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LVDS Repeater input failures

Joe Dumas
Prodigy 30 points
Other Parts Discussed in Thread: TPD2EUSB30A

Recently I have had several failures of TI SN65LVDS108DBT LVDS repeater IC's. The failure mode is reduced input impedance.

I have been troubleshooting for some time now and am currently under the impression that the root cause may be that the inputs are being driven with the parts not powered. The data sheet for the component indicates that with VCC < 1.5v outputs = high impedance, however this does not specifically eliminate failures if the inputs are driven.

Does anyone have experience with driving LVDS inputs while IC not powered?

  • 0
    TI__Guru 62910 points
    Hi Joe,

    I wouldn't expect issues with this chip when driving the inputs while powered down. That would typically be the case when a high-side diode (i.e., from pin to Vcc) is used internally for protection against overvoltages. The equivalent input circuits do not show this, though, and the "Absolute Maximum" ratings for the device do not reference Vcc. The power-off input current specs are very low as well (in the uA range), which I would think to be too low to result in physical damage to the silicon.

    You are specifically referring to driving the "A" and "B" inputs, right? Are there any kind of larger transients that may be occurring on these inputs that could be causing damage? What input impedance are you measuring on the failed units, and how are you measuring it? Are the failures permanent, or does power cycling clear the failure condition?

    Regards,
    Max
  • 0 in reply to Max Robertson
    Prodigy 30 points
    Thanks for the rapid response!

    Yes, the inputs that are failing have been the "A" & "B" pins.

    Unintended transients may be the cause. I cannot vouch for the conditions that these components are being subjected too, as I am looking at components that are being damaged in the field. I have not been able to replicate any failures at our facility.

    I have a taken readings with a Fluke Ohmmeter on a couple of samples. Typical (good) readings are A-B ~ 670K, IN-VDD ~ 330K & IN-GND~450K. While on a failed IC I have measured resistance pin-pin~ 100 Ohms &/or Pin-GND~11 Ohms.

    Once a part fails there is no clearing of the failure. The silicon is being damaged.

    Both serial resistance and a Thevenin divider on both inputs have been suggested as possible protection circuitry. Does TI have any recommendations with regards to this?
  • 0 in reply to Joe Dumas
    TI__Guru 62910 points
    Just to confirm, are those resistance measurements taken on an unpowered device that has been removed from the PCB?

    For protection, I would recommend the series resistances (placed between the LVDS termination and the A/B inputs) over the Thevenin divider, since the divider would reduce the input signal amplitude during normal operation. Other options for protection might be series FETs/relays that are controlled by the receiver's Vcc (if providing an input signal to an unpowered device is determined to be the problem) or TVS diodes (if transient voltages are causing the damage).

    Regards,
    Max
  • 0 in reply to Max Robertson
    Prodigy 30 points

    Yes, those resistances are measure with on unpowered components removed from the PCB.

    I have just started looking into TVS diode protection.  Would you have any specific recommendations on these?

    Thanks again for your assistance,

    Joe

  • 0 in reply to Joe Dumas
    TI__Guru 62910 points
    Joe,

    I don't have any first-hand experience with using TVS diodes with LVDS transceivers, but just browsing around available options it looks like TPD2EUSB30A could be a good fit:

    http://www.ti.com/product/tpd2eusb30a

    It's marketed specifically towards USB, but judging by the specs it should be usable for other higher-speed/lower-voltage differential interfaces.

    Best regards,
    Max