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TPS23754: PoE at 18W

Franz Schellhase
Expert 3605 points
Part Number: TPS23754
Other Parts Discussed in Thread: TPS23861, TPS23882, TIDA-00465, , TPS23756

Hi Team,

A customer is looking into developing a RFID reader PoE PD with 18W power consumption and max 100MBit/s. Can you check if the following statements are correct?

  1. Needed PoE type and class: IEEE 802.3at (alternative names: Type 2, PoE+) at power class 4 (PSE = 30W, PD = 12.95W – 25.5W)
  2. The minimum PD power (12.95W) does not mean the PD has to actually take the minimum of 12.95W at all times and if it doesn't (e.g. standby mode) the PSE powers down. But it rather means that minimum 12.95W have to reach the PD in order to comply with the classification.
  3. 2 pin pairs are needed only, not 4 pairs (this is for type 3 and 4)

They need a small solution size. Would you also suggest the following designs for this?

PSE: TIDA-00465

PD: TPS23754EVM-420

Thank you,

Franz

  • +1
    TI__Mastermind 26576 points

    Hi Franz, 

    For the PSE side, I recommend either TPS23861 or TPS23882(B). See below for more details:

    TPS23861: 4-channels, Type-2 2-pair 802.3at, autonomous mode, 0.255Ohm Rsense. For smaller design TIDA-00465 should work.

    TPS23882(B): 8-channels, Type-3 2-pair 802.3bt, autonomous mode with 'B' variant, 0.2Ohm Rsense, compatible with FIRMPSE

    Looping in PD experts to check on the statements you provided and give recommendations on designs. 

    Thanks and Regards,

    Raymond Lin

  • +1
    TI__Intellectual 2895 points

    Hi Franz,
    Statements 1 & 3 appear to be correct, but for some further clarification on statement 2:

    The PD does not have to draw a minimum of 12.95 W to function correctly, it can have a lower power.
    As long as it is higher than the PSE's Maintain Power Signature requirement, 570 mW:
    ( 10mA for 802.3at standard, 57V * 10mA = 570 mW )

    The 12.95 W value is to be considered more of an upper bound for Class 3 devices, rather than a strict lower bound for Class 4.
    It is fine for a Class 4 PD to draw a lower power, such as 5 W, for example.

    As for the PD, the TPS23754 design you linked would be a good fit.
    Hope this helps.

    Best,
    Sarah

  • 0 in reply to Sarah Lao
    Prodigy 60 points

    I bought a TPS23756 demo board during that time. It works with a 5 V output.  But we are also thinking about 24V/1A output. Do you have an application for this?

  • 0 in reply to Bernhard Schuessler
    TI__Intellectual 2895 points

    Hi Bernhard, 
    The TPS23756 will work for a 24 V output. Sadly, we don't have any reference designs on hand for a 24 V application,
    but we do have designs for a 12 V/1.25A output with the TPS23756 with the TIDA-00091

    I'm not sure if that answered what you were asking for, but let me know if I can help you any further!

    Best,
    Sarah