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LM5017: LM5017 flybuck switicing frequency go out of control

Part Number: LM5017

Hi,

recently we apply a demo board, which is built with "LM5017 flybuck" topology (An-2004)

the demo board proclaim it could be operated between 24V - 80V input voltage

I applied DC voltage to it from 48V to 24V slowly to observed the change of input current value by fluke electric meter, and meanwhile check the switching frequency by oscilloscope

i found out at the very point of 27V ~26V, the input current suddenly increase 6~8 mA (i think it should be a bit inverse linear with input voltage)

at the same time the switching frequency go out of control (nominal designed value should be 750KHz, but it just go to strange condition, around 2MHz)

please kindly help to give some direction of it

thanks for help.