now , referring to the accompanying drawings , a switching regulator according to an embodiment of the present invention is described below . fig1 illustrates the switching regulator having a soft - start function according to this embodiment . the switching regulator according to this embodiment includes a switching regulator control circuit 1 and peripheral circuits . a dc voltage source 34 is a power source of the switching regulator control circuit 1 and has a voltage range of from 0 . 9 v to 1 . 5 v , assuming a single dry cell as the dc voltage source 34 . between the dc voltage source 34 and gnd , a coil 33 and a power transistor 30 are connected . a connection point between the coil 33 and the power transistor 30 is connected to an output terminal vout via a diode 32 . to the output terminal vout , a resistor 35 and a resistor 36 , which form a feedback circuit , and an output capacitor 31 are connected . the switching regulator control circuit 1 has a first power supply terminal vin connected to the dc voltage source 34 , a second power supply terminal vout connected to the output terminal vout , an output terminal ext connected to a base of the power transistor 30 , and a feedback voltage terminal fb connected to an output terminal of the feedback circuit . the feedback circuit may be included in the switching regulator control circuit 1 . the switching regulator control circuit 1 includes an amplifier 11 , a soft - start circuit 12 , a reference voltage source 13 , an operational amplifier 14 , a ramp - wave oscillator 15 , a pwm comparator 16 , a voltage detection circuit 17 , a square - wave oscillator 18 , a multiplexer circuit 19 , and a buffer circuit 20 . the amplifier 11 and the soft - start circuit 12 together constitute a soft - start block 10 . a voltage of the first power supply terminal vin is a power supply voltage of the square - wave oscillator 18 . a voltage of the second power supply terminal vout is an output voltage of the switching regulator and also serves as a power supply voltage of the circuits other than the square - wave oscillator 18 . the feedback voltage terminal fb is connected to an output of the feedback circuit . the voltage detection circuit 17 monitors the voltage of the second power supply terminal vout . a detection signal vpg of the voltage detection circuit 17 is l when the voltage of the second power supply terminal vout is lower than a threshold voltage vth , and is h when the voltage of the second power supply terminal vout is higher than the threshold voltage vth . the square - wave oscillator 18 is a start - up oscillation circuit for outputting a square - wave pulse clk serving as a start - up switching signal . the square - wave oscillator 18 outputs the square - wave pulse clk when the detection signal vpg of the voltage detection circuit 17 is l . the square - wave oscillator 18 causes no problem in boosting operation even if an oscillation frequency fluctuates in a range of from several tens of khz to several hundreds of khz due to manufacturing fluctuations , temperature characteristics , or power supply voltage characteristics . accordingly , the square - wave oscillator 18 may be formed of a circuit capable of operation with a significantly low power supply voltage of 0 . 9 v . the operational amplifier 14 compares a voltage of the feedback voltage terminal fb input thereto with a reference voltage vref_ss output from the soft - start circuit 12 , and outputs a voltage verrout . the amplifier 11 is connected between input terminals of the operational amplifier 14 . the amplifier 11 is an amplifier with a gain of 1 and serves as a control circuit which operates so that the voltages of the input terminals of the operational amplifier 14 become equal to each other when the detection signal vpg of the voltage detection circuit 17 is l . the ramp - wave oscillator 15 is an oscillation circuit for outputting a ramp pulse vramp serving as a switching signal . the ramp pulse vramp of the ramp - wave oscillator 15 has an oscillation waveform with a constant slope , such as a triangular wave or a sawtooth wave . the pwm comparator 16 compares the voltage verrout from the operational amplifier 14 with the ramp pulse vramp from the ramp - wave oscillator 15 , and outputs a signal vpwm . the multiplexer circuit 19 is a switch circuit for selecting and outputting any one of the square - wave pulse clk , which is an output signal of the square - wave oscillator 18 , and the signal vpwm from the pwm comparator 16 . the multiplexer circuit 19 outputs the square - wave pulse clk when the detection signal vpg of the voltage detection circuit 17 is l , and outputs the signal vpwm when the detection signal vpg thereof is h . the buffer circuit 20 drives the power transistor 30 as a switching element of the switching regulator , based on a signal output from the multiplexer circuit 19 . referring to the drawings , an operation of the switching regulator having the above - mentioned configuration is described . fig2 is a graph illustrating the operation of the switching regulator of fig1 . before the switching regulator starts a boosting operation , the power transistor 30 is turned off and hence the output terminal vout has a voltage determined by subtracting a forward voltage vf of the diode 32 from the voltage vin of the dc voltage source 34 . when the diode 32 is a schottky barrier diode , the forward voltage vf is 0 . 