Hello Juan,
thank you for your comments; here my answers to your remarks:
1) sorry, I forgot to mention it. The circuit is oversized for a maximum current of 1.5A, but in reality the output current is never higher than 500÷600mA
2) The plot is referred to Vin (PIN 10); I didn't save the Vcc (PIN 9) screenshot but I checked it and nothing strange appeared.
3) Tried to increase it to 3uF, no effect. Already increased the cap on Vcc to 10uF (ceamic) with no effect.
4) I too have though to this; I well know thas passing signals through vias is not correct but the distances are very small and cannot do otherwise. The components, capacitors in particular, directly connected to the IC are as near as possible to its pins and, if on the opposite side, diretly under the IC. Anyway, I cannot see on the scope anything strange on the IC pins, which I would aspect in case of coupling of any nature.
5-6-7-8) you're obviously right and I investigate it FB signal as first thing; no spikes, noise, variation or strange things on it. The same on the other pins except COMP; I have already added ceramic capacitors on each DC pin with no effect. Also shorted the current feedback signal to GND directly on pin 1; the controller still works but again stops on surge. It doesn't stop only when R314 is disconnected or FB is shorted (without the MOS), but the same variation (maybe just smaller) on compensation pin appears. I don't understand why, without any FB variation, the COMP pin signal drops and stays low for so long.I cannot even understand whi the MOS command disappears and stays off fo so long after the surge impulse.
What worries me is that the LM3481 SEPIC evaluation board (with a varistor on its supply, a 200mA load and a smaller surge amplitude) has the very same behaviour, having a much better layout design than mine. My fear is that this could be a topology or component related problem, and I have to change the full power supply design on my application.
Again thank you for your attention and kind regards,
Luciano