LM5085: LM5085 Buck：Vin=60V,Vout=12V.However, after being powered on for a while, Vout will drop to 6V. Why?

Hi Wu,

So the voltage across the upper feedback resistor (R425, 130k) is changing? My recommendation is to check that there are no contaminants on the PCB (solder flux, PCB residues) that may result in a change in effective impedance from VOUT to FB. Such effects can occur some time after startup.

Also, I see you're using a 100uF electrolytic output capacitance - what is its ESR? Maybe you can probe the FB node to verify that the appropriate triangular ripple voltage is injected by coupling capacitor C425 as expected.

Regards,
Tim

Thank you firstly.
The voltage across the upper feedback resistor(R425,130K) is very stable when powered on. We do use the solder flux, but we also cleaned the board with PCB cleaner. We'll probe the FB node. Do you have any other suggestions?

So the FB voltage is changing from the normal 1.25V? You might also check that the VCC cap voltage is regulated as expected.

Are you sure you're not hitting current limit?

Please send the PCB layout for review.

In fact, there is no load at all.

Just have a look at the layout.