There are numerous applications like triac firing where you need to isolate the pulse source from the gate of the semiconductor device. The circuit described here can be best suited for such applications where you need to construct an isolated source from a single source. This can also be used for applications like power supply for the microcontroller. The Circuit uses transformer with 1:1 turns ratio for isolation so provides complete isolation.
In this circuit, a negative output voltage dc-dc converter generates a - S-V output at pin A. In order to generate - 5 V at point A. the primary of the transformer must fly back to a diode drop more negative than - 5 V. If the transformer has a tightly coupled I : 1 turns ratio. there will be a 5 V plus a diode drop across the secondary. The IN5817 rectifies this secondary voltage to generate an isolated 5-V output. The isolated output is not fully regulated since only the - 5 V at point A is sensed by the MAX635.

   Schematic

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