Abstract:
A DC-DC boost converter is provided that generally maintains discontinuous mode operation in a generally efficient manner. To accomplish this, a clamp generator, comparator, logic gates, a flip-flop, and counter are employed. These components generally operate together to determine if an over-limit condition has taken place, so that the ON time of the boost converters&#39; switch can be varied accordingly.

Description:
[0001]    This patent application claims priority from Japanese Patent Application No. 2008-039920, filed 21 Feb. 2008, the entirety of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention pertains to a DC-DC converter that converts input DC (direct-current) power to arbitrary DC (direct-current) power, and relates in particular to a DC-DC boost converter with which a high output voltage can be obtained from the input voltage. 
       BACKGROUND OF THE INVENTION 
       [0003]    DC-DC converters are small, lightweight, high-efficiency direct-current power sources that use semiconductor switching elements. They are widely used for electronic equipment and the like, and in recent years, the demand for small, lightweight and high-efficiency converters has increased. The basic principal of a DC-DC converter is to turn a switching element on and off at high frequency, variably control the On/OFF period ratio, that is, the duty ratio, and keep the direct-current output voltage at a constant level. Types with which a high output voltage is obtained from the input voltage with a so-called non-insulated or chopper system are called DC-DC boost converters or boosters. 
         [0004]    A conventional, representative DC-DC boost converter (booster) is shown in  FIG. 5 . This booster, broadly categorizing, is constituted with two parts: booster core  100  and control  102 . 
         [0005]    Booster core  100  is comprised of an inductor  104 , NMOS transistor  106 , diode  108  and output capacitor  110 . More specifically, inductor  104  and NMOS transistor  106  are connected in series through node N between an input terminal, to which direct-current input voltage V 1  is input, and a ground potential terminal, diode  108  is connected between node N and output terminal  112 , and output capacitor  110  is connected between output terminal  112  and a ground potential terminal. 
         [0006]    Control  102  is comprised of reference voltage generation circuit  114 , error amplifier  116 , integrator  118 , clamp voltage generation circuit  120 , ramp (sawtooth or triangular wave) generation circuit  122 , comparators  124  and  126 , logic gate circuit (AND gate)  128 , and gate drive circuit  130 . More specifically, reference voltage generation circuit  114  generates reference voltage V REF  at a constant voltage level and supplies it to one input terminal (+) of error amplifier  116 . Output voltage V O  is input from booster core  100  to the other terminal (−) of error amplifier  116 . Error amplifier  116  takes the difference or error between the two voltages V O  and V REF , and outputs an output voltage according to the error as error signal V e . Error signal V E  is time-integrated by integrator  118 , and is supplied to one input terminal (+) of comparator  124  as integrated error signal V ES . 
         [0007]    Ramp voltage, for example, sawtooth wave V RAMP , synchronized with clock CLK is supplied from ramp-generation circuit  122  to the other terminal (−) of comparator  124 . Comparator  124  compares the voltage levels of the two input signals V RAMP  and V ES , and outputs a binary signal or pulse at H level when V RAMP &lt;V ES , and at L level when V ramp &gt;V es , as pulse width control signal or PWM control signal V PWM . The PWM control signal V PWM  is sent to one of the input terminals of AND circuit  128 . 
         [0008]    Sawtooth wave V RAMP  output from ramp-generation circuit  122  is also supplied to one input terminal (−) of the other comparator  126 . Clamp voltage V CLAMP  with a constant voltage level is input from clamp voltage generation circuit  120  to the other input terminal (+) of comparator  126 . Comparator  126  compares the voltage levels of the two input signals V RAMP  and V CLAMP , and outputs a binary signal or pulse at H level when V RAMP &lt;V CLAMP , and one at L level when V RAMP &gt;V CLAMP , as ON period upper limit signal V LIMIT . ON period upper limit signal V LIMIT  is supplied to the other input terminal of AND circuit  128 . 
         [0009]    AND circuit  128  outputs, as switching drive signal V DRIVE , a binary signal or pulse which is at H level when both input signals V PWM  and V LIMIT  are at H level, and is at L level when one or both are at L level. Here, because both input signals V PWM  and V LIMIT  are synchronized with clock CLK, switching drive signal V DRIVE  is also synchronized with clock CLK. Gate drive circuit  130  outputs gate voltage V G  in response to switching drive signal V DRIVE  from AND circuit  128 , and NMOS transistor  106  of core part  100  is switched. 
         [0010]    In booster core  100 , the ON period is when gate voltage V G  is at H level. During this period, NMOS transistor  106  is on, and inductance current I L  flows to the ground potential terminal through inductor  104  and NMOS transistor  106  from the voltage input terminal, and is stored in inductor  104 . The ON period is when gate voltage V G  is at L level. During this period, NMOS transistor  106  is off, and electromagnetic energy stored in inductor  104  prior to that is discharged toward output capacitor  110 . That is, inductance current I L  from inductor  104  flows into output capacitor  110  through node N and diode  108 , and output capacitor  110  is charged. 
         [0011]    The basic operation of the booster is shown in  FIG. 6 . As illustrated, gate voltage V G , inductance current I L  and the potential V L  of node N are all synchronized with clock CLK that has constant cycle T S . 
         [0012]    That is, gate voltage V G  rises to H level from the prior L level at the start of each cycle of clock CLK, and NMOS transistor  106  comes on. During the period when gate voltage V G  maintains the H level and NMOS transistor is on (ON period), inductance current I L  increases with slope V I /L (L is the inductance of inductor  104 ). At this time, potential V L  of node N is at ground potential (zero volts). 
         [0013]    Then, during the relevant cycle, when gate voltage V G  changes from H level to L level, NMOS transistor  106  goes off, switching from the ON period to the OFF period. Then when node N is disconnected from the ground potential, and assuming that the voltage drop from diode  108  is ideally zero, potential V L  of node N rises to a level equal to output voltage V O , and the destination of inductance current I L  is switched to output capacitor  110  from the prior ground potential terminal destination. Here, inductance current I L  decreases at a slope of (V O −V 1 )/L. 
