This is a RS232 Line Driver Power Supply circuit. This circuit can gives an output of ± 12V at ± 40 mA and only requires an input voltage as low as 4.2V. The efficiency of this circuit is  better than 70%. This circuit provides a line regulation of ± 0.08% and a load regulation of ± 1.25% (from 10% to 100% of load). Besides that, this circuit has output voltage ripple of less than 40 mVp-p and a cycle-by-cycle current limit. Here is the circuit:

RS232 Line Driver Power Supply circuit schematic

This circuit use the feedback from the both output. One output is not left unregulated as in single feedback systems because of the  sharing of the output voltage regulation by each output. A linear regulator is not necessary used for output regulation because both sides of the output are requlated.

The primary inductance of transformer must high enough to prevent the output transistor switch from ramping higher than the transistor’s rating of 750mA. The transformer is selected for an output transistor “on” time of 0.4/f. To provide those requirement, Pulse Engineering PE-64287 was chosen. It has primary inductance of 50 µH and turns ratio of Np:Ns:Ns = 1:1.6:1.6. [Source: National Semiconductor Application Note]