Voltage Converter -48V to 5V(0.5A) for Telecom System

The IC is allowed to be biased at a -50 V rail with a shunt zener by the small current consumption of a MAX641 so that the IC can convert -50 to 5 V. In telecom systems, this is a common requirement where logic circuit must be powered from the central office battery voltage.

The feedback signal is shifted by a small high voltage pnp level from +5V output down to the pin MAX641, where the ground pin is tied to the 50V input (instead of true ground). After this step is done, EXT can drive the MOSFET to switch the inductor to -50V, making the circuit operate similar to a step up dc/dc converter. Feed forward compensation is provided by the 330 pF capacitor to stabilize the regulator control loop.