         [0014]    Then, when inductance current I L  has decreased to zero amperes and there is no flow, at that instant, potential V L  of node N changes to a level equal to input voltage V 1  (potential of the voltage input terminal) from a level approximately equal to output voltage V O  prior to that, and this non-current state is maintained until the end of the OFF period or cycle concerned. 
         [0015]    When the next cycle of clock CLK begins, gate voltage V G  again rises to H level from the L level prior to that, and the operation described above is repeated. However, because feedback-type PWM control is performed in control  102 , the ratio of the ON period and the OFF periods, that is, the duty ratio, is changed for every cycle.  FIG. 6  shows where inductance current I L  has returned to zero amperes and is interrupted (called “discontinuous mode” hereafter). 
         [0016]      FIG. 7  shows where inductance current I L  in each cycle continues to flow, without returning to zero amperes, until the next cycle starts (called “continuous mode” hereafter). Such a continuous mode is reached when the On duty period is made longer. 
         [0017]    It is generally considered that a transfer function of the booster core  100  in the discontinuous mode can be approximated as a primary pole system, and its operation is stable. On the other hand, a transfer function in the continuous mode not only operates as a secondary pole system, but has an RHP (right half-plane) zero point, and compensation for this is complicated and difficult. Therefore, stable operation and simplification of the circuit configuration are achieved by its configuration as a booster that will normally operate in discontinuous mode. 
         [0018]    Here, when the duty for the period when NMOS transistor  106  is on is D 1 , and the duty for the period after NMOS transistor  106  switches to off from on until inductance current I L  reaches zero amperes is D 2 , relative to cycle T S  of clock CLK, duties D 1  and D 2  are represented with the following formulas based on input voltage V 1 , output voltage V O , inductor  104  inductance L and load current I o : 
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         [0000]    Pertaining to ON duty D 1 , the condition to maintain a discontinuous mode is that D 1  be smaller than ON duty D in a continuous mode, so it is stipulated by following equation (3). 
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         [0019]    In the booster in  FIG. 5 , the ON period within each cycle is primarily influenced by PWM control signal V PWM  output from comparator  124 , and is ultimately determined by the logical product (AND condition) of PWM control signal V PWM  and ON period upper limit signal V LIMIT  output from comparator  126 . That is, the pulse width of PWM control signal V PWM  is subject to the restriction on the pulse width of ON period upper limit signal V LIMIT , that is, the ON period upper limit, through AND circuit  128 , so that the pulse width (ON period) of switching drive signal V drive  will not exceed the ON period upper limit. 
         [0020]    Therefore, from equation (3) above, by setting the ON period upper limit stipulated by ON period upper limit signal V LIMIT  to {1−(V 1 /V O )}·V S  (where V S  is the peak value of sawtooth wave V RAMP ), discontinuous mode operation can be realized wherein shifting to a continuous mode is restricted. In this case, duty D C  of ON period upper limit signal V LIMIT  is represented with equation (4) below. 
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         [0021]    An example of operation when load current I O  varies in the increasing direction above a steady value over a certain period T A  in the booster in  FIG. 5  is shown in  FIG. 8  with the waveforms of various parts. 
         [0022]    As shown in  FIG. 8 , when load current I O  varies in the increasing direction, output voltage V O  becomes lower than reference voltage V REF , the voltage level of error signal V ES  rises because of this, the pulse width (H level period) of PWM control signal V PWM  becomes larger, and the period for which NMOS transistor  106  is on (ON period) becomes longer. When the ON period becomes longer, inductance current I L  increases, more energy is supplied to output capacitor  110  from inductor  104 , and output voltage V O  rises toward reference voltage V REF . 
         [0023]    Then when output voltage V O  rises and exceeds reference voltage V REF , the length of time during which NMOS transistor  106  is on (ON period) becomes shorter, the energy supplied to output capacitor  110  from inductor  104  decreases, and output voltage V O  drops toward reference voltage V REF . 
         [0024]    In this feedback-type PWM control, when the pulse width of PWM control signal V PWM  exceeds pulse width T S ·D C  of ON period upper limit signal V LIMIT , it is subject to the upper limit restriction, and the pulse width of ON period upper limit signal V LIMIT , and then the ON period (ON period) of NMOS transistor  106  becomes T S ·D C  in order not to exceed it, that is, in order not to shift to continuous mode operation from discontinuous mode operation. 
         [0025]    In a conventional booster as described above, if each of the elements that constitute booster core  100  has the ideal characteristics, the conditions in equations (3) and (4) above are effective in restricting a shift to a continuous mode and for realizing normal discontinuous mode operation. In actuality, however, duty D C  for the required ON period changes due to parasitic components in inductor  104 , NMOS transistor  106 , diode  108  and output capacitor  110 , so the conditions above are not sufficient. 
         [0026]    Also, in applications where input voltage V I  or output voltage V O  is variable, when the effects of ambient temperature or process fluctuation are taken into account, setting duty D C  for the ON period upper limit uniquely, as with the conventional duty control method described above, is not appropriate. For example, while it is possible to set D C  to a smaller value beforehand, there is the risk that excessive restriction will diminish the load characteristics of output voltage V O  or the responsiveness to sudden load changes. On the other hand, when D C  is set to a larger value, the possibility of the operating mode shifting from discontinuous mode operation to continuous mode operation becomes greater, and there is the risk that the stability of booster operation will be diminished. 
         [0027]    An example of shifting to continuous operating mode from discontinuous operating mode with an abrupt load change and the output voltage becoming unstable in the booster in  FIG. 5  is shown with simulated waveforms in  FIG. 9 . 
