart illustrating input and output signals of a switch control circuit shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described hereinafter with reference to drawings. 
     According to an embodiment, a direct current to direct current (DC-to-DC) converter includes a power stage, a generating circuit, a voltage dividing circuit, a subtractor and a controller. The power stage converts an input voltage to a first output voltage. The generating circuit generates a reference voltage by selecting one of candidate voltages in accordance with a level of the first output voltage. The voltage dividing circuit divides the first output voltage to obtain a second output voltage. The subtractor calculates a differential voltage between the reference voltage and the second output voltage. The control circuit generates a control signal to control the level of the first output voltage based on the differential voltage. 
     In the drawings, the same constituent elements are denoted by the same respective reference numbers. Redundant explanation will be avoided. 
     First Embodiment 
     As shown in  FIG. 1 , a DC-to-DC converter according to the first embodiment includes a power stage  110 , a voltage dividing circuit  130 , a reference voltage generating circuit  140 , a subtractor  150 , and a switching control circuit  160 . The DC-to-DC converter converts an input voltage (V in ) applied by a voltage source  100  to an output voltage (V out ), and applies the output voltage (V out ) to a load  120 . The DC-to-DC converter shown in  FIG. 1  is for increasing the voltage (i.e., V in &lt;V out ), but may be used for decreasing the voltage (i.e., V in &gt;V out ). 
     The voltage source  100  includes a positive terminal and a negative terminal. The positive terminal of the voltage source  100  is connected to an input terminal of the DC-to-DC converter (i.e., an input terminal of the power stage  110 ). The negative terminal of the voltage source  100  is grounded. The voltage source  100  generates an input voltage (V in ), and applies the input voltage (V in ) to the DC-to-DC converter. 
     The load  120  may be any load. The load  120  is connected to an output terminal of the DC-to-DC converter (i.e., an output terminal of the power stage  110 ). The load  120  may be a secondary battery or an electronic device. 
     The power stage  110  includes an input terminal, a control terminal and an output terminal. The input terminal of the power stage  110  functions as an input terminal of the DC-to-DC converter, and is connected to the positive terminal of the voltage source  100 . The control terminal of the power stage  110  is connected to an output terminal of the switching control circuit  160 . The output terminal of the power stage  110  functions as an output terminal of the DC-to-DC converter, and is connected in common to the load  120 , an input terminal of the voltage dividing circuit  130 , and a control terminal of the reference voltage generating circuit  140 . The power stage  110  converts an input voltage (V in ) to an output voltage (V out ). 
     Specifically, the power stage  110  includes an inductor  111  (L), a switch  112  (SW 1 ), a diode  113  (D 1 ) and a capacitor  114  (C). 
     The inductor  111  (L) includes a first terminal and a second terminal. The first terminal of the inductor  111  (L) functions as an input terminal of the power stage  110 . The second terminal of the inductor  111  (L) is connected to the switch  112  (SW 1 ) and an anode of the diode  113  (D 1 ). 
     The inductor  111  (L) stores magnetic energy by a current flowing through the inductor  111  (L) while the switch  112  (SW 1 ) is in an ON state. The inductor  111  (L) discharges the stored magnetic energy as an electric energy while the switch  112  (SW 1 ) is in an OFF state. 
     The switch  112  (SW 1 ) includes a control terminal which functions as a control terminal of the power stage  110 . The switch  112  (SW 1 ) receives a switch control signal through the control terminal from the switching control circuit  160 , and performs ON/OFF operation in response to the switch control signal. Specifically, the switch  112  (SW 1 ) short-circuits or opens between the second terminal of the inductor  111  (L), the anode of the diode  113  (D 1 ), and the ground. 
     The diode  113  (D 1 ) includes an anode and a cathode. The anode of the diode  113  (D 1 ) is connected to both of the second terminal of the inductor  111  (L) and the switch  112  (SW 1 ). The cathode of the diode  113  (D 1 ) is connected to a first terminal of the capacitor  114  (C). 
     The diode  113  (D 1 ) allows a current supplied from the inductor  111  to flow into the capacitor  114  (C) after the switch  112  (SW 1 ) is switched to an OFF state from an ON state. The diode  113  (D 1 ) prevents a current from flowing back to the voltage source  100  from the capacitor  114  (C). 
     The capacitor  114  (C) includes a first terminal and a second terminal. The first terminal of the capacitor  114  (C) functions as the output terminal of the power stage  110 , and is connected in common to the cathode of the diode  113  (D 1 ), the load  120 , the input terminal of the voltage dividing circuit  130 , and the control terminal of the reference voltage generating circuit  140 . The second terminal of the capacitor  114  (C) is grounded. The capacitor  114  (C) is charged by a current supplied from the diode  113  (D 1 ) after the switch  112  (SW 1 ) is switched to the OFF state from the ON state. Accordingly, the voltage (V out ) of the first terminal of the capacitor  114  (C) increase. On the other hand, if current supply from the diode  113  (D 1 ) is stopped, the capacitor  114  (C) discharges. Accordingly, the voltage (V out ) of the first terminal of the capacitor  114  (C) decreases. 
     The voltage dividing circuit  130  includes an input terminal and an output terminal. The input terminal of the voltage dividing circuit  130  is connected in common to the output terminal of the power stage  110 , the load  120 , and the control terminal of the reference voltage generating circuit  140 . The output terminal of the voltage dividing circuit  130  is connected to an inverted input terminal of the subtractor  150 . 
     The voltage dividing circuit  130  divides the input voltage (V out ) by a predetermined voltage division ratio (N) to obtain an output voltage (V div ). The voltage dividing circuit  130  applies the output voltage (V div ) to the inverted input terminal of the subtractor  150 . If the voltage division ratio (N) is 1, the voltage dividing circuit  130  can be omitted. The following equation (1) is given for the input voltage (V out ) and the output voltage (V div ) of the voltage dividing circuit  130 :
 
