JP3229825B2 - Control device for chopper regulator and chopper regulator using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for a chopper regulator having a protection function from an overcurrent and for stably supplying a predetermined DC voltage to a load, and a chopper regulator using the same. is there.

[0002]

2. Description of the Related Art Conventionally, a constant voltage power supply circuit, for example,
It is widely used in electronic devices and the like to stably supply a DC voltage. These constant voltage power supply circuits
The method of stabilizing the voltage can be roughly classified into a dropper method and a chopper method.

[0003] The dropper system is a system in which a desired output voltage is obtained by lowering an input voltage by using, for example, a variable resistor itself or an equivalent variable resistor constituted by using a transistor or the like. This method has low noise and the circuit design is simple and easy. As a result, it is most effective in applications where the difference between the input voltage and the output voltage is small. On the other hand, in the dropper method, a voltage drop due to the variable resistor causes a loss in the constant voltage power supply circuit. Therefore,
If the difference between the input voltage and the output voltage is large, the efficiency will be poor. As a result, when the difference between the input voltage and the output voltage is relatively large, a chopper type constant voltage power supply circuit is often used.

The chopper system is also called a switching system, and is a system in which an input current is periodically intermittently output, and an output voltage after smoothing is controlled by a ratio between a conduction time and a cutoff time. In this method, loss due to a voltage drop does not occur unlike the dropper method, so that efficiency is high even in an application where the difference between the input voltage and the output voltage is large.

As shown in FIG. 11, the conventional constant voltage power supply circuit 101 is a step-down type chopper regulator integrated circuit (hereinafter simply referred to as a regulator IC) 103.
And a smoothing circuit 104 and a voltage dividing circuit 105 formed by members external to the regulator IC 103 such as coils and resistors. Regulator IC1
In 03, when a current is supplied from the input power supply 102, the npn-type output transistor 131 intermittently outputs the current at the designated duty. The intermittent current is smoothed by the smoothing circuit 104 and then supplied to the load 106 via the load terminal O.

On the other hand, the voltage V _{O at} the load terminal O is divided by the voltage dividing circuit 105 at a predetermined voltage dividing ratio, and the feedback voltage V _O
ADJ is applied to the feedback terminal _ADJ of the regulator IC 103. Regulator IC103
, The error amplifier 133 compares the feedback voltage V _ADJ with a predetermined reference voltage V _REF1, and outputs a voltage V _ERR corresponding to an error between the two. The PWM comparator 135 is
A triangular wave of a predetermined frequency given from the oscillator 134,
And comparing the error voltage V _ERR, and outputs a pulse signal V _PWM pulse width corresponding to the error voltage V _ERR. The pulse signal V _PWM is applied to the drive transistor 132 via the NAND circuit 136.

As a result, the duty of the output transistor 131 is controlled so that the error between the feedback voltage V _ADJ and the reference voltage V _REF1 is reduced. As a result, even if the input voltage V _IN or the load 106 fluctuates, the constant voltage power supply circuit 101 can stably supply the voltage V _O of a predetermined magnitude to the load 106.

By the way, the constant voltage power supply circuit 10 having the above configuration
1 changes the duty so that the feedback voltage V _ADJ , that is, the output voltage V _O has a predetermined value. Therefore, when the load terminal O is short-circuited or when the load 106 heavier than expected is connected to the load terminal O, the constant voltage power supply circuit 101 outputs the output transistor 131 to maintain the output voltage V _O. And a current exceeding the allowable amount of the smoothing circuit 104 or the like. As a result, each member of the constant voltage power supply circuit 101 such as the output transistor 131 and the smoothing circuit 104 may be damaged.

Therefore, the constant voltage power supply circuit 101 includes:
A function for limiting the supply current is essential. Specifically, in the regulator IC 103, the input terminal IN
And an output transistor 131, an overcurrent detection resistor 137 is provided, and an overcurrent detection circuit 138 is provided.
Monitors the voltage across the overcurrent detection resistor 137 to determine whether an overcurrent is flowing.

When the current flowing through the output transistor 131 increases and exceeds a predetermined value I _S0 , the overcurrent detection circuit 13
8 determines that an overcurrent is flowing, based on the voltage between both ends of the overcurrent detection resistor 137. In this case, the overcurrent detection circuit 138 sends a detection signal to the flip-flop 139, and the flip-flop 139 holds one of the outputs of the NAND circuit 136 at a low level until a reset signal is input from the oscillator 134. Further, the NAND circuit 136 shuts off the drive transistor 132 irrespective of the input from the PWM comparator 135. Thus, the output transistor 131 is always shut off while the overcurrent detection circuit 138 detects the overcurrent, and the constant voltage power supply circuit 101 is protected from the overcurrent. It is to be noted that the current value I _S0 is a constant voltage power supply circuit 1 in a normal state.
01 indicates the maximum output current I _{O that} can be supplied, and is determined according to the application.

[0011]

[SUMMARY OF THE INVENTION However, in the conventional constant voltage power supply circuit 101, the characteristics of the output current I _O at the time of short circuit, portions of the foot are formed, sufficiently reduce the output current I _O at short There is a problem that can not be.

More specifically, as shown by a broken line in FIG. 12, when the load terminal O is short-circuited and the current flowing through the output transistor 131 exceeds _IS0 , the overcurrent detection circuit 138
Limits the output current _IO . As a result, the output voltage V _O of the constant voltage power supply circuit 101 starts to decrease from the predetermined value V _C. Furthermore, the output current I _O, in accordance with the decrease in the output voltage V _O, gradually increases. As a result, the characteristic of the output current I _O with respect to the output voltage V _O at the time of short-circuit, that is, the overcurrent protection characteristic,
A portion (base) exceeding I _S0 indicating the maximum value of the current that can be supplied appears, and the output current I _S2 at the time of short-circuiting is equal to the current value I _S0.
Increase from _S0 .

Therefore, for example, a member externally attached to the regulator IC 103 such as a coil of the smoothing circuit 104 or a member constituting the constant-voltage power supply circuit 101 such as the output transistor 131 in the regulator IC 103 has a current rating. When determining, it is necessary to determine based on the current value I _S2 which is larger than the above-mentioned I _S0 . Generally, the size and price of the external member tend to increase as the current rating increases. Further, even for a member such as the output transistor 131 integrated with the regulator IC 103, when the current rating is increased, it is necessary to increase the size of the element or complicate the circuit. As a result, it is difficult to reduce the manufacturing cost and reduce the size of the constant voltage power supply circuit 101 having the above configuration.

In order to solve this problem, in the constant voltage power supply circuit 101, a comparator 140 is added to the regulator IC 103 in addition to the above configuration. The comparator 140 monitors the feedback voltage V _ADJ ,
It is determined whether or not the load terminal O is short-circuited. Upon detecting a short circuit, the comparator 140 activates the oscillator 134
To instruct the oscillation frequency to decrease.

As a result, as shown by the solid line in FIG.
The output current I _O of the constant voltage power supply circuit 101 is equal to the output voltage V _O
Is less than or equal to a predetermined value V _C2 , the frequency is suppressed by the frequency conversion of the oscillator 134. As a result, as shown by the broken line, the skirt portion is reduced as compared with the case where no frequency conversion is performed. Further, the current at the time of the short circuit also becomes I _S1 , which is smaller than the value I _S2 described above. In addition, the above-mentioned broken line
The case where there is no comparator 140 is shown.

However, even when frequency conversion is performed,
After the increase of the output current I _O is suppressed once, the output current I _O
Increases again as the output voltage V _O decreases. As a result, although the frequency is reduced as compared with the case where the frequency is not performed, a skirt portion remains in the characteristic of the output current _IO .

