LT8362 mode pin problem

hi,

I am prototyping for LT8362 for a "SEPIC doubled boost" converter for 80 V 100 mA output from 5 to 8 V input. All seems to work fine, EXCEPT that above roughly 7 V input the switching waveform becomes very unstable and "noisy".

This is the basic circuit:

I first assumed I had wrong compensation components, tried several things. The response looked pretty stable as long as I was at low enough Vin.

Then I figured I probably had too small an inductance (at first it was 6.8 uH and 33 uH so equivalent to a 5.6uH boost converter) and this might be too small and give subharmonic oscillation problem. I changed to the larger inductors above. Still trouble.

To rule out the loop gain / compensation I installed a 1 uF capacitor (only) for Vc pin. Stable at low Vin, but poor transient response expected of course. The jump to instability when Vin > about 7V is completely unchanged.

Finally, I guessed maybe the SYNC/MODE pin which is right next to SW pins may be picking up too much signal from SW pins and causing something bad. (Spurious syncing? SSFM? I don't know....)

Grounding SYNC/MODE has fixed the instability, the circuit works fine for well over 8 V input.

But, I'd like to use pulse-skip not burst mode, and not spread-spectrum, in this application. Is there any connection I can make at SYNC/MODE pin that will result in pulse-skip mode but be more immune to this SW pickup problem? Has anyone else seen this sort of issue with LT8362 or LT8361?

Thanks in advance,

       Gerard

p.s. I should mention, this is a hand-built prototype on copperclad board with a few "traces" carved by hand. There *is* a solid ground plane, but for sure the distances and capacitances are different than would be on a proper PCB, but if this problem happens on this prototype I believe there is not sufficient margin against it happening in a real board.

Parents
  • hello,

    My post was delayed for moderation and so never made the top of the list of open questions, it seems. Anyways I hope some expert could please comment on the possibility of troubles in floating the mode pin of LT8362, and what to do about it.

    In the meanwhile I have tentatively connected it to a 100 pF capacitor to ground, this does prevent the problem that I was seeing. DMM says the pin comes to 0.987 V. Is it a reasonable scheme to use a capacitor to ground to reliably select pulse-skip, non spread spectrum mode? (And I still have the question whether anyone else has had trouble with floating the mode pin on this part? If so, it would be nice if the datasheet explained a bit.)

    Thanks,

           Gerard

Reply
  • hello,

    My post was delayed for moderation and so never made the top of the list of open questions, it seems. Anyways I hope some expert could please comment on the possibility of troubles in floating the mode pin of LT8362, and what to do about it.

    In the meanwhile I have tentatively connected it to a 100 pF capacitor to ground, this does prevent the problem that I was seeing. DMM says the pin comes to 0.987 V. Is it a reasonable scheme to use a capacitor to ground to reliably select pulse-skip, non spread spectrum mode? (And I still have the question whether anyone else has had trouble with floating the mode pin on this part? If so, it would be nice if the datasheet explained a bit.)

    Thanks,

           Gerard

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