Abstract:
The voltage stabilizer for stabilizing a voltage of a circuit includes a first resistor, a second resistor, a voltage controller, and a current supply unit. The voltage controller keeps a voltage of the first resistor constant. The current supply unit supplies a first current to between the first resistor and the second resistor when a second current in the circuit becomes equal to or larger than a predetermined value.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1) Field of the Invention  
         [0002]     The present invention relates to a voltage stabilizer that stabilizes a voltage of a circuit.  
         [0003]     2) Description of the Related Art  
         [0004]     Conventionally, a power supply circuit that is connected to a battery has a problem in that after the power supply circuit passes an excessive current from the battery, the voltage of the battery drops rapidly and the output voltage of the power supply circuit becomes unstable. Therefore, a power supply control circuit is used to stabilize the output voltage of the power supply circuit.  
         [0005]     The power supply control circuit obtains signals of two systems concerning the current that flows through the power supply circuit and the output voltage of the power supply circuit. Based on the obtained signals of the two systems, the power supply control circuit determines the current and the voltage of the power supply circuit. When an excessive current flows through the power supply circuit, the power supply control circuit controls the current from the battery, thereby stabilizing the voltage of the power supply circuit (see, for example, Japanese Utility Model Application Laid-Open No. H5-25945).  
         [0006]      FIG. 11  is an example of a conventional power supply circuit. As shown in  FIG. 11 , this power supply circuit has a battery  10  that generates electromotive force, a load  20  that applies load to the power supply circuit, and a direct current to direct current (DC-DC) converter  30  that converts a direct current at a voltage of the battery  10  to a direct current at a different voltage.  
         [0007]     The DC-DC converter  30  has a capacitor  40 , a diode  50 , a transistor  60 , a coil  70 , voltage dividing resistors  80  and  90 , a current detecting circuit  100 , and a power supply control circuit  110 .  
         [0008]     The voltage dividing resistors  80  and  90  are used to detect an output voltage of the power supply circuit. The current detecting circuit  100  detects a size of a current that flows from the battery  10 . The power supply control circuit  110  is connected to the current detecting circuit  100 , an intermediate point between the voltage dividing resistors  80  and  90 , and the transistor  60 . The power supply control circuit  110  obtains a signal concerning a current from the current detecting circuit  100 , obtains a signal concerning an output voltage from the intermediate point between the voltage dividing resistors  80  and  90 , and controls the transistor  60 .  
         [0009]     In other words, the power supply control circuit  110  obtains signals of the two systems of the output voltage and the current of the power supply circuit, from the intermediate point between the voltage dividing resistors  80  and  90 , and from the current detecting circuit  100 . When an excessive current flows from the battery  10 , the power supply control circuit  110  controls the transistor  60  to prevent the flow of the excessive current, thereby stabilizing the output voltage of the power supply circuit.  
         [0010]     According to the conventional technique, however, circuit parts that are used generally cannot be used to prepare the power supply control circuit of the power supply circuit having an inherent characteristic. It is necessary to develop own circuit parts corresponding to individual power supply circuits, to prepare the power supply control circuit, which requires an enormous amount of cost.  
         [0011]     In integrating circuit parts and preparing the power supply control circuit using integrated circuit (IC) chips to respond to the requirement for miniaturization of the power supply control circuit, IC chips that can handle control signals of two or more systems are necessary. However, such IC chips are not available and therefore need to be developed, which is a more important task.  
         [0012]     In other words, it is considerably important to stabilize the output voltage of the power supply circuit using circuit parts that are generally used without providing a special power supply control circuit in the power supply circuit.  
       SUMMARY OF THE INVENTION  
       [0013]     It is an object of the present invention to at least solve the problems in the conventional technology.  
         [0014]     The voltage stabilizer according to an aspect of the present invention is a voltage stabilizer that includes a first resistor; a second resistor; a voltage controller that keeps a voltage of the first resistor constant; and a current supply unit that supplies a first current to between the first resistor and the second resistor when a second current in the circuit becomes equal to or larger than a predetermined value.  
         [0015]     The voltage stabilizing method according to another aspect of the present invention is a voltage stabilizing method for stabilizing a voltage of a circuit with a first resistor and a second resistor serially connected, and includes keeping a voltage of the first resistor constant; and supplying a first current to between the first resistor and the second resistor when a second current in the circuit becomes equal to or larger than a predetermined value.  
         [0016]     The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is an explanatory diagram of the concept of voltage stabilization according to a first embodiment of the present invention;  
         [0018]      FIG. 2  is an example of OR connection of circuits that control an output voltage;  
         [0019]      FIG. 3  is an example of application of a power supply circuit shown in  FIG. 1  to a power supply circuit explained in the conventional technique;  
         [0020]      FIG. 4  is a functional block diagram of a configuration of a power supply circuit according to the first embodiment;  
         [0021]      FIG. 5  is a functional block diagram of a configuration of a power supply circuit according to a second embodiment;  
         [0022]      FIG. 6  is a functional block diagram of a configuration of a power supply circuit according to a third embodiment;  
         [0023]      FIG. 7  is a functional block diagram of a configuration of a power supply circuit according to a fourth embodiment;  
         [0024]      FIG. 8  is a functional block diagram of a configuration of a power supply circuit according to a fifth embodiment;  
         [0025]      FIG. 9  is a functional block diagram of a configuration of a power supply circuit according to a sixth embodiment;  
         [0026]      FIG. 10  is a functional block diagram of a configuration of a power supply circuit according to a seventh embodiment; and  
         [0027]      FIG. 11  is an example of a conventional power supply circuit. 
     
    
     DETAILED DESCRIPTION  
       [0028]     Exemplary embodiments of a voltage stabilizer according to the present invention will be explained in detail below with reference to the accompanying drawings.  
