JP2012120289A - Switching power circuit and electronic apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply circuit that is of a current control system, depends on an output voltage and uses an oscillation circuit, capable of suppressing oscillation of an output voltage during a soft start while suppressing a cost increase.SOLUTION: An error amplifier EA10 into which a soft start voltage Vss, a reference voltage Vref and a feedback voltage Vadj are input switches a gain according to an output of a comparator 8 that compares between the soft start voltage Vss and the reference voltage Vs.

Description

  The present invention relates to a switching power supply circuit.

  FIG. 8 shows a circuit configuration of a conventional current control switching power supply circuit. The conventional switching power supply circuit shown in FIG. 8 includes a constant current source Ic, a capacitor Cs, an error amplifier EA100, a comparator 1, an oscillation circuit 2, a logic unit 3, buffers 4, 5, and switches SW1, SW2. A current amplifier 6, a slope compensation unit 7, a smoothing coil L, resistors R 10 and R 20, and a smoothing capacitor Co.

  A constant current source Ic and a capacitor Cs are connected in series between the ground and the power supply voltage VDD, and a soft start voltage Vss which is a voltage at a connection point between the constant current source Ic and the capacitor Cs is two non-inverting terminals of the error amplifier EA100. Is input to one of the The reference voltage Vref is input to the other non-inverting terminal of the error amplifier EA100. Further, a feedback voltage Vadj obtained by dividing the output voltage Vo by the resistors R10 and R20 is input to the inverting terminal of the error amplifier EA100. The power supply voltage VDD is generated from the input voltage VIN by a series regulator (not shown), for example.

  The error amplifier EA100 amplifies a difference between the reference voltage Vref input to the non-inverting terminal and the soft start voltage Vss, and the feedback voltage Vadj input to the inverting terminal, and compares the error amplifier output voltage Ve with a comparator. Output to 1. The current amplifier 6 amplifies and outputs the detected voltage of the coil current IL flowing through the smoothing coil L, and the current amplifier output voltage Vc whose output voltage is compensated by the slope compensator 7 is input to the comparator 1. In order to prevent low frequency oscillation, slope compensation is performed in the current feedback loop.

  The comparator 1 compares the error amplifier output voltage Ve and the current amplifier output voltage Vc, and outputs the comparison result to the logic unit 3. The oscillation circuit 2 changes the oscillation frequency according to the feedback voltage Vadj and outputs an oscillation signal to the logic unit 3. The logic unit 3 is configured by, for example, an RS flip-flop, generates a pulse signal based on the oscillation signal output from the oscillation circuit 2 and the output of the comparator 1, and outputs the pulse signal to the buffers 4 and 5. The buffers 4 and 5 generate drive signals for driving the switches SW1 and SW2 based on the pulse signals output from the logic unit 3, and output the drive signals to the switches SW1 and SW2. The switches SW1 and SW2 are controlled to turn on and off alternately, and a pulse signal having an amplitude from the input voltage VIN to the ground is generated at the node VSW which is a connection point between the switches SW1 and SW2. The generated pulse signal is smoothed by the smoothing coil L and the smoothing capacitor Co and output as the output voltage Vo.

  The oscillation circuit 2 that changes the oscillation frequency according to the feedback voltage Vadj is provided, but this is limited by the minimum ON pulse width by lowering the oscillation frequency when the output voltage Vo is lowered by an overcurrent protection function or the like. The purpose is to prevent the overcurrent set value from being exceeded by enabling operation with a duty ratio less than or equal to the specified duty ratio.

  In addition, the conventional switching power supply circuit shown in FIG. 8 has a soft start function that gently raises the output voltage Vo in order to suppress an inrush current from the input voltage VIN when the switching power supply circuit is activated. An example of a conventional soft start function is disclosed in Patent Document 1, for example.

Japanese Patent Laid-Open No. 10-257759

  However, the soft start function of the conventional switching power supply circuit has the following problems. 9 shows an example of the waveform of the conventional output voltage Vo, and the lower part of FIG. 9 shows an example of the waveform of the soft start voltage Vss. FIG. 10 shows a waveform example of each signal at the time of soft start in the conventional switching power supply circuit.