 V   div   =V   out   /N    (1)
 
     The reference voltage generating circuit  140  includes a control terminal and an output terminal. The control terminal of the reference voltage generating circuit  140  is connected in common to the output terminal of the power stage  110 , the load  120 , and the input terminal of the voltage dividing circuit  130 . The output terminal of the reference voltage generating circuit  140  is connected to a non-inverted input terminal of the subtractor  150 . 
     The reference voltage generating circuit  140  generates a reference voltage (V ref ) in accordance with the voltage to be applied to the control terminal. The reference voltage generating circuit  140  applies the reference voltage (V ref ) to the non-inverted input terminal of the subtractor  150 . 
     The subtractor  150  includes the non-inverted input terminal, inverted input terminal, and output terminal. The non-inverted input terminal of the subtractor  150  is connected to the output terminal of the reference voltage generating circuit  140 . The inverted input terminal of the subtractor  150  is connected to the output terminal of the voltage dividing circuit  130 . The output terminal of the subtractor  150  is connected to the input terminal of the switching control circuit  160 . 
     The subtractor  150  subtracts the voltage of the inverted input terminal (V div ) from the voltage of the non-inverted input terminal (V ref ). The subtractor  150  outputs a difference signal indicating a sign (positive or negative) of a differential voltage to the switching control circuit  160 . 
     The switching control circuit  160  includes the input terminal and the output terminal. The input terminal of the switching control circuit  160  is connected to the output terminal of the subtractor  150 . The output terminal of the switching control circuit  160  is connected to the control terminal of the power stage  110 . 
     The switching control circuit  160  receives a difference signal from the subtractor  150 . The switching control circuit  160  generates a switching control signal based on the difference signal. The switching control circuit  160  outputs the switching control signal to the power stage  110 . Specifically, if the difference signal represents a positive sign (i.e., V ref ≧V div ), the switching control circuit  160  generates a switching control signal to periodically switch ON and OFF states of the switch  112  (SW 1 ). The output voltage (V out ) of the power stage  110  increases in accordance with the switching control signal. On the other hand, if the difference signal represents a negative sign (i.e., V ref &lt;V div ), the switching control circuit  160  generates a switching control signal so that the switch  112  (SW 1 ) remains in the OFF state. The output voltage (V out ) of the power stage  110  consequently decreases in accordance with the switching control signal. 
     In other words, negative feedback control is performed to satisfy the following equation (2):
 
V div =V ref    (2)
 
     The equation (2) can be rewritten as the following equation (3) regarding the output voltage (V out ) of the power stage  110 :
 
 V   out   =N·V   ref    (3)
 
     The reference voltage generating circuit  140  selects a first candidate voltage (V ref1 ) as the reference voltage (V ref ) in a first phase where the output voltage (V out ) of the power stage  110  is lower than a first threshold voltage (V th ). On the other hand, the reference voltage generating circuit  140  selects a second candidate voltage (V ref2 ) as the reference voltage (V ref ) in a second phase where the output voltage (V out ) of the power stage  110  is equal to or greater than the first threshold voltage (V th ). 
     That is, the reference voltage (V ref ) is given as follows: 
     
       
         
           
             
               
                 
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     In  FIG. 1 , the reference voltage generating circuit  140  includes a reference voltage source  141 , a reference voltage source  142  and a switch  143  (SW 2 ). 
     The reference voltage source  141  is driven by the output voltage (V out ) of the power stage  110 , and generates the first candidate voltage (V ref1 ). The first candidate voltage (V ref1 ) is set to be equal to or greater than the output voltage (V div ) of the voltage dividing circuit  130  in the first phase. For example, the first candidate voltage (V ref1 ) may be the output voltage (V out ) of the power stage  110  itself or be generated by dividing the output voltage (V out ) by a voltage division ratio smaller than N. 
     The reference voltage source  142  is driven by the output voltage (V out ) of the power stage  110 , and generates the second candidate voltage (V ref2 ). For example, the reference voltage source  142  may be implemented by using a band gap reference (BGR) circuit. The second candidate voltage (V ref2 ) has a stable level regardless of change in the output voltage (V out ) of the power stage  110  in the second phase. In other words, the first threshold voltage (V th ) is set to be a level equal to or greater than the lower limit of a driving voltage so that the reference voltage source  142  operates normally (i.e., the level of the second candidate voltage (V ref2 ) is stable). 
     The switch  143  (SW 2 ) selects one of the first candidate voltage (V ref1 ) and the second candidate voltage (V ref2 ) as the reference voltage (V ref ) depending on the level of the output voltage (V out ) of the power stage  110 . Specifically, the switch  143  (SW 2 ) selects the first candidate voltage (V ref1 ) if the output voltage (V out ) of the power stage  110  is lower than the first threshold voltage (V th ). On the other hand, the switch  143  (SW 2 ) selects the second candidate voltage (V ref2 ) if the output voltage (V out ) of the power stage  110  is equal to or greater than the first threshold voltage (V th ). 
       FIG. 2  shows the relation between the output voltage (V out ) of the power stage  110  and the reference voltage (V ref ). When the output voltage (V out ) of the power stage  110  is greatly lower than the first threshold voltage (V th ), the reference voltage source  142  cannot operate normally, and the level of the second candidate voltage (V ref2 ) is extremely low. If the second candidate voltage (V ref2 ) the level of which is extremely low is used as the reference voltage (V ref ), the condition of V ref ≦V div  is always true, and the output voltage (V out ) of the power stage  110  cannot increase to a desired level. 
     Thus, as stated above, the reference voltage generating circuit  140  selects the first candidate voltage (V ref1 ) as the reference voltage (V ref ) in the first phase where the output voltage (V out ) of the power stage  110  is lower than the first threshold voltage (V th ). The first candidate voltage (V ref1 ) is set to be equal to or greater than the output voltage (V div ) of the voltage dividing circuit  130  in the first phase. In this case, if the first candidate voltage (V ref1 ) is used as the reference voltage (V ref ), the output voltage (V out ) of the power stage  110  can increase to a level sufficient for the reference voltage source  142  to operate normally. 
     In addition, as stated above, the second candidate voltage (V ref2 ) is selected as the reference voltage (V ref ) in the second phase where the output voltage (V out ) of the power stage  110  is equal to or greater than the first threshold voltage (V th ). The second candidate voltage (V ref2 ) has a stable level regardless of change in the output voltage (V out ) of the power stage  110  in the second phase. Accordingly, output voltage (V out ) of the power stage  110  can be stabilized through the negative feedback control. 
     As stated above, the DC-to-DC converter according to the first embodiment adaptively selects a reference voltage from among a plurality of candidate voltages having different characteristics. This DC-to-DC converter ensures stable operation even if the level of DC output voltage is minute. 
     Second Embodiment 
     As shown in  FIG. 3 , a DC-to-DC converter according to the second embodiment includes the power stage  110 , the voltage dividing circuit  130 , a reference voltage generating circuit  240 , the subtractor  150 , and the switching control circuit  160 . The DC-to-DC converter converts an input voltage (V in ) applied by a voltage source  100  to an output voltage (V out ), and applies the output voltage (V out ) to the load  120 . 
     The voltage source  100 , power stage  110 , load  120 , voltage dividing circuit  130 , subtractor  150  and switching control circuit  160  shown in  FIG. 3  may be the same as or similar to the voltage source  100 , power stage  110 , load  120 , voltage dividing circuit  130 , subtractor  150  and switching control circuit  160  shown in  FIG. 1 . The DC-to-DC converter shown in  FIG. 3  is for increasing the voltage (i.e., V in &lt;V out ), but may be used for decreasing the voltage (i.e., V in &gt;V out ). 
