# Choosing a large, reliable input capacitor for switching regulator

A project I am designing has to be reliable for many years. I have a switching power supply using a LM2596S-5.0/TR

I am having trouble deciding on the capacitors. I have placed a tantalum 10v 220 uF on the output but on the input I am having trouble.

The datasheet says:

A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin.

The example application lists a 660uF.

Looking on LCSC, I can only find a couple of SMD capacitors at 50V, 660 uF and they don't mention their ESR. It doesn't help that the datasheets are in Chinese.

I can't seem to find tantalums of this size and voltage either.

Can anyone recommend the types I could use here? Favouring reliability if possible.

• 50V 660uF is pretty massive. Why do you need to go with just one? Your only choice is aluminum polymers May 29, 2020 at 14:36
• No reason I could not go with more. I will have a search May 29, 2020 at 15:02

For a buck converter like this, it is very important to have a low ESR ceramic capacitor directly at the input of the converter. Otherwise you will have EMI problems.

So it is a common practice to put a small (10nF) ceramic cap directly at the input. For the bulk capacitance, you can then use the proposed 680uF (low cost) aluminium capacitor. (I would not recommend a tantalum capacitor for this).

The datasheet says — A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin but the example application lists a 660uF.

The data sheets lists a 680 uF capacitor and not 600 uF: -

The phrase "660" does not appear in the data sheet at all.

The phrase "680" appears in the data sheet 4 times and each is related to the 680 uF input capacitor or the 680 pF feedforward capacitor.

680 uF capacitor are as common as muck but, if you want long life then choose carefully following this answer.

• Sorry I was going on memory. May 29, 2020 at 15:03
• Philosophical question @GlennPierce - how do you know? May 29, 2020 at 15:05

Given your constraints, I would pick a UUD1C681MNL1GS SMT type from Nichicon, assuming 16V rating is adequate. 5000 hrs @105°C 90m$$\\Omega\$$ @100kHz

If you really need 50V or better rating and 680uF or more, a Nichicon UPW1J681MHD3 leaded type 18x20mm would make more sense than some huge SMT part. 5000 hrs @105°C 55m$$\\Omega\$$ @100kHz.