JP4282673B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device.

  Conventionally, a power supply output voltage obtained by converting a battery voltage as a power supply voltage source on the primary side into a set voltage is applied to a power supply circuit of a portable device that reproduces a CD (compact disc) or an MD (mini disc). In order to output to the load on the secondary side, a switching power supply device is employed. FIG. 11 shows a circuit configuration of a conventional switching power supply device (see, for example, Patent Document 1).

  The conventional switching power supply device shown in FIG. 11 adopts a step-up DC / DC converter configuration, and a power supply input voltage (battery voltage) VIN from a battery, which is a primary power supply voltage supply source BT, is used for driving a coil. The voltage is boosted to the set voltage by the switching operation of the bipolar transistor TrA and the field effect transistor (power MOSFET) TrB, and is output to the secondary load.

  In FIG. 11, the detection voltage of the secondary power supply output voltage VOUT detected by the division ratio of the resistors R2 and R3 is input to the FB terminal of the driver IC 101 for the DC / DC converter. The power supply output voltage VOUT is input to the VDD terminal as the power supply voltage of the driver IC 101. A pulse signal for controlling the switching operation of the coil driving transistor is output from the OUT terminal. FIG. 12 shows a pulse signal output from the OUT terminal.

  Based on the difference between the detection voltage input to the FB terminal and the internal reference voltage, the driver IC 101 determines the “H” level period (Ton) and the “L” level period (Toff) of the pulse signal output from the OUT terminal. By performing the control, a negative feedback operation is performed so that the voltage value obtained by dividing the power supply output voltage VOUT by the resistors R2 and R3 converges toward the internal reference voltage. The driver IC 101 outputs the pulse peak value Vm of the pulse signal output from the OUT terminal at substantially the same potential as the VDD terminal voltage.

  The PWM pulse control unit 102 converts the pulse signal output from the OUT terminal of the driver IC 101 into a current by the resistor R1 and outputs it to the base terminal of the bipolar transistor TrA, and directly outputs the pulse signal to the gate terminal of the field effect transistor TrB. Output. The bipolar transistor TrA and the field effect transistor TrB are turned on during the period (Ton) when the pulse signal is at the “H” level and turned off during the period (Toff) at the “L” level.

The operation of this conventional switching power supply device will be described below.
First, when the power supply voltage supply source BT is activated, assuming that the forward diode voltage of the Schottky barrier diode D1 is 0.2 (V), the VDD terminal of the driver IC 101 is
VIN-0.2
Is applied. However, VIN is a power supply input voltage (battery voltage).

Therefore, when the base-emitter voltage of the bipolar transistor TrA is VBE and the resistance value of the resistor R1 is R1, the base current Ib input to the base terminal of the bipolar transistor TrA at the start-up is
Ib≈ (VIN−0.2−VBE) / R1
Thus, the bipolar transistor TrA starts a switching operation.

Further, when the threshold voltage of the field effect transistor TrB is Vth, the power supply input voltage (battery voltage) VIN at the time of startup is
VIN−0.2 <Vth
Since the relationship between the pulse peak value Vm of the pulse signal output from the OUT terminal of the driver IC 101 and the threshold voltage Vth is the same, the field effect transistor TrB having a large threshold voltage Vth immediately after startup. Cannot start the switching operation.

After startup, the power supply output voltage VOUT is boosted by the switching operation of the bipolar transistor TrA,
VOUT ≧ Vth
Since the relationship between the pulse peak value Vm of the pulse signal output from the OUT terminal of the driver IC 101 and the threshold voltage is the same, the field effect transistor TrB starts the switching operation.

  As described above, a conventional switching power supply device including a bipolar transistor and a field effect transistor connected in parallel as a coil driving transistor, when starting up, first supplies the power output voltage VOUT on the secondary side to a predetermined voltage by the switching operation of the bipolar transistor. The voltage is boosted to (threshold voltage of the field effect transistor), and thereafter, the power supply output voltage VOUT on the secondary side is boosted to the set voltage mainly by the switching operation of the field effect transistor.

  In FIG. 11, the internal configuration of the driver IC 101 is not known, but the configuration of a general boost DC / DC converter is as shown in FIG. 4, and the power supply output voltage VOUT is changed from the power supply output voltage VOUT to resistors 41 and 42. The voltage value divided by is controlled so as to converge to the internal reference voltage Va of the reference voltage decrease 44.

  However, in the configuration of the conventional switching power supply device, the power input voltage is set by the same circuit in the standby mode and the normal operation mode even when used in an electric device having the standby mode and the normal operation mode. Will be converted to That is, the same set voltage is supplied to the load with the same current supply capability. Therefore, there is a case where a device (for example, a microcomputer, etc.) having a standby mode and a normal operation mode as a secondary side load and having a current consumption smaller than that in the normal operation mode is connected in the standby mode. However, in the conventional switching power supply configuration, the current consumption of the switching power supply itself cannot be reduced in the standby mode, and the current consumption (outflow current) from the power supply voltage source (battery) in the standby mode is reduced. Could not be reduced.

In addition, a device that requires an accurate constant voltage as a power supply input in the normal operation mode (for example, a microcomputer) is used as a load on the secondary side, and a battery whose voltage decreases over time is provided on the primary side. In the conventional switching power supply device composed of only the step-up DC / DC converter, the following problems occur. That is, in the conventional switching power supply device, when the battery voltage is higher than the voltage obtained by adding the forward diode voltage of the rectifying diode D1 to the set voltage, the power supply output voltage supplied to the load is changed from the battery voltage to the forward direction of the diode D1. Suspended to the voltage minus the diode voltage. When the battery voltage decreases with use time, the power supply output voltage also decreases accordingly, and a predetermined constant voltage cannot be supplied to the device in the normal operation mode.
JP 2001-69749 A

  In view of the above problems, the present invention provides a step-up DC / DC converter and a step-up / step-down DC / DC converter. In the standby mode, the step-up DC / DC converter has a set voltage (first setting) in the normal operation mode. A power supply output voltage that is lower than a set voltage (second set voltage) lower than the voltage) is supplied to the load. On the other hand, in the normal operation mode, the step-up / step-down DC / DC converter supplies the set voltage (first The power supply voltage of the switching power supply itself can be reduced in the standby mode, and the current consumption (outflow current) from the power supply source in the standby mode can be reduced. A switch that can supply a constant voltage that is not affected by the voltage of the power supply voltage supply to the load during normal operation mode And to provide a mode power supply unit.

  Further, in the conventional switching power supply device, the secondary side power supply output voltage is used as the circuit power supply of the step-up DC / DC converter. Therefore, when a battery is used as the primary side power supply voltage supply source, As described above, the power supply voltage of the step-up DC / DC converter is a voltage that drops from the battery voltage by the forward diode voltage of the diode, and the lower limit value of the battery voltage required at startup (when the battery is set) is There is a problem that the forward diode voltage becomes higher.

  Therefore, in view of the above problems, the present invention can operate even when the power supply input voltage at the time of start-up is low by directly using the power supply input voltage from the power supply voltage supply as the power supply voltage of the step-up DC / DC converter. An object of the present invention is to provide a switching power supply device capable of performing

  In addition, as shown in FIG. 4, a general boost DC / DC converter includes an amplifier (error amplifier). Therefore, in a conventional switching power supply device, current consumption occurs in the error amplifier even in the standby mode.

  Therefore, in view of the above problems, the present invention is configured such that the configuration of the boost DC / DC converter that operates in the standby mode does not include an amplifier (error amplifier) with respect to a general boost DC / DC converter. (See FIGS. 1 and 5), current consumption generated in the error amplifier in the standby mode can be reduced, and current consumption (outflow current) from the power supply voltage supply source in the standby mode can be reduced. An object of the present invention is to provide a switching power supply device that can be used.

  The switching power supply device according to claim 1 of the present invention includes an input terminal connected to a power supply voltage supply source, an output terminal connected to a load having a normal operation mode and a standby mode, and the power supply in the normal operation mode. A step-up / step-down DC / DC converter that supplies a power supply output voltage obtained by converting a power supply input voltage from a voltage supply source to a first set voltage to the load, and in the standby mode, the power supply output voltage is higher than the first set voltage. Supplying a power output voltage obtained by converting the power input voltage to the second set voltage to the load when the second set voltage is lower than the second set voltage, and when the power output voltage exceeds the second set voltage, A step-up DC / DC converter that supplies a power supply output voltage that has passed the power supply input voltage to the load without performing a conversion operation to a second set voltage; and the normal operation mode. And a mode changeover switch for outputting the output of the step-up / step-down DC / DC converter to the output terminal during a standby mode and outputting the output of the step-up DC / DC converter to the output terminal during the standby mode. And

  A switching power supply according to a second aspect of the present invention is the switching power supply according to the first aspect, wherein the step-up DC / DC converter uses the power supply input voltage as the power supply voltage.

  A switching power supply device according to claim 3 of the present invention is the switching power supply device according to claim 1 or 2, wherein the step-up DC / DC converter detects a power supply output voltage and outputs a detected voltage. A detection circuit that outputs, a triangular wave generation circuit that outputs a triangular wave signal, a comparator that compares a detection voltage of the detection circuit and a triangular wave signal from the triangular wave generation circuit, and outputs a comparison result signal; and A switching element that performs a switching operation according to the comparison result signal, a coil that is applied with the power supply input voltage at one end and is driven by the switching operation of the switching element, and a diode that rectifies the output of the coil In the standby mode, the output of the coil rectified by the diode is output to the output terminal. And wherein the door.

  The switching power supply device according to claim 4 of the present invention is the switching power supply device according to claim 3, wherein the step-up DC / DC converter includes a circuit that stops the switching element in an OFF state during the normal operation mode. Furthermore, it is characterized by comprising.

  According to the present invention, the step-up DC / DC converter and the step-up / step-down DC / DC converter are provided, and in the standby mode, the step-up DC / DC converter has a higher voltage than the set voltage (first set voltage) in the normal operation mode. A power supply output voltage higher than a low set voltage (second set voltage) is supplied to the load. On the other hand, in the normal operation mode, the step-up / step-down DC / DC converter has a set voltage (first set voltage) in the normal operation mode. Therefore, the current consumption of the switching power supply device itself can be reduced during the standby mode, and the current consumption from the power supply voltage supply source during the standby mode can be reduced. Therefore, when used in an electrical device that uses a battery as the power supply voltage source on the primary side, the battery life can be extended. In addition, when the battery is set and left unattended, if the device in which the standby mode is set is a secondary load, the battery voltage will drop if there is a large amount of current (outflow current) from the battery in the standby mode However, according to the present invention, the current consumption from the battery in the standby mode can be reduced, so that the battery leakage can be prevented and the electric leakage of the battery due to the battery leakage can be prevented. It is possible to prevent defects in quality.

  According to the present invention, in the normal operation mode, the power supply input voltage from the power supply voltage supply source is converted into the set voltage (first set voltage) by the step-up / step-down DC / DC converter. A constant voltage that is not affected by the current can be supplied to the load.

  Further, according to the present invention, since the power supply input voltage from the power supply voltage supply source is directly used as the power supply voltage of the step-up DC / DC converter, when a battery is used as the power supply voltage supply source on the primary side, It can operate even when the battery voltage is low (at startup).

  That is, in the conventional switching power supply device, since the secondary side power supply output voltage is used as the circuit power supply of the step-up DC / DC converter (see FIG. 11), the battery is used as the primary side power supply voltage supply source. The power supply voltage of the step-up DC / DC converter when the battery is set is a voltage that drops from the battery voltage by the forward diode voltage of the diode. According to the present invention, the power supply voltage of the step-up DC / DC converter As the battery voltage is directly used, the lower limit value of the battery voltage required at startup can be made lower than that of the conventional switching power supply device.

  In addition, as described above, in the conventional switching power supply device, the secondary side power supply output voltage is used as the circuit power supply of the step-up DC / DC converter, and therefore lower when the battery is used as the primary side power supply voltage supply source. A Schottky barrier diode having a small forward diode voltage is used as a rectifying diode so that the battery voltage can be started. However, according to the present invention, the battery voltage is used as the power supply voltage of the step-up DC / DC converter. Therefore, an inexpensive diode can be used as the rectifying diode. In addition, since a Schottky barrier diode is not used as a rectifying diode, when a step-up DC / DC converter is configured using a semiconductor integrated circuit, it is possible to incorporate the rectifying diode in the semiconductor integrated circuit. Become.

  Further, according to the present invention, the step-up DC / DC converter is configured not to include an amplifier (error amplifier) with respect to a general step-up DC / DC converter (see FIGS. 1 and 5). . Therefore, current consumption generated in the error amplifier in the standby mode can be reduced.

  Further, according to the present invention, the switching operation of the switching element built in the step-up DC / DC converter can be stopped in the OFF state during the normal operation mode, so that the step-up DC / DC converter built-in can be stopped during the normal operation mode. It is possible to prevent a malfunction that the switching element is turned on, reduce current consumption from the power supply voltage supply source, and avoid adverse effects on other circuits due to the switching operation of the switching element built in the step-up DC / DC converter.

  Hereinafter, a switching power supply according to an embodiment of the present invention will be described with reference to the drawings. The switching power supply device is used in an electric device using a battery as a primary power supply voltage supply source.

FIG. 1 is a diagram showing a schematic configuration of a switching power supply apparatus according to the present embodiment.
In FIG. 1, a microcomputer 1 is a load connected to a secondary output terminal (not shown) of the switching power supply device, and is built in an electrical device that uses the switching power supply device. The primary power supply voltage supply source 2 is connected to a primary input terminal (not shown) of the switching power supply device, and is a battery in the present embodiment. Moreover, as an electric device, it has a normal operation mode and a standby mode, and when the battery is left in a state where it is set, the standby mode is set, and most circuits of the microcomputer 1 are stopped, It is assumed that the remaining part of the circuit operates.

  Here, the specification of the power supply input from the switching power supply device of the microcomputer 1 will be described. In the standby mode, since the current consumption in the microcomputer 1 is small, the current supply capacity may be small (for example, 50 μA), and a second set voltage lower than the first set voltage required in the normal operation mode is applied. (For example, 2V or more). On the other hand, in the normal operation mode, the current consumption in the microcomputer 1 is large, so that the current supply capacity needs to be large (for example, 100 mA at the maximum), and a constant voltage (not affected by voltage fluctuations of the power supply voltage supply source (battery) 2). It is assumed that it is necessary to apply (first set voltage) (for example, 2.5 V).

  The output smoothing capacitor 3 is connected between the secondary output terminal of the switching power supply device and GND (ground), and smoothes the output from the step-up / step-down DC / DC converter 4 or the step-up DC / DC converter 5 and average power thereof. Is supplied to the load (microcomputer 1).

  The mode switching signal output circuit 6 outputs a mode switching signal indicating whether the electric device (microcomputer 1) is in the standby mode or the normal operation mode. Here, the mode switching signal is a binary signal whose potential is switched depending on whether it is in the standby mode or the normal operation mode, and becomes ‘H’ level in the standby mode and ‘L’ level in the normal operation mode. Note that the mode switching signal output circuit is built in an electric device and uses a circuit for setting the microcomputer 1 to the standby mode or the normal operation mode.

  The step-up / step-down DC / DC converter 4 supplies the microcomputer 1 with a power supply output voltage VOUT obtained by converting the power supply input voltage VIN from the power supply voltage supply source 2 into a first set voltage in the normal operation mode.

  When the power supply output voltage VOUT supplied to the microcomputer 1 is equal to or lower than a second set voltage lower than the first set voltage in the standby mode, the step-up DC / DC converter 5 receives power from the power supply voltage supply source 2. A power supply output voltage VOUT obtained by converting the power supply input voltage VIN into the second set voltage is output and supplied to the microcomputer 1 with a smaller current supply capability than the step-up / step-down DC / DC converter 4. On the other hand, when the power supply output voltage VOUT exceeds the second set voltage, the power supply input voltage VIN is passed through without being converted to the second set voltage and applied to the output smoothing capacitor 3. The step-up DC / DC converter 5 directly uses the power supply input voltage VIN from the power supply voltage supply source 2 as its power supply voltage.

  The mode changeover switch 7 has an A contact and a B contact. In accordance with the mode change signal from the mode change signal output circuit 6, the mode change switch 7 is connected to the A contact in the standby mode so that the power supply output voltage of the step-up DC / DC converter 5 is reduced. Supply to the computer 1 and connect to the B contact in the normal operation mode to supply the power output voltage of the step-up / step-down DC / DC converter 4 to the microcomputer 1.

  With the above configuration, power is supplied to the microcomputer 1 (load) by the step-up DC / DC converter with a small current consumption in the standby mode, so that the current consumption of the switching power supply itself can be reduced in the standby mode.

  That is, the conventional switching power supply device converts the power supply input voltage into the set voltage by the same circuit in the standby mode and in the normal operation mode. In other words, since the same set voltage is supplied to the load with the same current supply capability, even when power is supplied to a load with lower current consumption in standby mode than in normal operation mode, The current consumption of the switching power supply itself cannot be reduced, and therefore the current consumption (outflow current) from the power supply voltage supply source in the standby mode cannot be reduced. In contrast, in the present embodiment, as described above, the current consumption of the switching power supply device itself can be reduced in the standby mode, so that the current consumption from the power supply voltage supply source in the standby mode can be reduced. it can. Therefore, it is possible to set the consumption current from the power supply voltage supply source in the standby mode to be small as a specification of the electric equipment including the switching power supply device (for example, 200 μA when the power supply voltage supply source is 2.4 V).

  As described above, according to the switching power supply according to the present embodiment, the current consumption from the power supply voltage source on the primary side can be reduced in the standby mode, and the battery life is reduced when the battery is used as the power supply voltage supply source. Can be long.

  In addition, when the battery is set and left in a stand-by mode, if the device in which the standby mode is set is a load, if there is a large amount of current (outflow current) from the battery in the standby mode, the battery voltage will drop and the battery will Although liquid leakage may occur, according to the present embodiment, the current consumption from the battery in the standby mode can be reduced, so that the battery leakage is prevented and the quality of the electrical equipment due to the battery leakage is reduced. Can prevent malfunctions.

  Further, since the switching power supply apparatus directly uses the power supply input voltage VIN from the power supply voltage supply source 2 as the power supply voltage of the step-up DC / DC converter 5, when a battery is used as the primary power supply voltage supply source, It can operate even when the battery voltage is low when the battery is set (at startup).

  That is, in the conventional switching power supply device, the secondary side power supply output voltage is used as the circuit power supply of the step-up DC / DC converter. Therefore, when a battery is used as the primary side power supply voltage supply source, The power supply voltage of the step-up DC / DC converter in FIG. 2 is a voltage that drops from the battery voltage by the forward diode voltage of the rectifying diode (Schottky barrier diode). The switching power supply according to the present embodiment Since the battery voltage is directly used as the power supply voltage of the step-up DC / DC converter, the lower limit value of the battery voltage required at startup can be made lower than that of the conventional switching power supply device.

  Further, in the normal operation mode, the power supply input voltage VIN from the power supply voltage supply source 2 is converted into a set voltage (first set voltage) by the step-up / step-down DC / DC converter 4, so that the power supply voltage supply source (battery) 2 A constant voltage that is not affected by the voltage level can be supplied to the load.