     The reference voltage generating circuit  240  includes a control terminal and an output terminal. The control terminal of the reference voltage generating circuit  240  is connected in common to the output terminal of the power stage  110 , the load  120 , and the input terminal of the voltage dividing circuit  130 . The output terminal of the reference voltage generating circuit  240  is connected to the non-inverted input terminal of the subtractor  150 . 
     The reference voltage generating circuit  240  generates a reference voltage (V ref ) in accordance with the voltage to be applied to the control terminal. The reference voltage generating circuit  240  applies the reference voltage (V ref ) to the non-inverted input terminal of the subtractor  150 . 
     The reference voltage generating circuit  240  selects a first candidate voltage (V ref1 ) as the reference voltage (V ref ) in the aforementioned first phase. On the other hand, the reference voltage generating circuit  240  selects a second candidate voltage (V ref2 ) as the reference voltage (V ref ) in the aforementioned second phase. 
     In  FIG. 3 , the reference voltage generating circuit  240  includes the reference voltage source  141 , the reference voltage source  142 , the switch  143  (SW 2 ), an inverter  244 , a switch  245  (SW 3 ) and a switch  246  (SW 4 ). The reference voltage source  141 , reference voltage source  142 , and switch  143  (SW 2 ) may be the same as or similar to the reference voltage source  141 , reference voltage source  142  and switch  143  (SW 2 ) shown in  FIG. 1 . 
     The inverter  244  includes an input terminal and an output terminal. The input terminal of the inverter  244  is connected to a control terminal of the reference voltage generating circuit  240 . The output terminal of the inverter  244  is connected to a control terminal of the switch  245  (SW 3 ). The inverter  244  performs logical inversion on the output voltage (V out ) of the power stage  110 . 
     The switch  245  (SW 3 ) includes the control terminal. The output voltage of the inverter  244  is applied to the control terminal of the switch  245  (SW 3 ), and the switch  245  (SW 3 ) is switched between the ON and OFF states in accordance with the output voltage. Specifically, the switch  245  (SW 3 ) short-circuits between the control terminal of the reference voltage generating circuit  240  and the reference voltage source  141  in the first phase. On the other hand, the switch  245  (SW 3 ) opens between the control terminal of the reference voltage generating circuit  240  and the reference voltage source  141  in the second phase. That is, the switch  245  (SW 3 ) stops supply of a driving voltage to the reference voltage source  141  while the first candidate voltage (V ref1 ) is not selected as the reference voltage (V ref ). 
     The switch  246  (SW 4 ) includes a control terminal. The output voltage (V out ) of the power stage  110  is applied to the control terminal of the switch  246  (SW 4 ), and the switch  246  (SW 4 ) is switched between the ON and OFF states in accordance with the output voltage (V out ). Specifically, the switch  246  (SW 4 ) opens between the control terminal of the reference voltage generating circuit  240  and the reference voltage source  142  in the first phase. That is, the switch  246  (SW 4 ) stops supply of a driving voltage to the reference voltage source  142  while the second candidate voltage (V ref2 ) is not selected as the reference voltage (V ref ). On the other hand, the switch  246  (SW 4 ) short-circuits between the control terminal of the reference voltage generating circuit  240  and the reference voltage source  142  in the second phase. 
     As stated above, the DC-to-DC converter according to the second embodiment stops supply of a driving voltage to a specific candidate voltage while the specific candidate voltage is not selected as a reference voltage. Accordingly, this DC-to-DC converter reduces unnecessary current consumption in the reference voltage source to generate candidate voltages. That is, this DC-to-DC converter reduces current consumption and achieves high speed charging for a capacitor included in the power stage. 
     Third Embodiment 
     As shown in  FIG. 4 , a DC-to-DC converter according to the third embodiment includes the power stage  110 , the voltage dividing circuit  130 , a reference voltage generating circuit  340 , the subtractor  150 , and the switching control circuit  160 . The DC-to-DC converter converts an input voltage (V in ) applied by a voltage source  100  to an output voltage (V out ), and applies the output voltage (V out ) to the load  120 . 
     The voltage source  100 , power stage  110 , load  120 , voltage dividing circuit  130 , subtractor  150  and switching control circuit  160  shown in  FIG. 4  may be the same as or similar to the voltage source  100 , power stage  110 , load  120 , voltage dividing circuit  130 , subtractor  150  and switching control circuit  160  shown in  FIG. 3 . The DC-to-DC converter shown in  FIG. 4  is for increasing the voltage (i.e., V in &lt;V out ), but may be used for decreasing the voltage (i.e., V in &gt;V out ). 
