For some subscriber line-interface cards (SLICs), both the line and ringer voltages should be regulated under all conditions. The circuit shown in the figure meets this requirement. It accepts a 12-V ±10% input, and delivers 0 to 400 mA from a regulated −24-V ±5% output. From a regulated −75-V ±5% output, it provides 0 to 100 mA. Features of this circuit include a boost-controller IC in a transformer-flyback topology and an op amp in the inverting configuration. Used as a summing amplifier, this op amp derives shared feedback from the two regulated outputs. The transformer turns ratio is approximately 1:2:4.
Both outputs must remain in regulation even when one operates at full load and the other operates at no load. To ensure that this happens, the two outputs contribute to the IC’s feedback signal. The −24-V output generates the greater output power and two-thirds of the feedback current. Meanwhile, the −75-V output provides the remaining one third of the feedback current.
Such an arrangement allows the regulator to maintain a ±5% output-voltage tolerance on both outputs—for line variations of ±10%, and for any combination of output currents (i.e., zero to full load on either output). For fullload currents at both outputs and a 12-V input, the efficiency is 85%.