Abstract:
According to the present invention, there is provided a DC-DC converter converting input voltage into output voltage, including: an inductor having one terminal connected to the input voltage; a switch connected to the other terminal of the inductor and performing a switching behavior based on input of a periodic pulse signal; a monitor circuit detecting occurrence of overcurrent by converting a value of current flowing in the inductor into a monitor voltage value and comparing the monitor voltage value with a reference voltage value; a cancel out circuit fluctuating the reference voltage value so that the reference voltage value has negative correlation with fluctuation of the input voltage; and a regulator circuit fluctuating the reference voltage value so that the reference voltage value has positive correlation with fluctuation of a set value of the output voltage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a DC-DC converter, and more specifically, to a DC-DC converter having an overcurrent detection function. 
         [0003]    2. Description of Related Art 
         [0004]    A DC-DC converter having a function of detecting overcurrent to protect a circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2003-244941. Another technology of regulating a threshold value for overcurrent protection depending on fluctuation of input voltage is disclosed in Japanese Unexamined Patent Application Publications No. 2004-343900 and No. 2002-142456. In addition, a technology of regulating a threshold value for overcurrent protection depending on output voltage is disclosed in Japanese Unexamined Patent Application Publication No. 2005-20833. 
         [0005]    We have now discovered that the above-described related DC-DC converters have some problems explained below. 
         [0006]      FIG. 1  shows a configuration of a DC-DC converter  100  as well known in the art. The DC-DC converter  100  comprises an inductor  101 , a transistor  102 , a diode  103 , a capacitor  104 , and a resistance element  105 , and supplies output voltage to a circuit  200  connected to an output terminal  106 . The transistor  102  is turned on and off according to a periodic pulse signal PS input to a gate electrode. The output voltage of the DC-DC converter  100  can be controlled by changing a Duty ratio of the pulse signal PS. 
         [0007]    A current Im which flows in the transistor  102  of the DC-DC converter  100  is shown in  FIG. 2A . When a value of the current Im becomes larger than a threshold value Th, it is determined that excessive current flows in the transistor  102 . Then an operation of the DC-DC converter  100  is stopped to protect the circuit. The excessive current may be flowed from the output terminal  106  due to a failure of a circuit  200  connected to the output terminal  106 , for example. In such a case, a current waveform shows a behavior as shown in a dotted line in  FIG. 2A . 
         [0008]    A value of a ripple Irip shown in  FIG. 2A  can be evaluated from an expression as follows; 
         [0000]    
       
         
           
             
               I 
                
               
                   
               
                
               rip 
             
             = 
             
               
                 
                   
                     V 
                      
                     
                         
                     
                      
                     in 
                   
                   L 
                 
                 × 
                 T 
                  
                 
                     
                 
                  
                 on 
               
               = 
               
                 
                   
                     V 
                      
                     
                         
                     
                      
                     in 
                   
                   L 
                 
                 × 
                 
                   
                     
                       V 
                        
                       
                           
                       
                        
                       out 
                     
                     - 
                     
                       V 
                        
                       
                           
                       
                        
                       in 
                     
                   
                   
                     V 
                      
                     
                         
                     
                      
                     out 
                   
                 
                 × 
                 
                   1 
                   f 
                 
               
             
           
         
       
     
         [0000]    where Vin is the input voltage, VOUT is a set value of the output voltage, L is an inductance of the inductor  101 , Ton is a time length while the transistor  102  is on, and f is a frequency of the pulse signal PS. 
         [0009]    According to the equation above, when the input voltage or the set value of the output voltage fluctuates, the current waveform shown in a solid line in  FIG. 2A  changes. When the input voltage becomes smaller than it is shown in  FIG. 2A , the waveform of the current Im becomes the waveform shown in a dotted line in  FIG. 2B . When the input voltage becomes larger than it is shown in  FIG. 2A , the waveform of the current Im becomes the waveform shown in a dotted line in  FIG. 2C . When the set value of the output voltage becomes larger than it is shown in  FIG. 2A , the waveform of the current Im becomes the waveform shown in a dotted line in  FIG. 2D . When the set value of the output voltage becomes smaller than it is shown in  FIG. 2A , the waveform of the current Im becomes the waveform shown in a dotted line in  FIG. 2E . 
         [0010]    In the related techniques, the threshold value for overcurrent detection is left constant even though the waveform of the current Im changes due to the fluctuations of the input voltage or the set value of the output voltage. Therefore, in the related techniques, it may be determined that the overcurrent occurs even though it has not occurred or it may determined that the overcurrent has not occurred even though it has actually occurred. 
         [0011]    The input voltage becomes smaller when the input voltage supply is a battery and the battery has died, for example. On the other hand, the input voltage becomes larger when the input voltage supply is a rechargeable battery and the battery is overcharged, for example. 
         [0012]    The set value of the output voltage fluctuates as appropriate depending on a voltage value that is needed by the circuit  200  connected to the output terminal  106 . 
       SUMMARY 
       [0013]    According to one aspect of the present invention, there is provided a DC-DC converter that is capable of changing a threshold value for overcurrent detection in accordance with fluctuations of the input voltage and a set value of the output voltage. 
         [0014]    Therefore, it is possible to detect the overcurrent properly even when an waveform of current flowing in an inductor of the DC-DC converter changes. 
         [0015]    For example, the DC-DC converter of the present invention is the DC-DC converter converting input voltage into output voltage, including: an inductor having one terminal connected to the input voltage; a switch connected to the other terminal of the inductor and performing a switching behavior based on input of a periodic pulse signal; a monitor circuit detecting occurrence of overcurrent by comparing a monitor voltage value which is converted from a value of current flowing in the inductor with a reference voltage value; a cancel out circuit fluctuating the reference voltage value so that the reference voltage value has negative correlation with fluctuation of the input voltage; and a regulator circuit fluctuating the reference voltage value so that the reference voltage value has positive correlation with fluctuation of a set value of the output voltage. 
         [0016]    According to another aspect of the present invention, the DC-DC converter further includes a constant voltage supply; and a resistance element having one terminal connected to the constant voltage supply. The reference voltage value is voltage of the other terminal of the resistance element. The cancel out circuit is the circuit to decrease current flowing in the resistance element when the input voltage decreases and to increase current flowing in the resistance element when the input voltage increases. The regulator circuit is the circuit to flow current with a first current value to the resistance element when the set value of the output voltage is a first voltage value and to flow current with a second current value that is smaller than the first current value to the resistance element when the set value of the output voltage is a second voltage value that is larger than the first voltage value. 
         [0017]    As stated above, the cancel out circuit and the regulator circuit control the current flowing in the same resistance element. By having such a configuration, it is possible to change the threshold value for overcurrent detection in accordance with both the fluctuations of the input voltage and the set value of the output voltage in a simpler configuration than a configuration providing the cancel out circuit and the regulator circuit on separate bodies. 
         [0018]    According to the present invention, it is possible to set the threshold value for overcurrent detection properly even when whichever one of the input voltage and the set value of the output voltage may fluctuate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
           [0020]      FIG. 1  is a diagram to describe a DC-DC converter of a prior art; 
           [0021]      FIGS. 2A to 2E  are diagrams showing a current waveform of the DC-DC converter; and 
           [0022]      FIG. 3  is a diagram to describe a DC-DC converter of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    The invention will now be described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
         [0024]    A DC-DC converter according to the present embodiment will be described below with reference to  FIG. 3 . 
         [0025]    A DC-DC converter  1  comprises an input terminal Tin, an output terminal Tout, switches Tr 1 , Tr 2 , and Tr 3  composed of MOS transistors, an inductor  11 , a diode  12 , a capacitor  13 , resistance elements  14  and  15 , a pulse generator  16 , a buffer  17 , and a controller  18 . 
         [0026]    An input voltage Vin is applied to the input terminal Tin from an input voltage supply such as a battery CE. A lithium-ion battery can be used as the battery CE, for example. 
         [0027]    The switches Tr 2  and Tr 3  are turned on and off by a periodic pulse signal PS from the pulse generator  16 . The switch Tr 1  is turned on and off according to a signal from an overcurrent detection circuit  19  described below. 
         [0028]    The controller  18  includes an output voltage set register  181 . The controller  18  controls a Duty ratio of the pulse signal PS generated by the pulse generator  16  according to the value of output voltage set by the register. The DC-DC converter  1  converts the input voltage Vin into an output voltage Vout depending on the Duty ratio. Then the DC-DC converter  1  outputs the output voltage Vout from the output terminal Tout. 
         [0029]    While the switch Tr 3  is on, a monitor current Im flows in the resistance elements  14 . The monitor current Im corresponds to a magnitude of a current IL that flows in the inductor  11 . Voltage generated by an IR drop caused by the monitor current Im flowing in the resistance element  14  (monitor voltage value Vm) appears in one end of the resistance element  14  (node N 1 ). In the present invention, we assume that the monitor current Im is the current that flows in the resistance element  14 . However, it is not limited to this embodiment. For example, the monitor current Im may be the current that flows in the diode  12  or may be the current that flows in the switch Tr 2 . All that is required here is that the monitor current Im either directly or indirectly reflects the current that flows in the inductor  11 . In other words, all that is required here is that the monitor voltage value Vm is generated by converting the value of the current flowing in the inductor  11  into the voltage value. 
         [0030]    The monitor voltage value Vm is amplified by an amplifier  20 . Then the amplified monitor voltage value Vm is input to the comparator  21 . It is not absolutely necessary that the monitor voltage value Vm is amplified using the amplifier  20 . However, when the monitor voltage value Vm is amplified using the amplifier  20 , it is possible to make a comparison in a comparator  21  with a high degree of accuracy. The comparator  21  compares the amplified monitor voltage value Vma with a reference voltage value Vref. Then the comparator  21  outputs the comparison result RS to the overcurrent detection circuit  19 . 
         [0031]    When the comparison result RS shows that the amplified monitor voltage value Vma is larger than the reference voltage value Vref, the overcurrent detection circuit  19  turns off the switch Tr 1  and stops the operation of the DC-DC converter  1  to prevent the DC-DC converter  1  from being broken down due to the overcurrent. 
         [0032]    Now, a description will be made on how the reference voltage value Vref occurs and how to regulate the reference voltage value Vref in accordance with the fluctuations of the input voltage Vin and the set value of the output voltage Vout. 
         [0033]    A resistance element R 1  has one end connected to the constant voltage supply VREG. The other end of the resistance element R 1  (node N 2 ) has the reference voltage value Vref. The reference voltage value Vref is therefore expressed by the expression 
         [0000]        V ref= V reg− I 1* r 1, 
         [0000]    where Vreg is, the voltage value of the constant voltage supply VREG, I 1  is the current value that flows in the resistance element R 1 , and r 1  is the resistance value of the resistance element R 1 . Therefore, it is possible to regulate the reference voltage value Vref by changing the current value I 1  that flows in the resistance element R 1 . 
         [0034]    The current value I 1  can be regulated by a cancel out circuit  40  and a regulator circuit  60 . 
         [0035]    The cancel out circuit  40  includes a current mirror  41  composed of a pair of transistors Tr 4  and Tr 5 , a transistor Tr 6 , an operational amplifier  42 , resistance elements Rd 1  (resistance value rd 1 ) and Rd 2  (resistance value rd 2 ), R 2  (resistance value r 2 ), the constant voltage supply VREG, an input voltage terminal Tin 2  to which the input voltage Vin is supplied. One transistor Tr 5  which composes the current mirror  41  is connected to the resistance element R 1 . The current value I 1  can be controlled by controlling the current I 2  that flows in the transistor Tr 5 . 
         [0036]    Voltage of one input IN 1  of the operational amplifier  42  is the voltage of a contact point of the resistance element Rd 1  and the resistance element Rd 2  (node N 3 ). The voltage of the node N 3  is expressed by Vreg*rd 2 /(rd 1 +rd 2 ). Voltage of the other input IN 2  of the operational amplifier  42  is the voltage smaller than the input voltage Vin by the IR drop caused by the resistance element R 2 . The output of the operational amplifier  42  is connected to a gate electrode of the transistor Tr 6 . 
         [0037]    When the input voltage Vin becomes smaller in the cancel out circuit  40 , the current that flows in the current mirror circuit  41  becomes smaller, which makes the current I 2  that flows in the transistor Tr 5  smaller. When the current I 2  becomes smaller, the current I 1  that flows in the resistance element R 1  becomes smaller, which makes the reference voltage value Vref larger. In summary, when the input voltage Vin becomes smaller, the reference voltage value Vref becomes larger. On the contrary, when the input voltage Vin becomes larger, the current I 1  that flows in the resistance element R 1  becomes larger, which makes the reference voltage value Vref smaller. 
         [0038]    As stated above, the cancel out circuit  40  fluctuates the reference voltage value Vref so that the reference voltage value has negative correlation with the fluctuation of the input voltage Vin. 
         [0039]    The regulator circuit  60  includes a current mirror  61  composed of transistors Tr 7  and Tr 8 , a variable resistance element R 3  (variable resistance value r 3 ), an operational amplifier  62 , a transistor Tr 9 , and the constant voltage supply VREG. One transistor Tr 8  of the current mirror  61  is connected to the resistance element R 1 . The current value I 1  can be controlled by controlling a current I 3  that flows in the transistor Tr 8 . 
         [0040]    Voltage of one input IN 3  of the operational amplifier  62  is the voltage of the node N 3  described above. Voltage of the other input IN 4  of the operational amplifier  62  is the voltage smaller than the voltage Vreg of the constant voltage supply VREG by an amount of the IR drop caused by the variable resistance element R 3 . The output of the operational amplifier  62  is input to the gate electrode of the transistor Tr 9 . 
         [0041]    The resistance value r 3  of the variable transistor element R 3  is controlled by the controller  18 . The controller  18  controls the resistance value of the variable resistance element R 3  depending on the value of the output voltage Vout set in the output voltage set resistor  181 . 
         [0042]    When the controller  18  controls the resistance value r 3  of the variable resistance element R 3  to decrease, the current that flows in the current mirror  61  becomes larger, which the current I 3  that flows in the transistor Tr 8  becomes larger. When the current I 3  becomes larger, the current I 1  that flows in the resistance element R 1  becomes larger, which the reference voltage value Vref becomes smaller. On the contrary, when the controller  18  controls the resistance value r 3  of the variable resistance element R 3  to increase, the reference voltage value Vref becomes larger. As stated above, the reference voltage value Vref can be controlled by controlling the resistance value r 3  of the variable resistance element R 3 . 
         [0043]    When the set value of the output voltage Vout set by the output voltage set resistor  181  increases, the controller  18  increases the resistance value r 3  of the variable resistance element R 3 , for example. As a result, the regulator circuit  60  decreases the current I 1  that flows in the resistance element R 1 , which increases the reference voltage value Vref. 
         [0044]    On the contrary, when the set value of the output voltage Vout set by the output voltage set resistor  181  decreases, the controller  18  decreases the resistance value r 3  of the variable resistance element R 3 . As a result, the regulator circuit  60  increases the current I 1  that flows in the resistance element R 1 , which decreases the reference voltage value Vref. 
         [0045]    As stated above, the regulator circuit  60  fluctuates the reference voltage value Vref so that the reference voltage value has positive correlation with the fluctuation of the set value of the output voltage Vout. 
         [0046]    A behavior of the reference voltage value Vref against the input voltage Vin and the set value of the output voltage Vout can be expressed by the following equation. 
         [0000]    
       
