Electronics > Beginners
Why are specific types of capacitors called for in terms of buffering an LDO?
GeoffreyF:
Try different capacitors, watch the ripple with an oscilloscope. Experiment. Don't be afraid to fry something.
spec:
--- Quote from: jnz on October 19, 2018, 05:58:18 pm ---
Ignoring ESR for a moment. I can make that work. Lets say all the options have the same 5mOhm ESR.
Is there a reason an MLCC or Tant or Ceramic all of equal values buffer the input to an LDO like an electrolytic would?
edit: I'm interested in the question right there because I'm finding it extremely difficult to find a 40V+ rated 10uF that fits in my height requirements. If I can looked at other styles it would help. Unless someone has a 3mm tall 10uF electrolytic that's cheap!
--- End quote ---
OK, as you have the capacitance value and the ESR values established for your design, here are some points about the two capacitor types that you mentioned as possible replacements for aluminum electrolytic capacitors.
Tantalum (solid)
Tantalum capacitors would make good replacements as far as capacitance and ESR are concerned. But there is one problem. Tantalum capacitors should not be used where the current is not limited to a low value, or tantalum capacitors may fail and sick their guts all over the place- that is the general view anyway. This means that tantalum capacitors would not be suitable for your application.
Ceramic
Ceramic capacitor plus points
p1) high capacitance/volume ratio
p2) low ESR
p3) long life (practically infinite, unlike aluminum electrolytics)
Ceramic capacitor minus points.
m1) capacitance very dependent on applied voltage
m2) capacitance dependent on temperature
m3) microphonic
The degree of the above minus points is highly dependent on the dialectic material of ceramic capacitors.
To short a long explanation, the only dialectics worth considering for your application is X5R or preferably X7R.
There is another catch. In general, the smaller the ceramic capacitor, the worse are minus points, m1 and m2.
You should work out all these parameters from the capacitor data sheet but, as a general rule of thumb, whatever minimum capacitance you want, chose a ceramic capacitor of twice that value, with a maximum of 15% tolerance and X7R dialectic. Furthermore, chose a ceramic capacitor to be as big as possible (not good for your application) and as higher voltage rating as possible.
There is a lot of information about ceramic capacitor characteristics from the manufacturers, but to get a quick feel for ceramic capacitors have a look at page 15 of the data sheet for the LT1763 series, LDO voltage regulators:
https://www.analog.com/media/en/technical-documentation/data-sheets/1763fh.pdf
Don't let all the above put you off ceramic capacitors. They are amazing components, but you need to do a worst-case budget to ensure that a particular ceramic capacitor meets all the requirements of your application, and under all operating conditions. But, if your application needs low distortion, be very wary of ceramic capacitors!
jnz:
--- Quote from: T3sl4co1l on October 20, 2018, 02:12:58 am ---
--- Quote from: jnz on October 19, 2018, 11:27:38 pm ---Not really. It's 40V LDO, 12V automotive, 30-35V claming TVS, so I can get away with a 35V electrolytic. Nichicon makes a 3.95mm which is a hair too tall (literally, about a human hair). I MIGHT be able to lower the clamping on the TVS and use a lower rated cap, but this just risks blowig the TVS.
--- End quote ---
Is that all you're worried about? A 25V electrolytic will handle that fine. It won't even get noticeably hot or bulged during such a brief event.
Don't do the same with polymers -- they'll fail shorted. (Apparently OS-CONs can self-heal, but I wouldn't count on it.)
Ditto tantalum, of course, with more fireworks.
--- End quote ---
How can you be sure a 25V cap will hold up to automotive transients? My ESD should clamp at 32V or so, but the electrolytics I've seen that even specify surge/spikes usually say something like 1.15 * Rated Voltage.
Noted on polymer and tantalum. Yea, on tantalum I've decided that's at best on the 5V side of things if I use one at all.
I guess overall I'm trying to find out if MLCC / Ceramic / Tantalum / Polymer have the same low frequency characteristics as aluminum electrolytic - assuming apples to apples capacitance. For instance, if I have to buy a 25V 22uF MLCC to get 10uF actual and I can make the ESR work, that's fine, but the LDO mfg's specifically call out an electrolytic for a reason I would think.
jnz:
--- Quote from: GeoffreyF on October 20, 2018, 02:21:21 am ---Try different capacitors, watch the ripple with an oscilloscope. Experiment. Don't be afraid to fry something.
--- End quote ---
Nice idea, but I'm up on a deadline, will only have one shot at a prototype board, and space is an issue so applying many empty pads for different package types would be an iussue. There is a chance I'll be able to fit say a 1210 inside a 2917 for example so I can try those big polymer electrolytics and a handful of MLCCs and Tantalum at the same time. Not ideal though.
T3sl4co1l:
--- Quote from: jnz on October 22, 2018, 05:24:49 pm ---How can you be sure a 25V cap will hold up to automotive transients?
--- End quote ---
'Cuz I've done it. :-//
And there's all those times people have plugged 120V equipment into 240V mains, and the caps get hot but they don't bulge or vent for quite some time. I've even heard of e.g. 200V capacitors being reformed to operate at 350V or more (not that they'll necessarily meet any of their specs after such treatment).
Electrolytics are kind of like vacuum tubes: old, reliable (heh, well, "reliably unreliable" would perhaps be a better way to put it), able to take abuse for seconds, minutes even at a time. Just don't make a habit of it, and the overall lifetime won't be seriously impacted.
If you need design assurance -- no, I don't think you're going to find that in a datasheet or appnote anywhere, and the safe thing is to follow the datasheet as far as possible, and get manufacturer approval where the datasheet comes up lacking.
Hmm, on that note, this at least hints at it;
https://cn.tdk-electronics.tdk.com/download/540988/5f33d2619fa73419e2a4af562122e90c/pdf-generaltechnicalinformation.pdf
but the graph isn't to scale so I wouldn't draw any conclusions from it in regards to surge or transient rating. They do say they can provide transient ratings as needed, which may be a custom part request, so if you need a million pieces... Never hurts to ask if they have standard parts carrying such a rating, though.
Tim
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