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Original question:

TPS23750: Layout Questions

TPS23750: Aux power

Daniel Field
Prodigy 40 points
Part Number: TPS23750
Other Parts Discussed in Thread: TPS23753A

We are using the TPS23750 as a non-isolated design with 12.5 VDC of output power.  We are also using auxiliary power of the device using ORing Option 3 (the power going directly to the output side of the PoE power converter). 

If I am powering the device with just the aux power, everything works fine until I bring the voltage up to 12.8 V in which there is a 30 mA jump in current.  I also have one of the TPS23750 eval boards and I am seeing this same jump in current right around 12.8 V.  

A few questions: 

 - Is there a maximum Voltage that is allowed to power from Aux power if using ORing Option 3?

 - Do I risk damaging my PoE Circuitry if I power the device externally with 13 VDC?

 

  • 0
    TI__Genius 9230 points

    Hello Daniel,

    Before proceeding, could you give more clarity about your output voltage & current requirements? Additionally, have you considered TPS23753A? We have several tested & optimized non-isolated reference designs featuring the TPS23753A that you are free to copy.

    Regards,
    Tom Amlee
    PoE PD Apps Engineer

  • 0 in reply to Thomas Amlee
    Prodigy 40 points

    Tom, thanks for the response.  

    We are outputing 12.4 VDC with a current output of 0.7 A.  I like the reduced circuitry that the TPS23750 has for using the non-isolated design so am planning on sticking with the current part.

  • 0 in reply to Daniel Field
    TI__Genius 9230 points

    Hi Daniel,

    Where are you seeing the 30mA jump in current? (Input of the converter, output to load, etc.)

    Try the same conditions but with a diode on the output of the converter.

    Regards,
    Tom Amlee
    PoE PD Apps Engineer

  • 0 in reply to Thomas Amlee
    Prodigy 40 points

    Thomas, it is the converter that is sinking the 30 mA of current.  Looking at the datasheet, it doesn't mention there being any issues with powering directly at the output of the converter.

  • 0 in reply to Daniel Field
    TI__Genius 9230 points
    Daniel,

    Without the diode, the positive rail is moved 0.3V higher. The regulator thinks it is 0.3V out of regulation, so it is turning on the FET, sinking current into the inductor on the negative rail.

    Section 3.4 of SLVA306A says the adapter voltage must be less than the PoE output voltage to prevent power from flowing backwards. This is applicable to both buck & flyback. In the case of the flyback in the app note, we recommend turning off the synchronous rectifier. In a buck, we would need to use a diode or load switch.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Prodigy 40 points

    I've looked into this issue quite a bit more and found out the following:

     - When I am powering the unit just from an auxiliary power source, when the input power goes above 12.8 VDC, the TPS23750 is attempting to go into classification mode, hence the TPS23750's internal 10 V Regulator is enabled and supplying 10 VDC across RClass which is where my current is coming from.

    My RClass resistor has a high enough power rating that it will not be adversely affected by permanently being in the classification mode.  My question now is, would there be possible harm to the TPS23750's internal 10 V Regulator if the device is maintained in Classification Mode (Supplying 27 mA of current out of Class)?

  • 0 in reply to Daniel Field
    TI__Genius 9230 points
    Hi Daniel,

    Staying in classification will reduce your efficiency & may cause the part to get hotter, but it should not damage the IC.

    We recommend using TPS23753A so the APD pin can be used to turn off the PoE portion of the PD controller & give priority to the adapter.

    For more details about OR-ing implementations with TPS23753A, see our app note: www.ti.com/.../slva306a.pdf

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer