JPH10295074A - Dc-to-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC/DC converter which is composed of a synchronous rectifying type switching regulator and which proactively prevents the undershoot of an output voltage from being produced, while the output can be controlled in accordance with an external output control signal. SOLUTION: An output transistor 3 performs a switching operation in accordance with the output signal OUT1 of a control circuit 21. A smoothing circuit 20 smoothes the output current of the output transistor 3 and outputs a DC current. A synchronous rectifying transistor 4 is connected in parallel to a flywheel diode 6 and turned on, when the output transistor 3 is turned off based on an output signal OUT2 of the control circuit 21. An output control circuit 11 which turns off the output transistor 3, based on an output control signal A is connected to the control circuit 21. When the output transistor 3 is turned off, the control circuit 21 outputs the output signal OUT2 by which the synchronous rectifying transistor 4 is turned off.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a synchronous rectification type DC
/ DC converter. In a portable electronic device that operates using a battery as a power source, when it is necessary to supply a DC voltage different from the battery voltage to an internal circuit, a DC / DC converter that operates using a battery generates a desired DC voltage. ing. Some DC / DC converters use a synchronous rectification type switching regulator circuit to extend the life of the battery. It is necessary to operate the internal circuit stably based on the DC voltage output from such a synchronous rectification type switching regulator circuit.

[0002]

2. Description of the Related Art FIG. 6 shows an example of a conventional synchronous rectification type switching regulator circuit. The switching regulator circuit 1 includes a control circuit 2 formed on a one-chip integrated circuit, and a number of external elements.

The first output signal OUT1 of the control circuit 2
Is input to the base of an output transistor 3 composed of a PNP transistor, and the emitter of the output transistor 3 is connected to a power supply Vcc.

The collector of the output transistor 3 is N
The transistor 4 is connected to the drain of a synchronous rectification transistor 4 composed of a channel MOS transistor.
, The second output signal OUT of the control circuit 2
2 is input, and the source is connected to the ground GND.

[0005] The collector of the output transistor 3 is connected to an output terminal To via an output coil 5. Also,
The collector of the output transistor 3 is connected to the cathode of the flywheel diode 6, and the anode of the diode 6 is connected to the ground GND.

The output terminal To is connected to the ground GND via the capacitor 7 and to the ground GND via the resistors R1 and R2. The resistors R1, R2
Is to divide the output voltage Vo based on its resistance value in order to detect the output voltage Vo output from the output terminal To. Then, the divided voltage is applied to the control circuit 2
Is input as an input signal IN to the negative input terminal of the voltage control amplifier 8.

The voltage control amplifier 8 has first and second positive input terminals. Of the input voltages of both positive input terminals, a lower level input voltage and an input voltage of a negative input terminal are provided. And outputs an output voltage based on the potential difference. That is, if any positive input terminal voltage is higher than the negative input terminal voltage, the output voltage increases according to the potential difference, and if any positive input terminal voltage becomes lower than the negative input terminal voltage. The output voltage decreases according to the potential difference.

A first positive input terminal of the voltage control amplifier 8 is connected to a current source 9 for outputting a constant current I based on the supply of a power supply Vcc, and to a ground GND via a capacitor 10. Is done. The current source 9 and the capacitance 10
Thus, a soft-start circuit is configured to gradually raise the first input terminal voltage CS with a predetermined time constant when the power supply Vcc is turned on.

An output control transistor 11 composed of an NPN transistor is connected in parallel to the capacitor 10, and an output control signal A is input to the base of the transistor 11. When the output control signal A becomes H level, the output control transistor 11 is turned on, and the capacitance 1
By discharging the zero charge to the ground GND via the output control transistor 11, the first positive input terminal voltage CS of the voltage control amplifier 8 is reduced to the ground GND level. The output control transistor 11 uses a soft start circuit to output the output voltage V
Configure an output control circuit to control o.

