JP6145335B2 - Switching power supply circuit - Google Patents

Description

  The present invention relates to a switching power supply circuit including a soft start circuit that gently starts voltage supply on a load side.

  A switching power supply circuit is a power supply circuit that supplies power with low loss to a load by combining a switching element that repeats conduction / cutoff and an inductor, and is used in many fields.

<Conventional example 1>
FIG. 10 shows a configuration example of the step-down switching power supply circuit of Conventional Example 1. MP1 is a PMOS transistor as a switch element, and its conduction / cutoff is controlled by the drive circuit 1. L1 is an inductor that stores energy by the current IIN that flows when the transistor MP1 is turned on, supplies the energy to the capacitor C0 and the load RL, and rectifies the counter electromotive force that is generated when the transistor MP1 is cut off by the diode D1. Supply to RL. The voltage VOUT supplied to the load RL is divided by the feedback resistors R1 and R2, and the feedback voltage VFB obtained here is compared with the reference voltage VREF by the error amplifier AMP1. The output voltage V1 of the error amplifier AMP1 is compared with the fixed-cycle triangular wave voltage output from the triangular wave oscillator 2 by the comparator COMP1, and the drive circuit 1 is controlled according to the comparison result. In the case of the configuration of FIG. 10, when the output voltage V1 of the error amplifier AMP1 is increased, the period during which the transistor MP1 is conducting is increased during one period of the triangular wave, and when the output voltage V1 is low, the period is shortened. The output voltage VOUT is controlled so as to coincide with the reference voltage VREF input to the error amplifier AMP1. The resistor R3 and the capacitor C2 constitute a low pass filter.

In the switching power supply circuit of FIG. 10, the relationship between the output voltage VOUT and the ON / OFF time ratio D of the transistor MP1 and the input voltage VIN in an ideal state is obtained by the following equation by the negative feedback operation of the path including the error amplifier AMP1. become that way.
VOUT = D × VIN (1)
Further, the duty ratio D is represented by the following expression, where TON is the conduction time of the transistor MP1 and TS is the time of one cycle of the oscillation waveform of the triangular wave oscillator 2.
D = TON / TS (2)
In other words, in the ideal state, the output voltage VOUT of the switching power supply circuit increases as the period during which the transistor MP1 is conducting increases, and decreases as it decreases.

  The switching power supply circuit requires a capacitor C0 for smoothing the output voltage VOUT between the output terminal and GND due to its configuration. At the time of start-up, when the transistor MP1 starts a switching operation, it is necessary to first charge the capacitor C0 and raise the output voltage VOUT until the target voltage is reached. At this time, a very large current IIN has a voltage VIN of It flows from the power source in the direction of the load RL. This is called inrush current. If the inrush current is too large, the power supply voltage VIN may become unstable or a current exceeding the rating may flow through the transistor MP1.

  Therefore, the soft start circuit 3 is used to avoid such a problem. In the circuit of FIG. 10, the soft start circuit 3 includes a capacitor C1 and a resistor R4. At the time of start-up, the voltage at the non-inverting input terminal of the error amplifier AMP1 gradually rises from 0V with a time constant determined by the soft start circuit 3.

  As a result, the negative feedback of the entire circuit causes the output voltage VOUT to gradually rise from 0 V in a form proportional to the voltage at the non-inverting input terminal of the error amplifier AMP1, as shown in FIG. For this reason, the current IIN flowing into the capacitor C0 is suppressed to a low value.

<Conventional example 2>
On the other hand, in recent years, with the expansion of applications of light emitting diodes LED, the number of cases where the light emitting diode LED is driven using a switching power supply circuit is increasing. In order to drive the light-emitting diode LED with a constant luminance, it is necessary to control so that a constant current flows instead of applying a constant voltage between its anode and cathode. For this reason, when using light emitting diode LED for the load of a switching power supply circuit, it becomes a structure like FIG.

The resistor RS in the switching power supply circuit of FIG. 12 is a detection resistor for detecting the current ILED flowing through the light emitting diode LED. In normal operation, the current ILED flowing through the light emitting diode LED is determined as follows.
ILED = VREF / RS (3)

  FIG. 13 shows the change of the feedback voltage VFB with respect to the change of the output voltage VOUT of the switching power supply circuit of FIG. 10 and FIG. FIG. 13A shows the case of the switching power supply circuit of FIG. 10, and the feedback voltage VFB changes in proportion to the change of the output voltage VOUT. FIG. 13B shows the case of the switching power supply circuit of FIG. 12, in which the light emitting diode LED is driven. The light emitting diode LED hardly flows current until the voltage applied between its anode and cathode becomes equal to or higher than the threshold voltage of the light emitting diode LED. For this reason, when the output voltage VOUT is equal to or lower than the threshold voltage, the feedback voltage VFB is 0V.

  As a result, in the case of the switching power supply circuit of FIG. 12, even if the voltage at the non-inverting input terminal of the error amplifier AMP1 is 0V or a considerably low voltage, the feedback voltage VFB at the inverting input terminal remains 0V. Negative feedback operation through the entire circuit does not work. For this reason, the soft start circuit 3 does not function until the output voltage VOUT at start-up reaches the threshold voltage of the light emitting diode LED, and the current IIN flows as an excessive inrush current as shown in FIG.

<Conventional example 3>
Due to these problems, a conventional switching power supply circuit capable of driving a light emitting diode LED has a configuration as shown in FIG. In FIG. 15, a soft start circuit 3A including a current source I1, a capacitor C3, and a reset switch SW1 is added. COMP2 is a comparator, and the output voltage does not change unless the voltage at the two non-inverting input terminals is higher than the voltage at the inverting input terminal. For this reason, the output voltage of the comparator COMP2 is determined by the lower voltage of the voltages at the two non-inverting input terminals, and the output voltage VOUT gradually rises after activation. Therefore, the conduction time of the transistor MP1 gradually increases from a short state, and an excessive inrush current can be prevented from flowing through the capacitor C0.

  However, as a disadvantage of the switching power supply circuit provided with the soft start circuit 3A as shown in FIG. 15, a problem occurs when the normal constant voltage output is controlled.

  In the switching power supply circuit of FIG. 15, at the time of soft start, the output voltage V1 of the error amplifier AMP1 is higher than the output voltage V2 of the soft start circuit 3A. Thereafter, at the timing when the output voltage VOUT rises to the target voltage and shifts to a state in which the output voltage VOUT is controlled by the negative feedback operation of the path including the error amplifier AMP1, the output voltage V1 of the error amplifier AMP1 gradually decreases. Therefore, it is necessary to be lower than the output voltage V2 of the soft start 3A.

  However, the control switching timing is delayed due to the delay in the change of the output voltage V1 of the error amplifier AMP1, and as shown in FIG. 16, an overshoot voltage in which the output voltage VOUT is temporarily higher than the target voltage is generated. This rise in voltage VOUT may exceed the breakdown voltage of the load RL and may lead to destruction. That is, the switching power supply circuit provided with the soft start circuit 3A as shown in FIG. 15 is not very suitable for the purpose of outputting a constant voltage.

  As described above, in the conventional switching power supply circuit, the soft start circuit 3 is used when outputting a constant voltage (FIG. 10), and the light emitting diode is driven at a constant current (FIG. 15). Each of the soft start circuits 3A needs to be used. In other words, it is necessary to use different switching power supply circuits having different soft start circuits depending on the purpose of use.

  An object of the present invention is to solve such a problem, and to provide a switching power supply circuit that has a small overshoot voltage even in a constant voltage output application and functions appropriately in a light emitting diode driving application.

