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I have some question that i want to ask using an circuit example. enter image description here

  • What help provide an inductor in Vin and Vout, a short of noise filter? Can i change voltage regulator and maintain same inductors value?
  • If i change capacitors value, like from 0.1uF and 10uF to 0.01uF and 2.2uF like i see in other type of power supply of the same module, it change significantly the filter frequencies?
  • Instead of use 3 inductors, if i use multiple voltage regulators, can replace them with only 1 inductor at the power supply?
  • If i want to place a decoupling capacitor for this circuitry, which value should i use? and which footprint dimension(SMD)?
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  • \$\begingroup\$ Vin? Vout? Changing C will affect frequencies. What do you want to decouple? \$\endgroup\$ – Andy aka Jan 6 '15 at 13:22
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First, let's break down this circuit and see what it's actually doing. L2 is feeding C10 and C11, for the AVdd net, probably for some analog stuff. L1, C12, and C13 are doing the same thing for the DOVdd net, probably digital IO. These have nothing to do with the regulator other than they share a common power source.

That leaves the regulator itself. L3 and C14 form an LC filter on the input with a corner frequency of almost 88kHz. That may, or may not be particularly useful, hard to say without knowing the rest of the design. The same thing is happening on the output. L4, which is actually a ferrite bead, forms an LC filter with C15 and C16. Since a ferrite bead is only inductive to a certain point, that corner frequency could be from here to the moon. This is poor practice. The output cap needs to be right next to the regulator. Having that ferrite bead in between the regulator and output cap could result in poor output stability. If you must filter the output, this way, put an actual LC filter after the the output cap. For a linear regulator, that's not usually necessary, but a SMPS, it can be essential. Use ferrite beads along with the decoupling caps for power feeds to ICs where it makes sense to do so.

Can i change voltage regulator and maintain same inductors value?

You could, but lower the corner frequency would be preferable.

If i change capacitors value, like from 0.1uF and 10uF to 0.01uF and 2.2uF like i see in other type of power supply of the same module, it change significantly the filter frequencies?

The corner frequency has to change, physics and all. Whether or not that matters depends entirely on the application.

Instead of use 3 inductors, if i use multiple voltage regulators, can replace them with only 1 inductor at the power supply?

If you want to keep the LC filters and have multiple regulators, it would be pointless to only have one inductor. You wouldn't have an LC filter at that point.

If i want to place a decoupling capacitor for this circuitry, which value should i use? and which footprint dimension(SMD)?

The input cap is effectively the decoupling cap for the regulator. The datasheet recommends 1uF on the input and output. These should be ceramic and physically close to the regulator. It's also good practice to put some bulk capacitance, 10uF is reasonable starting place for a lot of applications.

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  • \$\begingroup\$ So, if want to use a TPS799xxxx voltage regulator i need to do this connections: - connect LC(3,3uH and 1uF) filter to Vin; - connect Vout to 2,2uF(like datasheet); - connect a FB; - connect 0,1uF and 10uF(like circuit schematics); - a bypass C of 0,01uf to NR. This circuit is made for an optic or image sensor, so i suppose that this frequencies(88Khz) may disturb sensor, that is right? ti.com/lit/ds/symlink/… \$\endgroup\$ – Yaroooo Jan 6 '15 at 15:34
  • \$\begingroup\$ Like this: i57.tinypic.com/2wp7chy.png \$\endgroup\$ – Yaroooo Jan 6 '15 at 15:37
  • \$\begingroup\$ Make the ferrite bead an inductor and you're in business. Make sure your input and output caps follow the recommendations in the datasheet. \$\endgroup\$ – Matt Young Jan 6 '15 at 15:38
  • \$\begingroup\$ But I don't know which value i need for change fb to inductor and this schematics advise me FB. Do you suggest me to use a 3.3uF inductor? Also, as you see in my image, i didn't used an input cap, but added just LC filter. Should i put a cap between LC and voltage regulator? Thank you. \$\endgroup\$ – Yaroooo Jan 6 '15 at 15:48
  • \$\begingroup\$ The problem with ferrite beads is they are generally inductive well into the MHz range, but that inductance isn't well defined, then they become resistive. If you really must put an LC filter on the output, I would use the 3.3uH since it's already on the BOM. The cap in your LC filter is your input cap at the moment. I would probably add a 10uF to have some bulk capacitance there as well. \$\endgroup\$ – Matt Young Jan 6 '15 at 15:59

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