  That is, in a conventional switching power supply device composed only of a step-up DC / DC converter, when a battery whose voltage decreases with time of use is used as a power supply voltage source on the primary side, a forward diode of a rectifying diode is used as a set voltage. If the battery voltage is higher than the added voltage, the power supply output voltage supplied to the load in the normal operation mode will rise to the voltage obtained by subtracting the forward diode voltage of the rectifying diode from the battery voltage, and the power Although the output voltage decreases, in this embodiment, since the step-up / step-down DC / DC converter is used in the normal operation mode, the battery voltage is higher than the voltage obtained by adding the forward diode voltage of the diode to the set voltage. Supply a constant voltage (first set voltage) with little fluctuation to the load even if the battery voltage fluctuates over time. It can become.

Next, the internal configuration of the step-up DC / DC converter 5 will be described.
In FIG. 1, a resistor 8 and a current source 9 constitute a detection circuit 10 that detects a power supply output voltage VOUT supplied to the microcomputer 1 and outputs a detection voltage. The triangular wave generation circuit 11 outputs a triangular wave signal having a predetermined peak value and a predetermined frequency.

  The comparator 12 inputs the detection voltage of the detection circuit 10 to the inverting input terminal, inputs the triangular wave signal from the triangular wave generation circuit 11 to the non-inverting input terminal, compares the two, and outputs a comparison result signal. Specifically, a pulse signal is output that is at the ‘H’ level when the triangular wave signal is above the detection voltage, and is at the ‘L’ level when the triangular wave signal is below the detection voltage.

  The comparison result signal of the comparator 12 is input to one end of the 2-input AND circuit 13. A mode switching signal from the mode switching signal output circuit 6 is input to the other end of the AND circuit 13. The output of the AND circuit 13 is input to the gate terminal of a MOS transistor (NchMOSFET) 14 that is a switching element for driving the coil. The output of the AND circuit 13 becomes a drive signal for the MOS transistor 14.

  In the standby mode, since the mode switching signal is at the “H” level, the MOS transistor 14 performs a switching operation according to the comparison result signal from the comparator 11. That is, the comparison result signal is a signal that controls the switching operation of the MOS transistor 14, and in this embodiment, the switching element is turned on during a period when the triangular wave signal exceeds the detection voltage, and the period during which the triangular wave signal falls below the detection voltage. Turn off.

  On the other hand, to the drain terminal of the MOS transistor 14, a coil 15 to which the power supply input voltage VIN from the power supply voltage supply source 2 is applied is connected to the other end, and a diode 16 is connected, and a source terminal is grounded. The diode 16 is connected to the output smoothing capacitor 3 via the mode switch 7 in the standby mode.

  The MOS transistor 14 drives the connection end of the coil 15 with the MOS transistor 14 by the switching operation, and the diode 16 rectifies the output voltage from the connection end. That is, when the MOS transistor 14 performs the switching operation, the energy from the power supply voltage supply source 2 is stored in the coil 15 during the ON period, and the energy stored in the coil 15 is output via the diode 16 during the OFF period. Is released.

  In the present embodiment, the resistance value of the resistor 8, the current value of the current source 9, and the upper limit value of the triangular wave signal are set so that the current consumption of the switching power supply device (step-up DC / DC converter 5) itself is reduced in the standby mode. It is set. Specifically, the detection voltage (corresponding to the power supply output voltage VOUT) of the detection circuit 10 when there is almost no current consumption in the microcomputer 1 is almost equal to the upper limit value (corresponding to the second set voltage) of the triangular wave signal. Set to match.

  Subsequently, with respect to the operation when the mode switching signal (“H” level signal) indicating the standby mode is output from the mode switching signal output circuit 6, there is little or no current consumption in the microcomputer 1. The description will be divided into (about 50 μA).

When there is almost no current consumption in the microcomputer 1, as described above, the detection voltage of the detection circuit 10 and the upper limit value of the triangular wave signal from the triangular wave generation circuit 11 are substantially the same, and the power supply output voltage VOUT is the second set voltage. It becomes. Therefore, as shown in FIG. 2, the period during which the MOS transistor 14 is turned on (the period during which the comparison result signal from the comparator 12 is at the “H” level) is extremely short and is output to the load (microcomputer 1). There is almost no average current (load current). Further, the power supply output voltage VOUT is expressed by assuming that the upper limit value of the triangular wave signal is V11max, the resistance value of the resistor 8 is R8, and the current value of the current source 9 is I9.
VOUT≈V11max + (R8 × I9)
It becomes.

  On the other hand, when there is some current consumption in the microcomputer 1 (about 50 μA), the voltage of the output smoothing capacitor 3 (power supply output voltage VOUT) decreases, so the detection voltage of the detection circuit 10 also decreases. Therefore, as shown in FIG. 3, the period during which the MOS transistor 14 is turned on becomes longer and the average current (load current) output to the load also increases, so that compared with the case where there is almost no current consumption in the microcomputer 1. Increases current supply capacity.

  The fluctuation range of the power supply output voltage VOUT between the state where there is almost no current consumption and the state where there is a little current consumption in the microcomputer 1, that is, the fluctuation range of the detection voltage of the detection circuit 10 depends on the amplitude of the triangular wave signal. To do. That is, since the oscillation frequency of the triangular wave signal output from the triangular wave generating circuit 11 is constant, if the amplitude is reduced, the ON period of the MOS transistor 14 can be sufficiently lengthened even with a small change in the detection voltage, and the current supply capability is increased. it can. Therefore, the fluctuation range of the power supply output voltage VOUT is also reduced. Therefore, in the present embodiment, the amplitude of the triangular wave signal output from the triangular wave generating circuit 11 is set to be small in order to reduce the influence of the current consumption in the microcomputer 1 on the power supply output voltage VOUT. For example, if the upper limit value of the triangular wave signal is 1.1 V and the lower limit value is 0.9 V, the fluctuation range of the detection voltage is about 0.2 V at the maximum, and the fluctuation range of the output voltage VOUT can be reduced.

Further, when the second set voltage in the standby mode is V2 and the voltage when the diode 16 is turned on (forward diode voltage) is VF, the voltage VIN of the power supply voltage supply source 2 is VIN> V2 + VF.
When the relationship shown in FIG.
VOUT ≒ VIN-VF
And the switching operation of the MOS transistor 14 is stopped, but the specification of the power supply input of the microcomputer 1 in the standby mode is the second as described above. There is no problem as long as the set voltage V2 or higher is applied.

  Next, the difference between the step-up DC / DC converter 5 in this embodiment and a general step-up DC / DC converter will be described. FIG. 4 shows a configuration of a general step-up DC / DC converter.

