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LM3485: LM3485: frequency switching problem

Part Number: LM3485

Hi,
I have a problem with LM3485 switching frequency. I made simulations in WEBENCH, in LTspice and everthing was ok - frequency was about 409 kHz. So I've designed custom board with LM3485 buck DC/DC converter 24 V - 5 V (max 2 A) I took parameters of components from my simulations (schematic form WEBENCH in attachments).

What I've got?:
Output voltage is 5 V, but switching frequency is about 4 MHz when I have Cff on PCB, and 1.3 MHz after I removed Cff! So temperature of transistor was very high (very poor efficeincy) when I reduced impedance of load and I must replaced PFET twice during tests.

I am attaching screens from my scope (without Cff and load was 430 Ohms) and schematic from WEBENCH (is the same on my PCB).