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TPS2375: TPS2375 Adaptor ORing Option 1

AI31
Expert 1600 points
Other Parts Discussed in Thread: TPS2375, TPS23758EVM-080, TPS23753A, TPS23755EVM-894, TPS23758

Hi, 

  • I'm designing a PoE device that should be powered either by the PoE connector or a DC adaptor (prefer 12V)
  • The required output DC voltage is 5V (VDD of my system).
  • For this I picked TPS2375 and I'm following the reference design: tidubr9 https://www.ti.com/lit/ug/tidubr9/tidubr9.pdf

In the Application note, it implements the adaptor ORing at the PoE rails, and it says the adaptor can be 12V. Meanwhile, in the application report SLVA306A https://www.ti.com/lit/an/slva306a/slva306a.pdf , it says Option 1 requires 48V at the top of page 3.

Question 1: Is a 12V adaptor sufficient to give the 5V at the output or not? Is there design change from tidubr9? I noticed RAPD1 and RAPD2 values don't match 12V (following the detailed design procedure in SLVA305C https://www.ti.com/lit/an/slva305c/slva305c.pdf )

Question 2: Can I power Up (start) the system by the DC adaptor (without PoE inserted) and vice versa? or does the PoE has to be there at first and then switch to the adaptor?

  • 0
    TI__Mastermind 21320 points

    Hello,

    Please use the TPS23758EVM-080. The TPS2375 is an older device. tidubr9 designs with the TPS23753A, this is probably the difference you see. 

    To use a 12V adapter for a 5V solution, you will need to step down the voltage. There are numerous ways to do this (LDO, buck, etc.). 

    Diode OR the adapter and the output of TPS23758EVM-080 , like in the TPS23755EVM-894.

    Yes you can power with adapter without PoE. 

    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
    Expert 1600 points

    Hi Michael,

    Why do I have to switch now to I TSP23753(A)? Is it going obsolete? 

    I'm ok to use TPS23758, but I also want to OR the adaptor before the flyback converter (Option1). This way I won't have to add another down converter and I'll make use of the protection Zener if the user accidently inserted the wrong adaptor (which is very common with a PoE device, they end up inserting a high voltage one). Is 12V Adaptor enough or do I need to raise its voltage?

    Thanks

  • 0 in reply to AI31
    TI__Mastermind 21320 points

    Hello,

    No you do not have to switch. You originally said you were using the TPS2375. That device is very old. The TPS23753A is still fine. I just recommended our newest part. 

    tidubr9  is OR-ing option 2, which is what I would recommend for you. It is still before the converter. This design will work - it says on page 6 the adapter voltage is 12V. 

    That is the confusion - tidubr9 is not option 1 it is option 2. Therefore 12V is okay. 

    The important thing to consider is you need a Wide-Vin transformer. This design uses one so i recommend do not change the transformer.

    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
    Expert 1600 points

    Yes I think I made a mistake when I said Option 1, it should be 2.

    Thanks for suggesting TPS23758, I like this solution as the circuit has less components,

    - in TPS23758EVM-080, does the circuit use adaptor Option 2? Why it says 48V adaptor? while tidubr9 says 12V?

    - What is the minimum adaptor voltage that can make tidubr9 work? (Outputs 5V after the transformer)

    Sorry I'm just having hard time understanding what sets the lowest input voltage spec for these chips

  • 0 in reply to AI31
    TI__Mastermind 21320 points

    Hello,

    Adapter, adapter power, auxiliary power, DC power, DC input, 12V input  - they are all just nomenclature for the same thing. 

    The difference is what i mentioned above, wide-Vin. The IC turns on at the VC threshold - 9V for TPS23753A and 8.25V for TP23758.

    What is most important is the transformer. DOes the transformer allow for 12V-57 input, 5V output?

    The TPS23758EVM-080 does not. The other design does. 

    Please consider using our reference design tool to find designs with input voltage of 10V-57V. That will help.

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments