JPH0326921B2 - - Google Patents

Description

Claim 1: A monolithic circuit comprising a first transistor to which a base current drive is supplied, comprising a step comprising a current source and a second transistor of the same conductivity type as the first transistor; The base and emitter of the two transistors are respectively coupled to the base and emitter of the first transistor, the collector of the second transistor is coupled to the current source, and the emitter of the first transistor is coupled to the emitter of the second transistor. When the base current exceeds a value that is N times the area and requires a collector current larger than the current that the second transistor can supply by the current source, the second transistor goes into saturation, in which the second transistor A method for limiting the current flowing through the collector-emitter of the first transistor, characterized in that the collector current flowing through the first transistor is limited to a value N times the magnitude of the current supplied from the current source.

2. An integrated circuit including an intermediate transistor amplifier stage providing current drive to an output transistor stage having an output transistor, including the step of providing an additional transistor having its base and emitter coupled respectively to the base and emitter of the output transistor; the emitter of the output transistor has an area N times the area of the emitter of the additional transistor; wherein the current drive of the output transistor saturates and limits the flow of current through the output transistor when the current drive exceeds a value such that the additional transistor requires a collector current greater than the current that the current source can supply. How to limit the collector current of a transistor.

3. An integrated circuit comprising a transistor having an emitter, a collector and a base coupled to a preceding amplifier stage, comprising an additional transistor having a base, an emitter and a collector, said base receiving base drive from a previous amplifier stage. the emitter is coupled to the emitter of the transistor, the emitter of the transistor having an area N times the area of the emitter of the additional transistor; a current source that supplies a current to the collector of the additional transistor, the additional transistor having a base drive above a limiting threshold that the current source cannot provide sufficient collector current required by the additional transistor. A circuit for limiting the collector current of a transistor, which is characterized in that when the voltage exceeds the voltage, the transistor becomes saturated and the collector current of the transistor is limited.

4. The circuit of claim 3, wherein the first current source is a resistor coupled between an operating potential and the collector of the additional transistor.

BACKGROUND OF THE INVENTION The present invention relates to methods for regulating current, and more particularly to circuits and methods for limiting current flow in semiconductor devices.

There are many applications where it is desirable to protect a transistor by limiting the maximum current flow in its collector-emitter path. For example, when a transistor is operating in its active region, a large voltage drop across the device's collector-emitter junction can exceed the limits of the transistor's power dissipation characteristics and damage the transistor. Otherwise, the transistor may be destroyed.
One way to prevent this from occurring is to limit the current through the transistor to a maximum value that is well within the power handling capabilities of the device.

One prior art technique for preventing high currents from being conducted into a high-gain transistor at the same time as a large voltage potential applied across the transistor's collector-emitter junction is to connect the emitter to the emitter with a feedback circuit coupled between the transistor's emitter and base. The idea is to use a sense resistor between it and the ground reference. In a typical application, the feedback circuit may be an additional transistor whose base and emitter are coupled across the sense resistor and whose collector is connected to the base of the transistor for protection. Therefore, if the current flowing through the high-gain transistor causes enough current across the sense resistor to turn on the feedback transistor, the base current of the high-gain transistor will flow through the collector-emitter of the feedback transistor. current, thereby limiting the current in the output transistor.

However, the prior art circuits described above have two drawbacks that may be unacceptable in some applications where it is necessary to protect output transistors commonly found in operational amplifiers, comparators, automotive circuits, etc. First, the voltage drop across the sense resistor is at least equal to the base-emitter voltage V _BE of the feedback transistor, which degrades the V _SAT characteristic of the circuit containing the transistor being protected. Therefore, the maximum output voltage swing of this prior art circuit is limited, which may be unacceptable. Second, adding a sense resistor introduces a loss in gm of the transistor amplifier, which is often undesirable.

Another prior art method of limiting current in an output transistor is shown in FIG.
This prior art circuit includes an output transistor 10 and a diode 12 and is commonly referred to as a current mirror. The anode and cathode of diode 12 are connected to the base and emitter of transistor 10, respectively. Transistor 10 is shown to have an emitter area that is N times the area of the anode region of diode 12, as will be understood. A previous or intermediate amplifier stage including a transistor 14 adapted to receive an input signal at its base 16 is coupled at its collector to the base of transistor 10 and to the anode of diode 12. A current source 18 supplying current I is connected between the power supply V _cc and the collector of transistor 14 .

In operation, the output current flowing through transistor 10 is always equal to NI'. However, I′ is diode 1
This is the current flowing through 2. Therefore, transistor 1
The maximum current flow in the collector of 0 is limited to NI. Therefore, this circuit suffers from a significant loss in the transistor's forward current gain factor, beta.

Therefore, the circuit exhibits excellent VSAT characteristics,
What is needed is a circuit and method for limiting current in a semiconductor device without significant loss of its gain.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved circuit and method for limiting the current flowing through a semiconductor device to a maximum predetermined value.

Another object of the invention is to provide a circuit and method for limiting current flow in a transistor.

In accordance with the above and other objects, a method is provided for limiting the flow of current through a first transistor, the method comprising a first transistor having a base and an emitter respectively coupled to a base and an emitter of the first transistor. providing a second transistor of the same conductivity type as the transistor, the emitter area of the first transistor being N times the emitter area of the second transistor, and the method further includes coupling a current source to the collector of the second transistor. The bases of the first and second transistors receive base current drive thereto.

SUMMARY OF THE INVENTION A circuit and method for limiting current through a transistor driven by a preceding amplifier stage is described. The circuit comprises an additional transistor (second transistor) having its base and emitter connected to the base and emitter of the first transistor, so that its current is limited.

The current source is supplied to the collector of the additional transistor, and the magnitude of the drive current supplied to the bases of the two transistors is such that the collector current required by the additional transistor cannot be supplied by the current source. When a limiting threshold such as exceeded is reached, the additional transistor becomes saturated.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram illustrating a prior art current limiter circuit.

FIG. 2 is a schematic diagram illustrating the current limiter circuit of the preferred embodiment.

FIG. 3 is a more detailed schematic diagram of the circuit of FIG.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to FIG. 2, a novel method and structure for limiting the collector current of transistor 10 to a predetermined value is shown. It will be seen that components in FIG. 2 that correspond to similar components shown in FIG. 1 are given the same reference numerals. As mentioned above, transistor 10 may be a pre-driver transistor amplifier or an output amplifier stage driven by intermediate amplifier stage 14. Current limiting is achieved by adding transistor 20. The base and emitter electrodes of transistor 20 are coupled to the base and emitter of transistor 10, respectively, to limit the collector current of transistor 10. The collector of transistor 20 is coupled to a current source 22 which supplies a constant current of value I' thereto. As shown, the emitter area of transistor 10 is N times the emitter area of transistor 20. The output of the circuit is taken from the collector of transistor 10 at node 24.

Under normal operating conditions, the input drive signal to the base input 16 of transistor 14 is such that transistors 10 and 20 operate in their active or cutoff regions. When operating in their active region, two transistors 1
The overall gain of the output stage including 0 and 20 is determined by the forward current gain beta of the matching transistor and the current source 1
8 multiplied by the base current drive supplied to it. Compared to its gain without transistor 20, there is some slight gain loss in the circuit of FIG. 2 due to the base current flowing through this transistor. However, the gain of the circuit remains approximately the same, reduced by a factor of 1/N+1.

When the input signal to transistor 14 is such that the conduction of this transistor is reduced to the threshold limit value, allowing excess base current to be supplied from current source 18 to transistor 20, the circuit operates in current limit mode. do. Therefore, at the limiting threshold, the magnitude of the portion of current I that is supplied to the base of transistor 20 is such that this transistor wants to supply a larger current at its collector than current source 22 can supply. make it happen. Therefore, the collector current available to transistor 20 is insufficient to meet its demands, and this transistor goes into saturation. In this state, the current at the collector of transistor 10 is
limited to the value of NI′. The excess base current drive available to transistors 10 and 20 when transistor 20 is saturated is transferred to the IC circuit through the parasitic PNP device formed by the base-collector junction of transistor 20, which is forward biased. drawn into the board. Therefore,
Although transistor 10 is protected by limiting the current flowing through it, the gain of this transistor stage is only slightly reduced. Furthermore, the V _SAT characteristic of the circuit is determined by the current limiting transistor 20
is not affected by the addition of .

Thus, the above-described circuit and method of current limiting allows the transistor whose collector current is limited to operate undisturbed up to the limiting threshold. The current flowing through the transistor is then limited to a predetermined maximum value.

FIG. 3 shows the circuit described and shown in FIG. 2 assembled in the form of a monolithic integrated circuit. The collector of the transistor 14 is a series resistor 28
Multi-collector PNP transistor 30 through
is attached to the base. Current source 22 is resistor 2
This is realized by 6. The collector of the output transistor 10 is connected to a multi-collector via the collector 30a.
It is coupled to VCC through PNP transistor 30. Collector 30b couples the base of transistor 30 to resistor 28.

In operation, the collector current of transistor 10 is limited to N times the current flowing through resistor 26. For example, if N is equal to the integer 4 (the emitter area of transistor 10 is four times the emitter area of transistor 20), then the approximate value of the current in transistor 10 in the limit state is:

I _MAX =4V _cc /R ₂₆ Thus, by varying the value of resistor 26, the maximum value of the current in transistor 10 is programmed.