         [0028]    In  FIG. 9 , when load current I o  is changed from 1 mA (milliamperes) to 29 mA at T=5 ms, output voltage V o  starts to drop, but output (error signal) V es  of error amplifier  116  begins to rise. Along with this, the pulse width of PWM control signal V pwm  becomes larger, and inductance current I L  also gradually increases. 
         [0029]    From T=5 ms to 5.05 ms, inductance current I L  returns to zero (where I L =0 A) at each cycle or switching cycle and operation is in discontinuous mode. After T=5.05 ms, however, rather than returning to I L =0 A in each cycle, there is a shift to continuous mode. With the shift to continuous mode, booster operation becomes unstable, and low-frequency fluctuation (ringing) appears in output voltage V o . 
         [0030]    In recent years, applications using source voltage boosted from a single lithium battery, such as light-emitting diodes (LED) mounted in portable electronic equipment, or liquid crystal displays (LCD), have increased. With such applications, many of the boosters that are used operate in discontinuous mode where load current is not that large, from several mA to around 20 mA. For the output voltage of this type of booster, stability with little AC fluctuation, rather than DC or absolute value accuracy, is generally considered important. 
         [0031]    The present invention was devised taking into consideration the problems in the prior art as described above, with the objective of providing a DC-DC boost converter (booster) that is not subject to the effects of the usage environment or variation in circuit elements, and with which discontinuous mode operation can be maintained efficiently without generally diminishing load characteristics or responsiveness. 
       SUMMARY OF THE INVENTION 
       [0032]    In accordance with a preferred embodiment of the present invention, DC-DC boost converter of the invention has: an inductance element, a switching element connected in series with the inductance element through a node between an input terminal to which direct-current voltage is input and a reference potential terminal, a rectifying element connected between the node and an output terminal, an output capacitor connected between the output terminal and a reference potential terminal, a switching control circuit that divides each cycle regulated by a main clock with a constant frequency into 2 parts, that is, variable ON period and OFF period, and that turns the switching element on during the ON period and turns the switching element off during the OFF period, a current-monitoring circuit that monitors whether current is still flowing toward the output capacitor from the inductance element at the end of each cycle, and an ON period upper limit control circuit that restricts the ON period to no more than a predetermined upper limit for each cycle, and variably controls the upper limit of the ON period in the next cycle according to the monitoring result produced by the current-monitoring circuit. 
         [0033]    The switching control circuit is turned on and off at a variable duty so that the voltage of the output capacitor, that is, the output voltage, will match a desired reference voltage. During this process the current-monitoring circuit monitors whether current is flowing from the inductance element toward the output capacitor at the end of each switching cycle. If no current is flowing, operation is determined to be in discontinuous mode, and if current is still flowing, operation is determined to be in continuous mode. The ON period upper limit control circuit variably controls the upper limit of the ON period in each cycle according to the monitoring result produced by the current-monitoring circuit. In particular, when a monitoring result from the current-monitoring circuit indicates that current is still flowing at the end of the cycle concerned, the upper limit of the ON period in the next cycle can be made lower, and a shift to continuous mode can be prevented in this way, or a return to discontinuous mode from continuous mode can be accomplished promptly. 
         [0034]    In a preferred embodiment of the present invention, the current-monitoring circuit has a first comparator that compares the potential of the node and the potential of the output terminal and outputs a binary signal indicating the size relationship between the two potentials, and a judgment circuit that judges that current is still flowing when the potential of the node is higher than the potential of the output terminal at the end of each cycle, and judges that the current is not flowing when the potential of the node is lower than the potential of the output terminal. In this case, it is even more preferable for the current-monitoring circuit to also have a latch circuit that, at the end of each cycle regulated by the main clock, fetches the output signal of the first comparator in response to the rising edge or falling edge of a secondary clock, which has the same frequency as the main clock. The rising edge or falling edge of the main clock would be set to within the period of the final 10% of each cycle regulated by the main clock, and could also be variably adjusted within that range. 
         [0035]    In a preferred embodiment of the present invention, the switching control circuit also has an error signal generation circuit into which the voltage of the output capacitor and a constant reference voltage are input and which generates an error signal representing the error between the two voltages, a pulse width control circuit that generates a pulse width control signal indicating the desired ON period for reducing the error in the next cycle, and a switching drive circuit which receives the pulse width control signal from the pulse width control circuit and also receives the ON period upper limit signal indicating the upper limit of the ON period produced by the ON period upper limit control circuit, and which drives the switching element to an on state according to the pulse width control signal when the desired ON period does not exceed the upper limit, and which drives the switching element to an on state according to the ON period upper limit signal when the desired ON period does exceed the upper limit. Preferably, an integration circuit that time-integrates the error signal between the error signal generation circuit and the pulse width control circuit could also be provided. 
         [0036]    In a preferred embodiment of the present invention, the pulse width control circuit has a ramp-generation circuit that generates a sawtooth wave or a triangular wave synchronized with the main clock, and a second comparator that compares the error signal and the sawtooth wave or triangular wave, and outputs, as the pulse width control signal, a binary signal that has a first logical value when the voltage level of the error signal is higher than the voltage level of the sawtooth wave or triangular wave, and has a second logical value when the voltage level of the error signal is lower than the voltage level of the sawtooth wave or the triangular wave. 
