JP6205596B2 - Soft start circuit and power supply device - Google Patents

Description

  The present invention relates to a soft start circuit used for the purpose of preventing an inrush current and the like and a power supply device configured using the soft start circuit.

  The synchronous rectification step-down switching regulator is generally configured as shown in the conceptual diagram of FIG. This switching regulator is controlled so that the voltage level divided by the resistors R1 and R2 matches the internal reference voltage VREF. Therefore, when the power supply device is started in a state where the output capacitor Cout is discharged, the output voltage VOUT rises sharply. At this time, the inflow of charge into the output capacitor Cout has a high density in time, which may lead to destruction or deterioration of the internal circuit and the inductor. Therefore, there is a demand for suppressing the peak value of the inrush current at the time of starting.

  Therefore, in general, the power supply circuit is often provided with a soft start function that gently raises the output voltage. FIG. 2 is a waveform diagram showing the change of the current value and the passage of time since the start. In FIG. 2, when the soft start function indicated by the solid line is not provided, electric charge is rapidly applied to the output capacitor Cout at the time of starting, so that the peak value of the output current IOUT increases. On the other hand, when the output voltage VOUT is gradually raised using the soft start function (indicated by a broken line in FIG. 2), the peak value of the output current IOUT becomes small.

  As a method of mounting this soft start function, there is a method of gradually raising the reference voltage VREF. The soft start function can be realized by a simple circuit as shown in FIG. In FIG. 3, the switch SW2 is turned on, the switch SW1 is turned off, and the output of the ramp circuit having a time constant composed of a constant current source (I1) and a capacitor (C1) is referred to as a reference voltage VREF. The voltage VREF is raised. As a result, the output voltage VOUT of the power supply circuit is made to follow the reference voltage VREF, thereby realizing a soft start function.

  However, since the reference voltage VREF exceeds the target voltage only with this, the comparator CMP1 is compared with the reference voltage VBGR generated by the band gap reference (BGR) or the like that is the target voltage. When it is detected that the lamp output voltage has reached a reference voltage VBGR such as a BGR output, the switch SW2 is opened and the switch SW1 is short-circuited. As a result, charging of the ramp circuit is stopped, and a reference voltage VBGR such as a BGR output is output as a target voltage to the reference voltage VREF.

  However, there is a problem with the configuration of the soft start circuit shown in FIG. That is, since there is a delay time in the comparator CMP1, even if the detection is performed using the BGR output that is the target voltage, the switch SW1 and the switch SW2 are switched at a slightly higher voltage. As a result, as shown in FIG. 4A, the reference voltage VREF causes an overshoot. Since the output voltage is controlled by this reference voltage VREF, overshoot occurs in the output voltage VOUT as well (FIG. 4B). The overshoot voltage is often tempered because the level causes the destruction of the device and the instability of convergence.

  As a technique for dealing with this problem, a circuit configuration as shown in FIG. 5 can be adopted. This is a technique introduced in Patent Document 1, in which another reference voltage slightly lower than the reference voltage is generated and set as a soft start completion target. According to this method, it is possible to cause the comparator CMP1 to make a determination at a voltage slightly lower than the target in consideration of the delay time, and overshoot does not occur. That is, when this function is implemented, the determination operation by the comparator CMP1 is performed by VREF2 shown in FIG. 6, and after the determination, it is switched to VREF1, which is the original target voltage.

JP 2007-159288 A

  However, the method disclosed in Patent Document 1 has a big problem. That is, the reference voltage system increases. BGR, which is a standard for power supply circuits, is often vulnerable to noise. Therefore, in the configuration as shown in FIG. 5, it is preferable to generate the voltage VREF2 by dividing the resistance in the subsequent stage of the buffer in consideration of the influence of noise. However, in this case, since it is necessary to constantly pass a current, a high resistance is required to reduce power consumption and a large area is required. It is also possible to generate a slightly lower voltage VREF2 in the process of generating the BGR voltage by circuit contrivance. In this case, the current and area do not increase, but the noise performance deteriorates when many reference voltage wires are routed. This can be avoided by inserting a buffer in VREF2, but there is still a problem that the area and current consumption increase.

  The present invention has been made as a solution to the problems of the conventional soft start circuit as described above, and the purpose of the present invention is to gently increase the voltage without adding a configuration or increasing the size, without causing an overshoot. And a soft start circuit capable of outputting an external output voltage that appropriately reaches a required voltage. Another object is to provide a power supply circuit using the soft start circuit.

The soft start circuit according to the present invention includes a predetermined voltage output unit that outputs a voltage having a predetermined voltage value, a gradually increasing high voltage output unit that gradually increases the voltage value as time elapses, and the predetermined voltage output. The output voltage of the part is taken into the reference voltage side terminal, a voltage lower than the taken-in voltage by a predetermined offset voltage is used as a reference voltage, and the reference voltage is compared with the output voltage of the gradually high voltage output part. A comparator that is an offset comparator that inverts an output signal when the output voltage of the high-voltage output unit is equal to or higher than the reference voltage, and a first switch and a second switch that are turned on / off by the output signal from the comparator, until the output signal from the comparator is inverted to turn on the first switch provided on the gradual voltage output unit The gradually an output voltage of the high voltage output section as an external output voltage from the external output terminal, wherein the progressively high voltage output by the output signal from the comparator to turn off the first switch to be reversed by the And disconnecting the output voltage of the unit from the path to the external output terminal and turning on the second switch between the predetermined voltage output unit and the path to the external output terminal. A voltage switching unit that performs switching to output the output voltage as an external output voltage, and the offset comparator includes a differential amplifier circuit, and is connected to two input terminals of the differential amplifier circuit. At least a transistor and a second pair of transistors respectively connected to the pair of transistors in the differential amplifier circuit Element size of each transistor are different from each other in the square of the pair.

The soft start circuit according to the present invention, the gradual voltage output unit includes a constant current source, and a capacitor connected in series to the constant current source, between said constant current source and the capacitor, wherein The first switch that is turned on / off by the output signal from the comparator is connected to output the potential of the capacitor.

  A power supply circuit according to the present invention is a power supply circuit that compares an output voltage with a reference voltage and controls the output voltage based on the comparison result, and provides a reference voltage to the comparator that performs the comparison. The soft start circuit described in 2 is used.

  In the power supply circuit according to the present invention, the power supply circuit is any one of a synchronous rectification step-down DCDC converter, an asynchronous rectification step-down DCDC converter, a series regulator, a synchronous rectification step-up DCDC converter, and an asynchronous rectification step-up DCDC converter. And

  In the present invention, the comparator takes in the output voltage of the predetermined voltage output unit to the reference voltage side terminal and performs comparison using the voltage that is lower than the taken-in voltage by a predetermined offset voltage as the reference voltage, so that there is a delay in the operation of the comparator. However, the overshoot in which the voltage value exceeding the reference voltage, which is the output voltage of the predetermined voltage output unit, is not generated. That is, according to the present invention, it is possible to output an external output voltage that appropriately reaches a required voltage by slowly increasing the voltage without adding a configuration or increasing the size, and without causing an overshoot.

