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AM26LV31E-EP: Questions about lightning protection design of the AM26LV31E-EP

user5233567
Prodigy 170 points
Part Number: AM26LV31E-EP
Other Parts Discussed in Thread: AM26LV31E, SN65LBC174A-EP, SN65HVD30-EP, SN65HVD10-EP, SN65HVD12-EP, SN55HVD75-EP, AM26LV31

Hi, 

I implement RS-422 interface using AM26LV31E-EP, and I am worried about the lightning protection design of the AM26LV31E-EP.

I tried to design each TVS(military of hi-rel grade) on the HI / LO line of AM26LV31E-EP.

Referring to the data sheet of the chip, Maximum input voltage of AM26LV31E-EP is 6V. 

But, Most TVS have a clamping voltage above 10V.(Stand off voltage: minimum 5V, Stand off voltage: minimum: 9V)

So, i think that most TVS will not be able to protect the AM26LV31E-EP.

(I think if the 9.2V is injection to the HI / LO pin of AM26LV31E-EP, AM26LV31E-EP will be damaged.)

Am I thinking wrong?

Is there a solution to the lightning protection on the AM26LV31E?

please reply my question. 

thanks.

  • 0
    TI__Guru 62930 points
    I believe your understanding is correct. I am checking with the commercial team to see if they have any recommended solutions.
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    TI__Guru 62930 points
    Unfortunately, we do not have a 3.3 quad line driver with enough absolute max headroom for TVS diode for lightning suppression.

    I did search through our other EP interface offerings. We do not have any 3.3V quad RS485 transceivers. We do have one 5V EP quad transceiver with enough head room to allow TVS diode protection.
    It is the SN65LBC174A-EP.
    It can handle +/- 30V for transients on bus.

    There are some 3.3V single channel RS485 devices that could work if single channel is option for you.
    The SN55HVD75-EP, SN65HVD30-EP, SN65HVD12-EP, and the SN65HVD10-EP.

    If this answers your question, please click "Verify it as the answer"
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    Prodigy 170 points
    Thank you for your kind reply.

    Is the SN65LBC174A-EP compatible with the RS-422 bus?
  • 0 in reply to Wade Vonbergen
    Prodigy 170 points
    I am checking the datasheet of AM26L31E-EP, and I seem to have misinterpreted the input voltage range.

    I think that the maximum input voltage in the AM26LV31E-EP datasheet is considered to be the input voltage of the input logic(1A, 2A, 3A, 4A).

    The schematic of the output logic in the datasheet, the output port consist of the drain of the MOSFET.

    Is the maximum input voltage of the output port same as the input port?

    If the two values are different, can I check for the correct value(Maximum Input Voltage for Output Port)?

    Regards,

    YS LEE
  • 0 in reply to user5233567
    TI__Genius 17115 points
    Yes. RS-485 is backward compatible with RS-422. RS-485 products just have larger transistors for more current drive to support multidrop applications.
  • 0 in reply to user5233567
    TI__Genius 17115 points
    The AM26LV31 is an industry standard RS-422 quad driver. The input pins are the single ended input signals and the enable pins. Abs max for those are 6V according to the datasheet.

    The output pins are the differential output bus. They also have an abs max rating of 6V.

    For indepth discussions on RS-422 vs. RS-485, fail safe and other design questions, please see this website which have IT apps notes that cover every topic.
    www.ti.com/.../technical-documents.html
  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 170 points
    Thank you for your reply.

    I have one more question.

    On page 6-7 of that 'SLLA292A' document, contains information about RS-485 circuit protection using a TVS.

    The document is written as follows

    'the maximum clamp voltages of TVS can range from 25V up to 35V, which is significantly higher than the maximum bus voltage of 14V of a standard transceiver.
    which is significantly higher than the maximum bus voltage of 14V of a standard transceiver.'

    Does this mean that the RS-422/485 Driver&Receiver with built-in ESC protection can withstand over voltage(that is occured by surge transients) that exceed the maximum input voltage?

    If possible, can you provide information on the ESD protection circuit for AM26LV31E-EP?
  • 0 in reply to user5233567
    TI__Genius 17115 points
    What the app note and datasheet are saying is that the driver outputs can withstand an ESD pulse, which is very short in duration (in the nanoseconds range) and that pulses of longer duration outside the specified voltage range could damage the part.

    The ESD tests used are industry standard and the conditions are listed in the datasheet. The ESD test pulse shapes can be found through an interent search.
  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 170 points
    I know the driver outputs can withstand an ESD pulse.

    The reason I am trying to connect the TVS to the AM26LV31E-EP is for lightning protection.

    So I want to know how long the AM26LV31E-EP can withstand the over voltage(about 10V)
  • 0 in reply to user5233567
    TI__Guru 62930 points
    YS,
    The 6V applies to DC case. Unfortunately, we do not have information on the ability for device to withstand a surge of greater than 6V other than the ESD case that Kirby has noted.

    Regards,
    Wade