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This is somewhat related to this question, but hopefully a bit broader.

I'm selecting components to fill out an lm2734. This regulator will take a 9-12V (11.1V nominal) input down to 5v @ 1A max. They provide guidance for selection on page 12 of the datasheet, and using the formulae there, I arrive at an inductance value of 4.9uH.

My question: what do I want directionally here (size and cost notwithstanding, and assuming the output current and DCR are correct), an inductance value that's within some percentage of the one calculated above, or just a value that's higher? Should I just be using the largest inductor I can fit to minimize ripple current?

To recheck my work, I looked at the sample circuits they provide in the DS, and calculated the inductance for them to compare to the inductor value they recommend. They seem to be taking inductor values as much as 2x greater than the formula suggests (which may also mean that I'm just doing it wrong). E.g.

  • for a 12V in / 3.3V out @ 1A configuration, I get 4.3uH, and they recommend a 4.7uH inductor
  • however, for an 18V in / 1.5V out @ 1A configuration, I get 3.1uH, and they recommend a 6.8uH part.
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kolosy
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3 Answers3

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These sorts of parts (i.e. parts with unspecified internal compensation) can be tricky because the size of the output inductor will determine the mode of operation (CCM or DCM) and the type of feedback compensation required, along with all of the other variables that have been touched on (core loss, saturation, etc.) Because of the vagueness, there are no hard-and-fast rules here; you need to do some empirical designing.

I offer some rules-of-thumb:

  • Choose an inductance value that will make the ripple current somewhere around 20% of the maximum load current, and the maximum switch current (DC + ripple) at least 20% below the internal current limit of the part. Many (most?) of the "off-the-shelf" DC/DC converter inductors on the market are designed with this sort of operation in mind (mostly CCM, low ripple, low ESR)

  • Choose (or design) a part that provides the inductance and current rating in an acceptable package size

  • Build the circuit and test under many corners (zero load, max load, overload, short circuit, input brownout, load transients, etc.) to ensure that the converter doesn't go unstable

Adam Lawrence
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Should I just be using the largest inductor I can fit to minimize ripple current?

Remember that, as a general rule, as you increase your inductors package size, your inductors DCR goes up.

There are various other aspects that should go into your inductor selection, such as switching frequency, inductor core material, output capacitors, etc...

Some things to make sure apart from the proper inductance value are:

  • Make sure the inductor has a proper thermal current rating at the inductance you want
  • Be extra careful about the saturation current rating the inductor.
  • Try to keep the DCR as low as possible.

Linears design tools, like LTpowerCAD II, will give you a general idea of what to look for in an inductor. It uses similar equations used in the datasheets to give you these values so you are on the right track.

What I would recommend as a next step is determine your most common operating Vin and then try and design, with other voltages in mind, around that. Various inductor manufacturers have their own tools to help you find the proper inductor for your application. I've always been a huge fan of Vishays inductors and they have a great tool to help you out.

In the end, you will need to do various testing on your power supply such as no-load, full load, in-between. If you want to test it a lot, you could do a loop-feedback test on the FB pin to see how your compensation network holds up.

Funkyguy
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I can try to answer your question in a different way. During the selection of inductor, you may have to choose the value that the manufacture suggests and verify it with your circuit calculations. The value of inductor here is also dependent on the capacitor you use at the output which together decide the switching frequency of the regulator. Also the inductor value you choose should have the saturation current more than the load current of your circuit. So here in the application circuits which you are referring to you may have to check the output capacitor value also.

IamDp
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