     The reference voltage generating circuit  340  includes a control terminal and an output terminal. The control terminal of the reference voltage generating circuit  340  is connected in common to the output terminal of the power stage  110 , the load  120 , and the input terminal of the voltage dividing circuit  130 . The output terminal of the reference voltage generating circuit  340  is connected to the non-inverted input terminal of the subtractor  150 . 
     The reference voltage generating circuit  340  generates a reference voltage (V ref ) in accordance with the voltage to be applied to the control terminal. The reference voltage generating circuit  340  applies the reference voltage (V ref ) to the non-inverted input terminal of the subtractor  150 . 
     The reference voltage generating circuit  340  selects a first candidate voltage (V ref1 ) as the reference voltage (V ref ) in the aforementioned first phase. The first candidate voltage (V ref1 ) is the output voltage (V out ) of the power stage  110 . On the other hand, the reference voltage generating circuit  340  selects the second candidate voltage (V ref2 ) as the reference voltage (V ref ) in the aforementioned second phase. 
     In  FIG. 4 , the reference voltage generating circuit  340  includes the reference voltage source  142 , the switch  143  (SW 2 ) and the switch  246  (SW 4 ). The reference voltage source  142 , switch  143  (SW 2 ) and switch  246  (SW 4 ) may be the same as or similar to the reference voltage source  142 , switch  143  (SW 2 ) and switch  246  (SW 4 ) shown in  FIG. 3 . Since the reference voltage generating circuit  340  uses the output voltage (V out ) of the power stage  110  as the first candidate voltage (V ref1 ), an element having a function corresponding to the reference voltage source  141  is not necessary. Since the condition where V ref1 =V out &gt;V div  is always true, the first candidate voltage (V ref1 ) is greater than the output voltage (V div ) of the voltage dividing circuit  130  in the first phase. 
     As stated above, the DC-to-DC converter according to the third embodiment uses the output voltage of the DC-to-DC converter as a specific candidate voltage. This DC-to-DC converter eliminates the need for providing a reference voltage source to generate the specific candidate voltage, thereby achieving simplification of the configuration and reduction of current consumption. 
     Fourth Embodiment 
     As shown in  FIG. 5 , a DC-to-DC converter according to the fourth embodiment includes the power stage  110 , a voltage dividing circuit  430 , a voltage dividing circuit  430 , a reference voltage generating circuit  440 , a subtractor  450 , and a switching control circuit  460 . The DC-to-DC converter converts an input voltage (V in ) applied by the voltage source  100  to an output voltage (V out ), and applies the output voltage (V out ) to the load  120 . 
     The voltage source  100 , power stage  110 , and load  120  shown in  FIG. 5  may be the same as or similar to the voltage source  100 , power stage  110 , and load  120  shown in  FIG. 4 . The DC-to-DC converter shown in  FIG. 5  is for increasing the voltage (i.e., V in &lt;V out ), but may be used for decreasing the voltage (i.e., V in &gt;V out ). 
     The voltage dividing circuit  430  includes an input terminal and an output terminal. The input terminal of the voltage dividing circuit  430  is connected in common to the output terminal of the power stage  110 , the load  120 , and a first control terminal of the reference voltage generating circuit  440 . The output terminal of the voltage dividing circuit  430  is connected to both of a second control terminal of the reference voltage generating circuit  440  and an inverted input terminal of the subtractor  450 . 
     The voltage dividing circuit  430  divides the input voltage (V out ) by a predetermined voltage division ratio (N) to obtain an output voltage (V div ). The voltage dividing circuit  430  applies the output voltage (V div ) to the second control terminal of the reference voltage generating circuit  440  and the inverted input terminal of the subtractor  450 . 
     Specifically, the voltage dividing circuit  430  includes a resistor  431  (R 1 ) and a resistor  432  (R 2 ). 
     A first terminal of the resistor  431  (R 1 ) functions as the output terminal of the voltage dividing circuit  430  and is connected to a second terminal of the resistor  431  (R 1 ). The second terminal of the resistor  431  (RD is grounded. 
     The first terminal of the resistor  432  (R 2 ) functions as the input terminal of the voltage dividing circuit  430 . The second terminal of the resistor  432  (R 2 ) functions as the output terminal of the voltage dividing circuit  430  and is connected to the first terminal of the resistor  432  (R 2 ). That is, the resistor  431  (R 1 ) and the resistor  432  (R 2 ) are connected in series. 
     The voltage division ratio (N) of the voltage dividing circuit  430  depends on a resistance (R 1 ) of the resistor  431  and a resistance (R 2 ) of the resistor  432 , as represented by the following equation (5): 
     
