Control circuit for DC-DC converter, DC-DC converter, and control method of DC-DC converter

A control circuit for a DC-DC converter includes: an output control circuit configured to control an output voltage of a DC-DC converter according to a reference voltage; a reference control circuit configured to control the reference voltage according to an open-circuit voltage of an external power supply coupled to the DC-DC converter; a limiting circuit configured to limit a current flowing from DC-DC converter to an external load; and a stopping control circuit configured to stop operation of the limiting circuit until the reference voltage reaches a given value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-072150, filed on Mar. 27, 2012, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the embodiments discussed herein is related to a control circuit for a DC-DC converter, a DC-DC converter, and a control method of a DC-DC converter.

BACKGROUND

A solar battery is known as a clean energy source. Since a solar battery generates an electric power by sunlight, the electric power to be obtained depends on illumination and changes according to a state of the weather, unlike other batteries. The difference between the changing electric powers is detected as a difference between open-circuit voltages, for example.

With respect to this, a patent document 1 (see Japanese Laid-Open Patent Publication No. 08-297516) discloses a technique that periodically stops the operation of a DC-DC converter connected to the solar battery, and measures the open-circuit voltage, for example. A patent document 2 (see Japanese Laid-Open Patent Publication No. 2010-81711) discloses a technique that performs switching control by using a PWM (Pulse Width Modulation) signal generated based on the open-circuit voltage of the solar battery, and flows a charging current from the solar battery to a storage battery.

The DC-DC converter which changes the output voltage of the solar battery may include a soft start function that secures an electric power used for the operation of a control circuit for the DC-DC converter at start-up, and limits a rush current to an output-side smoothing circuit. At this time, the limit value of a current is set up on the basis of a lower electric power in order to have a margin of the current. The start-up time until the output voltage of the DC-DC converter reaches a given rated value becomes long since a rate of increase of a voltage is low, as compared with the case where the limit value of the current is set up on the basis of a higher electric power.

With respect to this, a patent document 3 (see Japanese Laid-Open Patent Publication No. 2007-288979) discloses a power supply device that detects an input voltage from a battery, acquires a duty and a soft start period corresponding to the input voltage from a table, and outputs a PWM signal based on the duty until the soft start period elapses. The patent document 3 discloses that the power supply device changes to a normal mode after the soft start period elapses.

SUMMARY

According to an aspect of the present invention, there is provided a control circuit for a DC-DC converter, including: an output control circuit configured to control an output voltage of a DC-DC converter according to a reference voltage; a reference control circuit configured to control the reference voltage according to an open-circuit voltage of an external power supply coupled to the DC-DC converter; a limiting circuit configured to limit a current flowing from the DC-DC converter to an external load; and a stopping control circuit configured to stop operation of the limiting circuit until the reference voltage reaches a given value.

DESCRIPTION OF EMBODIMENTS

In the above-mentioned case, the DC-DC converter may include an overcurrent protection circuit in order to prevent an overcurrent caused by the short-circuit in a load-side circuit. However, as with the functions of securement of the above-mentioned electric power for operation and prevention of the above-mentioned rush current, the overcurrent protection circuit limits outputting the output current to the load, and hence the reduction of the start-up time of the DC-DC converter is prevented.

FIG. 1is a functional block diagram of a DC-DC converter according to a comparative example. A DC-DC converter9is a connected to an external solar battery4and a load Ld, and applies an output voltage Vout generated by converting an output voltage E of the solar battery4, to the load Ld.

The solar battery4converts a light into an electric power using a photoelectric effect. The solar battery4is made of silicon or another compound semiconductor, for example, and the material thereof is not limited to this.

The DC-DC converter9includes: a switch circuit92having at least one switch element; a control circuit91controlling switch operation of the switch circuit92; and a coil L and a capacitor C which are a smoothing circuit. The DC-DC converter9secures a consumption current Iq for the operation of the control circuit91at start-up, and limits a current Ipv inputted from the solar battery4in order to restrict a rush current Irush drawn in the coil L and the capacitor C.

