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I'm selecting parts for this boost converter circuit: enter image description here

The BOM lists these devices: enter image description here

I'm at a hobbyist level with electronics so I can't tell when design decisions are arbitrary and when they're deliberate.

The first capacitor is a 0402 package, but I can't solder that. Can I replace that with a similarly specified 0805?

The second capacitor is a 1206 package. Does that package size matter? Would a 0805 suffice (just out of curiosity)?

Here's what I'm looking at purchasing. Do these parts look interchangeable with the ones from the TI Workbench? enter image description here

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    \$\begingroup\$ If you're concerned about soldering 0402 packages, a WCSP IC will be a problem as well. \$\endgroup\$
    – user39382
    Commented Jan 31, 2019 at 6:39
  • \$\begingroup\$ @duskwuff I’m definitely concerned about that too. I figured I have to try it to learn. I suppose if I figure out how to reflow the WCSP, placing the 0402 into the solder paste probably isn’t too onerous. I ask some dumb questions here partially because I don’t have the basic EE knowledge to compare components and I don’t know how to get it. I look at a list of 68,000 capacitors and think, “how the heck am I supposed to narrow this down to a set of components that’ll work? Then how do I test it before I spend $50 on parts and boards that won’t do anything?” Thanks for taking the time to help! \$\endgroup\$
    – D. Patrick
    Commented Jan 31, 2019 at 6:49
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    \$\begingroup\$ except for very high frequencies, going up in package size is usually OK. Going down in size can run you into dissipation or current rating problems, and the more exotic dielectrics needed for smaller sizes can have some very unexpected falls in capacitance (down to 30% of rated) at rated voltage and high temperature. \$\endgroup\$
    – Neil_UK
    Commented Jun 14, 2019 at 14:32

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The first capacitor is a 0402 package, but I can't solder that. Can I replace that with a similarly specified 0805?

First, 0402 isn't that hard to solder. Get a good pair of tweezers and give it a try.

If you do need to substitute, the main advantage of 0402 over 0805 is lower package inductance. It's probably not critical in this application, but in case you see trouble with the 0805 solution, you might try five 1-uF 0805 capacitors in parallel instead of a single 4.7 uF part.

Another issue is availability. There's a worldwide shortage of multilayer ceramic capacitors (MLCCs) at the moment, and the big vendors like Murata are cutting production of larger sizes to focus on smaller sizes. So you may have trouble finding 1 uF in an 0805 package, or pay more for it than you expected to.

The second capacitor is a 1206 package. Does that package size matter? Would a 0805 suffice (just out of curiosity)?

Going down in size, the main risk is probably ripple current rating. If you can't find an 0805 with the same ripple current rating as the 1206 you're replacing, again you might need to parallel multiple smaller values (but then this could affect the regulator's stability, if they're counting on the ESR or ESL of the capacitor package to provide a pole or zero in the loop response at a certain frequency, so you might have to experiment)

This doesn't mean the substitutions totally won't work. In either case there's a good chance you'll have no problem. But you'll want to know what could go wrong and check for it when you build your circuit.

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  • \$\begingroup\$ That's great information! I even feel like I understood nearly all of that. I don't understand the inductance as it relates to package size and why putting them in parallel might fix it. But that's something I can probably google. :) How do I compare ESR, ESL, or ripple current? I'm looking at the datasheet for the cap I picked and the only thing it mentions about those is that they are low. :) "Low self-heating and high ripple resistance due to low ESR." Thanks again! \$\endgroup\$
    – D. Patrick
    Commented Jan 31, 2019 at 6:05
  • \$\begingroup\$ What's the part number of your proposed part? \$\endgroup\$
    – The Photon
    Commented Jan 31, 2019 at 6:07
  • \$\begingroup\$ These are the two caps I picked: mouser.com/ProductDetail/710-885012107009 mouser.com/ProductDetail/810-C2012X6S1C156M2A Thanks again! You're my hero. \$\endgroup\$
    – D. Patrick
    Commented Jan 31, 2019 at 6:27
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    \$\begingroup\$ For the Wuerth on, click on the link to "RedExpert" (and then "continue without signing in") to get more performance data. You can extract ESR and ESL values from the |Z| vs frequency chart. For the TDK one you can get similar info on TDK.com. \$\endgroup\$
    – The Photon
    Commented Jan 31, 2019 at 6:32
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    \$\begingroup\$ I don't see ripple current ratings for the Wuerth, but the TDK one gives temperature rise vs ripple current which gives you an idea where things could go wrong. \$\endgroup\$
    – The Photon
    Commented Jan 31, 2019 at 6:38

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