Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Patent
1991-05-21
1992-12-29
Stephan, Steven L.
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
323312, G05F 316
Patent
active
051754894
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a current-detecting circuit, incorporated in a bipolar type integrated circuit, for preventing an excessive current from flowing when a signal produced in the integrated circuit is output.
DISCUSSION OF THE RELATED ART
In an output circuit incorporated in a bipolar type integrated circuit, the output transistor may break down if an excessive current flows from an output terminal. In order to prevent the breakdown of the output transistor, the current flowing from the output terminal is monitored by a current-detecting circuit, and the operation of the output circuit is stopped if the value measured by the current-detecting circuit exceeds a predetermined value.
FIG. 1 is a schematic diagram of a conventional current-detecting circuit. As is shown in FIG. 1, a current-detecting resistor 41 is inserted in the path along which current i flows. If the voltage drop across the resistor 41 exceeds the base-emitter voltage V.sub.BE of an NPN transistor 42, this transistor 42 is turned on, and a signal indicating the flow of a predetermined amount of current appears at the collector of the transistor 42.
In the conventional current-detecting circuit mentioned above, a detection output is produced in accordance with the relationship between the voltage drop across the current-detecting resistor 41 and the base-emitter voltage V.sub.BE of the transistor 42. Since the base-emitter voltage V.sub.BE of the transistor 42 is about 0.7 V, the resistance of the resistor 41 should be as low as 0.7 .OMEGA. or so, so as to detect a current of 1 A.
In the meantime, the voltage and power which are lost in the current-detecting resistor 41 are inevitably large. In the case where a current of e.g. 1 A is detected, the voltage loss in the resistor 41 is as high as 0.7 V, while the power loss therein is as high as 0.7 W. This being so, it is desirable that the resistor 41 incorporated in the integrated circuit be a diffused resistor. However, since it is not easy to provide a diffused resistor with low resistance of 0.7 .OMEGA. or so, the resistor 41 has to be a discrete resistor externally connected to the integrated circuit. Due to the need to employ such a discrete resistor, structure is large, and the manufacturing cost is high, accordingly.
In the conventional current-detecting circuit, the value of a detection current is determined by the base-emitter voltage V.sub.BE of the transistor 42 and the resistance of the current-detecting resistor 41. Due to the temperature-dependent characteristic of voltage V.sub.BE, therefore, the value of the detection current is unstable. Let it be assumed that the temperature increases 100.degree. C. Since, in this case, the base-emitter voltage V.sub.BE decreases from 0.7 V to 0.5 V or so, the detection voltage decreases 28%, resulting in a decrease in the detection current.
The present invention has been developed in consideration of the above circumstances, and is intended to provide a current-detecting circuit which can be incorporated in an integrated circuit, has little voltage loss and little power loss, and produces a detection current that is not much dependent on the temperature.
SUMMARY OF THE INVENTION
A current-detecting circuit according to the present invention comprises: a current mirror circuit which is made up of first and second transistors that are of the first polarity and have their emitters connected to each other, and for which an input-to-output current ratio of M:1 (M: a real number equal to or larger than 1) is determined; a third transistor which is of the second polarity and has its collector and base connected to the collector of the first transistor of the current mirror circuit; a fourth transistor which is of the second polarity, has its collector connected to the collector of the second transistor of the current mirror circuit, and has its base connected to the base of the third transistor, and which has an emitter area N times (N: a real number equal to or larger than 1) wider than that of the thir
REFERENCES:
patent: 4308496 (1981-12-01), Nagano
patent: 5027004 (1991-06-01), Palara
"Toshiba Integrated Circuit Technical Data TA7272P" (2 pages), Toshiba Corporation (Sep. 26, 1989).
Berhane Adolf
Kabushiki Kaisha Toshiba
Stephan Steven L.
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