How to convert capacitor impedance to ESR?
You can't  directly get the equivalent series resistance (ESR) from the impedance.  You can estimate it from the measured impedance in the datasheet.

The datasheet impedance value includes impedance from the capacitor, the equivalent series inductance (ESL,) and the equivalent series resistance.  You could calculate the capacitor impedance for its value and the given frequency, and subtract that from the impedance given in the datasheet.

Take an example from your datasheet:



The 1uF, 50V capacitors from that series have an imepance of 4 ohms at 100kHz.  An ideal capacitor has an impedance of 1.59 ohms at 100 kHz.

The difference of 2.41 ohms is then the sum of the ESR and the ESL. There's no way really to separate the two.  In any case, I can't imagine that your LM2596 would be happy with it.

Another thing to look at is the rated ripple current.  This value should be part of the design process for your circuit.  The calculated ripple current for the capacitor must be lower than the ripple current rating in the datasheet.

The datasheet for the 2596 specifies low ESR capacitors, and recommends solid tantalum.  If it were me, I'd just use tantalum and be done with it.  Many folks don't like to use tantalum because they have a tendency to burst into flames if mistreated.  In that case, look around for aluminum electrolytics with about the same ESR as tantalum.



You should either get the ESR from the datasheet, or you should measure it.

The preference should be to get the ESR from the datasheet.  The manufacturer will have designed the part and tested enough of them that the value should be reliable.

If you measure the ESR yourself, you'll have to check all (or at least a representative sample) of every batch you buy.  If it isn't in the datasheet, then the manufacturer probably isn't making any effort to have the ESR be consistent - you will have to keep an eye on it if it is critical for your design.

For a one off hobby project you could probably put up with it.  If you are making thousands a week, not so much.

There are some general rules about ESR.  Certain types of capacitor tend to have certain ranges.  But, that won't necessarily help you.  Two aluminum SMD electrolytic capacitors could look nearly identical and have the same voltage and capacitance rating - and drastically different ESR ratings.

If you need a (relatively) high ESR, then you can add a small resistor in series with a low ESR capacitor.




Tantalum capacitors have (relatively) low ESR.
Aluminum capacitors can have very low ESR (lower than tantalum) but they can also have higher ESR - you must select the ESR you need.
Ceramic capacitors can have very low ESR, but the capacitance is limited and can vary with the applied voltage.