Author Topic: Vcc pin capacitors near strict are you?  (Read 664 times)

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Offline Faringdon

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Vcc pin capacitors near strict are you?
« on: June 18, 2021, 06:51:35 pm »
Pg 19 of the LIS2DW12 datasheet says it needs 100nF  and 10uF (Al elec) right by its Vcc pin.
But would the best thing to do (since we don’t want electrolytic caps) be to have the 100nF….and then a 10uF,0603 MLCC cap in series with a 470 milliOhm 0603 resistor? (ie instead of the 10uF  Al elec)

Offline james_s

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Re: Vcc pin capacitors near strict are you?
« Reply #1 on: June 18, 2021, 06:59:10 pm »
If the datasheet says to do something, I do it.

Why don't you want electrolytic capacitors? Remember a 10uF mlcc capacitor is not going to be anywhere near 10uF when you put DC across it. The datasheet ought to show you the chart but off the top of my head I'd guess it will probably end up being 2-3uF.
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Offline PeteH

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Re: Vcc pin capacitors near strict are you?
« Reply #2 on: June 18, 2021, 07:14:19 pm »
Situation dependent ...

If you have a plane and tons of bulk decoupling, I would say just local ceramic bypass is fine.
If you have nothing else on the rail, you probably need to ensure you meet the transient demand of the IC without significant droop... I'd say make sure you provide a low ripple supply and at least 10uF of distributed decoupling on the plane...
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Offline bobbydazzler

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Re: Vcc pin capacitors near strict are you?
« Reply #3 on: June 18, 2021, 09:52:11 pm »
If you were going to use mlcc with a resistor instead of aluminum electrolytic then I'd use at least 20uf mlcc instead of 10uf to account for capacitance drop with voltage/temp.
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Offline T3sl4co1l

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Re: Vcc pin capacitors near strict are you?
« Reply #4 on: June 18, 2021, 10:46:37 pm »
When a bulk cap is recommended, it rarely matters where it is, so long as the local cap doesn't resonate with it (for which the ESR is indeed a good idea).  They can be shared between devices, it might be the bulk cap on the supply regulator, etc.  As long as it's reasonably nearby.

Datasheets make these recommendations on the assumption that the designer is both ignorant and forgetful; reminding them simply to put a capacitor nearby, is more likely to produce a successful outcome, than not saying anything at all, or of using a more complicated but accurate model of the device's PDN requirements.

The device in question, likely needs very little current indeed, and just needs relative peace and quiet to operate its analog innards.  A bulk 10uF fed from literally anywhere on the board, is unlikely to help much in this regard, indeed I can quite quickly come up with counterexamples where it's likely to have no effect, or worsen things; maybe it can even cause it to oscillate, who knows.

The general truth is this: one cannot understand the device's supply from the 10uF down.  It's a complete network of interaction, and everything matters.

As for 0.1's -- on a GND-VCC plane build, very few indeed might be needed.  The inductance from pin to plane is dominant, and no amount of capacitors near those pins can fix the fact that the pins are inseparably there already.  At best they can save some via inductance, but you can just as well double up vias if needed.  Likewise any near-ish 0.1 looks like a low impedance plus some body, trace and via inductance, and these again can be reduced by using multiple vias in parallel.

The plane itself looks like a nearly ideal interconnect, with a small capacitance.  (The capacitance itself is something to be careful with, as it can resonate in the ~100MHz range against some nH of stray inductance.)  There is merely a small lateral spreading inductance, related to the distance between any given pairs of vias into the planes, on the order of log(distance) -- amounting to a few nH, even all the way across a board.

As long as you're using plane pairs, there's very little that 74HC, LVC and similar families, most CMOS devices (small ASICs, MCUs, FPGAs), and so on, will have troubles with.  Sprinkle some bypass caps around the planes and be done with it.  Can always put in more footprints as insurance, and a coax connector to measure actual supply ripple and verify that it's within tolerance.

Where local caps do the most good, is in 2-layer designs where only one net can be poured (GND), and even then it has to be poured on both sides and stitched with vias, religiously.  There's no place for VCC to pour, so it must be routed point-to-point like any other signal.  To maintain a low impedance, those routing traces need periodic capacitors to keep the impedance of the lumped-equivalent transmission line down.

