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,
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.