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For a power rail, we have used tantalum polymer caps whose ESR is as low as 50 mohm.

I see that a few of these capacitors have a 1 ohm resistor added in series, a significant value.

  1. What could be the reason for these resistors?

Is it for any damping functionality - damping the oscillations, if any?

Tantalum capacitors have low ESR, so wouldn't the parallel capacitors without series resistors conduct better, nullifying the intended damping?

  1. Would the capacitors with series resistors become less effective in handling ripple and transients due to the higher ESR effect?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Some crappy regulators rely on ESR for stability. Which one does yours use and what does the datasheet say about it? \$\endgroup\$
    – winny
    Commented Jan 30, 2023 at 12:43
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    \$\begingroup\$ The stability argument certainly doesn't apply in your case, because the caps with added ESR are shunted by the large 220µF caps, that lack this extra resistor. Usually the damping capacitance is at least about 10x larger than the low ESR caps. A very usual example are ~µF range MLCC in parallel with an Al electrolytic cap. \$\endgroup\$
    – tobalt
    Commented Jan 30, 2023 at 13:35
  • \$\begingroup\$ I'd strongly recommend AGAINST tantalum capacitors UNLESS you can guarantee thatyou will never ever not for a moment have a voltage spike even slightly above their voltage rating. See my answer re Tantalum capacitors here: electronics.stackexchange.com/a/99321/3288 \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 30, 2023 at 14:05
  • \$\begingroup\$ @tobalt: Does it need to be 10x higher? This ref says: "A general rule-of-thumb is to size the aluminum damping capacitor to at least three times larger than the value of the ceramic capacitor eetimes.com/… \$\endgroup\$
    – Dynamic_equilibrium
    Commented Apr 28, 2023 at 3:00
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    \$\begingroup\$ @DivyaK.S Y "At least three times larger" sounds about right IMO. It might not damp the ringing entirely or will need a very carefully chosen series resistor. 10x is safe and gives you more safe series resistance range. \$\endgroup\$
    – tobalt
    Commented Apr 28, 2023 at 7:42

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Older voltage regulator ICs were designed for use with aluminum electrolytic capacitors across the output. They can become unstable when a low ESR capacitor is placed across their output (such as a tantalum capacitor) and may oscillate, especially at low temperatures. If a circuit must use tantalum capacitors instead of aluminum electrolytic capacitors, resistors in series are added to emulate aluminum electrolytic capacitors. The spec sheet for the IC should explain in detail.

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    \$\begingroup\$ This is also frequently the case when using an old regulator with ceramic capacitors (as you can now get MLCCs into the 100s of μF) or aluminum polymer capacitors (which have very low ESR for aluminum caps). Conversely, cheaper solid tantalum capacitors may not need it, as they have higher ESR than tantalum polymer caps (anything with an ESR less than 100 mΩ is probably tantalum polymer). \$\endgroup\$
    – Hearth
    Commented Jan 30, 2023 at 19:41

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