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LM25017: LM25017 Design lead to the CE Issue

Steven Liu1
Expert 4380 points
Part Number: LM25017

Hi Team, 

My customer is design with LM25017, input is 24V, and output is 9V for powering the RS485. The SCH can seen as below.

The only loading part is the 9V output, and there no loading on the VDD output. 

The input 24V is from a RC powering as below:

Based on this design, the transmitting noise should be filtered by the L1 and L2. 

Now the CE issue is that:

If the 9V for RS485 output is connecting to the RS485 device with a long routine, the LM25017 design' CE signal would exceeded the requirement. 

As you can see as below, the CE's failure happened in the harmonic frequency which below 1MHz:

 

So we'd like to learn is there any issue with the LM25017's SCH design ,especially for those R, C values. 

If not, how could we solve this problem?

Please feel free to let me know if any comment! Thanks! 

  • 0
    TI__Mastermind 25590 points

    Steven,

    The schematic looks good for the LM25017. And the filtering looks sufficient. You may try shorting L2 so that your return path provides a good flat reference for the whole board. L2 can sometimes cause the entire board to ripple up and down with the voltage across L2 which causes bad common-mode noise, but this is usually a bigger problem in the higher frequencies.

    If the issue only occurs when you connect the load with a long wire, try adding a cap with some damping on the end of the wire (ceramic with small resistor in series or an electrolytic with some ESR). That will help to dampen any resonance caused by the parasitic inductance of the long wires.

    -Sam