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Topic: Do I need cap's around 3V3 regulator when source is regulated 5V? (Read 2 times) previous topic - next topic

Grumpy_Mike

Multilayer Ceramic Capacitors are not normally used when you want a capacitor with low ESR, that's why they don't put it in the data sheet. You will find in in the sheets of polarised capacitors. By and large if it isn't in the data sheet they are not proud of it.

maniacbug

Thanks, guys.  Man, electronics is bewildering...

Ok, so now I am looking at this part:  Lelon RXJ010M2ABK-0511P Aluminum Electrolytic Capacitors - Leaded 100V 1.0uF 105C 5x11 mm Datasheet.  It says this, "Low ESR, suitable for switching power supplies", and it has "Impedance" all over the place, which is I think what we're talking about, right?  7 Ohms for this guy.  Low enough?

RuggedCircuits

Quote
Multilayer Ceramic Capacitors are not normally used when you want a capacitor with low ESR, that's why they don't put it in the data sheet. You will find in in the sheets of polarised capacitors. By and large if it isn't in the data sheet they are not proud of it.


The opposite is true. MLCC's have extremely low ESR (see this presentation for example), lower than that of tantalum, and often too low for LDO's that expect a minimum ESR for loop stability.

The RXJ010M2ABK-0511P is definitely overkill for this application (100V??) but its impedance is fine -- 0.16 ohms at room temperature (I'm not sure where you got the 7 ohms number from).

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maniacbug


The RXJ010M2ABK-0511P is definitely overkill for this application (100V??) but its impedance is fine -- 0.16 ohms at room temperature (I'm not sure where you got the 7 ohms number from).


Ok, can you help me find where you got 0.16?  For my number, I looked at the tables on pages 2-3 until I found the section for this unit (100V (2A)), then looked at the row for 1uF, and followed over to the column for "Impedance / 20C".  Voila, 7!

Mouser has a category for "Low Electrolytic Capacitors", and this is the ONLY stocked product in that category at 1uF.

RuggedCircuits

Sorry, my bad, I thought you had picked out the 100uF capacitor. Indeed the 1uF capacitor has an impedance of 7 ohms, not really a good choice. I'd maybe get a 33uF 16V capacitor which only has an impedance of 1.3 ohms. 47uF 16V would be even better, with only 0.6 ohms of resistance.

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