         [0037]    In a preferred embodiment of the present invention, an ON period monitoring circuit is additionally provided that receives the pulse width control signal from the pulse width control circuit and monitors the size relationship between the ideal ON period and the upper limit of the ON period. The ON period upper limit control circuit then makes the upper limit of the ON period in the next cycle larger when no current is flowing toward the output capacitor from the node at the end of the cycle concerned, and the ideal ON period exceeds the ON period upper limit, according to the monitoring result produced by the current-monitoring circuit and the monitoring result produced by the ON period monitoring circuit. In this way the On duty can be increased, and responsiveness to abrupt load change can be increased, while maintaining a discontinuous mode. 
         [0038]    In a preferred embodiment of the present invention, the ON period monitoring circuit has a clamp voltage generation circuit that generates a clamp voltage with a predetermined voltage level, a ramp-generation circuit that generates a sawtooth wave or triangular wave synchronized with the main clock, and a third comparator that compares the clamp voltage and the sawtooth wave or triangular wave, and outputs, as an ON period upper limit signal indicating the upper limit of the ON period, a binary signal that has a first logical value when the voltage level of the clamp voltage is higher than the voltage level of the sawtooth wave or triangular wave, and has a second logical value when the voltage level of the clamp voltage is lower than the voltage level of the sawtooth wave or triangular wave. The ON period upper limit control circuit then has a clamp voltage control circuit that controls the clamp voltage generation circuit to vary the voltage level of the clamp voltage signal according to the monitoring result produced by the current-monitoring circuit and the monitoring result produced by the ON period monitoring circuit. 
         [0039]    In a preferred embodiment of the present invention, the clamp voltage generation circuit has an up/down counter that counts down when a monitoring result is produced from the current-monitoring circuit that current is still flowing toward the output capacitor from the node at the end of the cycle concerned, and counts up when a monitoring result is produced from the current-monitoring circuit that no such current is flowing at the end of the cycle concerned and a monitoring result is produced from the ON period monitoring circuit that the ideal ON period exceeds its upper limit. The clamp voltage generation circuit also has a digital-analog converter that converts the digital count value output from the up/down counter to an analog voltage signal. The up/down counter holds the count value without counting either down or up when a monitoring result is produced from the current-monitoring circuit that no current is flowing toward the output capacitor from the node at the end of the cycle concerned, and a monitoring result is produced from the ON period monitoring circuit that the ideal ON period does not exceed its upper limit. 
         [0040]    In a preferred embodiment of the present invention, the rectifying element is typically a diode, but it can also be replaced with a transistor. That is, it could be controlled synchronized with whether the switching element is on or off, so that when the switching element is on, the transistor is off, and when the switching element is off, the transistor is on. 
         [0041]    A DC-DC boost converter in another aspect of the present invention has an inductance element, a switching element connected in series with the inductance element via a node between a direct-current voltage input terminal and a reference potential terminal, a rectifying element connected between the node and an output terminal, an output capacitor connected between the output terminal and a reference potential terminal, and a switching control that divides each cycle regulated by a main clock with a constant frequency into 2 parts, that is, variable ON period and OFF period, and that turns the switching element on during the initial the ON period, and turns the switching element off during the subsequent the OFF period. The switching control has an error signal generation circuit into which the voltage of the output capacitor and a constant reference voltage are input, and which generates an error signal representing the error between the two voltages, a pulse width control circuit which generates a pulse width control signal indicating the desired ON period to make the error smaller in the next cycle according to the error signal from the error signal generation circuit, an ON period monitoring circuit which receives the pulse width control signal generated by the pulse width control circuit and an ON period upper limit signal indicating the desired upper limit and which monitors the size relationship between the desired ON period and the upper limit of the ON period, a switching drive circuit which drives the switching element to an on state according to the pulse width control signal when the desired ON period does not exceed the upper limit of the ON period, and which drives the switching element to an on state according to the ON period upper limit signal when the desired ON period does exceed the upper limit of the ON period according to the monitoring result produced by the ON period monitoring circuit, and an ON period upper limit control circuit which variably controls the upper limit of the ON period in the next cycle according to the monitoring result produced by the ON period monitoring circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0042]      FIG. 1  is a circuit diagram showing the constitution of a DC-DC boost converter (booster) in an embodiment of the present invention. 
           [0043]      FIG. 2  is a waveform diagram showing the waveforms of the various parts for explaining monitoring operation by the ON period monitoring circuit in the embodiment. 
           [0044]      FIG. 3  is a waveform diagram showing the waveforms of the various parts for explaining monitoring operation by the current-monitoring circuit in the embodiment. 
           [0045]      FIG. 4  is a simulated waveform diagram for showing an example of the functioning of the ON period upper limit control circuit in the embodiment. 
           [0046]      FIG. 5  is a circuit diagram showing the constitution of a conventional representative DC-DC boost converter (booster). 
           [0047]      FIG. 6  is a waveform diagram showing the waveforms of the various parts for explaining basic operation of the booster and a discontinuous mode operation state. 
           [0048]      FIG. 7  is a waveform diagram showing the waveforms of the various parts for explaining a booster continuous mode operation state. 
