I'm designing a PoE circuit that should be compatible with both active (802.3af/at) PoE and also 24V passive PoE. How can I do this?
Can I just use a PD chip that doesn't include a DC-DC converter? It looks like most of them are low-side MOSFET switches that only switch on when the PoE negotiation has successfully happened. But these PD chips support external adapters:
- TI TPS2379
- AD LTC4265
- ONsemi NCP1094
So maybe I could wire up the external input in some way to support passive PoE?
There are also plenty of integrated PD and DC-DC converter controller chips that are compatible with both active / standards-based PoE and external adapters. Maybe it would be possible that they can they be wired to work with non-standards based (passive) PoE?
- TI TPS23754 / TPS23756
- AD LTC4269 / LTC4278
- Microsemi PD70201 (has an 'external enable' pin)
- ONsemi NCP1083
- Akros AS1135
Alternatively, maybe I can wire my own circuit that performs the PD classification, inrush current limiting, etc. Is that difficult?