       
         
           
             
               
                 
                   
                     1 
                     N 
                   
                   = 
                   
                     
                       R 
                       1 
                     
                     
                       
                         R 
                         1 
                       
                       + 
                       
                         R 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     The reference voltage generating circuit  440  includes the first control terminal, the second control terminal and an output terminal. The first control terminal of the reference voltage generating circuit  440  is connected in common to the output terminal of the power stage  110 , the load  120 , and the input terminal of the voltage dividing circuit  430 . The second control terminal of the reference voltage generating circuit  440  is connected to the output terminal of the voltage dividing circuit  430 . The output terminal of the reference voltage generating circuit  440  is connected to a non-inverted input terminal of the subtractor  450 . 
     The reference voltage generating circuit  440  generates a reference voltage (V ref ) in accordance with the voltage (V div ) to be applied to the second control terminal. The reference voltage generating circuit  440  applies the reference voltage (V ref ) to the non-inverted input terminal of the subtractor  450 . 
     The reference voltage generating circuit  440  selects a first candidate voltage (V ref1 ) as the reference voltage (V ref )) in a first phase where the output voltage (V div ) of the voltage dividing circuit  430  is lower than a second threshold voltage (V d ). The first candidate voltage (V ref1 ) is the output voltage (V out ) of the power stage  110 . On the other hand, the reference voltage generating circuit  440  selects a second candidate voltage (V ref2 ) as the reference voltage (V ref ) in a second phase where the output voltage (V div ) of the voltage dividing circuit  430  is equal to or greater than the second threshold voltage (V d ). 
     In  FIG. 5 , the reference voltage generating circuit  440  includes a reference voltage source  442 , a switching circuit  443  (SW 2 ), switch  446  (SW 4 ), a resistor  481  (R 3 ), a diode  482  (D 2 ), a comparator  483 , and an inverter  484 . 
     The reference voltage source  442  is driven by the output voltage (V out ) of the power stage  110 , and generates the second candidate voltage (V ref2 ) while the switch  446  (SW 4 ) is in the ON state. Specifically, the reference voltage source  442  is implemented by using the BGR circuitry. The second candidate voltage (V ref2 ) has a stable level regardless of change in the output voltage (V out ) of the power stage  110  in the second phase. 
     The switching circuit  443  (SW 2 ) selects one of the first candidate voltage (V ref1 ) and the second candidate voltage (V ref2 ) as the reference voltage (V ref ) depending on the level of the output voltage (V out ) of the voltage dividing circuit  430 . Specifically, the switching circuit  443  (SW 2 ) selects the first candidate voltage (V ref1 ) if the output voltage (V div ) of the voltage dividing circuit  430  is lower than the second threshold voltage (V d ). On the other hand, the switching circuit  443  (SW 2 ) selects the second candidate voltage (V ref2 ) if the output voltage (V div ) of the voltage dividing circuit  430  is equal to or greater than the second threshold voltage (V d ). 
     The switching circuit  443  (SW 2 ) includes a switch  471  (SW 21 ) and a switch  472  (SW 22 ). 
     The switch  471  (SW 21 ) includes a control terminal. The output voltage of the inverter  484  is applied to the control terminal of the switch  471  (SW 21 ), and the switch  471  (SW 21 ) is switched between the ON and OFF states in accordance with the output voltage. Specifically, the switch  471  (SW 21 ) short-circuits between the first control terminal of the reference voltage generating circuit  440  and the output terminal of the reference voltage generating circuit  440  in the first phase. On the other hand, the switch  471  (SW 21 ) opens between the first control terminal of the reference voltage generating circuit  440  and the output terminal of the reference voltage generating circuit  440  in the second phase. That is, the switch  471  (SW 21 ) applies the first candidate voltage (V ref1 ) to the output terminal of the reference voltage generating circuit  440  as the reference voltage (V ref ) in the first phase. 
     The switch  472  (SW 22 ) includes a control terminal. An output signal of the comparator  483  is input to the control terminal of the switch  472  (SW 22 ), and the switch  472  (SW 22 ) is switched between the ON and OFF states in accordance with the output signal. Specifically, the switch  472  (SW 22 ) opens between the reference voltage source  442  and the output terminal of the reference voltage generating circuit  440  in the first phase. On the other hand, the switch  472  (SW 22 ) short-circuits between the reference voltage source  442  and the output terminal of the reference voltage generating circuit  440  in the second phase. That is, the switch  472  (SW 22 ) applies the second candidate voltage (V ref2 ) to the output terminal of the reference voltage generating circuit  440  as the reference voltage (V ref ) in the second phase. 
     The switch  446  (SW 4 ) includes a control terminal. An output signal of the comparator  483  is input to the control terminal of the switch  446  (SW 4 ), and the switch  446  (SW 4 ) is switched between the ON and OFF states in accordance with the output signal. Specifically, the switch  446  (SW 4 ) opens between the first control terminal of the reference voltage generating circuit  440  and the reference voltage source  442  in the first phase. That is, the switch  446  (SW 4 ) stops supply of a driving voltage to the reference voltage source  442  while the second candidate voltage (V ref2 ) is not selected as the reference voltage (V ref ). On the other hand, the switch  446  (SW 4 ) short-circuits between the first control terminal of the reference voltage generating circuit  440  and the reference voltage source  442  in the second phase. 
     The resistor  481  (R 3 ) includes a first terminal and a second terminal. The first terminal of the resistor  481  (R 3 ) is connected to the first control terminal of the reference voltage generating circuit  440 . The second terminal of the resistor  481  (R 3 ) is connected to both of an anode of the diode  482  (D 2 ) and an inverted input terminal of the comparator  483 . 
     The diode  482  (D 2 ) includes an anode and a cathode. The anode of the diode  482  (D 2 ) is connected to both of the second terminal of the resistor  481  (R 3 ) and the inverted input terminal of the comparator  483 . The cathode of the diode  482  (D 2 ) is grounded. That is, the resistor  481  (R 3 ) and the diode  482  (D 2 ) are connected in series. 
     The voltage of the second terminal of the resistor  481  (R 3 ) and that of the anode of the diode  482  (D 2 ) are used as the second threshold voltage (V d ). While the diode  482  (D 2 ) is in the OFF state, a current does not flow into the resistor  481  (R 3 ). In this case, the second threshold voltage (V d ) is equal to the output voltage (V out ) of the power stage  110 . While the diode  482  (D 2 ) is in the ON state, a current flows into the resistor  481  (R 3 ). In this case, the second threshold voltage (V d ) becomes lower than the output voltage (V out ) of the power stage  110 . Specifically, if the output voltage (V out ) of the power stage  110  is equal to or greater than a forward voltage (V f ) of the diode  482  (D 2 ), the second threshold voltage (V d ) is generally set to be the same as the forward voltage (V f ). 
     The second threshold voltage (V d ) is represented by equation (6) and  FIG. 6 . 
     
