JPH0854943A - Stabilized power supply circuit - Google Patents

Abstract

PURPOSE:To compose the low-pass filter for phase compensation of a feedback loop for current limiting without an exterior capacitor. CONSTITUTION:An FETQ 8 is provided between the collector of a transistor Q4 for excess current detection and a resistance R3 for current voltage conversion. Between the resistance R3 and the base of the differential FET Q5 of an error amplifier for excess current time, an FET Q9 composing the FET Q8 and a current mirror circuit is provided. Because the on-resistance of the FET Q9 is incommensurably larger than the resistance R3, the cut-off frequency of a low-pass filter for phase compensation becomes the product of the on- resistance of the FET Q9 and the capacitance of a capacitor C. As compared with a case where the cut-off frequency is determined by the resistance R3 and the capacitor C, the capacitor C becomes smaller and it becomes possible to be incoporated in a regulated power supply circuit IC. Because the FET Q8 and Q9 compose the current mirror circuit, the dispersion of the threshold voltage of the FET Q9 can be absorbed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stabilized power supply circuit having a current limiting function.

[0002]

2. Description of the Related Art FIG. 3 shows the configuration of a stabilized power supply circuit according to a first conventional example. This conventional example includes a first error amplifier including transistors Q1 and Q2 and constant current sources 1 to 3. The output voltage Vo output from the collector of the transistor Q3 is the resistance R1 and R
It is divided by a bleeder composed of 2 and fed back to the base of the transistor Q2 of the first error amplifier. Since the reference voltage Vref is applied to the base of the transistor Q1 which has a differential relationship with the transistor Q2,
The collector of the transistor Q2 has Vo for Vref
A voltage corresponding to the error of R2 / (R1 + R2) appears.
In this conventional example, this voltage is applied to the base of the transistor Q3 to obtain a stabilized output voltage Vo.

This conventional example has a function of limiting the current with a drooping characteristic as shown in FIG. 4 when the collector current (output current) Io of the transistor Q3 becomes an overcurrent. The overcurrent state of the output current Io occurs when a ground fault occurs on the side of the circuit driven by the stabilized output voltage Vo. For example, when a certain IC is driven by a stabilized power supply circuit, if there is a short-circuit point with the ground in the power supply wiring on the printed circuit board on which this IC is mounted, the output voltage Vo decreases and the output current Io increases. To do. When such a situation occurs, the transistor Q3 will be destroyed if it is noticeable. Of the circuits provided in FIG. 3, the transistor Q
The loop composed of 4, the resistor R3, the capacitor C and the transistor Q5 is a circuit for protecting the stabilized power supply circuit, particularly the output transistor Q3, from such a situation.

First, the transistors Q3 and Q4 form a current mirror circuit, and a current of output current Io × mirror ratio X flows through the collector of the transistor Q4. Resistance R
3 converts the collector current of the transistor Q4 into a voltage. The voltage drop due to the resistor R3 is applied to the base of the transistor Q5. The transistor Q5 constitutes a second error amplifier together with the transistor Q1 and the constant current sources 1 to 3. Vref is applied to the collector of the transistor Q5.
A voltage corresponding to the error of Io.X.R3 with respect to appears. This voltage is applied to the base of transistor Q3.

The second error amplifier operates when the output current Io becomes an overcurrent. That is, while the output current Io is smaller than the limit value (while normal), the base voltage of the transistor Q5 is lower than the base voltage of the transistor Q2, so that the transistor Q2 is on and the transistor Q5 is off. The amplifier works. When the output current Io increases and reaches the limit value, the base voltage of the transistor Q5 becomes higher than that of the transistor Q2, the transistor Q2 is turned off and the transistor Q5 is turned on. As a result, the second error amplifier operates. In the state where the second error amplifier is operating, the transistor Q3 is driven so that the output current Io is maintained at a constant value regardless of the output voltage Vo. Therefore, the current limit due to the drooping characteristic shown in FIG. Is realized.

FIG. 5 shows the configuration of a stabilized power supply circuit according to the second conventional example. This conventional example has a configuration in which a current mirror circuit composed of transistors Q6 and Q7 and a resistor R4 are added to the circuit of the first conventional example. While the output current Io is smaller than the limit value, the first error amplifier similar to the first conventional example operates. When the output current Io increases and reaches the limit value, the collector current of the transistor Q4 is converted into a voltage by the resistor R3, as in the first conventional example.
The voltage drop due to the resistor R3 is applied to the base of the transistor Q5. Therefore, as in the first conventional example, the second error amplifier operates.