           [0049]      FIG. 8  is a waveform diagram showing the waveforms of the various parts for explaining an example of operation when load current fluctuates in the conventional booster in  FIG. 5 . 
           [0050]      FIG. 9  is a simulated waveform diagram showing an example in which there is a shift from discontinuous mode to continuous operating mode and output voltage becomes unstable with abrupt load change in the conventional booster in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION  
       [0051]    With the DC-DC boost converter of this invention, the constitution and functioning as described above allows discontinuous mode operation to be maintained efficiently generally without its being subject to the effects of the usage environment or variation in circuit elements, and without the load characteristics or responsiveness being diminished. 
         [0052]    A preferred embodiment of the present invention is explained below with reference to  FIGS. 1-5 . 
         [0053]    The configuration of a DC-DC boost converter (booster) in an embodiment of the present invention is shown in  FIG. 1 . This booster, broadly categorized, is constituted in 2 parts: booster core  10  and control  12 . 
         [0054]    Booster core  10  is constituted with inductor  14 , NMOS transistor  16 , diode  18  and output capacitor  20 . More specifically, inductor  14  and NMOS transistor  16  are connected in series through node N between an input terminal, to which direct-current input voltage V 1  is input, and a ground potential terminal, diode  18  is connected between node N and output terminal  22 , and output capacitor  20  is connected between output terminal  22  and the ground potential terminal. 
         [0055]    Control  12  has reference voltage generation circuit  24 , error amplifier  26 , integrator  28 , clamp voltage generation circuit  30 , ramp (sawtooth wave or triangular wave) generation circuit  32 , comparators  34  and  36 , AND circuit  38 , and gate drive circuit  40  to form a PWM control circuit for matching output voltage V o  obtained by booster core  10  to the desired reference voltage. 
         [0056]    Reference voltage generation circuit  24  generates reference voltage V REF  with a constant voltage level, and this is supplied to one input terminal (+) of error amplifier  26 . Output voltage V O  is output from booster core  10  to the other terminal (−) of error amplifier  26 . Error amplifier  26  takes the difference between the two voltages V O  and V REF , and outputs as error signal V e , an output voltage corresponding to the error. Error signal V E  is time-integrated by integrator  28 , and is supplied to one input terminal (+) of comparator  34  as integrated error signal V ES . 
         [0057]    Ramp voltage, for example, sawtooth wave V RAMP , synchronized with main clock CLK is supplied from ramp-generation circuit  32  to the other terminal (−) of comparator  34 . Comparator  34  compares the voltage levels of the two input signals V RAMP  and V ES , and outputs, as a pulse width control signal or PWM control signal V pwm , a binary signal or pulse that is at H level when V RAMP &lt;V ES , and at L level when V RAMP &gt;V ES . PWM control signal V pwm  is supplied to one input terminal of AND circuit  38 . 
         [0058]    Sawtooth wave V RAMP  output from ramp-generation circuit  32  is also supplied to one input terminal (−) of the other comparator  36 . Clamp voltage V clamp  is input from clamp voltage generation circuit  30  to the other input terminal (+) of comparator  36 . Comparator  36  compares the voltage levels of the two input signals V RAMP  and V CLAMP , and outputs, as ON period upper limit signal V limit , a binary signal or pulse that is at H level when V RAMP &lt;V CLAMP , and is at L level when V RAMP &gt;V CLAMP . Note that clamp voltage V CLAMP  output from clamp voltage generation circuit  30  is variably controlled for each cycle of main clock CLK or PWM, as described in detail below. 
         [0059]    AND circuit  38  outputs, as switching drive signal V DRIVE , a binary signal or pulse that is at H level when both input signals V PWM  and V LIMIT  are at H level, and that is at L level when one or both are at L level. Here, both input signals V PWM  and V LIMIT  are synchronized with main clock CLK, so switching drive signal V DRIVE  is also synchronized with main clock CLK. Gate drive circuit  40  outputs gate voltage V G  according to switching drive signal V DRIVE  from AND circuit  38 , and switches NMOS transistor  16  in booster core  10 . 
         [0060]    In booster core  10 , the ON period is when gate voltage V G  is at H level. During this period, NMOS transistor  16  is on, inductance current I L  flows to the ground potential terminal through inductor  14  and NMOS transistor  16  from the voltage input terminal, and electromagnetic energy is stored in inductor  14 . The period when gate voltage V G  is at L level is then the OFF period. During this period, NMOS transistor  16  is off, and the electromagnetic energy stored in inductor  14  prior to that is discharged toward output terminal  22 . That is, inductance current I L  flows into output capacitor  20  through node N and diode  18  from inductor  14 , and output capacitor  20  is charged. 
         [0061]    Control  12  in this embodiment has, in addition to a PWM control circuit to match output voltage V O  of booster core  10  to reference voltage V REF  as described above, logic gate circuit  42 , RS-type flip-flop (FF)  44 , comparator  46 , RS-type flip-flop (FF)  48 , D type flip-flop (FF)  50 , logic gate circuit  52 , and N bit counter  54 , and also uses not only main clock CLK but also secondary clock CLKX %. 
         [0062]    Here, logic gate circuit  42  and RS-FF  44  constitute ON period monitoring circuit  56  that monitors whether the pulse width (desired ON period) of PWM control signal V PWM  exceeds the pulse width (ON period upper limit) of ON period upper limit signal V LIMIT  for each cycle of main clock CLK. 
         [0063]    More specifically, PWM control signal V PWM  from comparator  34  is input to one input terminal of logic gate circuit  42 , while ON period upper limit signal V LIMIT  from comparator  36  is also input to the other input terminal. Logic gate circuit  42  is composed of a NOR circuit and an input inversion circuit, and outputs, as over-upper-limit sensed pulse V over , a binary signal that is at H level when the pulse width of PWM control signal V PWM  exceeds the pulse width of ON period upper limit signal V LIMIT , that is, when V PWM =H level (logical value “1”) and V LIMIT =L level (logical value “0”). A table of logic gate circuit  42  true values is shown below. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Vpwm 
                 Vlimit 
                 Vover 
               