The circuit block diagram which shows the structure of the conventional synchronous rectification type | mold step-down switching regulator which does not use a soft start circuit. The waveform diagram which shows the time passage from the time of a start, and the change of an electric current value about two types with and without a soft start function. The circuit block diagram which shows the structure of the conventional soft start circuit. The wave form diagram which shows the overshoot by the conventional soft start circuit. The circuit block diagram which shows the structure of the conventional soft start circuit which considered overshoot. The wave form diagram which shows the operation | movement by the soft start circuit of FIG. 1 is a circuit block diagram of an embodiment of a soft start circuit according to the present invention. The wave form diagram which shows the operation | movement by the soft start circuit based on this invention. The block diagram of concrete embodiment of the offset comparator used for the soft start circuit which concerns on this invention. The circuit block diagram of the synchronous rectification type | mold step-down DCDC converter comprised using the soft start circuit based on this invention. The circuit block diagram of the asynchronous rectification type | mold step-down DCDC converter comprised using the soft start circuit based on this invention. The circuit block diagram of the series regulator comprised using the soft start circuit which concerns on this invention. The circuit block diagram of the synchronous rectification type | mold step-up DCDC converter comprised using the soft start circuit based on this invention. The circuit block diagram of the asynchronous rectification type | mold step-up DCDC converter comprised using the soft start circuit based on this invention.

  Embodiments of a soft start circuit and a power supply circuit using the same according to the present invention will be described below with reference to the accompanying drawings. In the drawings, the same components are denoted by the same reference numerals, and redundant description is omitted. FIG. 7 shows a circuit block diagram of an embodiment of the soft start circuit according to the present invention. This soft start circuit includes a predetermined voltage output unit 10, a gradually high voltage output unit 20, an offset comparator 30, and switches SW1 and SW2 as voltage switching units as main components.

  The predetermined voltage output unit 10 outputs a voltage VBGR having a predetermined voltage value, and includes a band gap reference (BGR) and a buffer buffer. The gradual high voltage output unit 20 outputs a voltage value that is gradually increased with time. The gradually high voltage output unit 20 can be configured by a clamp circuit in which a circuit in which a constant current source I1 and a capacitor C1 are connected in series is connected between a voltage source and a ground. This clamp circuit has a time constant composed of a constant current source I1, a capacitor C1, and the like, and outputs a voltage value that is gradually increased.

  The offset comparator 30 compares the output voltage of the predetermined voltage output unit 10 with the output voltage of the gradual high voltage output unit 20, and takes the output voltage of the predetermined voltage output unit 10 into the reference voltage side terminal. The comparison is performed using a voltage lower than the voltage by a predetermined offset voltage as a reference voltage.

  The switches SW1 and SW2, which are voltage switching units, gradually switch from the output voltage of the high voltage output unit 20 to the output voltage of the predetermined voltage output unit 10 based on the comparison result of the offset comparator 30 to obtain an external output voltage. That is, the switches SW1 and SW2 are turned on and off based on the output of the offset comparator 30.

  In the soft start circuit, when the switches SW1 and SW2 are both turned off, the switch SW2 is turned on by activation (time T1 in FIG. 8). At this time, the output voltage VREF of the high voltage output unit 20 gradually becomes smaller than the output voltage VBGR of the predetermined voltage output unit 10. Thereafter, electric charges are accumulated in the capacitor C1, and the voltage value of the output voltage VREF of the high-voltage output unit 20 gradually increases as shown in FIG.

When the output voltage VREF is determined offset value of the offset comparator 30 (time T2 in FIG. 8), the output is inverted offset comparator 30, the switch SW1 is set to ON state with the switch SW2 is turned OFF. As a result, the output voltage VBGR of the predetermined voltage output unit 10 is output to the external output terminal 40 (FIG. 8). As a result, the external output voltage output from the external output terminal 40 shifts to a predetermined voltage without overshooting.

  FIG. 9 shows a configuration diagram of a specific embodiment of the offset comparator. This offset comparator includes a differential amplifier circuit composed of (PMOS) transistors PM1, PM2 and (NMOS) transistors NM3, NM4, NM2, and a transistor PM3 which is a buffer circuit for receiving an output signal of the differential amplifier circuit, A first-stage inverter circuit composed of transistors PM4 and NM6 that amplify the output signal of the buffer circuit, and a second-stage inverter composed of transistors PM5 and NM7 that further amplify the output signal of the first-stage inverter circuit Circuit.

