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TPS2378: T2P; open collector to ground issues

Dirk-Willem (TI) van Gulik
Prodigy 10 points
Part Number: TPS2378
Other Parts Discussed in Thread: PMP11257

Tool/software:

We have a design with a TPS2378 for the PoE negotiation. Works well; and is known to work with a large range of ethernet switches. And the APD works; i.e. a supply from another source nicely quells the PoE, etc. With CDB helping a decent slow start.

However a red LED wired from the Vfeed (i.e. the positive 40-60 volt PoE voltage that goes to VDD) with a 22k resistor does not power on as expected. Varying the resistor a bit to 10k, etc does not help.

Am I misunderstanding the datasheet ? Schematic below; Vfeed/GndFeed are coming from the center taps on the ethernet transformers via a diode bridge. LED is a no-name 0603 SMD led, Forward voltage about 1.7volt at 2mAmp. Grounding pin 7 (or the kathode of the LED) to the ground, direct, or with a 10k resistor, both makes it light up as expected).

  • 0
    TI__Mastermind 37121 points

    Hi Dirk,

    Thanks for reaching out!

    1. T2P will be low if TPS2378 is connected to a Type-2/3/4 PSE. If the PSE is Type-1, or a passive injector, DC jack... T2P will remain high impedance.  

    2. If you are doing open load test with a PSE, be sure to add >0.8W dummy load at TPS2378's VDD - RTN side. Otherwise the PSE may disconnect the port due to lose of maintain power signature. 

    3. The 22 kOhm resistor could have 57V^2 / 22k = ~0.15W power dissipation. Please consider to use a high power rated package or use one with a higher resistance.  

    4. If you are using APD for adapter priority circuit. R67 - R69 APD resistor divider could be not valid. APD needs to be less than 18V always. Normally R67 is significantly higher than R69.

    5. PMP11257 is a buck converter using TPS2378 (https://www.ti.com/tool/PMP11257). It could be helpful to refer it.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 10 points

    So we know it does not dis-connect/provides power. And we can see in the switch that it draws the expected 1.2-1.4 Watt. So point 2 is moot I think.
    Point 3 - yes - taken - is a large 1206 package.
    Point 4 - VCC in this case is 5v - and that part of the circuit works.

    So that leaves 1 - so we're wondering if all the switches it is used to in the field all switch back/fall back to Type-1. Which would be odd - as most are modern, typical enterprise style, POE++ type of switches. We'll check that.

  • 0 in reply to Dirk-Willem (TI) van Gulik
    TI__Mastermind 37121 points

    Hi Dirk,

    Thanks for your reply.

    I think it is caused by your classification resistance set. Your class pin is set to Class-0 with 1270Ohm resistance, which will make a Type-2 (30W) PSE to only do Type-1 (15.4W) power handshake to your TPS2378. In this case T2P will be Hi-Z.

    You can set your CLS resistance to ~63.4Ohm to enable Type-2 handshake.

    Best regards,

    Diang