TPS23881: Using our custom TPS23881 board with the TPS2373-4EVM, cannot deliver more than 50W

Josh Elijah

Part Number: TPS23881

Hi there,

We have developed a custom PSE board using the TPS23811. The schematic is fairly vanilla, below is a screenshot, along with the key points:

Number of ethernet ports: 4
Onboard microcontroller: No
Configuration: Auto mode

We're aiming for this design to be able to work with any combination of different types of PoE devices connected. For example, a 2-pair 15W device on one port and a 4-pair 90W device on another port (or any other combination).

So far I'm testing this setup with the TPS2373-4EVM (PD eval board). I connect the TPS2373-4EVM to our custom board with a short RJ-45 cable and then simply change the jumpers (J7 and J10 set the PD type/class on the evm) on the TPS2372-EVM to configure it as different types of PD for testing.

I use an electronic load on the TPS2372-EVM to draw different power levels from it, while it is powered by our PSE board (using the TPS23811). Below is a diagram.

I'm able to verify it works for 40W delivered at the PD, 50W coming from PSE. However I can't seem to draw any more than that before the PSE shuts off power to the PD. I know the PD eval board is qualified to draw up to 90W from the PSE (71W at the PD), and I've checked all jumper settings.

So my question is, does the TPS23811 require anything else to be able supply 90W to a PD? I haven't changed anything on the SRAM, or have I written to any of the registers. Is this expected?

Thanks!

over 3 years ago

0 Penny Xu over 3 years ago

TI__Genius 15685 points

Hi Josh,

What's your auto pin's configuration? Please be aware that TPS23881 needs SRAM to be loaded in order to pass compliance test. So you will have to load SRAM patch through I2C after power on if you want to design an IEEE802.3bt compliant solution. Thanks.

Best regards,

Penny

0 Josh Elijah over 3 years ago in reply to Penny Xu

Prodigy 160 points

Hi Penny,

Thanks for your response. Our TPS23881 does not have an auto pin. I think the auto pin was included on a previous version (pin 52), but in the current datasheet, there is no auto pin. In our design, pin 52 is pulled to ground with a 7.68K resistor.

I understand we need to load the SRAM in order to pass compliance test, but what is the operation if we don't load the SRAM? Will TPS23881 output 90W without SRAM?

Can you help me access the SRAM, I can't seem to request it in the secure docs page.

Thanks,

Josh

0 Penny Xu over 3 years ago in reply to Josh Elijah

TI__Genius 15685 points

Hi Josh,

Please request access with your myTI account email through the TPS23881 product folder. This app note includes instructions on how to load the SRAM code through I2C. In the product advisory, you will find the limitations of only running the device in ROM without SRAM. I think you still should be able to output 90W. Please make sure you have 2 PSE channels connected to 1 RJ45 connecter. Thanks.

0 Josh Elijah over 3 years ago in reply to Penny Xu

Prodigy 160 points

Thanks Penny.

When I try to request the firmware while logged in with myTI account, I get the following message.

Thanks for clarifying about 90W, I do believe we have two 2 PSE channels connected to 1 RJ45 connector. Below is an image, can you check? PSE_N1 and PSE_N2 are connected to Drain 1 (pin 3) and Drain 2 (pin 5) on the TPS23881.

0 Josh Elijah over 3 years ago in reply to Josh Elijah

Prodigy 160 points

Forgot to attach the schematic image

0 Penny Xu over 3 years ago in reply to Josh Elijah

TI__Genius 15685 points

Josh,

You should have access now. Please check now.

Regarding the issue you are having, can you dump the registers values? Thanks.

0 Josh Elijah over 3 years ago in reply to Penny Xu

Prodigy 160 points

Thanks Penny, I can access the files now.

Give me a few days to get the register values, as I am away from our lab until Friday.

0 Penny Xu over 3 years ago in reply to Josh Elijah

TI__Genius 15685 points

Thanks for letting me. I will wait for your updates.

0 Josh Elijah over 3 years ago in reply to Penny Xu

Prodigy 160 points

Hi Penny,

I've setup my device to be able to read all the I2C registers and I have read the ID register as 0x55, which corresponds to the datasheet value for device ID

However, I see two addresses for the TPS23881, 0x20 and 0x21. I guess this is related to the configuration A and configuration B I2C modes, and it looks like the TPS23881 is in configuration A by default (based on table 25 in the datasheet).

I am not clear which address to use for the I2C register dump? Should I read all registers using address 0x20, then repeat that for 0x21? Or is 0x20 the only address to use? Sorry if this is a silly question, but it's not clear to me from the datasheet.

Thanks,

Josh

0 Penny Xu over 3 years ago in reply to Josh Elijah

TI__Genius 15685 points

Hi Josh，

Yes. Please use both 0x20(channel 1-4) and 0x21(channel 5-8) to read register values. Please note that I am out of office from Friday to Monday. Please expect some delays from my response. Thanks.

Best regards,

Penny

0 Josh Elijah over 3 years ago in reply to Penny Xu

Prodigy 160 points

Thanks Penny, I've just ordered another TPS2373EVM so I can test this again. Once I receive it I will dump the register values here.

0 Penny Xu over 3 years ago in reply to Josh Elijah

TI__Genius 15685 points

Thanks, Josh.

I will wait for your updates.

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TPS23881: Using our custom TPS23881 board with the TPS2373-4EVM, cannot deliver more than 50W