JP2007336731A - Overcurrent protective circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein it is difficult to prevent even the threshold for operating an overcurrent protective circuit to be set and prevent adverse effects on a control target circuit, when the current of the threshold flows for a long time, although this threshold is set larger than the normal current of the control target circuit, and to protect the control target circuit. <P>SOLUTION: A current I1 flowing through transistors Q1, Q2 is detected; and when the current I1 exceeds a reference current (I2+I3) to be taken in by a reference current circuit 18, a protective control circuit 22 is operated. Upon the start of its operation, by turning on a transistor Q14 to stop either of two current supply circuits constituting the reference current circuit 18, the protective control circuit 22 lowers the reference current to I2. The protective control circuit 22 suppresses the current to I2, which is lower than the threshold (I2+I3) at the start of operation, and the possible adverse effects on the control target circuit can be prevented regarding overcurrent. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an overcurrent protection circuit that protects an electric circuit from overcurrent.

  For example, when configuring a device with an integrated circuit (IC) with a built-in low-frequency amplifier, the output terminal of the low-frequency amplifier can be accidentally shorted to the power supply, ground potential, and other terminals. There is sex. In such a case, the current flowing through the output transistor of the low-frequency amplifier becomes larger than that in the normal state, which may cause a disadvantage that the IC is destroyed. Therefore, an overcurrent protection circuit is built in the IC.

  FIG. 3 is a circuit diagram of a low-frequency amplifier having a conventional overcurrent protection circuit. The overcurrent protection circuit detects the currents flowing through the output transistors Q1 and Q2 of the low frequency amplifier using a current mirror circuit using the transistors Q5 and Q6, and the total current I1 becomes a predetermined threshold current I4. Is increased, the bias currents of the output transistors Q1 and Q2 are controlled to prevent further increase of I1.

  The overcurrent protection circuit includes a transistor Q9, a protection control circuit 2, and a reference current circuit 4. Total current I1 flows toward the base of npn transistor Q9. The reference current circuit 4 is a current mirror circuit provided with a constant current source for the output current I4 on the input side, and the collector of the transistor Q11 on the output side of the current mirror circuit is connected to the base of Q9. Q9 has a function of a switch for turning on / off the operation of the protection control circuit 2, and is normally off. In a normal state, the current I1 flows through the collector of Q11 and becomes a current flowing through the output side path of the current mirror circuit of the reference current circuit 4. When the current I1 is smaller than I4, the reference current circuit 4 can absorb all of I1, so that Q9 is kept off. However, when I1 increases and reaches I4 due to abnormal connection or the like, The voltage rises and Q9 is turned on.

  When Q9 is turned on, for example, the protection control circuit 2 controls the operation of the amplifier circuit 6 of the low frequency amplifier to operate the bias currents of Q1 and Q2, and the total value I1 of the currents flowing through Q1 and Q2 is I4. Control not to exceed.

FIG. 4 is a graph showing changes in the output current of the low frequency amplifier provided with this overcurrent protection circuit. In the same figure, a graph showing the operation state of the protection control circuit 2 is also shown. Current flowing increases the output transistor Q1 or Q2 to be abnormal connection such as a short circuit of the output terminal OUT is generated, the output current I _OUT flowing through the output terminal OUT is increased. Correspondingly, I1 reaches a limit value corresponding to I4. Then, the protection control circuit 2 is operated to suppress operation of controlling so I _OUT does not exceed I4 is started (time t1). When the output current I _OUT is less than I4 Q9 returns to the OFF state, the suppressing operation by the overcurrent protection circuit is canceled (time t2).

  For example, in a low-frequency amplifier, a relatively large output current can flow at the moment when the volume is high even in a normal use state. The current limit value I4 of the overcurrent protection circuit is set with a certain margin so as not to suppress the large current that may occur in such a normal use state. Therefore, the current limit value can be relatively large. With this current limit value, it can be avoided that the circuit to be protected is immediately destroyed, but there is a problem in that if the state continues for a long time, the circuit may be destroyed or deteriorated due to heat generation or the like.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an overcurrent protection circuit that prevents the circuit from being destroyed or deteriorated even when an abnormal connection or the like is not immediately resolved. .

  An overcurrent protection circuit according to the present invention includes a current detection circuit that detects a control target current flowing in a target circuit, a reference current circuit that generates a reference current, and a current value of the control target current that is a current value of the reference current. A control circuit that performs a suppression operation for controlling the target circuit so as not to exceed the target circuit, and protects the target circuit from overcurrent, wherein the reference current circuit uses the first current as the reference current. Switching between outputting a current having a value and a current having a second current value lower than the first current value, and the control circuit increases the current to be controlled to the first current value. The suppression operation for the target circuit is started, and the reference current is switched from the first current value to the second current value. While an abnormal connection or the like is occurring, an unnecessary current is generated as the current to be controlled, and the control value is kept at the limit value full state. According to the configuration of the present invention, unnecessary current is reduced by lowering the upper limit after the start of the suppression operation.

  In the overcurrent protection circuit, when the control target current becomes less than the second current value, the suppression operation is stopped and the reference current is switched from the second current value to the first current value. A control circuit can be configured. According to this configuration, the overcurrent protection circuit returns to a state in which the current fluctuation below the first current value is enabled in conjunction with the cancellation of the abnormal connection or the like, and normal operation with a large dynamic range is restored. .

  In the overcurrent protection circuit according to the present invention, the reference current circuit has two constant current sources connected in parallel, and the control circuit stops one of the constant current sources during the suppression operation. It can be configured.

  According to the present invention, the first current value, which is a threshold value at which the suppression operation is started, can be set large, and a margin can be secured so that the suppression operation is not started unnecessarily with respect to the normal operation. On the other hand, after the start of the suppression operation, by limiting the control target current with the second current value smaller than the first current value as an upper limit, unnecessary current while abnormal connection or the like is continued is reduced. Further, it is possible to prevent the circuit from being destroyed or deteriorated due to, for example, the heat generation of the circuit due to the continuous large current.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

FIG. 1 is a schematic circuit diagram of a low frequency amplifier according to an embodiment of the present invention. In this circuit, an overcurrent protection circuit 12 is added to the original low-frequency amplifier 10 portion. The low frequency amplifier 10 is configured between the positive voltage source VCC and the ground potential GND. Transistors Q1 and Q2 are output power transistors. Q1 is composed of a pnp transistor, and has an emitter connected to VCC and a collector connected to the output terminal OUT. Q2 is composed of an npn transistor, and has an emitter connected to GND and a collector connected to the output terminal OUT. These Q1, Q2 performs a current amplification in response to the bias current supplied to the base, respectively, to generate an output current I _OUT from the output terminal OUT.

  The bias current supplied to the bases of Q1 and Q2 is controlled according to the differential output of the amplifier circuit 14. One of the differential output terminals of the amplifier circuit 14 is connected to the base of the transistor Q3, and the other is connected to the base of the transistor Q4. Q3 and Q4 are connected in series with each other's emitter-collector between the bases of Q1 and Q2. A resistor R1 is connected between the base of Q1 and VCC, and a resistor R2 is connected between the base of Q2 and GND.

The overcurrent protection circuit 12 uses the current flowing through the output transistors Q1 and Q2 of the low-frequency amplifier 10 as a main current to be controlled, a current detection circuit 16 for detecting this, a reference current circuit 18 for generating a reference current, and Q1 , Q2 and a control circuit 20 that performs a suppression operation for controlling the low-frequency amplifier 10 so that the sum of the emitter-collector currents I _CE1 and I _CE2 does not exceed the current value of the reference current.

The current detection circuit 16 is configured using transistors Q5 to Q8. Q5 forms a current mirror circuit with Q1, detects the emitter-collector current _ICE1 of Q1, and generates a current corresponding to _ICE1 at the output terminal of the current detection circuit 16. On the other hand, the emitter-collector current _ICE2 of Q2 is detected by Q2 and Q6 constituting a current mirror circuit. The detected current _ICE2 is turned back by a current mirror circuit composed of Q7 and Q8, and is output to the output terminal of the current detection circuit 16 with the same direction as the current output from Q5. As a result, a current I1 obtained by adding _ICE1 and _ICE2 is output to the output terminal of the current detection circuit 16.

  The reference current circuit 18 has a configuration in which two current supply circuits are provided in parallel. The first current supply circuit includes a constant current source A1 having an output current I2 and transistors Q10 and Q11. A1 is connected to the collector of Q10. Q10 and Q11 constitute a current mirror circuit, and currents flowing through the emitter of Q10 in the input side path and the emitter of Q11 in the output side path become equal. The collector of Q11 on the output side path becomes the output terminal of the reference current circuit 18. Q11 draws current from the output terminal. Q11 can draw a current up to an amount corresponding to I2 as a collector current.

  The second current supply circuit includes a constant current source A2 having an output current I3 and transistors Q12 to Q14. A2 is connected to the collector of Q13. Q13 and Q12 constitute a current mirror circuit, and currents flowing through the emitter of Q13 in the input side path and the emitter of Q12 in the output side path become equal. The collector of Q12 on the output side path is connected to the collector of Q11 of the first current supply circuit and constitutes the output terminal of the reference current circuit 18. Q12 draws current from the output terminal. Q12 can draw a current up to an amount corresponding to I3 as a collector current. The bases of Q12 and Q13 are configured to be grounded to GND via Q14. Q14 is controlled to be turned on / off in accordance with a control signal to the base. In the on state, Q12 and Q13 bases are short-circuited to GND, and Q12 and Q13 are turned off.

  That is, the reference current circuit 18 can draw a current corresponding to the generated reference current from the output terminal, and has a current drawing capability according to (I2 + I3) when Q14 is in an off state, while Q14 is on When in the state, it has a current drawing capability according to I2. The on / off state of Q14 is controlled by the control circuit 20 described below.

  The control circuit 20 includes a transistor Q9 and a protection control circuit 22. Q9 has a function of a switch for turning on / off the operation of the protection control circuit 22. The output terminal of the current detection circuit 16 and the output terminal of the reference current circuit 18 are connected to the base of Q9. When the current I1 flowing out from the current detection circuit 16 is within the current drawing capability of the reference current circuit 18, the current I1 is basically drawn into the reference current circuit 18, thereby keeping Q9 in the off state. On the other hand, when the current I1 reaches the current drawing capability of the reference current circuit 18 and attempts to exceed it, Q9 is turned on.

The protection control circuit 22 controls the low frequency amplifier 10 so that I1 does not exceed the limit current set by the reference current circuit 18. For example, the protection control circuit 22 controls the operation of the amplifier circuit 14, controls the bias currents of Q1 and Q2, and suppresses the emitter-collector currents I _CE1 and I _CE2 flowing in Q1 and Q2, respectively. So that I1 does not exceed the limit current. The current suppression operation by the protection control circuit 22 functions when Q9 is on, and is stopped when Q9 is off. Further, the protection control circuit 22 outputs a control signal to the base of Q14 together with the start of the suppression operation to turn on Q14.

Next, the overcurrent control operation in the present low frequency amplifier will be described with reference to FIG. FIG. 2 shows the time change of whether the overcurrent protection circuit 20 (or the protection control circuit 22) is in the operating state (ON state) or the stopped state (OFF state), and the output terminal OUT of the low frequency amplifier 10. It is a typical graph which shows the time change of the flowing output current _IOUT . In the figure, the horizontal axis is the time axis, and time passes to the right.

  In the initial state, the output terminal OUT of the low frequency amplifier 10 is in a normal state. In this state, I1 is basically drawn into the reference current circuit 18, so that Q9 is off and the protection control circuit 22 is also off. Further, since the protection control circuit 22 is in the off state, Q14 is also kept in the off state, the first current supply circuit and the second current supply circuit of the reference current circuit 18 operate together, and the current of the reference current circuit 18 is The supply capacity is (I2 + I3).

