Part Number:LMZ21701
What could keep the switcher in oscillation? Vin 12V steady, output 5V5 with enable pin used. Current around 500mA max normally.
When enabled, PG oscillate and also Vout.
If the switcher is working ok on more than 100 boards of older production, and this new production of the exact same board/parts would have issues with the switcher.
The only thing we think is that some parts tolerance (including the LMZ21701SIL) can be different on that production (more than 16 cards) so the inrush current is little bit higher when enabling the switcher so that cause it to shutdown, and retry...shutdown...retry...etc.
Why it can't come back on track by himself?
By pursuing the idea of inrush current, I've changed the SS cap from 3.3nF to 100nF. It seems to work the first time at boot, but when disabling and re-enabling the supply again, it come back in oscillation.
If disabling and re-enabling again, it come back on track....doing it again, it works...
so at first boot with 100nF it's working, but after that it fail/work/fail/work in alternate way after each enable/disable.
We just found a weird thing on some cards (not each) that replacing a ferrite bead on the board for a short makes the switcher not fail...which is really weird...
I hope to have some clue here!
thanks