Power Supplies

Switched Power Supply using LM2575 or LM1575

To derive a 5 V line from, say, a 15-25 V supply, National Semi-4 conductor’s LM1575 or LM2575 is ideal. In this article i have explained the Switched Power Supply Circuit.

The LM2575 device greatly simplifies the design of switching power supplies by conveniently providing all the active functions needed for a step-down (buck) switching regulator in an integrated circuit. LM1575 integrated circuits that provide all the active functions. Versions for a step-down (buck) switching regulator etc.


How to setup the circuit?

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 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 m Vpp.

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.

Switched Power Supply Circuit Diagram:

Switched Power Supply


Circuit Description:

The choke is a ready-made 330 µH 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/OFF: if that is wanted, connect this pin to a 2.4-15 V potential instead of to earth.


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