In this state, when the terminal OUT is short-circuited with the other terminals and wiring (time _{t0), I OUT} begins to _increase, in conjunction with the _{I OUT} I1 also increases. The first limit value I _H of the reference current circuit 18 in accordance with the amount retractable current (I2 + I3) until I _OUT reaches the, Q9 is off. _However, if the _{I OUT} reaches the limit value _{I H,} Q9 is turned on, the protection control circuit 22 starts the current control operation (time t1). Further, the protection control circuit 22 turns on Q14, whereby the reference current circuit 18 stops the function of one of the current supply circuits, and the current supply capability is lowered to I2.

Protection control circuit 22 executes a suppressing operation of the I _OUT a second limit value I _L corresponding to a current supply capability value I2 which is the lowered as a target value. As a _{result, I OUT} is reduced or suppressed to _{I L} (time t2). Then, the protection control circuit 22 is followed by a short-circuit state of the terminal OUT, and while it by trying to flow excessive _{I OUT,} kept _on, continue to suppress the _{I OUT} to _{I L} (period t2 to t3) .

When the short circuit of the terminal OUT is eliminated, I _OUT can be less than I _L (period t3 to t4). When the timing when I _OUT becomes less than I _L occurs, Q 9 returns to the off state, the protection control circuit 22 is turned off, and the current suppressing operation is released. The Q14 is restored to the OFF state, by this, the limit value of the current suppressing set for the overcurrent protection circuit 12 is first limit value I _H again. Thus, in conjunction with a short circuit is in the normal operating condition is eliminated, since the return to the limit value is high state, I _OUT is it is avoided that limited by low limit value I _L in the normal operating state A suitable low-frequency amplification operation with a large dynamic range is realized.

  Since the overcurrent protection circuit of the present invention is thus set to a relatively high current limit value until an overcurrent due to abnormal connection or the like is detected, the normal operation of the control target circuit is unnecessarily limited. Is avoided. On the other hand, after the overcurrent is detected, the current limit value is lowered to prevent the circuit from being destroyed or deteriorated due to heat generation or the like.

The second threshold I _L may be same as the first threshold value I _H, set to at least the maximum value of the output current signal at terminal OUT in the normal operation of the low-frequency amplifier 10. The second threshold value I _L may be set to such a value that the output current signal exceeds temporarily in normal operation. Be set in this way, the detection of the overcurrent is conducted on the basis of the high threshold I _H, and after changing the threshold I _L, if Shojire output current signal even temporarily in a state below this threshold from returning again to a high value I _H, the overcurrent protection circuit 12 is low normal suppress possible output signals unnecessarily during operation of the low-frequency amplifier 10.

1 is a schematic circuit diagram of a low-frequency amplifier according to an embodiment of the present invention. It is a graph explaining operation | movement of the overcurrent protection circuit in the low frequency amplifier which is embodiment of this invention. It is a circuit diagram of the low frequency amplifier provided with the conventional overcurrent protection circuit. It is a graph explaining operation | movement of the low frequency amplifier provided with the conventional overcurrent protection circuit.

Explanation of symbols

  10 Low frequency amplifier, 12 Overcurrent protection circuit, 14 Amplifier circuit, 16 Current detection circuit, 18 Reference current circuit, 20 Control circuit, 22 Protection control circuit.

Claims (3)

A current detection circuit for detecting a control target current flowing in the target circuit; a reference current circuit for generating a reference current; and controlling the target circuit so that a current value of the control target current does not exceed a current value of the reference current A control circuit that performs a suppression operation, and an overcurrent protection circuit that protects the target circuit from an overcurrent.
The reference current circuit is configured to be capable of switching between outputting a current of a first current value and a current of a second current value lower than the first current value as the reference current,
The control circuit starts the suppression operation for the target circuit when the control target current increases to the first current value, and switches the reference current from the first current value to the second current value;
An overcurrent protection circuit. The overcurrent protection circuit according to claim 1,
The control circuit stops the suppression operation and switches the reference current from the second current value to the first current value when the control target current becomes less than the second current value.
An overcurrent protection circuit. In the overcurrent protection circuit according to claim 1 or 2,
The reference current circuit has two constant current sources connected in parallel,
The control circuit stops one of the constant current sources during the suppression operation;
An overcurrent protection circuit.

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