         
           
             
               
                 V 
                  
                 
                     
                 
                  
                 ref 
               
               = 
               
                 
                   V 
                    
                   
                       
                   
                    
                   reg 
                 
                 - 
                 
                   
                     
                       
                         V 
                          
                         
                             
                         
                          
                         in 
                       
                       - 
                       
                         K 
                         × 
                         V 
                          
                         
                             
                         
                          
                         reg 
                       
                     
                     
                       r 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   × 
                   r 
                    
                   
                       
                   
                    
                   1 
                 
               
             
              
             
               
                 
                   
                     V 
                      
                     
                         
                     
                      
                     reg 
                   
                   - 
                   
                     K 
                     × 
                     V 
                      
                     
                         
                     
                      
                     reg 
                   
                 
                 
                   r 
                    
                   
                       
                   
                    
                   3 
                 
               
               × 
               r 
                
               
                   
               
                
               1 
             
           
         
       
     
         [0047]    A second term of the above equation is a contribution made by the cancel out circuit  40  and a third term of the above equation is a contribution made by the regulator circuit  60 . Note that K=rd 2 /(rd 1 +rd 2 ). 
         [0048]    Note that the DC-DC  1  converter includes at least one of the cancel out circuit  40  and regulator circuit  60 . 
         [0049]    Note that a process variation of the voltage Vreg of the constant voltage supply VREG can be made small by trimming a fuse after being produced, for example. The process variation of the resistance values of the resistance elements R 1 , R 2 , R 3 , Rd 1 , and Rd 2  cancels with each other. Therefore, it is possible to decrease the variation that appears in the reference voltage value Vref, which to generate the reference voltage value Vref that has high accuracy. 
         [0050]    It is apparent that the present invention is not limited to the above embodiment, but may be modified and changed without departing from the scope and spirit of the invention.