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UCC12050: Why the output voltage is wrong?

Part Number: UCC12050

Hi experts,

My customer reported a strange issue with UCC12050: the output voltage is not selected value.

Their target VISO is 5.0V, and the applied VINP is +5V. EN is shorted to VINP, SEL is shorted to VISO. However VISO voltage is measured about 3.3V, even all load on it has been disconnected. 

It seems the IC itself is good since it function properly when soldered on another board (different PCB).

I am not sure what is the possible root cause? I am guessing if it is due to device incorrectly reading the voltage of SEL pin due to some reason? Such as too slow rising edge of VINP. I cannot find any requirement for VINP rising edge.

Thank you!