FIG. 2is a graph illustrating a current-voltage characteristic of the solar battery4concerning the comparative example. InFIG. 2, a horizontal axis indicates the output voltage E (V) and a vertical axis indicates the output current I (A). A code “GH” indicates the current-voltage characteristic when the illumination is high. A code “GL” indicates the current-voltage characteristic when the illumination is low.

The solar battery4has a characteristic in which the current increases as the voltage becomes low, and the current is saturated when the voltage reaches a fixed value. In the solar battery4, the electric power is changed according to the illumination. A short-circuit current Isc (H) is an output current when the illumination is high and terminals are short-circuited. A short-circuit current Isc (L) is an output current when the illumination is low and the terminals are short-circuited. An open-circuit voltage Voc (H) is an output voltage when the illumination is high and the terminals are opened. An open-circuit voltage Voc(L) is an output voltage when the illumination is low and the terminals are opened.

A limit value Imax of the input current Ipv is the total of the above-mentioned consumption current Iq and a permissible rush current Irush, and is set on the basis of a current when the illumination is low. Therefore, a surplus current which is not used for the start-up of the DC-DC converter9among the output current I of the solar battery4exists when the illumination is high. Here, a voltage Va (H) is an output voltage when the illumination is high. A voltage Va (L) is an output voltage when the illumination is low.

FIG. 3is a timing chart illustrating the operation of the DC-DC converter9according to the comparative example. InFIG. 3, a horizontal axis indicates a time and a vertical axis indicates a current or voltage. With respect to each of the current and the voltage, “high illumination” indicates the operation of the DC-DC converter9when the illumination is high. On the contrary, “low illumination” indicates the operation of the DC-DC converter9when the illumination is low. This is a notation for simply illustrating the illuminations as two different values for convenience, and in subsequent explanation and drawings, the above-mentioned notation is also maintained.

Here, the rush current Irush is indicated as an average value of the current. With respect to the rush current Irush, the notation of the “high illumination” and the “low illumination” is also maintained in subsequent drawings.

With respect to the voltage E of the solar battery4, the voltage Va (H) when the illumination is high is larger than the voltage Va (L) when the illumination is low.

In the case where the illumination is high and in the case where the illumination is low, the rush currents Irush are limited to the same limit value Imax. Therefore, the output voltages Vout of the DC-DC converter9in the case where the illumination is high and in the case where the illumination is low have the same start-up period t0. Thus, the limit value Imax is uniformly set up on the basis of the case where the illumination is low, so that shortening of the start-up period when the illumination is high is prevented.

FIG. 4is a graph illustrating a current-voltage characteristic of the solar battery4concerning the present embodiment. InFIG. 4, the elements corresponding to those inFIG. 2are designated by identical reference numerals, and detailed description thereof is omitted.

In the present embodiment, a limit value Imax (H) when the illumination is high and a limit value Imax (L) when the illumination is low are provided as limit values of the current. The rush current Irush (H) when the illumination is high is larger than the rush current Irush (L) when the illumination is low, by a difference All, so that the start-up period when the illumination is high is shortened, compared with the comparative example mentioned above.

On the other hand, an overcurrent limit value Ilim is based on an overcurrent protection function which limits a current flowing to an external load Ld from a DC-DC converter. The overcurrent protection function prevents an accident, such as ignition, when a short circuit or the like has occurred in a circuit of the external load Ld.

The overcurrent limit value Ilim is set larger than the above-mentioned limit value Imax (L), and is set smaller than the above-mentioned limit value Imax (H) by a difference ΔI2. Therefore, the start-up period when the illumination is high becomes longer during operation of the overcurrent protection function than during un-operating of the overcurrent protection function. Therefore, in the present embodiment, the overcurrent protection function is controlled so as to stop during the start-up of the DC-DC converter.

FIG. 5is a circuit diagram of the DC-DC converter according to the present embodiment. A DC-DC converter2is connected to a solar battery4which is an external power supply, and an external load Ld. The DC-DC converter2includes a control circuit for DC-DC converter1(hereinafter referred to as the “control circuit1”), an IN conversion circuit20, one or more switching elements SW4and SW5, a voltage detecting resistor Rs3, a coil L, a capacitor C, and an output switch SW6. Although in the embodiment, the DC-DC converter2is a current mode (C-mode) system, the DC-DC converter2is not limited to this, but may be a voltage mode (V-mode) system.

