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Original question:

SN65HVD3086E: Data communication

SN65HVD3086E: Data communication

Sandra Yip
Prodigy 20 points
Part Number: SN65HVD3086E
Other Parts Discussed in Thread: LM22675, LM25011

Dear Parker

We did further tests and would like to share our results. Request your advice.

1) During further tests, we find when the RS422 signals and DC_24V are separated and connected using separate cables, this problem does not occur. The RS422 signals is OK with cable length of 200 to 300 meters, when DC_24V is not present on the same cable.

2) On our board have used DC-DC converter IC  LM25011MYX/NOPB 9V-30V to 5.00V @ 1A. We had followed the design as shown here https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=80D4213D6E367940

3) We tested with a older generation product  that uses a different DC-DC converter LM22675. This problem is not seen.

4) Our device connect using CAT5/CAT6 cable to the remote master unit. This is the RJ45 cable connection signal & pin details.

Pin# Signal RJ45 Cable
1 Tx+ White Green
2 Tx- Green
3 Rx+ White Orange
4 DC_24V Blue
5 DC_24V White Blue
6 Rx- Orange
7 GND White Brown
8 GND Brown

Regards

Sandra

  • 0
    Guru 292040 points

    Apparently, the LM25011 generates more switching noise. I guess you have to add more filtering in front of it. (Use an oscilloscope to find out the noise frequencies.)

    Using the same signal for both conductors of a twisted pair will cause common-mode noise. DC_24V and GND should be paired, two times.

  • 0
    TI__Mastermind 47997 points

    Hi Sandra,

    Thanks for the detailed update!

    So basically based on your results the SMPS is generating a lot of switching noise when you are using the LM25011 and this problem isn't seen with the LM22675. So based on your webench design - the 25011 is switching at >x2 the switching frequency of the LM22675 - so my  first thought would be to turn the switching frequency since the LM25011 can go slower (you will also get a better efficiency by turning it down - https://webench.ti.com/power-designer/switching-regulator/customize/30) The regulator switching speed is the largest source of noise and a lower frequency seems to be a large difference between the current design  and the older one that worked.

    Extra filtering caps at the output could help but that may affect the soft-start if that is something that is important in your design. 

    I think turning down the frequency of the switching regulator would be my first step to see if that helps - because the >1MHz switching speed may be causing too much interference with data signal. If that doesn't solve your problem extra filtering at the output is a possibility but that will effect the regulator response a bit as well. The absolute best way to remove SMPS noise is to follow it with an LDO - its not great from an efficiency point of view but those parts do a great job at cleaning up SMPS noise. 

    Please let me know if you have any other questions and if any of these solutions help and if not we will see what else can be looked into!

    Best,

    Parker Dodson