In order to achieve the above object, a switching power supply circuit according to a first aspect of the present invention includes a switching element for conducting / cutting off a voltage supplied to a load, and a feedback voltage corresponding to the voltage supplied to the load as a reference voltage. An error amplifier that outputs an error voltage obtained by comparison, a first soft start circuit that outputs a voltage that gradually rises at the time of start-up, and the output voltage of the first soft start circuit and the error voltage that are low A first comparator that compares the voltage of the other side with a triangular wave voltage having a constant period, and a drive circuit that controls conduction / cutoff of the switch element according to a comparison result of the first comparator, In a switching power supply circuit that supplies a constant voltage to a load in accordance with conduction / cutoff of a switch element, the reference voltage is changed to a first value when an output voltage of the first soft start circuit reaches a predetermined value. Characterized by comprising a second soft start circuit to start to raise from the voltage value slowly to a second voltage value as a target.
According to a second aspect of the present invention, there is provided a switching power supply circuit comprising: a switching element that conducts / cuts off a voltage supplied to a load; and an error voltage obtained by comparing a feedback voltage corresponding to the voltage supplied to the load with a reference voltage. An error amplifier for output, a first soft start circuit for outputting a voltage that gradually rises at the time of start-up, a lower one of the output voltage of the first soft start circuit and the error voltage, and a triangular wave having a fixed period A first comparator for comparing the voltage and a drive circuit for controlling conduction / cutoff of the switch element according to a comparison result of the first comparator, and according to conduction / cutoff of the switch element In the switching power supply circuit for supplying a constant voltage to the load, the reference voltage is gradually increased from the first voltage value to the target second voltage value when the feedback voltage reaches a predetermined value. Characterized by comprising a second soft start circuit that.
According to a third aspect of the present invention, in the switching power supply circuit according to the second aspect , the first soft start circuit includes the second soft start circuit after the feedback voltage exceeds the first voltage value. During the period until the output voltage exceeds the first voltage value, the rate of increase of the output voltage is set to be higher than the rate of increase during the other periods.
According to a fourth aspect of the present invention, there is provided a switching power supply circuit comprising: a switching element that conducts / cuts off a voltage supplied to a load; and an error voltage obtained by comparing a feedback voltage corresponding to the voltage supplied to the load with a reference voltage. An error amplifier for output, a first soft start circuit for outputting a voltage that gradually rises at the time of start-up, a lower one of the output voltage of the first soft start circuit and the error voltage, and a triangular wave having a fixed period A first comparator for comparing the voltage and a drive circuit for controlling conduction / cutoff of the switch element according to a comparison result of the first comparator, and according to conduction / cutoff of the switch element In the switching power supply circuit for supplying a constant voltage to the load, a second comparator for comparing the error voltage with the output voltage of the first soft start circuit, and the second comparator Second soft- ware which starts to gradually increase the reference voltage from the first voltage value to the target second voltage value when it is determined that the pressure is lower than the output voltage of the first soft-start circuit. A start circuit is provided.
According to a fifth aspect of the present invention, there is provided a switching power supply circuit that outputs an error voltage by comparing a switching element that conducts / cuts off a voltage supplied to a load and a feedback voltage corresponding to a voltage supplied to the load with a reference voltage. An error amplifier; a third comparator that compares the error voltage with a triangular wave voltage having a constant period; and a drive circuit that controls conduction / cut-off of the switch element according to a comparison result of the third comparator. A switching power supply circuit that supplies a constant voltage to a load in accordance with conduction / cutoff of the switch element, a first soft start circuit that outputs a voltage that gradually rises at startup, and the feedback voltage is set to a predetermined value And a second soft start circuit that starts gradually increasing the reference voltage from the first voltage value to the target second voltage value by reaching the switch element, Characterized in that so as to be controlled in accordance with the comparison result of the output voltage is high Itoki said drive circuit said third comparator of said first soft start circuit than the voltage corresponding to the current flowing in .
According to a sixth aspect of the present invention, there is provided a switching power supply circuit that outputs an error voltage by comparing a switching element that conducts / cuts off a voltage supplied to a load and a feedback voltage corresponding to the voltage supplied to the load with a reference voltage. An error amplifier; a third comparator that compares the error voltage with a triangular wave voltage having a constant period; and a drive circuit that controls conduction / cut-off of the switch element according to a comparison result of the third comparator. A switching power supply circuit that supplies a constant voltage to a load according to conduction / cutoff of the switch element, a first soft start circuit that outputs a voltage that gradually rises at startup, and one or more of the triangular wave voltages over a period of cycles, the output voltage of the first soft start circuit than the voltage corresponding to the current flowing through the switching element is high Itoki, the reference voltage the first voltage value A second soft start circuit starts to be gradually increased to a second voltage value to Luo target, characterized in that it comprises a.

  According to the present invention, the first soft start circuit can function properly even for light emitting diode driving applications, and the second soft start circuit can reduce the overshoot voltage even for constant voltage output applications.

