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TPS23756EVM some explanation

Semir Fazlagic
Intellectual 290 points
Other Parts Discussed in Thread: TPS23756EVM, TPS23756

Dear sirs

We want to incorporate TPS23756EVM design because it contains all we need.

I read more doc. about 237xx family and from SLVA306A I understand that for Option2 ORING

for POE priority for wide range (12-48V AUX) we need add some circuits for sensing POE supply

(see page 7 and 8). I can't see these  circuits. Is they really necessary or can be omitted ?

My second question is: My customer needs some Power ON/OFF switch. I know that it request is unusual

but I should implement it. I need  simple don't allow TPS23756 to start regardless AUX or POE supply.

I will have some OC , but where to apply ?

Thank in advance

  • 0
    TI__Expert 7990 points

    In the case where the adapter voltage can be larger than the PoE voltage, you need a solution like figure 5 of tha app note.

    The ON/OFF switch implementation depends on what the customer considers true ON/OFF.  A 3PST switch cutting off the two PoE inputs and an adapter input (maybe a 2PST would work) is the most conservative, keeping all the circuits unpowered.  If allowing some circuits to be powered is acceptable, disabling teh PoE by holding DEN low and holding CTL low might be sufficient.

  • 0 in reply to Martin Patoka
    Intellectual 290 points

    Thank you very match.
    We didn't buy EVM yet but surely we will buy, so I had doubt.

    Can you in this moment confirm that Option 2 (preference Adapter) (figure 8) is OK

    for range 12V-48V . In my opinion yes.

    Concerning ON-OFF it is true that is not simple, I will check yet with customer to clarify.

    Thank you one more

  • 0 in reply to Semir Fazlagic
    TI__Expert 7990 points

    Reading that section of the app note, you see considerations of voltage qualification and the voltage rating of the APD pin.

    It sounds like you plan to use with both 12V and 48V adapters.  Consider the case of a PD design set up to start with 10V on the adapter.  Now imagine that a 48V adapter is pluginto the pd, but not the ac.  Now plug the adapter in. 

    Questions for you are:

    "will your adapter voltage rise faster than the startup delay of the PD converter?"  "Will this be true for a (AC) power glitch with restart?"

    For example, if the adapter voltage rises to 10V in 5ms, and the PD starts, will there be a latchup?

    "If the converter starts before the adapter voltage is mostly on, can the adapter source enough current to complete turnon (avoid load-line latchup)?"  

    These are generic (physics-based) power system considerations.

  • 0 in reply to Martin Patoka
    Intellectual 290 points

    Thank you very match.

    Yes, our goal is wide range for AUX input.

    We even think about supporting also AC input which will be via separated bridge and diodes rectified to dc

    To be honest, I didn't think about all these issues about you are talking. We simple started from your reference

    design, where is noted wide range input 10-57. We expected that it means that any input in this range can be used an nominal.

    Unfortunately, there is not some additional user manual which explains these issues.

    As I understand you, for 48V input we should select mounting D22 ) zener, instead of R29 (0) Ohm ?, what is of course

    impossible for "generic input" 10-57 V.

    Now, when you know our goal, is some other configuration (maybe POE prefers APD) could be more adequate for us,

    even using more complex circuits ?

    Hoping to hear you soon, best regards