2 v to 0 . 3 v . such a digital circuit as the multiplexer circuit 19 and the buffer circuit 20 is capable of operating even if the voltage vin is 0 . 9 v and the voltage of the output terminal vout is lower than 0 . 9 v by 0 . 2 v to 0 . 3 v . first , an operation when the switching regulator starts up at a start - up time ts is described . here , the threshold voltage vth of the voltage detection circuit 17 is set to 1 . 5 v . in other words , when the voltage vin falls within a range of from 0 . 9 v to 1 . 5 v , the detection signal vpg of voltage detection circuit 17 is l . accordingly , the square - wave oscillator 18 starts its operation to output the square - wave pulse clk . further , the multiplexer circuit 19 selects the output of the square - wave oscillator 18 . then , the multiplexer circuit 19 outputs the square - wave pulse clk of the square - wave oscillator 18 . in response to the square - wave pulse clk output to the output terminal ext , the power transistor 30 is driven and the switching regulator starts the boosting operation . a period when the switching regulator performs the boosting operation with the square - wave pulse clk is referred to as a start - up period t 1 . in the start - up period t 1 , the switching regulator according to the present invention does not perform negative feedback control , but performs the boosting operation with the square - wave pulse clk until the output voltage of the output terminal vout exceeds the threshold voltage vth . at the same time , each of the soft - start circuit 12 and the amplifier 11 constituting the soft - start block 10 starts its operation as well . the amplifier 11 is set to have an operating state so as to output , to the reference voltage vref_ss , a voltage approximating the feedback voltage fb from the start - up time ts . however , immediately after the start - up time ts , the output voltage of the output terminal vout is too low to allow the amplifier 11 to operate normally . accordingly , there is a large potential difference between the reference voltage vref_ss and the feedback voltage fb . the operational amplifier 14 is an operational amplifier for performing , when constituting a feedback circuit , feedback control so that a potential difference between an inverting input and a non - inverting input thereof is 0 v . specifically , the operational amplifier 14 decreases the voltage verrout when the reference voltage vref_ss is higher than the feedback voltage fb , and increases the voltage verrout when the reference voltage vref_ss is lower than the feedback voltage fb . in other words , if the potential difference between the inverting input and the non - inverting input of the operational amplifier 14 is substantially zero at a time point when the operational amplifier 14 starts the feedback control after entering a soft - start period t 2 , the feedback control accompanying large fluctuations is not required , to thereby make a stable transition from the start - up period t 1 to the soft - start period t 2 . fig3 is a circuit diagram illustrating an example of the soft - start block 10 according to this embodiment . the soft - start block 10 includes the amplifier 11 and the soft - start circuit 12 . the amplifier 11 includes a differential amplifier circuit , which is formed of transistors 100 to 104 and a constant current source 112 , and a switch circuit , which is formed of transistors 105 and 110 and an inverter 111 . the soft - start circuit 12 includes constant current sources 113 , 114 , and 115 , a dc voltage source 108 , a capacitor 107 , and transistors 106 and 109 . in the differential amplifier circuit , the feedback voltage fb and the reference voltage vref_ss as the output of the soft - start circuit 12 are input to the input transistors 101 and 102 , respectively . the transistor 100 , to which the detection signal vpg is input , controls the operation and suspension of the differential amplifier circuit . the transistors 110 and 105 , to which the detection signal vpg is input via the inverter 111 , switch between an output of the differential amplifier circuit and an output of the constant current source 113 , which is then output . when a start - up signal en is input from a start - up circuit ( not shown ) to a gate of the transistor 106 , the charge / discharge of the capacitor 107 is controlled . the transistor 109 has a gate terminal controlled by a voltage ss_cap of the capacitor 107 . the soft - start