  This offset comparator includes a constant current source IS1 and a transistor NM1 connected between the constant current source IS1 and the ground as bias control means. The offset comparator includes transistors NM2 and NM5 that are current mirrors of the transistor NM1 provided in the bias control means. The transistor NM2 determines the drive current of the differential amplifier circuit composed of PM1, PM2, NM3, and NM4, and the transistor NM5 determines the drive current of the buffer circuit composed of the transistor PM3.

  In the offset comparator, a pair of transistors NM3 and NM4 respectively connected to the two input terminals INP and INM of the differential amplifier circuit, and a pair of transistors NM3 and NM4 respectively connected in the differential amplifier circuit. In at least one pair of the second pair of transistors PM1 and PM2, the element sizes of the transistors are different.

  In a normal comparator, the element size of a transistor in a differential amplifier circuit composed of transistors PM1, PM2, NM3, and NM4 is generally [PM1 = PM2] and [NM3 = NM4]. That is, by taking this balance, the comparator threshold is set so that the output of the comparator is inverted when the signal at the input terminal is INP = INM.

  On the other hand, the circuit configuration of the offset comparator of the present invention is the same as that of a general comparator as shown in FIG. However, as described above, the element size of the transistor in the differential amplifier circuit including the transistors PM1, PM2, NM3, and NM4 is [PM1 ≠ PM2], [NM3 ≠ NM4], or both. Thus, the threshold value of the comparator is set so as to be inverted when the signal at the input terminal is INP ≠ INM, and the output OUTP is changed at a voltage slightly lower than the target voltage. By applying a comparator that intentionally offsets the threshold in this way to this soft-start circuit, the soft-start function can be simplified without causing an increase in circuit scale, system change, or deterioration in noise immunity. And avoiding the occurrence of overshoot.

  The soft start circuit according to the present invention can be applied to various power supply circuits. That is, the present invention can be applied to a power supply circuit that compares an output voltage with a reference voltage and controls the output voltage based on the comparison result.

  First, the present invention can be applied to a synchronous rectification step-down DCDC converter as shown in FIG. This converter compares the output of the error amplifier Error AMP and the output signal of the internal oscillator OSC in the comparator Comp. Using this comparison result, the control circuit (Control Logic) performs pulse width modulation to create a time signal corresponding to a fixed ON time. The output signal of the control circuit (Control Logic) is amplified by the buffer Buffer, and the high side switch S1 and the low side switch S2 are turned ON / OFF. By controlling the ON / OFF time according to the difference voltage between the output voltage VOUT and the internal reference voltage VREF, a target voltage is obtained. The soft start circuit 100 according to the present invention can be used as a circuit for generating the internal reference voltage VREF.

  Second, the present invention can be applied to the asynchronous rectification step-down DCDC converter shown in FIG. The asynchronous rectification step-down DCDC converter having this configuration realizes the same function as the synchronous rectification step-down DCDC converter shown in FIG. 10, but facilitates its control by using a diode instead of the low-side switch S2. The configuration is adopted.

  The synchronous rectification step-down DCDC converter as shown in FIG. 10 and the asynchronous rectification step-down DCDC converter shown in FIG. 11 may be configured as a ripple control converter that does not use an error amplification circuit or an oscillator by giving a ripple to the feedback signal. Is possible.

  Third, it can be applied to the series regulator shown in FIG. In this series regulator, a voltage obtained by dividing the output voltage VOUT by resistors R1 and R2 and a reference voltage VREF are compared by an error amplifier AMP, and a target voltage is output by applying a voltage corresponding to the error to the gate of the transistor PM1. Is. The soft start circuit 100 according to the present invention can be used as a circuit for generating the internal reference voltage VREF.

