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I was browsing Seeed Studio's Open Parts Library and found the ETA3406, which is a 1.5 MHz 1.2 A Step-Down Converter.

It's pretty cheap, and looks very useful for something I want to build, but the datasheet only shows it using a 2.2 µH inductor, which curiously is not in the Open Parts Library.

The datasheet also doesn't provide any guidance when it comes to selecting the inductor, all it says is:

If much smaller values are used, inductor current rises, and a larger output capacitance may be required to suppress output ripple. Larger values than LIDEAL can be used to obtain higher output current, but typically with larger inductor size.

I assume LIDEAL is that 2.2 µH value, but it's not very clear. Anyone have any tips on how to make the selection, given the scarcity of data?

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  • \$\begingroup\$ Generally any BUck datasheet will provide equations to findout Inductor values for perticular combination of Vin,Vout,Ripple Current,freq and duty cycle requirements. Which gives min Inductance required to maintain ripple current. As you increase the Inductance values beyond that generally your output ripple current will be less, so, required output capacitance will be lower, slow transient response, Copper losses also different. Lower values of inductance will give good transient response,More losses,requires more output capacitance. Idc,Irms important, At peak currents L value change. \$\endgroup\$
    – user19579
    Commented Oct 13, 2014 at 7:13

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If you zoom the datasheet, you can see that the diagrams actually recommends the use of a 2.2uH inductor!

enter image description here

Although, it seems that Open Parts Library doesn't have these inductos either (but you can buy them cheap from Digikey, Mouser or any other distributor). To select one of them, is very important to take note on the maximum current the inductor can manage (as these ICs can have up to 1-2A as inductor current), so be careful!

This value is tipycal for step-down converters (I have used some of TI and other manufacturers and it's a standard value for a wide range of applications). I strongly believe you can use the datasheets of those parts to search for tips on value selection.

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  • \$\begingroup\$ Thanks! I edited the question to say 2.2 µH, very good to know that I mis-read the datasheet to begin with. :) \$\endgroup\$
    – unwind
    Commented Oct 9, 2014 at 17:46
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For switch-mode power supplies (SMPS) inductor current rating and current saturation are key parameters. The inductance value won't matter too much, if you use anything from 1uH to 10uH it will probably work depending on the ripple voltage you can tolerate. I'd stick with 2.2uH just to be safe. The converter switches at 1.5MHz which is pretty fast, and means you can get away using low value inductors such as 2.2uH without horrible output ripple.

For current rating, the inductor needs to be able to handle the maximum amount of current your load will draw, then add in another 25-50% margin for reliability. If your load is 1A, make the inductor rated for at least 1.2A, 1.5A would be better.

Saturation current is the amount of current the inductor can handle before the inductance value starts to plummet. Very bad in a SMPS. It has to do with saturating the ferromagnetic core of the component. Again, you want this rating to be 25-50% higher than what you expect your load to be.

Another somewhat important parameter is equivalent series resistance and it is exactly what it sounds like. Imagine a series resistor in front of your inductor. The larger this is the less efficient the SMPS is. You want this to be as small as reasonably possible. For a 1A load 100 - 200mΩ is probably fine.

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  • \$\begingroup\$ nice descriptions of stuff one needs to remember! \$\endgroup\$
    – cjferes
    Commented Oct 9, 2014 at 17:59

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