JP2012080685A - Synchronous rectification-type power supply circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a high speed response in a duty range of 0 to 100% becomes difficult when a load fluctuates at high speed and a voltage of a battery drops.SOLUTION: A transistor exclusive control circuit 1-1 outputs a drive voltage for exclusively turning on/off a transistor 3 and a transistor 4 by a pulse having a time width equivalent to fluctuation voltage of a voltage at a series connection point of the transistor 3 and the transistor 4. A time measuring counter 2 measures a duty of the transistor 3 by the pulse. A mode switch control circuit 1-2 supplies the drive voltage to the transistors 3 and 4 for a switching operation when the duty is in a prescribed upper/lower limit range. At a linear mode, the mode switch control circuit 1-2 supplies the fluctuation voltage for driving the transistor 3 and supplies ground potential to the transistor 4 for a linear operation.

Description

  The present invention relates to a synchronous rectification type power supply circuit, and more particularly to a synchronous rectification type power supply circuit in which an operation range of a switching transistor is expanded.

  FIG. 5 shows an example of a general synchronous rectification type power supply circuit. The high-side transistor 3 and the low-side transistor 4 are alternately switched in response to a signal from the transistor exclusive control circuit 1-1, and outputs a DC voltage to the load device 18. A part of the output voltage is fed back to the error amplifier 12 through the phase correction filter 11, and the error amplifier 12 inputs an error voltage with respect to the reference voltage Vref to the PWM comparator 14. The PWM comparator 14 slices the triangular wave signal generated by the ramp clock oscillation circuit 13 with the error voltage, and supplies the obtained time width signal to the transistor exclusive control circuit 1-1. The transistor exclusive control circuit 1-1 controls on / off of the high-side transistor 3 and the low-side transistor 4 by this signal.

  By the way, when the load fluctuates at a high speed or the battery voltage decreases, the ratio (duty) of the on time to the off time of the high side transistor 3 may be 0% or nearly 100%. However, when the two transistors 3 and 4 for synchronous rectification are turned on at the same time, a through current flows from the power supply input to the ground side. Therefore, a dead time is provided to avoid this, and generally a duty of 10% or less is provided. In some cases, driving with a duty of 90% or more is not possible.

  That is, in general, the power transistor has a slow switching speed, cannot be driven with a minimum pulse width, has a limitation on the minimum pulse width of the duty of the high side transistor 3 at a low current, and is dead dead synchronized with the low side transistor 4. Since the time cannot be secured, the maximum pulse width of the duty of the high-side transistor 3 is limited when the potential difference between input and output is small.

  In order to solve such a problem, two PWM circuits are provided, each of which generates a first PWM signal and a second PWM signal that are shifted from each other by a half cycle and have an amplitude corresponding to the error signal from the error amplifier. A power supply circuit and a PWM circuit are disclosed in which a PWM signal having a frequency twice the predetermined frequency is obtained by synthesizing the PWM signals.

JP 2003-284329 A

  The problem to be solved is that when the load fluctuates at a high speed or the voltage of the battery decreases, a high-speed response with a duty range of 0 to 100% is difficult.

  The main feature of the present invention is to switch the DCDC converter to a linear operation when the duty exceeds a predetermined range in order to enable a high-speed response in the 0 to 100% duty range.

  Since the synchronous rectification type power supply circuit of the present invention switches the DCDC converter to linearly operate when the duty exceeds a predetermined range, even when the load current decreases or the battery voltage decreases to the output voltage. There is an advantage that the power supply usage range can be expanded while using low-cost low-speed transistors, and power supply control responding at high speed can be performed.

  Further, since the linear operation is performed, it is possible to stably supply power with little ripple noise in the low power mode by intermittent operation of the load device, and the heat generation of the transistor is small and the heat sink can be made small.

It is a circuit diagram which shows Example 1 of the synchronous rectification type | mold power supply circuit of this invention. It is an operation | movement waveform diagram at the time of the low power operation | movement of the synchronous rectification type | mold power supply circuit of this invention. It is an operation waveform diagram at the time of battery voltage drop of the synchronous rectification type power supply circuit of the present invention. It is a circuit diagram which shows the other Example of the synchronous rectification type | mold power supply circuit of this invention. It is a circuit diagram which shows an example of the conventional synchronous rectification type power supply circuit.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a first embodiment of a synchronous rectification type power supply circuit according to the present invention. This power supply circuit stably supplies to the load device 18 an output voltage obtained by converting the voltage of the battery 19 by a DCDC converter including the high-side transistor 3 and the low-side transistor 4. A current detection resistor 16 is inserted between the battery 19 and the high side transistor 3, and an output inductor 5 and an output capacitor 6 are connected between the connection point of the high side transistor 3 and the low side transistor 4 and the load device 18. , A two-stage smoothing circuit including a secondary output inductor 7 and a secondary output capacitor 8 is inserted.

