LM2575 switching regulator - ideal inductor and COUT sizes

[TokTok12]

Hi all,

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

Eagle schematic file: https://sites.google.com/site/bgedsadata/EASYDAQ-XBEE%20v4sch.sch?attredirects=0&d=1
Eagle board file: https://sites.google.com/site/bgedsadata/EASYDAQ-XBEE%20v4sch.brd?attredirects=0&d=1

See the following links for screenshots of the schematic and the boards respectively:

Schematic screenshot:



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 atatched 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 my attachments, 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.

I appreciate any assistance.

Regards,
TokTok.

 

[KlausST]

Hi,

Your boardhouse is a good one, they check your PCB...

1) Inductor:
Your package size is 0805 wich is smaller than 3mm x 2mm and is SMD.
I just looked for PE-52629. It is 18 x 18 mm and THM.

Maybe you should draw the inductor package in 1:1 and try to "solder" it on you PCB (printout).

2) look for a capacitor that fitts your electrical needs. Then look into the datasheet and check what size it is.
Use the same package size in your PCB layout.

3) Look into the datasheet of LM2575. There is a section called "output capacitor selection" Then look at your dealer what type is available.


++++

BTW: I strongly reccomend to use a GND plane. especially with switched, high current, low voltage... signals.

If you want to sell the pcb then you have to comply with EMI standards...

Hope this helps
Klaus

 

[TokTok12]

Hi,

Your boardhouse is a good one, they check your PCB...

1) Inductor:
Your package size is 0805 wich is smaller than 3mm x 2mm and is SMD.
I just looked for PE-52629. It is 18 x 18 mm and THM.

Maybe you should draw the inductor package in 1:1 and try to "solder" it on you PCB (printout).

2) look for a capacitor that fitts your electrical needs. Then look into the datasheet and check what size it is.
Use the same package size in your PCB layout.

3) Look into the datasheet of LM2575. There is a section called "output capacitor selection" Then look at your dealer what type is available.


++++

BTW: I strongly reccomend to use a GND plane. especially with switched, high current, low voltage... signals.

If you want to sell the pcb then you have to comply with EMI standards...

Hope this helps
Klaus

Hi Klaus,

Thanks for the reply. Yes the board house guys have been very helpful especially considering I'm of a junior level in terms of my electronics and I'm all alone on this project.

I think I've found a capacitor that fits my needs based on the section of the datasheet you refer to i.e. . It has a voltage rating of at least 12V, its form factor is much larger than 0805 i.e. it is a radial can - SMD and it is of the correct capacitance i.e. 330uF plus its Cadsoft Eagle library files are available which is a big plus. Here it is https://uk.farnell.com/panasonic/eeefc1c331p/cap-alu-elec-330uf-16v-smd-reel/dp/2254344

Based on my earlier uploaded schmatic design and my earlier post do you think the above cap would be helpful?

Regarding the inductor now I see the difference between the sizes of an inductor with a 0805 form factor to that of the PE-52629. In saying this I'm still clueless with respect to finding an inductor that has a larger form factor than 0805 and that has the closest properties to those I mention in my post i.e. 67127050 from Schott, PE-52629 from Pulse Eng and RL1950 from Renco.

Thanks,
TokTok.

 

[KlausST]

Hi,

for the capacitor you can use a tantalum LOW ESR.
TPS series of AVX is a good choice. TPSD in size "D" or TPSE in size "E".
Look for low ESR. Here the tantalum have benefits instead of conventional electrolyte...
Even better are caramics. but for 330uF ????

inductor:
If you dont find the package for the inductor and are not familiar with drawing your own package, then simply use two eyes or testpoints and set them on the right positions in the layout.
(drawing your own package is not that difficult....)

I have my own library for passives/inductors that i can send you.
With packages from WURTH: WE-PD, WE-PD3, WE-PD4. you can find that inductors at farnell.

Hope that helps

Klaus

 

[FvM]

My inductor standard for similar applications would be Wuerth PD series, e.g. 74477126, Farnell 1635916. It's 12x12 mm shielded SMT. similar types available from various manufacturers.

LM2575 is a first generation "simple switcher" IC. It's cheap, but the low switching frequency involves respectively large inductor and capacitor values. But it has also low EMI potential, so it's probably a good choice for simple double layer layouts without dedicated supply voltage filter elements.

The capacitor is basically O.K. As you are already using a throughplated switcher IC, a leaded capacitor is usually cheaper and better available than the SMD version.

 

[TokTok12]

Hi,

for the capacitor you can use a tantalum LOW ESR.
TPS series of AVX is a good choice. TPSD in size "D" or TPSE in size "E".
Look for low ESR. Here the tantalum have benefits instead of conventional electrolyte...
Even better are caramics. but for 330uF ????

inductor:
If you dont find the package for the inductor and are not familiar with drawing your own package, then simply use two eyes or testpoints and set them on the right positions in the layout.
(drawing your own package is not that difficult....)

I have my own library for passives/inductors that i can send you.
With packages from WURTH: WE-PD, WE-PD3, WE-PD4. you can find that inductors at farnell.

Hope that helps

Klaus

Hi Klaus,

Thanks for the reply. I would not mind you sending me the library for passives/inductors. My email is bgedsa12@gmail.com.

Regarding the caps I'll look into the caps you have suggested.

As for the inductor I’ve found the PE-52629 inductor (https://uk.farnell.com/pulse-engine...h-smt-0-85a-11-7x18mm/dp/2215934?Ntt=PE-52629) which has a larger form factor compared to the inductor I selected in my last design i.e. L1. I selected the PE-52629 after following the design procedure as per page 13 of the LM2575 datasheet. The challenge is finding its the Cadsoft Eagle files. I'll try and draw up the package based on what I've found.

Regards,
TokTok

- - - Updated - - -

Hi FvM

I seem to have found the inductor I wnat based on your suggestions from the Wuerth PD series i.e. the WURTH ELEKTRONIK - 74458268 - CHOKE, SMD, 680UH which is of the right inductance i.e. 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 - INDUCTOR, 680UH, 20%, SMD which has similar properties with a difference being it is sheilded.

Do you think the WURTH ELEKTRONIK - 74458268 - CHOKE, SMD, 680UH is a solid choice?

Cheers,
Toktok.

 

[FvM]

The inductors look like clones of the classical unshielded Coilcraft storage inductors and are cheaper than shielded types, also larger in size. If you don't mind some magnetical stray field they should be O.K.

 

[TokTok12]

The inductors look like clones of the classical unshielded Coilcraft storage inductors and are cheaper than shielded types, also larger in size. If you don't mind some magnetical stray field they should be O.K.

Yep. I'll go ahead with using the Wurth part but I'll darw up the Multicomp part too.

 

[KlausST]

Hi,

did you receive my library?

BTW: Multicomp is not a really existing manufacturer. Afaik its a farnell dummy name where farnell buys at MULTIple COMPanies.
Nothing wrong with that. It seems like farnell creates the datasheets with both names "farnell" and "multicomp".

Klaus