JP6198442B2 - Constant current protection circuit - Google Patents

Description

  The present invention relates to a constant current protection circuit, and more particularly to a circuit for improving response speed.

As a constant current circuit and an output current protection circuit thereof, a circuit configuration well known in the art includes, for example, that shown in FIG.
The conventional circuit will be described below with reference to FIG.
First, the constant current circuit 20A is constituted by a constant voltage source 22A, a constant current driving error amplifier 21A, a constant current driving transistor Qd, and a current setting resistor R1 as main components.
In the constant current circuit 20A, the constant current driving transistor Qd is controlled so that the reference voltage VREF1 set by the constant voltage source 22A is equal to the feedback voltage VREF2 corresponding to the output current.

  The output current of the constant current circuit 20A is set as a value obtained by dividing the feedback voltage VREF2 by the current setting resistor R1, and the output current is, for example, a load circuit 10A composed of a plurality of LEDs 11A connected in series. It is to be supplied to.

An overcurrent protection circuit 30A as described below is added to the constant current circuit 20A.
The overcurrent circuit 30A includes the current sense transistor Qs, the current sense resistor R2, and the comparator 31A as main components.
In the overcurrent circuit 30A, the current sense transistor Qs forms a current mirror with the constant current drive transistor Qd of the constant current circuit 20A, and the current of the constant current drive transistor Qd is current mirrored via the current sense transistor Qs. Thus, a current that is twice the size ratio of the transistors (Qd size: Qs size) flows through the current sense resistor R2.

The voltage VREF5 corresponding to the voltage drop generated in the current sense resistor R2 is input to the non-inverting input terminal of the comparator 31A, and this voltage VREF5 is the reference voltage VREF3 set by the constant voltage source 32A connected to the inverting input terminal. If exceeded (see FIG. 13B), the operation of the constant current drive error amplifier 21A is limited by the comparator 31A. As a result, the operation of the constant current drive transistor Qd is similarly limited, and the output current ILED is limited to a predetermined set value as shown in FIG.
Examples of this type of protection circuit include those disclosed in Patent Document 1 and the like.

JP 2009-193414 A (page 4-8, FIGS. 1 to 6)

  However, in the above-described conventional circuit, the comparator 31A starts operating after the output current becomes a current value equal to or greater than the threshold value, and the output current is limited. In order to limit the output current in this way, not only the occurrence of a delay time due to passing through the comparator 31A, but also the detection of an overcurrent due to factors such as the phase compensation capacity inside the constant current circuit 20A, There is also the occurrence of a delay time in the current limiting operation of the current circuit 20A.

For this reason, since the current limitation by the overcurrent protection circuit 30A is not performed during the delay time, the output current increases.
In such a case, if the output terminal voltage Vout fluctuates abruptly for some reason, as shown in the timing chart of FIG. 14, an output current (see FIG. 14B) greater than the set value flows to the load. Will lead to problems.

As described above, the cause of the steep fluctuation of the output terminal voltage Vout is, for example, when the power supply is switched from the battery power supply to the AC adapter and the power supply voltage is switched, or when the power supply voltage is switched sharply due to noise superposition. It may occur.
Also in the conventional circuit as shown in FIG. 8, problems of output voltage fluctuation and output current overcurrent occur as described below.
The circuit shown in FIG. 8 is configured so that the number of LED lamps of the load can be switched. Except for this point, the other configuration is the same as the conventional circuit shown in FIG.

In this conventional circuit, for example, when a white LED is used, the forward voltage VF is about 3 to 4 V, the power supply voltage of the LED is lowered for some reason, and the voltage applied to the LED is VF. If it falls below, the output terminal voltage Vout becomes 0 V, and current cannot flow.
Therefore, in this conventional circuit, in order to avoid turning off all the LEDs, an input voltage monitoring circuit 12 for detecting a decrease in the power supply voltage is provided, whereby the output terminal voltage Vout can be secured by switching the number of LED lamps. I am doing so.
However, since the output terminal voltage Vout varies by VF corresponding to the number of switched LED lamps, an overcurrent problem occurs as in the conventional circuit of FIG.

  The present invention has been made in view of the above circumstances, and provides a constant current protection circuit capable of limiting an output current at a high response speed.

In order to achieve the above object of the present invention, a protection circuit for a constant current circuit according to the present invention comprises:
A protection circuit for a constant current circuit having a constant current driving transistor provided so that a constant current output can be supplied to a load via a drain by controlling a gate voltage ,
A current sense transistor having a gate terminal in common with the constant current driving transistor and having a current value determined by a size ratio with the constant current driving transistor;
A voltage fluctuation monitoring circuit that detects a steep voltage fluctuation due to a transient response of the drain terminal of the constant current driving transistor through capacitive coupling ;
While detecting a steady overcurrent of the constant current drive transistor by the current sense transistor,
The voltage fluctuation monitoring circuit detects a transient overcurrent generated by a sudden voltage increase at the drain terminal of the constant current drive transistor, and depending on the detection result, the gate voltage of the constant current drive transistor is reduced, or The gate voltage of the constant current driving transistor can be limited.

According to the present invention, since the output current is limited when a steep rise in the output voltage is detected, the output current is stopped at a faster response speed than in the past, or The restriction can be made, and the effect of more reliable and accurate circuit protection can be obtained.
Also, unlike the conventional case, even when the power supply voltage is not applied to the constant current circuit and the output terminal voltage rises sharply, the constant current protection circuit operates and circuit protection can be achieved. .
In addition, by using a parasitic capacitance of a transistor for capacitive coupling, a circuit can be configured without using a capacitor, and a layout area can be reduced.
Further, in the configuration in which the circuit operation is controlled in accordance with the feedback signal corresponding to the output current, the constant current circuit is configured so that when the fluctuation of the drain voltage of the constant current driving transistor is detected, the constant current circuit corresponds to the feedback signal. By stopping the circuit operation of the current circuit, it is possible to prevent an overshoot of the output current when returning from the output current stop state, and to secure a stable circuit operation.
In addition, since the clamp circuit is connected to a node where the fluctuation of the drain voltage of the constant current drive transistor is detected through capacitive coupling, the voltage of the node is clamped to a certain voltage or less, so that There is an effect that the low withstand voltage can be easily reduced.

It is a circuit diagram which shows the example of a basic circuit structure of the constant current protection circuit in embodiment of this invention. It is a circuit diagram which shows the more concrete 1st circuit structural example of the constant current protection circuit in embodiment of this invention. It is a circuit diagram which shows the more concrete 2nd circuit structural example of the constant current protection circuit in embodiment of this invention. It is a circuit diagram which shows the more concrete 3rd circuit structural example of the constant current protection circuit in embodiment of this invention. It is a circuit diagram which shows the more concrete 4th circuit structural example of the constant current protection circuit in embodiment of this invention. It is a circuit diagram which shows the more specific 5th circuit structural example of the constant current protection circuit in embodiment of this invention. It is a circuit diagram which shows the circuit example of the protection circuit of the conventional constant current circuit. It is a circuit diagram which shows the other circuit example of the protection circuit of the conventional constant current circuit. FIG. 9 is a timing chart showing a change example of a signal of a main part in the first and third specific circuit configuration examples shown in FIG. 2 and FIG. 4, and FIG. 9A is a timing chart showing a change in the output terminal voltage; FIG. 9B is a timing chart showing changes in load current, and FIG. 9C is a timing chart showing changes in the gate voltage of the output current control transistor. FIGS. 10A and 10B are timing charts showing examples of changes in signals of main parts in the second specific circuit configuration example shown in FIG. 3, FIG. 10A is a timing chart showing changes in output terminal voltage, and FIG. FIG. 10C is a timing chart showing changes in the gate voltage of the output current control transistor. FIGS. 11A and 11B are timing charts showing an example of changes in signals of main parts in the fourth specific circuit configuration example shown in FIG. 5, FIG. 11A is a timing chart showing changes in output terminal voltage, and FIG. FIG. 11C is a timing chart showing a change in voltage at a node where a change in the drain voltage of the constant current driving transistor is detected by capacitive coupling. FIGS. 12A and 12B are timing charts showing an example of changes in signals of main parts in the fifth specific circuit configuration example shown in FIG. 6, FIG. 12A is a timing chart showing changes in output terminal voltage, and FIG. FIG. 12C is a timing chart showing changes in the gate voltage of the output current control transistor. FIG. 13A is a timing chart showing a change example of signals of a main part of the conventional circuit shown in FIG. 7, FIG. 13A is a timing chart showing a change in load current, and FIG. 13B constitutes an overcurrent circuit. It is a timing chart which shows the change of the source voltage of a current sense transistor. FIG. 14A is a timing chart showing a change example of a signal of a main part of the conventional circuit shown in FIG. 8, FIG. 14A is a timing chart showing a change in output terminal voltage, and FIG. 14B shows a change in load current. FIG. 14C is a timing chart showing a change in the source voltage of the current sense transistor constituting the overcurrent circuit.

Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 6 and FIGS. 9 to 12.
The members and arrangements described below do not limit the present invention and can be variously modified within the scope of the gist of the present invention.
First, an example of a basic circuit configuration of a constant current protection circuit according to an embodiment of the present invention will be described with reference to FIG.
FIG. 1 shows a basic circuit configuration example of a constant current protection circuit 40 according to an embodiment of the present invention, together with a constant current circuit 20 to be protected and an overcurrent protection circuit 30 thereof.

First, the constant current circuit 20 is configured to be able to supply a constant current to the load circuit 10, and includes a constant current driving error amplifier 21, a constant current driving transistor (indicated as "Qd" in FIG. 1) 1, and The current setting resistor (indicated as “R1” in FIG. 1) 15 and a constant voltage source 22 are configured, and the basic configuration is the same as that of the conventional circuit.
Hereinafter, a specific circuit configuration will be described. First, the positive side of the constant voltage source 22 is connected to the non-inverting input terminal of the constant current driving error amplifier 21 using an operational amplifier or the like. The negative electrode side is connected to the ground.

  The output terminal of the constant current drive error amplifier 21 is connected to the gate of the constant current drive transistor 1. In the embodiment of the present invention, an N-channel MOS FET is used as the constant current driving transistor 1, and the load circuit 10 is connected to the drain thereof, while the source is connected to the ground via the current setting resistor 15. In addition, it is connected to the inverting input terminal of the constant current driving error amplifier 21.

  The load circuit 10 is formed by connecting a plurality of LEDs 11 in series. Among the plurality of LEDs 11 connected in series, the anode at one end is a power source (not shown) and the cathode at the other end is driven by a constant current. Each transistor is connected to the drain.

The overcurrent protection circuit 30 performs current limitation so that the output current of the constant current circuit 20 does not exceed a predetermined current, and includes a current sense transistor (indicated as “Qs” in FIG. 1) 2, a current sense resistor (Indicated as “R2” in FIG. 1) 16, a comparator 31, and an overcurrent protection constant voltage source 32, and the basic configuration is the same as that of a conventional circuit. .
Hereinafter, a specific circuit configuration will be described. First, an N-channel MOS FET is used for the current sense transistor 2, and its drain is connected to the drain of the constant current drive transistor 1 constituting the constant current circuit 20. Similarly, the gate is connected to the gate of the constant current driving transistor 1.

The source of the current sense transistor 2 is connected to the source of the constant current drive transistor 1 via the current sense resistor 16 and to the non-inverting input terminal of the comparator 31.
The comparator 31 has an inverting input terminal connected to the positive side of the overcurrent protection constant voltage source 32 and the negative side of the overcurrent protection constant voltage source 32 connected to the ground.
Further, the output terminal of the comparator 31 is connected to the control terminal of the constant current driving error amplifier 21, and the operation of the constant current driving error amplifier 21 can be controlled according to the output of the comparator 31.

The operations of the constant current circuit 20 and the overcurrent protection circuit 30 described above are basically the same as those of the conventional circuit, and will be briefly described below.
First, in the constant current circuit 20, the constant current drive transistor 1 is a constant current drive error amplifier 21 so that the reference voltage VREF1 set by the constant voltage source 22 and the feedback voltage VREF2 corresponding to the output current are the same. It is controlled by.
The output current of the constant current circuit 20 is set as a value obtained by dividing the feedback voltage VREF2 by the current setting resistor 15, and the output current is supplied to the load circuit 10.

  On the other hand, in the overcurrent protection circuit 30, when the voltage of the current sense transistor 2 exceeds the reference voltage VREF3 set by the overcurrent protection constant voltage source 32, the comparator 31 causes the constant current drive error amplifier 21 to operate. Limited. As a result, the operation of the constant current drive transistor Qd is similarly limited, and the output current ILED is limited to a predetermined set value.

Next, the constant current protection circuit 40 limits the output current of the constant current circuit 20 at a faster response speed than the overcurrent protection circuit 30, and is a voltage fluctuation monitoring circuit (“V-MON” in FIG. 1). (Notation) 41 and an output current control circuit (indicated as “I-CON” in FIG. 1) 42.
In such a configuration, when the output terminal voltage Vout of the constant current circuit 20 sharply increases, the voltage monitoring circuit 41 detects an increase in the output terminal voltage Vout, and the detection signal is input to the output current control circuit 42. The output current control circuit 42 limits the output current.

FIG. 2 shows a more specific first specific circuit configuration example. Hereinafter, the first specific circuit configuration example will be described with reference to FIG.
The same components as those shown in FIG. 1 are denoted by the same reference numerals, detailed description thereof is omitted, and different points will be mainly described below.
This first specific circuit configuration example shows a specific circuit configuration example of the voltage fluctuation monitoring circuit 41 and the output current control circuit 42. The constant current circuit 20, the overcurrent protection circuit 30, and the load circuit 10 are shown in FIG. Is the same as the configuration example shown in FIG.

More specifically, the output terminal voltage monitoring capacitance element (indicated as “C1” in FIG. 2) 18 and the output terminal voltage monitoring resistor (indicated as “R3” in FIG. 2) 17 are used to provide a voltage fluctuation monitoring circuit 41. The output current control circuit 42 is configured by an output current control transistor (indicated as “Q1” in FIG. 2) 5.
The output terminal voltage monitoring capacitive element 18 has one end connected to the drains of the constant current drive transistor 1 and the current sense transistor 2, and the other end connected to the ground via the output terminal voltage monitoring resistor 17. It has become a thing.

The connection point between the output terminal voltage monitoring capacitor 18 and the output terminal voltage monitoring resistor 17 is connected to the gate of the output current control transistor 5.
In this circuit configuration example, an N-channel MOS FET is used as the output current control transistor 5.
Further, the drain of the output current control transistor 5 is connected to the gate of the constant current driving transistor 1 together with the gate of the current sense transistor 2, while the source is connected to the ground.
The capacitance value of the output terminal voltage monitoring capacitor 18 is, for example, 4 pF, and the resistance value of the output terminal voltage monitoring resistor 17 is about 100 kΩ.

Next, the operation in this configuration will be described.
For example, in the constant current circuit 20, if the output terminal voltage Vout shows a voltage change equal to or higher than the threshold voltage of the output current control transistor 5 at a rising speed of several tens of μs or less, the capacitively coupled capacitive element for output terminal voltage monitoring 18, the gate voltage VREF4 of the output current control transistor 5 rises, and as a result, the output current control transistor 5 enters an operating state.
When the output current control transistor 5 is in the operating state, the gate voltage VGATE of the constant current drive transistor 1 is lowered, and supply of the output current to the load circuit 10 by the constant current drive transistor 1 is stopped.

In this circuit configuration example, as shown in the timing chart of FIG. 9, the circuit operates when the output terminal voltage Vout rises (see FIG. 9A) (FIG. 9B and FIG. 9). 9 (C)), the protection operation is performed more quickly than the conventional circuit.
Further, for example, in the conventional circuit having the configuration as shown in FIG. 7, when the output terminal voltage Vout rises sharply when the power supply voltage is not applied to the constant current circuit 20A, the overcurrent protection circuit 30A does not operate, the gate voltage rises via the drain-gate capacitance of the constant current drive transistor Qd, and an unintended current may flow to the load circuit 10A.

  On the other hand, in the circuit according to the embodiment of the present invention, even when the power supply voltage is not applied to the constant current circuit 20, the operation of the output current control transistor 5 causes the constant current drive transistor 1 to operate. Since the gate voltage VGATE is lowered, the output current is surely stopped.

Next, a second specific circuit configuration example will be described with reference to FIG.
The same components as those shown in FIGS. 1 and 2 are denoted by the same reference numerals, detailed description thereof will be omitted, and different points will be mainly described below.
This second specific circuit configuration example uses an output terminal voltage monitoring transistor (indicated as “Q2” in FIG. 3) 6 instead of the output terminal voltage monitoring capacitive element 18 in the configuration shown in FIG. It is what was made up.

  That is, the output terminal voltage monitoring transistor 6 using an N-channel MOS FET has its gate and source connected to each other, and its connection point is connected to the ground via the output terminal voltage monitoring resistor 17. , Drains are connected to the drains of the constant current drive transistor 1 and the current sense transistor 2. The drain of the constant current driving transistor 1 is connected to the gate of the output current control transistor 5 through the parasitic capacitance of the output terminal voltage monitoring transistor 6, and the output terminal voltage monitoring transistor in FIG. The same function as that of the capacitive element 18 is achieved.

A voltage adjusting transistor 7 using an N-channel MOS FET is diode-connected.
That is, the voltage adjusting transistor 7 has its gate and drain connected to each other, and its connection point is connected to the gate of the current sense transistor 2, while its source is connected to the drain of the output current control transistor 5. It has been made.

Next, the operation in this configuration will be described.
For example, in the constant current circuit 20, if the output terminal voltage Vout shows a voltage change equal to or higher than the threshold voltage of the output current control transistor 5 at a rising speed of several tens of μs or less, the parasitic capacitance of the output terminal voltage monitoring transistor 6 causes As a result, the gate voltage VREF4 of the output current control transistor 5 rises, and as a result, the output current control transistor 5 enters an operating state.
When the output current control transistor 5 is in the operating state, the gate voltage VGATE of the constant current drive transistor 1 is lowered, and supply of the output current to the load circuit 10 by the constant current drive transistor 1 is limited.

  In the second specific circuit configuration example, by using the parasitic capacitance of the transistor, it is not necessary to use a capacitive element, and the layout area can be reduced correspondingly. Further, when the output terminal voltage Vout is a high voltage, the configuration using the capacitive element needs to be physically increased in order to secure the withstand voltage of the capacitive element. The layout area can be reduced by the amount that the capacitive element is unnecessary.

  Further, since the voltage adjusting transistor 7 in the diode connection state is used, only the threshold voltage Vt of the voltage adjusting transistor 7 is applied to the gate voltage VGATE of the constant current driving transistor 1. Therefore, unlike the first specific circuit configuration example shown in FIG. 2, when the output terminal voltage Vout suddenly rises as shown in FIG. 10, the output current is not stopped but the output current is not stopped. Is limited to a certain size (see FIGS. 10A and 10B).

Next, a third specific circuit configuration example will be described with reference to FIG.
The same components as those shown in FIGS. 1 and 2 are denoted by the same reference numerals, detailed description thereof will be omitted, and different points will be mainly described below.
In this third specific circuit configuration example, the specific circuit configuration of the constant current circuit 20 is different as described below, and only the constant current protection circuit 40 is used without using the overcurrent protection circuit 30 (see FIG. 2). It is comprised as follows.

The constant current circuit 20 includes the constant current driving transistor 1 in the same way as the above-described specific circuit configuration examples, except that the configuration does not use output feedback.
That is, the constant current circuit 20 is provided with a current mirror transistor (indicated as “Q4” in FIG. 4) 8 using an N-channel MOS FET, and its gate and drain are connected to each other. The connection point is connected to the gate of the constant current driving transistor 1.

The drain of the current mirror transistor 8 is supplied with a current from a constant current source 23 connected to a power source (not shown), while the source is connected to the ground.
The current mirror transistor 8 and the constant current drive transistor 1 constitute a current mirror.
The constant current protection circuit 40 is the same as the circuit configuration previously shown in FIG. 2, and detailed description thereof will be omitted here.

Next, the operation in this configuration will be described.
In the constant current circuit 20 in this circuit configuration example, when the output terminal voltage Vout increases sharply, the gate voltage increases via the parasitic capacitance C2 between the drain and gate of the constant current driving transistor 1 (see FIG. 4). Although the output current increases, the constant current protection circuit 40 can protect the circuit as described below.

That is, in the constant current circuit 20, if the output terminal voltage Vout shows a voltage change equal to or higher than the threshold voltage of the constant current driving transistor 1 at a rising speed of several tens of μs or less (see FIG. 9A), capacitive coupling is performed. The gate voltage VREF4 of the output current control transistor 5 is increased by the output terminal voltage monitoring capacitor 18 (see FIG. 9C), and as a result, the output current control transistor 5 is in an operating state.
When the output current control transistor 5 is in an operating state, the gate voltage VGATE of the constant current drive transistor 1 is lowered, and supply of the output current to the load circuit 10 by the constant current drive transistor 1 is stopped (FIG. 9 (B)).

Next, a fourth specific circuit configuration example will be described with reference to FIG.
The same components as those shown in FIGS. 1 and 2 are denoted by the same reference numerals, detailed description thereof will be omitted, and different points will be mainly described below.
The fourth specific circuit configuration example is the same as the first specific circuit configuration example shown in FIG. 2, except that the constant current protection circuit 40 includes a current limit hold circuit (“I-HOL” in FIG. 5). (Notation) 43 is provided.

  That is, the current limit hold circuit 43 has an input stage connected to a connection point between the output terminal voltage monitoring capacitor 18 and the output terminal voltage monitoring resistor 17, while the output stage is an output current control transistor. 5 is connected to a terminal for inputting a signal for controlling the operation of the constant current drive error amplifier 21.

Next, the operation in this configuration will be described.
For example, in the constant current circuit 20, when the output terminal voltage Vout changes in voltage at a rising speed of several tens of μs or less, the voltage VREF 4 rises by the capacitively coupled output terminal voltage monitoring capacitor element 18.
When the voltage VREF4 rises, the current limit hold circuit 43 outputs a signal of a level corresponding to the logical value High, and as a result, the output current control transistor 5 enters an operating state and the constant current drive error amplifier 21 Stopped.
On the other hand, when the output current control transistor 5 is in the operating state, the gate voltage VGATE of the constant current drive transistor 1 is lowered, and the supply of the output current to the load circuit 10 by the constant current drive transistor 1 is stopped. It becomes.

  For example, in the case where the operation of the constant current circuit 20 is controlled by the feedback signal as shown in FIG. 2, when the output current is stopped by reducing only the gate voltage of the constant current driving transistor 1, the constant current The drive error amplifier 21 operates so as to raise the gate of the constant current drive transistor 1 to a voltage higher than the steady state. For this reason, when returning from the protection operation, there is a possibility that the gate voltage is raised excessively and the overshoot is output before the control of the constant current circuit 20 is stabilized.

In the fourth specific circuit configuration example, the constant current drive error amplifier 21 and the constant current drive transistor 1 are stopped, and the feedback signal loop of the constant current circuit 20 is reset once to recover from the output current stop state. Occasionally output current overshoot is prevented.
Further, as shown in FIG. 11B, the current limit hold circuit 43 can secure an output stop time longer than a certain time, so that the constant current drive error amplifier 21 is surely stopped. .
This is because, when the output terminal voltage Vout changes sharply, for example, when the constant current protection circuit 40 operates for 100 ns or less, the protection operation is released before the charge is completely drawn from the constant current drive error amplifier 21. Therefore, it is effective to surely avoid such a situation.

As a method of providing the output stop time without using the current limit hold circuit 43, for example, it is conceivable to increase the time constants of the output terminal voltage monitoring capacitance element 18 and the output terminal voltage monitoring resistor 17. In the method, particularly when the output terminal voltage monitoring capacitive element 18 has a high breakdown voltage, there is a concern that the element size and the layout area are increased.
On the other hand, in the fourth specific circuit configuration example, by using the current limit hold circuit 43, the output stop time can be generated by a circuit with a small area, and the current limit hold circuit 43 can The reset time of the current circuit 20 can be adjusted.

