Tantalum caps

A friend of mine, a good old analog engineer always talks about using tantalum caps as bypass in power supply. I wonder if that's still a good practice and if they needs a 100n in parallel to improve the high-frequency behavior.

I try to avoid electrolytic caps (Al) if I don't need a large capacity. Instead I prefer to use ceramic (up to several uF) because they *should* have a better frequency response.

Where is the truth? I'm talking about standard power supply filter, near an LDO or even a small DC/DC.

Thanks Marco

Reply to
Marco Trapanese
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IIRC some of the LDO's will oscillate with an electro hung on the output. I found out the hard way.

Reply to
Dennis

Tant's are pretty good - but they do have a bit of a reputation for going leaky, although AFAIK this is usually rare unless subjected to reverse voltage.

Bear in mind that in a fast switcher, rectifier Trr can become critical - even very tiny negative spikes can upset a tant.

Reply to
Ian Field

"Marco Trapanese"

** It never was good advice and still is not.

Fitting a 100nF cap in parallel is totally superfluous in most cases.

** A ceramic or film cap of a few uF will have a lower ESR than an electro ( tant or Al ) of the same value. But a 100uF ( or higher ) low ESR electro will cost way less and do the job of bypassing way better.
** Tants are so fragile that if there is ANY chance of a large current spike, a short or the application of reverse voltage of even 1 volt - then use an Al electro. Also it is wise to derate the voltage by a factor of 2 or 3 times.

Fact is, most tantalums will not stand repeated shorts when charged to their rated voltage while Al electros laugh at such treatment.

A jaded engineer once said: " Tantalum caps are perfectly reliable, long as you put a fuse in series and a reverse diode in parallel".

... Phil

Reply to
Phil Allison

Il 21/09/2011 14:24, Phil Allison ha scritto:

The advice you're talking about is the use of tantalum cap or the 100n in parallel?

Ok, I got it.

Good point. So, changing the question: when would you use a tantalum cap?

LOL :)

Marco

Reply to
Marco Trapanese

"Marco Trapanese"

** Both.

All a parallel 100nF cap will do is create a narrow impedance dip at about

5 MHz.

** They are sensibly used where the DC supply voltage is low, there is no chance of a charge or discharge current spike nor a polarity reversal AND the small size is crucial. Eg a mobile phone.

Low leakage apps are often touted for tants - however leakage is not c " Tantalum caps are perfectly reliable, long as you put a fuse in series and a reverse diode in parallel ".

He avoided them entirely.

... Phil

Reply to
Phil Allison

Some LDOs, and some older regulators even, can be unstable with ceramic loads. If the datasheet isn't specific, beware. Older datasheets were printed before large-value ceramic caps became popular. Some regulator datasheets specifically call out tantalum load caps, usually in some tiny footnote on page 29 or something.

Tantalums have nice low but not too low ESRs and low ESLs so tend to keep regulators stable, especially switchers that need a little ESR in the loop.

MnO2 dry tantalums unfortunately tend to detonate. This is usually caused by excess dV/dT, namely too much surge current. They are very reliable if used at a fraction of rated voltage (1/2 is good, 1/3 better) or if they are downstream of something that soft starts.

Incidentally, for a 3t regulator like an LM317 or LM1117, if you put a cap across the lower Vset resistor, they soft start, and have better high frequency rejection.

Aluminums are OK but can dry out over time, and their ESR skyrockets at low temperatures when the electrolyte freezes or something. Polymer aluminums are great, but their ESR is so low they act sort of like giant ceramic caps.

John

Reply to
John Larkin

I avoid tantalum because they are very prone to shorts. They also may burn a hole in a PCB if you are unlucky.

Ceramic is a good choice but sometimes the ESR of a ceramic is too low. For example: I like to use LM1117 LDO's but I need to put a 0.5 Ohm resistor in series with the 10uf ceramic at the output.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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Reply to
Nico Coesel

Il 21/09/2011 15:44, Phil Allison ha scritto:

Ok, thank you! Marco

Reply to
Marco Trapanese

I like tantalum capacitors for good performance in the temperature range and the convenient SMT packaging. Despite of horror stories told by some people, I never had any issues with capacitors blown or an LDO oscillation. I attribute those stories to poorly made design and/or board layout.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

formatting link

Reply to
Vladimir Vassilevsky

They detonate from high dV/dT. The MNO2 is an oxidizer and tantalum powder is fuel. Too much pulse current ignites it. But it's erratic, and people who have never experienced it tend to discount the possibility.

Board layout has nothing to do with it. Design with MNO2 tantalums should avoid high dV/dT or derate voltage by 2:1 or better 3:1.

ftp://jjlarkin.lmi.net/Fried_Tant_1.JPG

ftp://jjlarkin.lmi.net/Fried_Tant_2.JPG

John

Reply to
John Larkin

IME, it's less of a problem where you have internal relatively wimpy power supplies.

When you give the customer power supply connections or allow hot plugging of PCBs it's more likely to show up.

Reply to
Spehro Pefhany

On a sunny day (Wed, 21 Sep 2011 13:16:09 -0500) it happened Vladimir Vassilevsky wrote in :

I agree.

Reply to
Jan Panteltje

Why would board layout affect whether a tantalum explodes?

John

Reply to
John Larkin

Failure to put 15" of 10-mil wide 1/2 oz copper in series with the cap.

Reply to
Spehro Pefhany

You mean, if LDO oscillates, then the cap is OK ?

Reply to
Vladimir Vassilevsky

That's about 1.5 ohms. I'd go for a little more to be safe, 60" maybe.

I've seen appnotes from the cap manufacturers who seriously suggest adding a series resistor of 1 or 2 ohms per volt of supply rail. Kinda alters the concept.

John

Reply to
John Larkin

It's more like, if the cap doesn't bypass, it's OK.

John

Reply to
John Larkin

It usually doesn't. Tants can be fine for a few months. Then one fine morning the birds start to chirp, fresh coffee smell wafts through the lab ... tsst ... *POW* ... a smoke alarm starts blaring, a stench wafts through the rooms and *POP* .. *BAM* ...

A VP of production once said "And then it was like popcorn".

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Tantalums are wonderful, reliable parts if you respect their dV/dT failure mechanism. They are IEDs if you don't.

John

Reply to
John Larkin

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