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TPS2375: Getting a switching output with ~52V peak to peak

Abhinav Ranjan
Prodigy 10 points
Part Number: TPS2375

Tool/software:

Hi 

I am using TPS2375 for PoE in a PD. Below is the related schematic:

PD class is set to Class 0.

At first I had 0E in place of 2.2uH then I was seeing below output across 100uF capacitor on the output of TPS2375:

After I replaced the 0E with 2.2uF I'm getting below output:

What am I missing here? What could be the reason I'm getting such an output?

Thanks in advance!

  • 0
    TI__Mastermind 37121 points

    Hi Abhinav,

    Thanks for reaching out!

    The reply may be delayed due to the limited resource and Lunar New Year. We will get back to you in two weeks. Thanks for your patience! 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37121 points

    Hi Abhinav,

    Could you try to add a load between POE_OUT+ and DGND? PSE needs a minimum maintain power signature current (>14mA) drawing from PD or PSE may turn off the PD port. A  ~3kOhm resistor with power rating > 3W could be considered.

    Best regards,

    Diang 

  • 0 in reply to Diang Xing
    Prodigy 10 points

    HI Diang,

    Tried adding a load of 3.3K but the output hasn't changed yet.

    I'm still getting 0-52V switching output as shown in the previously attached images.

    Regards,

    Abhinav

  • 0 in reply to Abhinav Ranjan
    TI__Mastermind 37121 points

    Hi Abhinav

    The ILIM resistor now is 1M which is higher than max rating. Could you try with a lower resistance between 62.5k - 500k? 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 10 points

    Hi DIang,

    I have tried that already, switched to 160K resistor value for ILIM but the behavior didn't changed.

    Regards,

    Abhinav

  • 0 in reply to Abhinav Ranjan
    TI__Mastermind 37121 points

    Hi Abhinav,

    Could you try with two 3.3kOhm resistors in parallel to increase the load? Or can you read the PSE side what is the failure cause?

    I did not see other issues on the schematic. One concern is the Bob-Smith terminal ground is normally not short with VSS, it normally is earth/chassis/output GND/floating depends on applications.  But it should not be related to your issue

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 10 points

    Hi Diang,

    Your point is right, I had tried increasing the load after ~3.3K load and the voltage was stable this time. However, I had chosen 50mA load, now I will try reducing it to find out the optimum value.

    Also, I would like to seek your advice on one more thing; to have this issue resolved do I need to add a resistor between PoE_OUT+ and DGND? That would be a continuous power wastage, can you suggest an alternative to this?

    Thanks,

    Abhinav

  • 0 in reply to Abhinav Ranjan
    TI__Mastermind 37121 points

    Hi Abhinav,

    Thanks for your reply. 

    The minimum DC current should > 14mA theoretically.  In this case 50V / 3.3k = 15.2mA should meet. But maybe there are some minor factors, like voltage is smaller than 50V or current sense resistor error. Using a 2.5k resistor may be enough.

    The PSE will check if the PD is still actively drawing power or the PSE will turn off the port. In most of cases you do not need to add a dummy load. Normally during start up, the PD needs to switch from open-load status to load status within 300ms to avoid PSE turning off.

    Best regards,

    Diang