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TPS2378: PoE+ LLDP protocoling, implement or not?

Øystein Kristiansen
Prodigy 70 points
Part Number: TPS2378
Other Parts Discussed in Thread: TPS23731, , TPS23734

Hi

We are designing a product that will be connected over Ethernet throguh an RJ45 connector and be powered over Ethernet with PoE+ class 4. We are looking at ICs for power negotiating and from what I can read up on the Internet it seems like its two ways to negotiate PoE+, type, class 4 power = 30W.

Eiher HARDWARE BASED or SOFTWARE BASED/LLDP. 

And from what I can read up it seems like the 802.at standard specifies that PoE+ PSE equimpment has to at least implemnt one of them, but PoE+ PD equipment has to implement both.

Based on this it seems like like if we are going to be compliant with  802.at standard, as a PD equipment we have to implement both, but almost all IC we have looked at for power negotiating only supports HARDWARE based negotating. 

Does this mean that PoE+, class 4 power negotating is almost always done with HARDWARE BASED negotiating, and that LLDP is very uncommon and unlikely to experience for typical PSE equipment?

Anyt reccomendation for PoE+ ICs to use would be appreciated

  • 0
    TI__Genius 15295 points

    Hi Oystein!

    Currently there is no PD in the industry that supports LLDP and negotiation is most always done hardware based. This is due to the need to implement a microcontroller and software in order to complete negotiation, which adds a lot of cost and complexity and is not necessary. If your system has a microcontroller in order to complete negotiation with PSE then that could potentially work if you have PSE that supports LLDP.

    I would also like to clarify IEEE802.3at standard is the old standard for PoE, the newest standard is IEEE802.3bt which is backwards compatible with 802.3at standard but allows for added functionality and higher power.

    If you are looking for PoE+ Class 4 solution, there are several products that will suffice (TPS23734, TPS23731, and TPS2378)  but will depend on system spec to determine which is the best selection. Does the design need isolation? Does the design favor being cost effective or high efficiency? With this details I can guide in the right direction for selecting a device!

    Very Respectfully,

    Brandon Beader

  • 0 in reply to Brandon Beader
    Prodigy 70 points

    Hi Slight smile

    The requirement is, based on that LLDP is very uncommon, and we do not need any complaint PoE+ standard power supply:

    • PoE+ class 4, 30Watt
    • Hardware based negotiation
    • High efficiency, we need a power thats highly efficient because temperature is measured to be an issue. We have a prelimionary design today with PoE modules installed with only 77% efficieny, and its gets too hot, we are redsisnging the heat sink solutions, but higher efficiency would be appreciated
    • Since we don't need to be IEEE 802.at complaint, I think we can drop the required isolation requirement from the standard, then we can use non-isolated DC/DC design, we need to go from 48V DC to 5V DC  

    BR

    Øystein

  • 0 in reply to Øystein Kristiansen
    TI__Genius 15295 points

    Hi Oystein,

    Based off your need for high efficiency, I would recommend going with an active clamp forward design which also helps with thermals. However, if you would still like to go with buck converter design I will make two different recommendations, both of these devices utilized hardware based negotiation.

    • TPS23734 - Class 4 IEEE802.3bt standard which is optimized for ACF DCDC design (High Efficiency)
    • TPS2378 - Class 4 IEEE802.3at standard standalone PD which allows for external DCDC controller or buck topology

    I would also like to note for IEEE standard 30W is based off the PSE side, therefore the minimum received at the PD is 25W based on 100M of cable. Please keep this in mind as you will not see 30W at the PD with Class 4, you will see slightly less depending on cable loss.

    Very Respectfully,

    Brandon Beader