  The source of the N-type MOS high-side transistor 3 is connected to the current detection resistor 16, the source of the N-type MOS low-side transistor 4 is connected to the ground, and the drains of the high-side transistor 3 and the low-side transistor 4 are coupled together. The high-side transistor 3 and the low-side transistor 4 are alternately switched. However, when the gate drive signal is less than 10% and more than 90%, and when the current flowing out from the battery 19 is less than a predetermined value, the high-side transistor 3 is linear. The operation is performed and the low-side transistor 4 is turned off.

  The voltage obtained by subtracting the drop voltage of the output inductor 5 from the voltage at the drain coupling point of the high-side transistor 3 and the low-side transistor 4 (referred to as output voltage) is divided by the feedback resistors 9 and 10 and passes through the phase correction filter 11. The error amplifier 12 and the linear error amplifier 13 are led to inputs at one end. The phase correction filter 11 stabilizes the feedback loop of the output voltage. The error amplifier 12 inputs an error voltage between the reference voltage Vref1 and the voltage obtained by subtracting the voltage drop of the phase correction filter 11 from the divided voltage to one end of the PWM comparator 14. The reference voltage Vref1 is a voltage equal to the divided voltage in the steady state. Therefore, the error voltage is a voltage corresponding to the variation of the divided voltage.

  The ramp clock oscillator 13 outputs a triangular wave signal, and the PWM comparator 14 slices the triangular wave signal with the error voltage from the error amplifier 12 and outputs a time width signal to the transistor controller 1 and the time measurement counter 2. The linear error amplifier 15 outputs an error voltage between the reference voltage Vref2 and the voltage obtained by subtracting the voltage drop of the phase correction filter 11 from the divided voltage to the transistor controller 1. The reference voltage Vref2 is set so that the error voltage remains within a range in which the high-side transistor 3 performs a linear operation, and is equal to the reference voltage Vref1 if there is no variation in components.

  The time measurement counter 2 includes a time measurement counter 2-1 and two comparators 2-2 and 2-3. The time measurement counter 2-1 measures the duty of the output voltage based on the time width signal from the PWM comparator 14. At this time, the time measurement counter 2-1 inputs a sine wave signal as a reference for counting how many clocks the time width signal from the PWM comparator 14 has. Comparator 2-2 detects a duty exceeding 90%, and comparator 2-3 detects a duty of less than 10%, and notifies transistor controller 1 of each. The current detection circuit 17 detects the current flowing out from the battery 19 based on the voltage drop across the current detection resistor 16 and notifies the transistor controller 1 of the current.

  The transistor controller 1 performs control to switch the DCDC converter so as to perform a switching operation or a linear operation. For this purpose, a transistor exclusive control circuit 1-1, a mode switching control circuit 1-2, a first switch 1-3, and a second switch 1-4 are provided.

  The transistor exclusive control circuit 1-1 outputs a switch signal for turning on / off the high-side transistor 3 and the low-side transistor 4 for the time determined by the time width signal from the PWM comparator 14 as the first switch 1-3 and the second switch Output to switch 1-4. At this time, control is performed so that the high-side transistor 3 and the low-side transistor 4 are not turned on simultaneously. The mode switching control circuit 1-2 enables the mode switching signal when receiving a notification with a duty less than 10% or exceeding 90% from the time measurement counter 2 or when the current value notified from the current detection circuit 17 is a predetermined value or less. And

  The first switch 1-3 normally leads the switch signal from the DCDC converter transistor exclusive control circuit 1-1 to the gate of the high-side transistor 3, but when the mode switching signal is enabled, the linear error amplifier 15 Is supplied to the gate of the high-side transistor 3. Thereby, the gate voltage is switched so that the high-side transistor 3 can function as a linear regulator. The second switch 1-4 normally leads the switch signal from the DCDC converter transistor exclusive control circuit 1-1 to the gate of the low side transistor 4, but when the mode switching signal is enabled, the ground potential is set to the low side. This is supplied to the gate of the transistor 4.