  The soft start voltage Vss rises from the timing t1, which is the start timing of the switching power supply circuit, and the soft start voltage Vss reaches the reference voltage Vref at the timing t2 (lower stage in FIG. 9). A period from timing t1 to timing t2 is a soft start period. In this period, the error amplifier EA100 amplifies the difference between the soft start voltage Vss and the feedback voltage Vadj and outputs an error amplifier output voltage Ve. The soft start voltage Vss also rises after the timing t2, and becomes a constant voltage at a certain timing. After timing t2, the error amplifier EA100 amplifies the difference between the reference voltage Vref and the feedback voltage Vadj and outputs an error amplifier output voltage Ve. The normal operation period is after timing t2.

  In some cases, the gain of the error amplifier EA100 is set to a lower value in order to prevent oscillation. In this case, the error amplifier output voltage Ve, which is the output of the error amplifier EA100, does not follow the output voltage Vo when the soft start function starts (see FIG. 10). Then, the oscillation frequency is increased by the oscillation circuit 2 without tracking, the ripple of the current amplifier output voltage Vc corresponding to the coil current IL is reduced, and the output current Io that is the average value of the coil current IL is increased. Due to the rapid increase of the output current Io, the output voltage Vo overshoots to a value higher than the set value by the soft start voltage Vss and oscillates. An example of the oscillation waveform of the output voltage Vo is shown in the upper part of FIG.

  In view of the above problems, the present invention provides a switching power supply circuit that uses an oscillation circuit that depends on an output voltage in a current control method, and that can suppress oscillation of the output voltage during soft start while suppressing an increase in cost. The purpose is to provide.

  In order to achieve the above object, the present invention provides an error amplifier that receives a soft start voltage that gradually increases at start-up, a first reference voltage, and a feedback voltage corresponding to the output voltage, and an oscillation frequency that depends on the feedback voltage. Generating a pulse signal for switching based on the output of the oscillation circuit, the first comparator for comparing the output of the error amplifier and the detection signal of the current by switching, and the output of the first comparator and the output of the oscillation circuit A switching power supply circuit comprising: a pulse signal generation unit; and a second comparator that compares the soft start voltage with a second reference voltage, wherein the error amplifier changes a gain according to an output of the second comparator A configuration is possible.

  According to such a configuration, by increasing the gain of the error amplifier at the time of soft start, the output of the error amplifier follows the output voltage, the oscillation frequency by the oscillation circuit is increased, and the ripple of the current detection signal is reduced. Even if this occurs, the output voltage does not rise rapidly, and oscillation of the output voltage can be suppressed. Further, the configuration for changing the gain of the error amplifier can be realized by adding a small number of circuits, and an increase in cost can be suppressed.

  In the above configuration, the error amplifier includes an input transistor that is a differential input section, a tail current transistor that allows a tail current to flow, and a resistor connected between the source or emitter of the input transistor and the tail current transistor. And whether the resistor is short-circuited or not can be switched according to the output of the second comparator.

  Further, in the above configuration, the error amplifier includes a switch, a current limiting resistor whose connection is switched in accordance with switching of the switch, a current mirror circuit that converts a current limited by the current limiting resistor into a tail current, The switch may be switched according to the output of the second comparator.

  Further, in the above configuration, the error amplifier includes an input transistor that is a differential input unit, and a gain setting resistor connected to a drain or a collector of the input transistor, according to the output of the second comparator. Thus, the gain setting resistor may be configured to switch whether or not to short-circuit.

  Further, in the above configuration, the error amplifier includes an input transistor that is a differential input unit, a source follower or emitter follower that is provided at one stage of the input transistor and that receives the feedback voltage, and the other of the input transistors. And a source follower or emitter follower that receives the soft start voltage and the first reference voltage as input, and the second reference voltage may be the first reference voltage.

  In addition, an electronic apparatus according to the present invention includes a switching power supply circuit having any one of the above configurations.

  According to the present invention, in a switching power supply circuit using an oscillation circuit that depends on an output voltage in a current control system, it is possible to suppress oscillation of the output voltage during soft start while suppressing an increase in cost.