However, in this conventional example, the transistor Q4
The collector current of is not determined only by the output current Io and the mirror ratio X of the current mirror circuit composed of the transistors Q3 and Q4. That is, it is also limited by the current flowing through the resistor R4 (corresponding to the output voltage Vo). Therefore, the limit value Ilimit in this conventional example is as follows. The first term on the right side of this equation is the transistors Q3 and Q4.
Is a term related to the current mirror circuit composed of, and represents the output current Io at the time when the output voltage Vo becomes 0, that is, the short-time current Ishort. The second term is related to the current mirror circuit composed of the transistors Q6 and Q7. Therefore, the current limiting characteristic is a "F" -shaped characteristic as shown in FIG. However, VBE is the base-emitter voltage of the transistor Q6.

[0008]

## EQU1 ## Ilimit = Vref / R3.X + (Vo-VBE)
/ R4 ・ X

[0009]

These conventional examples include the following:
In both cases, the capacitor C has a large capacity, and when it is made into an IC, it has to be externally attached, and the number of parts increases, which is a problem.

First, in the first conventional example, in order to remove the high frequency component from the output of the stabilized power supply circuit by reducing the gain frequency characteristic extending up to several MHz to several tens to several hundreds Hz, the collector of the transistor Q3 is used. It is necessary to provide a capacitor of several tens of μF between the ground and a low-pass filter for phase compensation in the feedback loop from the output transistor Q4 through the resistor R3 to the transistor Q5. When the mirror ratio X of the current mirror circuit composed of the transistor Q3 and the transistor Q4 is 1000, the capacitor C that constitutes the low-pass filter together with the resistor R3 has a large electrostatic capacity of about tens of thousands of pF.
It is difficult to configure in the IC.

In particular, in the second conventional example, even when the transistor Q5 is off, the transistor Q3 passes through the transistors Q6, Q7 and the resistor R3, and then the transistor Q3.
Since positive feedback is added to the path up to 5, phase compensation is essential in this sense as well. The current flowing through the resistor R3 is μA
If it is on the order, the value of the resistor R3 is on the order of kΩ, and the capacitor C also has a large electrostatic capacity of about tens of thousands of pF, so that it is difficult to configure it in the IC.

The present invention has been made to solve the above problems, and when a low-pass filter for phase compensation is provided in a feedback loop for current limitation, an active element is used for this low-pass filter. This configuration eliminates the need to use a phase compensation capacitor having a large electrostatic capacity and realizes a stabilized power supply circuit suitable for downsizing and IC implementation.

[0013]

In order to achieve such an object, the present invention provides a resistor for converting a detected output current into a voltage by a current limiting feedback loop and a high frequency component of the output current. An active element that turns on and inserts an on-resistance at least when the output current of the output transistor becomes excessive is provided between the capacitors to be removed.

The present invention further provides that the active element is a FET.
Is characterized in that. The present invention is characterized by comprising a current mirror circuit together with the active element, and comprising an FET for supplying a current having a value corresponding to the detected output current to the active element. The present invention is characterized by including means for limiting the current flowing through the resistor according to the output voltage.

[0015]

In the present invention, the active element having the ON resistance is inserted between the resistor for converting the detected output current into the voltage and the capacitor for removing the high frequency component of the output current. As a result, the low-pass filter composed of the resistor, the capacitor and the active element is inserted in the feedback loop for current limitation. The cutoff frequency of this low-pass filter is determined by the values of the on resistances of the resistors, capacitors and active elements. Therefore, compared to the configuration in which the cutoff frequency is determined only by the resistance and the capacitor, the capacitance of the capacitor can be reduced by the ON resistance of the active element, so that at least the same current limiting performance and frequency characteristics as before can be maintained. However, it becomes possible to realize a stabilized power supply circuit suitable for miniaturization and IC implementation.

In particular, when an FET is used as the active element, its on-resistance becomes very large, so that the electrostatic capacitance of the capacitor can be made very small, and the resistance for current-voltage conversion in designing the cutoff frequency. You can ignore the value of. Further, by providing an FET that supplies a current having a value corresponding to the detected output current to the FET so as to configure the current mirror circuit, it is possible to absorb variations in the threshold voltage of the FET that configures the low-pass filter.

The current limiting characteristic of the present invention can be a drooping characteristic or a fold-back characteristic. In the case of the "F" -shaped characteristic, the current flowing through the resistance for current-voltage conversion may be limited according to the output voltage.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. The same components as those of the conventional example shown in FIGS. 3 to 6 are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 shows the configuration of a stabilized power supply circuit according to the first embodiment of the present invention, and FIG. 2 shows the configuration of a stabilized power supply circuit according to the second embodiment. The current limiting characteristic of the first embodiment is a drooping characteristic, and the current limiting characteristic of the second embodiment is a fold-back characteristic. These examples are
Alternatively, it is a configuration in which transistors Q8 and Q9 are added to the second conventional example. The transistors Q8 and Q9 form a current mirror circuit. An FET is used as the transistor Q5, like the newly added transistors Q8 and Q9.