  Next, the operation of the synchronous rectification type power supply circuit configured as described above will be described. The high-side transistor 3, the low-side transistor 4, the phase correction filter 11, and the transistor controller 1 perform a synchronous rectification operation using a ramp clock oscillation circuit 13 having a frequency corresponding to the response characteristic of the component.

  The PWM comparator 14 always operates and generates a pulse having a width proportional to the output voltage even when synchronous rectification is not performed. The time measurement counter 2 measures the duty by the above pulse, and when the duty is within the set upper and lower limits, the high side transistor 3 and the low side transistor 4 are driven by the output of the PWM comparator 14 (switching) mode).

  However, when the output current decreases, the PWM comparator 14 outputs a pulse with a small duty, and the duty may exceed the lower limit. Further, when the input / output voltage approaches, the PWM comparator 14 outputs a pulse with a large duty, and the duty may exceed the upper limit. In these cases, the mode switching control circuit 1-2 changes the operation mode, drives the high-side transistor 3 with the linear error amplifier 15, and stops the operation of the low-side transistor 4 (linear mode). The high-side transistor 3 allows a current to flow in response to fluctuations in the divided voltage caused by the feedback resistors 9 and 10 and stabilizes the output voltage.

  Details will be described below. The current supplied from the battery 19 is supplied to the source of the high side transistor 3 via the current detection resistor 16. The current is detected by the current detection circuit 17 and input to the mode switching control circuit 1-2.

  When the mode switching signal is disabled, the DCDC converter operates in the switching mode. The high side transistor 3 functions as a switching regulator. It is turned on / off in response to a switch signal from the first switch 1-3, and when it is on, the current supplied from the battery 19 is output to the smoothing circuit. On the other hand, the low-side transistor 4 is turned on / off exclusively from the high-side transistor 3 in response to a switch signal from the second switch 1-4, and forms a current path from the ground to the smoothing circuit when turned on. To do.

  The output voltage, specifically, the voltage at the connection point of the output inductor 5 and the secondary output inductor 6 constituting the smoothing circuit is input to the error amplifier 12 and the linear error amplifier 15 via the phase correction filter 11. The error amplifier 12 outputs an error voltage between the reference voltage Vref1 and the output voltage of the phase correction filter 11 to the PWM comparator 14.The PWM comparator 14 slices the triangular wave signal from the ramp clock oscillator with this error voltage, and the time The width signal is output to the counter 2-1 of the time measurement counter 2 and the transistor exclusive control circuit 1-1 of the transistor controller 1. The linear error amplifier 15 outputs an error voltage between the reference voltage Vref2 and the output voltage of the phase correction filter 11 to one input terminal of the first switch 1-3 of the transistor controller 1.

  The transistor exclusive control circuit 1-1 sends a switch signal for turning on / off the high-side transistor 3 and the low-side transistor 4 for a time determined by the time width signal to the first switch 1-3 and the second switch 1-4. Output to one of the input terminals. The counter 2-1 measures the duty by the time width signal, the comparator 2-2 detects that the duty exceeds 90%, and the comparator 2-3 detects that the duty is less than 10% and outputs it to the mode switching control circuit 1-2.

  If the mode switching control circuit 1-2 does not detect a duty exceeding 90%, the comparator 2-3 does not detect a duty less than 10%, and the current value from the current detection circuit 17 is not less than a predetermined value, the mode switching signal is output. Keep enabled and continue the previous switching mode. The first switch 1-3 and the second switch 1-4 use the switch signal supplied from the transistor exclusive control circuit 1-1 at the upper input terminal shown in FIG. As a result, the high-side transistor 3 and the low-side transistor 4 continue to operate in the switching mode.

  Now, when the load device 18 enters a low power standby state, the load current decreases, so the output voltage increases, the error voltage output by the error amplifier 12 increases in the positive direction, and the time width signal from the PWM comparator 14 Becomes smaller. As a result, the duty measured by the counter 2-1 decreases. Then, since the load current further decreases, the duty continues to decrease as shown in FIG. FIG. 2 is a waveform diagram showing the drive voltage of the high-side transistor 3 (the output of the first switch 1-3) together with the output voltage in this case.

  When the drive voltage of the high side transistor 3 is at a high level, the high side transistor 3 is on and the low side transistor 4 is off, so that the smoothing circuit is charged and the output voltage rises. On the other hand, when the drive voltage is at a low level, the high-side transistor 3 is off and the low-side transistor 4 is off, so that a current path from the ground voltage to the smoothing circuit is formed, and the output voltage drops.