  In FIG. 4, resistors 41 and 42 constitute a detection circuit 43 that outputs a detection voltage obtained by dividing the power supply output voltage VOUT of this general step-up DC / DC converter. The reference voltage source 44 outputs an internal reference voltage.

  The internal reference voltage of the reference voltage source 44 is input to the non-inverting input terminal of the amplifier (error amplifier) 45, and the detection voltage of the detection circuit 43 is input to the inverting input terminal. The amplifier 45 is a negative feedback type differential amplifier whose output is fed back to the inverting input terminal via a resistor 46 and a capacitor 47. With this configuration, the amplifier 45 outputs an error voltage signal obtained by amplifying the difference between the internal reference voltage and the detection voltage with a predetermined amplification factor.

  The comparator 48 inputs the error amplified signal from the amplifier 45 to the non-inverting input terminal, inputs the triangular wave signal from the triangular wave generating circuit 49 to the inverting input terminal, compares the two, and outputs a comparison result signal. Specifically, a pulse signal is output that is ‘H’ level when the voltage of the error amplification signal is higher than the triangular wave signal, and ‘L’ level when the voltage of the error amplification signal is lower than the triangular wave signal. This comparison result signal is input to the gate terminal of a MOS transistor (NchMOSFET) 50 which is a switching element for driving the coil. The MOS transistor 50 performs a switching operation according to the comparison result signal from the comparator 48.

  That is, the comparison result signal is a signal for driving and controlling the switching operation of the MOS transistor 50. Here, the switching element is turned on during the period when the voltage of the error amplification signal exceeds the triangular wave signal, and the voltage of the error amplification signal is the triangular wave. It is turned off during the period below the signal.

  On the other hand, to the drain terminal of the MOS transistor 50, a coil 52 to which the power input voltage VIN from the power voltage supply source (battery) 51 is applied is connected to the other end, a diode 53 is connected, and a source terminal is grounded. The The diode 53 is connected to the output smoothing capacitor 54.

  The MOS transistor 50 drives the connection end of the coil 52 with the MOS transistor 50 by the switching operation, and the diode 53 rectifies the output from the connection end. That is, when the MOS transistor 50 performs a switching operation, energy from the power supply voltage supply source 51 is stored in the coil 52 during the ON period, and energy stored in the coil 52 is output via the diode 53 during the OFF period. Is released.

  As described above, the general boost type DC / DC converter detects the power supply output voltage VOUT fed back from the secondary side by the detection circuit 43, and detects the detection voltage of the detection circuit 43 and the internal reference voltage of the reference voltage source 44. The error amplified signal obtained by amplifying the difference between the two is compared with the triangular wave signal from the triangular wave generating circuit 49, and a comparison result signal is output as shown in FIG. That is, as the power supply output voltage VOUT decreases and the error amplification signal rises, the period of the comparison result signal that is at the “H” level, that is, the ON period of the MOS transistor 50 is set longer, thereby setting the power supply output voltage VOUT. Converge to voltage.

  Specifically, when the voltage of the reference voltage source 44 is Va and the resistance values of the resistors 41 and 42 are R41 and R42, the power supply output voltage VOUT of this general boost type DC / DC converter is a value determined by the following equation: Converge to.

VOUT≈Va × {(R41 + R42) / R42}
That is, the lower the power supply output voltage VOUT, the higher the level of the error amplification signal, the longer the period of the comparison result signal 'H' level, the longer the period in which the MOS transistor 50 is on, and the power output The voltage VOUT tends to rise. Similarly, the higher the power supply output voltage VOUT, the smaller the level of the error amplification signal, the shorter the “H” level period of the comparison result signal, and the shorter the period during which the MOS transistor 50 is on. The power supply output voltage VOUT tends to decrease. With the above operation, the power supply output voltage VOUT converges to a value determined by the above equation.

  The general boost type DC / DC converter shown in FIG. 4 has been described above. The configuration of the boost type DC / DC converter 5 of the switching power supply according to the present embodiment is generally as shown in FIG. Such a step-up DC / DC converter does not include an amplifier (error amplifier). Therefore, current consumption generated in the error amplifier in the standby mode can be reduced, so that when the battery is used as the power supply voltage source on the primary side, the battery life can be extended, and further due to battery leakage. It is possible to improve defects in the quality of electrical equipment.

  Further, as described above, in the conventional switching power supply apparatus, the power supply output voltage VOUT is used as the circuit power supply of the step-up DC / DC converter. Therefore, when a battery is used as the power supply voltage supply source on the primary side, the battery voltage is lower. Is used as a rectifying diode, a Schottky barrier diode having a low forward diode voltage (forward diode voltage; about 0.2 V) is used. Therefore, an inexpensive silicon diode (forward diode voltage; about 0.6 to 0.7 V) can be used. In addition, since a Schottky barrier diode is not used as a rectifying diode, when a step-up DC / DC converter is configured using a semiconductor integrated circuit, it is possible to incorporate the rectifying diode in the semiconductor integrated circuit. Become.

  Further, according to the AND circuit 13, the switching operation of the coil driving MOS transistor 14 can be stopped in the OFF state in the normal operation mode. That is, in the normal operation mode, since the mode switching signal is at the ‘L’ level, the ‘L’ level signal is always input to the gate terminal of the MOS transistor 14, and the switching operation is stopped in the off state. Therefore, in the normal operation mode, the malfunction of turning on the MOS transistor 14 can be prevented, current consumption from the power supply voltage supply source 2 can be reduced, and adverse effects on other circuits due to the switching operation of the MOS transistor 14 can be avoided. Can do.

  In the present embodiment, in the standby mode, when the power supply input voltage VIN from the power supply voltage supply source 2 exceeds the voltage obtained by adding the forward diode voltage of the diode 16 to the second set voltage, the boost type Since the power supply input voltage VIN via the diode 16 built in the DC / DC converter 5 is applied to the output smoothing capacitor 3, the power supply output voltage VOUT to the microcomputer 1 in this case is changed from the power input voltage VIN to the diode 16. It is raised to a voltage that has dropped by the forward diode voltage. Therefore, it is necessary to pay attention to the specifications when the microcomputer 1 is in the standby mode.

  Next, the internal configuration of the step-up / step-down DC / DC converter 4 will be described with reference to FIG. However, FIG. 6 shows only the configuration in which the internal configuration of the step-up / step-down DC / DC converter 4 is mainly used in the normal operation mode. The power supply voltage supply source 2, the output smoothing capacitor 3, and the mode switching signal output circuit 6 are described. The mode switch 7 and the triangular wave generating circuit 11 are the same as those described with reference to FIG.

