DP83849 TD pin bias voltage

John11513

Other Parts Discussed in Thread: DP83849I

Hi team,

In my customer's application ckt, the TD pins of DP83849 are biased to 3.3V through 49.9R ressistor at device side, however on transformer side, which is located on another board and has been kept as is, the center tap of transformer is pulled to 2.5V. Can DP83848 work properly in this condition?

Thank you,

John

over 9 years ago

0 Aniruddha Khadye over 9 years ago

TI__Mastermind 18425 points

Hi John,

The PHY design requirement is to keep both the voltages at VDD. I would strongly recommend to pull the center tap of the transformer to 3.3V.

-Regards,
Aniruddha

0 John11513 over 9 years ago in reply to Aniruddha Khadye

TI__Expert 5720 points

Hi Aniruddha,

Thank you for reply.

Unfortunately they cannot change the bias voltage on the center tap of the transformer, it is on another board.

I am wondered if it is possible to keep the transformer center tap at 2.5V and bias the TP pins of PHY at 3.3V, while block the two differente bias voltage through 0.1uF caps? see below ckt:

0 Rob Rodrigues over 9 years ago in reply to John11513

TI__Mastermind 19780 points

Hi John,

Your solution with AC coupling capacitors does solve the issue of DC bias. Another problem would arise with return loss of the system and losing compliance with IEEE 802.3 specs.

There is an app note on capacitive coupling of DP8384x devices. www.ti.com/.../snla088a.pdf

In this case you are combining two isolation methods and the function of the device cannot be guaranteed.

Best Regards,

0 John11513 over 9 years ago in reply to Rob Rodrigues

TI__Expert 5720 points

Hi Bob,

Thank you for help.

I think in point of view of AC signal there is no difference between this ckt and recommended design. So I do not think it will result return loss problem. I will have customer checked the compliance of this ckt.

Regards,

John

0 john adams over 9 years ago in reply to Rob Rodrigues

Prodigy 40 points

hi, thanks foe response...
I selected dp83849i for design, but I saw this PHY have not TXER(MII transmit error)pin. but other PHY for example KSZ8895 have this pin.
thanks.

0 Ross Pimentel over 9 years ago in reply to john adams

TI__Mastermind 23675 points

Hi John,

The DP83849 does not have TX_ER because the device will automatically corrupt frames that have errors. The error will be propagated and detected by the link partner using the RX_ER pin.

Kind regards,
Ross

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DP83849 TD pin bias voltage