DC/DC converter and subscriber line interface circuit

A subscriber line interface circuit, comprising:

• a loop circuit for accumulating electromagnetic energy in an inductor when an electric current is supplied and generating a negative DC voltage by causing a loop electric current no flow by using the electromagnetic energy accumulated in said inductor when no electric current is supplied;
• a power transistor for supplying the electric current supplied from a drive power source line to said loop circuit when the potential of a control pulse signal is at an active level;
• an SLIC control circuit for generating said control pulse signal of a duty ratio corresponding to the value of the DC voltage generated by said loop circuit;
• an abnormal high temperature detection circuit for detecting a normal/abnormal state of temperature of said power transistor;
• and
• an abnormality processing circuit for making said power transistor not to supply the electric current to said loop circuit when said abnormal high temperature detection circuit has detected that the temperature of said power transistor is abnormal,
• wherein said inductor is connected by one terminal thereof to one main electrode of said power transistor and by the other terminal thereof to a ground line, and
• wherein said loop circuit comprises said inductor, a capacitor having one terminal thereof connected to the other terminal of said inductor, and a diode having an anode thereof connected to the other terminal of said capacitor and a cathode thereof connected to one terminal of said inductor.