As a result, the problem that the output current I _O at the time of short circuit cannot be sufficiently reduced has not been solved. Therefore, it is necessary to set the current rating of the external members and the like higher than in the case where the current rating is set based only on the supply current in a normal state, and to reduce the manufacturing cost and the size of the constant voltage power supply circuit 101. It is hindering.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide a chopper regulator having an overcurrent protection circuit, in which the output current drooping characteristic during the overcurrent protection operation is improved, and The purpose is to reduce the current.

[0019]

According to a first aspect of the present invention, there is provided a control apparatus for a chopper regulator, wherein a current supplied from an input side is intermittently switched by a switching element and then smoothed by a smoothing circuit and output. A control device for a regulator, characterized in that the following measures are taken in order to solve the above-mentioned problems.

That is, control means for controlling the duty of the switching element so as to stabilize the output voltage, current detecting means for detecting a current flowing to the switching element, and a current detected by the current detecting means. The value is compared with a preset overcurrent detection level to detect that excessive current is flowing to the switching element.
Then, the conduction of the switching element is prevented and the switch is turned off.
While the switching element is shut off,
When the detected current value falls below the overcurrent detection level, the switch
Overcurrent detection means for resuming conduction of the switching element, an oscillator for instructing the control means to shut off the switching element when an overcurrent is detected, and an oscillator for instructing the control means when an output voltage falls below a predetermined level. And a short-circuit detecting means for lowering the oscillation frequency of the signal.

Furthermore, the overcurrent detection means, said overcurrent
Current detection level, when the short circuit detecting means detects a short circuit, when the short circuit detecting means does not detect a short circuit.
Unlike the case where the short-circuit detecting means does not detect the short-circuit, it is set lower .

In the above configuration, for example, the switching element such as the output transistor intermittently outputs the current supplied from the input side at the duty specified by the control device of the chopper regulator. The current is, for example,
The power is supplied to the load via a smoothing circuit including a coil, a catch diode, a capacitor, and the like.

On the other hand, the control means detects a change in the output voltage, for example, by monitoring a feedback voltage obtained by dividing the output voltage, and controls the duty of the switching element to cancel the change. Thus, the chopper regulator can output a stable voltage at a desired value during a period in which no short circuit or overcurrent occurs. Hereinafter, a state in which no short circuit or overcurrent has occurred is referred to as a normal state.

The overcurrent detecting means detects a current flowing through the switching element based on, for example, a voltage between both ends of a resistor provided in series with the switching element. In this case, it is determined that an overcurrent has occurred. On the other hand, the oscillator indicates, for example, a period in which conduction of the switching element is prevented in the case of overcurrent by, for example, periodically applying a reset signal. The control unit shuts off the switching element for a designated period, for example, between the time when the overcurrent is detected and the time when the reset signal is applied. As a result, the output current of the chopper regulator is limited, and the chopper regulator is protected from overcurrent.

When the output voltage is higher than the predetermined level, the short-circuit detecting means sends the signal to the oscillator and the overcurrent detecting means.
Notifies that no short circuit has occurred. Based on this,
The oscillation frequency of the oscillator oscillates at a higher frequency than at the time of short circuit. The overcurrent detection means detects an overcurrent at a detection level set higher than that at the time of short circuit.

Since the detection levels of the overcurrent detection means are different from each other when a short circuit occurs and when a non-short circuit occurs, the detection level during a non-short circuit can be set in accordance with a current value that can be supplied by a normal chopper regulator. Therefore, when the current flowing through the switching element becomes excessive due to a change in load or input voltage during a non-short circuit, the overcurrent detection means can immediately detect the occurrence of the overcurrent, and the chopper regulator is switched from the overcurrent to the overcurrent. Can be reliably protected. Furthermore, the oscillation frequency of the oscillator is set to be relatively high, unlike at the time of short circuit. Therefore, when the current flowing through the switching element is no longer excessive, the control means can immediately restart the duty control of the switching element. As a result, the time required for the chopper regulator to return from the overcurrent state to the normal state can be maintained as in the conventional case.

On the other hand, when the output terminals are short-circuited, the output voltage decreases and the output current starts to increase. At this point, the short-circuit detection means has not yet detected the short-circuit, and the overcurrent detection level of the overcurrent detection means and the oscillation frequency of the oscillator are:
It is kept the same as when no short circuit occurs. Therefore, the overcurrent detecting means detects the overcurrent at the detection level at the time of non-short circuit,
The oscillator indicates, at the frequency at the time of non-short circuit, a period during which conduction of the switching element is blocked. As a result, the output current of the chopper regulator is limited, and the output voltage starts to drop. In addition, the output current gradually increases as the output voltage decreases.

Subsequently, when the output voltage drops and falls below a predetermined level, the short-circuit detecting means notifies the overcurrent detecting means and the oscillator that a short-circuit has occurred. Based on this, the overcurrent detection means sets the overcurrent detection level lower than when no short circuit occurs. Similarly, the oscillator lowers the oscillation frequency. As a result, the output current of the chopper regulator is
Once greatly reduced.

By the way, even after the frequency of the oscillator is converted, the output current of the chopper regulator increases again as the output voltage decreases. However, the detection level of the overcurrent detection means is set lower than when no short circuit occurs.
Therefore, the rate of increase in the output current is suppressed lower than when no short-circuit occurs, and the hem portion of the overcurrent protection characteristic can be reduced as compared with the related art. As a result, the output current at the time of short circuit can be reduced, and can be suppressed to be lower than the current that can be supplied at the time of non-short circuit.

In the above configuration, the hem portion of the overcurrent protection characteristic can be reduced as compared with the related art, so that heat generation of the switching element at the time of overcurrent can be suppressed. As a result, the safety of the chopper regulator can be improved.
Further, the current rating of the switching element can be reduced, and the control device and the switching element can be formed relatively easily in one package. Further, for example, the current rating of members constituting the smoothing circuit such as a coil and a catch diode can be reduced. As a result,
It is possible to use a member that is less expensive and smaller in size, so that the cost and size of the chopper regulator can be reduced.

According to a second aspect of the present invention, there is provided a control apparatus for a chopper regulator, wherein a current supplied from an input side is interrupted by a switching element and then smoothed by a smoothing circuit and output. However, in order to solve the above-described problem, when an overcurrent is detected,
Pulse-by-pulse operation at the cycle of the
Medium, flip-flop for preventing conduction of the switching element
Control means for controlling the duty of the switching element so that the output voltage is stabilized, current detection means for detecting a current flowing to the switching element, and a current value detected by the current detection means. Is compared with a preset overcurrent detection level, and when it is detected that an excessive current is flowing to the switching element,
Blocking the conduction of the switching element,
And the current detected by the current detecting means.
When the value falls below the overcurrent detection level, the switching
Overcurrent detection means for restarting the conduction of the element, if the output voltage falls below a predetermined level, is characterized in that it comprises a short circuit detecting means for reducing the oscillation frequency of the oscillator.

In the above configuration, when the short circuit occurs, the oscillation frequency of the oscillator
Since both the wave number and the overcurrent detection level decrease,
As in item 1, the base of the overcurrent protection characteristics has been reduced.
it can. This allows safe, low-cost and compact
A regulator can be realized.

According to a third aspect of the present invention, in the control device for a chopper regulator according to the first or second aspect of the present invention, the overcurrent detecting means passes with the current flowing through the switching element. An overcurrent detection resistor that increases or decreases the current, a detection unit that outputs a current corresponding to the voltage between both ends of the overcurrent detection resistor, and a predetermined value of current when the short circuit detection unit detects a short circuit. The constant current source to be output and the sum of the currents supplied from the detection unit and the constant current source determine whether or not exceeds a predetermined value.
A determination unit for detecting an overcurrent.

The overcurrent detecting resistor is, for example,
The current flowing through the overcurrent detection resistor is provided in series with the switching element, and changes as the current flowing through the switching element increases and decreases.