Transistor Q9 in this embodiment is used as a resistor. That is, when a large drain current flows through the transistor Q8, a drain current of the mirror ratio times the transistor Q9 flows. In this state, that is, when the transistor Q9 is on, the on-resistance of the transistor Q9 is inserted into the low-pass filter for phase compensation on the feedback loop for current limitation. Here, since FET is used as the transistor Q9,
The on-resistance can be on the order of tens of MΩ.
Since this resistance value is sufficiently larger than the value of the resistor R3 for current / voltage conversion (on the order of several kΩ), the cut-off frequency of the low-pass filter for phase compensation in this embodiment is exclusively
It is determined by the on-resistance of the transistor Q9 and the capacitance of the capacitor C. Since the on-resistance of the transistor Q9 is about 10,000 times larger than that of the resistor R3, the capacitance of the capacitor C can be reduced to 1 / th of that of the conventional example without causing a change in the base voltage of the transistor Q5 as in the case where the resistor R3 is simply increased. It can be reduced to about 10,000, that is, several pF to several tens of pF. As a result, the capacitor C can be configured in the stabilized power supply circuit IC, the number of external parts can be reduced, and the cost can be reduced.

Further, since the current mirror circuit is constituted by the transistors Q8 and Q9, the following formula is established, and the variation of the ON resistance due to the variation of the threshold voltage of the transistor Q9 can be absorbed.

[0022]

[Equation 2] L9 / W9 = Ron / {(2.L8 / W8) .K
P · ID} ^1/2 where L9, W9: effective gate length and width of transistor Q9 L8, W8: effective gate length and width of transistor Q8 Ron: on-resistance of transistor Q9 KP: transconductance of transistors Q8 and Q9 : The drain current of the transistor Q8 The reason why the transistor Q5 is an FET is that the offset becomes significant when it is a bipolar transistor. In addition, the polarity of each bipolar transistor is PN by design.
It can be P or NPN and each bipolar transistor can be replaced by a FET.

[0023]

As described above, according to the present invention,
An active element having an on-resistance is inserted between the resistor for converting the detected output current to a voltage and the capacitor for removing the high frequency component of the output current, and the low-pass filter composed of the resistor, the capacitor and the active element is used as the current. Since it is inserted into the feedback loop for limiting, the capacitance of the capacitor can be reduced while maintaining at least the current limiting characteristics and frequency characteristics similar to the conventional one, realizing a stabilized power supply circuit suitable for miniaturization and IC integration. it can.

In particular, when an FET is used as the active element, its on-resistance becomes very large, so that the electrostatic capacitance of the capacitor can be made extremely small, and the resistance for current-voltage conversion in designing the cutoff frequency. You can ignore the value of. Further, by providing an FET that supplies a current having a value corresponding to the detected output current to the FET so as to configure the current mirror circuit, it is possible to absorb variations in the threshold voltage of the FET that configures the low-pass filter. Further, the current limiting characteristic of the present invention can be a drooping characteristic or a fold-back characteristic.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a stabilized power supply circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a stabilized power supply circuit according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a configuration of a stabilized power supply circuit according to a first conventional example.

FIG. 4 is a diagram showing a current limiting characteristic of a first conventional example.

FIG. 5 is a circuit diagram showing a configuration of a stabilized power supply circuit according to a second conventional example.

FIG. 6 is a diagram showing a current limiting characteristic of a first conventional example.

[Explanation of symbols]

 Vcc Power supply voltage Vref Reference voltage Vo Output voltage Io Output current 1-3 Constant current source Q1-Q9 Transistor R1-R4 Resistor C Capacitor

Claims (5)

[Claims] 1. An output transistor for outputting a stabilized output voltage to the outside, an output voltage of the output transistor is detected, an error of the output voltage is detected with reference to a predetermined reference voltage, and the detected error is Voltage stabilizing means for driving the output transistor to be suppressed, and detecting the output current, detecting an excess of the output current with reference to the reference voltage, and driving the output transistor so that at least the detected excess is canceled. The current limiting means includes a resistor for converting the detected output current into a voltage, a capacitor connected in parallel with the resistor for removing high frequency components of the output current, and at least an output current of the output transistor being excessive. And an active element that inserts the on-resistance between the resistor and the capacitor when Stabilized power supply circuit for the butterflies. 2. A current limiting circuit for detecting an output current of a stabilized power supply circuit, detecting an excess of the output current with reference to a predetermined reference voltage, and controlling the output current so as to cancel at least the detected excess. , A resistor that converts the detected output current into a voltage, a capacitor that is connected in parallel with the resistor to remove high frequency components of the output current, and that turns on when at least the output current becomes excessive. A current limiting circuit, comprising: 3. The circuit according to claim 1, wherein the active element is a FET. 4. The FE according to claim 3, wherein a current mirror circuit is formed with the active element to supply a current having a value corresponding to the detected output current to the active element.
A circuit comprising T. 5. The circuit according to claim 1, further comprising means for limiting a current flowing through the resistor according to an output voltage.