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A Switching Power Supply Using The IC As Its Frame 

Build a tiny switching power supply - get 3.3 volts at 300mA from a 9.6 volt battery source

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Contributed by Puddledud.

The Article

The concept being explored is to build the whole power supply using the IC itself as the framework.

The first build resulted in a supply about 35mm long; 15-20 mm wide and 25 - 30 mm high. No surface mount components were used. The dimensions of the finished product will vary according to the size and placement of the components. I expect that it is possible to further reduce the size of the finished supply.

Note that this use sits right down near the mimimum voltage and current potentials of the IC and for this reason I have not been much concerned about heat in the circuit. If a higher output voltage or a higher output current is needed then it may well be necessary to pay careful attention to ensuring proper heatsinking of the device.

This exercise in perspective -

The impetus for this project derived from a plan to refit an incandescent torch to take a LED light source. That project involves fitting a heatsink to the IC to deal with both the LED generated heat and any IC generated heat but that part of the exercise is still to be attended to.

At this stage it looks like the refit is possible but that it will probably mean developing a design for a different reflector. This is due to the constraint presented by the 35 mm length of the finished supply.


1    source the components

IC                            LM2576T -ADJ [TO-220]

Input capacitor        100 uF            (electrolytic - 50 volt or better)

Output capacitor      680 uF            (electrolytic - 25 volt or better)

Schottky diode        1N5820            (I used a 1N5819 in this example)

Resistor R1             2.7K ohm        (metal film - 0.5 watt)

Resistor R2            1.6K ohm        (metal film - 0.5 watt)

Inductor                 470 uH            (keep the series resistance low)

and - the datasheet for the LM2576T - ADJ from National Semiconductor. Take special note of the circuit diagram - Figure 2 page 9 of the datasheet.


Electrostatic charge can damage the LM2576T-ADJ so wear a grounding wrist strap and handle the chip using insulated tools.

The LM2576T-ADJ will absorb heat amounting to just 10 sec at 260 degrees C - let the chip cool between soldering stages.

Soldering and a possible consequent exposure to lead is an inherently dangerous activity - take proper precautions. Keep the workplace clean;  wear protective clothing; avoid breathing vapours; work in a well ventilated location environment and wash up carfefully afterwards.

Do not have food or drink in such a working environment.

2    start the build

DIAGRAM 1- pin 5 bent

Wrap pin 5 - the
far left hand pin when the chip is lying flat on its base with the pins pointing away - over so that it makes contact with the back plane of the IC.

Carefully solder the pin to the backplane.

This connection serves to lock the chip in the ON state.

3.    Pin 2 adjustment

Bend pin 2 so that it is vertical.

  DIAGRAM 2 - pin 2 bent

4.    Prepare the diode Wrap the diode lead - the 1N5820 - around a paperclip to coil it.

The coil should be close to the diode body and on the banded end of the diode.

Twist the coil slightly to one side of the diode so that it ends up parallel to the length of the diode.

 Leave the long piece of diode lead which exits the coil in place - this is handy to use to position the diode and later provides a useful takeoff point for the 3.3 volt +ve connection.

Cut the lead at the other end of the diode and bend it over to form an angle.

  DIAGRAM 3 - diode bend.

5.    Solder the diode in place

Place the diode so that the coil goes over pin 2 and the angle rests on the back of pin 3.

Solder the bent angle to pin 3 - the bend rests on the top of pin 3.

Solder the coil to pin 2.

    DIAGRAM 4 - diode placement.

The diode should end up vertical alongside pin 2.

The diode band ends up pointing to pin 2.

6.    Place the input capacitor

Attach the 100 uF input capacitor to the ends of pins 1 and 3.

The -ve points to pin 3.

Use short leads - the capacitor should end up mounted vertically above the ends of the pins as shown in the sketch.

DIAGRAM 5 input capacitor placement.

7.    Place the inductor

The inductor is to sit on top of the chip itself.

Make sure that the ends of the inductor leads are clean.

Tin the ends of the leads with solder to make sure that this is the case.

Solder one end of the inductor to the top of pin 2

DIAGRAM 6 - inductor placement.

8.    Place the resistor RI

Line up the restistor R1 so that it lies along pin 4.

Solder one end of R1 to the back of pin 4 - just forward of the body of the IC.

The next step involves soldering R2 to this same attachment point.

Solder the other end of R1 to the end of pin 3.

I used the solder point that was put in place for the input capacitor.

    DIAGRAM 7 - input resistor (R1) placement.

9.    Place the resistor R2

Mount R2 vertically at the back of pin 4 where R1 connects to pin 4.

Connect the free end of the inductor to R2.

   DIAGRAM 8 - output resistor (R2) placement.

10.    Position the output capacitor.

Attach the output capacitor to the top of R2 (+ve side) and the (-ve side) to pin 3.

Put a piece of heat shrink on the lead which climbs up to connect to the top of R2.

DIAGRAM 9 - output capacitor placement

The capacitors should end up upright over the ends of the IC pins.

11.     Check the layout

Pay particular attention to the direction of the band on the Shottky diode

and the -ve direction of both the input and output capacitors

Also look for obvious shorts.

12.     Test the regulator

Connections are as follows:

output:    +ve from the diode lead extending from the top of pin 2 I bent it to face in the same direction as the IC pins.)

-ve from the TAB case at the top of the IC

input:    +ve to pin 1.

-ve to the TAB case at the top of the IC

  Run the regulator for a period of time and satisfy yourself that it doesn't get too hot.

First prototype

Pictures of the prototype (below)




Read the datasheetcarefully - this outline is largely concerned with the fabrication and placement of components.

This use doesn't stretch the chip  - 300 mA is tiny compared to a 3A current ceiling and 3.3 volts is also in the very low region of the voltage output range.

Changing the parameters may result in creating a significant heat dispersion problem - particularly with the components compressed together in this way - be cautious in the event of making such changes!


The datasheet is very careful to indicate that quality components should be used and, although tempted, I chose not to use tantalum capacitors - even for this use.

The output capacitor value was set at 680 uF despite the calculated value being much lower. This decision relates to the need to  keep the ripple voltage low.

The datasheet recommends using low ESR rated capacitors and this is one reason why the voltage characteristics of the capacitors have been chosen to be as high as they are - higher voltage capacitors tend to have better ESR characteristics.
Note that on page 9 of the datasheet a 75 volt input capacitor is specified but that on page 10 reference is made to a 25 volt input capacitor.


Sourcing the inductor was a tad difficult. The torroidal inductors are often a bit large but a search through scrap boards at length located a 1.6 mH inductor which I was able to unwind until I obtained the 470 uH value that I was   after. (A capacitence meter comes in handy at this point.)

One of the constraints with respect to the inductor was to choose an inductor type which would have a low resistance.

Saturation of the inductor is to be avoided.

This supply may have EMI issues - I have not investigated this possibility.


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Article copyright ©2012 by Puddledud, Web page layout copyright ©2011 Richard Cappels All Rights to layout Reserved. Find updates at Return to HOME

First posted in June 6, 2012 (120606)

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Keywords: Buck Converter, Forward Converter, Swithcing Power Supply,  Low Voltage Power Supply, Compapct Power Spply, No PCB.

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