It is a circuit diagram of the switching power supply circuit of Example 1 of this invention. It is an operation | movement waveform diagram at the time of starting of the constant voltage output of the switching power supply circuit of Example 1. FIG. It is an operation | movement waveform diagram at the time of starting when the load of the switching power supply circuit of Example 1 is made into the light emitting diode. FIG. 3 is a circuit diagram of a specific example of a main part of the switching power supply circuit according to the first embodiment. It is a circuit diagram of the switching power supply circuit of Example 2 of this invention. FIG. 10 is a circuit diagram of a modification of the main part of the switching power supply circuit according to the second embodiment. It is a circuit diagram of the switching power supply circuit of Example 3 of this invention. It is a circuit diagram of the switching power supply circuit of Example 4 of this invention. It is a circuit diagram of the switching power supply circuit of Example 5 of this invention. It is a circuit diagram of the switching power supply circuit of Conventional Example 1. It is an operation | movement waveform diagram at the time of starting of the constant voltage output in the switching power supply circuit of the prior art example 1. FIG. It is a circuit diagram of the switching power supply circuit of Conventional Example 2. It is an operation | movement waveform diagram at the time of starting in the switching power supply circuit of the prior art examples 1 and 2. It is an operation | movement waveform diagram at the time of starting in the switching power supply circuit of the prior art example 2. FIG. It is a circuit diagram of the switching power supply circuit of Conventional Example 3. It is an operation | movement waveform diagram at the time of starting in the switching power supply circuit of the prior art example 3. FIG.

<Example 1>
FIG. 1 shows a switching power supply circuit according to Embodiment 1 of the present invention. The same components as those described in FIGS. 10, 12, and 15 are denoted by the same reference numerals, and detailed description thereof is omitted. In the first embodiment, a detection circuit 4A that detects that the output voltage V2 of the soft start circuit 3A has increased to the threshold voltage VTH is provided. The output voltage V3 gradually rises from the voltage VREFS near the GND voltage from the time when the output voltage V2 of the soft start circuit 3A reaches the threshold voltage VTH, and finally reaches the value of the reference voltage VREF. A second soft start circuit 3B that outputs the output voltage V3 is provided.

  In the switching power supply circuit according to the first embodiment, when the output voltage VOUT is controlled to a constant voltage, the voltage and current of each unit are as shown in FIG. Before startup, the output voltage VOUT is 0V, and the voltage V2 across the capacitor C3 of the soft start circuit 3A is also 0V. At the same time, the output voltage V3 of the soft start circuit 3B may be 0 V. However, in order to stabilize the output voltage of the error amplifier AMP1, the initial voltage VREFS is set to about 1/10 of the reference voltage VREF.

  When the switching power supply circuit starts operating at time t0, first, the output voltage V2 of the soft start circuit 3A starts to rise gently. At this time, since the feedback voltage VFB is 0V, the output voltage V1 of the error amplifier AMP1 rises due to the output voltage V3 (= VREFS) of the soft start circuit 3B and is higher than the output voltage V2 of the soft start circuit 3A. Become.

  The comparator COMP2 has one inverting input terminal and two non-inverting input terminals, and compares the low voltage of the two non-inverting input terminals with the voltage of the inverting input terminal and outputs the result. Therefore, immediately after startup, the duty ratio of conduction / cutoff of the transistor MP1 is determined based on the output voltage V2 of the soft start circuit 3A, which is a voltage lower than the voltage V1 of the error amplifier AMP1.

  Thereafter, the output voltage VOUT rises by the conduction / cutoff operation of the transistor MP1, the feedback voltage VFB rises in proportion thereto, and the time t1 when the feedback voltage VFB reaches the initial voltage VREFS of the soft start circuit 3B. Then, the output voltage V1 of the error amplifier AP1 decreases and becomes equal to or lower than the output voltage V2 of the soft start circuit 3A.