In the above configuration, when the short-circuit detecting means does not detect a short-circuit, the constant current source does not supply a current to the determination unit. Therefore, the determining unit determines that an overcurrent occurs when the current output from the detecting unit exceeds a predetermined value.
On the other hand, when the short-circuit detecting means detects a short-circuit, the constant current source supplies a predetermined current to the determination unit. Therefore, even if the current output from the detection unit does not exceed the predetermined value, the determination unit determines that the overcurrent is not exceeded if the sum of the current and the output current of the constant current source exceeds the predetermined value. Is determined. As a result, the overcurrent detection level at the time of short circuit can be set lower than at the time of non-short circuit.

By the way, various configurations can be considered as the configuration for selecting the overcurrent detection level in the overcurrent detection means. For example, an overcurrent detection resistor provided in series with a switching element, a voltage source that generates two levels of voltage, and a voltage between both ends of the overcurrent detection resistor and each voltage level generated by the voltage source are compared. Then, the overcurrent may be determined. However, when a circuit for comparing voltages by transistors or the like is configured, each voltage level is limited to a voltage between the base and the emitter of the transistor, and it is difficult to set the voltage level to an excessively low value. Therefore, the higher voltage level tends to be higher than the voltage level when overcurrent is detected by a conventional chopper regulator. As a result,
It is necessary to set the resistance value of the overcurrent detection resistor higher than before, and increase the voltage between both ends of the overcurrent detection resistor, which leads to an increase in power consumption.

On the other hand, in the configuration according to the third aspect of the invention, when a short circuit occurs, current is supplied to the determination unit from both the constant current source and the detection unit. Therefore, the resistance value of the overcurrent detection resistor can be suppressed as in the related art.
As a result, overcurrent can be detected at two different detection levels without increasing power consumption.

Furthermore, a chopper regulator according to a fourth aspect of the present invention is the chopper regulator according to the first, second or third aspect , wherein the control device and the switching element are connected to one another.
It is characterized by being formed in a package.

In the above configuration, most of the members necessary for forming the chopper regulator, such as the switching element, the control means, and the overcurrent detection means, are formed in one package, and the smoothing circuit and the like are externally provided. By itself, a high-performance chopper regulator can be manufactured. Since the number of components that make up the chopper regulator can be reduced, handling and manufacturing labor can be reduced.

Furthermore, since the hem portion is reduced in the overcurrent characteristic as compared with the conventional chopper regulator, the rating of external members such as a catch diode and a coil can be set lower. As a result, the size of the entire chopper regulator can be reduced.

[0041]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention.
The following is a description based on FIG.
That is, the constant voltage power supply circuit (chopper regulator) according to the present embodiment is used for stably supplying a desired DC voltage in, for example, an electronic device.

As shown in FIG. 1, the constant voltage power supply circuit 1
Is an input power supply 2 for supplying an input voltage V _IN , a step-down chopper regulator integrated circuit (hereinafter simply referred to as a regulator IC) 3 for intermittently outputting a current supplied from the input power supply 2, The output voltage V _OUT of the regulator IC 3 is smoothed, and a smoothing circuit 4 that outputs the output voltage O from the output terminal O of the load 6 and the output voltage V _O of the constant voltage power supply circuit 1 are divided to generate a feedback voltage V _ADJ . Voltage dividing circuit 5
And In the constant voltage power supply circuit 1, for example,
When the input voltage V _IN or the load 6 fluctuates, the output voltage V _O of the constant voltage power supply circuit 1 tends to change. However, based on the feedback voltage V _ADJ , the regulator IC 3
To detect the fluctuation of the output voltage V _O and suppress the fluctuation,
The ratio of the period during which the regulator IC 3 is conductive and the period during which the regulator IC 3 is shut off is controlled. As a result, even if the input voltage V _IN or the load 6 fluctuates, the constant voltage power supply circuit 1
_O can be output from the output terminal O. Hereinafter, the output terminal O of the constant voltage power supply circuit 1 is referred to as a load terminal O to distinguish it from the output terminal OUT of the regulator IC 3.

The smoothing circuit 4 includes a regulator IC 3
And a catch diode 12 whose anode is connected to the end of the coil 11 on the side of the regulator IC 3 and whose cathode is grounded. And an output capacitor 13 having one end connected to the load terminal O and the other end grounded. The voltage dividing circuit 5 is composed of resistors 14 and 15 connected in series with each other. The resistor 14 side of the voltage dividing circuit 5 is connected to the load terminal O,
The resistor 15 side is grounded. Furthermore, both resistors 14
The connection point 15 is connected to the feedback terminal ADJ of the regulator IC3. As a result, the output voltage V _O of the constant voltage power supply circuit 1 can be divided at a predetermined voltage division ratio and applied to the regulator IC 3 as the feedback voltage V _ADJ .

The current rating of each of the members 11 to 15 is as follows:
The regulator IC 3 is set so as not to be damaged even when the largest current is supplied. In general, the larger the current rating, the larger the dimensions, and the higher the price, of the coils, capacitors, resistors, diodes, and the like constituting each of the members 11 to 15. Therefore,
When designing the constant voltage power supply circuit 1, the current ratings of the members 11 to 15 are set as small as possible after a sufficient margin is provided for safety.

The regulator IC 3 is, for example, as shown in FIG.
As shown in (a) and (b), it is packaged in one chip. Specifically, the circuit section 21 formed of a semiconductor wafer or the like and on which a circuit of the regulator IC 3 described later is formed is, for example, a bonding section 2 formed of Ag paste or the like.
2 and is fixed on the metal frame 23. One end of the metal frame 23 is extended to form an outer lead frame 24 that serves as a ground terminal GND. Furthermore, regulator IC
Reference numeral 3 includes outer lead frames 25 to 27 serving as an input terminal IN, an output terminal OUT, and a feedback terminal ADJ, respectively. The outer lead frames 24 and 25 to 27 are arranged in parallel with each other.

The circuit section 21 has contact sections 21a to 21d for connecting to the outside.
Each of the contact portions 21a to 21d is made of a metal wire 28.
Are wire-bonded to any of the outer lead frames 24 to 27, respectively. Specifically, the grounding contact portion 21a is connected to the metal frame 23, the input contact portion 21b is connected to the outer lead frame 25, and the output contact portion 21c is connected to the outer lead frame 2.
6 respectively. Further, the feedback contact portion 21d is connected to the outer lead frame 27 serving as a feedback terminal ADJ.

Further, the circuit portion 21 and the metal frame 2
3 and one end of the outer lead frames 24 to 27 are covered with a package 29. The package 29 is made of, for example, an exterior resin such as an epoxy resin, and is formed by a process such as transfer molding.

The regulator IC 3 according to the present embodiment comprises:
As shown in FIG. 1, the components 11 to 15 constituting the smoothing circuit 4 and the voltage dividing circuit 5 are externally connected to the output terminal OUT and the feedback terminal ADJ of the regulator IC 3 as shown in FIG. Thus, the constant voltage power supply circuit 1 can be configured relatively easily. Also, since the regulator IC3 is integrated into one package,
Handling becomes simple and mounting of the constant voltage power supply circuit 1 becomes easy.

On the other hand, when viewed as an electric circuit, the regulator IC 3 is an npn-type output transistor (switching element) 31 provided between the input terminal IN and the output terminal OUT, and a pn for driving the output transistor 31.
and a p-type drive transistor 32. Both transistors 31, 32 is Darlington-connected, by the base potential V _B of the drive transistor 32 can conduct / cut off between the input terminal IN and the output terminal OUT.

