How "low" depends on the efficiency and reliability you are trying to achieve. For low ESR capacitors the manufacturer will supply the values.

Search for Nichicon low ESR capacitors and you will find parts that have a low ESR. The VR series is not a low series resistance capacitor. The PM Series is and the ESR is specified in the datasheet. Nichicon (which makes excellent capacitors) may have some newer series.

The ESR is critical to the life of the capacitor since as ESR increases the temperature of the capacitor will increase which will decrease its life.

(* jcl *)

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It's 'equivalent series resistance', and is somewhat frequency dependent. Basically it's the unavoidable ordinary resistance that comes along with the capacitor.

Lower ESR means that the capacitor is more like an ideal circuit element. Resistance just dissipates power, which results in heat, which is generally no good for capacitors, especially electrolytics.

Just speculating now - On that data sheet you linked, the salient parameter looks like 'tangent of loss angle'. If one assumes that 'loss angle' is the angle away from a purely capacitive reactance, then the tangent of that angle would be the series resistance divided by the capacitive reactance, in which case this number being low would imply ESR to be low.

In this case I think the low ESR is needed to get a low ripple on the output voltage.

The current through the inductor will have some ripple, and assuming a constant output current the capacitor will have to absorb or supply the difference to the output. Multiply this ripple current with the ESR and you get the ripple voltage.

You can measure ESR by charging and discharging the capacitor with a relatively high current switched by a function generator, and then measure the ripple voltage with an oscilloscope.

I've seen 170 mOhm in practice for a low ESR SMD electrolytic capacitor. If I remember correctly the voltage difference was 0.5V so the current ripple must have been 3A (limited by the power supply).

If a datasheet just says 'low ESR' without specifying a value you are usually fine with any style of capacitor relatively low ESR. All this really means is that you should avoid cheap unrated aluminum electrolytic caps since their ESR is terribly high, can be several ohms.

In this case it wants a 'low ESR' cap for the 1000uF output cap. I don't think i've ever seen a ceramic 1000uF cap and a 1000uF tantalum would probably cost $50 so you are going to have to track down a low ESR aluminum cap for this application. The output ripple will decrease linearly with the ESR of the cap, so lower is better up to whatever price you want to pay.

As an aside, that is a ridiculously high required output capacitance for a switching regulator in that voltage range. You may want to take a look around for a regulator that meets your needs but is stable without such a requirement. Don't get me wrong, usually more capacitance the better but 1000uF is really high for a 1.5A supply.