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As far as I know, normally when you directly supply the circuit, it must have decoupling capacitors to deal with noise. If I use an LDO instead of directly supplying it, do I need to have decoupling capacitors? In most datasheets they will recommend the best value of capacitor for best stability. Since inside most of LDOs is a CMOS FET, I am concerned that adding more capacitance (decoupling) than recommend is going to mess up the capacitance of the LDO input. Am I just worrying too much?

Here is my circuit LDO is driving ICs about 350mA max, supply is given to the terminal block (J4):

enter image description here

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    \$\begingroup\$ Just follow the recommendations for the given part. You will not know more about its internals than the people who designed it. \$\endgroup\$
    – Lundin
    Jul 15, 2022 at 9:10
  • \$\begingroup\$ Are you confusing "decoupling" caps with some other power supply caps (maybe bulk storage)? Decoupling caps are typically smallish in the 10's to 100's of nF, and there's 1 or 2 of them placed immediately adjacent to each pair of power supply pins on each IC in your circuit. You don't lump all the decoupling together in one place on your board where it could be replaced by a single component (LDO or otherwise). \$\endgroup\$
    – brhans
    Jul 15, 2022 at 9:13
  • \$\begingroup\$ nope those bypass cap near ICs is no problem their as their placement is near those ICs i mean the big one maybe Tantalum with 1 to 1 uF that place near supply source (connector maybe?) \$\endgroup\$
    – Settasak
    Jul 15, 2022 at 9:24
  • \$\begingroup\$ @brhans, capacitors to decouple fast switching ICs from a PSU connected by PCB tracks/planes, like logic chips with spike currents, are typically 10..100 nF, as you say. But capacitance to decouple a board from a PSU, connected say by cable, is much larger. That bulk capacitance is often 10s/100s uF or much more, depends on the currents drawn and load frequencies. That's to be placed by power input connector(s) and distributed near loads, including any power out connectors. So the sizes, location and distribution of all decoupling C across PCB are selected for the functions and PCB layout. \$\endgroup\$
    – TonyM
    Jul 15, 2022 at 11:56
  • \$\begingroup\$ so lets say I have 2 tantalum capacitor align at the power connector power trace after those capacitor going in to LDO and LDO will have 2 mlcc capacitor at input and output each i will algin theme next to LDO is this ok? \$\endgroup\$
    – Settasak
    Jul 15, 2022 at 12:00

3 Answers 3

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I am concerned that adding more capacitance (decoupling) than recommend is going to mess up the capacitance of the LDO input.

You can't have too much capacitance on the LDO input. This capacitance just provides a reservoir of charge for the LDO to draw from when it needs to supply current to its load. A bigger reservoir won't cause any problems with the LDO.

What it might cause is a high inrush current when you first connect or turn on the external supply. Whether that's a problem and how you might want to deal with it depend on what's providing the external supply, how much inductance is in the wires between the supply and your board, etc.

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  • \$\begingroup\$ so a big no for decoupling capacitor at connector? \$\endgroup\$
    – Settasak
    Jul 15, 2022 at 15:08
  • \$\begingroup\$ @Settasak, it won't cause a problem for your LDO's. It might cause a problem for your upstream supply. You haven't told us enough for to be able to say. I wouldn't expect anything below 500 uF to cause a problem. And 100-400 uF might be helpful if your loads produce very large switching transients. \$\endgroup\$
    – The Photon
    Jul 15, 2022 at 15:12
  • \$\begingroup\$ thank you. for your help \$\endgroup\$
    – Settasak
    Jul 15, 2022 at 15:17
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    \$\begingroup\$ There could be problems if the output capacitor is much larger than that on the input of the regulator. When input power is disconnected, the charge on the output will inject reverse current through the regulator. It might be good to add a diode from output to input to take care of this. \$\endgroup\$
    – PStechPaul
    Jul 15, 2022 at 18:57
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The input caps can be shared, if the regulators are positioned close together. The output caps, plus whatever the attached loads are bypassed with, is necessary for stability.

Note that, if J4 is subject to hot-plugging, you may want a larger electrolytic, or a TVS (e.g. SMAJ5.0A) to protect against inrush peak voltages.

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If I use an LDO instead of directly supplying it, do I need to have decoupling capacitors?

Yes, because LDOs generally aren't designed to regulate away high frequency noise. This makes sense: high frequencies can be attenuated easily by adding capacitors, and that is a lot simpler and more cost effective than making a faster LDO.

As @The Photon points out, LDOs don't mind large capacitors in the input, but the rest of your circuit might.

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