Further, in order to control the conduction / interruption of the output transistor 31, that is, to control the base potential V _B of the driving transistor 32, the regulator IC 3 includes the reference voltage V _REF1 applied from the constant voltage source 33 a and the feedback voltage V _REF1. An error amplifier 33 for detecting an error from V _ADJ , an oscillator 34 for generating a triangular wave of a predetermined frequency, and an error amplifier 3
Compared 3 of the output V _OSC output V _ERR and the oscillator 34, PWM for generating a pulse signal V _PWM of width corresponding to the error (Pulse Width Modulation) comparator 35
And Output V _{PWM of} PWM comparator 35
Is applied to the base of the drive transistor 32 via the NAND circuit 36. Thus, the output transistor 31 is turned on / off by the pulse width control in the voltage mode. In addition, each said member 31
To 36 and a flip-flop 39 described later correspond to the control means described in the claims. The oscillator 34 is also the oscillator described in the claims.

The regulator IC 3 according to the present embodiment comprises:
As described later, a circuit is provided for protecting the regulator IC 3, the smoothing circuit 4, the voltage dividing circuit 5, and the like from overcurrent. However, here, before describing these circuits and the operation at the time of protection from overcurrent, the entire constant voltage power supply circuit 1 in a state where no short circuit or overcurrent has occurred (hereinafter, referred to as a normal state). About the operation of
A brief description will be given with reference to the waveform diagram of FIG.

That is, as shown in FIG. 3A, when an input voltage V _IN is applied from the input power supply 2 to the input terminal IN, the regulator IC 3 is activated, and the output transistor 31 is repeatedly turned on / off. . As a result, the regulator IC3 starts supplying the current supplied from the input terminal IN from the output terminal OUT. The intermittently supplied current is
The smoothing is performed by the smoothing circuit 4. Thereby, the constant voltage power supply circuit 1 outputs the smoothed voltage V via the load terminal O.
_O can be applied to the load 6. The voltage dividing circuit 5 divides the potential of the load terminal O (output voltage V _O ) at a predetermined voltage dividing ratio and applies the divided voltage to the feedback terminal ADJ of the regulator IC 3.

In the regulator IC3, the error amplifier 33 is connected to the potential of the feedback terminal ADJ (the feedback voltage VJ).
_ADJ ) is compared with a reference voltage V _REF1 having a predetermined value, and an error voltage V _ERR corresponding to an error between the two is output as shown in FIG. On the other hand, in a normal state, the oscillator 34 outputs a triangular wave V _OSC having a predetermined frequency. Further, the PWM comparator 35 _outputs V _ERR and V
During the period when V _ERR is larger than _OSC , a high-level pulse signal V _PWM is generated. Note that the pulse signal V _PWM is at a low level during a period in which V _OSC is larger.

As will be described later, in the normal state, the NA
One input of the ND circuit 36 is always kept at a high level. Therefore, the NAND circuit 36 inverts the pulse signal V _PWM and applies it as a drive signal V _B to the base of the drive transistor 32 as shown in FIG. During the drive signal V _B is at a low level, the drive transistor 32 to conduct the output transistor 31 conducts. As a result, the output transistor 31 repeats conduction / interruption, and outputs an intermittent current I _OUT from the output terminal OUT as shown in FIG.

The current I _OUT is supplied to the output transistor 31
Is supplied to the load 6 via the coil 11 of the smoothing circuit 4 during the conduction. On the other hand, the output transistor 31
In the cutoff period, the energy stored in the coil 11 during the conduction period is returned, and as shown in FIG.
The current _ID flows through the catch diode 12. The current _ID is supplied to the load 6 via the coil 11. As a result, as shown in FIG. 3F, during the conduction period and the interruption period of the output transistor 31, the coil 11
, A current _IL flows. Further, the output capacitor 13
Absorbs the AC component of the current I _L , so that a smooth output current I _O is output from the load terminal O of the constant voltage power supply circuit 1.

On the other hand, the voltage V _{O at} the load terminal O is divided by the voltage dividing circuit 5 and applied to the regulator IC 3 as the feedback voltage V _ADJ . Further, the error amplifier 33
Outputs an error voltage V _ERR corresponding to the error between the feedback voltage V _ADJ and the reference voltage V _REF1 . Further, as shown in FIG. 3B, the frequency and amplitude of the triangular wave V _OSC in the normal state are constant. Therefore, the conduction period w of the output transistor 31 is increased or decreased by the output voltage V _ERR of the error amplifier 33.

For example, when the output voltage V _O becomes smaller than a desired value V _C due to a change in the input voltage V _IN or the load 6, the output voltage V _ERR of the error amplifier 33 increases, and the conduction period w of the output transistor 31 increases. Becomes longer. on the other hand,
If the value becomes larger than the desired value V _C , the error voltage V _ERR becomes smaller, and the conduction period w becomes shorter.

Output transistor 31 according to the present embodiment
The switching frequency is the same as the frequency of the triangular wave V _OSC and is constant while the load terminal O is not short-circuited.
Therefore, the duty of the output transistor 31 changes in the same manner as in the conduction period w. Note that the duty is the conduction period w / cycle T of the output transistor 31.

Thus, when the output voltage V _O changes slightly due to a change in the input voltage V _IN or the load 6, the regulator IC 3 can change the duty of the output transistor 31 so as to suppress the change. As a result, the output voltage V _O of the constant voltage power supply circuit 1 is maintained at a desired value V _C during the normal state.

The voltage value V _C is equal to the resistance 1 of the voltage dividing circuit 5.
It is set by the resistance values R14 and R15 of 4.15, and as shown in the following equation (1), V _C = V _REF1 × (R14 + R15) / R14 (1) Also, the duty D of the output transistor 31
Is as follows: D = (V _C + V _f ) / (V _IN −V _{CE (sat)} + V _f ) (2) In the above equation (2), _Vf is a quasi-directional voltage of the catch diode 12, and _{Vf
CE (sat)} is a collector-emitter saturation voltage of the output transistor 31.

Incidentally, the constant voltage power supply circuit 1 controls the duty D of the output transistor 31 so that the output voltage V _O can be maintained at a desired value V _C. Therefore, when an extremely heavy load 6 is connected to the load terminal O, or when the load terminal O is short-circuited, an excessive current may flow through the output transistor 31 in order to maintain the voltage value V _C. There is. This overcurrent is caused by the output transistor 31 itself or the constant voltage power supply circuit 1 such as the coil 11 and the catch diode 12 of the smoothing circuit 4.
May be damaged. Therefore,
For a chopper regulator like the constant voltage power supply circuit 1,
Overcurrent protection is essential.

Hereinafter, a configuration for realizing the overcurrent protection function and an operation at the time of overcurrent protection will be described in detail.

That is, as shown in FIG. 1, the regulator IC 3 further includes an overcurrent detecting resistor 37 provided between the collector of the output transistor 31 and the input terminal IN, and the overcurrent detecting resistor 37. Overcurrent detection circuit (overcurrent detection means) 38 for detecting an overcurrent based on the voltage V _R across the
And a feedback voltage V
_A comparator (short-circuit detection means) 40 for detecting a short-circuit of the load terminal O based on _ADJ and transmitting the short-circuit to the overcurrent detection circuit 38 and the oscillator 34 is provided.

When the oscillator 34 receives the output signal V _CMP indicating a short circuit from the comparator 40, the oscillator 34 can set the oscillation frequency of the triangular wave V _OSC to be lower than that when no short circuit occurs. In the present embodiment, as an example of both oscillation frequencies, the frequency at the time of non-short circuit is set to 100 kHz, and the frequency at the time of short circuit is set to 50 kHz.

The overcurrent detection circuit 38 monitors the voltage V _R across the overcurrent detection resistor 37. When an overcurrent is detected, the high-level set signal V is sent to the set terminal S of the flip-flop 39. Can output _SET . Further, when detecting an overcurrent, the overcurrent detection circuit 38 can set different detection levels for a short circuit and a non-short circuit. The detection level at the time of short-circuiting is set lower than the detection level at the time of non-short-circuiting.
While receiving the output signal V _CMP indicating a short circuit from the controller, it can be determined that an overcurrent occurs even with a smaller current.