  In FIG. 6, the mode changeover switch 7 is connected to the B contact in the normal operation mode in which the step-up / step-down DC / DC converter 4 operates, as described above, and the power supply output voltage of the step-up / step-down type DC / DC converter 4 is connected to the microcomputer. 1 is supplied.

  The pulse control unit 61 controls the switching operation of the MOS transistors 77 to 80 which are switching elements for driving the coil for converting the power supply input voltage VIN from the power supply voltage supply source 2 to the first set voltage. The output smoothing capacitor 3 smoothes the induced voltage of the coil 81 obtained by the switching operation and converts it into a DC voltage. Note that the pulse control unit 61 uses the power supply output voltage VOUT on the secondary side as the power supply voltage, and uses the voltage of the power supply voltage supply source 2 as the power supply voltage directly for the other circuits of the step-up / step-down DC / DC converter.

  The resistors 62 and 63 in the pulse control unit 61 constitute a detection circuit 64 that outputs a detection voltage obtained by dividing the power supply output voltage VOUT supplied to the load (microcomputer 1). The reference voltage source 65 outputs an internal reference voltage.

  The internal reference voltage of the reference voltage source 65 is input to the non-inverting input terminal of the amplifier (error amplifier) 66, and the detection voltage of the detection circuit 64 is input to the inverting input terminal. The amplifier 66 is a negative feedback type differential amplifier whose output is fed back to the inverting input terminal via a resistor 67 and a capacitor 68. With this configuration, the amplifier 66 outputs an error voltage signal obtained by amplifying the difference between the internal reference voltage and the detection voltage with a predetermined amplification factor.

  The first comparator 69 is used for step-down, and the error amplified signal from the amplifier 66 is input to the inverting input terminal, and the triangular wave signal from the triangular wave generating circuit 70 is input to the non-inverting input terminal. A comparison result signal is output. Specifically, a pulse signal is output that is ‘H’ level when the triangular wave signal exceeds the voltage of the error amplification signal, and ‘L’ level when the triangular wave signal is lower than the voltage of the error amplification signal. This step-down comparison result signal becomes a signal for controlling the switching operation of the step-down switching element.

  On the other hand, the second comparator 71 is for boosting, and inputs the error amplification signal from the amplifier 66 to the non-inverting input terminal, and also reverses the triangular wave signal from the triangular wave generating circuit 70 by the inverting circuit 72 (inverted triangular wave signal). ) Is input to the inverting input terminal, and both are compared and a comparison result signal for boosting is output. Specifically, a pulse signal is output that is ‘H’ level when the voltage of the error amplification signal is higher than the inverted triangular wave signal, and is ‘L’ level when the voltage of the error amplification signal is lower than the inverted triangular wave signal. This boosting comparison result signal becomes a signal for controlling the switching operation of the boosting switching element.

  The output terminal voltage detection circuit 73 detects the power supply output voltage VOUT on the secondary side of the step-up / step-down DC / DC converter and outputs a circuit operation switching signal for switching the circuit operation in accordance with the voltage. That is, when the power supply output voltage VOUT is equal to or lower than a predetermined voltage (for example, a hysteresis of about 1.8 V or less and usually about 0.1 V is applied), a signal for switching to the starting circuit operation is output. On the other hand, when the voltage exceeds the predetermined voltage, the pulse control unit 61 outputs a signal for switching to the normal circuit operation for converting the power input voltage VIN to the first set voltage.

  When the switching circuit 74 for start-up receives a signal for switching to normal circuit operation from the output terminal voltage detection circuit 73, it outputs the comparison result signals for boosting and stepping down from the comparators 69 and 71. On the other hand, when a signal for switching to the circuit operation for starting is received, instead of the output of the step-down comparator 69, a GND level signal ('L' level signal) is output, and the output of the step-up comparator 71 is output. Instead, a pulse signal (starting pulse signal) from the triangular wave generation circuit 11 in the boost DC / DC converter 5 is output. The triangular wave generation circuit 11 has a function of outputting a pulse signal (for example, duty 25%) that becomes ‘H’ level during the falling period of the triangular wave signal, for example.

  The DC / DC converter 75 for driving the output transistor supplies a voltage obtained by boosting the power supply input voltage VIN from the power supply voltage supply source 2 to the level shift circuit 76, so that the first to fourth MOS transistors as switching elements are supplied. (Nch MOSFET) 77 to 80 can be driven with sufficiently low on-resistance.

  The level shift circuit 76 converts the “H” level of the signal from the activation switching circuit 74 into the voltage level output from the DC / DC converter 75, and outputs the step-down comparison result signal or the GND level signal to the first level. Is output to the gate terminal of the second MOS transistor 78, the inverted signal is output to the gate terminal of the second MOS transistor 78, and the comparison result signal for boosting or the pulse signal for starting is output to the gate terminal of the third MOS transistor 79. The inverted signal is output to the gate terminal of the fourth MOS transistor 80.

  The first and second MOS transistors 77 and 78 are step-down switching elements, the source terminal of the first MOS transistor 77 is grounded, and the drain terminal of the second MOS transistor 78 is connected to the power supply voltage supply source 2. The power source input voltage VIN is applied, the drain terminal of the first MOS transistor 77 and the source terminal of the second MOS transistor 78 are connected, and one end of the coil 81 is connected to the connection point.

  On the other hand, the third and fourth MOS transistors 79 and 80 are step-up switching elements, the source terminal of the third MOS transistor 79 is grounded, and the drain terminal of the fourth MOS transistor 80 is connected to the mode switch 7. The drain terminal of the third MOS transistor 79 and the source terminal of the fourth MOS transistor 80 are connected, and the other end of the coil 81 is connected to the connection point.

  As described above, the step-up / step-down DC / DC converter has a configuration of a synchronous rectification type DC / DC converter. In the step-down mode, the power source input voltage VIN from the power source voltage supply source 2 is supplied by the MOS transistors 77 and 78. In the boost mode, the power supply input voltage VIN from the power supply voltage supply source 2 is converted into the first set voltage in the boost mode. On the other hand, the power supply voltage of the pulse control unit 61 at the time of startup is the voltage of the power supply voltage supply source 2. When a battery is used as the power supply voltage supply source on the primary side, the battery voltage at startup (when the battery is set) When the pulse control unit 61 is lower than a voltage at which switching control is sufficiently possible, the step-up / step-down DC / DC converter forcibly activates the boosting MOS transistors 79 and 80 by the activation pulse signal from the triangular wave generation circuit 11. The switching operation is performed, and the power supply output voltage VOUT is boosted to a voltage at which the pulse control unit 61 can sufficiently perform switching control.

  Further, the mode switching signal from the mode switching signal output circuit 6 is input to the DC / DC converter 75 and the level shift circuit 76, and an “H” level signal is input from the mode switching signal output circuit 6 in the standby mode. Then, the DC / DC converter 75 that supplies the voltage serving as the power supply voltage of the level shift circuit 76 stops its operation, and at the same time, the level shift circuit 76 outputs an 'L' level signal to the gate terminals of the MOS transistors 77-80. Then, the MOS transistors 77 to 80 are stopped in the off state. Therefore, in the standby mode, it is possible to prevent a malfunction in which the MOS transistors 77 to 80 are turned on, reduce current consumption from the power supply voltage supply source 2, and adversely affect other circuits by the switching operation of the MOS transistors 77 to 80. Can be avoided.

Next, the operation of this step-up / step-down DC / DC converter will be described.
At the time of startup, the level shift circuit 76 outputs an “L” level signal (a signal for stopping the switching element in the OFF state) to the first MOS transistor 77 and the third MOS transistor 79, and the second MOS transistor 78. Then, an inverted signal “H” level signal (a signal for stopping the switching element in the ON state) is output to the fourth MOS transistor 80.

  When the voltage of the set battery is low and the power supply output voltage VOUT at startup is equal to or lower than a predetermined voltage, the output terminal voltage detection circuit 73 outputs a signal for switching to the circuit operation for startup. The start switching circuit 74 receives the signal and outputs a GND level signal ('L' level signal) and a start pulse signal from the triangle generation circuit 11.

  FIG. 7 shows an operation waveform diagram when the power supply output voltage VOUT at the time of startup is equal to or lower than a predetermined voltage set by the output terminal voltage detection circuit 73. As shown in FIG. 7, the start switching circuit 74 outputs a start pulse signal, and instead of the step-down comparison result signal, an “L” level signal for start-up (the switching element is turned off) The signal to stop at is output.

  The level shift circuit 76 receives the output from the DC / DC converter 75 and outputs a level-shifted (voltage converted) start pulse signal to the third MOS transistor 79, and outputs the inverted pulse signal to the fourth MOS transistor 80. Output to the first MOS transistor 77 in place of the comparison result signal for step-down and to the first MOS transistor 77, and an inverted “H” level signal (the switching element is turned on). Signal to be stopped in the state) is output to the second MOS transistor 78. As a result, the power supply output voltage VOUT of the step-up / step-down DC / DC converter is boosted by switching control of the start pulse signal.

  In general, the first and second MOS transistors 77 and 78 are not turned on at the same time, and the third and fourth MOS transistors 79 and 80 are not turned on at the same time. A period for preventing penetration (for example, about 50 ns at the time of 100 kHz signal) called dead time in which both gate voltages become 'L' level when the pulse polarity is switched is provided.

  When the secondary-side power supply output voltage VOUT rises to a predetermined voltage (for example, 1.8 V) set by the output terminal voltage detection circuit 73, the power supply output voltage VOUT is changed to the first set voltage by the operation described below. Converge. At this time, the pulse control unit 61 using the power supply output voltage VOUT as the power supply voltage increases the power supply output voltage VOUT to a predetermined voltage (for example, 1.8 V) set by the output terminal voltage detection circuit 73. Sufficient switching control is possible.

  When the secondary power supply output voltage VOUT rises to a predetermined voltage set by the output terminal voltage detection circuit 73, the output terminal voltage detection circuit 73 outputs a signal for switching to normal circuit operation. The start switching circuit 74 receives the signal and outputs a comparison result signal from the step-down comparator 69 and a comparison result signal from the step-up comparator 71.

  The step-up / step-down DC / DC converter determines whether to operate as a step-up DC / DC converter or a step-down DC / DC converter based on the voltage relationship between the first set voltage and the power supply output voltage VOUT.

  FIG. 8 shows an operation waveform diagram in the step-down mode when the secondary side power supply output voltage VOUT is higher than the first set voltage. As shown in FIG. 8, the higher the secondary power supply output voltage VOUT, the lower the output of the amplifier 66 (error amplified signal), and the step-down voltage comparing the error amplified signal with the triangular wave signal from the triangular wave generating circuit 70. The comparison result signal 'H' level period becomes longer. This step-down comparison result signal is level-shifted and output to the first MOS transistor 77, and its inverted signal is output to the second MOS transistor 78. On the other hand, the comparison result signal for boosting that compares the error amplification signal and the inverted triangular wave signal from the inverting circuit 72 is fixed to the “L” level, so that the signal fixed to the “L” level is output to the third MOS transistor 79. The inverted signal (fixed to the “H” level) is output to the fourth MOS transistor 80. Therefore, in this case, the step-up / step-down DC / DC converter operates as a step-down DC / DC converter.

  As a result, when the resistance values of the resistors 62 and 63 are R62 and R63, and the internal reference voltage of the reference voltage source 65 is Va, the power supply output voltage VOUT converges to a value (first set voltage) determined by the following equation. .

VOUT≈Va × {(R62 + R63) / R63}
FIG. 9 shows an operation waveform diagram in the boost mode when the power supply output voltage VOUT on the secondary side is lower than the first set voltage. As shown in FIG. 9, the lower the power supply output voltage VOUT on the secondary side, the higher the output of the amplifier 66 (error amplified signal), and the boosted voltage comparing the error amplified signal with the inverted triangular wave signal from the inverting circuit 72. The comparison result signal 'H' level period becomes longer. This boosting comparison result signal is level-shifted and output to the third MOS transistor 79, and its inverted signal is output to the fourth MOS transistor 80. On the other hand, since the step-down comparison result signal obtained by comparing the error amplification signal and the triangular wave signal from the triangular wave generating circuit 70 is fixed to the “L” level, the signal having the fixed “L” level is output to the first MOS transistor 77. The inverted signal (a signal fixed at the “H” level) is output to the second MOS transistor 78. Therefore, in this case, the step-up / step-down DC / DC converter operates as a step-up DC / DC converter.

  As a result, when the resistance values of the resistors 62 and 63 are R62 and R63, and the internal reference voltage of the reference voltage source 65 is Va, the power supply output voltage VOUT converges to a voltage (first set voltage) determined by the following equation. .

VOUT≈Va × {(R62 + R63) / R63}
Thus, since the power supply output voltage VOUT converges to the same value in both the step-down mode and the step-up mode, a constant voltage that is not influenced by the voltage of the power supply voltage supply source can be supplied to the load. Note that although the boost mode and the step-down mode have been described, as a configuration, a period in which both modes operate simultaneously may be provided so that the modes are switched smoothly when the modes are switched.

