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Bypass capacitors

Hello, I've been trying to find some bypass caps that I plan on using with a DC/DC Converter (ADuM5000). And the datasheet recommend a low inductance 0.1uF and 10uF. So, I've been googling around to try and find out what I should choose here, and I'm not sure.

First I ran into this article

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which says "we recommend using aluminum electrolytics. These capacitors open when they fail" but then I ran into web pages that say electrolytics short when they fail....

Question 1: Is there a type of cap that is better for bypassing power supply rails because they fail open?

Ok, forgetting the fail open stuff I started to look for low esl. This led me to the reverse geometry stuff, and stacked caps, but... when I look up a low esl reverse geometry cap on digikey, the voltage is damn low, I couldn't find them over 10V. My DC/DC converter will have 5V on both input and output rails.

Question 2: How much higher should the voltage spec be for a bypass cap than the intended voltage it will be used for

Questions 3: What are the general, important basics for choosing a proper bypass cap

My guess for Question 3: Low ESL, adequate voltage spec and that it fails open. Those are the three I've been trying to use (I've also mainly been looking at ceramics). but I've been having a hard time nailing all 3.

much thanks!

Reply to
panfilero
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Use a ceramic cap, and it won't fail.

But yikes, 180 MHz and "up to 33% efficiency" ! The price is similar to a 1-watt SIP converter that will be over 80% efficient.

John

Reply to
John Larkin

Should I look for a low ESL ceramic, or not even worry about that part? Yup, not the most efficient part, but I like that its standalone, doesn't need any extra parts, I'm mainly using it for isolation.

much thanks!

Reply to
panfilero

"panfilero = wanker "

** True - but " fail" = die of old age.

** Only if severely abused by over voltage or over temp operation.

Try reading the context - f****it.

... Phil

Reply to
Phil Allison

An ordinary 0603 or 0805 cap has very low ESL. Keep the leads short and fat.

I like this series:

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John

Reply to
John Larkin

Is SIP referring to a package, or some sort of topology.

Reply to
miso

it is a package, think the "sip" is for system in a package sometime is it just a pcb with pins on the side so it can be mounted horisontal on a pcb, or like this:

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-Lasse

Reply to
langwadt

I used those (ADuM5000s); thirteen on one 16.5" x 1.25" board. ;-) Ceramic caps work quite well. Following ADI's guidelines for board layout we passed FCC class-A test with a *lot* of margin (~20dB).

Ceramics.

Depends on the material. Ceramics, zero (if you're sure of the extremes). Tantalums, 100-200%. Aluminums, maybe 50%.

Depends on the situation. In this particular case, use the manufacturer's data to look at the impedance at the frequency of interest. In the case of your ADuM5000s, select the cap with the lowest impedance at 2 x 180MHz (the frequency at the rectifiers). I use AVX's SpyCap for this.

Reply to
krw

Ceramics often tolerate voltages well in excess of rated, but the capacitance goes to nil. This is physically identical to an inductor which might tolerate pulsed currents well in excess of rated, but the inductance goes to nil past saturation.

On principle, I don't use any caps worse than X7R. X5R, Z5U and other very-high-k ceramics have atrocious temperature and voltage coefficients; at rated temperature, value typically drops by 80%, and the same for voltage -- presumably, around 96% reduction under both conditions, where you might as well not have any capacitor at all anymore. X7R has a modest

20% reduction over the same ranges, which still sucks, but is tolerable.

Fortunately, big values in relatively big voltages are cheap and compact. X7R comes in 0603 package up to 25V, 1uF or so, and 0805 isn't much larger if you need somewhat more. If you need a lot of capacitance, aluminum polymer is good for that.

This works great for general purposes when you aren't crunched on space, price or hard things like that.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

I use aluminums, wet and polymer, at rated voltage and don't see any problems. Tantalums are fine at rated voltage *if* they don't see any big surge currents, which this dinky ADI device couldn't do.

Hi-K ceramics don't fail at reasonable (like, 2x or so) voltage but they do lose capacitance, even at rated voltage.

I tried destroying some 16 volt ceramics, but ran out of power supply at 120 volts.

John

Reply to
John Larkin

Witches' Brew: Paint remover + acetone for a few days. I lost the little toroidal transformer picking the gunk off.

John

Reply to
John Larkin

Appreciate all the good info!

Reply to
panfilero

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