               
                   
               
             
             
               
                 ∘ 
                 x 
                 L 
               
               
                 1 
                 ∘ 
                 H 
               
               
                 1 
                 1 
                 L 
               
               
                   
               
             
          
         
       
     
         [0064]    With RS-FF  44 , over-upper-limit sensed pulse V over  from logic gate circuit  42  is input to the set input terminal (S), while main clock CLK is also input to the reset input terminal (R), and a binary signal that will be at H level when V OVER =H level (logical value “1”) and CLK=L level (logical value “0”) is output as count-up instruction signal V UP . 
         [0065]    The monitoring operation by ON period monitoring circuit  56  ( 42 ,  44 ) is shown in  FIG. 2  with a waveform diagram. In the example illustrated, operation when load current I o  varies in the increasing direction above a steady value across a certain period Ta is shown. 
         [0066]    As shown in  FIG. 2 , when load current I o  varies in the increasing direction, output voltage V O  becomes lower than reference voltage V REF , the voltage level of error signal V ES  rises because of this, pulse width (H level period) P PWM  of PWM control signal V PWM  becomes larger, and the ON period (ON period) T S ·D 1  of NMOS transistor becomes longer. When ON period T S ·D 1  becomes longer, inductance current I L  increases, more energy is supplied from inductor  14  to output capacitor  22 , and output voltage V O  rises toward reference voltage V REF . 
         [0067]    In feedback PWM control such as this, when pulse width P PWM  of PWM control signal V PWM  does not exceed pulse width T S ·D C  of ON period upper limit signal V LIMIT , over-upper-limit sensed pulse V over  from logic gate circuit maintains an L level (logical value “0”) state, RS-FF  44  remains reset with the rise of clock CLK, and output (Q) count-up instruction signal V UP  is held at L level (logical value “0”). 
         [0068]    However, when pulse width P PWM  of PWM control signal V PWM  exceeds pulse width T S ·D C  of ON period upper limit signal V LIMIT , RS-FF  44  is reset with the timing at which over-upper-limit sensed pulse V over  changes from L level to H level, and output (Q) count-up instruction signal V UP  is set to H level. Then at the rise of the next clock CLK, RS-FF  44  is reset, and count-up instruction signal V UP  is returned to L level. 
         [0069]    In this way, ON period monitoring circuit  56  ( 42 ,  44 ) monitors the size relationship between pulse width P PWM  of PWM control signal V PWM  and pulse width T S ·D C  of ON period upper limit signal V LIMIT , and holds count-up instruction signal V UP  at L level when P PWM &lt;T S ·D C , and sets count-up instruction signal V UP  to H level when P PWM &gt;T S ·D C . 
         [0070]    In control  12 , comparator  46 , RS-FF  48  and D-FF  50  constitute current-monitoring circuit  58  that monitors whether inductance current I L  is still flowing from inductance element  14  toward output capacitor  22  in booster core  10  at the end of each cycle regulated by main clock CLK. 
         [0071]    More specifically, one input terminal (+) of comparator  46  is connected to the positive pole terminal of output capacitor  22  or to output terminal  22 , and the other input terminal (−) is connected to node N. Comparator  46  compares potential V L  of node N and output voltage V O , and outputs binary signal V COM  to be at H level when V L &lt;V O , and at L level when V L &gt;V O . 
         [0072]    For RS-FF  48 , switching drive signal V DRIVE  is input from AND circuit  38 , with polarity inverted, to the set input terminal (S), output signal V COM  from comparator  46  is input to the reset input terminal (R), and a binary signal is output, as monitor signal V MON , to be at H level when V DRIVE =L level (logical value “0”) and V COM =L level (logical value “0”). That is, RS-FF  48  is reset when switching drive signal V DRIVE  falls from H level to L level, and output (Q) monitor signal V MON  rises to H level from the L level prior to that. It is reset when output signal V COM  of comparator  46  then changes from L level to H level (that is, when potential V L  of node N becomes lower than output voltage V o ), so that output (Q) monitor signal V MON  will return to L level from H level. 
         [0073]    For D-FF  50 , monitor signal V MON  from RS-FF  48  is input to the data input terminal (D), while secondary clock CLKX % from the auxiliary clock generation circuit (not shown) is also input to the clock input terminal (C). The rising edge of secondary clock CLKX % is used as the reference point, and binary count-down instruction signal V DW  is output so that if monitor signal V MON  is at H level (logical value “1”) at that point, V DW =H level (logical value “1”), and if monitor signal V MON  is at L level (logical value “0”) at that point, V DW =L level (logical value “0”). 
         [0074]    Note that secondary clock CLKX % has the same frequency as main clock CLK, and the phase is offset by a predetermined value. That is, the rising edge of secondary clock CLKX % could normally be set to within the period of the remaining 10% at the end of the main clock CLK cycle. As a variation, when D-FF  50  latches monitor signal V MON  at the falling edge of secondary clock CLKX %, the falling edge of secondary clock CLKX % could be set to the end of the main clock CLK cycle. 
         [0075]    Monitoring operation by current-monitoring circuit  58  ( 46 ,  48 ,  50 ) is shown in  FIG. 3  with a waveform diagram. With the example illustrated, operation when load current I o  varies in the increasing direction above a steady value across a certain period T a  is shown. 
         [0076]    In  FIG. 3 , when switching drive signal V DRIVE  changes from H level to L level within each cycle of main clock CLK, output (Q) of RS-FF  48 , that is, monitor signal V MON , changes to H level from the L level prior to that. That is, when switching drive signal V DRIVE  changes from H level to L level, an H level signal is input to the set input terminal (S) of RS-FF  48 . At the same time, NMOS transistor  16  turns off in booster core  10  and the energy stored in inductor  14  is discharged to output capacitor  20 . That is, inductance current I L  flows toward output capacitor  20  through diode  18  from node N. For this reason, potential V L  of node N becomes higher than the voltage of output capacitor  20   k , that is, output voltage V O , output signal V COM  of comparator  46  changes from H level to L level, and an L level signal is input to the reset input terminal (R) of RS-FF  48 . Because of this, RS-FF  48  is set, and monitor signal V MON  changes from L level to H level. 
         [0077]    After switching drive signal V DRIVE  changes from H level to L level in this way, the relationship V L &gt;V O  continues while inductance current I L  from inductor  14  is flowing to output capacitor  20  through node N and diode  18 , so the state where V COM =L level and V MON =H level is maintained. 
         [0078]    Then when inductance current I L  decreases to zero amperes, potential V L  of node N at that point drops instantly to potential V 1  of the voltage output terminal, which is lower than output voltage V O , and output signal V COM  of comparator  46  changes from L level to H level. Then RS-FF  48  is reset, and monitor signal V MON  changes from H level to L level. 
         [0079]    When the booster is operating in discontinuous mode, because of the return to I L =0 A before the end of each cycle, monitor signal V MON  at data input terminal (D) of D-FF  50  is at L level with the rise of secondary clock CLKX %, and output (Q) count-down instruction signal V DW  remains at L level. 
         [0080]    However, with a shift from discontinuous mode to continuous mode, because there is no return to I L =0 A even at the end of each cycle, that is, because monitor signal V MON  at data input terminal (D) remains at H level, D-FF  50  sets output (Q) count-down instruction signal V DW  to H level at the rise of secondary clock CLKX %. Then, this state, that is, the H level state of count-down instruction signal V DW , is maintained until there is a return to discontinuous mode from continuous mode. 
         [0081]    In this way, current-monitoring circuit  58  ( 46 ,  48 ,  50 ) monitors whether inductance current I L  is still flowing toward output capacitor  20  from inductance element  14  in booster core  10  at the end of each cycle of main clock CLK, based on secondary clock CLKX %, output voltage V O  and potential V L  of node N in booster core  10 , and outputs count-down instruction signal V DW  to be at L level when the booster is operating in discontinuous mode, and at H level when operating in continuous mode. 
         [0082]    In control  12 , logic gate circuit  52  and N bit counter  54  constitute ON period upper limit control circuit  60  that variably controls or calibrates pulse width T S ·D C  of ON period upper limit signal V LIMIT  for each cycle of main clock CLK based on the monitoring results produced by ON period monitoring circuit  56  and current-monitoring circuit  58 . Clamp voltage generation circuit  30  has a digital-analog (D/A) converter to convert digital clamp voltage signal V COUNT  provided by ON period upper limit control circuit  60  to an analog voltage signal (clamp voltage V CLAMP ). 
         [0083]    Count-up instruction signal V UP  from ON period monitoring circuit  56  and count-down instruction signal V DW  from current-monitoring circuit  58  are input to logic gate circuit  52 , and a pair of output signals, that is, count enable signal V ENB  and up/down selection signal V UP/DW , are output based on the true value table below. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Vup 
                 Vdw 
                 Venbl 
                 Vup/dw 
               
