DP83848I: Pull up resistor influence discussion of transformer

Lin_Charles

Part Number: DP83848I

Hi team,

My customer need to pass surge 2kV common mode between RJ connector and Earth on DP83848I board, they found that transformer will be very hot after surge test, when adding pull up ressitor 4.7k resistor betwwen 3.3V and TDC_P will lower the temperature. My customer have two questions about this ressitor,

1.Can we add pull up resistor? What is the influence if they add pull up ressitor?

2.They found some data loss when resistor is 4.7k, what is the value we recommend to pass surge test?

Thanks.

BR,

Charles Lin

over 6 years ago

0 Justin Lazaruk over 6 years ago

TI__Expert 7040 points

Hi Charles,

Adding a resistor to the transformer common mode is not recommended in the TI design guidelines. This circuit is not characterized by our team so I cannot comment on the implications it may have on surge protection or data transfer.

Regards,
Justin

0 Lin_Charles over 6 years ago in reply to Justin Lazaruk

TI__Expert 4260 points

Hi Justin,

Customer is pushing us about the effect if they connect this resistor, would you appreciate it if you can discuss with team to let customer know what risk they need to take. Thanks.

BR,

Charles Lin

0 Lin_Charles over 6 years ago in reply to Lin_Charles

TI__Expert 4260 points

Hi Justin,

Do you have some idea about the question from customer's side? Thanks.

BR,

Charles Lin

0 Justin Lazaruk over 6 years ago in reply to Lin_Charles

TI__Expert 7040 points

Hi Charles,

The IR drop across a series resistor to the transformer common mode is likely what is causing BER issues in your design. This is not a circuit that TI has characterized and is not recommended.

How have you choosen a 4.7k resistor? Have you tried using a ferrite bead or additional capacitance?

Regards,
Justin

0 Lin_Charles over 6 years ago in reply to Justin Lazaruk

TI__Expert 4260 points

Hi Justin,

We have tried the ferrite bead solution, but the issue cannot be resolved. So we use pull up resistor, we reduce resistor to 1.2k, but the data loss also cannot be resolved. What do you mean by additional capacitance here?

Any other solution can be shared to solve thermal performance here? Thanks.

BR,

Charles Lin

0 Justin Lazaruk over 6 years ago in reply to Lin_Charles

TI__Expert 7040 points

Hi Charles,

Two solutions that were implemented during the DP83822 system level surge testing were ensuring there is no ground plane under the magnet and MDI signal traces as well as isolating system ground from the shield ground using a 4700pF cap and 1Mohm resistor as shown in the picture below.

The DP83848 was release over 15 years ago, and surge testing may not have been a standard at the time of release. If this is a new design, I recommend using the DP83822 or DP83826 for robust industrial applications.

Regards,
Justin

0 Lin_Charles over 6 years ago in reply to Justin Lazaruk

TI__Expert 4260 points

Hi Justin,

I did not see your picture, could you share me that picture to me again? Thanks.

BR,

Charles Lin

0 Justin Lazaruk over 6 years ago in reply to Lin_Charles

TI__Expert 7040 points

Regards,

Justin

Interface

Interface forum

DP83848I: Pull up resistor influence discussion of transformer