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TPS2375: TPS2375 not working with a TPS23861

Daniel Riches
Prodigy 20 points
Part Number: TPS2375
Other Parts Discussed in Thread: TPS23861

Hi.

I have a simple circuit connected up exactly as the datasheet shows for a class 3 PD using the TPS2375, it's connected to a PoE switch (TL-SF1008P) which uses the TPS23861. The web site for the PSE product says it's 802.3af compliant however the TPS23861 is 802.3at compliant. I'm having real problems getting the PD to switch on. I've now removed the RDet resistor from the TPS2375 and placed it directly across the two diode bridges in series with a 220 ohm resistor and the PSE is now trying to switch on as it briefly hits 48V and then drops out to detection mode but only when I cut the track to the TPS2375 power rail and the rest of the circuit!! With the 2375 in the circuit all I get is frequent pulses around 5.6V and that's it.

So basically I can only get to a 48V almost working state is by placing the 24.9K + 220Ohm resistor across the HD01 bridge outputs (dual bridges from a H2019 transformer)

Do you know any reason why this is happening and should I be using another TI part as the PD front end instead of the TPS2375? I can't supply schematics as this is plain old school prototyping (so far it's proved to be the quickest route to production!). I've also replaced the 2375 in case I'd shorted out the N/C pin etc and broken it. Any information would be greatly appreciated.

Kind Regards, Dan

  • 0
    Prodigy 20 points
    Well, after spending days on this and getting no response from anyone to give me even any pointers, I decided to design my own detection, classification and power on circuit using a mosfet and a few zeners and it works perfectly under all the test conditions I can think of. Nice to get some help TI (add sarcasm here!)....

    It appears that the detection works on the TPS2375 but it won't pass classification and then go on to power up, I tried all sorts but to no avail. Plus since it's a fault latching device it makes it very difficult and time consuming to figure out what's wrong. Hence, I'm giving up with the TI PoE PD devices completely, my circuit may take up 3 times the space and cost a little more but at least it actually works!!
  • 0 in reply to Daniel Riches
    TI__Mastermind 41760 points
    Hi Daniel,

    We do apologize for the delayed reply, as many of our apps engineers were not available this past weekend.
    The TPS23861 is .at compliant which means it should be interoperable with both Type 2 and Type 1 (some prefer to refer to Type 1 as '802.3af') power levels.

    For Type 1 applications the TPS2375 is widely used for PD-only designs. It has also been tested by UNH-IoL (3rd party tester for IEEE compliance) and passed.

    If the PSE is not turning on to ~48V, then it's not passing detection. For this case, I typically place a detection voltage (from a power supply) at the PoE front end to see how far from the acceptable detection current it is. If the current is too high, I would check that there is no extra loading from any components (or added components) on VDD-VSS that might be adding to the detection current.

    By looking at the waveform VDD-VSS, if the voltage ever goes to ~48V, then it has passed detection and the issue could be from the load between (VDD-RTN).

    There is also an EVM available for the TPS2375 that could be used for prototyping out a solution. This is in the link below.
    store.ti.com/TPS2375EVM-TPS2375-IEEE8023AF-POE-Powred-Device-Evaluation-Module-P549.aspx