  As described above, in the normal operation mode, even if the power supply input voltage VIN varies with the usage time as in the case where the primary power supply voltage supply source is a battery (for example, from 3.3 V to 1.5 V). Range), a constant voltage (for example, 2.5 V) can be generated as the power supply output voltage VOUT by the operation of the step-up DC / DC converter.

  Further, until the pulse control unit 61 using the power supply output voltage VOUT as the power supply voltage starts switching control, the power supply output voltage VOUT is boosted by a circuit using the power supply voltage supply source directly as the power supply voltage. After the power supply output voltage VOUT is boosted to a voltage that allows sufficient switching control, the pulse control unit 61 boosts the power supply output voltage VOUT, so that the pulse control unit can perform switching control even when the power supply voltage supply voltage is low. The power supply output voltage VOUT can be easily boosted to a voltage that achieves a long battery life. Further, even when the switching power supply device is used in an electric device in which the normal operation mode is set at the time of activation and is in a heavy load state, it can be activated without any problem. Further, if the 'H' level period of the starting pulse signal from the triangular wave generation circuit inside the boost DC / DC converter is set long (the on period of the third MOS transistor is lengthened and the off period of the fourth MOS transistor is lengthened). Setting it longer) makes it easier to start.

  In the normal operation mode, in order to reduce the current consumption from the power supply voltage supply source, the operation other than the circuit shared by the step-up / step-down DC / DC converter is stopped among the internal circuits of the step-up DC / DC converter. desirable.

  FIG. 10 shows a schematic configuration of a switching power supply apparatus according to another embodiment. In the switching power supply device shown in FIG. 1, the connection point between the mode changeover switch 7 and the diode 16 is connected to the resistor 8 for feedback, but the connection point between the mode changeover switch 7 and the output smoothing capacitor 3 as shown in FIG. May be connected to the resistor 8 for feedback.

  In this embodiment, the output smoothing capacitor is provided on the output side of the mode change switch 7, but it may be provided on the input side of the mode change switch 7. Further, they may be provided in the step-up DC / DC converter 5 and the step-up / step-down DC / DC converter 4, respectively. Further, in this case, step-up / step-down DC / DC converter 4 may be provided with boosting and step-down output smoothing capacitors, respectively.

  The switching power supply according to the present invention can reduce the current consumption of the switching power supply itself in the standby mode, can reduce the current consumption (outflow current) from the power supply voltage supply source in the standby mode, and the normal operation mode. Sometimes a constant voltage that is unaffected by the voltage level of the power supply voltage supply can be supplied to the load, and a battery with a large voltage fluctuation is used as the power supply voltage supply source on the primary side. It is sufficient that a predetermined voltage or more is applied as a voltage, and it is useful for an electric device that needs to apply a constant voltage with high accuracy as a power supply output voltage in the normal operation mode.

The figure which shows schematic structure of the switching power supply device which concerns on embodiment of this invention Operation waveform diagram for explaining the operation in the standby mode of the switching power supply according to the embodiment Operation waveform diagram for explaining the operation in the standby mode of the switching power supply according to the embodiment The figure which shows the structure of a general step-up DC / DC converter Operation waveform diagram for explaining the operation of a general step-up DC / DC converter The figure which shows the internal structure of the buck-boost type DC / DC converter of the switching power supply which concerns on embodiment of this invention Operation waveform diagram for explaining the operation in the normal operation mode of the switching power supply according to the embodiment Operation waveform diagram for explaining the operation in the normal operation mode of the switching power supply according to the embodiment Operation waveform diagram for explaining the operation in the normal operation mode of the switching power supply according to the embodiment The figure which shows schematic structure of the switching power supply device which concerns on other embodiment of this invention. The figure which shows schematic structure of the conventional switching power supply device The figure which shows the pulse signal which drives the transistor for coil drive of the conventional switching power supply device

Explanation of symbols

1 Microcomputer (load)
2, 51 Supply voltage supply source 3, 54 Output smoothing capacitor 4 Step-up / step-down DC / DC converter 5 Step-up DC / DC converter 6 Mode switching signal output circuit 7 Mode switching switch 8 Resistance 9 Current source 10, 43, 64 Detection circuit 11 , 49, 70 Triangular wave generation circuit 12, 48 Comparator 13 AND circuit 14, 50 MOS transistor 15, 52, 91 Coil 16, 53 Diode 41, 42, 62, 63 Resistor 44, 65 Reference voltage source 45, 66 Amplifier 46, 67 Resistor 47, 68 Capacitor 61 Pulse control unit 69 Comparator 71 Comparator 72 Inverting circuit 73 Output terminal voltage detection circuit 74 Start-up switching circuit 75 DC / DC converter 76 Level shift circuit 77, 78, 79, 80 MOS transistor 101 Driver IC
102 PWM pulse controller

Claims (4)

An input terminal connected to a power supply voltage source;
An output terminal connected to a load having a normal operation mode and a standby mode;
A step-up / step-down DC / DC converter for supplying, to the load, a power supply output voltage obtained by converting a power supply input voltage from the power supply voltage supply source into a first set voltage in the normal operation mode;
In the standby mode, when the power output voltage is equal to or lower than the second set voltage lower than the first set voltage, the power output voltage converted from the power input voltage to the second set voltage is supplied to the load. When the power supply output voltage exceeds the second set voltage, the step-up DC / DC that supplies the power supply output voltage that has passed the power supply input voltage to the load without performing the conversion operation to the second set voltage. A DC converter;
A mode switch for outputting the output of the step-up / step-down DC / DC converter to the output terminal during the normal operation mode and outputting the output of the step-up DC / DC converter to the output terminal during the standby mode;
A switching power supply device comprising:   2. The switching power supply device according to claim 1, wherein the step-up DC / DC converter uses the power supply input voltage as the power supply voltage. The step-up DC / DC converter is
A detection circuit that detects a power supply output voltage and outputs a detection voltage;
A triangular wave generating circuit for outputting a triangular wave signal;
A comparator that compares the detection voltage of the detection circuit with the triangular wave signal from the triangular wave generation circuit and outputs a comparison result signal;
A switching element that performs a switching operation according to a comparison result signal from the comparator;
A coil in which the power supply input voltage is applied to one end and the other end is driven by a switching operation of the switching element;
A diode for rectifying the output of the coil;
3. The switching power supply device according to claim 1, wherein an output of the coil rectified by the diode is output to the output terminal in the standby mode. 4. The switching power supply device according to claim 3, wherein the step-up DC / DC converter further includes a circuit that stops the switching element in an OFF state during the normal operation mode.

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