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DS16F95QML-SP: TVS diode protection vs signal integrity

Colton Beery
Prodigy 30 points

Part Number: DS16F95QML-SP

Hello TI Team, 

I was hoping you could help me answer some questions about TVS diode protection on the RS485 interface. My team is currently designing an RS485 interface using DS16F95QML-SP that looks like below: 

image.png

Our application forces us to use large TVS diodes to protect from lightning, but we also are also trying to get a high data rate. Both the driver and receiver will be DS16F95QML-SP (each in a dedicated direction), but they do not share a common ground. (unfortunately the customer is driving the grounding scheme.)

My questions are 

1) Large TVS diodes mean large capacitance. Aside from testing in the lab, is there a recommended approach for determining the maximum capacitance allowable vs the data rate? There doesn't seem to be much guidance online about sizing TVS diodes other than "smaller is better..." I know this is a bit tricky because any capacitance will slow the rise/fall time and it gets into "how degraded is too degraded," but any tips would be useful.

2) If there is a large ground potential difference between the two transceivers (because they're not sharing a common ground), will the TVS protect the transceivers from the common-mode voltage in this setup? If yes, will data transmission still be possible (presumably with degraded signal integrity)?

  • 0
    TI__Guru* 80691 points
    Colton Beery said:
    1) Large TVS diodes mean large capacitance. Aside from testing in the lab, is there a recommended approach for determining the maximum capacitance allowable vs the data rate? There doesn't seem to be much guidance online about sizing TVS diodes other than "smaller is better..." I know this is a bit tricky because any capacitance will slow the rise/fall time and it gets into "how degraded is too degraded," but any tips would be useful.

    We generally suggest to keep external caps around 100pF or lower on the differential pins (not including the cable loading of course). I think in most cases, the TVS diode's loading won't affect the bus that much, generally the cap is lower than 100pF.

    If your customer is operating the bus at lower speeds like 115kbps then the cap loading is even more minimal, as the max distance is more affected by the cable's parasitic resistance. 

    Colton Beery said:
    2) If there is a large ground potential difference between the two transceivers (because they're not sharing a common ground), will the TVS protect the transceivers from the common-mode voltage in this setup? If yes, will data transmission still be possible (presumably with degraded signal integrity)?

    Keep in mind that the TVS diodes are there for clamping short transient events (generally in the ns time frame). GND shift can be a longer time frame event that would just break the TVS diodes.  If you're worried about lightning strikes then a TVS diode may not survive this kind of event since the power dissipated during this event is longer than what a TVS diode is generally set up for.

    You generally want to add additional external components to protect the bus further like MOVs and TBUs.

    A series pulse proof resistor (10 ohms and rated for higher wattage) in series of the RS driver would also be a good choice but will affect the max distance/speed much more than a TVS diode would. 

    There is a TI design that walks through this.

    https://www.ti.com/lit/ug/tiduas1b/tiduas1b.pdf?ts=1769462655343

    -Bobby