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| Using an MLCC above its self resonant frequency for decoupling |
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| hazuki:
I'm looking at using a 3MHz switcher, Analog Devices' ADP2108 https://www.analog.com/media/en/technical-documentation/data-sheets/ADP2108.pdf In the datasheet they recommend some output capacitors on page 14. These are 10uF 0603s. It just so happens that at 3MHz these capacitors are past their self resonant frequency and appear more like inductors! Check out the datasheet of one of the recommended caps here: https://product.tdk.com/en/search/capacitor/ceramic/mlcc/info?part_no=C1608JB0J106K080AB What would happen if I use one of the caps they recommend? Since we are above the SRF it no longer makes sense to talk about the MLCCs having a capacitance at these frequencies. These should still filter out the switching noise though since it's really the impedance at frequency that matters. Does this all sound correct? Thanks for the help. |
| T3sl4co1l:
What do you think will happen? How could you figure it out? SPICE simulation is a good exercise here. :) Keep in mind the SRF isn't a very meaningful quantity, as it will be considerably lower in a real circuit that has trace lengths. You must approximate these impedances as well. Tim |
| magic:
It literally means that the voltage drop required to change the direction of current through the capacitor will be larger than the normal ripple voltage. And that their phase relation will be different. |
| KE5FX:
They appear like really awful inductors, which means they still exhibit a usefully low impedance well beyond their SRF. (Notice how SRF curves almost always appear on log-X plots.) It's more instructive to think about the capacitors' physical construction than it is to run them through SPICE and call it a day. An MLCC is just that -- a multilayered construction in which numerous smaller capacitors, each with a somewhat-higher individual SRF, are effectively paralleled. A SPICE model that looks like a capacitor in series with an inductor and a resistor isn't entirely adequate to capture all of the subtleties. |
| AndyC_772:
You might find this article useful. |
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