The Si9121 simplifies the -48-V to +5-V or +3.3-V converter design for ISDN application by integrating the floating feedback error amplifier providing direct output voltage regulation. This approach eliminates the need for an external shunt regulator. The Si9121 also integrates a high voltage depletion mode MOSFET which allows the converter to be powered directly from the high input bus voltage without requiring an external start-up circuit. Combined with simple magnetic design due to its non-isolated topology, the Si9121 provides a one-chip solution for complete ISDN power supply.