A reference voltage Vs is input to a second positive input terminal of the voltage control amplifier 8. Reference voltage V
s is a voltage level lower than the power supply Vcc, and is set to a voltage obtained by dividing a desired output voltage Vo by the resistors R1 and R2.

The output signal of the voltage control amplifier 8 is input to the negative input terminal of the first PWM comparator 12. The output signal of the voltage control amplifier 8 is constantly boosted by a boosting power supply 14 and input to the negative input terminal of the second PWM comparator 13 with a constant voltage width.

An output signal of the voltage control amplifier 8 is input to a negative input terminal of the voltage control amplifier 8 via a phase compensation capacitor 17 connected as an external element. Oscillation is prevented.

The first and second PWM comparators 12, 1
A triangular wave having a constant frequency is input from the oscillator 15 to the positive input terminal 3. The first and second PWM comparators 12 and 13 are activated by the bias current supplied from the bias circuit 16 based on the turning on of the power supply Vcc.

The first PWM comparator 12 compares the negative-side input terminal voltage with the positive-side input terminal voltage. If the positive-side input terminal voltage becomes higher than the negative-side input terminal voltage, the first PWM comparator 12 outputs an H-level output signal. OUT1 is output. If the positive input terminal voltage is lower than the negative input terminal voltage, the output signal OUT1 at the L level is output.

Accordingly, the output signal OUT1 becomes a pulse signal having the same frequency as the output signal of the oscillator 15, and the time width of the H level becomes narrower as the output voltage level of the voltage control amplifier 8 rises.

The second PWM comparator 13 compares the negative input terminal voltage with the positive input terminal voltage. If the positive input terminal voltage is higher than the negative input terminal voltage, the second PWM comparator 13 outputs an H level output signal. OUT2 is output. When the positive input terminal voltage becomes lower than the negative input terminal voltage, the output signal OUT2 at L level is output.

Therefore, the output signal OUT2 becomes a pulse signal having the same frequency as the output signal of the oscillator 15, and the time width of the H level becomes narrower as the output voltage level of the voltage control amplifier 8 increases. Further, since the negative input terminal voltage of the second PWM comparator 13 is always higher than the negative input terminal voltage of the first PWM comparator 12 by the boosting power supply 14, the output signal OUT2 becomes the output signal. OU
The output signal OUT1 rises after T1 rises.
Fall before falling.

In the synchronous rectification type switching regulator circuit configured as described above, the power supply Vcc is supplied while the output control signal A is maintained at the L level and the output control transistor 11 is turned off. And the voltage control amplifier 8 outputs an output signal based on the potential difference between the input signal IN and the reference voltage Vs, and outputs the output signal O from the first and second PWM comparators 12 and 13 based on the output signal.
Pulse signals are output as UT1 and OUT2.

Then, the output transistor 3 performs a switching operation based on the output signal OUT1 of the first PWM comparator 12. The output current of the output transistor 3 is changed by the switching operation of the output transistor 3
And the capacitance 7. When the output transistor 3 is turned off, the output voltage Vo is smoothed by the current supplied from the capacitor 7 to the output coil 5 via the flywheel diode 6.

When the output transistor 3 is turned off, the transistor 4 for synchronous rectification is turned on by the output signal OUT2 of the second PWM comparator 13, and the forward voltage drop of the flywheel diode 6 is almost "0". ”To improve the smoothing efficiency.

At this time, the synchronous rectification transistor 4 is turned on after the output transistor 3 is turned off, and is turned off before the output transistor 3 is turned on.
Output transistor 3 and synchronous rectification transistor 4
No through current flows to the ground GND via.

With such an operation, the input signal IN of the voltage control amplifier 8 based on the output voltage Vo changes to the reference voltage Vs.
If it is lower, the output voltage of the voltage control amplifier 8 increases,
Output signal OU of first and second PWM comparators 12 and 13
The time width of the L level of T1 and OUT2 increases.

Then, the ON time of the output transistor 3 becomes longer, and the output voltage Vo rises. Also, the output voltage V
When the input signal IN of the voltage control amplifier 8 based on o is higher than the reference voltage Vs, the output voltage of the voltage control amplifier 8 decreases, and the output signals OUT1 and OUT2 of the first and second PWM comparators 12 and 13 are reduced. , The time width of the H level increases.
Then, the ON time of the output transistor 3 is shortened, and the output voltage Vo is reduced.

By such an operation, the output voltage Vo converges to an output voltage Vo such that the input signal IN of the voltage control amplifier 8 matches the reference voltage Vs, and becomes a constant voltage. When the power supply Vcc is turned on, the input signal C of the voltage control amplifier 8 is actuated by the action of the current source 9 and the capacitor 10 as shown in FIG.
The voltage level of S gradually increases. Then, the input signal I
Even if N is at the ground GND level, the output signal of the voltage control amplifier 8 does not rise sharply, and the ON time of the output transistor 3 does not become much longer than the OFF time.

Then, as shown in FIG.
Also, regardless of the load connected to the output terminal To, the output signal Vo rises slowly with the rise of the input signal CS. When the input signal CS exceeds the reference voltage Vs, the output signal Vo becomes a constant voltage.

Therefore, an adverse effect on the load circuit due to the rapid rise of the output voltage Vo when the power supply Vcc is turned on is prevented. When the output control signal A is set to the H level while the output voltage Vo is maintained at a constant voltage, the transistor 11 is turned on, and the charge of the capacitor 10 flows to the ground GND via the output control transistor 11. The input signal CS of the voltage control amplifier 8 is, as shown in FIG.
It drops sharply to the ground GND level.

Then, the output signal of the voltage control amplifier 8 sharply drops below the lowest level of the output signal of the oscillator 15, and the output signals OUT1 and OUT1 of the PWM comparators 12 and 13 are output.
OUT2 is maintained at the H level.

As a result, the output transistor 3 is turned off, and the synchronous rectification transistor 4 is turned on.
The output voltage Vo is reduced to the ground GND level. Therefore, by inputting the H level output control signal A, the output voltage Vo can be forcibly reduced to the ground GND level.

Also, by adding an output control transistor 11 to the soft start circuit, the output voltage Vo can be forcibly reduced to the ground GND level without increasing the number of input / output terminals of the control circuit 2. A control circuit is configured.

[0030]

In the synchronous rectification type switching regulator as described above, the output control signal A is set to H level.
When the output voltage Vo is lowered to the level of the ground GND as the level, the synchronous rectification transistor 4 is kept on. Then, as shown by a dotted line in FIG.
A discharge current Id flows through D.

As a result, as shown in FIG.
Since o falls sharply and undershoot Pu occurs, there is a problem that a load circuit connected to the output terminal To is damaged or malfunctions.

An object of the present invention is to provide a DC / DC converter constituted by a synchronous rectification type switching regulator, in which output control can be performed based on an output control signal input from the outside, and occurrence of output voltage undershoot is prevented. Another object of the present invention is to provide a DC / DC converter that can prevent the above problem.

[0033]

FIG. 1 is a diagram for explaining the principle of claim 1. That is, the output transistor 3 performs a switching operation based on the first output signal OUT1 output from the control circuit 21. The smoothing circuit 20 smoothes the output current of the output transistor 3 and outputs a DC voltage. The transistor 4 for synchronous rectification is connected in parallel to the flywheel diode 6 constituting the smoothing circuit 20, and a second output signal O output from the control circuit 21.
It is turned on when the output transistor 3 is turned off based on the UT 2 to improve the smoothing efficiency of the flywheel diode 6. The control circuit 21 includes the smoothing circuit 2
The on-time of the output transistor 3 is controlled so that the DC output voltage Vo output from 0 is maintained constant.
An output control circuit 11 for turning off the output transistor 3 based on an output control signal A is connected to the control circuit 21. The control circuit 21 controls the output control signal A
When the output transistor 3 is turned off on the basis of the input of (1), a second output signal OUT2 for turning off the synchronous rectification transistor 4 is output.

According to the second aspect, the output transistor performs a switching operation based on the first output signal output from the control circuit. The smoothing circuit includes an output coil interposed between the output transistor and an output terminal, a capacitor connected between the output terminal and a low-potential-side power supply, and a capacitor connected between the output transistor and the low-potential-side power supply. And a flywheel diode connected to the output transistor and smoothes the output current of the output transistor to output a DC output voltage.
The synchronous rectification transistor is connected in parallel with the flywheel diode, and is turned on when the output transistor is turned off based on a second output signal output from the control circuit, to improve smoothing efficiency. The control circuit controls an on-time of the output transistor so as to maintain a constant DC output voltage output from the smoothing circuit. The control circuit includes an output control circuit that turns off the output transistor based on an output control signal. The control circuit, of the preset reference voltage and the output voltage of the soft start circuit, any low-level voltage, a voltage control amplifier that outputs a signal based on the potential difference between the DC output voltage, A first PW for comparing the output signal of the voltage control amplifier with the output signal of the oscillator to generate and output the first output signal;
An M comparator, and a second PWM comparator for comparing the output signal of the voltage control amplifier and the output signal of the oscillator to generate and output the second output signal. The soft start circuit includes a current source and a capacitor connected in series between a high-potential power source and a low-potential power source, and is charged with a constant current output from the current source when the power is turned on. A charging voltage of a certain capacity is output to the voltage control amplifier.
The output control circuit includes a switching element that is turned on based on the output control signal and discharges the charge of the capacitor. An output noise prevention circuit that turns off the synchronous rectification transistor with the second output signal based on the ON operation of the switching element is connected to the second PWM comparator.

According to a third aspect of the present invention, the output noise prevention circuit inputs the charging voltage of the capacitor of the soft start circuit to the second PWM comparator, and when the charge of the capacitor is discharged, And outputting a second output signal for turning off the synchronous rectification transistor, based on a comparison between the charge voltage of the capacitor and the output signal of the voltage control amplifier regardless of the output signal.

According to a fourth aspect, the output noise prevention circuit outputs the output signal of the second PWM comparator to the synchronous rectification transistor as a second output signal via a switch circuit. When the capacity of the soft start circuit is discharged, the transistor becomes non-conductive and turns off the synchronous rectification transistor.

According to a fifth aspect of the present invention, in the output noise prevention circuit, a switch circuit is interposed between the bias circuit for supplying a bias current to the second PWM comparator and the second PWM comparator. The circuit becomes non-conductive when the capacitance of the soft start circuit is discharged, and the second P
The synchronous rectification transistor is turned off by inactivating the WM comparator.

(Function) In the first aspect, the output control circuit 11
Therefore, when the output transistor 3 is turned off, the synchronous rectification transistor 4 is turned off by the control circuit 21, so that generation of noise in the output voltage Vo output from the smoothing circuit 20 is prevented.

According to the present invention, when the switching element of the output control circuit is turned on by the output control signal and the charge of the capacitance of the soft start circuit is discharged, the output voltage of the soft start circuit and the DC output voltage of the smoothing circuit are reduced. Is output from the voltage control amplifier, and the transistor for synchronous rectification is turned off based on the second output signal output from the second PWM comparator by the ON operation of the switching element.

According to the third aspect, when the switching element is turned on and the capacitance of the soft start circuit is discharged, the second PWM comparator compares the charge voltage of the capacitance with the output signal of the voltage control amplifier. Thus, a second output signal for turning off the synchronous rectification transistor is output.

According to the fourth aspect, when the switching element is turned on and the capacitance of the soft start circuit is discharged, the switch circuit becomes non-conductive, and the second output signal of the second PWM comparator becomes a synchronous rectification transistor. , The synchronous rectification transistor is turned off.