As described above, the consumption current Iq flows to each element of the control circuit1from the solar battery4, and then the rush current Irush flows to the coil L and the capacitor C. The control circuit1generates a PWM signal based on an output voltage Vout of the DC-DC converter2fed back from the smoothing circuit C and L, and an error signal Verr generated based on reference voltages Vref1and Vref2. Then, the DC-DC converter2generates the output voltage Vout by providing on/off-control for the switching elements SW4and SW5based on the PWM signal. In the following, a description is given of the control circuit1in detail.

The control circuit1includes a reference control circuit10, an output control circuit11, a limiting circuit12, a stopping control circuit13, a constant current source CC, voltage dividing resistors Rs1and Rs2, a PWM comparator14, a slope compensation circuit15, a PWM signal generating circuit16, and an oscillator17. The control circuit1may be provided in a single semiconductor chip, or may be composed of a plurality of elements provided in the circuit substrate.

The reference control circuit10controls the reference voltage Vref1of the output control circuit11according to the open-circuit voltage (see Voc (H), Voc(L) inFIG. 4) of the solar battery4which is an external power supply connected to the DC-DC converter2. The reference control circuit10includes a voltage detector100, switches SW1and SW2, and the capacitor elements Cs1and Cs2.

Each of the switches SW1and SW2is a FET (Field Effect Transistor). One ends of the switches SW1and SW2are connected to the constant current source

CC, and another ends thereof are connected to the capacitor elements Cs1and Cs2, respectively. The constant current source CC is a transistor, is connected to the solar battery4, generates a current indicative of a certain value from the consumption current Iq, and sends the current to the switches SW1and SW2. The capacitor element Cs1is connected between the switch SW1and a ground GND, and the capacitor element Cs2is connected between the switch SW2and the ground GND.

The voltage detector100detects the open-circuit voltage and performs A/D conversion (i.e., Analog-Digital conversion) on a value of the detected voltage before controlling a first reference voltage Vref1. The voltage detector100provides on/off-control for the switches SW1and SW2according to the values d1and d2acquired by the A/D conversion. For example, when the illumination is high and the open-circuit voltage is large, the voltage detector100turns on the switch SW1and turns off the switch SW2. On the contrary, when the illumination is low and the open-circuit voltage is small, the voltage detector100turns on the switches SW1and SW2. Thereby, the reference control circuit10selects at least one of capacitance values of the capacitor elements Cs1and Cs2connected between the solar battery4and the output control circuit11according to the open-circuit voltage. Therefore, the reference control circuit10can easily control increment of the first reference voltage Vref1. Here, although decode values d1and d2of the open-circuit voltage are 2 bits as an example, the decode values are not limited to this, but may be provided according to the range of voltage values to be detected.

The voltage detector100outputs a notification signal S1notifying that the selection of the capacitance values of the capacitor elements Cs1and Cs2has been completed, to the output control circuit11, the constant current source CC, the PWM comparator14and the PWM signal generating circuit16. The output control circuit11, the constant current source CC, the PWM comparator14and the PWM signal generating circuit16stop the operation until the notification signal S1is inputted to them, and begin the operation after the input of the notification signal S1. Thereby, the operation of the DC-DC converter2in an unstable electrical state is prevented.

Here, it is assumed that the consumption current of the voltage detector100is small to such an extent that it does not influence the open-circuit voltage of the solar battery4substantially. The voltage detector100may perform the A/D conversion on the voltage value of the open-circuit voltage so that a change amount of the open-circuit voltage by its own consumption current is compensated.

The output control circuit11is an amplifier including two noninverting input terminals (+) and a single inverting input terminal (−). One noninverting input terminal (+) is connected between the constant current source CC and the switches SW1and SW2, and the first reference voltage Vref1based on the input voltage from the solar battery4is applied to the one noninverting input terminal (+). A constant second reference voltage Vref2is applied to another noninverting input terminal (+). A voltage acquired by dividing a feedback output voltage Vout of the DC-DC converter2with the voltage dividing resistors Rs1and Rs2is applied to the inverting input terminal (−).

