LM5070: POE does not power on when plugged in

Daniel Briscoe

Part Number: LM5070
Other Parts Discussed in Thread: TPS23758, TPS23753A

I am working on a custom implementation of POE on a IOT device. I received the boards for this unit, however the POE appears to not power on when it is plugged in.

Because this device is a prototype, I don't know the exact current needed and have the device marked as class 0. I believe it should be drawing 5W on average.

While it was plugged in I checked the waveform on an oscilloscope, and it seemed to fluctuate between -50 and 50mV. I attached this waveform to this support request. I also attached the schematic, which I believe is functionally the same as the example application given on the datasheet.

Do you have any insight on what makes this design not power up?

over 5 years ago

0 Daniel Briscoe over 5 years ago

Prodigy 30 points

0 Michael Pahl over 5 years ago in reply to Daniel Briscoe

TI__Mastermind 21320 points

Hello,

I am assuming "the waveform" is PoE+ and PoE-. This looks like the basic waveform on the cable while the PSE is waiting for a valid detection. So your device is not giving a valid detection.

Another check you can do is to power your board with a DC power supply to ensure the DC-DC works properly.

Here is one of our reference designs to compare. https://www.ti.com/lit/ug/snva094c/snva094c.pdf?ts=1595601274659&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM5070

Additionally, we have multiple reference designs for 3.3Vout Type 1 PD's, but they use newer IC's (TPS23753A and TPS23758). I would highly encourage you to copy one of these designs.

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Daniel Briscoe over 5 years ago in reply to Michael Pahl

Prodigy 30 points

Thanks for your assistance,

I ended up figuring out the the issue was with the POE jack (not rated for type A), and not my circuit. Sorry to waste your time.

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LM5070: POE does not power on when plugged in