It is a figure which shows the structural example of the switching power supply circuit which concerns on embodiment of this invention. It is a figure which shows the example of a waveform of each part signal at the time of the soft start in the switching power supply circuit which concerns on embodiment of this invention. It is a figure which shows the example which applied the switching power supply circuit which concerns on embodiment of this invention to LED lighting apparatus. It is a figure which shows the structural example of the error amplifier which concerns on this invention. It is a figure which shows the structural example of the error amplifier which concerns on this invention. It is a figure which shows the structural example of the error amplifier which concerns on this invention. It is a figure which shows the structural example of the error amplifier which concerns on this invention. It is a figure which shows the structural example of the conventional switching power supply circuit. It is a figure which shows the example of a waveform of the output voltage in the past and this invention, and the example of a waveform of a soft start voltage. It is a figure which shows the example of a waveform of each part signal at the time of the soft start in the conventional switching power supply circuit.

  Embodiments of the present invention will be described below with reference to the drawings. In the drawings used for the following description, the same portions are denoted by the same reference numerals. Since their names and functions are the same, detailed description thereof will not be repeated.

  A configuration example of a switching power supply circuit according to an embodiment of the present invention is shown in FIG. The switching power supply circuit shown in FIG. 1 has a configuration in which an error amplifier EA10 is used as an error amplifier and a comparator 8 is added to the conventional current control switching power supply circuit shown in FIG. The soft start voltage Vss is input to the inverting terminal of the comparator 8, and the reference voltage Vs is input to the non-inverting terminal (the reference voltage Vs is the same value as the reference voltage Vref). Then, the comparator output voltage Vcomp that is the output of the comparator 8 is input to the error amplifier 10.

  The error amplifier EA10 has a function of increasing the gain when the error amplifier output Vcomp is at a high level and decreasing the gain when the error amplifier output Vcomp is at a low level. FIG. 2 shows a waveform example of each signal at the time of soft start in the switching power supply circuit according to the embodiment of the present invention. 9 shows an example of the waveform of the output voltage Vo of the switching power supply circuit according to the embodiment of the present invention. When the switching power supply circuit is activated and the soft start voltage Vss is lower than the reference voltage Vs (= Vref), the error amplifier output Vcomp becomes High level, and the error amplifier EA10 increases the gain. As a result, the error amplifier output voltage Ve follows the output voltage Vo. Therefore, even if the ripple of the current amplifier output voltage Vc decreases due to the increase of the oscillation frequency by the oscillation circuit 2, the output current Io does not increase rapidly. (FIG. 2). Therefore, the output voltage Vo does not overshoot, and the oscillation of the output voltage Vo can be suppressed (lower stage in FIG. 2, middle stage in FIG. 9).

  FIG. 3 shows a configuration example in which the switching power supply circuit according to the embodiment of the present invention is applied to an LED lighting device. The AC voltage Va is converted into an input voltage VIN which is a DC voltage by the diode bridge DB1 and the smoothing capacitor C1, and is input to the switching power supply circuit. An LED module LED1 composed of one or a plurality of LEDs is connected to the output side of the switching power supply circuit. By suppressing the oscillation of the output voltage Vo at the time of soft start by the switching power supply circuit according to the embodiment of the present invention, it is possible to reduce noise mixing into the AC line and radiation noise.

  FIG. 4 shows a configuration example of the error amplifier EA10 in the switching power supply circuit according to the embodiment of the present invention shown in FIG. The error amplifier shown in FIG. 4 includes NchMOS transistors N1 and N2 that are differential inputs, resistors R1, R2, R3, and R4 related to gain, an NchMOS transistor N3 that supplies a tail current, and a PchMOS that divides the tail current into two. Transistors P1 and P2, switching NchMOS transistors N4 and N5, a minimum voltage selection circuit 9, and an error amplifier output stage 10 are provided. The minimum voltage selection circuit 9 compares the input reference voltage Vref and the soft start voltage Vss, and outputs the smaller voltage to the gate of the NchMOS transistor N2. A feedback voltage Vadj is input to the gate of the Nch MOS transistor N1. The error amplifier output stage 10 receives the output of the differential circuit, amplifies the difference between them, and outputs an error amplifier output voltage Ve.

  A connection point between the source of the Nch MOS transistor N1 and the drain of the Nch MOS transistor N4 and a connection point between the source of the Nch MOS transistor N2 and the drain of the Nch MOS transistor N5 are connected in series by a resistor R3 and a resistor R4. A connection point between the resistors R3 and R4 is connected to a connection point between the source of the Nch MOS transistor N4 and the source of the Nch MOS transistor N5. A connection point between the source of the Nch MOS transistor N4 and the source of the Nch MOS transistor N5 is connected to the drain of the Nch MOS transistor N3. Comparator output voltage Vcomp is input to the gates of NchMOS transistor N4 and NchMOS transistor N5, respectively.

