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THVD1400: Maximum allowable TVS Capacitance on the THVD1400

Neet
Intellectual 612 points
Part Number: THVD1400

Tool/software:

Hello E2E Community,

We are trying to improve the Surge & Transient immunity of our sensor nodes that are interfaced using THVD1400.

We have a TVS diode with following specifications - based on your expert knowledge kindly suggest to us how would implementation of these TVS diodes affect the performance of the network?

  • Maximum cable length: 20m CAT6 UTP
  • Maximum nodes: 6
  • Maximum data speed: 20kbps

We want to implement low cost & 2kV surge immune system hence we are considering the said TVS Diode.

  • 0
    Guru 314190 points

    The THVD1400 itself does not care.

    Capacitance on the bus lines limits the speed (as does the capacitance of the cable itself), but a speed of 0.02 Mbps is fine.

  • 0 in reply to Clemens Ladisch
    Intellectual 612 points

    Hello ,

    Alright that confirms my assumptions - my concern now is the variation in TVS Diode's part to part capacitance on A & B it should too be just fine, right?

    (about 250pF variation might be present)

  • 0 in reply to Neet
    TI__Genius 11742 points

    Hi Neet,

    I would recommend our TVS diode TSD05DYFR instead. It is the same package, has a much lower 19pF capacitance, better clamping voltage at 50A, and it is cheaper. The frequency can be what causes the variation of the capacitance. Because both A and B should have the same frequency, this should not be a concern.

    Let me know if this helps or if you have any further questions.

    Best,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Intellectual 612 points

    Hello  & ,

    Yes, we have used TSD05D in past designs - the issue is this TVS is out of stock from both Digikey & TI Website until January.. Hence we are searching for similar footprint TVS diode.

    Can you help us confirm if the Capacitance variation on A & B due to TVS Diode's part to part variation will degrade the waveform significantly at out target speeds?

    The issue is one TVS on "A" may have 720pF while other on "B" may have 820pF (assuming part to part variation in capacitance), will this create any major issue for THVD1400 to perform on field given our use case details?

  • 0 in reply to Neet
    Guru 314190 points

    This can affect the propagation delay, but due to the differential transmission, rising and falling edges of the original signal are affected by the same amount, so it does not really matter.

  • 0 in reply to Clemens Ladisch
    Intellectual 612 points

    Hello ,

    Thank you for confirming - so if I understand it correctly - using TVS diodes with two different Capacitance (let's say 720pF on A & 900pF on B) will not affect the performance of the THVD1400, correct?

    I am confirming this again - as any mistake in interpretations here might turn out to be rather costly for us.

  • +1 in reply to Neet
    Guru 314190 points

    Yes. The receiver simply compares the voltages at the A and B inputs; when you have different capacitances, the edges on one line will be slower, so the time when the two edges cross will be different (somewhere between where they would be with 2× 720 pF and with 2× 820 pF).

    Asymmetries on the A/B lines can introduces common-mode noise, which you might want to filter out with common-mode chokes for EMI reasons. But the THVD1400 is a low-speed device with slow edges (and the capacitances will slow them down even further), so that noise will probably not be at sensitive frequencies and have a very low level.