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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: -

enter image description here

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As a note, series inductances are generally very slow for these components (i.e. 300 pH as shown); you need to be more concerned about the stray inductance in your traces. ESR means Equivalent Series Resistance, which, along with inductance, is a non-ideal characteristic due to subsequent power losses. Such devices known as super capacitors have ultra-low ESR values. –  sherrellbc Jun 13 '14 at 17:24

1 Answer 1

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.


simulate this circuit – Schematic created using CircuitLab

If you are able to show the schematic, I might be able to recommend options.

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Andy, L2 comes from the wire of course, but why is there a C2 for the inductor/resistor model? Does this is anyway have to do with the contacts? If yes, how so? –  sherrellbc Jun 13 '14 at 17:25
@sherrellbc an inductor has a self resonant frequency due to the proximity of the coils with each other - the capacitance is anything from 1pF to several tens of pF (typically in that range for small inductors in the tens of nH to tens of uH). For an 0805 resistor, self capacitance might be around 0.1pF. –  Andy aka Jun 13 '14 at 17:29
Oops so used to Mega Ohms as opposed to milli. Hope this link works link –  jwhitmore Jun 13 '14 at 17:40
schematic –  jwhitmore Jun 13 '14 at 17:43
@jwhitmore I've added the picture to your question so that folk can review it and make recommendations. Bad lad for thinking Mega ohms hehehe!!! –  Andy aka Jun 13 '14 at 17:45

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