Overcurrent detection circuit

Abstract

An overcurrent detection circuit for detecting an overcurrent condition in an output transistor connected in series with an electrical load includes a pair of transistors having input terminals connected together. The pair of transistors is interposed between a current mirror circuit and a resistor and between the current mirror circuit and a detection transistor capable of being turned on at the same time as the output transistor. When a voltage is applied to the input terminals of the pair of the transistors, output terminals of the current mirror circuit are fixed at the same potential. Thus, even when an early effect occurs in the current mirror circuit, an electric current flowing through the resistor becomes equal to that flowing through the detection transistor. The overcurrent detection circuit can accurately detect the overcurrent condition based on a voltage drop across the resistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

FIG. 1 is a schematic diagram of an overcurrent detection circuit according to a first embodiment of the present invention;

FIGS. 2A-2C are schematic diagrams of a current mirror circuit in the overcurrent detection circuit of FIG. 1;

FIG. 3 is a schematic diagram of a typical op-amp;

FIG. 4 is a schematic diagram of an overcurrent detection circuit to which an improvement is made to overcome a problem of a conventional overcurrent detection circuit of FIG. 22;

FIG. 5 is a schematic diagram of an op-amp to which an improvement is made to overcome a problem of the conventional overcurrent detection;

FIG. 6 is a schematic diagram of an op-amp in an overcurrent detection circuit according to a second embodiment of the present invention;

FIG. 7 is a schematic diagram of an op-amp in an overcurrent detection circuit according to a third embodiment of the present invention;

FIG. 8 is a schematic diagram of an overcurrent detection circuit according to a fourth embodiment of the present invention;

FIG. 9 is a schematic diagram of an op-amp of an overcurrent detection circuit according to a fifth embodiment of the present invention;

FIG. 10 is a schematic diagram of an overcurrent detection circuit according to a sixth embodiment of the present invention;

FIG. 11 is a schematic diagram showing a current flow in the overcurrent detection circuit of FIG. 1 observed when an open-circuit occurs;

FIG. 12 is a schematic diagram of an overcurrent detection circuit according to a seventh embodiment of the present invention;

FIG. 13 is a schematic diagram of an overcurrent detection circuit according to an eighth embodiment of the present invention;

FIG. 14 is a schematic diagram of an overcurrent detection circuit according to a ninth embodiment of the present invention;

FIG. 15 is a schematic diagram of an overcurrent detection circuit according to a tenth embodiment of the present invention;

FIGS. 16A-16D are schematic diagrams of a switch in the overcurrent detection circuit of FIG. 15;

FIGS. 17A-17D are schematic diagrams of a switch in the overcurrent detection circuits of FIGS. 13 and 14;

FIG. 18 is a schematic diagram of an overcurrent detection circuit according to an eleventh embodiment of the present invention;

FIG. 19 is a schematic diagram of an overcurrent detection circuit according to a twelfth embodiment of the present invention;

FIG. 20A is a top view of a semiconductor substrate to which an overcurrent detection circuit is formed by trench isolation, and FIG. 20B is a cross-sectional view of FIG. 20A;

FIG. 21A is a top view of a semiconductor substrate to which an overcurrent detection circuit is formed by pn-junction isolation, and FIG. 21B is a cross-sectional view of FIG. 21A; and

FIG. 22 is a schematic diagram of the conventional overcurrent detection circuit.

Claims

1. An overcurrent detection circuit for detecting an overcurrent condition in an output transistor connected in series with an electrical load between a power supply and a first ground, the output transistor having a first output terminal connected to the load, a second output terminal connected to the first ground, and an input terminal, the overcurrent detection circuit comprising: a detection transistor having a first output terminal, a second output terminal connected to the second output terminal of the output transistor, and an input terminal connected to the input terminal of the output transistor so that the output transistor and the detection transistor can be turned on at a time;an operational amplifier having a non-inverting input terminal connected to the first output terminal of the output transistor, an inverting input terminal connected to the first output terminal of the detection transistor, and an output terminal;a current mirror circuit including a pair of first and second transistors having first output terminals connected together to the output terminal of operational amplifier, second terminals, and input terminals connected together, the second output terminal of the first transistor being connected to the input terminal of the first transistor and the first output terminal of the detection transistor;current detection means connected to the second output terminal of the second transistor to detect a current flowing through the second transistor; andan early effect cancel circuit including a pair of third and fourth transistors, the third transistor having a first output terminal connected to the current mirror circuit, a second output terminal connected to the detection transistor, and an input terminal, the fourth transistor having a first output terminal connected to the current mirror circuit, a second output terminal connected to the current detection means, and an input terminal connected to the input terminal of the third transistor, whereinthe first output terminal of each of the third and fourth transistors has an electric potential fixable by an application of a predetermined voltage to the input terminal of each of the third and fourth transistors.

2. The overcurrent detection circuit according to claim 1, wherein the overcurrent detection circuit is connected between the power supply and a second ground different from the first ground.

3. The overcurrent detection circuit according to claim 2, wherein the operational amplifier includes a pair of fifth and sixth transistors for providing an input stage of the operational amplifier, each of the fifth and sixth transistors having an output terminal connected to the first ground directly or through a resistor.

4. The overcurrent detection circuit according to claim 2, further comprising: an error prevention circuit for detecting an electrical change caused by an open-circuit in a wire connected to the first ground and for disabling an output of the current detection means in response to the electrical change.

5. The overcurrent detection circuit according to claim 4, wherein the error prevention circuit disables the output of the current detection means when an electrical potential of the first ground exceeds a predetermined potential value.

6. The overcurrent detection circuit according to claim 4, further comprising: a voltage buffer circuit for applying the voltage to the cancel circuit, whereinthe error prevention circuit disables the output of the current detection means when an output current of the buffer circuit exceeds a predetermined current value.

7. The overcurrent detection circuit according to claim 4, wherein the error prevention circuit disconnects an electrical connection between the output terminal of the operational amplifier and the current mirror circuit in response to the electrical change.

8. The overcurrent detection circuit according to claim 4, wherein the error prevention circuit disconnects an electrical connection between the current detection means and the current mirror circuit in response to the electrical change.

9. The overcurrent detection circuit according to claim 4, wherein the current detection means includes a resistor and a voltage sensor for detecting a voltage produced across the resistor by the current flowing through the second transistor, andthe error prevention circuit shorts out the resistor in response to the electrical change.

10. The overcurrent detection circuit according to claim 1, wherein the overcurrent detection circuit is provided on a SOI substrate having a trench portion filled with an electrical insulating material, andcircuit elements constructing the overcurrent detection circuit are isolated from each other by the trench portion.