       
         
           
             
               
                 
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     The comparator  483  includes a non-inverted input terminal, inverted input terminal, and output terminal. The non-inverted input terminal of the comparator  483  is connected to the output terminal of the voltage dividing circuit  430 . The inverted input terminal of the comparator  483  is connected to both of the second terminal of the resistor  481  (R 3 ) and the anode of the diode  482  (D 2 ). The output terminal of the comparator  483  is connected to the control terminal of the switch  446  and the input terminal of the inverter  484 . 
     The comparator  483  compares the voltage (V div ) of the non-inverted input terminal and the voltage (V d ) of the inverted input terminal. The comparator  483  generates an output signal corresponding to a high-level digital signal if the voltage (V div ) of the non-inverted input terminal is equal to or greater than the voltage (V d ) of the inverted input terminal. On the other hand, the comparator  483  generates an output signal corresponding to a low-level digital signal if the voltage (V div ) of the non-inverted input terminal is lower than the voltage (V d ) of the inverted input terminal. 
     In other words, if V out &lt;V f , V out /N=V div &lt;V d =V out . In addition, even if V out ≧V f , if the level of the output voltage (V out ) of the power stage  110  is relatively low, there may be the case where V out /N=V div &lt;V d  ≈V is true. In such a case, since the level of the output signal of the comparator  483  is low, the first candidate voltage (V ref1 ) is used as the reference voltage (V ref ). 
     On the other hand, if V out ≧V f , and the level of the output voltage (V out ) of the power stage  110  is relatively high, V f ≈V d ≦V div =V out /N. In such a case, since the level of the output signal of the comparator  483  is high, the second candidate voltage (V ref2 ) is used as the reference voltage (V ref ). The level (≈NV f ) of output voltage (V out ) of the power stage  110  when V out /N=V div =V d ≈V f  is true can be regarded as the first threshold voltage (V th ) 
       FIG. 7  shows the relation between the output voltage (V out ) of the power stage  110  and the reference voltage (V ref ). When the output voltage (V out ) of the power stage  110  is considerably lower than the first threshold voltage (V th ), the reference voltage source  442  cannot operate normally, and the level of the second candidate voltage (V ref2 ) is extremely low. If the second candidate voltage (V ref2 ) the level of which is extremely low is used as the reference voltage (V ref ) the condition of V ref ≦V div  is always true, and the output voltage (V out ) of the power stage  110  cannot increase to a desired level. 
     Accordingly, the reference voltage generating circuit  440  selects the first candidate voltage (V ref1 ) as the reference voltage (V ref ) in the first phase where the output voltage (V div ) of the voltage dividing circuit  430  is lower than the second threshold voltage (V d ), as stated above. The first candidate voltage (V ref1 ) is set to be equal to or greater than the output voltage (V div ) of the voltage dividing circuit  430  in the first phase. In this case, if the first candidate voltage (V ref1 ) is used as the reference voltage (V ref ), the output voltage (V out ) of the power stage  110  can increase to a level sufficient for the reference voltage source  442  to operate normally. 
     In addition, as stated above, the second candidate voltage (V ref2 ) is selected as the reference voltage (V ref ) in the second phase where the output voltage (V div ) of the voltage dividing circuit  430  is equal to or greater than the second threshold voltage (V d ). The second candidate voltage (V ref2 ) has a stable level regardless of change in the output voltage (V out ) of the power stage  110  in the second phase. Accordingly, output voltage (V out ) of the power stage  110  can be stabilized through the negative feedback control. 
     The inverter  484  includes an input terminal and an output terminal. The input terminal of the inverter  484  is connected to a control terminal of the reference voltage generating circuit  440 . The output terminal of the inverter  484  is connected to a control terminal of the switch  471  (SW 21 ). The inverter  484  performs logical inversion on an output signal of the comparator  483 . 
     The subtractor  450  includes the non-inverted input terminal, inverted input terminal, and output terminal. The non-inverted input terminal of the subtractor  450  is connected to the output terminal of the reference voltage generating circuit  440 . The inverted input terminal of the subtractor  450  is connected to the output terminal of the voltage dividing circuit  430 . The output terminal of the subtractor  450  is connected to the input terminal of the switching control circuit  460 . 
     The subtractor  450  subtracts the voltage of the inverted input terminal (V div ) from the voltage of the non-inverted input terminal (V ref ). The subtractor  450  outputs a difference signal indicating a sign (positive or negative) of a differential voltage to the switching control circuit  460 . 
     Specifically, the subtractor  450  is implemented by a comparator. The subtractor  450  outputs a difference signal (V CMP ) corresponding to a high-level digital signal to the switching control circuit  460  when a differential voltage shows a positive sign. On the other hand, the subtractor  450  outputs a difference signal (V CMP ) corresponding to a low-level digital signal to the switching control circuit  460  when the differential voltage shows a negative sign. 
     The switching control circuit  460  includes the input terminal and the output terminal. The input terminal of the switching control circuit  460  is connected to the output terminal of the subtractor  450 . The output terminal of the switching control circuit  460  is connected to the control terminal of the power stage  110 . The switching control circuit  460  is implemented by using a Constant On circuit, for example. 
     The switching control circuit  460  receives a difference signal (V CMP ) from the subtractor  450 . The switching control circuit  460  generates a switching control signal based on the difference signal (V CMP ). The switching control circuit  460  outputs the switching control signal to the power stage  110 . Specifically, if the level of the difference signal (V CMP ) is high, the switching control circuit  460  generates a switching control signal (V SW1 ) to periodically switch ON and OFF states of the switch  112  (SW 1 ). As shown in  FIG. 8 , a switching control signal (V SW1 ) corresponding to a pulse signal having predetermined ON periods and OFF periods is generated. This switching control signal (V SW1 ) increases the output voltage (V out ) of the power stage  110 . On the other hand, if the level of the difference signal (V CMP ) is low, the switching control circuit  460  generates a switching control signal (V SW1 ) so that the switch  112  (SW 1 ) remains in the OFF state. This switching control signal (V SW1 ) decreases the output voltage (V out ) of the power stage  110 . 
     As stated above, the DC-to-DC converter according to the fourth embodiment adaptively selects a reference voltage from among a plurality of candidate voltages having different characteristics. This DC-to-DC converter ensures stable operation even if the level of DC output voltage is minute. 
     In addition, the DC-to-DC converter stops supply of a driving voltage to a specific candidate voltage while the specific candidate voltage is not selected as a reference voltage. Accordingly, this DC-to-DC converter reduces unnecessary current consumption in the reference voltage source to generate candidate voltages. That is, this DC-to-DC converter reduces current consumption and achieves high speed charging for a capacitor included in the power stage. 
     Furthermore, the DC-to-DC converter uses the output voltage of the DC-to-DC converter as a specific candidate voltage. This DC-to-DC converter eliminates the need for providing a reference voltage source to generate the specific candidate voltage, thereby achieving simplification of the configuration and reduction of current consumption. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.