More specifically, for example, as shown in FIG. 4, the overcurrent detection circuit 38 supplies a constant voltage source 51 for generating a predetermined voltage V _S and a voltage V _R across the overcurrent detection resistor 37. A detection unit 52 that outputs currents I _C1 and I _C2 having a corresponding difference,
Based on the ratio of the two currents I _C1 · I _C2, a determination unit 53 whether the end-to-end voltage V _R reaches the detection level, in accordance with the instruction of the determination unit 53, the set signal V to the flip-flop 39 An output unit 54 for outputting _SET , and a comparator 4
When an output signal V _CMP indicating a short circuit is received from 0,
A constant current source 55 for adding a predetermined value of current I _C3 to the current I _C2 ;

The detecting section 52 has a current mirror circuit composed of pnp transistors Q11 and Q12 whose bases are connected to each other. Transistor Q1
A collector and a base are connected to each other;
The emitter is connected to one end of the overcurrent detection resistor 37 on the output transistor 31 side. On the other hand, transistor Q
The twelve emitters are connected to the other end of the overcurrent detection resistor 37.

The determination section 53 includes a current mirror circuit including npn-type transistors Q21 to Q23 whose bases are connected to each other. Transistor Q
21 collectors are connected to the base, and
The said collector and base, a constant voltage V _S is applied from the constant voltage source 51 through a resistor R21. On the other hand, the collector of the transistor Q22 is connected to the collector of the transistor Q11 provided in the detection unit 52. Similarly, the collector of transistor Q23 is connected to the collector of transistor Q12. Note that the emitters of the transistors Q21 to Q23 are grounded.

The emitter area of each of the transistors Q21 to Q23 is set such that the emitter area of the transistor Q23 is larger by a predetermined factor than the remaining transistors Q21 and Q22. Hereinafter, a case where the emitter area of each of the transistors Q21 to Q23 is 1: 1: 4 will be described as an example of the emitter area. Since the collector current required for normal operation of the transistor increases as the emitter area increases, as shown in FIG. 5, the transistor Q23 is connected to the remaining transistors Q21 and Q21 in order to operate normally.
It requires four times the collector current as compared with 22.

In addition, the output section 54 is connected to an npn transistor which is turned on / off in accordance with the collector potential of the transistor Q23.
Transistors Q31 and Q32. Specifically, the base of the transistor Q31 is
Is connected to the collector of the transistor Q23. A constant voltage V _S is applied to the collector of the transistor Q31 from the constant voltage source 51 via the resistor R31. Further, the collector of the transistor Q31 and the resistor R3
The connection point of the transistor 1 is connected to the base of the transistor Q32. The voltage V _S is applied to the collector of the transistor Q32 from the constant voltage source 51 via the resistor R32. The connection point between the transistor Q32 and the resistor R32 is connected to the flip-flop 39 shown in FIG.
Is connected to the set terminal S. The emitters of both transistors Q31 and Q32 are grounded.

Further, the constant current source 55 is connected to the output voltage V _{CMP of the} comparator 40 shown in FIG.
It comprises a pn transistor Q41 and pnp transistors Q42 and Q43 whose bases are connected to each other, and a current mirror circuit for transmitting the collector current of the transistor Q41 to the determination section 53.
The collector of the transistor Q42 is connected to its own base, and further connected to the collector of the transistor Q41 via a resistor R41. on the other hand,
The collector of the transistor Q43 is connected to the collector of the transistor Q23 in the determination section 53. Note that the emitter of each transistor Q42 · Q43, are applied from the constant voltage source 51 a predetermined voltage V _S is, the emitter of the transistor Q41 is grounded.

When a high-level output signal V _CMP is applied from the comparator 40 to the base of the transistor Q41, the transistor Q41 conducts, and its collector current I _C3 is supplied to the current mirror circuit composed of the transistors Q42 and Q43. The signal is output to the transistor Q23 of the determination unit 53 via the output terminal. The collector current I
_C3 is determined by the output voltage V _S of the constant voltage source 51, the resistance value of the resistor R41, and the like. On the other hand, the output signal V
_{When the CMP} goes low, the transistor Q41 is shut off, and no collector current flows. Therefore, the constant current source 55 stops operating and does not supply current to the determination unit 53. As a result, the constant current source 55 starts operating only when the output signal V _CMP is at a high level, that is, when the comparator 40 detects a short circuit,
A predetermined current I _C3 can be supplied to

In the overcurrent detection circuit 38 having the above configuration,
When the current flowing through the output transistor 31 is small, the voltage drop of the overcurrent detection resistor 37, that is, the voltage V _R
Is also low. In this state, in the detection unit 52, the base-emitter voltages V _BE (Q12) and V _BE (Q11) of both transistors Q11 and Q12 are equal to V _BE (Q1
2) ΔV _BE (Q11). Both transistors Q
Since the emitter areas of 11 and Q12 are the same, the collector currents I _C1 and I _C2 of both transistors Q11 and Q12 are
_C2 = _IC1 .

The current I _C1 is controlled by the current I _C0 flowing through the current mirror circuit of the judgment section 53. That is, the collector current I _C0 of the transistor Q21 is equal to the output voltage V _S of the constant voltage source 51 and the resistance R21.
And is maintained at a constant value.
In the present embodiment, since the emitter areas of the transistors Q21 and Q22 are kept the same, I _C0 = I _C1 . Therefore, when the voltage V _R is small, I
_C0 = _IC1 = _IC2 .

On the other hand, in the judgment section 53, the emitter area of the transistor Q23 is equal to the remaining transistor Q21 ·
It is set to be larger than Q22. Therefore, in this state, although the transistors Q21 and Q22 are operating normally, the transistor Q23 cannot operate normally. Specifically, the transistor Q23
As shown in FIG. 5, when the collector current is I _C1 , the collector-emitter voltage V _CE (Q23) becomes V _CE (Q2
3) = V ₀ <0.7V This value is less than the required V _BE = 0.7 V for driving the transistor Q31 connected to the collector of the transistor Q23.

As a result, in the output section 54, the transistor Q31 is turned off and the transistor Q3
The base potential of 2 becomes _VS. Therefore, transistor Q32 conducts, and its collector potential, ie, set signal V _SET is kept at a low level.

On the other hand, when the current flowing through the output transistor 31 increases, the voltage V _R across the overcurrent detection resistor 37 also increases. Here, the collector current I _C1 of the transistor Q11 is the same as the constant current I _C0 of the determination unit 53,
It is kept constant. Therefore, transistor Q11
, The base-emitter voltage V _BE remains unchanged at about 0.7V. On the other hand, the emitter potential of the transistor Q12 is compared to the transistors Q11, is higher by V _R,
The base-emitter voltage V _BE (Q
12), the V _R + V _BE. Therefore, as shown in FIG. 6, the collector current I _C2 of the transistor Q12 increases sharply with an increase in the voltage V between both ends.

The current I _C2 increases, and the current I _C (Q23) flowing to the transistor Q23 of the judgment unit 53 becomes 4 ·
When I _C1 is exceeded, as shown in FIG. 5, V _CE (Q23)>
0.7V. This value is the V _{BE of} transistor Q31.
(Q31) is over 0.7V, and the transistor Q31 of the output unit 54 can be sufficiently driven.

As a result, in the output section 54, the transistor Q31 is turned on, and the resistor R31 is connected to the transistor Q31.
31 collector currents flow. Therefore, the resistance R31
, The base potential of the transistor Q32 drops, and the transistor Q32 is cut off. As a result, the collector potential of the transistor Q32, that is, the set signal V _SET goes high.

In the overcurrent detecting circuit 38, the detecting section 52
Increases the current I _C2 according to the increase of the voltage V _R. The determining unit 53 determines whether the current I _C (Q23) flowing through the transistor Q23 has exceeded a predetermined value. Further, the output unit 54 sends the set signal V to the flip-flop 39 shown in FIG.
_{Send SET} . The current I _C (Q23) is detected by the detecting unit 5
2 and the current I _C2 · I _C3 supplied from the constant current source 55, and if I _C3 does not change, I _C (Q23)
_Varies with I _C2 . Therefore, the overcurrent detection circuit 38 can notify the flip-flop 39 whether or not the current flowing through the output transistor 31 has exceeded the detection level.

