JPS62150912A - Protection circuit - Google Patents

Abstract

PURPOSE:To ensure the protection of a power supply side output transistor (TR) by discharging an electric charge of a capacitor forcibly constituting a time constant circuit at the detection of an abnormarity so as to decrease the inoperation time of the protection circuit. CONSTITUTION:An abnormality of the power supply side output TR of an amplification circuit is detected, a power application time at abnormality is reduced to ensure the protection of the TR. Since a control signal VC goes to a low level at the occurrence of a abnormality, TRs Q24-Q26 are all turned off. A TR Q21 is turned off, a power application path from a resistor R1 to a capacitor C1 is interrupted and a TR Q22 is turned by a bias voltage supplied via a resistor R5 at the same time to apply forced discharge of the capacitor C1. Further, the TR Q23 is turned on and the TR Q32 is turned off. The operation at an abnormal state is executed immediately in this way and the time interval of non-operation of the protection circuit 21 is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an image stabilization circuit suitable for use in an amplifier circuit or the like that obtains a high output signal such as a power IC.

[Background technology]

The output circuit of a power IC is often configured as a push-pull power amplifier, and one of the failures of power amplifiers is short circuit protection. For example, if the intermediate terminal of a push-pull power amplifier is grounded or if the power supply voltage rises abnormally, the voltage between the collector and emitter of the output transistor on the power supply side will become large, and if this is left untreated, , the above transistor is destroyed. Regarding the above-mentioned short circuit protection, etc., please refer to "Analog Integrated Circuits" (4th edition published on February 1, 1980, published by Kindai Kagakusha, pp. 218--
219) for details.

By the way, the present inventor constructed a time constant circuit consisting of a resistor and a capacitor for the above-mentioned short circuit protection, and automatically energized the output transistor and cut off the power by using the charging time and discharging time when an abnormality is detected. can be done to
A so-called automatic reset type protection circuit was considered.

The protection operation will be explained with reference to FIG. 3. When an abnormality occurs at time %11, the protection circuit operates and the output transistor power supply (T+ connection) is cut off.

Next, the protection circuit becomes inoperable and the output transistor is energized in accordance with the time constant of the time constant circuit (17).
while). The above-mentioned protective operation is automatically repeated until the cause of the abnormality is removed. However, if the period of 12 is large, an abnormally high voltage will remain between the collector and emitter of the output transistor for a long time. The inventor's studies have revealed that the output transistor is likely to be destroyed if the voltage is not applied.

The above problem can be solved by shortening the time during which the protection circuit is inoperative, as indicated by T,' in FIG.

The present inventor has focused on the above points and has proposed the present invention.

[Purpose of the invention]

An object of the present invention is to provide a protection circuit that can reliably reduce destruction of transistors to be protected, such as output transistors.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[Summary of the invention]

A brief summary of the invention disclosed in this application is as follows:
The following electricity is applied.

In other words, the collector-emitter voltage of the output transistor on the power supply side of the amplifier circuit is detected by an abnormality detection circuit, and when an abnormality is detected, the capacitor that constitutes the time constant circuit is forcibly discharged to reduce the non-operation time of the protection circuit. By doing so, the object of the present invention, which is to reliably protect the power supply side output transistor, is achieved.

〔Example〕

Hereinafter, one embodiment of a protection circuit to which the present invention is applied will be described with reference to FIGS. 1 to 3. Note that FIG. 1 is a circuit diagram of an amplifier circuit provided with a protection circuit configured by an IC, and FIGS. 2 and 3 are characteristic diagrams for explaining the circuit operation.

The feature of this embodiment is that an abnormality is detected in the output transistor on the power supply side of the amplifier circuit, and the energization time in the event of an abnormality is shortened to ensure protection of the transistor.

First, referring to the overall circuit configuration, 1 is a preamplifier, and an input signal Vin is supplied through the 1st terminal. 2
is a class A amplifier, which drives the output circuit 3 at the next stage.

In the output circuit 3, D, , D are level sick, Darlington connected transistors QI%Q are power supply side output transistors, and Darlington connected transistors Q, , Q are ground side output transistors. be. Further, the constant current circuit C81 supplies a drive current to the transistor Q3.

In a steady state, everything from the preamplifier 1 to the output circuit 3 operates normally, and the output signal Vout is output from the No. 3 terminal.
is output, and the speaker 4, which is a load, is driven. Note that Ca is an output capacitor.

By the way, under normal conditions, the DC voltage level at terminal 3 is
It is 1/2■CC. However, if the No. 3 terminal is grounded as shown by the dotted line due to some kind of accident, the collector-emitter voltage Vce of the transistor Q becomes approximately Vcc.
rises to .

Such an abnormal voltage is compared with the reference voltage Vref by the abnormality detection circuit 11. The voltage level of the abnormality detection signal Vc changes from, for example, a normal level to a low level when an abnormality is detected.

What should be noted here is that the time constant control circuit 21 according to the present invention
The circuit operation is as follows.

The resistor R1 and the capacitor C5 correspond to the time constant circuit according to the present invention, and the transistor Q,1 corresponds to the first switching transistor according to the present invention, and the current is supplied to the capacitor C8, in other words, to the time constant circuit. This is for energizing and switching between power sources. and the first transistor QtI
The on/off control of is performed by the operating transistor Qt.

In addition, transistor Q! 4 operates in an on state when the control signal Vc is at a high level, in other words, during normal operation, and holds the transistor Qu in an off state.

Transistor Q2! corresponds to the second switching transistor in the present invention, and forcibly discharges the capacitor C1. The transistor Qvs corresponds to the third switching transistor according to the present invention, and controls the output voltage Vo to a low level and a high level.

During normal operation, the control signal Vc is at a high level, so the transistors Qt4 to Qta operate in an on state.

And transistor Q□ is not turned off, capacitor C8
There is no short circuit and charging is possible.

Since the transistor Q+ is not on and the transistor Qts is off, the capacitor CI is charged via the resistor R8, and the output voltage VO becomes high level.

On the other hand, the transistors Q8+1 and Ql+2 constitute a Schmitt circuit, and since the output voltage Vo is at a high level, the transistor Qay operates in an on state.

The resistance value of the resistor RI and the resistor R1 is Rh (R,.

is set to .

Therefore, the voltage Va is: ■a=-horse, /RI+(R,s+
R, 4) −(Vcc, −Vcesat Q, , )
In this case, a bias voltage sufficient to drive the output transistor Qss cannot be obtained.

The transistor Qas remains off, and the output signal is transmitted from the drive circuit 3 to the output circuit 4, and normal amplification operation is performed.

Next, the circuit operation when an abnormality occurs will be explained.

In this case, since the control signal Vc becomes low level, the transistors Q and 4 to Qta are all turned off.

Transistor Qz+ is not turned off, and at the same time, the current path from the resistor hole to capacitor C1 is cut off, and at the same time, transistor Qz+ is turned off. 2 is turned on by the bias voltage supplied via the resistor RIl, and the capacitor C1 is forced to discharge.

Furthermore, since the transistor Qts is also turned on by the bias voltage supplied through the resistor R8, the output voltage 0 remains at a low level and the transistor Q0 is turned off.

In this case, a bias voltage is supplied to the transistor Qs+ via the resistor R1, and the transistor Qs+ is turned on.

Since the resistances R,,,R1,t have the above relationship, the resistance R
+, 8. The amount of current flowing through R1 increases.

For convenience of explanation, the voltage Va when
The voltage determined by Vcct-VcesatQ□) is at a high level compared to the voltage Va.

The output transistor Qss operates in an on state, and the output terminal of the drive circuit 3 is almost grounded, and signal transmission to the output circuit 4 is cut off.

As a result, transistor Q is not driven and detection circuit 1
The control signal Vc obtained from 1 becomes high level again,
Transistor Q! <~Qta operates in the on state.

Since the capacitor CI has been discharged once, the charging time constant is almost constant, and the output voltage Vo is V as shown in Figure 3.
, the transistor Qsz remains off and the transistor Qss remains on. When the value exceeds 71, the transistor Qsz is turned on and the transistor Qss is turned off. However, when the output circuit 4 is driven and the control signal Vc is switched to low level, the abnormal operation of the transistors Qt1 to Q26 is immediately performed, so the time width during which the protection circuit 21 is inactive is It is shortened as shown between T and 7 in the figure.

That is, the time period during which a high voltage that would destroy the output transistor Q is applied is significantly shortened, and the protection of the output transistor Q, in other words, the protection of the IC itself, is reliably performed.

〔effect〕

(1) Transistor Qv! is forced by transistor Q7. By cutting off the current supply, the charging time becomes almost constant #1,
The effect of accurately increasing the operating time of the protection circuit is obtained.

(2) When an abnormality is detected, reducing the output voltage Vo of the time constant circuit that forms part of the protection circuit using the transistor Qts has the effect of shortening the non-operating time of the protection circuit. It will be done.

(3) With (1) and (2) above, it is possible to obtain the effect that the transistor to be protected is reliably protected.

Although the invention made by the present inventor has been specifically explained above based on examples, the present invention is not limited to the above-mentioned examples, and can be modified in various ways without departing from the gist of the invention. Needless to say. For example, the seamit circuit may be any other waveform shaping circuit.

[Application field]

Above, we have explained the case where the invention made by the present inventor is applied to a protection circuit for an output transistor of an amplifier circuit, which is the field of application that formed the background of the invention. Can be widely used.

The present invention can utilize at least a linear IC and a power ICK having an output circuit.

[Brief explanation of drawings]

Fig. 1 shows a circuit diagram illustrating an embodiment of a protection circuit to which the present invention is applied, Fig. 2 shows a characteristic diagram illustrating the operating time of the protection circuit, and Fig. 3 shows the circuit operation of the protection circuit. It shows a voltage characteristic diagram to be explained. 11... Abnormality detection circuit, 21... Protection circuit %QI~
Q33...Transistor, C1...Capacitor, R
3-R04...Resistance, Va...Voltage, ■o...Output voltage, Vc...Control signal. Figure 1 Figure 2 Figure 3

Claims (1)

[Scope of Claims] 1. (1) An abnormality detection circuit that detects the collector-emitter voltage of a transistor to be destroyed and generates an abnormal signal when an abnormal collector-emitter voltage is detected; (2) A first transistor that controls energization and power cutoff to a time constant circuit consisting of a resistor and a capacitor, a second transistor that forcibly discharges the capacitor when power is cut off to the time constant circuit, and an output voltage caused by the abnormality signal. A third switching transistor that changes the level from the first level to the second level, and a switching transistor that drives the first, second, and third transistors as described above using the abnormality detection signal. A protection circuit comprising: a constant circuit; and a protection circuit, wherein the protection circuit reduces the destruction of the transistor to be protected against destruction by shortening the discharge time of the time constant circuit.