• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 62 subscribers
  • Views 497 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS2373: How does LLDP tie into the design of the TPS2373 controller?

Daniel DeLuca
Prodigy 220 points
Part Number: TPS2373
Other Parts Discussed in Thread: TPS23880, , , TPS2378

We're looking to integrate POE++ into our device, but are need some expertise on how LLDP fits in the equation. Based on what I've read about POE++, when we plug into a POE port, the PSE applies a small DC voltage between the wire pairs and measures the received current. This is done at a regular interval to ensure the PD is still there. Then once were discovered, the PSE initiates a classification test where we indicate what power we need (based on the resistor values we choose for the PD controller). The PSE will then indicate what it can allocate to us and demote us if necessary. There are 3 outputs on the PD controller that will tell us what power we were allocated. For our application, if the allocation is 40W+ we can then use that to turn on the DC-DC controller and boot our SOM. From there the SOM can begin negotiating over LLDP? 

The LLDP part is not clear to me. Are there switches that solely rely on LLDP to negotiate power? Or is this hardware negotiation standard on every 802.3bt switch? Do we even need LLDP at all? 

Thanks!

  • 0
    TI__Mastermind 21320 points

    Hello,

    Just to clarify something upfront: IEEE802.3.bt and POE++ are not equivalent. They are both standards for sending high power (>25W) in PoE, but they are not interchangeable. We have a training video on this topic:

     

    In 802.3.bt and POE++, the power negotiation is done through a hardware handshake, like you described above. We have a training video on that topic as well. So for these two methods, LLDP is not required to send power. LLDP can be used, for example, to fine tune the power limit, to communicate more information with the PSE, and so on. It can be used instead of the hardware handshake. But it will be implemented with an MCU to handle the communication. 

     

    My suggestion is to get one of our TPS2373-4EVM-758 and our TPS23880 EVM to play around with. 

    Additionally, the TPS23730 can support up to 51W, so this IC can support your design as well. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 220 points

    The videos and your explanation helped clear that up very much, thank you. With regard to the LLDP only solution, what power is allocated by the PSE if there is no hardware handshake? Class 0? Class 1? Basically if we went the LLDP only route, what power does our MCU have available to negotiate that power?

    Dan

  • 0 in reply to Daniel DeLuca
    TI__Mastermind 21320 points

    Hello, 

    So initially the PD and the PSE will negotiate Class 0 power - 13W. This low power state is used to power the MCU, which can then negotiate more power through LLDP. 

    So the detection and class resistor still must be set on the PD to get the initial power. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 220 points

    Makes sense, thank you.

    We've run into issues in previous products that used the TPS2378 where we set the classification resistors to indicate we are a Class 4 device, but are initially not given the power from the customers PSE. At that point we allow our MCU to take over and negotiate the Class 4 power we need via LLDP. What could be causing that behavior? We show up on the PSE as a IEEE Class 4 PD when we ping the switch, but are allocated less until we negotiated over LLDP.

    Thanks,

    Dan

  • 0 in reply to Daniel DeLuca
    TI__Mastermind 21320 points

    Hello Daniel, 

    If you are doing hardware classification, to set the TPS2378 to class 4, the only thing to set is the class resistor. We can prove that if you probe VDD_VSS and see the two class fingers during the PoE handshake. That would solidify that the PSE is sending the correct power.

    There is something in PoE called power demotion. It is where the PSE can detect a PD at a certain class level, but allocate less power. This could be happening. 

    If you are doing it through LLDP, and the PSE is expecting LLDP, then it may make sense that the PSE does not allocate more power until LLDP.

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments