TPS2384: Port fault if there is another connection to the same powered device.

Karlis Tucs

Part Number: TPS2384
Other Parts Discussed in Thread: TPS23861, TPS2376

Hi. I have rather challenging problem regarding PoE.

I have 2 identical devices which use TPS2384 for PSE, and TPS2376D as PD chip. Both devices has 5 Ethernet ports.

If I connect these devices with Ethernet cable one acts as PSE and other as PD, everything is fine, but when I connect second cable connecting two PSE ports of both devices, I get fault on primary port which is powering the second device. See image below.

For this failure to happen I don't even need connect all pins of cable 2-2, just the DC- which is brown & brown/white in B mode. POE_N net in schematics below.

I have another device in which I have used TPS23861 as PSE and on that device this failure does not happen.

Circuitry regarding POE protection and ethernet transformers are same for both, but I noticed one difference.

When I connect cable 2-2 on TPS2384 device I get this voltage spike on P net of PoE connection.

When I connect cable 2-2 on TPS23861 device I get this voltage spike:

I thought maybe this longer duration voltage spike in TPS2384 case would cause Over voltage fault, but then again I dont know why it shows 101 error code.

Below are schematics for my PoE application:

Any advice will be appreciated.

Karlis

over 3 years ago

0 Darwin Fernandez over 3 years ago

TI__Mastermind 41690 points

Hi Karlis, thanks for posting on the forums! Do you also have a schematic of the TPS2376? I'd like to see how that fits into the system as well and its interaction with the TPS2384 if any. Thanks!

0 Karlis Tucs over 3 years ago in reply to Darwin Fernandez

Prodigy 65 points

Device with TPS23861 works like a PoE injector on all its ports.

I dont know why our TPS2384+TPS2376D device works ok with this TPS23861 but not with another TPS2384+TPS2376D device.

You will notice that TPS23861 application does not have Bob Smith termination, but it does not play a role in this issue. I removed it from TPS2384 device PSE ports and it didn't matter.

0 Darwin Fernandez over 3 years ago in reply to Karlis Tucs

TI__Mastermind 41690 points

Hi Karlis, thanks for the TPS23861 design as well! I'm still having trouble seeing the TPS2376 connection to the PSEs. Do you have a schematic of the TPS2376 design as well and how it is related to the PSE system (if any connection like the RTN ground or something else)? Thanks!

0 Karlis Tucs over 3 years ago in reply to Darwin Fernandez

Prodigy 65 points

Ohh yea sorry, I forgot to add image with PoE PD part with TPS2376 from device 1 and device 2.

0 Darwin Fernandez over 3 years ago in reply to Karlis Tucs

TI__Mastermind 41690 points

Thanks Karlis! Let me take a look at the schematics and I'll let you know might thoughts. But I will say, at a high level, a PSE should NOT be trying to power a PSE so this is unusual.

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TPS2384: Port fault if there is another connection to the same powered device.