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I'm developing a PCB in Cadsoft Eagle as seen in the brd and sch files as per the links below:

Eagle schematic file

Eagle board file

See the images below for screenshots of the schematic and the board respectively.

Eagle schematic screenshot

Eagle board screenshot

My issue is with feedback I received from my board house regarding the sizes of my inductor and Cout being too small in relation to the switching frequency of my LM2575 switching regulator which as per page 7 of the datasheet is 52 kHz.

For the switching regulator sub-circuit design I carefully utilised the design procedure as per pages 13 to 18 of the LM2575 datasheet.

Based on the provided design guidance within this pages the stand out selection factors in relation to picking an inductor are:

  • The inductor is rated for operation with a current rating (Irating) of 1.15 x Iload whereby I load in my case is 0.4A hence Irating equals 0.46A.

  • The inductor is rated to operate with the LM2575 switching frequency of 52kHz.

The pages also provide inductor part numbers from three different manufacturers. Selection of the part number is based on the inductor value which in turn is based on the desired LM2575 Vout, maximum input voltage into the LM2575 and Iload. In my case my desired LM2575 Vout is 5V, my maximum input voltage is +24V and my Iload is 0.4A hence the inductor value as per the figure 28 on the LM2575 datasheet is 680uH which as per table 2 on page 16 of the LM2575 datasheet means the following inductors can be used: 67127050 from Schott, PE-52629 from Pulse Eng and RL1950 from Renco.

1st question: What exactly is wrong with the size of my current inductor selection as per my .sch and .brd files or above screenshots? I'm using an inductor of package 0805 as per the eagle package.

2nd question: Does anyone have any idea where I can find the Cadsoft Eagle library files for any of the three inductors? I feel one of these three should be of the right size in relation to the feedback from my boardhouse.

As for the selection of Cout, the design guidance within pages 13 - 18 doesn't mention anything regarding the size of the Cout capcaitor to be used. In my design as per links above, I'm utilising a standard 0805 capacitor which I have used all across my schematic.

3rd question: Any idea which Cout capacitor size I should be selecting?

Note: I would like to stick to using the LM2575 as I've gone quite far with integrating it within my design plus the availability of its Cadsoft Eagle library file.

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    \$\begingroup\$ I'd like to share my design philosophy which might help you out a little bit in the future. I always start by designing the schematics. Get the values of your components right and so on. After that, I search for availability of components, and their footprints which I have to use for the PCB design. Personally, I browse farnell.com If you are a member of their community it is often possible to download the eagle files from their website as well (very helpful). I recommend search for the components before starting your PCB design. \$\endgroup\$ – Matthijs Jun 3 '14 at 9:43
  • \$\begingroup\$ @Matthijs: Thanks. I look to implement your well structured design philosophy as I move on. \$\endgroup\$ – user34395 Jun 3 '14 at 11:06
  • \$\begingroup\$ I seem to have found the inductor I've been searching for i.e. the WURTH ELEKTRONIK - 74458268 which is of the right inductance - 680uH and it can handle the current with a decent margin i.e. a current rating of 720mA which covers the result of 460mA (1.15 * 400mA) current rating as per the design procedure (page 13 of the LM2575 datsheet). In addition it's library availability in eagle is a plus! The other option was the MULTICOMP - MCBF7330-681MU which has similar properties. Do you think the WURTH ELEKTRONIK - 74458268 is a better choice than the MULTICOMP - MCBF7330-681MU? \$\endgroup\$ – user34395 Jun 3 '14 at 15:54
  • \$\begingroup\$ It's your own choice really. Try to compare both datasheets to each other and try to determine which is the best. Another important factor could be costs, maybe you want to keep things low cost, then choose the cheapest. Although from experience, I know Wurth delivers pretty neat material :) \$\endgroup\$ – Matthijs Jun 3 '14 at 17:49
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Without spending a lot of time on this, there simply do not exist parts in that package size (0805) which match the electrical requirements for the inductor or Cout.

You do not get to pick the package arbitrarily, you must look to see what packages are available and practical, and pick one of those. That part will have a part number and manufacturer, and you should list that on the BOM (Bill of materials).

For example, you might choose an aluminum polymer part for Cout such as the Panasonic 10SVP330M, which is 8mm in diameter and stands 12mm tall. There are several technologies that could be used for the capacitor and multiple manufacturers, and you should pick at least one specific one ( preferably more than one ) and design that in.

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  • \$\begingroup\$ thanks. I get your point i.e. check available packages, select one and design around it. I've had a look at the 10SVP330M capacitor and I noticed it is a 10V rated aluminum polymer capacitor which is fine but to follow the advice in the datasheet which mentions low ESR capacitor types are recommended whereby low value caps e.g. <12V usually have a high ESR so I'll be searching for a similar cap to that you proposed with a higher voltage rating. \$\endgroup\$ – user34395 Jun 3 '14 at 11:05
  • \$\begingroup\$ It would be good to have some idea of the requirements. Aluminum polymer caps all have relatively low ESR compared to general purpose aluminum electrolytics, 17m\$\Omega\$ in the case of the one I mentioned, but it's not the lowest available, nor probably the smallest, the longest life, the most available, the cheapest, the nicest color or whatever else might be important to you. The advice about voltage rating is a bit bizarre and certainly dated. Most motherboards have 6.3V low ESR caps. \$\endgroup\$ – Spehro Pefhany Jun 3 '14 at 11:24
  • \$\begingroup\$ Ah so ideally a 6.3V rated cap would be close to ideal? As long as the cap is 330uF and is rated for the expected ripple current. \$\endgroup\$ – user34395 Jun 3 '14 at 11:41
  • \$\begingroup\$ Maybe, they're certainly made in huge quantity. \$\endgroup\$ – Spehro Pefhany Jun 3 '14 at 11:46
  • \$\begingroup\$ A couple of 6.3V rated options uk.farnell.com/panasonic/eeefc0j331p/… OR uk.farnell.com/panasonic/eeefk0j331ap/… taht culd be potential matches to the Cout requirements. \$\endgroup\$ – user34395 Jun 3 '14 at 11:58
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Per either inductor's datasheet, can you live with the 1ohm series resistance of the inductor? Both inductors look equivalent, but the Multicomp gives you more details on the part performance.

Also, you may consider using shielded inductors in this design to mitigate any EMI/high frequency issues? It looks like you are attempting to build some type of wireless control card.

Regarding the caps, give yourself some margin. Although a 6.3V cap on a 5V line may suffice, consider a 10V cap to allow derating. You can lower the effective ESR by using a large electrolytic in parallel with a small ceramic. So maybe a 330uF and a 0.1uF or similar.

Sidenote: Although it's convenient to simply import the Eagle footprint you want, don't let that confine you to components that aren't ideal. You should get comfortable building a footprint for the component you want.

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  • \$\begingroup\$ thanks for the post. Yes I am developing a wireless control card. I'll go with the WURTH ELEKTRONIK - 74458268 for now and if this causes any EMI issues then switch to the MULTICOMP - MCBF7330-681MU which I will draw up in Eagle. As for the cap I will go for the 10V rated cap - uk.farnell.com/panasonic/eeefk1a331ap/… to deal with derating as you mentioned. \$\endgroup\$ – user34395 Jun 4 '14 at 8:36

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