circuit 12 outputs the soft - start reference voltage vref_ss . the amplifier 11 controls the reference voltage vref_ss to be substantially equal to the feedback voltage fb when the detection signal vpg becomes h to switch a boosting state . the detection signal vpg is l at the start - up time ts , and hence the transistor 100 is in a conducting state and the differential amplifier circuit starts its operation . further , the transistor 105 is in a conducting state while the transistor 110 is in a non - conducting state , and hence an output node of the differential amplifier circuit is connected to the gate of the transistor 109 via the transistor 105 . on this occasion , the differential amplifier circuit , the transistor 109 , and the constant current sources 114 and 115 together constitute an operational amplifier . in the operational amplifier , a source of the transistor 109 serves as an output , a gate of the transistor 102 serves as an inverting input , and an input of the transistor 101 serves as a non - inverting input . therefore , the operational amplifier constitutes a voltage follower circuit and outputs a voltage equal to the feedback voltage fb to an output terminal thereof . in other words , the reference voltage vref_ss becomes equal to the feedback voltage fb . through the boosting operation described above , the output voltage of the output terminal vout exceeds the threshold voltage vth of the voltage detection circuit 17 , entering the soft - start period t 2 . when the detection signal vpg of the voltage detection circuit 17 becomes h , the transistors 100 and 105 become nonconductive so that the differential amplifier circuit suspends its operation . the transistor 110 becomes a conducting state and the constant current source 113 starts charging the capacitor 107 . therefore , the voltage of the node ss_cap starts increasing gradually . at this time , the constant current sources 114 and 115 continue supplying currents , and hence the transistor 109 operates as a source follower circuit and outputs , as the reference voltage vref_ss , a voltage which is reduced from the voltage of the node ss_cap by a voltage vgs of the transistor 109 . through the above - mentioned operation , the reference voltage vref_ss reaches to a voltage corresponding to the feedback voltage fb in the start - up period t 1 , and immediately after entering the soft - start period t 2 , starts increasing with a form depending on the charge waveform at the node ss_cap . because the transistor 109 has a drain connected to the dc voltage source 108 , once the reference voltage vref_ss increases to a voltage vref of the dc voltage source 108 , the reference voltage vref_ss does not increase any more . therefore , the voltage vref is obtained as a stable potential , ending the soft - start period t 2 . then , in the start - up period t 1 , when the detection signal vpg becomes h , the square - wave oscillator 18 suspends its operation so that the multiplexer circuit 19 outputs the signal vpwm . in other words , the switching regulator performs the boosting operation under normal pwm control . as described above , in the switching regulator according to the present invention , the soft - start reference voltage vref_ss is equal to the feedback voltage fb at the time of switch from the start - up period t 1 to the soft - start period t 2 . therefore , a stable transition from the start - up period t 1 to the soft - start period t 2 is made . note that , the description is given above on the configuration of the switching regulator according to the present invention , in which the soft - start reference voltage vref_ss is equal to the feedback voltage fb at the time of switch from the start - up period t 1 to the soft - start period t 2 . however , another configuration in which the reference voltage vref_ss becomes higher than the feedback voltage fb at that time may be employed . for example , the amplifier 11 may be set to have a gain of 1 or larger , in other words , the amplifier 11 may be used as a non - inverting amplifier circuit for amplifying the feedback voltage fb at a desired gain . this configuration allows for a margin with respect to performance fluctuations of the amplifier 11 , such as an offset voltage , to thereby make a stable transition from the start - up period t 1 to the soft - start period t 2 . further , the amplifier 11 may be formed of a source follower circuit in which an input is connected to a feedback voltage vfb and an output is connected to the reference voltage vref_ss . in general , however , the source follower circuit has a gain of 1 or smaller and outputs a voltage substantially determined by subtracting from an input voltage a threshold voltage of a mos transistor used in the source follower circuit . therefore , a gain close to 1 may be obtained if the source follower circuit uses a transistor having a low threshold , such as a depletion type transistor .