Here, control of negative feedback of the entire switching power supply circuit is started via the error amplifier AMP1. When the voltage VOUTS is the output voltage of the switching power supply circuit before the soft start circuit 3B operates, the feedback voltage VFB at that time is
VFB = VREFS = VOUTS × R1 / (R1 + R2) (4)
Therefore, the output voltage VOUTS is a voltage determined by the following expression and is once stabilized.
VOUTS = VREFS × (1 + R2 / R1) (5)

  At time t2, when the output voltage V2 of the soft start circuit 3A reaches the threshold voltage VTH, this is detected by the detection circuit 4A, and the output voltage V3 of the soft start circuit 3B starts to rise from VREFS. In response to the rise, the output voltage VOUT also rises, and when the soft start end time t3 when the output voltage V3 of the soft start circuit 3B reaches the reference voltage VREF is reached, the output voltage VOUT also reaches the target voltage at the same time. For this reason, the output voltage overshoot which becomes a problem in the switching power supply circuits of FIGS. 12 and 15 hardly occurs.

Here, the output voltage V3 of the soft start circuit 3B needs to satisfy the following conditions.
(1) The time (t0 to t2) for the feedback voltage VFB to reach the initial voltage VREFS of the soft start circuit 3B from the start of operation is set to a sufficient time.
(2) The increase of the output voltage V3 of the soft start circuit 3B is proportional to the increase of the output voltage V2 of the soft start circuit 3A or according to the rising slope.

  Here, at the timing t2 when the output voltage V2 of the soft start circuit 3A reaches the threshold voltage VTH, the feedback voltage VFB needs to reliably reach the initial voltage VREFS of the soft start circuit 3B.

In the step-down switching power supply circuit, when the target value of the output voltage VOUT is set close to the power supply voltage VIN, the time ratio approaches 1, so the threshold voltage VTH needs to be set high. For this reason, the threshold voltage VTH needs to be set to the value of the following expression, where VOSCA is the amplitude of the output voltage of the triangular wave oscillator 2 and VOSCL is the lower limit value of the output voltage.
VTH ≧ VOSCA × (VREFS / VREF) + VOSCL (6)

  The above condition (2) means that, for example, when the rise time of the soft start circuit 3A is increased, it is necessary to increase the rise time of the soft start circuit 3B accordingly. If the rise time of the soft start circuit 3B is too short, the output voltage V1 of the error amplifier AMP1 is increased to the output voltage V2 of the soft start circuit 3B while the output voltage VOUT is rising in accordance with the output voltage of the soft start circuit 3B. In some cases, the control of the output voltage VOUT returns to the control by the output voltage V2 of the soft start circuit 3B. For this reason, it is considered that the output voltage VOUT rises and an overshoot or the like occurs.

  Next, the case where the light emitting diode LED is driven at a constant current in the switching power supply circuit of FIG. 1 will be described. At this time, the load resistor RL in FIG. 1 is replaced with the light emitting diode LED, the resistors R1 and R2 are disconnected, and the series detection resistor RS is connected to the light emitting diode LED. That is, the output circuit portion is replaced with the circuit described in FIG. The voltage and current of each part are as shown in FIG.

  In this case, the feedback voltage VFB is 0 V until the time t2 when the output voltage VOUT reaches the threshold voltage of the light emitting diode LED even after the activation at the time t0. Therefore, the comparison result of the comparator COMP2 is determined by the output voltage V2 of the soft start circuit 3A, and the conduction time of the transistor MP1 becomes longer as the output voltage V2 increases. Although the output voltage V3 of the soft start circuit 3B starts to rise at the time t1 in the middle, the feedback voltage VFB remains 0V, so that the comparison result of the comparator COMP2 is not affected. After that, when the output voltage VOUT reaches the threshold voltage of the light emitting diode LED at time t2, the feedback voltage VFB starts to increase rapidly, and reaches the output voltage V3 of the soft start circuit 3B at time t3 during the increase. To do. At this time t3, the output voltage V1 of the error amplifier AMP1 decreases and becomes equal to or lower than the output voltage V2 of the soft start circuit 3A, and negative feedback including the error amplifier AMP1 works, and thereafter, the output voltage V3 of the soft start circuit 3B. The output voltage VOUT changes in response to the change of, and reaches the target voltage. The soft start operation ends at time t4 when the voltage V3 becomes the voltage VREF.

  By the operation as described above, the switching power supply circuit according to the first embodiment can perform an optimum soft start operation in a power supply application for constant voltage output and a power supply application for driving the light emitting diode LED with a constant current.