That is, for a route with mostly equal distances between loads, a capacitor at each node gives an LC ladder circuit, with the L's being the trace inductance between caps -- ballpark 1nH/mm of trace length.  The characteristic impedance is then Zo = sqrt(L/C), and for a linear (chain) routing topology, one or both ends of that route should be terminated with a bulk cap Cbulk >> Cbyp(tot) and ESR = Zo.  A star or tree topology is also acceptable, given that every spoke/branch is similarly terminated at the end (the nodes will have low impedances, where the branches are acting in parallel, and bulk caps will be ineffective there).

A branch can also be series-terminated by using a series filter inductor L ~ Zo^2 * Cbulk, in parallel with a resistor Zo.  This doesn't afford much actual filtering, and inductors are more expensive than capacitors, so it's rarely used.  It does have the advantage that the network can be isolated somewhat from very low impedance elements (like low-ESR ceramic or polymer bulk caps, such as you'd use at the output of a regulator to achieve low ripple voltage).

You can design actual filter networks in the same way, using a relatively large series L and terminating it into a Cbulk + ESR, with ESR = Zo chosen the same way, this time using sqrt(Lfilt / Cbyp(tot)).

The meaning of impedance here, is this: consider a step change in a load.  Maybe it goes from 0 to 100mA in a few nanoseconds or so.  If it's nominal 3.3V supply, and ripple should be less than 5% of that, then peak voltage should be under 165mV, so the supply impedance should be under 1.65Ω.  We could use 0.1uF bypass caps with a maximum mean length between them of about 27cm -- in practice we'd probably have loads every couple cm so this will be met easily.  Note that 0.1's have an ESR around 50mΩ, so the Q factor will be quite high (~30?) if we do not employ a lossy bulk cap.  If we have ten loads on this chain (Cbyp(tot) = 1uF), a Cbulk of say >= 3.3uF will do.  If the mean distance between caps turns out to be 2cm, Zo will be closer to 0.447Ω so choose ESR = 0.5Ω give or take -- easily afforded with ceramic + resistor, or tantalum or polymer.

(Note that both tant and poly are available in wide ranges of C and ESR, the polys generally clustering in lower values, but there is significant overlap in available ESRs.  Shop around, you'll find something that works.)

Electrolytic is less preferable, because ESR increases considerably at low temperature, or past end of life, and the capacitance tolerance is awful.  A good strategy then is using more than needed, so that its Cbulk hardly matters ("Cbulk >> Cbyp" as in 100 times or more) and dominates other reactances on the net, and so also its ESR dominates (which can be chosen say 2-3x lower than ideal, for this reason).

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Offline Manul

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Re: Vcc pin capacitors near strict are you?
« Reply #5 on: June 18, 2021, 11:09:06 pm »
It depends. In this case I'm inclined to think, that the idea of recommended capacitors is more about filtering external supply noise, not so much for the current surges of the IC itself. It kinda suggests me to consider Vdd as "analog grade" supply. It is partly analog IC after all, so for best performance clean supply must be important. In any case, for maximum success, things need to be understood before doing. For example, by reading T3sl4co1l posts. I mean, what can I say... Excellent as always.
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Offline WattsThat

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Re: Vcc pin capacitors near strict are you?
« Reply #6 on: June 19, 2021, 03:55:09 am »
Tim, an absolutely brilliant post. It connected the dots of several issues that I’ve never seen tied together in such a linear, quantified manner.

Thank you. Much appreciated.
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Online tszaboo

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Re: Vcc pin capacitors near strict are you?
« Reply #7 on: June 19, 2021, 08:18:46 am »
I regularly have to ignore datasheet recommendations for capacitors. Sometimes large capacitors are just unsafe, if you operate in an explosive environment. I read the datasheet, they write a capacitance there, then use best judgement, and testing to reduce that to safe levels. I also put it in an environmental chamber and cool it down to it's minimum working temperature for example.
The least amount of effort they spend on writing the datasheet, and explaining why you need that, the less likely those values matter in the first place.
If they ask for electrolytic capacitor, they are usually want to get rid of ringing. If your power supply doesn't ring, you don't need that.
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Online Siwastaja

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Re: Vcc pin capacitors near strict are you?
« Reply #8 on: June 19, 2021, 01:06:09 pm »
10uF MLCC with explicit series resistor (say, 1 ohm) is fine. Being in small 0603 case, it doesn't even carry high risk of cracking which is the usual problem when designers go to "improve reliability" in harsh conditions by replacing elcaps with large 1210 or even larger MLCCs.
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