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High frequency buck regulators and MLCC frequency behaviour

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justanothername:
Hi.
So I recently thought about some of those high frequency buck modules for a very space constrained application and I stumbled across this nice device:
http://www.ti.com/lit/ds/symlink/tps82085.pdf
Switching at 2.4 MHz. Would you look at the recommended output capacitor from the datasheet, it is this type:
https://product.tdk.com/de/search/capacitor/ceramic/mlcc/info?part_no=C2012X7S1A226M125AC
What is going on? The capacitance seems to drop significantly above 1MHz. As stated in the datasheet, I should, after considering the bias effect, provide a Cout of at least 8µF.
What am I missing?

T3sl4co1l:
The capacitance mainly matters to the control loop, which should roll off somewhere below 1MHz; in any case, it would only need to be compensated against ESL in the cap, which is what's causing the capacitance to flip.  And that's not really a problem, as ESL causes a phase lead, not lag, so if anything it tends to help with compensation.

At high frequencies, all that's really needed is a low impedance.  The phase angle (capacitive, resistive, inductive) doesn't really matter.  At a couple (2 or 3) milliohms, it's quite low. :)

Mind, at very high frequencies (100s MHz), the ESL becomes relatively significant, and switching edges are let through.  The same goes for trace lengths and ground paths, which can generate common mode or ground loop noise in the surrounding circuit.  To prevent this, place components close together, over solid ground plane, and if possible, arrange the input and output filter capacitors so they are nearby (acting to keep the input and output voltages as near to the same ground point as possible).  Add another LC filter stage (to both input and output) if necessary.

Tim

justanothername:
Thank you T3sl4co1l.
Judging from several datasheets those types of high frequency switchers seem to work well above the resonant frequencies of their output capacitors.
However, it is difficult for me to understand how the capacitor is working as capacitor since the inductance becomes dominant.
Maybe it just does not have to be capacitive for the switching frequency but for the maximum transient frequency of the load and the control loop.
Will it work right right at the self-resonant frequency when it is basically a short?

EDIT: It seems that my question is answered here:
https://www.eevblog.com/forum/projects/ceramic-capacitor-characteristics/
short: Impedance matters at the switching frequency no matter what kind of.

OM222O:
my main question would be: why on earth would you run it at that frequency?
There are a lot of modules with crazy high switching frequencies, but that is to allow for more phases with smart doublers which basically chop that signal to multiple phases; e.g: chop 600KHz to 300KHz signals for 2 phases.

If you have a look at any high power buck converter, the sweet spot is that 300KHz to have low ripple without throwing out the entire efficiency of the module (higher switching frequencies = higher switching loss and it's pretty linear!) the ripple is further suppressed by chokes (inductors) and capacitors which are tuned for the application to allow good transient response as well. You can have a look at any motherboard / GPU power delivery. they are purely buck convertors as the main supply rail is 12v and silicon chips require something like 1 to 1.5 volts (dangerous territory!).

justanothername:
@OM222O
as INITIALLY said: space constrained application where every mm² counts -> smaller inductors. In this case it is a die embedded into a pcb with inductor mounted ontop. 2A @ 2x2mm
and: that is NOT the main question. The main question was how this specific product works with the capacitor stated in the datasheet, not why anyone would use it.
if you ask my grandmother (if she would be still alive) she would say that the MAIN QUESTION is why on earth anyone would sit in front of a computer and how could that be called "work".

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