  FIG. 4 shows the simplest example of the soft start circuit 3B and the detection circuit 4A used in the switching power supply circuit of FIG. The detection circuit 4A includes an NMOS transistor MN1 that inputs the output voltage V2 of the soft start circuit 3A to the gate. The soft start circuit 3B includes a switch SW2 that switches between the reference voltage VREF and the output voltage of the NMOS transistor MN1, a comparator COMP5 that controls the switch SW2, a current source I2, and a resistor R6.

  At startup, the source of the transistor MN1 is connected to the resistor R6 via the switch SW2 of the soft start circuit 3B. An initial voltage VREFS (≈VREF / 10) is generated at both ends of the resistor R6 due to the current flowing from the current source I2. When the output voltage V2 of the soft start circuit 3A reaches the threshold voltage VTH, the transistor MN1 becomes conductive, and the voltage generated in the resistor R6 due to the current flowing through the transistor MN1 rises from the initial voltage VREFS according to the output voltage V2. Thereby, when the voltage generated in the resistor R6 exceeds the reference voltage VREF, the switch COMP2 is switched to the reference voltage VREF side by the comparator COMP5, and the output voltage V3 of the soft start circuit 3B is fixed to the reference voltage VREF.

  With such an operation, the circuit of FIG. 2 can serve as the detection circuit 4A and the soft start circuit 3B of the switching power supply circuit of FIG. Particularly in this configuration, since the capacitor C3 for setting the delay time can be shared, there is an advantage that only the value of the capacitor C3 needs to be changed when the soft start time is changed.

<Example 2>
FIG. 5 shows a configuration example of the switching power supply circuit according to the second embodiment. In the switching power supply circuit of FIG. 1, in the configuration with the constant voltage output, a time period in which the increase of the output voltage VOUT is temporarily stopped until the increase of the output voltage V3 of the soft start circuit 3B is started (time t1 in FIG. 2). There is a time T1) of ~ t2.

  Therefore, in the switching power supply circuit of the second embodiment, a detection circuit 4B that detects that the feedback voltage VFB has reached the initial voltage VREFS of the soft start circuit 3B is provided, and when this detection circuit 4B detects the initial voltage VREFS, The operation of the soft start circuit 3B is started. Accordingly, the soft start circuit 3B starts sweeping at time t1 in FIG. 2, and the period in which the increase in the output voltage VOUT is temporarily stopped can be shortened.

  Here, FIG. 6 shows a circuit of a modified example that performs the operation of eliminating the stop time zone in the same manner as the circuit of FIG. 5 described above. In the circuit configuration of FIG. 6, in addition to the circuit of FIG. 4, the soft start circuit 3A is replaced with a soft start circuit 3A ′ having a new current source I3 that increases the charging current to the capacitor C3. Further, a detection circuit 4C for controlling the current source I3 is newly provided. The detection circuit 4C includes an AND circuit AND1 that controls operation / stop of the current source I3, comparators COMP6 and COMP7, and a voltage source of the start voltage VREFS.

  At the start of operation, the AND circuit AND1 operates the current source I3 when the feedback voltage VFB exceeds the start voltage VREFS even if the output voltage V3 of the soft start circuit 3B is lower than the start voltage VREFS. Charging the capacitor C3 is started. Thereafter, when the output voltage V3 of the soft start circuit 3B exceeds the start voltage VREFS, charging of the capacitor C3 is stopped.

  As a result, the switching power supply circuit of FIG. 1 has a time zone in which the increase of the output voltage VOUT temporarily stops during the constant voltage output operation. In the switching power supply circuit having the configuration of FIG. The current source I3 can be shortened by increasing the charging current to the capacitor C3.

<Example 3>
FIG. 7 shows a configuration example of the switching power supply circuit according to the third embodiment. In the switching power supply circuit of FIG. 5 of the second embodiment described above, in the configuration in which a capacitor for phase compensation is connected between the output terminal and the inverting input terminal of the error amplifier AMP1, the operation start of the soft start circuit 3B is accurately controlled. Can not.

