TPS23754: Differences between TPS23756 and TPS23754

Tool/software:

Hello TI:

        I am planning to use a PoE PD controller which needs to support up to class 4, and I am checking the differences between TPS23754 and TPS23756, in the datasheet, I found only converter start-up voltage is different.

        I need your help to clarify below Qs:

        1. Is this "converter start-up voltage" refer to the V between VDD1 and RTN? For 23754 converter start if VDD1-RTN > 15V, and for 23756, it is 9V, right?

        2. My current understanding is that, I need to consider converter start-up voltage only when I use external adapter in parallel with POE power like the SCH shows in datasheet, if I use PoE only, I don't need to care about the converter start-up voltage because the PoE power is always >15V when the negotiation complete, right?

       3. Is there any other difference between TPS23756 and TPS23754? If I use PoE power input only, TPS23754 and TPS23756 are drop-in replacement for each other, right?

       Thanks.