  When the comparator 2-3 is notified that the duty has reached less than 10%, the mode switching control circuit 1-2 enables the mode switching signal. In response to the enabling of the mode switching signal, the first switch 1-3 uses the output of the linear error amplifier 15 as the gate of the high side transistor 3, and the second switch 1-4 uses the ground voltage as the gate of the low side transistor 4. The DCDC converter is switched to operate as a linear regulator. The linear error amplifier 15 operates to limit the current of the high side transistor 3 and keep the output voltage constant.

  In general, when a battery is used for a long period of time, discharge proceeds and internal resistance increases, and the terminal voltage of the battery decreases when the same current flows. When the battery terminal voltage decreases, the output voltage decreases, so the error voltage output from the error amplifier 12 increases in the negative direction, and the time width signal from the PWM comparator 14 increases. As a result, the duty measured by the counter 2-1 increases. Then, since the load current increases, the duty continues to increase as shown in FIG. FIG. 3 is a waveform diagram showing the drive voltage (output of the first switch 1-3) of the high side transistor 3 in this case together with the output voltage.

  When the drive voltage of the high side transistor 3 is at a high level, the high side transistor 3 is on and the low side transistor 4 is off, so that the smoothing circuit is charged and the output voltage rises. On the other hand, when the drive voltage is at a low level, the high-side transistor 3 is off and the low-side transistor 4 is off, so that a current path from the ground voltage to the smoothing circuit is formed, and the output voltage drops.

  When the comparator 2-3 is notified that the duty exceeds 90%, the mode switching control circuit 1-2 enables the mode switching signal. In response to the enabling of the mode switching signal, the first switch 1-3 uses the output of the linear error amplifier 15 as the gate of the high side transistor 3, and the second switch 1-4 uses the ground voltage as the gate of the low side transistor 4. The DCDC converter is switched to operate as a linear regulator. The linear error amplifier 15 operates to limit the current of the high side transistor 3 and keep the output voltage constant.

  As described above, since the duty is less than 10% and the operation is switched to the linear operation before the operation limit of the transistor operating frequency, even when the power consumption of the load device 18 is reduced, the operation as the synchronous rectification type power supply circuit is performed. It can be performed stably. Further, ripple noise can be reduced in the linear error amplifier 15. In addition, since the high-side transistor 3 is configured to perform a linear operation only when the current is small, it generates less heat and is suitable for downsizing the transistor, and can operate as both a switching transistor and a linear transistor. It is. These advantages apply to the case where the supply voltage from the battery 19 decreases because the duty is over 90% and the operation is switched to the linear operation before the operation limit of the transistor operating frequency.

  When the duty returns to 10% or more or 90% or less after switching to the linear operation, the operation returns to the switching operation by the operation opposite to the above operation.

  Further, in the above description, the mode switching control circuit 1-2 switches the operation mode based only on the duty from the time measurement counter 2, but whether or not the current detected by the current detection circuit 17 is equal to or greater than a predetermined value. The operation mode may be switched by.

  Furthermore, the mode switching control circuit 1-2 switches the operation mode more precisely if both the duty from the time measurement counter 2 and whether or not the current detected by the current detection circuit 17 is equal to or greater than a predetermined value are used as a switching condition. Can be realized.

  Next, as another embodiment of the present invention, the basic configuration is as described above. FIG. 4 shows an example in which a feedback loop to the linear error amplifier 15 is devised. In this example, feedback resistors 9 and 10 are added immediately before the output inductor 5, and a phase correction filter 20 is connected for the linear error amplifier 15. The linear error amplifier 15 operates to limit the current of the high-side transistor 3 and keep the output voltage constant, but the phase correction filter 20 can perform feedback operation faster than the frequency characteristics of the switching operation of the high-side transistor 3. It is configured.

  In general, it is conceivable to perform intermittent operation in the low power mode, but when a large current and a small current fluctuate in a specific cycle, the voltage variation increases depending on the frequency characteristics of the feedback cycle. In this example, since the linear error amplifier 15 is braked, an effect is obtained that a stable voltage can be supplied even during intermittent fluctuation operation configured systematically at a low current.

  A part or all of the above-described embodiment can be described as in the following supplementary notes, but is not limited thereto.

