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I am about to build voltage booster (1.5V battery to 3.3V) with NCP1402.
Datasheet recommends 68μF tantalum or two 22μF capacitors for output. Schematic looks like this:
enter image description here

Output capacitor should have low ESR (page 16):

The output capacitor is used for sustaining the output voltage when the internal MOSFET is switched on and smoothing the ripple voltage. Low ESR capacitor should be used to reduce output ripple voltage. In general, a 47 μF to 68 μF low ESR (0.15 Ω to 0.30 Ω) Tantalum capacitor should be appropriate. For applications where space is a critical factor, two parallel 22 μF low profile SMD ceramic capacitors can be used.

I have smd ceramic 22μF caps, but I do not know whether they are suitable or not. I measured ESR of those caps with LCR meter:

8.30Ω @100Hz
0.63Ω @1kHz
0.04Ω @10kHz
0.04Ω @100kHz

ESR is fine for frequencies over 10kHz. But it is larger than recommended value at 1kHz and lower.

Is this capacitor suitable? Is it possible to ignore higher ESR at lower frequencies? Should I buy different/better caps?

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    \$\begingroup\$ If stability is not mentioned, the converter will work but you will get more ripple with higher ESR. \$\endgroup\$
    – Vladimir Cravero
    Commented Feb 25, 2017 at 18:20
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    \$\begingroup\$ You can also lower the collective ESR by using more caps of a smaller value. \$\endgroup\$
    – Los Frijoles
    Commented Feb 25, 2017 at 18:22
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    \$\begingroup\$ Switch mode converters typically operate at high frequency to keep the magnetics small, and the one you linked to looks to run around 100 kHz, so the low frequency readings are pretty much meaningless for this application. I wouldn't really trust those readings either, I think that might be a limitation of your LCR meter measuring ESR of low value capacitors at low frequency. \$\endgroup\$
    – Evan
    Commented Feb 25, 2017 at 18:52
  • \$\begingroup\$ Yes, that convertor runs at around 100kHz. That's why I was hoping that I can ignore measurements at frequencies lower than 10kHz. \$\endgroup\$
    – Chupacabras
    Commented Feb 25, 2017 at 21:06

1 Answer 1

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First, I doubt that the ESR measurement at low frequency is accurate. Ceramic caps have much lower ESR than Tantalum, which can affect phase margin and transient response since your ESR zero moves way higher in frequency.

Second, ceramics caps can show very large reductions in capacitance with DC bias, so depending on the dielectric you have you may have much lower capacitance than you think.

Third, there is significant current ripple in the output caps of a boost converter. If you do use a tantalum cap use a polymer tantalum and check the ripple current rating. Ordinary tantalum caps are known to short and ignite with excess ripple current (sometimes even within their ripple current rating.)

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  • \$\begingroup\$ +1. 22µF MLCC will have ESR below 10 mOhm. They are capable of very large ripple currents. Also, very low ESL makes them good at reducing HF noise from your DCDC. HOWEVER they make nice piezoelectric transducers, too. If the load draws pulsed current at audible frequencies, MLCC cap may buzz. Pay attention to capacitance loss under bias! \$\endgroup\$
    – bobflux
    Commented Feb 25, 2017 at 19:58
  • \$\begingroup\$ I am using "DER EE DE-5000" LCR meter. It is not Fluke, but I considered it accurate enough for my purpose. I measured many different types of capacitors I have. 10uF tantalum has 2.3Ω at 100Hz, 1.00Ω at 1kHz, 0.87Ω at 10kHz, 0.67Ω at 100kHz, 33uF electrolyte has 4.4Ω at 100Hz, 1.95Ω at 1kHz, 1.43Ω at 10kHz, 1.13Ω at 100kHz, 10uF ceramic X7R had 7.1Ω at 100Hz, 0.44Ω at 1kHz, 0.02Ω at 10kHz, 0.03Ω at 100kHz. \$\endgroup\$
    – Chupacabras
    Commented Feb 25, 2017 at 20:52

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