Abstract:
The invention provides a bootstrap circuit which enables adequate charging of a capacitor used in the bootstrap circuit even during light load or no load conditions, and which does not impede the performance of a step-down converter proper, as well as a step-down converter using the bootstrap circuit. A capacitor charge/discharge path formation mechanism is provided in the bootstrap circuit that enables a terminal of a capacitor used in the bootstrap circuit to be separated and made independent from a step-down converter circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a bootstrap circuit which, in order to perform switching control by applying a voltage from a driver to a gate of a switching device which uses an N-channel MOSFET having a drain to which an input voltage is supplied, has a capacitor which steps up a power supply voltage of the driver to the input voltage or higher, as well as a step-down converter using this circuit, and in particular this invention enables adequate charging of a capacitor used in a bootstrap circuit even during light load or no load. 
         [0002]    In a step-down converter (step-down type DC-DC converter) which uses an N-channel MOSFET as a switching device, a circuit (generally called a bootstrap circuit), having a capacitor which steps up the power supply voltage of the driver to the input voltage for input to the switching device or higher, in order to apply the high-side driver voltage to the gate of the switching device and perform switching control, is necessary.  FIG. 3A  through  FIG. 3C  are diagrams which explain the configuration and operation of a step-down converter comprising a bootstrap circuit of the prior art. In general, a step-down converter drives a driver (Q 1  driver)  12  according to a PWM (Pulse Width Modulation) signal  11 , as shown in  FIG. 3A , and by supplying an inductor current I L  to the inductance L 1  ( 15 ) from the input voltage VCC during the on interval of the switching device Q 1  ( 13 ), energy is stored in the inductance L 1  ( 15 ), and the stored energy is discharged to the load, via the path of ground potential→inductance L 1  ( 15 )→load during the off interval of the switching device Q 1  ( 13 ) (hereafter, this circuit is called a “step-down converter circuit”), to realize a step-down converter. Here, the diode D 1  ( 14 ) (in  FIG. 4A  described below, the on-state switch Qs ( 23 ), or a PN junction diode  24  fabricated by semiconductor processes when manufacturing the switch Qs ( 23 )) provides a current path for current to flow from the inductance L 1  ( 15 ) to the load during the off intervals of the switching device Q 1  ( 13 ). The capacitor  16  functions as a smoothing capacitor to smooth the output voltage. 
         [0003]    As shown in  FIG. 3A , a bootstrap circuit  10  of the prior art comprises a power supply VREG ( 2 ), diode D B  ( 4 ), and capacitor C B  ( 6 ); the capacitor C B  ( 6 ) used in the bootstrap circuit is charged by current I CB  from the power supply VREG ( 2 ) via the diode D B  ( 4 ). The bootstrap circuit  10  is used as a power supply by the driver (Q 1  driver)  12  which operates the high-side switching device Q 1  ( 13 ), and by driving the driver (Q 1  driver)  12  according to PWM signals  11 , on/off control of the switching device Q 1  ( 13 ) is executed to realize a step-down converter.  FIG. 3B  explains operation of the step-down converter shown in  FIG. 3A  during intervals in which the switching device Q 1  is on, and  FIG. 3C  explains operation during intervals in which the switching device Q 1  is off. 
         [0004]    When, as shown in  FIG. 3C , the N-channel MOSFET Q 1  ( 13 ) is turned off, the capacitor C B  ( 16 ) used in the bootstrap circuit is charged by the current I CB  from the power supply VREG ( 2 ), via the diode D B  ( 4 ). On the other hand, when as in  FIG. 3B  the N-channel MOSFET Q 1  ( 13 ) is turned on, the voltage (VREG-VFB) (where VFB is the forward-direction voltage of the diode D B  ( 4 )) across the capacitor C B  ( 6 ) used in the bootstrap circuit, added to the input voltage VCC (VREG−VFB+VCC), is used to drive the high-side driver (Q 1  driver)  12 , to perform switching control of the N-channel MOSFET Q 1  ( 13 ). This bootstrap circuit can operate on the same principle in the conventional synchronous rectification-type step-down converter shown in  FIG. 4A  through  FIG. 4C , or in the conventional diode rectification-type step-down converter shown in  FIG. 5A  through  FIG. 5C . 
         [0005]    When charging the capacitor C B  ( 6 ) used in the bootstrap circuit in the circuit shown in  FIG. 3A  or  FIG. 4A , first D 1  ( 14 ) in  FIG. 3A  or Qs ( 23 ) or the PN junction diode  24  in  FIG. 4A  must be made conducting, and the potential at the CB-terminal must be set to GND level (strictly speaking, the voltage shifted from GND level by the voltage drop of D 1  ( 14 ), the PN junction diode  24 , or Qs ( 23 )) and fixed. Further, when there is light load or no load, the load current Io decreases, and even when the diode D 1  ( 14 ) is conducting during the off interval of the switching device Q 1  ( 13 ) in  FIG. 3C , an adequate charging current I CB  can no longer be secured. That is, the charging current I CB  is a portion of the inductor current I L  (I CB &lt;I L ), and the average value of the inductor current I L  is equal to the average value of the load current Io, so that when the load current Io is small, the charging current I CB  can no longer be made large. Also, when the inductor current I L  becomes zero, the CB-terminal cannot be held at GND potential, so that the capacitor C B  ( 6 ) cannot be charged adequately, the charged voltage of the capacitor C B  ( 6 ) used in the bootstrap circuit falls, and ultimately the switching device Q 1  ( 13 ) can no longer be driven. Hence a circuit is also necessary to avoid insufficient charging of the capacitor C B  ( 6 ) used in the bootstrap circuit. 
         [0006]      FIG. 4A  through  FIG. 4C  explain the configuration and operation of a synchronous rectification-type step-down converter comprising a bootstrap circuit of the prior art.  FIG. 4A  shows the configuration of the synchronous rectification-type step-down converter comprising the conventional bootstrap circuit,  FIG. 4B  explains operation during intervals in which the switching device Q 1  is on in the synchronous rectification-type step-down converter shown in  FIG. 4A , and  FIG. 4C  explains operation during intervals in which the switching device Q 1  is off.  FIG. 4A  through  FIG. 4C  are graphs equivalent to  FIG. 3A  through  FIG. 3C  respectively, and the configuration and operation are the same other than for the portions of the switch Qs ( 23 ) and the diode D 1  ( 14 ). 
         [0007]    In the synchronous rectification-type step-down converter of  FIG. 4A  through  FIG. 4C , during no load or light load, a reverse inductor current I L  flows during an interval in which the switching device Q 1  ( 13 ) is off, worsened efficiency may result, and so it is necessary to detect reverse flow of the inductor current I L  and cut off the switch Qs ( 23 ) on the synchronous rectification side. However, when such a cutoff function is added, if the load current Io is very small, then the current charging the capacitor C B  ( 6 ) used in the bootstrap circuit is limited by the inductor current I L  in the intervals in which the switching device Q 1  ( 13 ) is off and moreover the synchronous rectification-side switch Qs ( 23 ) is on, and so similarly to the case of  FIG. 3C , the capacitor C B  ( 6 ) used in the bootstrap circuit can no longer be charged. Therefore, in general control of the switch Qs ( 23 ) is executed such that the flow of the inductor current I L  is intentionally reversed, as shown in  FIG. 4C , during an interval sufficient to enable charging of the capacitor C B  ( 6 ) used in the bootstrap circuit. As an example of this type of technique of the prior art, for example, the circuit described in the Specification of U.S. Pat. No. 6,747,441 is known. That is, as indicated in FIG. 4 and FIG. 5 of U.S. Pat. No. 6,747,441, the low-side transistor permits reverse flow of current to secure a time period for charging the capacitor  76  of the bootstrap circuit. 