  The comparator output voltage Vcomp is at a high level at the time of soft start, both the Nch MOS transistors N4 and N5 are turned on, and the resistors R3 and R4 are short-circuited. This increases the gain of the error amplifier. At the end of the soft start and normal operation, the comparator output voltage Vcomp is at the low level, the NchMOS transistors N4 and N5 are both turned off, the resistors R3 and R4 are activated, and the gain of the error amplifier is reduced. .

Here, when the transconductance of the Nch MOS transistors N1 and N2 is gm and the resistance values of the resistors R3 and R4 are r, the transconductance Gm of the differential input section is (1) when the resistors R3 and R4 are not short-circuited. It can be expressed as an expression.
Gm = gm / (1 + gm · r) (1)
In addition, when the resistors R3 and R4 are short-circuited, Gm can be expressed by the equation (2).
Gm = gm (2)
Therefore, the gain of the error amplifier is higher when the resistors R3 and R4 are short-circuited.

  By applying the error amplifier having the configuration shown in FIG. 4 to the switching power supply circuit shown in FIG. 1, the oscillation of the output voltage Vo at the time of soft start can be suppressed. In the error amplifier having the configuration shown in FIG. 4, an npn bipolar transistor is used instead of the Nch MOS transistors N1 and N2 in the differential input section, the collector of the npn bipolar transistor is connected to the resistors R1 and R2, and the emitter is the resistor R3. It may be connected to R4, and the feedback voltage Vadj and the output of the minimum voltage selection circuit 9 may be input to the base.

  FIG. 5 shows another configuration example of the error amplifier EA10 in the switching power supply circuit according to the embodiment of the present invention shown in FIG. The error amplifier shown in FIG. 5 includes NchMOS transistors N1 and N2, which are differential inputs, resistors R1 and R2 related to gain, an NchMOS transistor N3 that passes a tail current, and PchMOS transistors P1 and P2 that divide the tail current into two. And tail current determining resistors R5 and R6, a current mirror circuit NchMOS transistor N6, an inverter INV, a switch PchMOS transistor P3, a minimum voltage selection circuit 9, and an error amplifier output stage 10. Have.

  The gates of the Nch MOS transistors N3 and N6 are connected to each other, the gate and the drain of the Nch MOS transistor N6 are connected, and the sources of the Nch MOS transistors N3 and N6 are connected to the ground to constitute a current mirror circuit. The power supply voltage VDD is connected to one end of the resistor R5 and the source of the Pch MOS transistor P3. One end of a resistor R6 is connected to the drain of the PchMOS transistor P3, and the other end of the resistor R6 and the other end of the resistor R5 are connected to the drain of the NchMOS transistor N6. Further, the comparator output voltage Vcomp is input to the inverter INV, and the output of the inverter INV is input to the gate of the PchMOS transistor P3.

  The comparator output voltage Vcomp is at a high level at the time of soft start, the PchMOS transistor P3 is turned on by the inverter INV, and the resistor R6 and the resistor R5 are connected in parallel. As a result, the tail current increases and the gain of the error amplifier increases. At the end of the soft start and normal operation, the comparator output voltage Vcomp is at a low level, the inverter INV turns off the PchMOS transistor P3, the resistor R6 becomes ineffective, the tail current decreases, and the error amplifier gain is low. Become.

If the tail current is It, the transconductance gm of the Nch MOS transistors N1 and N2 can be expressed by equation (3).
gm = √ (It · β) (3)
However, β is a constant according to the process and shape of the Nch MOS transistors N1 and N2, and therefore the gain of the error amplifier increases as the tail current It increases.

  By applying the error amplifier having the configuration shown in FIG. 5 to the switching power supply circuit shown in FIG. 1, oscillation of the output voltage Vo at the time of soft start can be suppressed.

  FIG. 6 shows still another configuration example of the error amplifier EA10 in the switching power supply circuit according to the embodiment of the present invention shown in FIG. The error amplifier shown in FIG. 6 includes NchMOS transistors N1 and N2 that are differential inputs, resistors R1, R2, R7, and R8 related to gain, an NchMOS transistor N3 that passes a tail current, and a PchMOS that divides the tail current into two. Transistors P1 and P2, switching PchMOS transistors P7 and P8, a minimum voltage selection circuit 9, and an error amplifier output stage 10 are provided.

