RMS Ripple Current Rating (RRCR) Output Cap

[chimera_786]

Well I am been designing a product at work and we've decieded to use an LM2576 ADJ. Everything is fine. However, I am not satisfied when it comes of to the RMS Current rating of the output/input caps. According to the data sheet of LM2576:

1- Input Cap should have a RMS ripple current rating of 2.14 A (RMS) and 1000uF rating (overkill)
2- Output cap should have a RMS ripple current rating of 1.65 A (RMS) and 1000uF rating (overkill)

I figured I can use same rating for both the output cap and input cap (2.14 A-rms being the case).

However, my question is:

IF I parallel say, two 680uF (two on the input and two on the output), would they share the RMS ripple current requirement? As in, would the RMS requirement be split into two becaz now I have two "banks" for the circuits current requirement.

OR should I just use 1000uF @ the required RMS current rating? (kind of an expensive option though)

Price vs Reliability .. the age old dilemma

All insight is appreciated!!

[amyk]

Yes, the ripple current will be divided between the two caps.

The ESR will also be halved.

[chimera_786]

Yes, the ripple current will be divided between the two caps.

The ESR will also be halved.

Thanks and I also confirmed that with the following info. Even though I am using the LM2576, the same principle applies based on another family member of the Simple Switcher family of National Semi (now owned by TI).

Excerpt from LM2676 data sheet:

The RMS current flowing through the input capacitor is equal to one-half of the maximum dc load current so the capacitor should be rated to handle this. Paralleling multiple capacitors proportionally increases the current rating of the total capacitance. The voltage rating should also be selected to be 1.3 times the maximum input voltage. Depending on the unregulated input power source, under light load conditions the maximum input voltage could be significantly higher than normal operation and should be considered when selecting an input capacitor.

The input capacitor should be placed very close to the input pin of the LM2676. Due to relative high current operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create ringing signals at the input terminal which could possibly propagate to the output or other parts of the circuitry.