  FIG. 7 of the third embodiment is a configuration for solving such problems. The output voltage V2 of the soft start circuit 3A and the output voltage V1 of the error amplifier AMP1 are compared by the comparator COMP3, and the output voltage V1 of the error amplifier AMP1 is compared. Is lower than the output voltage V2 of the soft start circuit 3A, that is, when the transistor MP1 is controlled by the output voltage V1 of the error amplifier AMP1, the operation of the soft start circuit 3B is started. Thereby, even in the configuration in which the phase compensation capacitor C4 is connected, the operation start of the soft start circuit 3 can be accurately controlled.

<Example 4>
FIG. 8 shows a configuration example of the switching power supply circuit according to the fourth embodiment. In the switching power supply circuits of the first to third embodiments (FIGS. 1, 5, and 7), a comparator for comparing the output voltage V2 of the soft start circuit 3A, the output voltage of the triangular wave oscillator 2, and the output voltage V1 of the error amplifier AMP1. COMP3 was used.

  On the other hand, the switching power supply circuit of FIG. 8 is provided with a detection circuit 4C that detects a current IIN flowing through the detection resistor RT while the transistor MP1 is in a conductive state and outputs an output voltage V4 proportional to the current value. A comparator COMP4 is provided for comparing the detected voltage V4 with the output voltage V2 of the soft start circuit 3A. Further, a NOR circuit NOR1 for controlling the drive circuit 1 by inputting the outputs of the comparators COMP1 and COMP4 is provided.

  The comparator COMP4 sets its output to “H” when the output voltage V4 of the detection circuit 4C is higher than the output voltage V2 of the soft start circuit 3A. As described above, after the feedback voltage VFB reaches the initial voltage VREFS, the output voltage of the comparator COMP1 repeats “L” and “H” according to the change in the output voltage of the triangular wave oscillator 2.

  The value of the current IIN flowing through the transistor MP1 increases according to the length of the conduction time, but as the output voltage V2 of the soft start circuit 3A increases, the output of the comparator COMP4 becomes “L”. The conduction time of the transistor MP1 gradually increases from a short state, and a soft start operation similar to that of the switching power supply circuit of FIG. 1 can be performed.

  Also in this case, the feedback voltage VFB is detected by the detection circuit 4B, and when the feedback voltage VFB reaches the initial voltage VREFS, the soft start circuit 3B outputs the reference voltage VREF. The same effects as those of the switching power supply circuit can be obtained.

<Example 5>
FIG. 9 shows a configuration example of the switching power supply circuit according to the fifth embodiment. FIG. 9 shows a configuration in the case where the switching power supply circuit of FIG. 8 realizes characteristics equivalent to those of the soft start circuit 3B of FIG. In the circuit of FIG. 9, even if the output voltage V2 of the soft start circuit 3A and the output voltage V1 of the error amplifier AMP1 are compared, the timing at which the transistor MP1 is cut off is controlled by either comparison result of the comparators COMP1 and COMP4. It is not possible to judge whether it has been done.

  However, when the output of the comparator COMP1 repeats “L” and “H” to control the conduction / cutoff of the transistor MP1, the output voltage V4 of the detection circuit 4C is reduced by the transistor MP1 being cut off. As a result, the output voltage V2 of the soft start circuit 3A cannot be reached, and the output of the comparator COMP4 remains fixed at the “L” state.

  Therefore, although the output voltage of the comparator COMP1 repeats “L” and “H”, the output voltage of the comparator COMP4 does not change from “L” in one cycle or several cycles of the triangular wave oscillator 2. If it is detected by the detector 4D, it can be determined from the detection result that the transistor MP1 is controlled by the output voltage V1 of the error amplifier AMP1. Then, if it is detected that the state continues, the same effect as that of the switching power supply circuit of FIG. 7 of the third embodiment can be obtained by starting the operation of the soft start circuit 3B.

  The present invention is a switching power supply circuit that can be easily used for applications such as constant voltage output and constant current control of light emitting diodes without changing the circuit configuration, and can be used in a wide range of applications such as lighting using light emitting diodes and automotive lighting. Can be used.