The output control circuit11detects an error between the voltage of the noninverting input terminal (+) to which a smaller voltage is applied, among the two noninverting input terminals (+), and the voltage of the inverting input terminal (−), and outputs the error signal Verr based on the error. That is, the output control circuit11detects the error between the smaller voltage among the first reference voltage Vref1and the second reference voltage Vref2, and the output voltage Vout of the DC-DC converter2.

The first reference voltage Vref1is based on the input voltage inputted from the solar battery4. On the other hand, the second reference voltage Vref2is constant. Therefore, the output voltage Vout of the DC-DC converter2is controlled according to the first reference voltage Vref1in time of the start-up. On the other hand, after the first reference voltage Vref1reaches the second reference voltage Vref2, i.e., in time of normal operation after the start-up, the output voltage Vout of the DC-DC converter2is controlled according to the second reference voltage Vref2.

As described above, the reference control circuit10selects the capacitor element (Cs1and/or Cs2) to be connected to the noninverting input terminal (+) corresponding to the first reference voltage Vref1, according to the open-circuit voltage Voc of the solar battery4. Therefore, in time of the start-up, a time period for charging the capacitor elements Cs1and Cs2by a current from the constant current source CC changes according to the open-circuit voltage Voc of the solar battery4. A time period until the first reference voltage Vref1reaches the second reference voltage Vref2(i.e., start-up period by soft start) also changes similarly.

The limiting circuit12is a clamp circuit, for example, and functions as an overcurrent protection function which limits the current flowing to the external load Ld from the DC-DC converter2. An inverting input terminal (−) of the limiting circuit12is connected to an output terminal of the output control circuit11. A voltage Vlim is applied to a noninverting input terminal (+) of the limiting circuit12. Thereby, the limiting circuit12limits a voltage of the error signal Verr inputted from the output control circuit11, to a limit value Vlim. Also, an output terminal of the limiting circuit12is connected to an inverting input terminal (−) of the PWM comparator14as described later.

FIG. 6is a graph illustrating the current-voltage characteristic of the limiting circuit12. InFIG. 6, a vertical axis indicates the voltage of the error signal Verr, and a horizontal axis indicates an output current lout flowing to the load Ld. When the voltage of the error signal Verr reaches the limit value Vlim, the limiting circuit12prevents the voltage from being equal to or more than the limit value Vlim by a clamp function. Here, the output current Tout when the voltage is the limit value Vlim corresponds to the above-mentioned overcurrent limit value Ilim.

FIG. 7is a graph illustrating an output voltage-output current characteristic of the DC-DC converter when the limiting circuit12has operated. A vertical axis indicates the output voltage Vout of the DC-DC converter2, and a horizontal axis indicates the output current Iout flowing to the load Ld. The DC-DC converter2generally outputs a rated voltage Eo. However, when the output current Iout increases by a short circuit or the like which has occurred in a circuit of the load Ld, and becomes the overcurrent limit value Ilim, the output voltage Vout decreases by an overcurrent protection operation. This is because the voltage is limited to the limit value Vlim corresponding to the overcurrent limit value Ilim by the clamp function of the limiting circuit12when the voltage of the error signal Verr increases.

Referring toFIG. 5again, the operation of the limiting circuit12is permitted by a permission signal S2inputted from the stopping control circuit13. Unless the operation is permitted, the limiting circuit12is not performed the clamp function mentioned above, and the overcurrent protection operation is not performed.

The stopping control circuit13stops the operation of the limiting circuit12until the first reference voltage Vref1reaches a given value Vth. The stopping control circuit13is a comparator, for example, and compares the first reference voltage Vref1with the given value Vth. When the first reference voltage Vref1is identical with the given value Vth, the stopping control circuit13outputs the permission signal S2permitting the operation of the limiting circuit12, to the limiting circuit12. An inverting input terminal (−) of the stopping control circuit13is connected to the noninverting input terminal (+) of the output control circuit11, and the first reference voltage Vref1is applied to the inverting input terminal (−) of the stopping control circuit13. On the other hand, the given value Vth (i.e., a threshold voltage Vth) is applied to a noninverting input terminal (+) of the stopping control circuit13. An output terminal of the stopping control circuit13is connected to the limiting circuit12and the switch SW6.

The permission signal S2is outputted to the limiting circuit12and the switch SW6when the first reference voltage Vref1reaches the given value Vth (i.e., a threshold voltage Vth). Therefore, the threshold voltage Vth is set to the second reference voltage Vref2of the output control circuit11or a value close to the second reference voltage Vref2, so that the operation of the limiting circuit12can be stopped during the start-up of the DC-DC converter2.

The output switch SW6is a FET (Field Effect Transistor), for example, and connected between the switching elements SW4and SW5, and the external load Ld. Specifically, the output switch SW6is connected between the coil L and the external load Ld. The DC-DC converter2is electrically connected to the external load Ld when the output switch SW6is ON, and separated from the external load Ld when the output switch SW6is OFF. Therefore, by setting the threshold voltage Vth as described above, the stopping control circuit13can provide off-control for the switch SW6so that the output voltage Vout is not applied to the external load Ld until the first reference voltage Vref1reaches the given value Vth.

Thereby, when the DC-DC converter2is electrically separated from the external load Ld during the start-up of the DC-DC converter2, and the short circuit or the like occurs in the circuit of the external load Ld, it is possible to prevent the overcurrent from flowing to the external load Ld. This is effective in order that the overcurrent protection fanction does not work by the stop of the operation of the limiting circuit12during the start-up of the DC-DC converter2. Here, the output switch SW6may be provided in the circuit of the external load Ld.

The PWM comparator14is a comparator. A waveform control signal Vslp is inputted to a noninverting input terminal (+) of the PWM comparator14from the slope compensation circuit15, and the error signal Verr is inputted to the inverting input terminal (−) of the PWM comparator14from the output control circuit11. The

PWM comparator14outputs a control signal Vp to the PWM signal generating circuit16according to a comparison result of the waveform control signal Vslp and the error signal Verr.

The PWM signal generating circuit16operates based on a clock signal CLK inputted from the oscillator17. The PWM signal generating circuit16generates

PWM signals Tp and Tn to be outputted to the switching elements SW4and SW5, based on the control signal Vp inputted from the PWM comparator14. Each of the PWM signals Tp and Tn has a pulse width according to the control signal Vp.

The switching element SW4is a P-channel FET, for example, and turned on or off by the PWM signal Tp. On the other hand, the switching element SW5is a N-channel FET, for example, and turned on or off by the PWM signal Tn. A drain terminal of the switching element SW4is connected to the solar battery4via the voltage detecting resistor Rs3, and a gate terminal of the switching element SW4is connected to the PWM signal generating circuit16.

The I/V conversion circuit20is connected to both ends of the voltage detecting resistor Rs3, and the slope compensation circuit15. When the switching element SW4is in an ON state, the I/V conversion circuit20detects a current value flowing to the voltage detecting resistor Rs3as a voltage value Vs, and outputs the voltage value Vs to the slope compensation circuit15. When a duty ratio of the PWM signal Tp is equal to or more than 50 (%), the slope compensation circuit15calculates a compensation value which compensates inclination of the waveform of a current flowing through the coil L, and outputs the compensation value as the waveform control signal Vslp, in order to prevent subharmonic oscillation.

A source terminal of the switching element SW4is connected to a drain terminal of the switching element SW5, and a source terminal of the switching element SW5is connected to the ground GND. One end of the coil L is connected between the switching elements SW4and SW5, and another end of the coil L is connected to the output switch SW6and one end of the capacitor C. Another end of the capacitor C is connected to the ground GND.

When the switching operation of the switching elements SW4and SW5is performed based on the PWM signals Tp and Tn, the capacitor C is charged and discharged and the coil L is magnetized, so that the output voltage Vout is generated. When the output switch SW6is ON, the output voltage Vout is applied to the external load Ld. In addition, the output voltage Vout is fed back to the output control circuit11in order to generate the error signal Verr.

FIGS. 8 to 10are timing charts illustrating the operation of the DC-DC converter according to the present embodiment. First, referring toFIGS. 8 and 9, the voltage detector100detects the open-circuit voltages Voc(H) and Voc(L) of the solar battery4, and outputs values dl and d2acquired by A/D-converting the open-circuit voltages Voc(H) and Voc(L), to the switches SW1and SW2as digital signals. Thereby, the capacity to be connected to the noninverting input terminal (+) of the output control circuit11is selected according to each open-circuit voltage. Here, in the present embodiment, the capacitor elements Cs1and Cs2are used as the capacity of wiring, but the capacity that the wiring itself has (i.e., wiring capacity) may be used.

After the selection of the capacity, the voltage detector100outputs the notification signal S1to the output control circuit11, the constant current source CC, the PWM comparator14and the PWM signal generating circuit16. Thereby, each of the output control circuit11, the constant current source CC, the PWM comparator14and the PWM signal generating circuit16begins the operation. At this time, an output voltage E of the solar battery4decreases by the consumption current of the DC-DC converter2.

The first reference voltage Vref1increases as the input voltage inputted from the solar battery4increases. Since a charging period of the capacitor elements Cs1and Cs2is determined according to each open-circuit voltage, times tH and tL in which the first reference voltage Vref1reaches the second reference voltage Vref2are also determined according to the illumination. The reaching period tH when the illumination is high is shortened from the reaching period tL when the illumination is low.

The first reference voltage Vref1is smaller than the second reference voltage Vref2, the output control circuit11outputs the error signal Verr based on an error between the first reference voltage Vref1and the output voltage Vout. Therefore, the rush current Irush is limited according to the pulse widths W(H) and W(L) of the PWM signals Tp and Tn generated based on the error signal Verr. The limit value Imax (H) of the rush current Irush when the illumination is high is larger than the limit value Imax (L) of the rush current Irush when the illumination is low. With respect to a reaching period in which the output voltage Vout reaches the rated voltage Eo, the reaching period tH when the illumination is high is shorter than the reaching period tL when the illumination is low, as is the case with the first reference voltage Vref1. Therefore, unlike the comparative example, the start-up period of the DC-DC converter2is changed according to the illumination. The start-up period of the DC-DC converter2when the illumination is high is shortened, compared with the start-up period of the DC-DC converter2when the illumination is low.

Here, after the first reference voltage Vref1reaches the second reference voltage Vref2(see after time tH), the output control circuit11outputs the error signal Verr based on an error between the constant second reference voltage Vref2and the output voltage Vout. The pulse widths W(H) and W(L) of the PWM signals Tp and Tn do not depend on the illumination, but are the same as each other. The output voltage Vout becomes the rated voltage Eo.

Next, a description is given of the operation of the limiting circuit12and the stopping control circuit13with respect toFIG. 10. Since the overcurrent limit value Ilim by the limiting circuit12is larger than the limit value Imax (L) of the rush current Irush when the illumination is low and smaller than the limit value Imax (H) of the rush current Irush when the illumination is high, as previously explained with reference toFIG. 4, a problem occurs in the case where the illumination is high. Therefore,FIG. 10illustrates only the current and the voltage in the case where the illumination is high. Here, inFIG. 10, a threshold voltage Vth of the stopping control circuit13is the same as the second reference voltage Vref2, but the threshold voltage Vth may be different from the second reference voltage Vref2.

Before the first reference voltage Vref1reaches the threshold voltage Vth, i.e., in the start-up period form 0 to tH, the stopping control circuit13maintains the voltage of the permission signal S2at a low level (‘L’). At this time, since the limiting circuit12stops the operation, an overcurrent protection function does not work. Therefore, the rush current Irush is limited by the limit value Imax (H) larger than the overcurrent limit value Ilim.

When the voltage of the permission signal S2is the low level, the output switch SW6is turned off. Therefore, even when the short circuit or the like occurs in the circuit of the load Ld during the start-up of the DC-DC converter2, a current does not flow from the DC-DC converter2to the load Ld. The PWM signals Tp and Tn, and the output voltage Vout are stated above.