Next, a fifth specific circuit configuration example will be described with reference to FIG.
The same components as those shown in FIGS. 1 and 2 are denoted by the same reference numerals, detailed description thereof will be omitted, and different points will be mainly described below.
In the fifth specific circuit configuration example, a clamp circuit (indicated as “CLAM” in FIG. 6) 44 is added to the constant current protection circuit 40 in the first specific circuit configuration example shown in FIG. It is provided.
That is, the clamp circuit 44 is connected to a connection point (node) between the output terminal voltage monitoring capacitor 18 and the output terminal voltage monitoring resistor 17. That is, the clamp circuit 44 is connected to a node where the fluctuation of the drain voltage of the constant current drive transistor 1 is detected through capacitive coupling.

Next, the operation in this configuration will be described.
For example, in the constant current circuit 20, if the output terminal voltage Vout shows a voltage change equal to or higher than the threshold voltage of the output current control transistor 5 at a rising speed of several tens of μs or less, the capacitively coupled capacitive element for output terminal voltage monitoring 18, the gate voltage VREF4 of the output current control transistor 5 rises, and as a result, the output current control transistor 5 enters an operating state.
When the output current control transistor 5 is in the operating state, the gate voltage VGATE of the constant current drive transistor 1 is lowered, and supply of the output current to the load circuit 10 by the constant current drive transistor 1 is stopped.

In the case of the fifth specific circuit configuration example, the clamp circuit 44 clamps the gate voltage VREF4 of the output current control transistor 5 to a predetermined voltage or lower (see FIG. 12C).
Therefore, in the constant current protection circuit 40, for example, a low withstand voltage element can be used for an output current control transistor, and a circuit configuration using a small element size can be realized.

  It can be applied to a constant current source circuit in which output current limitation with a faster response speed is desired.

DESCRIPTION OF SYMBOLS 1 ... Constant current drive transistor 2 ... Current sense transistor 5 ... Output control transistor 6 ... Output terminal voltage monitoring transistor 7 ... Voltage adjustment transistor 8 ... Current mirror transistor 18 ... Output terminal voltage monitoring capacitance element 20 ... Constant current Circuit 40 ... Constant current protection circuit

Claims (4)

A protection circuit for a constant current circuit having a constant current driving transistor provided so that a constant current output can be supplied to a load via a drain by controlling a gate voltage ,
A current sense transistor having a gate terminal in common with the constant current driving transistor and having a current value determined by a size ratio with the constant current driving transistor;
A voltage fluctuation monitoring circuit that detects a steep voltage fluctuation due to a transient response of the drain terminal of the constant current driving transistor through capacitive coupling ;
While detecting a steady overcurrent of the constant current drive transistor by the current sense transistor,
The voltage fluctuation monitoring circuit detects a transient overcurrent generated by a sudden voltage increase at the drain terminal of the constant current drive transistor, and depending on the detection result, the gate voltage of the constant current drive transistor is reduced, or A protection circuit for a constant current circuit, characterized in that the increase in the gate voltage of the constant current drive transistor can be limited. 2. The protection circuit for a constant current circuit according to claim 1 , wherein a parasitic capacitance of the transistor is used for capacitive coupling for detecting a fluctuation in drain voltage of the constant current driving transistor . The constant current circuit includes a constant current drive error amplifier that controls the operation of the constant current drive transistor according to a feedback signal corresponding to an output current, and when an overcurrent of the constant current drive transistor is detected 3. The protection of the constant current circuit according to claim 1, wherein the operation of the constant current drive error amplifier is stopped while the gate voltage of the constant current drive transistor is lowered. circuit. A clamp circuit is connected to the other node that is different from a node that is a connection point with the drain terminal of the constant current driving transistor, and a voltage at the connection point is a constant voltage. by clamping below, claim 1, characterized by being capable avoid application of overvoltage against the element connected via the capacitor, according to claim 2, or the constant current circuit according to claim 3, wherein Protection circuit.

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