         [0029]      FIG. 1  is an explanatory diagram of the concept of voltage stabilization according to the first embodiment. As shown in  FIG. 1 , a power supply circuit  300  has the load  20 , a diode  55 , the voltage dividing resistors  80  and  90 , a detecting circuit  210 , an amplifier  220 , a current limiting resistor  230 , and a control circuit  240 . The load  20  and the voltage dividing resistors  80  and  90  are similar to the load and the voltage dividing resistors explained in the conventional technique, and therefore, their explanation is omitted.  
         [0030]     The diode  55  is an electronic part that passes a current to one direction, like the diode  50  explained in the conventional technique. According to the first embodiment, Vf denotes a drop voltage of the diode  55 . The diode  55  is connected to an intermediate point between the voltage dividing resistors  80  and  90 , and to the current limiting resistor  230 . A current flows from the current limiting resistor  230  to the voltage dividing resistor  90 .  
         [0031]     The detecting circuit  210  detects a current that flows through the power supply circuit  300 , and passes a signal of the detected current of the power supply circuit  300  to the amplifier  220 . The amplifier  220  amplifies a signal of a current obtained from the detecting circuit  210 .  
         [0032]     The current limiting resistor  230  suitably adjusts a total gain of a feedback circuit at the time of controlling the voltage. The current limiting resistor  230  prevents the total gain of the feedback circuit from increasing from the gain during a normal operation at the time of controlling the voltage, thereby preventing the voltage control from becoming unstable.  
         [0033]     The control circuit  240  keeps the voltage between both ends of the voltage dividing resistor  90  constant. The control circuit  240  is connected to an intermediate point between the voltage dividing resistor  80  and the voltage dividing resistor  90 . According to the first embodiment, the control circuit  240  keeps the voltage (a reference voltage) between both ends of the voltage dividing resistor  90  at Vr. Therefore, the size of a current  12  that flows to the voltage dividing resistor  90  becomes always constant.  
         [0034]     Assume that R 1  denotes a resistance of the voltage dividing resistor  80 , and R 2  denotes a resistance of the voltage dividing resistor  90 . Then, an output voltage Vo of the power supply circuit  300  is expressed as follows.  
             Vo   =           R   ⁢           ⁢   1     +     R   ⁢           ⁢   2         R   ⁢           ⁢   2       ×   Vr             (   1   )             
 
         [0035]     When a current I 3  flows from the diode  55  to the voltage dividing resistor  90 , the control circuit  240  controls a current I 2  to become constant. Therefore, a current I 1  that flows from the voltage dividing resistor  80  decreases. As a result, the voltage between both ends of the voltage dividing resistor  80  drops, and the output voltage of the power supply circuit  300  drops.  
         [0036]     Assume that Va denotes an output voltage of the amplifier  220 . Then, when an output voltage Va of the amplifier  220  is smaller than a sum of a drop voltage Vf of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output voltage Vo of the power supply circuit  300  is expressed as follows.  
             Vo   =           R   ⁢           ⁢   1     +     R   ⁢           ⁢   2         R   ⁢           ⁢   2       ×   Vr             (   1   )             
 
 When the output voltage Va is equal to or larger than the sum of the drop voltage Vf of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output voltage Va is expressed as follows.  
             Vo   =             R   ⁢           ⁢   1     +     R   ⁢           ⁢   2         R   ⁢           ⁢   2       ×   Vr     -         R   ⁢           ⁢   1       R   ⁢           ⁢   3       ×     (     Va   -   Vf   -   Vr     )                 (   2   )             
 
         [0037]     In other words, when the output voltage Va is equal to or larger than the sum of the drop voltage Vf and the reference voltage Vr, the output voltage Vo of the power supply circuit  300  drops. In the expression (2), R 3  denotes a resistance of the current limiting resistor  230 .  
         [0038]     The output voltage Va of the amplifier  220  becomes large when the current that flows through the power supply circuit  300  becomes large. When an excessive current flows through the power supply circuit  300 , the output voltage Va becomes larger than the sum of the drop voltage Vf and the reference voltage Vr, thereby lowering the output voltage Vo of the power supply circuit  300 . Consequently, the voltage of the power supply circuit  300  can be stabilized.  
         [0039]     A circuit that controls the output voltage can be OR connected between the voltage dividing resistors  80  and  90 .  FIG. 2  is an example of OR connection of circuits that control an output voltage, between the voltage dividing resistors. As shown in  FIG. 2 , a power supply circuit  310  has diodes  55   a  and  55   b , current limiting resistors  230   a  and  230   b , amplifiers  221  and  222 , and detecting circuits  210   a  and  210   b . Other configurations and operations are similar to those of the power supply circuit  300  shown in  FIG. 1 , and therefore, like reference numerals denote like constituent elements and their explanation is omitted.  
         [0040]     The diodes  55   a  and  55   b , the current limiting resistors  230   a  and  230   b , the amplifiers  221  and  222 , and the detecting circuits  210   a  and  210   b  are similar to the diode  55 , the current limiting resistor  230 , the amplifier  220 , and the detecting circuit  210 , shown in  FIG. 1  respectively. Therefore, their explanation is omitted.  
         [0041]     As explained above, two circuits that control the output voltage are OR connected between the voltage dividing resistors  80  and  90 . With this arrangement, even when the resistance of the current limiting resistor  230   a  decreases, the diode  55   b , the current limiting resistor  230   b , the amplifier  222 , and the detecting circuit  210   b  can keep the current that flows from the amplifier  221  constant. While the OR connection of the two circuits that control the output voltage is shown in  FIG. 2 , two or more circuits can be connected between the voltage dividing resistors  80  and  90 .  
         [0042]      FIG. 3  is an example of application of the power supply circuit shown in  FIG. 1  to the power supply circuit explained in the conventional technique. As shown in  FIG. 3 , the amplifier  220  of a power supply circuit  320  is connected to a current detecting circuit  100  and the current limiting resistor  230 . The current limiting resistor  230  is connected to an intermediate point between the voltage dividing resistor  80  and  90 , via the diode  55 .  
         [0043]     When an excessive current flows to the power supply circuit  320  (when the voltage of the amplifier  220  becomes equal to or larger than the sum of the drop voltage of the diode  55  and the voltage between both ends of the voltage dividing resistor  90 ), the current flows from the diode  55  to the voltage dividing resistor  90 , and the current that flows to the voltage dividing resistor  80  decreases. Therefore, the output voltage of the power supply circuit  320  drops.  
         [0044]      FIG. 4  is a functional block diagram of the configuration of the power supply circuit according to the first embodiment. As shown in  FIG. 4 , a power supply circuit  330  has resistors  100   a ,  220   a ,  220   b ,  220   c , and  220   d , a capacitor  220   e , and an operational amplifier  220   f . Other configurations and operations are similar to those of the power supply circuit  320  shown in  FIG. 3 , and therefore, like reference numerals denote like constituent elements and their explanation is omitted. In  FIG. 4 , while a DC-DC converter is a step-down switching power supply circuit, a DC-DC converter of other system, such as a step-up switching power supply circuit, can be also used.  
         [0045]     The resistor  100   a  is a current detecting resistor. The resistors  220   a ,  220   b ,  220   c , and  220   d  are used to determine amplification of the operational amplifier  220   f . The capacitor  220   e  is a compensation capacitor for the operational amplifier  220   f.    
         [0046]     The operational amplifier  220   f  is a high-gain amplifier that amplifies a difference between voltages of a plus terminal (a noninverting input terminal) and a minus terminal (an inverting input terminal) to about 10 3  to 10 6  times, and outputs the amplified result.  
         [0047]     When a current lo flows to the output of the power supply circuit  330 , a potential of lo×R 8  occurs (where R 8  is a resistance of the resistor  100   a  according to the first embodiment) between both ends of the resistor  100   a . In this case, the output voltage Va of the operational amplifier  220   f  is expressed as A×lo×R 8 , where A is the amplification of the operational amplifier  220   f.    
         [0048]     Assume that Vf denotes a drop voltage of the diode  55 , Vr denotes voltage (hereinafter, a reference voltage) between both ends of the voltage dividing resistor  90 , and Va denotes an output voltage of the operational amplifier  220   f  (where the value of Va is equal to the value of A×lo×R 8 ). When the output voltage Va is smaller than the sum of the drop voltage Vf and the reference voltage Vr, the diode  55  blocks the output of the operational amplifier  220   f , and a current cannot be passed to the voltage dividing resistor  90 .  
         [0049]     In other words, under the following condition shown in an expression (3), the output voltage of the power supply circuit  330  does not undergo the voltage control operation according to the present invention, and the power supply circuit  330  executes a normal output operation.  
             Io   &lt;       (     Vf   +   Vr     )       A   ×   R   ⁢           ⁢   8               (   3   )             
 
         [0050]     On the other hand, when the condition shown in the expression (3) is not satisfied (in other words, the output voltage Va becomes equal to or larger than the sum of the drop voltage Vf and the reference voltage Vr), the output of the operational amplifier  220   f  passes through the diode  55 , and the current flows to the voltage dividing resistor  90 . Therefore, the current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  330 .  
         [0051]     As described above, according to the first embodiment, the control circuit  240  of the power supply circuit  330  keeps the voltage between both ends of the voltage dividing resistor  90  constant. When the output current lo increases to make the output voltage Va of the operational amplifier  220   f  equal to or larger than the sum of the drop voltage Vf of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the operational amplifier  220   f  passes through the diode  55 , and flows to the voltage dividing resistor  90 . Since the current that flows through the voltage dividing resistor  90  is constant, the current that flows to the voltage dividing resistor  80  decreases, and the output voltage of the power supply circuit  330  drops. Therefore, the output voltage can be stabilized without using a special power supply control circuit.  
         [0052]     A second embodiment of the present invention is explained next. A power supply circuit according to the second embodiment of the present invention controls the output voltage by controlling an input current when the input current increases.  
         [0053]      FIG. 5  is a functional block diagram of the configuration of the power supply circuit according to the second embodiment. As shown in  FIG. 5 , a power supply circuit  340  has the diode  55 , the resistors  100   a ,  220   a ,  220   b ,  220   c , and  220   d , the capacitor  220   e , the operational amplifier  220   f , and the current limiting resistor  230 . Other configurations and operations are similar to those of the power supply circuit  320  shown in  FIG. 3 , and therefore, like reference numerals denote like constituent elements and their explanation is omitted. In  FIG. 5 , while a DC-DC converter is a step-down switching power supply circuit, a DC-DC converter of other system, such as a step-up switching power supply circuit, can be also used.  
         [0054]     The diode  55 , the resistors  100   a ,  220   a ,  220   b ,  220   c , and  220   d , the capacitor  220   e , the operational amplifier  220   f , and the current limiting resistor  230  are similar to the diode, the resistors, the capacitor, the operational amplifier, and the current limiting resistor shown in  FIG. 4 , respectively, and therefore, like reference numerals denote like elements and their explanation is omitted.  
         [0055]     When a current li flows to the input of the power supply circuit  340 , a potential of li×R 8  occurs (where R 8  is a resistance of the resistor  100   a  according to the second embodiment) between both ends of the resistor  100   a . In this case, the output voltage of the operational amplifier  220   f  is expressed as A×li×R 8 , where A is the amplification of the operational amplifier  220   f.    
         [0056]     Assume that Vf denotes a drop voltage of the diode  55 , Vr denotes voltage (a reference voltage) between both ends of the voltage dividing resistor  90 , and Va denotes an output voltage of the operational amplifier  220   f  (where the value of Va is equal to the value of A×li×R 8 ). When the output voltage Va is smaller than the sum of the drop voltage Vf and the reference voltage Vr, the diode  55  blocks the output of the operational amplifier  220   f , and a current cannot be passed to the voltage dividing resistor  90 .  
