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Original question:

TPD6E004: ESD Diode selection

TPD6E004: Designing with TPD6E004

Sv
Intellectual 515 points
Part Number: TPD6E004

Tool/software:

Hello Team,
This post is a continuation of one of my previous post link

As per the application note slva680a, the ESD /TVS diode must be connected to the chassis ground.

In our system, these diodes are connected to the system ground only.
1). Is there any problem with this?.
2). Which is the best way to connect the ESD diode?
3). Also, can I directly connect the chassis Ground to the Earth Ground?.
4). In our design, the shield of the USB, RJ45 are connected to the chassis ground directly and these shields are connected to the singal ground through resistor parallel to capacitor in multiple places.
Will this create any ground loop in the system?.
5). Can you please review the ground diagram below.?



Looking for your reply

  • 0
    TI__Expert 5290 points

    Hi Sv,

    Please see the answers to your questions below:

    1) According to the document you linked earlier, it would be best to avoid the system ground being used for both signals and ESD protection. Please see the following excerpt from page 3:

    "Having a chassis ground for the TVS that is separated from the digital and/or analog ground by inductance provides very good protection against ESD related failures. Yet it presents great challenges when routing high speed circuits across multiple ground planes. For this reason, many designs use one common ground for the protected circuits... Electrical connections to an earth grounded chassis, like a PCB grounded through-hole for a chassis screw, immediately adjacent to the TVS ground and the ESD Source's ground (for example, a connector shield) provide a sound methodology in keeping ground shifts at Protected ICs to a minimum."

    2) The optimal ESD diode connection would be as close to the source of ESD as possible, with the ground of the TVS being connected to a specific chassis ground plane for TVS protection. In addition, there should be ESD protection featured on nearby signal lines to minimize the potential for EMI coupling (as shown in the first image you attached). 

    3) Yes, an earth grounded chassis is acceptable and encouraged!

    4) Just confirming here, this means that the chassis ground and the signal ground are connected by an RC circuit? I believe that a chassis ground can be connected to the signal ground for shielding purposes, but as mentioned above the best case scenario would be to have a separate ESD chassis ground that is not connected to the signal ground.

    5) For this grounding diagram, which grounds are on the same layer? Are all of the chassis grounds, signal grounds, and earth ground a separate layer or are some shared? I would like to understand this layout before providing any feedback.

    Please let me know if you have any additional questions!

    Best Regards,

    Josh Prushing

  • 0 in reply to Josh Prushing
    Intellectual 515 points


    Hello Josh,
    Thank you for your reply.
    Can you please answer some doubt we have?

    Josh Prushing said:
    3) Yes, an earth grounded chassis is acceptable and encouraged!

    In some systems (in ESD strap also), there is a resistance between the chassis GND and the earth GND. Whys so?.
    In my laptop I am using, the shield of the USB is connected to the body of the laptop and then to the earth pin (3rd pin in the DC side of the adapter). I can see a resistance of approximately 1M between this pin and the earth pin (in the AC chord)

    Josh Prushing said:
    4) Just confirming here, this means that the chassis ground and the signal ground are connected by an RC circuit?

    In our case, we have USB and RJ45 connectors
    The shield of these connectors are connected to the chassis GND directly and to the signal GND through an R//C connection.
    If I understand you correctly, we should separate the chassis GND (shield of USB) with signal ground and the ESD shall be connected across the chassis and the signal only.
    Please correct me if I am wrong.
    Also, if we connect the chassis GND with signal GND in multiple locations, will it create any GND loop?.
    Following is a snippet from the launch pad of MSP432 MCU (SLAU748B).
    In launch pad, the ESD diodes are connected to the signal ground and the shield of USB and RJ45 are connected to the signal GND via a R//C network.
     



    We have chassis GND on all board, probably on edges of the PCB on all layers and they connect to the shield of all connectors and to mounting holes.
    The Earth GND will be some wire from the back panel that goes to the EARTH of the building.
    Do we need to connect the chassis GND with the earth GND directly?.


    Looking for your reply

  • 0 in reply to Sv
    TI__Expert 5290 points

    Hi Sv,

    The resistance is present to prevent charge building up between the two grounds if I am not mistaken. This resistance isolates the grounds so that the ESD or anything else being shorted to ground is not made present on the other ground.

    Sv said:
    If I understand you correctly, we should separate the chassis GND (shield of USB) with signal ground and the ESD shall be connected across the chassis and the signal only.

    I am saying that the ESD ground should be separate from the signal ground for the best results.

    Sv said:
    Also, if we connect the chassis GND with signal GND in multiple locations, will it create any GND loop?.

    I am not 100% confident, but I do worry that this could create a ground loop, which is why I am recommending these grounds be separate.

    Sv said:
    In launch pad, the ESD diodes are connected to the signal ground and the shield of USB and RJ45 are connected to the signal GND via a R//C network.

    The RC network may mitigate the risk of a ground loop, but I would need to conduct testing to confirm this. If possible, I believe it is safer to just keep the chassis ground and signal ground unconnected.

    Sv said:
    The Earth GND will be some wire from the back panel that goes to the EARTH of the building.
    Do we need to connect the chassis GND with the earth GND directly?.

    I believe this connection can be direct, but using a high impedance (1MOhm) will mitigate any potential charge buildup I believe.

    Please let me know if you have any additional questions.

    Best Regards,

    Josh Prushing

  • 0 in reply to Josh Prushing
    Intellectual 515 points

    Hello Josh,
    Thank you for your reply.

    We need a closed path to flow the current.
    When static electricity is built in our body, sometimes we get electric shock when we touch the door handle.
    The door handle may not be always connected to a metal door or to GND. Sometimes it can be a glass door or wooden door but still we get shock. So what will be the return path of the static electricity built on the body?.

    Looking for your reply

  • 0 in reply to Sv
    TI__Expert 5290 points

    Hi Sv,

    I think this is where connecting the chassis ground to the earth ground comes into play. Without this connection, the ESD events shunted to ground will just be present on the chassis, and produce these shocks on the handle. If this ground is connected to the earth ground, these ESD events should be dispersed away from the handle and into the earth of the building.

    Best Regards,

    Josh Prushing

  • 0 in reply to Josh Prushing
    Intellectual 515 points

    Hello Josh,
    Thank you for your reply.
    As per the e2e thread (link), the ESD diodes are recommended to connect with the USB Ground (System GND).
    But, connecting the TVS/ESD to chassis makes more sense.
    In our system, we have an isolated GND.
    We have ESD diodes in both the sides.



    So if we connect ESD diodes to chassis from both the sides, will it create any issue in isolation?.

    Looking for your reply

  • 0 in reply to Sv
    TI__Expert 5290 points

    Hi Sv,

    The image you have shown above aligns with my understanding of how the ground needs to be configured. I do not believe there will be any issue in isolation.

    Best Regards,

    Josh Prushing