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THVD1451: Inquiry Regarding Use of THVD1451DRBR with Custom USB Type-C Pinout Carrying RS422 Signals

Prem Kumar
Prodigy 120 points
Part Number: THVD1451


Tool/software:

Hello Sir/Ma'am,

Dear TI Support Team,

We are considering using the THVD1451DRBR in our design, where RS-422 signals are routed through a USB Type-C connector with a custom pinout configuration tailored to our application. The pinout mapping is as follows:

 

A1 GND B12 GND
A2 NC B11 NC
A3 NC B10 NC
A4 RS422- A B9 VBUS
A5 RS422- B B8 NC
A6 RS422- Z B7 RS422- Y
A7 RS422- Y B6 RS422- Z
A8 NC B5 RS422- B
A9 VBUS B4 RS422- A
A10 NC B3 NC
A11 NC B2 NC
A12 GND B1 GND

According to the  standard Type C connector pinouts,

We understand that A4 and B4 are typically assigned to VBUS in the USB Type-C standard. In our design, however, these pins are repurposed to carry RS422-A signals, which connect directly to pin 8 of the THVD1451DRBR.

Our concern is the scenario where a user inadvertently connects a standard USB Type-C power source (e.g., a power brick), which would apply 5V to pin A4/B4. Given that the THVD1451DRBR supports a common-mode voltage range of ±7V, we anticipate that this may not damage the device. However, we would like to confirm:

  1. Will applying 5V to the RS422-A input (pin 8 of THVD1451DRBR) cause any damage or malfunction?
  2. If this poses a risk, could you please recommend an alternate IC that can tolerate such a condition or suggest a protection strategy?



  • 0
    Guru 317630 points

    It will not cause immediate damage. But the short circuit current (see the IOS specification) might lead to thermal problems.

    Please note that you should route differential signals (A/B, Y/Z) through differential pairs in a USB cable (2/3, 6/7, 10/11); not doing this will lead to EMI problems.

  • 0 in reply to Clemens Ladisch
    TI__Guru 52592 points

    Hi Prem,

    Clemens is correct - please let me know if you have any other questions!

    Best,

    Parker Dodson

  • 0 in reply to Clemens Ladisch
    Prodigy 120 points

    Hello Clemens,

    Thank you for your response.

    Regarding the concern about short-circuit current (IOS):
    In our design, the RS422-A signal routed to A4/B4 of the USB Type-C connector is connected to pin 8 of the THVD1451DRBR, which is the receiver input, not the driver output. As per the datasheet, the IOS specification applies to the driver side, so we believe this condition should not trigger the IOS-related thermal concerns.

    As for the EMI consideration:
    We appreciate your input. This interface is intended solely for UART diagnostics in our application and will not be subject to EMI certification. Therefore, while we acknowledge the potential for EMI issues due to non-differential routing, it is acceptable within the scope of our use case.

    Please let us know if there are any additional concerns or recommendations.

  • 0 in reply to Prem Kumar
    TI__Guru 52592 points

    Prem,

    At the end of the day it is your design - and if you are okay with the risks pointed out by Clemens then you are okay to proceed - it isn't best practice it sounds like you don't really need to be concerned of those risks in this application - but he is correct on best practice.

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 120 points

    Parker,

    Can you respond to the first point i.e. Regarding the concern about short-circuit current (IOS)?

  • +1 in reply to Prem Kumar
    TI__Guru 52592 points

    Hi Prem,

    The "A" pin is the receiver input - so the short circuit current really shouldn't apply. There is a common mode voltage rating of -7V to 12V because this is actually an RS-485 part, which can do RS-422, but it is more robust. If you short 5V to A the device is protected and the thermal concerns would be minimal. The concern would be functional - i.e. you will get garbage data most likely if a short to 5V happens, but I don't think by what I have read on this thread that you are trying to short this to 5V - its more that it might get shorted to 5V and that is a fault condition. If that type of fault happens you are protected. 

    Please let me know if you have any other questions. 

    Best,

    Parker Dodson