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I was planning to use this part (datasheet) (basically MC34063 with thermal shutdown,) but I don´t have enough room above the PCB. I need to keep the components under 2.2mm.

According to my calculation I need like 70uF in Cout.

I was going to use a tantalum capacitor, but then I read on page 9 in the datasheet:

"Capacitor CO should be a low equivalent series resistance (ESR) electrolytic designed for switching regulator applications."

Can I use a tantalum with low ESR instead? What is "low"? Under 3 ohm? Under 1 ohm? Or 300mohm?

Is it better to use ceramic, or do Ihave to use electrolytic? (It is hard to find short electrolytics.)

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    \$\begingroup\$ The question title kind of sounds like you (personally) are too tall. Unless that was your intention, let's rename the question to something like "Switching regulator on a height constrained PCB." \$\endgroup\$
    – akwky
    Commented Nov 24, 2021 at 12:10
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    \$\begingroup\$ The SOIC-8 package dimension table in your datasheet claims the height (C) of the IC to be just 1.75 mm. Isn't that OK? \$\endgroup\$
    – akwky
    Commented Nov 24, 2021 at 12:13
  • \$\begingroup\$ it´s not the height of the IC but of the capacitor that is the problem. \$\endgroup\$
    – mistermister
    Commented Nov 24, 2021 at 12:22
  • \$\begingroup\$ With a more modern switching IC having higher switching frequency, you'll be able to shrink both the inductor and input/output caps and use all ceramic caps. \$\endgroup\$
    – bobflux
    Commented Nov 24, 2021 at 12:22
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    \$\begingroup\$ @akwky I like a nice humorous (but clear) title :) \$\endgroup\$
    – Marcus Müller
    Commented Nov 24, 2021 at 12:35

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'Low' is probably in the 10 mΩ range. The design does not need ESR to be above a certain value.

That IC was designed before large ceramic capacitors (MLCC) were available or practical. Now they are practical and suitable for this part.

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  • \$\begingroup\$ sorry for newbie question: One would want the ESR to be low because of heat? or something else? but high for some regulators because of some something with the control loop? correct? you mention ceramic. But can low ESR tantalum work? \$\endgroup\$
    – mistermister
    Commented Nov 24, 2021 at 12:26
  • \$\begingroup\$ Is there such a thing as "too low" ESR? I'd swear I saw a (linear?) regulator which was only stable in certain "envelope". \$\endgroup\$
    – akwky
    Commented Nov 24, 2021 at 12:38
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    \$\begingroup\$ With old style LDOs, you can have too low ESR, for stability. New style LDOs should be able to cope with ceramics. With a switcher, lowest ESR is good, for ripple. Beware, ceramic capacitors, especially large values in small packages, may only deliver 20-30% of their advertised capacitance at their rated voltage. It sucks but it's true, high-K ceramics usually have a vicious voltage coefficient, which together with tempco and tolerance can leave you with waaay less capacitance than you thought. \$\endgroup\$
    – Neil_UK
    Commented Nov 24, 2021 at 13:49
  • \$\begingroup\$ @Neil_UK, spot on with the ceramic capacitor warning. MLCC capacitance in certain parts varies dramatically with applied voltage and it's a lesser-known characteristic in industry. \$\endgroup\$
    – TonyM
    Commented Nov 24, 2021 at 16:41
  • \$\begingroup\$ so if at the rated max voltage they lose 20% to 30% : at what voltage are they using for the advertised capacitance? any special voltage or just what gives the most capacitance? \$\endgroup\$
    – mistermister
    Commented Nov 24, 2021 at 20:40

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