         [0057]     In other words, under the following condition shown in an expression (4), the output voltage of the power supply circuit  340  does not undergo the voltage control operation according to the present invention, and the power supply circuit  340  executes a normal output operation.  
             Ii   &lt;       (     Vf   +   Vr     )       A   ×   R   ⁢           ⁢   8               (   4   )             
 
         [0058]     On the other hand, when the condition shown in the expression (4) is not satisfied (in other words, the output voltage Va of the operational amplifier  220   f  becomes equal to or larger than the sum of the drop voltage Vf and the reference voltage Vr), the output of the operational amplifier  220   f  passes through the diode  55 , and the current flows to the voltage dividing resistor  90 . Therefore, the current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit.  
         [0059]     As described above, according to the second embodiment, the control circuit  240  of the power supply circuit  340  keeps the voltage between both ends of the voltage dividing resistor  90  constant. When the input current li increases to make the output voltage Va of the operational amplifier  220   f  equal to or larger than the sum of the drop voltage Vf of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the operational amplifier  220   f  passes through the diode  55 , and flows to the voltage dividing resistor  90 . Since the current that flows through the voltage dividing resistor  90  is constant, the current that flows to the voltage dividing resistor  80  decreases, and the output voltage of the power supply circuit  340  drops. Therefore, the output voltage can be stabilized without developing a special power supply control circuit.  
         [0060]     A third embodiment of the present invention is explained next. A power supply circuit according to the third embodiment controls the output voltage when the input voltage of the power supply circuit drops, thereby protecting the input.  
         [0061]      FIG. 6  is a functional block diagram of the configuration of the power supply circuit according to the third embodiment. As shown in  FIG. 6 , a power supply circuit  350  has batteries  10  and  15 , the load  20 , the diode  55 , a comparator  220   g , and the current limiting resistor  230 . Other configurations and operations are similar to those of the DC-DC converter shown in  FIG. 4 , and therefore, like reference numerals denote like constituent elements and their explanation is omitted. In  FIG. 6 , while a DC-DC converter is a step-down switching power supply circuit, a DC-DC converter of other system, such as a step-up switching power supply circuit, can be also used. In place of the comparator, an amplifier having an optional amplification A can be used.  
         [0062]     The battery  10 , the load  20 , the diode  55 , and the current limiting resistor  230  are similar to the battery  10 , the load  20 , the diode  55 , and the current limiting resistor  230  shown in  FIG. 5 , and therefore, their explanation is omitted. The comparator  220   g  is a circuit that compares a voltage of the battery  10  with a voltage of the battery  15 . The battery  15  is used to generate a reference voltage Vb.  
         [0063]     Assume that Vf denotes a drop voltage of the diode  55 , Vr denotes voltage (a reference voltage) between both ends of the voltage dividing resistor  90 , and Va denotes an output voltage of the comparator  220   g . When the output voltage Va is smaller than the sum of the drop voltage Vf and the reference voltage Vr, the diode  55  blocks the output of the comparator  220   g , and a current cannot be passed to between the voltage dividing resistors  80  and  90 .  
         [0064]     In other words, when an input voltage Vin of the power supply circuit  350  is larger than the reference voltage Vb (Vin&gt;Vb), the output voltage of the power supply circuit  350  does not undergo the voltage control operation according to the present invention, and the power supply circuit  350  executes a normal output operation.  
         [0065]     However, when the input voltage Vin is equal to or smaller than the reference voltage Vb, the output voltage Va of the comparator  220   g  becomes equal to or larger than the sum of the drop voltage Vf of the diode  55  and the voltage Vr between both ends of the voltage dividing resistor  90 . The output of the comparator  220   g  passes through the diode  55 , and flows to the voltage dividing resistor  90 . Therefore, the current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  350 .  
         [0066]     As described above, according to the third embodiment, the control circuit  240  of the power supply circuit  350  keeps the voltage between both ends of the voltage dividing resistor  90  constant. When the input voltage Vin decreases to make the output voltage Va of the comparator  220   g  equal to or larger than the sum of the drop voltage Vf of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the comparator  220   g  passes through the diode  55 , and flows to the voltage dividing resistor  90 . Since the current that flows through the voltage dividing resistor  90  is constant, the current that flows to the voltage dividing resistor  80  decreases, and the output voltage of the power supply circuit  350  drops. Therefore, the output voltage can be stabilized without developing a special power supply control circuit.  
         [0067]     A fourth embodiment of the present invention is explained next. A power supply circuit according to the fourth embodiment controls the output voltage when the input current of the power supply circuit becomes equal to or larger than a certain value or when the input voltage becomes equal to or smaller than a certain value, thereby protecting the input.  
         [0068]      FIG. 7  is a functional block diagram of the configuration of the power supply circuit according to the fourth embodiment. As shown in  FIG. 7 , a power supply circuit  360  has the batteries  10  and  15 , the load  20 , the diodes  55  and  56 , the resistors  100   a ,  220   a ,  220   b ,  220   c , and  220   d , the capacitor  220   e , the operational amplifier  220   f , the comparator  220   g , and the current limiting resistors  230  and  235 . Other configurations and operations are similar to those of the DC-DC converter shown in  FIG. 4 , and therefore, like reference numerals denote like constituent elements and their explanation is omitted. In  FIG. 7 , while a DC-DC converter is a step-down switching power supply circuit, a DC-DC converter of other system, such as a step-up switching power supply circuit, can be also used. In place of the comparator, an amplifier having an optional amplification A can be used.  
         [0069]     The batteries  10  and  15 , the load  20 , the diodes  55  and  56 , the resistors  100   a ,  220   a ,  220   b ,  220   c , and  220   d , the capacitor  220   e , the operational amplifier  220   f , the comparator  220   g , and the current limiting resistors  230  and  235  are similar to the batteries  10  and  15 , the load  20 , the diodes  55  and  56 , the resistors  100   a ,  220   a ,  220   b ,  220   c , and  220   d , the capacitor  220   e , the operational amplifier  220   f , the comparator  220   g , and the current limiting resistor  230  shown in  FIG. 5  or  FIG. 6 , and therefore, their explanation is omitted.  
         [0070]     When a current li flows to the input of the power supply circuit  360 , a potential of li×R 8  occurs (where R 8  is a resistance of the resistor  100   a  according to the fourth embodiment) between both ends of the resistor  100   a . The output voltage Va of the operational amplifier  220   f  is expressed as A×li×R 8 , where A is the amplification of the operational amplifier  220   f.    
         [0071]     Assume that Vf denotes a drop voltage of the diode  55 , Vr denotes voltage (a reference voltage) between both ends of the voltage dividing resistor  90 , and Va denotes an output voltage of the operational amplifier  220   f . When the output voltage Va is smaller than the sum of the drop voltage Vf and the reference voltage Vr, the diode  55  blocks the output of the operational amplifier  220   f , and a current cannot be passed to the voltage dividing resistor  90 .  
         [0072]     In other words, under the following condition shown in an expression (4), the output voltage of the power supply circuit  360  does not undergo the voltage control operation according to the present invention, and the power supply circuit  360  executes a normal output operation.  
             Ii   &lt;       (     Vf   +   Vr     )       A   ×   R   ⁢           ⁢   8               (   4   )             
 
         [0073]     When the input voltage Vin is higher than the reference voltage Vb of the comparator  220   g  (Vin&gt;Vb), the diode  56  blocks the output of the comparator  220   g , and a current cannot be passed to the voltage dividing resistor  90 . Therefore, when the condition of the expression (4) is satisfied, or when the input voltage Vin is higher than the reference voltage Vb of the comparator  220   g , the output voltage of the power supply circuit  360  does not undergo the voltage control operation according to the present invention, and the power supply circuit  360  executes a normal output operation.  
         [0074]     However, when any one of the two conditions is not satisfied, a current flows to the voltage dividing resistor  90  from the operational amplifier  220   f  or the comparator  220   g , thereby controlling the output voltage of the power supply circuit  360 .  
         [0075]     Specifically, when the input current li does not satisfy the condition of the expression (4), the output voltage Va of the operational amplifier  220   f  becomes equal to or larger than the sum of the drop voltage Vf and the reference voltage Vr. The output of the operational amplifier  220   f  passes through the diode  55 , and flows to the voltage dividing resistor  90 . Therefore, the current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  360 .  
         [0076]     On the other hand, when the input voltage Vin becomes equal to or smaller than the reference voltage Vb, the output voltage of the comparator  220   g  becomes equal to or larger than the sum of a drop voltage Vf 2  (where Vf 2  is a drop voltage of the diode  56  according to the fourth embodiment) of the diode  56  and the reference voltage Vr. Therefore, the output of the comparator  220   g  passes through the diode  56 , and flows to the voltage dividing resistor  90 . The current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  360 .  
         [0077]     As described above, according to the fourth embodiment, the control circuit  240  of the power supply circuit  360  keeps the voltage between both ends of the voltage dividing resistor  90  constant. When the input voltage Vin decreases to make the output voltage of the comparator  220   g  equal to or larger than the sum of the drop voltage Vf 2  of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the comparator  220   g  passes through the diode  56 , and flows to the voltage dividing resistor  90 . When the input current li increases to make the output voltage Va of the operational amplifier  220   f  equal to or larger than the sum of the drop voltage Vf of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the operational amplifier  220   f  passes through the diode  55 , and flows to the voltage dividing resistor  90 . Since the current that flows through the voltage dividing resistor  90  is constant, the current that flows to the voltage dividing resistor  80  decreases, and the output voltage of the power supply circuit  360  drops. Therefore, the output voltage can be stabilized without developing a special power supply control circuit.  
         [0078]     A fifth embodiment of the present invention is explained next. A power supply circuit according to the fifth embodiment controls the output voltage when either the input current or the output current of the power supply circuit increases excessively, thereby protecting the input and the output.  
         [0079]      FIG. 8  is a functional block diagram of the configuration of the power supply circuit according to the fifth embodiment. As shown in  FIG. 8 , a power supply circuit  370  has the battery  10 , the load  20 , the diodes  55  and  56 , the resistors  100   a ,  100   b ,  220   a ,  220   b ,  220   c ,  220   d ,  220   i ,  220   j ,  220   k , and  220   l , the capacitors  220   e  and  220   m , the operational amplifiers  220   f  and  220   h , and the current limiting resistors  230  and  235 . Other configurations are similar to those of the DC-DC converter shown in  FIG. 4 , and therefore, like reference numerals denote like constituent elements and their explanation is omitted. In  FIG. 8 , while a DC-DC converter is a step-down switching power supply circuit, a DC-DC converter of other system, such as a step-up switching power supply circuit, can be also used.  
         [0080]     The battery  10 , the load  20 , the diodes  55  and  56 , the resistors  100   a ,  100   b ,  220   a ,  220   b ,  220   c ,  220   d ,  220   i ,  220   j ,  220   k , and  220   l , the capacitors  220   e  and  220   m , the operational amplifiers  220   f  and  220   h , and the current limiting resistors  230  and  235  are similar to the battery  10 , the load  20 , the diodes  55  and  56 , the resistors  100   a ,  220   a ,  220   b ,  220   c , and  220   d , the capacitor  220   e , the operational amplifier  220   f , and the current limiting resistors  230  and  235  shown in  FIG. 4  or  FIG. 5 , and therefore, their explanation is omitted.  