  1: driving circuit, 2: triangular wave oscillator, 3, 3A, 3B: soft start circuit, 4A, 4B, 4C, 4D: detection circuit

Claims (6)

A switching element that conducts / cuts off the voltage supplied to the load, an error amplifier that outputs an error voltage obtained by comparing a feedback voltage corresponding to the voltage supplied to the load with a reference voltage, and gradually rises at startup A first soft start circuit that outputs a voltage; a first comparator that compares a lower one of the output voltage of the first soft start circuit and the error voltage with a triangular wave voltage having a constant period; A switching power supply circuit for controlling a conduction / cutoff of the switch element according to a comparison result of the first comparator and supplying a constant voltage to a load according to the conduction / cutoff of the switch element; ,
A second soft start that starts gradually increasing the reference voltage from the first voltage value to the target second voltage value when the output voltage of the first soft start circuit reaches a predetermined value. A switching power supply circuit comprising a circuit. A switching element that conducts / cuts off the voltage supplied to the load, an error amplifier that outputs an error voltage obtained by comparing a feedback voltage corresponding to the voltage supplied to the load with a reference voltage, and gradually rises at startup A first soft start circuit that outputs a voltage; a first comparator that compares a lower one of the output voltage of the first soft start circuit and the error voltage with a triangular wave voltage having a constant period; A switching power supply circuit for controlling a conduction / cutoff of the switch element according to a comparison result of the first comparator and supplying a constant voltage to a load according to the conduction / cutoff of the switch element; ,
A switching power supply comprising: a second soft start circuit that gradually raises the reference voltage from a first voltage value to a target second voltage value when the feedback voltage reaches a predetermined value. circuit. The switching power supply circuit according to claim 2 ,
The first soft start circuit has an output voltage during a period from when the feedback voltage exceeds the first voltage value to when the output voltage of the second soft start circuit exceeds the first voltage value. A switching power supply circuit characterized in that the rate of increase is set to be higher than the rate of increase in other periods. A switching element that conducts / cuts off the voltage supplied to the load, an error amplifier that outputs an error voltage obtained by comparing a feedback voltage corresponding to the voltage supplied to the load with a reference voltage, and gradually rises at startup A first soft start circuit that outputs a voltage; a first comparator that compares a lower one of the output voltage of the first soft start circuit and the error voltage with a triangular wave voltage having a constant period; A switching power supply circuit for controlling a conduction / cutoff of the switch element according to a comparison result of the first comparator and supplying a constant voltage to a load according to the conduction / cutoff of the switch element; ,
A second comparator for comparing the output voltage of the first soft start circuit and the error voltage;
When the second comparator determines that the error voltage is lower than the output voltage of the first soft start circuit, the reference voltage is gradually increased from the first voltage value to the target second voltage value. A switching power supply circuit comprising a second soft start circuit that starts the operation. A switching element that conducts / cuts off a voltage supplied to the load; an error amplifier that outputs an error voltage by comparing a feedback voltage corresponding to the voltage supplied to the load with a reference voltage; and a triangular wave having a fixed period and the error voltage A third comparator for comparing the voltage and a drive circuit for controlling conduction / cutoff of the switch element according to a comparison result of the third comparator, and according to conduction / cutoff of the switch element In a switching power supply circuit that supplies a constant voltage to the load,
A first soft start circuit that outputs a slowly rising voltage at startup;
A second soft start circuit that starts gradually increasing the reference voltage from the first voltage value to the target second voltage value when the feedback voltage reaches a predetermined value;
That the output voltage of the first soft start circuit than the voltage corresponding to the current flowing through the switching element is high Itoki the drive circuit has to be controlled in accordance with the comparison result of the third comparator Switching power supply circuit characterized.

A switching element that conducts / cuts off a voltage supplied to the load; an error amplifier that outputs an error voltage by comparing a feedback voltage corresponding to the voltage supplied to the load with a reference voltage; and a triangular wave having a fixed period and the error voltage A third comparator for comparing the voltage and a drive circuit for controlling conduction / cutoff of the switch element according to a comparison result of the third comparator, and according to conduction / cutoff of the switch element In a switching power supply circuit that supplies a constant voltage to the load,
A first soft start circuit that outputs a slowly rising voltage at startup;
Over a period of one or more cycles of the triangular wave voltage, the target output voltage is high Itoki of the first soft start circuit than the voltage corresponding to the current flowing through the switching element, the reference voltage from the first voltage value A second soft start circuit that starts to slowly increase to a second voltage value,
A switching power supply circuit comprising:

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