  Resistors R8, R1, R2, and R7 are connected in series between a connection point between the drain of the PchMOS transistor P1 and the drain of the NchMOS transistor N1 and a connection point between the drain of the PchMOS transistor P2 and the drain of the NchMOS transistor N2. . A connection point between the gates of the PchMOS transistors P1 and P2 is connected to a connection point of the resistors R1 and R2. The source of PchMOS transistor P7 is connected to the connection point of resistors R8 and R1, and the drain of PchMOS transistor P7 is connected to the connection point of the drain of PchMOS transistor P1 and the drain of NchMOS transistor N1. The source of PchMOS transistor P8 is connected to the connection point of resistors R2 and R7, and the drain of PchMOS transistor P8 is connected to the connection point of the drain of PchMOS transistor P2 and the drain of NchMOS transistor N2. The comparator output voltage Vcomp is input to the gates of the Pch MOS transistors P7 and P8.

  The comparator output voltage Vcomp is at a high level at the time of soft start, the Pch MOS transistors P7 and P8 are turned off, and the resistors R8, R1, R2, and R7 are connected in series, thereby increasing the gain of the error amplifier. At the end of the soft start and the normal operation, the comparator output voltage Vcomp is at a low level, the Pch MOS transistors P7 and P8 are turned on, and the resistors R8 and R7 are short-circuited, so that the gain of the error amplifier is lowered.

When the resistance values of the resistors R8 and R7 are r1, and the resistance values of the resistors R1 and R2 are r2, the gain Gain before the error amplifier output stage 10 can be expressed by the following equation (4).
Gain = gm · (r1 + r2) (4)
Therefore, the gain of the error amplifier is increased by connecting the resistors R8, R1, R2, and R7 in series.

  By applying the error amplifier having the configuration shown in FIG. 6 to the switching power supply circuit shown in FIG. 1, the oscillation of the output voltage Vo at the time of soft start can be suppressed. In the error amplifier having the configuration shown in FIG. 6, an npn bipolar transistor is used instead of the Nch MOS transistors N1 and N2 in the differential input section, the collector of the npn bipolar transistor is connected to the resistors R8 and R7, and the emitter is the NchMOS transistor N3. The feedback voltage Vadj and the output of the minimum voltage selection circuit 9 may be input to the base.

  FIG. 7 shows still another configuration example of the error amplifier EA10 in the switching power supply circuit according to the embodiment of the present invention shown in FIG. FIG. 7 also shows the connection of the comparator 8. The error amplifier shown in FIG. 7 includes NchMOS transistors N1 and N2 that are differential inputs, resistors R1, R2, R3, and R4 related to gain, an NchMOS transistor N3 that supplies a tail current, and a PchMOS that divides the tail current into two. Transistors P1 and P2, switching NchMOS transistors N4 and N5, error amplifier output stage 10, constant current sources I1 and I2, and source follower PchMOS transistors P4, P5, and P6 are provided.

  The constant current source I1 and the gate of the NchMOS transistor N1 are connected to the source of the PchMOS transistor P4, the drain of the PchMOS transistor P4 is connected to the ground, and the feedback voltage Vadj is input to the gate of the PchMOS transistor P4. As a result, a source follower having the feedback voltage Vadj as an input is formed before the NchMOS transistor N1.

  The constant current source I2 and the gate of the NchMOS transistor N2 are connected to the source of the PchMOS transistor P5, the drain of the PchMOS transistor P5 is connected to the ground, and the soft start voltage Vss is input to the gate of the PchMOS transistor P5. As a result, a source follower having the soft start voltage Vss as an input is formed before the NchMOS transistor N2. The constant current source I2 and the gate of the NchMOS transistor N2 are connected to the source of the PchMOS transistor P6, the drain of the PchMOS transistor P6 is connected to the ground, and the reference voltage Vref is input to the gate of the PchMOS transistor P6. As a result, a source follower having the reference voltage Vref as an input is formed before the NchMOS transistor N2.

