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THVD1451: Can I use this part without external TVS and/or series-resistors for RS485?

Quan Gan
Prodigy 30 points
Part Number: THVD1451

Hello,

I am designing a T shaped repeater that uses the THVD1451 to read the data from the trunk's A/B and then directly sends that signal back out of the Y/Z ports (by tying the R and the D lines together) to a branch (to minimize stub length since our products may be some distance away from the trunk line).

Typically, I have seen RS485 designs with bi-directional TVS diodes and sometimes 47ohm resistors in series to the bus lines.

With the THVD1451, are these components necessary?

According to the datasheet, it says "THVD14xx is a family of noise-immune RS-485/RS422 transceivers designed to operate in rugged industrial environments. The bus pins of these devices are robust to high levels of IEC electrical fast transients (EFT) and IEC electrostatic discharge (ESD) events, eliminating the need for additional system level protection components."

We have a simplified RS485 system where there is only 1 master controller at the beginning of the trunk and all other devices (active repeating Ts) on the line are only receiving.

Here's our topology:

Since this is a critical component, I just want to double-check with TI before finalizing the design. Can we leave TVS diodes off of the THVD1451 since it states it's inherently protected?

 

Thank you

Quan

  • 0
    Guru 269810 points

    The external protection components protect against events that are stronger than what the THVD's internal protections can handle.

    You have to decide yourself whether such events can happen in your system. (You should not use less protection than you have chosen to use in the other nodes on the bus.)

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Clemens Ladisch said:

    The external protection components protect against events that are stronger than what the THVD's internal protections can handle.

    You have to decide yourself whether such events can happen in your system. (You should not use less protection than you have chosen to use in the other nodes on the bus.)

    I'm simply asking if I can get the same level of protection as I used to get (SP3041EE + external TVS and resistors) with the THVD1451 by removing the external protection? I'm not looking for additional protection. I just simply want to match the previous protection levels while simplifying the design and reducing the component count.

  • 0 in reply to Quan Gan
    TI__Mastermind 25705 points

    Quan,

    What external TVS did you choose to work with SP3041EE? THVD1451 is rated with IEC 61000-4-2 +/-18kV contact and +/-25kV air gap and IEC 61000-4-4 +/-4kV. If you expect the ESD events are less severe than the spec, you can spare the TVS.

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 30 points

    We were using little fuse equivalent:

    While the external says to protect up to +/-30kV, I think the stock rating is fine. So if I decide to leave off the external protection, I should be able to directly have the chip's I/O pins exposed to the physical connector pins on my device, correct? Our system is typically not connected to external devices at both ends... only the controller master is feeding the trunk signal and is grounded. All other devices on the bus are receiving from that bus and repeating it out to branched light fixtures. All of these light fixtures are connected to a single power supply that's also from the end where the signal source resides.

  • 0 in reply to Quan Gan
    TI__Mastermind 25705 points

    Quan,

    THVD1451's ESD rating is not as high as the little fuse's TVS you mentioned. However in the application as you described, the protection might not need to so well. In my opinion, the HBM/CDM spec matter more for the RS485 in your system.

    Regards,

    Hao