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I am designing a buck converter for an application that has several different loads. Depending on what devices are connected, the capacitance seen at the output of the buck converter can vary significantly (200-500uF). This makes designing a consistently fast and stable ac performance very tricky.

This answer suggests adding an LC filter to decouple the bulk/load capacitance from the control loop. I would like to use this approach, but how can I determine a value for the inductor? Some applications I've seen use a very small value (100-200nH with a capacitance of 100uF), others larger (470nH to 2.2uH (see section 2.3 of this application note (PDF)), but don't specify how they arrive at this.

(I'm not so concerned about noise suppression, which most of these app notes are tailored for, just transient performance.)

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  • \$\begingroup\$ Make the converter deliver an output current, and think of the load as having a minimum load capacitance, so 200 to infinity. That tells you where to put your other unavoidable time constants to maintain stability as the output C changes. \$\endgroup\$ – Neil_UK Jun 14 at 15:18
  • \$\begingroup\$ What is the controller for the buck converter? \$\endgroup\$ – Voltage Spike Jun 14 at 15:28
  • \$\begingroup\$ A variation of only 200 to 500 uF is barely anything at all and any adequately designed buck converter should easily deal with this range with virtually no signs of instability. \$\endgroup\$ – Andy aka Jun 14 at 17:41
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    \$\begingroup\$ A buck converter is a switched RLC series resonant circuit, therefore all values for every component can make a significant effect of the Q and overshoot on startup and demand special attention to loop compensation filters ( lead/lag) Nit on,y the ESR,DCR, Ron values but also the parasitic ESL and Miller capacitance. These can be modelled in your favourite Bode simulator. Mine is Falstad’s. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 14 at 18:13

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