         [0008]      FIG. 5A  through  FIG. 5C  explain the configuration and operation of a diode rectification-type step-down converter comprising a bootstrap circuit of the prior art.  FIG. 5A  shows the configuration of another diode rectification-type step-down converter comprising a bootstrap circuit of the prior art;  FIG. 5B  explains operation of the diode rectification-type step-down converter shown in  FIG. 5A  during an interval in which the switching device Q 1  is turned on; and  FIG. 5C  explains operation during an interval in which the switching device Q 1  is turned off.  FIG. 5A  through  FIG. 5C  are equivalent to  FIG. 3A  through  FIG. 3C , respectively, and other than the switch Q B  ( 33 ) and the driver thereof (Q B  driver)  32 , the configuration and operation are the same. In contrast with the synchronous rectification design in  FIG. 4A  through  FIG. 4C , in the case of the diode rectification-type step-down converter of  FIG. 5A  through  FIG. 5C , to the CB-terminal of the capacitor C B  ( 6 ) used in the bootstrap circuit are added a switch Q B  ( 33 ) and a driver therefor (Q B  driver)  32 , to connect the CB-terminal to ground in order to secure a current path during charging. By this means, similarly to the principle of synchronous rectification of  FIG. 4A  through  FIG. 4C , by turning the switch Q B  ( 33 ) on during intervals in which the switching device Q 1  ( 13 ) is off, as shown in  FIG. 5C , charging of the capacitor C B  ( 6 ) used in the bootstrap circuit is made possible, even when there is no inductor current I L . As an example of the prior art of this type, for example, the circuit described in U.S. Pat. No. 6,798,269 is known. That is, the switch Qs shown in FIG. 6 of U.S. Pat. No. 6,798,269 is equivalent to the switch Q B  of  FIG. 5A  through  FIG. 5C , and similarly to the switch Q B  of  FIG. 5A  through  FIG. 5C , by turning the switch Qs on during intervals in which the switching device Q is off, charging of the capacitor C B  used in the bootstrap circuit is possible even when there is no inductor current. 
         [0009]    Further, in the prior art step-down converters comprising a bootstrap circuit such as that described in Japanese Patent Laid-open No. 10-56776 are known. That is, in a step-down converter comprising a bootstrap circuit described in Japanese Patent Laid-open No. 10-56776, when loading becomes light, the switching frequency is lowered and time to charge the capacitor used in the bootstrap circuit is secured. 
         [0010]    Because during light load or no load of step-down converters of the prior art, including those of the above-described U.S. Pat. No. 6,747,441 and U.S. Pat. No. 6,798,269, the capacitor C B  used in the bootstrap circuit is charged, during off intervals of the switching device Q 1  control is executed to turn on switch QS in a synchronous rectification-type device and to turn on switch Q B  in a diode rectification-type device. In this case, by changing the source-side potential of the switching device Q 1 , that is, by changing the inductor current, the current path of the step-down converter itself is affected, so that compared with the step-down converter proper without a bootstrap circuit, power supply efficiency worsening, increases in output ripple, and other side-effects occur, and so there is the problem that the performance of the step-down converter proper is impeded. 
         [0011]    In control during light load of the step-down converter in the above-described Japanese Patent Laid-open No. 10-56776, because the ratio of the time during which the capacitor is being charged to the time during which the capacitor cannot be charged does not change, the average charged voltage remains low. During light load, the charging time is lengthened to a certain extent, so that instantaneous driving capacity can be secured, but on the other hand, because the time during which charging is not possible (that is, the discharge interval) is also lengthened, the charged voltage falls immediately, and as the frequency is lowered, there is the problem that the time over which driving capacity is insufficient is also longer. 
       SUMMARY OF THE INVENTION 
       [0012]    The invention provides a bootstrap circuit which enables adequate charging of the capacitor used in the bootstrap circuit even during light load or no load, and which does not impede the performance of the step-down converter proper, as well as a step-down converter using such a circuit. 
         [0013]    In a preferred embodiment, a bootstrap circuit in accordance with the invention, having a capacitor which steps up a power supply voltage of a driver to an input voltage or higher, in order to perform switching control by applying a voltage from the driver to a gate of a switching device employing an N-channel MOSFET having a drain to which the input voltage is supplied, includes a capacitor charge/discharge path formation mechanism, which forms, independently of a step-down converter circuit, a charge/discharge path for charging the capacitor in synchronization with an off state of the switching device, and for discharging the capacitor in synchronization with an on state of the switching device for application as the power supply voltage to the driver. 
         [0014]    In a bootstrap circuit of this invention, the CB-terminal of the capacitor C B  used in the bootstrap circuit is connected, via the capacitor charge/discharge path formation means, to the step-down converter circuit, and by this means the path for charging the capacitor C B  used in the bootstrap circuit is made independent. As a result, effects on the step-down converter during charging of the capacitor C B , that is, the occurrence of power supply efficiency worsening, increases in output ripple, and other side effects, can be avoided. Moreover, the capacitor C B  used in the bootstrap circuit can always be charged with stability, regardless of the load state, such as for example when the load is light or there is no load. 
         [0015]    Further, a step-down converter including a bootstrap circuit of this invention includes a bootstrap circuit having capacitor charge/discharge path formation mechanism; the CB-terminal of the capacitor C B  used in the bootstrap circuit is connected, via the capacitor charge/discharge path formation mechanism, to the step-down converter circuit, and by this mechanism the current path to charge the capacitor C B  used in the bootstrap circuit is made independent. As a result, effects on the step-down converter during charging of the capacitor C B , that is, the occurrence of power supply efficiency worsening, increases in output ripple, and other side effects, can be avoided, so that stable operation and improved power supply efficiency of the step-down converter circuit can be expected. Moreover, the capacitor C B  used in the bootstrap circuit can always be charged with stability, regardless of the load state, such as for example when the load is light or there is no load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention will now be described with reference to certain preferred embodiments thereof and the accompanying drawings, wherein: 
           [0017]      FIG. 1A  shows the configuration of a first embodiment of a step-down converter comprising a bootstrap circuit of an aspect of the invention; 
           [0018]      FIG. 1B  explains operation during on intervals of a switching device Q 1  in the step-down converter of the first embodiment shown in  FIG. 1A ; 
           [0019]      FIG. 1C  explains operation during off intervals of the switching device Q 1  in the step-down converter of the first embodiment shown in  FIG. 1A ; 
           [0020]      FIG. 2A  shows the configuration of a second embodiment of a step-down converter comprising a bootstrap circuit of an aspect of the invention; 
           [0021]      FIG. 2B  explains operation during on intervals of a switching device Q 1  in the step-down converter of the second embodiment shown in  FIG. 2A ; 
           [0022]      FIG. 2C  explains operation during off intervals of the switching device Q 1  in the step-down converter of the second embodiment shown in  FIG. 2A ; 
           [0023]      FIG. 3A  shows the general configuration of a step-down converter comprising a bootstrap circuit of the prior art; 
           [0024]      FIG. 3B  explains operation during on intervals of a switching device Q 1  in the step-down converter shown in  FIG. 3A ; 
           [0025]      FIG. 3C  explains operation during off intervals of the switching device Q 1  in the step-down converter shown in  FIG. 3A ; 
           [0026]      FIG. 4A  shows the configuration of a synchronous rectification-type step-down converter comprising a bootstrap circuit of the prior art; 
           [0027]      FIG. 4B  explains operation during on intervals of a switching device Q 1  in the synchronous rectification-type step-down converter shown in  FIG. 4A ; 
           [0028]      FIG. 4C  explains operation during off intervals of the switching device Q 1  in the synchronous rectification-type step-down converter shown in  FIG. 4A ; 
           [0029]      FIG. 5A  shows the configuration of a diode rectification-type step-down converter comprising a bootstrap circuit of the prior art; 
           [0030]      FIG. 5B  explains operation during on intervals of a switching device Q 1  in the diode rectification-type step-down converter shown in  FIG. 5A ; and, 
           [0031]      FIG. 5C  explains operation during off intervals of the switching device Q 1  in the diode rectification-type step-down converter shown in  FIG. 5A . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    A bootstrap circuit in accordance with the invention, which is the bootstrap circuit  100  shown in  FIG. 1A  or  FIG. 2A , comprises, in addition to the power supply VREG ( 2 ), diode D B  ( 4 ), and capacitor C B  ( 6 ) used in the bootstrap circuit, which are the constituent components of the bootstrap circuit  10  of the prior art shown in  FIG. 4A  or  FIG. 5A , a configuration which connects the CB-terminal of the capacitor C B  ( 6 ) used in the bootstrap circuit to the drains of the P-channel MOSFET Qx ( 112 ) and the N-channel MOSFET Qy ( 114 ), connects the gate of the P-channel MOSFET Qx ( 112 ) to the output side of the Qx driver ( 111 ) which drives the switch Qx, connects the source of the P-channel MOSFET Qx ( 112 ) to the source terminal of the switching device Q 1  ( 13 ), and on the other hand connects the gate of the N-channel MOSFET Qy ( 114 ) to the output side of the Qy driver ( 113 ) which drives the switch Qy, and grounds the source of the N-channel MOSFET Qy ( 113 ). 