  Fourthly, the present invention can be applied to the synchronous rectification step-up DCDC converter shown in FIG. In this converter, a switch SW1 is provided in a path for leading the power supply voltage to the output terminal via the coil L. One end of the switch SW2 is connected to the line of the coil L and the switch SW1, and the other end of the switch SW2 is connected to the converter SW2. It is grounded. The switches SW1 and SW2 are turned ON / OFF by the output of the buffer Buffer. Other configurations are the same as those of the converter shown in FIG. In this converter having such a configuration, the ON / OFF ratio of the switches SW1 and SW2 is generated according to the feedback voltage. Based on this ratio, the switch SW1 and the switch SW2 are alternately turned ON / OFF to determine the power supply voltage. This circuit obtains a high desired voltage output. The soft start circuit 100 according to the present invention can be used as a circuit for generating the internal reference voltage VREF.

  Fifth, the present invention can be applied to the asynchronous rectification step-up DCDC converter shown in FIG. This converter is provided with a diode D in place of the switch SW1 in the converter of FIG. 13 and realizes the same function as the synchronous rectification step-up DCDC converter shown in FIG. 13, but has a configuration that facilitates its control. Adopted.

  In the power supply circuit described above, the internal reference voltage VREF is generated by the soft start circuit 100 according to the present invention, and the internal reference voltage VREF is compared with the feedback voltage at the time of start-up in response to a gradual rise of the internal reference voltage VREF. Output characteristics. Therefore, in the power supply circuit including the soft start circuit according to the present invention, the specific internal configuration and control method are not particularly limited, and various types can be used.

  As described above, the soft start circuit according to the present invention is mounted on various power supply circuits. However, when realizing the soft start in which the reference voltage is gradually raised, the effect of the soft start circuit according to the present invention can be obtained. . That is, it is possible to simply realize the soft start function and avoid the occurrence of overshoot without causing an increase in circuit scale, system change, and further deterioration in noise resistance.

10 Predetermined voltage output unit 20 Gradual high voltage output unit 30 Offset comparator 40 External output terminal 100 Soft start circuit

Claims (4)

A predetermined voltage output unit that outputs a voltage having a predetermined voltage value;
A gradual high voltage output unit that gradually increases the voltage value over time and outputs it,
The output voltage of the predetermined voltage output unit is taken into a reference voltage side terminal, and a voltage lower than the taken-in voltage by a predetermined offset voltage is used as a reference voltage to compare the reference voltage with the output voltage of the gradually high voltage output unit. A comparator which is an offset comparator that inverts an output signal when the output voltage of the gradually high voltage output unit is equal to or higher than the reference voltage;
The first switch that is configured by a first switch and a second switch that are turned on / off by the output signal from the comparator, and that is gradually provided in the high-voltage output unit until the output signal from the comparator is inverted. By turning on, the output voltage of the gradually high voltage output section is output as an external output voltage from the external output terminal, and when the output signal from the comparator is inverted, the first switch is turned off. The output voltage of the gradual high voltage output section is disconnected from the path to the external output terminal, and the second switch between the predetermined voltage output section and the path to the external output terminal is turned on to turn on the second switch. A voltage switching unit that performs switching to output the output voltage of the voltage output unit as an external output voltage,
The offset comparator includes a differential amplifier circuit,
In at least one pair of a pair of transistors respectively connected to the two input terminals of the differential amplifier circuit and a second pair of transistors connected to the pair of transistors in the differential amplifier circuit A soft start circuit characterized in that each transistor has a different element size. The gradually high voltage output unit includes a constant current source and a capacitor connected in series to the constant current source ,
Between the constant current source and the capacitor, the first switch is turned on and off by the output signal from the comparator is connected to claim 1, characterized in that the output potential of the capacitor The soft start circuit described. In the power supply circuit that compares the output voltage with the reference voltage and controls the output voltage based on the comparison result,
3. A power supply circuit using the soft start circuit according to claim 1 or 2 as a circuit for supplying a reference voltage to a comparator for comparison.   The power supply circuit is any one of a synchronous rectification step-down DCDC converter, an asynchronous rectification type step-down DCDC converter, a series regulator, a synchronous rectification type step-up DCDC converter, and an asynchronous rectification type step-up DCDC converter. Power supply circuit.

Images