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Location and value of decoupling capacitors (not BGA)

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T3sl4co1l:

--- Quote from: exmadscientist on July 08, 2020, 05:05:26 pm ---
* Bulk decoupling: add on each rail one large capacitor (actual value in µF being defined by the application) with substantial ESR (>1 ohm)....

* I usually prefer electrolytic capacitors for rails that do go off-board. They're larger, but more rugged. They're also usually more expensive in the grades worth buying (105°C+, 5000+ hours, or equivalent). Be careful to avoid super-low-ESR parts; they're not what you want here and it'd be a shame to add a series resistor to an electrolytic....
* I don't find much place in general use for polymer capacitors, either tantalum or aluminum. They're a bit more expensive and have less of the bulk ESR I'm looking for.
--- End quote ---

Just a refinement to the first quoted point -- the desired ESR is of course whatever the circuit needs.  Vcore often needs much lower, for example; whereas an old school analog circuit might be most comfortable with much more (e.g. bypassing a LM317).

Electrolytics have all sorts of ESRs, so you can easily toss in a small shitty one to do the latter, or a big and stout one for the former (or polymers :) ).

Regarding the other points -- hmm, shop around!  I think you'll find polymers are available in a wide range of ESRs, just like tantalums are.  As a group, they do cluster lower, but there is much overlap. :-+

Also, if you do "screw up" with having too little impedance, you can always add series resistance.  Say you have a large common supply rail, which requires polymers or ceramics to meet its ripple spec.  Then you have smaller loads starred off from this node.  To keep those loads happy, you can either add series impedance at the connection, making a one-side-open, one-side-terminated filter; or parallel impedance, making a one-side-terminated, one-side-shorted filter.

For the first case, use a lossy inductance (R || L).  You need to know total capacitance on either side of this point, and desired impedance or cutoff frequency.  Then you can solve for L and R.  The load's plane can have low ESR caps all over (subject to the above limitations on split resonances, of course).

For the second, use some series L (may be very little, like 10-100nH of trace inductance), and a lossy bulk cap in parallel with the plane, at the load end.

Nothing wrong with doing both, of course (doubly terminated filter)!

The downside to this is, an R||L element has poor attenuation at high frequencies.  You can use higher order filter sections to account for this (say L + R||L, or more CLC sections, damped with R+Cs, etc.).

And of course, if you can afford the DC drop, an L with sufficient DCR to do the damping, is perfectly fine!

Tim

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