Switched 5 V supply

To derive a 5 V line from, say, a 15-25 V supply, National Semiconductor’s LM1575 or LM2575 is ideal. These switched 5-V regulators need only a choke, a freewheeling diode and a couple of electrolytic capacitors. In fact, they go a long way to prove that the industry has overcome most of the problems switched power supplies have suffered fired from; both the regulators mentioned worked first time without any problems whatsoever.

The only thing that is still not as good as in linear power supplies is the ripple: in the prototypes, this amounted to 20 mV PP.

The measured efficiency of the prototypes is 75%, whereas National Semiconductor’s give a figure of 82%. This figure does, of course, depend to some extent on the components used.

The diode is a Type 1N5822 or BYW29 from Philips: any fast diode will do, however: do not use a common or garden 1N4001 or similar.

The choke is a ready-made 330 pH type that must be able to handle currents of at least 1 A.

Build the supply with a central, star-shaped common earthing point as indicated in the diagram. Use good quality electrolytic capacitors (perhaps two in parallel). The more compact you keep the supply. the better it will work.

The input voltage can lie between 8 V and 32 V. The maximum current the regulator can provide is about 1 A. Pin 4 is the feedback input.

Pin 5 may be used for switching the supply on/of; if that is wanted, connect this pin to a 2.4-15 V potential instead of to earth.

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