               
                   
                   
               
             
             
               
                   
                 ◯ 
                 ◯ 
                 L 
                 L 
               
               
                   
                 1 
                 ◯ 
                 H 
                 H 
               
               
                   
                 X 
                 1 
                 H 
                 L 
               
               
                   
                   
               
             
          
         
       
     
         [0084]    N bit counter  54  receives count enable signal V ENB  and up/down selection signal V UP/DW  at their respective control input terminals, while main clock CLK is also input to the clock input terminal (CK). Counting down or counting up is selectively performed according to the logic of the two control signals V ENB1  and V UP/DW , and N bit count value V COUNT  is output as a digital clamp voltage signal. 
         [0085]    In ON period upper limit control circuit  60 , more specifically, when V UP =L level (logical value “0”) and V DW =L level (logical value “0”), V ENB1 =L level, and only then will N bit counter  54  hold the current count value (value of V COUNT ) without either counting up or down. When V UP =H level (logical value “1”) and V DW =L level (logical value “0”), V ENB1 =L level and V UP/DW =H level, and N bit counter  54  operates as an up counter and increments the count value (value of V COUNT ) by 1 (1 step worth) at the rise of main clock CLK. When V DW =H level (logical value “1”), N bit counter  54  operates as a down counter regardless of the state of V UP , and decrements the count value (value of V COUNT ) by 1 (1 step worth) at the rise of main clock CLK. 
         [0086]    In this way, at each cycle, when a monitoring result is produced from ON period monitoring circuit  56  that pulse width P PWM  of PWM control signal V PWM  has not exceeded pulse width T S ·D C  of ON period upper limit signal V LIMIT , and a monitoring result is produced from current-monitoring circuit  58  that booster core  10  is operating in discontinuous mode, ON period upper limit control circuit  60  continues to hold the value of clamp voltage signal V COUNT . Therefore, at the next cycle the voltage level of clamp voltage V CLAMP  generated by clamp voltage generation circuit  30  will not change, and thus pulse width T S ·D C  of ON period upper limit signal V LIMIT  also will not change. 
         [0087]    However, when a monitoring result is produced from ON period monitoring circuit  56  that pulse width P PWM  of PWM control signal V PWM  has exceeded pulse width T S ·D C  of ON period upper limit signal V LIMIT , and a monitoring result is produced from current-monitoring circuit  58  that booster core  10  is operating in discontinuous mode in a given cycle, ON period upper limit control circuit  60  increments the value of clamp voltage signal V COUNT  by 1. Then the voltage level of clamp voltage V CLAMP  generated by clamp voltage generation circuit  30  rises 1 step at the next cycle, and pulse width T S ·D C  of ON period upper limit signal V LIMIT  rises 1 step. 
         [0088]    Also, when a monitoring result is produced from current-monitoring circuit  58  that booster core  10  is operating in continuous mode in a given cycle, ON period upper limit control circuit  60  decrements the value of clamp voltage signal V COUNT  by 1 regardless of the monitoring result produced by ON period monitoring circuit  56 . Then, the voltage level of clamp voltage V CLAMP  generated by clamp voltage generation circuit  30  drops 1 step at the next cycle, and pulse width T S ·D C  of ON period upper limit signal V LIMIT  decreases 1 step. 
         [0089]    PWM control with which the ON period or On duty of booster core  10  can be varied as soon as possible or to the maximum limit (to nearly 100% of the ON period in discontinuous mode) is possible while essentially maintaining discontinuous mode by variably controlling pulse width T S ·D C  of ON period upper limit signal V LIMIT  for each cycle of main clock CLK by means of ON period upper limit control circuit  60  as described above. 
         [0090]      FIG. 4  illustrates an example of the functioning of ON period upper limit control circuit  60  in this embodiment with simulated waveforms. The example illustrated shows the waveforms of the various parts when load current I O  increases suddenly while the booster is operating in discontinuous mode. 
         [0091]    In  FIG. 4 , when load current I O  increases stepwise at point A, output voltage V O  subsequently drops. Then feedback PWM control by control  12  works as described above, the voltage level of error signal V E  rises, and the pulse width of switching drive signal V DRIVE  increases from point A to point B. Nonetheless, at point B, although operation is still in discontinuous mode, the pulse width (ON period or On duty) of switching drive signal V DRIVE  increases additionally afterward, and goes to nearly 100% of the ON period duty rate in discontinuous mode. Then at point C it is operating at near the maximum duty (100% of the ON period in discontinuous mode) in discontinuous mode. The interval from point B to point C in  FIG. 4  is shown enlarged in  FIG. 5 . 
         [0092]    At an interval a the pulse width of V DRIVE  is narrower than the pulse width (T S ·D C ) of ON period upper limit signal V LIMIT , and there is operation in discontinuous mode, so count-down signal V DW  and count-up signal V UP  are, respectively, V DW =L level and V UP =L level. The value of clamp voltage signal V COUNT  output from ON period upper limit control circuit  60  does not change, and clamp voltage V CLAMP  output from clamp voltage generation circuit  30  is kept at the same value before that (about 1.0 volt). 
