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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 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
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
LM2576T -ADJ [TO-220]
Input capacitor 100
(electrolytic - 50 volt or better)
Output capacitor 680 uF
(electrolytic - 25 volt or better)
1N5820 (I used
a 1N5819 in this example)
(metal film - 0.5 watt)
1.6K ohm (metal film - 0.5 watt)
(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
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
Do not have food or drink in such a working environment.
2 start the
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
Bend pin 2 so that it is vertical.
pin 2 bent
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.
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 -
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
The inductor is to sit on top of the chip
Make sure that the ends of the inductor leads
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 -
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
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.
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.
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.
Pay particular attention to the direction of the
band on the Shottky diode
and the -ve direction of both the input and output
Also look for obvious shorts.
Connections are as follows:
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
Run the regulator for a period of time and satisfy
yourself that it doesn't get too hot.
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
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
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
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Article copyright ©2012 by Puddledud, Web page layout copyright
©2011 Richard Cappels All Rights to
layout Reserved. Find updates
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First posted in June 6,
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email to me at
"(at)" with "@" before mailing.
Keywords: Buck Converter, Forward Converter, Swithcing Power
Supply, Low Voltage Power Supply, Compapct Power Spply, No PCB.
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