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LTC4366-2 output regulation problem

Category: Hardware
Product Number: LTC4366-2

We are designing surge protection circuit for meet MIL-STD-1275 standards. In this design we used LTC4366 with example circuit at datasheet page 15. We are adjust timer capacitor to hold on surge time during test.

We have some test cases.
1. With no load, we run MIL-STD-1275 test procedure voltages with power supply (28V-100V-40V), Outpu voltage clamping works fine.
2. While 1A or more loaded, @28V we can shutdown circuit via SHDN pin with no problem.
3.While 1A or more loaded, we increase the input voltage above 45 V, output voltage clamping does not work properly and output voltage oscillates as figure below.

Our observation is at high input voltage (above 45V) and load 1A and more circuit does not properly works. I added osiloscope visuals.
Could you help me with this?

No load or light loaded case

1 A or more loaded case