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SN65C3232E: RS-232 long-term overvoltage protection

Andy Liu
Prodigy 100 points
Part Number: SN65C3232E
Other Parts Discussed in Thread: ESDS552, ESD451, THVD2412V

Hi Ti support team,

I use SN65C3232E to support RS-232 line (Tx and Rx only),
The project requirements need to support output short circuit testing.
The test conditions are as follows,
1. RS-232 output will connect to 35V or ground
2. RS-232 input will connect to 35V or -1V
All I need to consider is the condition that the input and output should be connected to 35V for a long time (up to 1-2 hours not transient during this test).

I believe this can be supported by some protection components, such as Zener Diodes.
Can you provide me with relevant suggestions based on the output characteristics of SN65C3232E?

Thanks,

Andy

  • 0
    Guru 297310 points

    Normal signal voltages must be left alone, but voltages above the absolute maximum ratings must be avoided.

    Zeners will shunt the current to ground. I assume that that 35 V source is not strong enough to burn out the diodes.

    The bus receiver pins must be clamped to a voltage between 15 V and 25 V. The bus driver pins must be clamped to a voltage between about 6 V and 13.2 V. So use, e.g., 20 V Zeners like the 1N5357B/SMBJ5357B and  9.1 V Zeners like the 1N5346B/SMBJ5346B.

  • 0
    TI__Guru 63020 points

    Hi Andy,

    Clemen's suggestion is correct about the clamping voltages. The absolute max rating for the Rin pins are +25V, clamping them before that voltage will be the the recommended approach. For the DOUT pins, We recommend an absolute max of about 13V, the clamp should occur above 6V. 

    I will reassign this thread to our protection devices product line to see if TI has any recommended solutions they can provide. 

    -Bobby

  • 0 in reply to BOBBY
    TI__Expert 5290 points

    Hi Andy,

    In terms of the recommended ESD devices from TI, please see below:

    • For Bus Receiver pins, we recommend ESDS552, as the working voltage is 12V and clamps between 19 and 23.5V. 
    • For the Rin pins, we recommend ESD451, as the working voltage is 5.5V and clamps between 7.2 and 10.4V.

    These are not Zener diodes, are Zener diodes required for your application? We are currently working to bring these to our portfolio. 

    Please let me know if you have any questions!

    Best Regards,

    Josh Prushing

  • 0
    Prodigy 100 points

    Hi All,

    The test is to connect about 35V to the RS-232 input and output signals for a long time, not for transients.
    Therefore ESD protection components cannot be used in this application.

    I found the idea of ​​using a Zener diode elsewhere, but a Zener diode can only handle a small amount of current. It is necessary to add a series resistor to limit the current, but too large a resistor will cause a voltage drop in signal transmission on RS-232, so I don’t know how to achieve this requirement through discrete parts.

    I also found a material that can meet this specification (MAX13223E), but the price is very expensive
    Can you provide relevant advice?

    Thanks,
    Andy

  • 0 in reply to Andy Liu
    TI__Guru 63020 points

    Hi Andy,

    Unfortunately we don't have an RS232 transceiver that supports that level of fault protection like the comp device you referenced.

    Our next best solution would be to move from RS232 to RS485 which has more options on the fault protection feature. From the TI portfolio, I would probably look at THVD2412V. The main drawback for RS485 would be the cabling would be more expensive with the twisted pairs.

    -Bobby