According to the fifth aspect, when the switching element is turned on and the capacitance of the soft start circuit is discharged, the switch circuit becomes non-conductive, and no bias current is supplied to the second PWM comparator. Since the PWM comparator is inactivated and the second output signal of the second PWM comparator is not output to the synchronous rectification transistor, the synchronous rectification transistor is turned off.

[0043]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) FIG. 2 shows a first embodiment of a synchronous rectification type switching regulator embodying the present invention. The same components as those in the conventional example are denoted by the same reference numerals, and description thereof will be omitted.

In the control circuit 21a of this embodiment, first and second positive input terminals are provided in the second PWM comparator 13a, and the first positive input terminal is provided in the same manner as in the conventional example. The output signal of the oscillator 15 is input, and the input signal CS is input to a second positive input terminal. Then, the second PWM comparator 13a outputs an output signal obtained by comparing the low-level input terminal voltage of any of the first and second positive-side input terminals with the negative-side input terminal voltage.

In the synchronous rectification type switching regulator configured as described above, when the power supply Vcc is continuously supplied, the input signal CS is almost at the power supply Vcc level, so that the voltage control amplifier 8 and the second PWM comparator 1
3a operates in the same manner as the conventional example.

Therefore, the output voltage Vo becomes a constant voltage based on the switching operation of the output transistor 3, and when the output transistor 3 is turned off, the synchronous rectification transistor 4 is turned on to increase the rectification effect.

When the power supply Vcc is turned on, as shown in FIG. 3, the input signal CS is supplied to the ground GND in the same manner as in the conventional example.
The voltage gradually rises from the level, and the voltage control amplifier 8 and the first PWM comparator 12 operate in the same manner as in the conventional example.
Since the input signal CS is input to the second positive input terminal, the output signal OUT2 of the second PWM comparator 13a is maintained at the L level when the input signal CS is at the low level. Transistor 4 is off.

When the H-level output control signal A is input while the constant voltage output voltage Vo is being output, the input signal CS sharply drops to the ground GND level.
Then, the voltage control amplifier 8 and the first PWM comparator 1
2 operates similarly to the conventional example, and the output transistor 3 is turned off.

In the second PWM comparator 13a, the input signal CS drops sharply to the ground GND and falls below the output level of the oscillator 15, so that the output signal OUT2 is maintained at the L level, and the synchronous rectification transistor 4 is turned on. Turned off.

Therefore, the charge of the capacitor 7 is equal to the resistance R1,
Since the discharge is gradually performed based on the time constant of R2 and the load circuit, the output voltage Vo decreases gradually. As a result,
It is possible to prevent undershoot from occurring at the output voltage Vo. (Second Embodiment) FIG. 4 shows a second embodiment of the present invention. The same components as those of the conventional example,
The same reference numerals are given and the description is omitted.

In this embodiment, the second PWM comparator 1
3 is output via the switch circuit 18 to the output signal OU.
T2, and the output terminal of the switch circuit 18 is connected to the ground GND via the resistor R3.
Reference numeral 8 denotes a configuration for performing opening / closing control based on the input signal CS.

The switch circuit 18 conducts when the input signal CS exceeds a threshold set between the power supply Vcc level and the ground GND level, and turns off when the input signal CS falls below the threshold. It is configured to be. Such a switch circuit 18 can be constituted by, for example, an N-channel MOS transistor used as a switch element or a transfer gate.

In such a synchronous rectification type switching regulator circuit, at the time of soft start and when the power Vcc is continuously supplied, the switch circuit 18 is turned on, and the switching circuit 18 is turned on similarly to the first embodiment. Operate.

When the output control signal A at the H level is input from the state where the constant output voltage Vo is being output, the output control transistor 11 is turned on, and when the input signal CS falls to the ground GND level, The output transistor 3 is turned off based on the operations of the control amplifier 8 and the first PWM comparator 12.