Further, the constant current source 55 supplies the current I
_C3 is supplied, and no current is supplied when not short-circuited. Therefore, at the time of a short circuit, even if the current I _C2 is smaller, the current received by the determination unit 53 exceeds the above-described predetermined value. As a result, at the time of short-circuit, the detection level for detecting overcurrent can be reduced as compared with the case of no short-circuit.

More specifically, when a short circuit occurs, the judgment unit 53
The current is supplied from both the transistor Q12 of the detection unit 52 and the transistor Q43 of the constant current source 55 to the transistor Q23. Therefore, as shown in FIG. 7, the collector currents from the transistors Q12 and Q43 are changed to I _C (Q12) and I _C (Q43), respectively.
Then, the current I supplied to the transistor Q23 is
_C (Q12 + Q43) is the sum of the two. Therefore, the current I _C (Q12) required to make transistor Q23 sufficiently conductive is kept at a relatively low value.
In the figure, I _C (Q23) is a load line of the transistor Q23.

On the other hand, at the time of non-short circuit, constant current source 55 is not operating, and only transistor Q12 is connected to transistor Q12.
23 is supplied with current. Therefore, as shown in FIG. 8, the current I _C (Q12) required to make transistor Q23 sufficiently conductive is relatively larger than I _C (Q12) shown in FIG.

As a result, the determination section 53 supplies a current to the output section 54 even when the collector current I _C2 is smaller in a short circuit than in a non-short circuit. Therefore,
The collector current I _C2 at which the judging section 53 starts to supply the current and the voltage V _R across the overcurrent detection resistor 37 are lower in the short circuit state than in the non-short circuit state.

Thus, the overcurrent detection circuit 3 shown in FIG.
8 is an output signal V _CMP from the comparator 40 indicating a short circuit.
Is notified, the detection level for detecting the overcurrent can be set lower than when no short circuit occurs. Further, the overcurrent detection circuit 38 detects the current flowing through the output transistor 31 based on the voltage V _R across the overcurrent detection resistor 37, and, as shown in FIG. , A high-level set signal V _SET can be applied to the set terminal S of the flip-flop 39.

On the other hand, in the flip-flop 39, the reset terminal R is connected to the oscillator 34, and receives a reset signal from the oscillator 34 every one cycle of the triangular wave _VOSC . The inverted output terminal Q of the flip-flop 39 is connected to one input of the NAND circuit 36. As a result, the flip-flop 39 outputs the low-level signal from the reception of the high-level set signal V _SET to the reception of the reset signal.
Circuit 36.

In the comparator 40, the inverting input terminal is connected to the feedback terminal ADJ, and the feedback voltage V _ADJ is applied. The non-inverting input terminal receives a constant reference voltage V from the constant voltage source 40a.
_REF2 is applied. Thereby, the comparator 40
When the feedback voltage V _ADJ becomes lower than the reference voltage V _REF2 , a high-level output signal V _CMP is applied to the oscillator 34 and the overcurrent detection circuit 38 to short-circuit the load terminal O to both members 34 and 38. Can be notified.

The operation of each unit in the constant voltage power supply circuit 1 having the above configuration when overcurrent is detected will be described below. In the description, the constant voltage power supply circuit 1
FIG. 10 showing the characteristics of the output voltage I _O with respect to the output voltage V _O of FIG.

First, at the time of non-short circuit, the regulator I
C3 is based on the feedback voltage V _ADJ as described above.
The duty of the output transistor 31 is controlled to keep the output voltage V _O at a constant value V _C. In this state, the feedback voltage V _ADJ is not lower than V _REF2 . As a result, the comparator 40 sets the output signal V _CMP to the low level, and notifies the oscillator 34 and the overcurrent detection circuit 38 that the load terminal O is not short-circuited.

Therefore, the oscillation frequency of the oscillator 34, that is, the switching frequency of the output transistor 31, is set to a higher frequency than that at the time of short circuit. Also,
In the overcurrent detection circuit 38, the constant current source 55 shown in FIG. 4 does not supply the current I _C3 . As a result, the detection level at the time of overcurrent detection is set to a higher level than the detection level at the time of short circuit.

The overcurrent detection circuit 38 monitors the voltage V _R across the overcurrent detection resistor 37 and outputs
It is determined whether the current flowing through is excessive. For example, the output current I _O is I _S0 following areas shown in FIG. 10,
The voltage V _R across the overcurrent detection resistor 37 is increased or decreased by the current flowing through the output transistor 31, but does not exceed the detection level. Therefore, as shown in FIG. 1, the overcurrent detection circuit 38 applies the low-level set signal V _SET to the flip-flop 39. As a result, the conduction of the output transistor 31 is not prevented, and the constant voltage power supply circuit 1 can keep the output voltage V _O at the predetermined value V _C without any problem.

On the other hand, when the voltage V _IN of the input power supply 2 increases or the load 6 becomes heavy, the output transistor 31
Increases, the voltage V _R across the overcurrent detection resistor 37 also increases. The overcurrent detection circuit 38 sets the set terminal S of the flip-flop 39 when the voltage V _R increases and exceeds the non-short detection level.
, A high-level signal is applied. A reset signal is applied to the reset terminal R of the flip-flop 39 from the oscillator 34 every cycle. Thus, when an overcurrent is detected, the flip-flop 39 performs a pulse-by-pulse operation, and continuously applies a low-level signal to the NAND circuit 36 for one cycle. As a result, N
The AND circuit 36 controls the base potential V _B of the driving transistor 32 regardless of the output of the PWM comparator 35.
Keep high level. Accordingly, in the switching cycle, the conduction of the output transistor 31 is blocked. Although the flip-flop 39 is reset every cycle, the overcurrent detection circuit 38 continues to send the low-level set signal V _SET to the flip-flop 39 while detecting the overcurrent. , The conduction of the output transistor 31 is blocked.

In this state, the conduction time of the output transistor 31 becomes shorter than the instruction of the PWM comparator 35. As a result, the constant voltage power supply circuit 1 is protected from overcurrent. Thereafter, when the cause of the overcurrent is eliminated and the current flowing through the output transistor 31 falls below the detection level, the overcurrent detection circuit 38 applies the low-level set signal V _SET to the flip-flop 39. As a result, conduction of the output transistor 31 is resumed, and the constant voltage power supply circuit 1
Return to normal status.

In general, when the cause of the overcurrent is an increase in the input voltage V _IN or a change in the load 6, the output current _IO changes slowly and the period of the overcurrent state is short. Further, when the overcurrent state is resolved, it is required that the regulator IC3 immediately returns to a normal state and resumes the duty control.

In the constant voltage power supply circuit 1 according to the present embodiment,
While the comparator 40 does not detect a short circuit, the oscillator 3
The oscillation frequency of No. 4 and the detection level of the overcurrent detection circuit 38 are kept high as in the conventional case. Therefore, the overcurrent detection circuit 38 can immediately detect a decrease in the current flowing through the output transistor 31. Further, the cycle when the flip-flop 39 performs the pulse-by-pulse operation is set to be short as in the related art, and when the overcurrent detection circuit 38 reports that the overcurrent state has been eliminated, the flip-flop 39 performs the NAND operation. It is possible to immediately instruct the circuit 36 to restart the duty control. As a result, the constant voltage power supply circuit 1
Can immediately return to a normal state when the overcurrent state is resolved.

