Electronic – Capacitor selection from Linear Technology generated design

capacitor

I'll preface this by saying I'm a SW Engineer. I've used the Linear Technologies Software to design a step down DC power supply from 12V to 6V. The package has given me a design but I'm a tad confused by the output capacitors given by the design.

The first output cap is a ceramic but is illustrated on the design as polarised. I thought ceramic caps weren't bothered with positive and negative connections. On top of that the spec for the cap is 10uF, ESR = 5M, and ESL = 0.3nH.

The second "Output Bulk Cap" is polarised, but I assume it can be electrolytic, and has values of 100uF, ESR = 15M and ESL = 1nH.

I normally do my shopping at Farnell's UK site and can't figure out what I need as there's nothing that comes close to 5 Mega Ohm ESR. Am I looking for a special type of cap which I'm not familiar with? I'd be very grateful for any pointers.

EDIT – this is the circuit: –

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Best Answer

It means 5 milli-ohm not mega-ohm. Imagine the capacitor part were shorted out. You'd be left with a resistor, the effective series resistor. If that we're 5 mega ohm then it's a nonsense value even for the lightest of loads on the output. Here's the equivalent circuits for capacitors, inductors and resistors. Sometimes they are shown differently but these are generally the most applicable to common circuits.

schematic