At this time, based on the fall of the input signal CS, the switch circuit 18 becomes non-conductive, and the output signal OUT
2 is at the ground GND level. Then, the synchronous rectification transistor 4 is turned off.

Therefore, the charge of the capacitor 7 is equal to the resistance R1,
The output voltage Vo is gradually discharged based on the time constant of R2 and the load circuit, and the output voltage Vo gradually decreases.
The occurrence of undershoot in o can be prevented beforehand.

Since the second PWM comparator 13 only needs two inputs, the number of elements is reduced compared to the second PWM comparator 13a of the first embodiment, and the circuit area of the control circuit 21b is reduced. Can be reduced. (Third Embodiment) FIG. 5 shows a third embodiment of the present invention. The same components as those of the conventional example,
The same reference numerals are given and the description is omitted.

In this embodiment, the second PWM comparator 1
3 is connected to ground GND via a resistor R4, a bias current is supplied from a bias circuit 16 to a first and second PWM comparators 12 and 13 via a switch circuit 19, and the switch circuit 19 Is configured to perform opening / closing control based on the input signal CS.

The switch circuit 19 conducts when the input signal CS exceeds a threshold set between the power supply Vcc level and the ground GND level, and turns off when the input signal CS falls below the threshold. It is configured to be. Such a switch circuit 19 can be composed of, for example, an N-channel MOS transistor used as a switch element or a transfer gate.

In such a synchronous rectification type switching regulator circuit, at the time of soft start, a bias current is supplied to the first and second PWM comparators 12 and 13 until the input signal CS exceeds the threshold value of the switch circuit 19. Therefore, the output transistor 3 is not turned on, and the output voltage Vo does not rise.

The input signal CS is applied to the switch circuit 19
Is exceeded, the bias current is supplied to the PWM comparators 12 and 13, the output transistor 3 starts switching operation based on the output signal OUT1 of the PWM comparators 12 and 13, and the output voltage Vo rises. I do.

When the power Vcc is continuously supplied, the switch circuit 18 becomes conductive, and outputs a constant output voltage Vo as in the first embodiment. From the state where the constant voltage output voltage Vo is output,
The H-level output control signal A is input, the output control transistor 11 is turned on, and the input signal CS is
When the voltage drops to the ND level, the switch circuit 19 becomes non-conductive, and the PWM comparators 12 and 13 become inactive.

Then, the output signal OU of the PWM comparator 12
T1 is in an undefined state, and the output transistor 3 is turned off. Although the output signal of the PWM comparator 13 is undefined, the gate potential of the synchronous rectification transistor 4 is almost at the ground GND level by the resistor R4, and the synchronous rectification transistor 4 is turned off.

Therefore, the charge of the capacitor 7 is equal to the resistance R1,
The output voltage Vo is gradually discharged based on the time constant of R2 and the load circuit, and the output voltage Vo gradually decreases.
The occurrence of undershoot in o can be prevented beforehand.

Since the second PWM comparator 13 needs only two inputs, the number of elements is reduced compared to the second PWM comparator 13a of the first embodiment, and the circuit area of the control circuit 21c is reduced. Can be reduced.

[0066]

As described above in detail, the present invention relates to a DC / DC converter constituted by a synchronous rectification type switching regulator, which enables output control while enabling output control based on an output control signal input from the outside. And a DC / DC converter capable of preventing the occurrence of undershoot in advance.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a circuit diagram showing a first embodiment.

FIG. 3 is a waveform chart showing an operation of the first embodiment.

FIG. 4 is a circuit diagram showing a second embodiment.

FIG. 5 is a circuit diagram showing a third embodiment.

FIG. 6 is a circuit diagram showing a conventional example.

FIG. 7 is a waveform chart showing the operation of the conventional example.

