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I'm using a TI SN6501 transformer driver to provide an isolated power supply for a project I'm working on. However, it is imperative that the output power rail be as clean and noiseless as possible, so I've added an LC filter onto the output of the TPS76333 (a 3.3V LDO). The SN6501 datasheet says that it switches at 410kHz, but the switching frequency can range "between 300 kHz and 620 kHz for VCC = 5 V ±10%" depending on things like ambient temperature. While I'm fairly certain my project will never be used outside of room temperature, I still want to make sure that I'm doing a passable job of blocking this entire frequency range. Will the schematic excerpt shown below work? If so, how do I best pick an inductor to block these frequencies- what specs, physical sizes, and other factors should I consider?

Edit: Added a new picture of my schematic, hopefully this one is more readable.

5V in, need 3.3V out Rearranged schematic on white background

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  • \$\begingroup\$ A decent circuit that is readable would help. Red lines on a black background is really crappy. \$\endgroup\$
    – Andy aka
    Jan 22, 2015 at 8:37
  • \$\begingroup\$ Sorry, that's default for EAGLE, and only readable if you open the image in a new tab. Is the second one better? \$\endgroup\$ Jan 23, 2015 at 6:23

2 Answers 2

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I can't read your circuit too well, but I'd suggest the following course of action. Figure out your current requirements - this will influence the physical size of the inductor, as you will need a larger core to prevent saturation as more current gets pushed through the inductor.

Next, determine the transfer function of your filter in the frequency domain. Use this to draw a bode plot by hand or using matlab. This will give you some intuition about what frequencies get cut off as you change up your values.

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  • \$\begingroup\$ If you right click on the image, and open it in a new tab, you can see the schematic more clearer. \$\endgroup\$
    – efox29
    Feb 25, 2015 at 16:06
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Your circuit has a parallel tuned circuit L3 C9 in the output which isn't common on practical switchers BUT on your variable frequency scheme it wont work reliably because the frequency does vary so where would you set it up ? inbetween ? well it would never be right in production SO just do a LC filter setup for a cut frequency of say 40KHz and you could place it before the linear post reg Also I would consider increasing all your caps to 1microfarad then 20microhenry inductance will be good the inductor must be rated at the output current BUT because the flux is rather steady at this part of your circuit then you can use a cheap lossy coil if you want to in fact you are better to run a lossy coil here

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