         [0081]     When the current li flows to the input of the power supply circuit  370 , the potential of li×R 8  occurs (where R 8  is a resistance of the resistor  100   a  according to the fifth embodiment) between both ends of the resistor  100   a . An output voltage Va 1  of the operational amplifier  220   f  is expressed as A×li×R 8 , where A is the amplification of the operational amplifier  220   f.    
         [0082]     Assume that Vf 1  denotes a drop voltage of the diode  55 , Vr denotes voltage (a reference voltage) between both ends of the voltage dividing resistor  90 , and Va 1  denotes an output voltage of the operational amplifier  220   f . When the output voltage Va 1  of the operational amplifier  220   f  is smaller than the sum of the drop voltage Vf 1  and the reference voltage Vr, the diode  55  blocks the output of the operational amplifier  220   f , and a current cannot be passed to the voltage dividing resistor  90 .  
         [0083]     In other words, under the following condition shown in an expression (5), the output voltage of the power supply circuit  370  does not undergo the voltage control operation according to the present invention, and the power supply circuit  370  executes a normal output operation.  
             Ii   &lt;       (       Vf   ⁢           ⁢   1     +   Vr     )       A   ×   R   ⁢           ⁢   8               (   5   )             
 
         [0084]     When the current lo flows to the output of the power supply circuit  370 , a potential of lo×R 14  occurs (where R 14  is a resistance of the resistor  100   b ) between both ends of the resistor  100   b . An output voltage Va 2  of an operational amplifier  220   h  is expressed as A×lo×R 14 , where A is the amplification of the operational amplifier  220   h.    
         [0085]     Assume that Vf 2  denotes a drop voltage of the diode  56 , Vr denotes the reference voltage, and Va 2  denotes the output voltage of the operational amplifier  220   h . When the output voltage Va 2  is smaller than the sum of the drop voltage Vf 2  and the reference voltage Vr, the diode  56  blocks the output of the operational amplifier  220   h , and a current cannot be passed to the voltage dividing resistor  90 .  
         [0086]     In other words, under the following condition shown in an expression (6), the output voltage of the power supply circuit  370  does not undergo the voltage control operation according to the present invention, and the power supply circuit  370  executes a normal output operation.  
             Io   &lt;       (       Vf   ⁢           ⁢   2     +   Vr     )       A   ×   R   ⁢           ⁢   14               (   6   )             
 
         [0087]     In other words, when any one of the expression (5) and the expression (6) is not satisfied, a current flows to the voltage dividing resistor  90  from the operational amplifier  220   f  or  220   h , thereby controlling the output voltage of the power supply circuit  370 .  
         [0088]     Specifically, when the input current li does not satisfy the condition of the expression (5), the output voltage Va 1  of the operational amplifier  220   f  becomes equal to or larger than the sum of the drop voltage Vf 1  and the reference voltage Vr. The output of the operational amplifier  220   f  passes through the diode  55 , and flows to the voltage dividing resistor  90 . Therefore, the current that flows to the voltage dividing resistor  80  decreases, and the output voltage of the power supply circuit  370  drops.  
         [0089]     On the other hand, when the output current lo does not satisfy the condition of the expression (6), the output voltage Va 2  of the operational amplifier  220   h  becomes equal to or larger than the sum of the drop voltage Vf 2  and the reference voltage Vr. Therefore, the output of the operational amplifier  220   h  passes through the diode  56 , and flows to the voltage dividing resistor  90 . The current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage of the power supply circuit  370 .  
         [0090]     As described above, according to the fifth embodiment, the control circuit  240  of the power supply circuit  370  keeps the voltage between both ends of the voltage dividing resistor  90  constant. When the input current li increases to make the output voltage Va 1  of the operational amplifier  220   f  equal to or larger than the sum of the drop voltage Vf 1  of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the operational amplifier  220   f  passes through the diode  55 , and flows to the voltage dividing resistor  90 . When the output current lo increases to make the output voltage Va 2  of the operational amplifier  220   h  equal to or larger than the sum of the drop voltage Vf 2  of the diode  56  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the operational amplifier  220   h  passes through the diode  56 , and flows to the voltage dividing resistor  90 . Since the current that flows through the voltage dividing resistor  90  is constant, the current that flows to the voltage dividing resistor  80  decreases, and the output voltage of the power supply circuit  370  drops. Therefore, the output voltage can be stabilized without developing a special power supply control circuit.  
         [0091]     A sixth embodiment of the present invention is explained next. A power supply circuit according to the sixth embodiment controls the output voltage when the input voltage of the power supply circuit decreases or when the output current of the power supply circuit increases excessively, thereby protecting the input and the output.  
         [0092]      FIG. 9  is a functional block diagram of the configuration of the power supply circuit according to the sixth embodiment. As shown in  FIG. 9 , a power supply circuit  380  has the batteries  10  and  15 , the load  20 , the diodes  55  and  56 , the resistors  100   a ,  220   i ,  220   j ,  220   k , and  220   l , the operational amplifier  220   f , the comparator  220   g , the capacitor  220   m , and the current limiting resistors  230  and  235 . Other configurations and operations are similar to those of the DC-DC converter shown in  FIG. 4 , and therefore, like reference numerals denote like constituent elements and their explanation is omitted.  
         [0093]     In  FIG. 9 , while a DC-DC converter is a step-down switching power supply circuit, a DC-DC converter of other system, such as a step-up switching power supply circuit, can be also used. In place of the comparator, an amplifier having an optional amplification A can be used.  
         [0094]     The batteries  10  and  15 , the load  20 , the diodes  55  and  56 , the resistors  100   a ,  220   i ,  220   j ,  220   k , and  220   l , the operational amplifier  220   f , the comparator  220   g , the capacitor  220   m , and the current limiting resistors  230  and  235  are similar to the batteries  10  and  15 , the diodes  55  and  56 , the resistors  100   a ,  220   a ,  220   b ,  220   c , and  220   d , the capacitor  220   e , the operational amplifier  220   f , the comparator  220   g , and the current limiting resistors  230  and  235  shown in  FIG. 4 ,  FIG. 5  or  FIG. 6 , and therefore, their explanation is omitted.  