  (Supplementary note 1) In a synchronous rectification type power supply circuit in which a high-side transistor for supplying battery voltage to a load and a low-side transistor for supplying ground voltage to the load are connected in series to constitute a DCDC converter. When the on / off of the high-side transistor exceeds a predetermined upper and lower limit range, the high-side transistor is controlled to be switched from a switching operation to a linear operation, and the switching operation of the low-side transistor is stopped. Synchronous rectification type power supply circuit.

  (Supplementary Note 2) In a synchronous rectification type power supply circuit in which a high side transistor for supplying battery voltage to a load and a low side transistor for supplying ground voltage to the load are connected in series to constitute a DCDC converter. An error amplifier that detects a fluctuation voltage of the voltage at the series connection point, a PWM comparator that outputs a pulse having a time width equivalent to the fluctuation voltage, and the high-side transistor and the low-side transistor are exclusively turned on by the pulse. A transistor exclusive control circuit that outputs a drive voltage for turning on / off, a time measurement counter that measures on / off (duty) of the high-side transistor by the pulse, and the duty is within a predetermined upper and lower limit range Is in the switching mode, but the duty has a predetermined upper and lower limit. In other words, the mode switching control circuit for switching to the linear mode, the driving voltage is supplied to the high-side transistor and the low-side transistor in the switching mode, and the variable voltage is supplied to the high-side transistor in the linear mode. A synchronous rectification type power supply circuit comprising: a switch that is supplied for driving and supplies a ground potential to the low-side transistor.

  (Supplementary note 3) In a synchronous rectification type power supply circuit in which a high-side transistor for supplying battery voltage to a load and a low-side transistor for supplying ground voltage to the load are connected in series to constitute a DCDC converter. An error amplifier that detects a fluctuation voltage of the voltage at the series connection point, a PWM comparator that outputs a pulse having a time width equivalent to the fluctuation voltage, and the high-side transistor and the low-side transistor are exclusively turned on by the pulse. A transistor exclusive control circuit for outputting a driving voltage for turning off / off, a current flowing out from the battery as a detection current detection circuit, and when the current exceeds a predetermined value, a switching mode is set. A mode switching control circuit that switches to the linear mode when the value is less than the value, and the switching mode The driving voltage is supplied to the high-side transistor and the low-side transistor, and in the linear mode, the fluctuation voltage is supplied for driving the high-side transistor and a ground potential is supplied to the low-side transistor. And a synchronous rectification type power supply circuit.

  (Supplementary Note 4) In a synchronous rectification type power supply circuit in which a high side transistor for supplying battery voltage to a load and a low side transistor for supplying ground voltage to the load are connected in series to constitute a DCDC converter. An error amplifier that detects a fluctuation voltage of the voltage at the series connection point, a PWM comparator that outputs a pulse having a time width corresponding to the fluctuation voltage, and the high-side transistor and the low-side transistor are exclusively turned on by the pulse. Transistor exclusive control circuit for outputting a drive voltage for turning on / off, a time measuring counter for measuring on / off (duty) of the high-side transistor by the pulse, and detecting current flowing from the battery A circuit, the duty is within a predetermined upper and lower limit range, and the current is A switching mode when a value exceeds a predetermined value, but a mode switching control circuit that switches to a linear mode when the duty exceeds a predetermined upper or lower limit or the current falls below a predetermined value, and the driving voltage when the switching mode is selected Is supplied to the high-side transistor and the low-side transistor, and in the linear mode, the variable voltage is supplied for driving the high-side transistor, and a switch for supplying a ground potential to the low-side transistor is included. Synchronous rectification type power supply circuit.

  (Supplementary Note 5) The error amplifier inputs a resistance division voltage with respect to a voltage via the output inductor constituting the smoothing circuit from the series connection point via a phase correction filter, and changes the error voltage with respect to a predetermined reference voltage to the fluctuation The synchronous rectification type power supply circuit according to any one of appendices 2 to 4, wherein the voltage is a voltage.

  (Supplementary Note 6) A linear error amplifier is provided that inputs a resistance division voltage with respect to the voltage at the series connection point via a second phase correction filter and detects an error voltage with respect to a predetermined reference voltage. The synchronous rectification type power supply circuit according to any one of appendices 2 to 5, wherein the output of the linear error amplifier is sometimes supplied for driving the high side transistor.

