Many systems require a low-power supply in addition to the main supply. A typical example is when an analog front-end amplifier needs ±5 V, while the main digital circuitry requires +5 V only. For reasons of cost, inventory management, electromagnetic compatibility, and so on, a separate —5-V converter may not be appropriate. So, some means must be found to provide extra power rails from the main supply.
Implementing a step-down IC converter's switching action may derive one or more outputs, isolated or non-isolated, quasi-regulated or unregulated. Auxiliary output currents of 10% to 30% of the main output are quite possible.
Step-Down Waveforms
A review of the waveforms found in a working step-down converter will identify the voltage and currents that may be used to generate additional outputs (Fig. 1).
At the LX pin, there's a switching-voltage waveform of amplitude:
Voltage across the main inductor, L1, during the power cycle (LX connected to VIN), is:
