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TPS2378: Class 4 minimum power requirement

Frank Liu60
Prodigy 40 points
Part Number: TPS2378

According to table 1 of the TPS2378 datasheet, the class 4 minimum power requirement is 12.95W. We have an application that "switches" between low-power (~5W) and high-power modes (~15W). As a result, we may not meet the minimum power requirements for a class 4 powered device at all times. We're not very familiar with the PoE standard - what effect will this have on our power circuitry?

Thanks for your help.

--Frank

  • 0
    TI__Expert 5615 points
    Frank, That is all correct.

    In terms of designing the PD side, the impact of this information is that you would design (scale all components as appropriate) for a Class 4 application.

    If needing further clarification, or discussion, I could loop in our PD Application Engineer.

    Thanks you for designing with PoE products from Texas Instruments,
  • 0 in reply to Thomas Lewis
    Prodigy 40 points
    Thanks Thomas, we'll look to design specifically for class 4 then.
    Just out of curiosity, if our design does not meet the minimum power requirements, will the chip simply cut power to the rest of the board?
    --Frank
  • 0 in reply to Frank Liu60
    TI__Expert 5615 points
    Frank,

    The PSE will supply whatever power is requested from the PD side during the detection and classification phase (based on resistor values you place on the 'Class' pin).

    Should you want dynamic power control (the ability to vary what current...AKA power...is being requested) this would require a robust data link layer (AKA DLL) which could interrupt the PSE's normal operation, via an external MCU, and make a call for lower power.

    Thanks,