Electronic – Is this capacitor suitable for boost converter (low ESR)

boostcapacitoresrpower supply

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?

Best Answer

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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