DESCRIPTION OF SYMBOLS 1 Transistor control circuit 2 Time measurement counter 3 High side transistor 4 Low side transistor 5 Output inductor 6 Output capacitor 7 Secondary output inductor 8 Secondary output capacitor 9 Feedback resistance
10 Feedback resistor
11 Phase correction filter
12 Error amplifier
13 Ramp clock oscillator
14 PWM comparator
15 Linear error amplifier
16 Current detection resistor
17 Current detection circuit
18 Load device
19 battery
20 Phase correction filter
1-1 Transistor control circuit
1-2 Mode switching circuit
1-3 First switch
1-4 Second switch
2-1 Counter
2-2 Comparator
2-3 Comparator

Claims (6)

In a synchronous rectification type power supply circuit in which a high side transistor for supplying battery voltage to a load and a low side transistor for supplying ground voltage to the load are connected in series to form a DCDC converter.
When the on / off of the high-side transistor exceeds a predetermined upper and lower limit range, the high-side transistor is controlled to be switched from a switching operation to a linear operation, and the switching operation of the low-side transistor is stopped. Synchronous rectification type power supply circuit. In a synchronous rectification type power supply circuit in which a high side transistor for supplying battery voltage to a load and a low side transistor for supplying ground voltage to the load are connected in series to form a DCDC converter.
An error amplifier for detecting a fluctuation voltage of the voltage at the series connection point;
A PWM comparator that outputs a pulse having a time width corresponding to the fluctuation voltage;
A transistor exclusive control circuit that outputs a driving voltage for exclusively turning on and off the high-side transistor and the low-side transistor by the pulse;
A time measurement counter for measuring on / off (duty) of the high-side transistor by the pulse;
A switching mode when the duty is within a predetermined upper and lower limit range, but a mode switching control circuit that switches to a linear mode when the duty exceeds a predetermined upper and lower limit;
In the switching mode, the driving voltage is supplied to the high-side transistor and the low-side transistor, and in the linear mode, the fluctuation voltage is supplied to drive the high-side transistor and the low-side transistor is grounded. A synchronous rectification type power supply circuit comprising a switch for supplying a potential. In a synchronous rectification type power supply circuit in which a high side transistor for supplying battery voltage to a load and a low side transistor for supplying ground voltage to the load are connected in series to form a DCDC converter.
An error amplifier for detecting a fluctuation voltage of the voltage at the series connection point;
A PWM comparator that outputs a pulse having a time width corresponding to the fluctuation voltage;
A transistor exclusive control circuit that outputs a driving voltage for exclusively turning on and off the high-side transistor and the low-side transistor by the pulse;
A current detection circuit for detecting a current flowing out of the battery;
A mode switching control circuit that switches to a linear mode when the current is less than a predetermined value when the current exceeds a predetermined value;
In the switching mode, the driving voltage is supplied to the high-side transistor and the low-side transistor, and in the linear mode, the fluctuation voltage is supplied to drive the high-side transistor and the low-side transistor is grounded. A synchronous rectification type power supply circuit comprising a switch for supplying a potential. In a synchronous rectification type power supply circuit in which a high side transistor for supplying battery voltage to a load and a low side transistor for supplying ground voltage to the load are connected in series to form a DCDC converter.
An error amplifier for detecting a fluctuation voltage of the voltage at the series connection point;
A PWM comparator that outputs a pulse having a time width corresponding to the fluctuation voltage;
A transistor exclusive control circuit that outputs a driving voltage for exclusively turning on and off the high-side transistor and the low-side transistor by the pulse;
A time measurement counter for measuring on / off (duty) of the high-side transistor by the pulse;
A current detection circuit for detecting a current flowing out of the battery;
When the duty is within a predetermined upper and lower limit range and the current exceeds a predetermined value, the switching mode is set. A mode switching control circuit for switching;
In the switching mode, the driving voltage is supplied to the high-side transistor and the low-side transistor, and in the linear mode, the fluctuation voltage is supplied to drive the high-side transistor and the low-side transistor is grounded. A synchronous rectification type power supply circuit comprising a switch for supplying a potential.   The error amplifier inputs a resistance division voltage with respect to a voltage via the output inductor constituting the smoothing circuit from the series connection point via a phase correction filter, and sets an error voltage from a predetermined reference voltage as the fluctuation voltage. The synchronous rectification type power supply circuit according to claim 2. A resistance-divided voltage with respect to the voltage at the series connection point is input via a second phase correction filter, and a linear error amplifier that detects an error voltage with a predetermined reference voltage is provided.
6. The synchronous rectification type power supply circuit according to claim 2, wherein the switch supplies an output of the linear type error amplifier for driving the high side transistor in a linear mode.

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