On the other hand, when the load terminal O is short-circuited, the output voltage V _O of the constant voltage power supply circuit 1 decreases, and accordingly, the feedback voltage V _ADJ also decreases. Further, since the difference between the input voltage V _IN and the output voltage V _O increases, the current flowing through the output transistor 31 greatly increases. Therefore,
Similarly to the overcurrent state at the time of non-short circuit, the overcurrent detection circuit 38
Detects an overcurrent and causes the flip-flop 39 to prevent the output transistor 31 from conducting.

In this state, the conduction time of the output transistor 31 decreases, and the constant voltage power supply circuit 1
_O and can not be kept at a constant value V _C. When the conduction of the output transistor 31 is blocked, the current flowing through the output transistor 31 decreases. Overcurrent detection circuit 38
Restarts conduction of the output transistor 31 when the current falls below the detection level. However, when the output voltage V _O decreases, the PWM comparator 35 sets the error amplifier 3
According to the instruction of No. 3, the duty of the output transistor 31 is controlled to be high. Therefore, at the time of overcurrent, the output current I _O of the constant voltage power supply circuit 1 increases as the output voltage V _O decreases (in FIG. 10, the output voltage V _O becomes V
_{From C} to _VREF2 ).

Further, the output voltage V _O and the feedback voltage V
_{When ADJ} decreases and the feedback voltage V _ADJ falls below the reference voltage V _REF2 , the comparator 40 sets the output signal V _CMP to a high level and sets the oscillator 34 and the overcurrent detection circuit 38.
(In FIG. 10, the output voltage V _O is V
_{At REF2} ).

According to this, the oscillator 34 converts the oscillation frequency and inputs a reset signal to the reset terminal R of the flip-flop 39 at a frequency lower than the frequency when no short circuit occurs. This causes the flip-flop 39 to output the pulse
The period of the bi-pulse operation becomes longer, and the conduction of the output transistor 31 is blocked for a longer period. As a result, as shown in FIG. 10, the output current I _O of the constant voltage power supply circuit 1 drops significantly when the output voltage V _O falls below V _REF2 .

On the other hand, the overcurrent detection circuit 38 outputs the output signal V
_{When the CMP} goes high, the detection level at the time of overcurrent detection is changed to a value lower than that at the time of non-short circuit. Further, the overcurrent detection circuit 38 detects the voltage V _R across the overcurrent detection resistor 37.
Exceeds the detection level, the output transistor 3
1 is blocked.

By the way, even when the overcurrent detection level is set low, as the output voltage V _O decreases, the output current I _O increases. However, since the detection level is set low, the overcurrent detection circuit 38 outputs the output transistor 3 until the output current _IO becomes lower.
1 is suppressed. Therefore, the slope of the output current I _O with respect to the output voltage V _O becomes steeper than when no short circuit occurs.

As a result, in the overcurrent protection characteristic shown in FIG. 10, the skirt portion is greatly reduced as compared with the conventional characteristic (broken line). Further, the current I _S1 at the time of short-circuit is reduced as compared with the conventional short-circuit current I _S2 , and is suppressed to be equal to or less than the output current I _O at the time of short-circuit detection.

Since the short-circuit current due to the drooping characteristic can be suppressed, the heat generation of the output transistor 31 at the time of short-circuit can be greatly suppressed as compared with the related art. As a result, the safety of the regulator IC 3 is improved. Further, for example, a member for cooling the regulator IC 3 such as a radiation fin becomes smaller or unnecessary. Further, since the current rating of the output transistor 31 can be reduced,
For example, it becomes easy to form another member constituting the regulator IC 3 such as the error amplifier 33 and the output transistor 31 in the same package. Further, for example, the current rating of a member externally attached to the regulator IC 3 such as the catch diode 12 and the coil 11 constituting the smoothing circuit 4 can be reduced. As a result, a smaller and cheaper constant voltage power supply circuit 1 can be provided.

In the present embodiment, the overcurrent detection circuit 38 shown in FIG. 4 has been described as an example, but the present invention is not limited to this. For example, the overcurrent detection circuit 38 outputs one of two different reference voltages according to the instruction of the comparator 40, and compares the reference voltage with the voltage V _R across the overcurrent detection resistor 37. Thus, it may be determined whether the current flowing through the output transistor 31 is excessive. When the comparator 40 notifies the short circuit, the overcurrent detection circuit 38
If the detection level for detecting overcurrent can be reduced as compared with the case where no short circuit occurs, the same effect as that of the present embodiment can be obtained.

However, the above configuration has a problem that it is difficult to lower both reference voltages. For example, by using a transistor, when comparing the end-to-end voltage V _R and the two reference voltages, the base of the transistor - is limited to emitter voltage can not be set both reference voltage too low.
Therefore, the higher reference voltage tends to be higher than the reference voltage when detecting an overcurrent with a conventional chopper regulator. As a result, it is necessary to set the resistance value of the overcurrent detection resistor 37 higher than before, and increase the voltage V _R across the overcurrent detection resistor 37. However, a voltage drop in the overcurrent detection resistor 37 causes a loss in the constant voltage power supply circuit 1. Therefore, when the resistance value of the overcurrent detection resistor 37 is increased, power consumption increases.

On the other hand, the overcurrent detection circuit 38 shown in FIG. 4 includes a detection unit 52 that outputs a current I _C2 corresponding to the voltage V _R at both ends, and a constant supply of a predetermined current I _C3 when a short circuit occurs. A current source 55 and a determination unit 53 that detects an overcurrent based on whether the current supplied from the detection unit 52 and the constant current source 55 exceeds a predetermined value are provided.

In this configuration, when a short circuit occurs, the detecting unit 5
2 and the constant current source 55 supply a current to the determination unit 53. Therefore, while maintaining the resistance value of the overcurrent detection resistor 37 as in the related art, the detection level at the time of short-circuit is lower than that at the time of non-short-circuit Can be reduced. As a result, overcurrent can be detected at two different detection levels without increasing power consumption.

Although the output transistor 31 according to the present embodiment is formed integrally with the regulator IC 3, the present invention is not limited to this, and the output transistor 31 may be provided separately. However, when both are formed integrally, the number of components constituting the regulator IC 3 is reduced, so that it is possible to reduce labor in handling and manufacturing.

When the current flowing to the output transistor 31 is large, heat is generated, and it is difficult to form the output transistor 31 and the remaining circuit of the regulator IC 3 such as the error amplifier 33, for example. However, in the present embodiment, the output current at the time of overcurrent can be reduced and the short-circuit current can be reduced as compared with the related art. Therefore, heat generation of the output transistor 31 is reduced, and the current rating can be set low. As a result, the output transistor 31 and the remaining members can be integrally formed without any problem.

Further, in this embodiment, the short-circuit current is reduced, and the current rating of the coil and the like constituting the smoothing circuit 4 can be reduced. Therefore, a member externally attached to the regulator IC 3 can also be reduced in size. As a result, a small and inexpensive constant voltage power supply circuit 1 can be realized.

[0111]

As described above, the control device for the chopper regulator according to the first aspect of the present invention controls the duty of the switching element so as to stabilize the output voltage, and the current flowing to the switching element. Current detection means for detecting the current value, the detected current value is compared with a preset overcurrent detection level, and when it is detected that an excessive current is flowing to the switching element Blocking the conduction of the switching element
The switching element is cut off, and the current is detected.
If the current value detected by the means falls below the overcurrent detection level
An overcurrent detecting means for resuming conduction of the switching element, an oscillator for instructing the control means to shut off the switching element upon detection of an overcurrent, and an output voltage falling below a predetermined level. Short-circuit detection means for lowering the oscillation frequency of the oscillator, wherein the over-current detection means sets the over-current detection level, and when the short-circuit detection means detects a short circuit , the short-circuit detection means
Unlike the case where no short circuit is detected,
Is set lower than when no short circuit is detected.

In the above configuration, the hem portion of the overcurrent protection characteristic can be reduced as compared with the related art, so that heat generation of the switching element at the time of overcurrent can be suppressed. Therefore, there is an effect that the safety of the chopper regulator can be improved.

