1. Field of the Invention
The present invention relates to an electronic circuit board having a TMR element, a magnetic sensor using the same, and a method for protecting a power supply of an electronic circuit.
2. Description of the Related Art
Magnetic sensors having a magneto-resistive element such as TMR element or GMR element are used in various fields of industry, and in the automotive field, for example, they are used for detecting steering angle or speed.
As disclosed in Japanese Unexamined Patent Application Publication No. 2002-303536, such a magnetic sensor has a circuit configuration in which a magneto-resistive element is connected between a power supply (Vcc) and a ground (GND) (see FIG. 8). Here, when a GMR element or an AMR element is used as the magneto-resistive element, they allow an electric current to flow parallel to a film surface, so that even if a magnetic film is broken by a failure, an open circuit occurs only between the power supply and the ground and therefore the whole electronic circuit will never be fatally damaged by the failure.
When a TMR element is used as the magneto-resistive element, on the other hand, since it is constructed to have two magnetic layers opposed to each other across a tunnel barrier, a breakdown of the tunnel barrier due to a failure causes a short circuit, i.e., short between the power supply and the ground. Therefore, this failure may result not only in fatally damaging the whole circuit but also in causing fire, for example, at the power supply and leading to a dangerous situation such as fire disaster. Since the occurrence of such a situation may be fatal to humans, particularly, in the above-described automotive application, it is totally unacceptable.
From such a viewpoint, it has been increasingly demanded to realize a so-called fail-safe for the magnetic sensor using the TMR element. Moreover, since the above circuit configuration is widely applicable and not limited to the magnetic sensor, the fail-safe should be realized also in the field of other devices using the TMR element.
As a protecting circuit regarding the magneto-resistive element, for example, Japanese Unexamined Patent Application Publication No. 9-288805 discloses the one in which a diode is connected in parallel to a magneto-resistive element or a fuse is connected in series to a magneto-resistive element. However, this circuit is intended to protect the magneto-resistive element from an overcurrent but not to protect a power supply from the overcurrent. Accordingly, a short circuit accident which may be caused by a failure of the TMR element cannot be prevented by this circuit.
On the other hand, Japanese Unexamined Patent Application Publication No. 2008-202988 discloses a circuit which is to be controlled to stop operation upon detecting a change in an output signal of a magnetic field change detection element, thereby realizing a fail-safe in case of a failure of the magnetic field change detection element. With this method, however, it is difficult to effectively prevent an instantaneous overcurrent during the time from the failure detection to the stop control, and since a complicated control means is required, it is also disadvantageous for miniaturization and cost-reduction.