  In FIG. 1, the reference voltage Vs is input to the non-inverting terminal of the comparator 8, but in FIG. 7, the reference voltage Vref is input to the non-inverting terminal of the comparator 8 so that the reference voltage Vs is unnecessary.

  Here, the current of the constant current sources I1 and I2 is i1, β is a constant according to the process and shape of the PchMOS transistors P4, P5, and P6, and Vth is the threshold voltage of the PchMOS transistors P4, P5, and P6. At this time, the overdrive voltage of the PchMOS transistor P4 becomes √ (2 · i1 / β), and the gate voltage of the NchMOS transistor N1 becomes Vadj + Vth + √ (2 · i1 / β).

  When the output of the comparator 8 is switched to Vss = Vref, the same current flows through the PchMOS transistors P5 and P6. Therefore, the overdrive voltage of the PchMOS transistors P5 and P6 becomes √ (i1 / β), and the gate voltage of the NchMOS transistor N2 Becomes Vref + Vth + √ (i1 / β).

  Further, when Vss> Vref, no current flows to the PchMOS transistor P5 when Vss is higher than Vref by √ (i1 / β). When no current flows in the Pch MOS transistor P5, the overdrive voltage of the Pch MOS transistor P6 becomes √ (2 · i1 / β), and thus the gate voltage of the Nch MOS transistor N2 becomes Vref + Vth + √ (2 · i1 / β). This gate voltage is a reference voltage to be compared with the gate voltage of the Nch MOS transistor N1 during normal operation. The gain of the error amplifier can be switched by switching the output of the comparator 8 at Vss which is lower than the reference voltage Vss by the gate voltage of the Nch MOS transistor N2 by √ (i1 / β).

  In the error amplifier configured as shown in FIG. 7, an emitter follower using a pnp bipolar transistor may be provided instead of a source follower using a PchMOS transistor.

  The switching power supply circuit according to the present invention can be mounted on all electronic devices, but is particularly suitable for use in electronic devices such as LED lighting devices, optical storage devices, and liquid crystal televisions.

DESCRIPTION OF SYMBOLS 1 Comparator 2 Oscillator 3 Logic part 4 Buffer 5 Buffer 6 Current amplifier 7 Slope compensation part 8 Comparator 9 Minimum voltage selection circuit 10 Error amplifier output stage Ic Constant current source Cs Capacitor L Smoothing coil Co Smoothing capacitor R10, R20 Resistor SW1 , SW2 switch EA10, EA100 Error amplifier

Claims (6)

An error amplifier that receives a soft start voltage that gradually increases at start-up, a first reference voltage, and a feedback voltage corresponding to the output voltage; an oscillation circuit whose oscillation frequency depends on the feedback voltage; and an output of the error amplifier; A switching power supply comprising: a first comparator that compares a current detection signal by switching; and a pulse signal generator that generates a pulse signal for switching based on the output of the first comparator and the output of the oscillation circuit In the circuit
A switching power supply circuit comprising: a second comparator for comparing the soft start voltage with a second reference voltage, wherein the error amplifier can change a gain according to an output of the second comparator.   The error amplifier includes an input transistor that is a differential input section, a tail current transistor that allows a tail current to flow, and a resistor connected between the source or emitter of the input transistor and the tail current transistor, The switching power supply circuit according to claim 1, wherein whether or not the resistor is short-circuited is switched according to an output of the second comparator.   The error amplifier includes a switch, a current limiting resistor whose connection is switched according to switching of the switch, and a current mirror circuit that converts a current limited by the current limiting resistor into a tail current, 2. The switching power supply circuit according to claim 1, wherein the switch is switched in accordance with an output of the two comparators.   The error amplifier includes an input transistor which is a differential input unit, and a gain setting resistor connected to a drain or a collector of the input transistor, and the gain setting resistor is set according to an output of the second comparator The switching power supply circuit according to claim 1, wherein whether or not to short-circuit is switched.   The error amplifier includes an input transistor that is a differential input unit, a source follower or emitter follower that is provided in one preceding stage of the input transistor and that receives the feedback voltage, and is provided in the other preceding stage of the input transistor. 2. The switching power supply according to claim 1, further comprising: a source follower or an emitter follower having a soft start voltage and the first reference voltage as inputs, wherein the second reference voltage is the first reference voltage. circuit.   An electronic apparatus comprising the switching power supply circuit according to claim 1.

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