         [0033]    A configuration (capacitor charge/discharge path formation means)  110  is added which, by turning on the switch Qx ( 112 ) in synchronization with the on intervals of the switching device Q 1  ( 13 ) according to PWM (Pulse Width Modulation) signals  11 , the CB-terminal is connected to the source terminal of the switching device Q 1  ( 13 ), and by turning on the switch Qy ( 114 ) in synchronization with the off intervals of the switching device Q 1  ( 13 ) grounds the CB-terminal, so that the CB-terminal of the capacitor C B  ( 6 ) used in the bootstrap circuit is separated and made independent from the step-down converter circuit. Here, “step-down converter circuit” means the circuit which, by means of the above-described PWM signals  11 , drives the switching device Q 1  ( 13 ) via the high-side driver (Q 1  driver)  12 , and by supplying the inductor current I L  from the input voltage VCC to the inductance L 1  ( 15 ) during on intervals of the switching device Q 1  ( 13 ), stores energy in the inductance L 1  ( 15 ), and which discharges stored energy to the load and/or capacitor  16  through the path of the ground potential→inductance L 1  ( 15 )→load during off intervals of the switching device Q 1  ( 13 ). 
         [0034]    The switch Qs ( 23 ) is driven by inverting the PWM signals  11  via the low-side driver (Qs driver)  22 , and the switching device Q 1  ( 13 ) and switch Qs ( 23 ) are turned on and off in a complementary manner, so that both are never turned on simultaneously. Further, the low-side driver (Qs driver)  22  functions to turn off the switch Qs ( 23 ) when a protection circuit, not shown, detects backflow of the inductor current I L . 
         [0035]    Thus in the bootstrap circuit of this aspect of the invention, capacitor charge/discharge path formation means is provided, and by connecting the CB-terminal of the capacitor C B  used in the bootstrap circuit to the step-down converter circuit via this capacitor charge/discharge path formation means, the CB-terminal of the capacitor C B  used in the bootstrap circuit can be separated and made independent from the step-down converter circuit. Because the current path to charge the capacitor C B  used in the bootstrap circuit is made independent, effects on the step-down converter circuit, that is, the occurrence of power supply efficiency worsening, increases in output ripple, and other side effects, can be avoided. Moreover, the capacitor C B  used in the bootstrap circuit can always be charged with stability, regardless of the load state, such as for example when the load is light or there is no load. 
