This circuit is a 3.3V/10mA regulator that extracts its power from a 4mA to 20mA loop. This circuit demonstrates how an LTC1474/LTC1475-based regulator is easily optimized for such low current applications . The 2W sense resistor limits the peak inductor current to 40mA to minimize current ripple and provide good efficiency (84%). The 330mH inductor is a good value to use at this current level to keep the frequency low enough to avoid excessive switching losses without being so large that DCR losses are significant (see inductor section of the data sheet). The Zener diode at the input clamps the input voltage to 12V, which is then converted to 3.3V. This enables the 4mA (min) input current to be more than doubled at the output.