MC34063 SMD Inductor Selection

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Cosford

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EDIT: Whoops, should be in project guidance! A move would be appreciated, if someone can?

Hi,

I'm designing a board using the MC34063 switching regulator, to bring a 7.2v (or 9.6v, currently undecided) NiMh battery down to 5v, as per the example circuit given on page 7 here (http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF). I need to allow for up to ~

I've had this working previously, but it's with a pretty large, through hole inductor. I'd like to replace this with a smaller, SMD one, but I'm unsure which characteristics are important?

I'm of the understanding that;
-> I need a MINIMUM of 220uH inductance.
-> I'll need to allow for an average consumption of up to 500mA or so (so, inductor needs to be able to sustain 1A current throughput?)
-> A lower DC resistance is beneficial.

What is the significance of inductor shielding? I see there are several types; what is special about 'Power inductors'? Are these the type I need?

Does anyone have any recommendations on a specific inductor from http://uk.farnell.com to use for this purpose?

Thanks in advance,
Cosford.
 
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For those parameters, a 220 uH typical is OK. Typically, inductors have a tolerance of +/- 10 to +/- 20 %. That's OK.
For a load current of 500 mA, the peak inductor current may be close to 1 A, although the rms ('average') current will be closer to 500 mA. You need an inductor that can handle 1 A peak current without saturating (losing inductance at high currents).

It needs to handle ~ 600 mA rms without overheating. DC resistance minimizes heating and helps efficiency.

A shielded inductor will radiate less EMC (radio noise), although other portions of the circuit may still radiate.

If you don't have size constraints, something like this MCSDRH125B-221MHF will work.
 
Great, thanks for that.

Interestingly, the MCSDRH12B was the part I picked out (which at least shows I'm on the right track), although, it has quite a large footprint at 12.5mm x 12.5mm. Do you have any other recommendations at all for a smaller part or is that a pretty normal size for an inductor of those specifications?
 
For a chip running at 100 kHz, normally a pretty large inductor is necessary.

Generally, higher frequencies allow for smaller inductors. PCB layout becomes more critical as the frequency increases. 50, 100 and sometimes even up to 200 can be fairly flexible on layout. At frequencies like 500 to 800 or higher, layout and minor details become critical.
 
I didn't search any deeper -- generally you just need one that can handle the peak current without saturating; size is related to losses (efficiency), and EMC radiation; if those are of less concern, then perhaps there is a suitable smaller one. If you are designing for volume production, you probably need to be more cautious than if building a 1-off board for home use.
 
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