         [0095]     When the output current lo flows to the power supply circuit  380 , the potential of lo×R 8  occurs (where R 8  is a resistance of the resistor  100   a  according to the sixth embodiment) between both ends of the resistor  100   a . The output voltage Va 1  of the operational amplifier  220   f  is expressed as A×lo×R 8 , where A is the amplification of the operational amplifier  220   f.    
         [0096]     Assume that Vf 1  denotes a drop voltage of the diode  56 , Vr denotes voltage between both ends of the voltage dividing resistor  90 , and Va 1  denotes an output voltage of the operational amplifier  220   f . When the output voltage Va 1  is smaller than the sum of the drop voltage Vf 1  and the reference voltage Vr, the diode  56  blocks the output of the operational amplifier  220   f , and a current cannot be passed to the voltage dividing resistor  90 .  
         [0097]     In other words, under the following condition shown in an expression (6), the output voltage of the power supply circuit  380  does not undergo the voltage control operation according to the present invention, and the power supply circuit  380  executes a normal output operation.  
             Io   &lt;       (       Vf   ⁢           ⁢   1     +   Vr     )       A   ×   R   ⁢           ⁢   8               (   7   )             
 
         [0098]     When the input voltage Vin is higher than the reference voltage Vb of the comparator  220   g  (Vin&gt;Vb), the diode  55  blocks the output of the comparator  220   g , and a current cannot be passed to the voltage dividing resistor  90 . Therefore, when the condition of the expression (6) is satisfied, or when the input voltage Vin is higher than the reference voltage Vb of the comparator  220   g , the output voltage of the power supply circuit  380  does not undergo the voltage control operation according to the present invention, and the power supply circuit  380  executes a normal output operation.  
         [0099]     However, when any one of the two conditions is not satisfied, a current flows to the voltage dividing resistor  90  from the operational amplifier  220   f  or the comparator  220   g , thereby controlling the output voltage of the power supply circuit  380 .  
         [0100]     Specifically, when the output current lo does not satisfy the condition of the expression (6), the output voltage Va 1  of the operational amplifier  220   f  exceeds the sum of the drop voltage Vf 1  and the reference voltage Vr. The output of the operational amplifier  220   f  passes through the diode  56 , and flows to the voltage dividing resistor  90 . Therefore, the current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  380 .  
         [0101]     When the input voltage Vin becomes equal to or smaller than the reference voltage Vb, the output voltage of the comparator  220   g  becomes equal to or larger than the sum of the drop voltage Vf 2  of the diode  56  and the reference voltage Vr. Therefore, the output of the comparator  220   g  passes through the diode  55 , and flows to the voltage dividing resistor  90 . The current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  380 .  
         [0102]     As described above, according to the sixth embodiment, the control circuit  240  of the power supply circuit  380  keeps the voltage between both ends of the voltage dividing resistor  90  constant. When the input voltage Vin decreases to make the output voltage of the comparator  220   g  equal to or larger than the sum of the drop voltage Vf 2  of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the comparator  220   g  passes through the diode  55 , and flows to the voltage dividing resistor  90 . When the output current lo increases to make the output voltage Va 1  of the operational amplifier  220   f  equal to or larger than the sum of the drop voltage Vf 1  of the diode  56  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the operational amplifier  220   f  passes through the diode  56 , and flows to the voltage dividing resistor  90 . Since the current that flows through the voltage dividing resistor  90  is constant, the current that flows to the voltage dividing resistor  80  decreases, and the output voltage of the power supply circuit  380  drops. Therefore, the output voltage can be stabilized without developing a special power supply control circuit.  
         [0103]     A seventh embodiment of the present invention is explained next. A power supply circuit according to the seventh embodiment controls the output voltage when the input voltage of the power supply circuit decreases, when the input current of the power supply circuit increases, or when the output current of the power supply circuit increases excessively, thereby protecting the input and the output.  
         [0104]      FIG. 10  is a functional block diagram of the configuration of the power supply circuit according to the seventh embodiment. As shown in  FIG. 10 , a power supply circuit  390  has the batteries  10  and  15 , the load  20 , the diodes  55 ,  56 , and  57 , the resistors  100   a ,  100   b ,  220   a ,  220   b ,  220   c ,  220   d ,  220   i ,  220   j ,  220   k , and  220   l , the capacitors  220   e  and  220   m , the comparator  220   g , the operational amplifiers  220   f  and  220   o , and the current limiting resistors  230 ,  235 , and  236 . Other configurations and operations are similar to those of the DC-DC converter shown in  FIG. 4 , and therefore, like reference numerals denote like constituent elements and their explanation is omitted.  
         [0105]     In  FIG. 10 , while a DC-DC converter is a step-down switching power supply circuit, a DC-DC converter of other system, such as a step-up switching power supply circuit, can be also used. In place of the comparator, an amplifier having an optional amplification A can be used.  
         [0106]     The batteries  10  and  15 , the load  20 , the diodes  55 ,  56 , and  57 , the resistors  100   a ,  100   b ,  220   a ,  220   b ,  220   c ,  220   d ,  220   i ,  220   j ,  220   k , and  220   l , the capacitors  220   e  and  220   m , the comparator  220   g , the operational amplifiers  220   f  and  220   o , and the current limiting resistors  230 ,  235 , and  236  are similar to the batteries, the load, the diodes, the resistors, the capacitor, the comparator, the operational amplifier, and the current limiting resistors shown in  FIG. 4  or  FIG. 7 , and therefore, their explanation is omitted.  
         [0107]     When the current lo flows to the output of the power supply circuit  390 , a potential of lo×R 15  occurs (where R 15  is a resistance of the resistor  100   b  according to the seventh embodiment) between both ends of the resistor  100   b . In this case, the output voltage of the operational amplifier  220   o  is expressed as A×lo×R 15 , where A is the amplification of the operational amplifier  220   o.    
         [0108]     Assume that Vf 1  denotes a drop voltage of the diode  57 , Vr denotes voltage (hereinafter, a reference voltage) between both ends of the voltage dividing resistor  90 , and Va 1  denotes an output voltage of the operational amplifier  220   o . When the output voltage Va 1  is smaller than the sum of the drop voltage Vf 1  and the reference voltage Vr, the diode  57  blocks the output of the operational amplifier  220   o , and a current cannot be passed to between the voltage dividing resistors  80  and  90 .  
         [0109]     In other words, under the following condition shown in an expression (7), the output voltage of the power supply circuit  390  does not undergo the voltage control operation according to the present invention, and the power supply circuit  390  executes a normal output operation.  
             Io   &lt;       (       Vf   ⁢           ⁢   1     +   Vr     )       A   ×   R   ⁢           ⁢   15               (   8   )             
 
         [0110]     When the current li flows to the input of the power supply circuit  390 , the potential of li×R 8  occurs (where R 8  is a resistance of the resistor  100   a  according to the seventh embodiment) between both ends of the resistor  100   a . In this case, an output voltage of the operational amplifier  220   f  is expressed as A×li×R 8 , where A is the amplification of the operational amplifier  220   f.    
         [0111]     Assume that Vf 2  denotes a drop voltage of the diode  55 , Vr denotes the reference voltage, and Va 2  denotes an output voltage of the operational amplifier  220   f . When the output voltage Va 2  is smaller than the sum of the drop voltage Vf 2  and the reference voltage Vr, the diode  55  blocks the output of the operational amplifier  220   f , and a current cannot be passed to between the voltage dividing resistors  80  and  90 .  
         [0112]     In other words, under the following condition shown in an expression (8), the output voltage of the power supply circuit  390  does not undergo the voltage control operation according to the present invention, and the power supply circuit  390  executes a normal output operation.  
             li   &lt;       (     Vf2   +   Vr     )       A   ×   R8               (   9   )             
 
         [0113]     When the input voltage Vin is higher than the reference voltage Vb of the comparator  220   g  (Vin&gt;Vb), the diode  56  blocks the output of the comparator  220   g , and a current cannot be passed to the voltage dividing resistor  90 . Therefore, when the conditions of the expression (7) and the expression (8) are satisfied, or when the input voltage Vin is higher than the reference voltage Vb of the comparator  220   g , the output voltage of the power supply circuit  390  does not undergo the voltage control operation according to the present invention, and the power supply circuit  390  executes a normal output operation.  
         [0114]     However, when any one of the above three conditions is not satisfied, a current flows to the voltage dividing resistor  90  from the operational amplifiers  220   f  and  220   o  or the comparator  220   g , thereby controlling the output voltage of the power supply circuit  390 .  
         [0115]     Specifically, when the output current lo does not satisfy the condition of the expression (7), the output voltage Va 1  of the operational amplifier  220   o  becomes equal to or larger than the sum of the drop voltage Vf 1  and the reference voltage Vr. The output of the operational amplifier  220   o  passes through the diode  57 , and flows to the voltage dividing resistor  90 . Therefore, the current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  390 .  
         [0116]     When the input current li does not satisfy the condition of the expression (8), the output voltage Va 2  of the operational amplifier  220   f  becomes equal to or larger than the sum of the drop voltage Vf 2  and the reference voltage Vr. Therefore, the output of the operational amplifier  220   f  passes through the diode  55 , and flows to the voltage dividing resistor  90 . The current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  390 .  
         [0117]     When the input voltage Vin becomes equal to or smaller than the reference voltage Vb, the output voltage of the comparator  220   g  becomes equal to or larger than the sum of a drop voltage Vf 3  (where Vf 3  is a drop voltage of the diode  56  according to the seventh embodiment) of the diode  56  and the reference voltage Vr. Therefore, the output of the comparator  220   g  passes through the diode  56 , and flows to the voltage dividing resistor  90 . The current that flows to the voltage dividing resistor  80  decreases, thereby dropping the output voltage Vo of the power supply circuit  390 .  
         [0118]     As described above, according to the seventh embodiment, the control circuit  240  of the power supply circuit  390  keeps the voltage between both ends of the voltage dividing resistor  90  constant. When the input voltage Vin decreases to make the output voltage of the comparator  220   g  equal to or larger than the sum of the drop voltage Vf 3  of the diode  56  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the comparator  220   g  passes through the diode  56 , and flows to the voltage dividing resistor  90 .  
         [0119]     When the input current li increases to make the output voltage Va 2  of the operational amplifier  220   f  equal to or larger than the sum of the drop voltage Vf 2  of the diode  55  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the operational amplifier  220   f  passes through the diode  55 , and flows to the voltage dividing resistor  90 .  
         [0120]     When the output current lo increases to make the output voltage Va 1  of the operational amplifier  220   o  equal to or larger than the sum of the drop voltage Vf 1  of the diode  57  and the reference voltage Vr between both ends of the voltage dividing resistor  90 , the output of the operational amplifier  220   o  passes through the diode  57 , and flows to the voltage dividing resistor  90 . Since the current that flows through the voltage dividing resistor  90  is constant, the current that flows to the voltage dividing resistor  80  decreases, and the output voltage of the power supply circuit  390  drops. Therefore, the output voltage can be stabilized without developing a special power supply control circuit.  
         [0121]     The power supply circuits according to the first to the seventh embodiments can stabilize the output voltage by using circuit parts that are generally used, without using a special power supply control circuit. Therefore, development cost can be decreased substantially.  
         [0122]     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.