[Explanation of symbols]

 Reference Signs List 3 output transistor 4 transistor for synchronous rectification 6 flywheel diode 11 output control circuit 20 smoothing circuit 21 control circuit OUT1 first output signal OUT2 second output signal Vo DC output voltage

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takashi Matsumoto 2-1844-2 Kozoji-cho, Kasugai-shi, Aichi Prefecture Inside Fujitsu VSI Co., Ltd. (72) Inventor Toshiyuki Matsuyama 2-1844-2 Kozoji-cho, Kasugai-shi, Aichi Fujitsu VLSI Corporation

Claims (5)

[Claims] An output transistor that performs a switching operation based on a first output signal output from a control circuit; a smoothing circuit that smoothes an output current of the output transistor and outputs a DC voltage; For synchronous rectification, which is connected in parallel to the flywheel diode to be configured and is turned on when the output transistor is turned off based on a second output signal output from the control circuit, thereby improving the smoothing efficiency of the flywheel diode. A transistor, the control circuit controls an on-time of the output transistor so as to maintain a constant DC output voltage output from the smoothing circuit; and the control circuit, based on an output control signal, A DC connected to an output control circuit for turning off the output transistor.
/ DC converter, wherein the control circuit outputs a second output signal for turning off the synchronous rectification transistor when turning off the output transistor based on the input of the output control signal. DC / DC converter. 2. An output transistor that performs a switching operation based on a first output signal output from a control circuit; an output coil interposed between the output transistor and an output terminal; A capacitor connected between a power supply and a flywheel diode connected between the output transistor and a low-potential-side power supply, smoothing an output current of the output transistor,
A smoothing circuit that outputs a DC output voltage, and is connected in parallel to the flywheel diode, and is turned on when the output transistor is turned off based on a second output signal output from the control circuit, to improve smoothing efficiency. The synchronous rectification transistor to be improved, and the control circuit controls an on-time of the output transistor so as to maintain a constant DC output voltage output from the smoothing circuit. A DC / DC having an output control circuit for turning off the output transistor based on a control signal;
A DC converter, wherein the control circuit outputs a signal based on a potential difference between any one of a low-level voltage of a preset reference voltage and an output voltage of a soft start circuit and the DC output voltage. A control amplifier; a first PWM comparator that compares an output signal of the voltage control amplifier with an output signal of the oscillator to generate and output the first output signal; A second PWM comparator that compares an output signal with an output signal of an oscillator to generate and output the second output signal, wherein the soft start circuit includes a high-potential power supply and a low-potential power supply. And a current source and a capacitor connected in series between
A charge voltage of a capacity charged with a constant current output from a current source based on power-on is output to the voltage control amplifier, and the output control circuit is turned on based on the output control signal to output the capacity. The second PWM comparator has an output noise prevention circuit that turns off the synchronous rectification transistor with the second output signal based on an ON operation of the switching element. A DC / DC converter to which a circuit is connected. 3. The output noise prevention circuit inputs a charging voltage of a capacitance of the soft start circuit to the second PWM comparator, and outputs an output signal of the oscillator when a charge of the capacitance is discharged. Irrespective of the above, based on a comparison between the charging voltage of the capacitor and an output signal of the voltage control amplifier, a second output signal for turning off the synchronous rectification transistor is output. DC described in 2
/ DC converter. 4. The output noise prevention circuit outputs an output signal of the second PWM comparator as a second output signal to the synchronous rectification transistor via a switch circuit.
3. The DC / DC converter according to claim 2, wherein the switch circuit is turned off when the capacitance of the soft start circuit is discharged, and turns off the synchronous rectification transistor. 5. The output noise prevention circuit includes a switch circuit interposed between a bias circuit for supplying a bias current to the second PWM comparator and the second PWM comparator. 3. The synchronous rectifying transistor according to claim 2, wherein the synchronous rectifying transistor is turned off by inactivating the second PWM comparator when the capacitance of the soft start circuit is discharged. DC / DC converter.