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TPS23751: About Adapt IEEE 802.3af

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Guru 21045 points
Part Number: TPS23751
Other Parts Discussed in Thread: TPS2375

Hi Team,

 

We understand that TPS23751 is a suitable device for IEEE 802.3at.

Therefore, TPS23751 can’t use for IEEE 802.3af PSE even if set CLASS0 to 3(less than 25.5W).

Is my understanding correct?

 

And, we understand that TPS2375 is a suitable device for IEEE 802.3af.

Could you please let us know if you have any recommended device?

 

[Image]


 

Regards,

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  • 0
    TI__Expert 5615 points

    Hide,

    The TPS23751 can inter operate with an .af switching hub no problem.  It will, of course, be limited to Type 1 (Class 3) power levels - 12.95W at the input to the load - but it will pass connection check and power up to Class 3 (or 2 or 1 depending on resistor setting).

    You can feel free to purchase one of our TPS23751 EVMs for testing with the .af PSE equipment of your choice to test for any interoperability concerns you might have but later versions of the standard were defined in order to allow for backward compatibility.

    Thank you for considering PoE products from Texas Instruments,

  • 0 in reply to Thomas Lewis
    Guru 21045 points

    Hi Thomas-san,

     

    Thank you for your prompt reply.

    I understood it as follows.

    If PES is IEEE802.3af and TPS23751 is set CLASS0,1,2,3, correct operation.

    If PES is IEEE802.3af and TPS23751 is set CLASS4, incorrect operation.

     

    Regards,

    Hide

  • 0 in reply to Hide
    TI__Expert 5615 points

    Hide,

    If PSE is IEEE 802.3af (Type 1) and is connected to the TPS23751 (Class 4, Type 2, IEEE 802.3at) PD, power demotion will occur (which is a feature of PoE - not "incorrect operation").  Power demotion is:

    "any time a PD is allocated power from a PSE that is lower than what the PD requested. And its benefits is that it allows the PSE to supply power to a PD  even if the PSE does not have all of the PD's requested power available. Also it allows higher power PDs to operate in a reduced mode when connected to  lower power PSEs."

    I recommend you watch the following video on power demotion in order to develop a deeper understanding of the operation.  The video discusses the matter from the perspective of an .at PSE connected to a .bt PD...but the same operation will occur when an .af PSE is connected to .at PD.  I think the video explains this well.  Once you watch it, feel free to ask any follow up questions.  It is part of our larger PoE training series.

    A sample of the video script (around 3:45) reads as follows:

    "Power demotion is not really a new feature. In the current IEEE802.3at standard, type 2 PDs could demote to type 1 when connected to a type 1 PSE or a power-limited type 2 PSE. So in this case, the PSE will present one class finger to the PD. For the new IEEE802.3 standard, since the class levels are increased, the PSE will be able to demote type 3 and type 4 PDs a similar way, which we'll talk about in the next slide."

    There is also an E2E FAQ on this topic you can read.

    Thanks again for considering PoE products from Texas Instruments,

  • 0 in reply to Thomas Lewis
    Guru 21045 points

    Hi Thomas-san,

    Thank you for the detail information.

    I was able to understand that well.

    I greatly appreciate your cooperation.

    Regards,

    Hide

  • 0 in reply to Hide
    TI__Expert 5615 points

    Of course Hide.  My pleasure.