         [0036]      FIG. 1A  through  FIG. 1C  show a first embodiment of a step-down converter comprising a bootstrap circuit of an aspect of the invention; in the first embodiment, the invention is applied to a synchronous rectification-type step-down converter.  FIG. 1A  shows the configuration of the first embodiment of a step-down converter comprising a bootstrap circuit of an aspect of the invention,  FIG. 1B  explains operation during on intervals of the switching device Q 1  in the step-down converter of the first embodiment shown in  FIG. 1A , and  FIG. 1C  explains operation during off intervals of the switching device Q 1 . The first embodiment of course comprises the bootstrap circuit  100  of the aspect of the invention described above. Similarly to the synchronous rectification-type step-down converter of the prior art shown in  FIG. 4A  through  FIG. 4C , in the synchronous rectification-type step-down converter of  FIG. 1A  to  FIG. 1C  also, the switching device Q 1  ( 13 ) is driven by PWM signals  11  via the driver (Q 1  driver)  12 , and by supplying an inductor current I L  from the input voltage VCC to the inductance L 1  ( 15 ) during on intervals of the switching device Q 1  ( 13 ), energy is stored in the inductance L 1  ( 15 ), and energy stored in the inductance L 1  ( 15 ) is discharged to the load and/or capacitor  16  during off intervals of the switching device Q 1  ( 13 ) to realize the step-down converter. Here, the PN junction diode  24  fabricated by semiconductor processes when manufacturing the on-state switch Qs ( 23 ) or switch Qs ( 23 ) provides a path for current flowing from the inductance L 1  ( 15 ) to the load during intervals in which the switching device Q 1  ( 13 ) is off, and the capacitor  16  functions as a smoothing capacitor to smooth the output voltage. 
         [0037]    During intervals in which the above-described switching device Q 1  ( 13 ), which operates according to the PWM signals  11 , is turned off, the bootstrap circuit  100  drives the switch Qy ( 114 ) by inversion of the PWM signals  11  via the Qy driver ( 113 ), as shown in  FIG. 1C , so that the switch Qy ( 114 ) is turned on and the CB-terminal is grounded in synchronization with the off intervals of the switching device Q 1  ( 13 ). By this means, the capacitor C B  ( 6 ) used in the bootstrap circuit can be charged by the current I CB , via the path from the power supply VREG ( 2 ) through the diode D B  ( 4 ), capacitor C B  ( 6 ) and switch Qy ( 114 ). 
         [0038]    Further, during on intervals of the switching device Q 1  ( 13 ), by using the PWM signals  11  to drive the switch Qx ( 112 ) via the Qx driver ( 111 ) as shown in  FIG. 1B , to turn on the switch Qx ( 112 ) in synchronization with the on intervals of the switching device Q 1  ( 13 ), the CB-terminal is connected to the source terminal of the switching device Q 1  ( 13 ). By this means, the gate terminals of the high-side driver (Q 1  driver)  12  and switching device Q 1  ( 13 ) are driven by the voltage resulting by adding the voltage to which the capacitor C B  ( 6 ) used in the bootstrap circuit is charged and the input voltage VCC, and the switching device Q 1  ( 13 ) can be turned on. By turning on the switching device Q 1  ( 13 ), the inductor current I L  from the input voltage VCC is supplied to the inductor L 1  ( 15 ), and energy can be stored in the inductance L 1  ( 15 ). The switches Qx ( 112 ) and Qy ( 114 ) are turned on and off in a complementary manner, so that both are never turned on simultaneously. 
         [0039]    In this first embodiment of a step-down converter comprising a bootstrap circuit of an aspect of this invention, a bootstrap circuit is comprised having capacitor charge/discharge path formation mechanism or means, and by connecting the CB-terminal of the capacitor C B  used in the bootstrap circuit to the step-down converter circuit via the capacitor charge/discharge path formation mechanism, the current path to charge the capacitor C B  used in the bootstrap circuit can be made independent. As a result, effects on the step-down converter circuit, that is, the occurrence of power supply efficiency worsening, increases in output ripple, and other side effects, can be avoided, so that stable operation and improved power supply efficiency of the step-down converter circuit can be expected. Moreover, the capacitor C B  used in the bootstrap circuit can always be charged with stability, regardless of the load state, such as for example when the load is light or there is no load. 
         [0040]      FIG. 2A  through  FIG. 2C  show a second embodiment of a step-down converter comprising the bootstrap circuit of an aspect of the invention; in the second embodiment, the invention is applied to a diode rectification-type step-down converter.  FIG. 2A  shows the configuration of the second embodiment of the step-down converter comprising the bootstrap circuit of an aspect of the invention,  FIG. 2B  explains operation during on intervals of the switching device Q 1  in the step-down converter of the second embodiment shown in  FIG. 2A , and  FIG. 2C  explains operation during off intervals of the switching device Q 1 . The second embodiment of course comprises the bootstrap circuit  100  of the aspect of the invention described above. Similarly to  FIG. 3A  through  FIG. 3C  or to the diode rectification-type step-down converter of the prior art shown in  FIG. 5A  through  FIG. 5C , in the diode rectification-type step-down converter of  FIG. 2A  to  FIG. 2C  also, the switching device Q 1  ( 13 ) is driven by PWM signals  11  via the driver (Q 1  driver)  12 , and by supplying an inductor current I L  from the input voltage VCC to the inductance L 1  ( 15 ) during on intervals of the switching device Q 1  ( 13 ), energy is stored in the inductance L 1  ( 15 ), and energy stored in the inductance L 1  ( 15 ) is discharged to the load and/or capacitor  16  during off intervals of the switching device Q 1  ( 13 ) to realize the step-down converter. Here, the diode D 1  ( 14 ) provides a path for current flowing from the inductance L 1  ( 15 ) to the load during intervals in which the switching device Q 1  ( 13 ) is off, and the capacitor  16  functions as a smoothing capacitor which smoothes the output voltage. 
         [0041]    During intervals in which the above-described switching device Q 1  ( 13 ), which operates according to the PWM signals  11 , is turned off, the bootstrap circuit  100  drives the switch Qx ( 112 ) by inversion of the PWM signals  11  via the Qx driver ( 111 ), as shown in  FIG. 2C , so that the switch Qy ( 114 ) is turned on and the CB-terminal is grounded in synchronization with the off intervals of the switching device Q 1  ( 13 ). By this means, the capacitor C B  ( 6 ) used in the bootstrap circuit can be charged by the current I CB , via the path from the power supply VREG ( 2 ) through the diode D B  ( 4 ), capacitor C B  ( 6 ) and switch Qy ( 114 ). 
         [0042]    Further, during on intervals of the switching device Q 1  ( 13 ), by using the PWM signals  11  to drive the switch Qx ( 112 ) via the Qx driver ( 111 ) as shown in  FIG. 2B , to turn on the switch Qx ( 112 ) in synchronization with the on intervals of the switching device Q 1  ( 13 ), the CB-terminal is connected to the source terminal of the switching device Q 1  ( 13 ). By this means, the gate terminals of the high-side driver (Q 1  driver)  12  and switching device Q 1  ( 13 ) are driven by the voltage resulting by adding the voltage to which the capacitor C B  ( 6 ) used in the bootstrap circuit is charged and the input voltage VCC, and the switching device Q 1  ( 13 ) can be turned on. By turning on the switching device Q 1  ( 13 ), the inductor current I L  from the input voltage VCC is supplied to the inductor L 1  ( 15 ), and energy can be stored in the inductance L 1  ( 15 ). Further, the switches Qx ( 112 ) and Qy ( 114 ) are turned on and off in a complementary manner, so that both are never turned on simultaneously. 
         [0043]    In this second embodiment of a step-down converter comprising a bootstrap circuit of an aspect of this invention, a bootstrap circuit is comprised having capacitor charge/discharge path formation mechanism or means, and by connecting the CB-terminal of the capacitor C B  used in the bootstrap circuit to the step-down converter circuit via the capacitor charge/discharge path formation mechanism, the current path to charge the capacitor C B  used in the bootstrap circuit can be made independent. As a result, effects on the step-down converter circuit, that is, the occurrence of power supply efficiency worsening, increases in output ripple, and other side effects, can be avoided, so that stable operation and improved power supply efficiency of the step-down converter circuit can be expected. Moreover, the capacitor C B  used in the bootstrap circuit can always be charged with stability, regardless of the load state, such as for example when the load is light or there is no load. 
         [0044]    The invention has been described with reference to certain preferred embodiments thereof. It will be understood, however, that modifications and variations are possible within the scope of the appended claims. 
         [0045]    This application is based on, and claims priority to, Japanese Patent Application No: 2007-277022, filed on Oct. 24, 2007. The disclosure of the priority application, in its entirety, including the drawings, claims, and the specification thereof, is incorporated herein by reference.