Then, after the first reference voltage Vref1reaches the threshold voltage Vth, i.e., after the time tH, the stopping control circuit13maintains the voltage of the permission signal S2at a high level (‘H’). At this time, although the output switch SW6is turned on and the current flows from the DC-DC converter2to the load Ld, the overcurrent protection function works since the operation of the limiting circuit12is permitted by the permission signal S2. Therefore, at the time of the normal operation after the start-up (i.e., after time tH), a current larger than the overcurrent limit value Ilim is prevented from flowing from the DC-DC converter2to the load Ld.

FIG. 11is a timing chart illustrating the operation of the DC-DC converter when the limiting circuit12operates during the start-up of the DC-DC converter. InFIG. 11, unlike the present embodiment, it is assumed that the stopping control circuit13always maintains the voltage of the permission signal S2at the high level (‘H’), for comparison.

In this case, the overcurrent protection function always works since the operation of the limiting circuit12is permitted by the permission signal S2. Therefore, the rush current Irush is limited by not the limit value Imax (H) according to the reference voltage Vref1but the overcurrent limit value Ilim smaller than the limit value Imax (H). Therefore, compared withFIG. 10, the pulse widths of the PWM signals Tp and Tn are spread, and a time period until the output voltage Vout reaches the rated voltage Eo is prolonged by only a difference Δt. Thus, the start-up period is shortened by stopping the operation of the limiting circuit12during the start-up of the DC-DC converter, as illustrated inFIG. 10.

As described above, the output control circuit11controls the output voltage Vout of the DC-DC converter according to the reference voltage Vref1on the basis of the input voltage inputted from the solar battery4. Therefore, the control circuit1secures its own consumption current Iq and realizes soft start by limiting the rush current Irush flowing to the smoothing circuit L and C of an output side according to increase of the input voltage. Moreover, since the limiting circuit12limits the current flowing from the DC-DC converter2to the external load Ld, the overcurrent protection function is realized.

Then, the reference control circuit10controls the above-mentioned reference voltage Vref1according to the open-circuit voltage Voc of the solar battery4. Moreover, the stopping control circuit13stops the operation of the limiting circuit12until the reference voltage Vref1reaches the given value Vth. The rush current Irush is limited by the suitable limit values Imax (H) and Imax (L) according to the open-circuit voltage, without being subjected to limitation by the limiting circuit12. According to the present embodiment, the limit value of the input current inputted from the solar battery4is controlled according to the limitation determining the open-circuit voltage, so that the start-up period of the DC-DC converter2can be shortened and the overcurrent protection function can quickly work after the start-up.

This effect is similarly acquired by the following control method of the DC-DC converter. That is, the output voltage Vout of the DC-DC converter2is controlled according to the reference voltage Vref1, and the reference voltage Vref1is controlled according to the open-circuit voltage Voc of the solar battery4. Then, the operation of the limiting circuit12that limits the current flowing from the DC-DC converter2to the external load Ld stops until the reference voltage Vref1reaches the given value Vth.

In the above-mentioned embodiment, timing for detecting the open-circuit voltage Voc of the solar battery4, and timing for operating the output control circuit11or the like are managed by the voltage detector100. However, a management method is not limited to this, and these timing may be managed by an external circuit.FIG. 12is a circuit diagram of the DC-DC converter according to another embodiment. InFIG. 12, elements corresponding to those of the above-mentioned embodiment are designated by the same reference numerals, and description thereof is omitted.

A voltage detector101of the another embodiment is notified of the timing for detecting the open-circuit voltage Voc by a detection instruction signal S0from an external timing control circuit5. That is, the reference control circuit10detects the open-circuit voltage Voc of the solar battery4which is an external power supply, according to timing notified from an outside (i.e., the external timing control circuit5). Therefore, the voltage detector101can detect the open-circuit voltage Voc according to timing managed accurately.

Instead of the voltage detector101, the timing control circuit5may output the above-mentioned notification signal S1to the output control circuit11and so on. Thereby, the DC-DC converter2can be operated according to timing managed accurately. Here, the timing control circuit5may output the detection instruction signal S0and the notification signal S1by using a timer, for example.