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SN74LVC2G34: Hot Insertion

Garret Holt
Intellectual 1050 points
Part Number: SN74LVC2G34
Other Parts Discussed in Thread: ESD351

Hi Team,

We are working with SN74LVC2G34 as a buffer for the TxD and RxD UART signals from the MCU to drive a cable between two boards separated by about 12 inches. The voltage is about 3.3V and the data rate is 250kHz. When hot inserting the cable the buffer is damaged. Would use suggest a different device to drive the cable? They currently have a TVS diode with a clamping voltage of 7.5V, could this be resolved by using a lower clamping voltage device such as ESD351?

Regards,

Garret

  • 0
    Guru 243220 points

    7.5 V is above the absolute maximum ratings, so a lower-voltage diode might be better.

    But are you sure that this is the reason? Are the connectors constructed to ensure that GND is connected first? Is there a current-limiting resistor between the TVS diode and the logic input? Wouldn't a plain diode between the input and VCC be simpler?

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 1050 points

    Hi Clemens,

    Thanks for your reply. We are still looking into alternative causes. The connector is not constructed such that the GND is connected first. There currently is no current limit resistor. Are you referring to a series connected resistor on the data line? What value would you recommend for a 3.3VDC signal with <250kHz data rate? Would this be located on the receiver end? Do you have any other general advice for protecting the interface? Thank you!

    Regards,


    Garret

  • 0 in reply to Garret Holt
    Guru 243220 points

    If the grounds are different, then any voltage could be much larger when seen relative to the other ground.

    A resistor between the external diode and the pin (with its internal diode) would ensure that the external diode takes most of the current, and that the voltage at the pin is lower. The resistor forms a RC low-pass filter together with any parasitic capacitances; check the resulting waveform with an oscilloscope.