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TPS23754/6 Questions

Johnny Lau
Intellectual 2465 points
Other Parts Discussed in Thread: TPS23754, TPS23756

Team,

Customer needs POE+ PD solution.  How do we determine the best part for them to use? 54 or 56?

They need 20W usable power at PD and will also need to support a 12V local power adaptor.

Questions once we pick a part:

1) Is there a way to determine on the PD side if high power (>13W) is available?  Is this the T2P pin?  They want to know so that the PD can turn on options that require more than 13W if available.

2) Is the TPS23754/6 also backwards compatible with 802.3af?  I assume yes but want to verify

3) are the reference design EVM the most efficient designs we have?

4) On the EVM user guides looking at the efficiency curves why are the curves different for Converter 48V, Adaptor 48V, and PoE 48V?  Seems like it should be the same if input voltage is the same.

thanks

 

  • 0
    TI__Expert 7990 points

    The TPS23756 Vc startup threshold permits startup from a 12V adapter.  The wide input-range converter used with the 12V adapter (10V - 57V) will be somewhat larger/less efficient.  A 48V adapter is the best all-around if you can go there.

    1)  If T2P is active after powerup,  a 2-event class has occurred before startup  indicating a high power (type 2) PSE is the source.  Most likely it is a midspan.  It is incumbent on a type 2 PD to try and sync up with the PSE through software to negociate power.  Lacking a 2-event hardware class indication on powerup, the PD must run at 13W or less until it can negociate higher power from the PSE.

    2)  Yes.  IEEE 802.3-2008 clause 33 is encompased within IEEE 802.3.at as a type 1 device, roughly as a subset.  Thus, the TPS23754/6 works with both '2008 standard and '.at standards.  This includes use of the '754/6 as a (hardware) class 0 - 3 device simply by using the correct Class program resistor.  Inherent is this is that the '754/6 runs perfectly well even if set as Class 4 from a type 1 PSE.

    3)  Talk to Apps and Design Services - make sure you compare designs of like requirements and design goals (colloquially known as apples-to-apples).

    4)  When comparing efficiency numbers, you must know the assumptions.  Most other vendors do not provide this.  Applications has kindly provided data for just the converter stage, converter stage powered through the adapter interface, which usually includes a blocking diode (1V/50V = 2%), and the efficiency of powering through the PoE interface which includes input magnetics loss, full diode bridge, and hotswap losses.    Always compare apples-to-apples.