I know this is fairly simple, and I get why these components are there. However, I do not fully understand why the certain values are picked and what the end filtering result is.

  1. Is the inductor in series or parallel with the capacitors? It seems to be in series with them to ground, but parallel to the Vin pin (one end connected to Vcc, other to caps and Vin).

  2. Is this a series LC filtering circuit? How would you determine which frequencies are being restricted? I know the basic equations, but not necessarily how to apply them.

  3. I understand that the caps shunt AC to ground, and the inductors 'block' AC with high impedance. The end result is that the L and C are filtering AC noise, but why are these particular values chosen?

  4. Why are several caps used in parallel, and why is one in front of the inductor?

  5. On the output, why are two capacitors used, one that is 50V and one that is 25V rated?

Supply filtering caps and inductors


1) Neither, this C - L -C structure is a low pass filter, it blocks high frequency voltage variations from +15V_ISOL to reach the chip.

2) You could calculate a bandwidth for the filter consisting of 6.8 uH and 11.1 uF (the sum of all capacitors) formula: Fc = 1/(2piSQRT(LC)) = 18.4 kHz So at 18.4 kHz a signal would roughly be halved in amplitude. How much the most left 10 uF cap filters depends on the impedance of the +15V line.

3) That depends on the expected frequency components on the +15V line, how sensitive the IC is for certain frequencies (it might supress low frequencies already by itself but might not be able to supress high frequencies). Available board space, cost of components, etc...

4) Several in parallel because a 10 uF cap is usually not so effective at higher frequencies, this is where the 0.1 uF comes in, it is more suited for high frequencies. Look at a datasheet of any capacitor and you will see it will only behave as a capacitor within a certain frequency range. By combining several caps, the effective frequency range is extended (the combination gives a better capacitor). The 10 uF at the left also gives some filtering.

5) for small value caps like 0.1uF a 25 V version might not be available or a 50 V version was already used elsewhere in the design. As long as the voltage rating is high enough any rating can be used. The choice depends on price, availability etc.

  • \$\begingroup\$ This is a great answer, thank you for the effort. I did the Fc calculation before posting, but wasn't sure why 18.4 kHz would be chosen specifically. I thought I may have done that wrong. Was this 6.8uH inductor likely chosen because it is a common value (such as the 0.1, 1, and 10uF caps)? \$\endgroup\$ – jareddbh Oct 7 '15 at 23:34
  • \$\begingroup\$ I don't know why 6.8 uH was chosen, maybe it was used elsewhere in the design, maybe they had a whole bunch of them already, maybe these were the smallest they could fit on the PCB, maybe it was the cheapest they could get away with. There are many variables that determine which component is chosen. \$\endgroup\$ – Bimpelrekkie Oct 8 '15 at 5:58

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