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I am using the SCT2331 (a clone of the Diodes AP63200, with 400KHz switching frequency instead of 500KHz). The design works with an inductor from Coilcraft (LPS3314-103), but when I try a different, same value inductor, Bourns SRR4828A-100M, the regulator doesn't generate the correct voltage, and is stuck in LDO mode. The layout supports the Bourns footprint (larger). Both inductors have ferrite core, the Bourns has lower DCR, (100mOhm vs 330mOhm) and lower SRF (30MHz vs 55MHz). What can I do to get the design to work with the Bourns part?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ When you say the design works, What is the Output current you are seeing? \$\endgroup\$
    – user19579
    May 25 at 8:05
  • \$\begingroup\$ 100 to 200mA - Well below Isat of both inductors. \$\endgroup\$
    – Lior Bilia
    May 25 at 10:17
  • \$\begingroup\$ Links to any component data sheets saves many other people having to serach for them. \$\endgroup\$
    – Russell McMahon ♦
    May 26 at 11:53
  • \$\begingroup\$ ISat is a bit low on both coils. This can be relevant at start up. The 330mOhm coil may saturate later and UOut will come up just before this is critical. \$\endgroup\$
    – Jens
    May 26 at 15:10

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The big question is what is your loop phase margin and breakpoint.

The part with higher DCR has a lower Q almost critically damped with 44uF and SRF makes no difference. The Coilcraft has a peak resonant gain of 2.5 dB while Bournes is 9 dB with both peaking around 6kHz with good attenuation 6 octaves up at 400 kHz (6x12dB/oct) but a slower response time to step loads with more overshoot possibly.

  • the fix must be in your loop lead-lag compensation to improve phase margin.

There may be better L/C choices too.

enter image description here

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