Further, since the base portion is reduced as compared with the related art, the short-circuit current can be reduced, and the current that can be supplied by the chopper regulator can be suppressed when no short-circuit occurs. As a result, the current rating of the members constituting the chopper regulator can be set lower. As a result,
This has the effect of greatly contributing to cost reduction and size reduction of the chopper regulator.

As described above, the control device for the chopper regulator according to the second aspect of the present invention generates a signal when an overcurrent is detected.
Pulse-by-pulse operation at the cycle of the
Medium, flip-flop for preventing conduction of the switching element
Control means for controlling the duty of the switching element so that the output voltage is stabilized, current detection means for detecting a current flowing to the switching element, and a current value detected by the current detection means. Is compared with a preset overcurrent detection level, and when it is detected that an excessive current is flowing to the switching element,
Blocking the conduction of the switching element,
And the current detected by the current detecting means.
When the value falls below the overcurrent detection level, the switching
Overcurrent detection means for restarting the conduction of the element, if the output voltage falls below a predetermined level, a configuration in which a short circuit detecting means for reducing the oscillation frequency of the oscillator.

In the above configuration, when the short circuit occurs,
Since both the wave number and the overcurrent detection level decrease,
As in item 1, the base of the overcurrent protection characteristics has been reduced.
it can. This allows safe, low-cost and compact
A regulator can be realized.

According to a third aspect of the present invention, in the control device for a chopper regulator according to the first or second aspect of the present invention, the overcurrent detecting means is provided in accordance with a current flowing through the switching element. An overcurrent detection resistor that increases or decreases the passing current, a detection unit that outputs a current corresponding to a voltage between both ends of the overcurrent detection resistor, and a predetermined value when the short circuit detection unit detects a short circuit. A constant current source that outputs a current, and a determination unit that determines whether the sum of the currents supplied from the detection unit and the constant current source exceeds a predetermined value and detects an overcurrent. Configuration.

In the above configuration, when a short circuit occurs, the current is supplied from both the constant current source and the detection unit to the determination unit, so that the resistance value of the overcurrent detection resistor can be suppressed as in the related art. As a result, it is possible to detect an overcurrent at two different detection levels without increasing power consumption.

A chopper regulator according to a fourth aspect of the present invention has a configuration in which the control device of the chopper regulator according to the first, second or third aspect and the switching element are formed in one package.

In the above configuration, since the skirt portion is reduced in the overcurrent characteristics, the rating of the external members such as the catch diode and the coil can be set low. Furthermore, since the number of components constituting the chopper regulator can be reduced, the handling and manufacturing labor can be reduced. As a result, there is an effect that the entire chopper regulator can be reduced in size as compared with the related art.

[Brief description of the drawings]

FIG. 1, showing an embodiment of the present invention, is a block diagram illustrating a main part of a step-down chopper regulator.

FIG. 2 is a schematic view showing a package of the step-down chopper regulator.

FIG. 3 is a waveform diagram showing an operation of each unit in a normal state in the step-down chopper regulator.

FIG. 4 is a circuit diagram showing a configuration example of an overcurrent protection circuit in the step-down chopper regulator.

FIG. 5 is a graph showing I _C -V _CE characteristics of a transistor constituting a determination unit in the overcurrent protection circuit.

FIG. 6 is a graph showing I _C -V _BE characteristics of a transistor constituting a detection unit in the overcurrent protection circuit.

FIG. 7 is a graph showing the relationship between the collector characteristics of the transistors forming the detection section and the transistors forming the constant current source, and the load lines of the transistors forming the determination section in the overcurrent protection circuit. .

FIG. 8 is a graph showing a relationship between a collector characteristic of a transistor included in the detection unit and a load line of a transistor included in the determination unit in the overcurrent protection circuit.

FIG. 9 is a waveform diagram showing an input signal to a set terminal when an overcurrent is detected in the flip-flop of the step-down chopper regulator.

FIG. 10 shows the step-down chopper regulator.
4 is a graph showing overcurrent protection characteristics.

FIG. 11 shows a conventional example, and is a block diagram illustrating a main part of a step-down chopper regulator.

FIG. 12 shows the step-down chopper regulator according to the present invention.
4 is a graph showing overcurrent protection characteristics.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 constant voltage power supply circuit (chopper regulator) 3 integrated circuit for step-down type chopper regulator (control device) 4 smoothing circuit 31 output transistor (switching element) 33 error amplifier (control means) 34 oscillator (oscillator; control means) 35 PWM comparator (Control means) 36 NAND circuit (Control means) 37 Overcurrent detection resistor 38 Overcurrent detection circuit (Overcurrent detection means) 39 Flip-flop (Control means) 40 Comparator (Short circuit detection means) 52 Detection section 53 Judgment section 55 Constant Current source

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-150763 (JP, A) JP-A-5-336743 (JP, A) JP-A-1-281353 (JP, A) JP-A-9-99 140126 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H02M 3/155 H02H 9/02 H02H 9/08

Claims (4)

(57) [Claims] 1. A control device for a chopper regulator, wherein a current supplied from an input side is intermittently switched by a switching element, and then smoothed by a smoothing circuit and output, wherein the switching is performed so that an output voltage is stabilized. Control means for controlling the duty of the element; current detection means for detecting a current flowing to the switching element; and a current value detected by the current detection means, which is compared with a preset overcurrent detection level. When it is detected that an excessive current is flowing to the switching element ,
Blocking the conduction of the switching element and
And the current detected by the current detecting means is
When the level is lower than the overcurrent detection level, the switching element
Overcurrent detection means for resuming conduction of the oscillator, an oscillator for instructing the control means to shut off the switching element when an overcurrent is detected, and oscillation of the oscillator when an output voltage falls below a predetermined level. Short-circuit detecting means for lowering the frequency. The over-current detecting means sets the over-current detection level to the short-circuit detecting means when the short-circuit detecting means detects a short circuit.
Unlike when the stage does not detect a short circuit,
A control device for a chopper regulator, wherein the setting is made lower than when a short circuit is not detected by the means . 2. After the current supplied from the input side and intermittently at the switching element, a control device for a chopper regulator and outputting the smoothed by the smoothing circuit, upon detection of overcurrent, a cycle of oscillator Pulse by Pal
The switching element is turned on during the period.
Includes a flip-flop to prevent, so that the output voltage is stabilized, and control means for controlling the duty of the switching element, a current detecting means for detecting a current flowing to the switching element at the current detecting means, detecting The detected current value is compared with a preset overcurrent detection level, and when it is detected that an excessive current is flowing to the switching element, the switching is performed .
Blocking the conduction of the switching element and
And the current value detected by the current detecting means is
When the output current falls below the overcurrent detection level, the switching element
Overcurrent detection means to restart conduction, if the output voltage falls below a predetermined level, and a short circuit detection means for decreasing the oscillation frequency of the oscillator, said overcurrent detection means, the excessive the current detection level, when the <br/> short circuit detection means detects a short circuit, the short circuit detection hand
Unlike when the stage does not detect a short circuit,
A control device for a chopper regulator, wherein the control device sets a value lower than a case where a short circuit is not detected by the means . 3. An overcurrent detecting means, comprising: an overcurrent detecting resistor whose passing current increases and decreases according to a current flowing through the switching element; and a current corresponding to a voltage between both ends of the overcurrent detecting resistor. A detection unit that outputs a constant current source that outputs a current of a predetermined value when the short circuit detection unit detects a short circuit; and a sum of currents supplied from the detection unit and the constant current source is a predetermined value. 3. The control device for a chopper regulator according to claim 1, further comprising: a determination unit configured to determine whether or not the current value has exceeded the threshold value and detect an overcurrent. 4. A chopper regulator, wherein the control device for the chopper regulator according to claim 1, 2 or 3, and the switching element are formed in one package.