         [0093]    Only the value of the  2  least significant bits [1:0] of clamp voltage signal V COUNT  are indicated represented with V COUNT  [1] and V COUNT  [0]. Also, potential V L  of node N at interval a in  FIG. 5  is ringing centered on the level of V 1  (about 3.0 volts). This is because even though the flow of inductance current I L  is stopped, some energy still remains in inductor  14  and is discharged; there is no effect on operation. 
         [0094]    At interval or cycle b, potential V L  of node N is still higher than output voltage V O  at the monitoring point (rising edge of secondary clock CLKX %) set to 95% (5% remaining) of the clock cycle, so a monitoring result is produced from current-monitoring circuit  58  that a shift to continuous mode has occurred (V DW =H level). This monitoring result is received, and at the next interval (cycle) c, ON period upper limit control circuit  60  decrements the value of clamp voltage signal V COUNT  by 1, that is, reduces the value of the 2 least significant bits from [1:0] to [0:1], and clamp voltage V CLAMP  falls to a step one level lower (about 0.95 volt) from the value prior to that (about 1.0 volt) because of this. When clamp voltage V CLAMP  falls one step, pulse width T S ·D C  of ON period upper limit signal V LIMIT  output from comparator  36  becomes smaller by one step. 
         [0095]    Although there is also some time lag until upper limit T S ·D C  of the ON period decreases after ON period upper limit control circuit  60  decrements the value of clamp voltage signal V COUNT , a monitoring result is produced from current-monitoring circuit  58  at interval c that continuous mode is in effect, the same as at interval b. The result is that, at the next interval (cycle) d, ON period upper limit control circuit  60  further decrements the value of clamp voltage signal V COUNT  by 1, and the value of its least 2 significant bits is reduced from [0:1] to [0:0]. With this, clamp voltage V CLAMP  further drops to a level one step lower (about 0.90 volt), and pulse width T S ·D C  of ON period upper limit signal V LIMIT  becomes even smaller by one step. 
         [0096]    In this way, at interval d, while V UP =H level by virtue of the pulse width of switching drive signal V DRIVE  effectively being subject to the restriction of upper limit T S ·D C , a monitoring result is produced from current-monitoring circuit  58  that discontinuous mode is in effect from the fact that V L &lt;V O  at the current monitoring point (the point at 5% remaining in the clock cycle). Because of this, at the next interval (cycle) e, ON period upper limit control circuit  60  increments the value of clamp voltage signal V COUNT  by 1, setting the value of its 2 least significant bits to [0:1] from [0:0]. Because of this, clamp voltage V CLAMP  rises to a level one step higher (about 0.95 volts), and pulse width T S ·D C  of ON period upper limit signal V LIMIT  becomes one step higher. 
         [0097]    After this, the state in interval d and the state in interval e are alternately repeated, and operation at an ON duty near 100% of the ON period in discontinuous mode can be continued while essentially maintaining the discontinuous mode. In fact, the On duty upper limit can be stopped near 95%, for example, by adjusting the timing or phase of the monitoring point in current-monitoring circuit  58 , that is, the rising edge of secondary clock CLKX %, in a forward direction, and discontinuous mode can be held absolutely reliably. 
         [0098]    Because of operating while essentially maintaining a discontinuous mode in this way, as shown in  FIG. 4 , no ringing as shown in  FIG. 9  appears in output voltage V O  and very stable output characteristics are obtained in terms of AC. Note that output voltage V O  ultimately drops to around 4.8 volts as shown in  FIG. 4 , but this drop is −4% relative to the target voltage (5 volts), and is not a particular problem with normal applications. 
         [0099]    As described above, the booster in this embodiment monitors the size relationship between the pulse width of PWM control signal V PWM  and pulse width T S ·D C  of ON period upper limit signal V LIMIT , with PWM control for matching output voltage V O  of booster core  10  to reference voltage V REF  while also monitoring whether there is operation in either continuous mode or discontinuous mode, and variable control is performed as appropriate of the On duty or pulse width of switching drive signal V DRIVE  according to the monitoring results. Thus even when the ambient temperature fluctuates or there is variation in the circuit element characteristics in booster core  10 , discontinuous mode operation can be maintained stably generally without diminishing load characteristics or responsiveness. 
         [0100]    The present invention also has the advantage that shifting to continuous mode can be restricted and discontinuous mode can be maintained even though there is no complicated, large-scale phase compensation circuitry in the booster, where stabilization of operation is complicated and difficult with continuous mode, so that the layout area of the circuitry can be made smaller. 
         [0101]    Alternatively, in booster core  10 , diode  108  constituting the rectifying element could be replaced with an NMOS transistor. In this case, the rectifying transistor could be turned on and off synchronized with the switching operation of driving switching element  16  complementarily to it or in inverse phase. That is, it could be turned on and off so that when NMOS transistor  16  is on, the rectifying transistor is off, and when NMOS transistor  16  is off, the rectifying transistor is on. 
         [0102]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.