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Inputs to SN65LBC172A when device is powered down

Daniel Carlson
Prodigy 370 points
Other Parts Discussed in Thread: SN65LBC172A

Hello,

A customer is using a quad RS485 driver (SN65LBC172A) and he wants to know if the input pins can tolerate a high input signal when the device is powered down. The datasheet says the maximum voltage the input pins can tolerate is Vcc + 0.5 V, but I am assuming this refers to Vcc when the device is turned on.

We just want to make sure that the device will not be damaged when the input signal is high and Vcc = 0.

Thanks,

Daniel

  • 0
    TI__Guru 62910 points
    Hi Daniel,

    How high of an input signal would you expect? Would it be in the standard RS-485 signal range, or would it be a higher voltage?

    Max
  • 0 in reply to Max Robertson
    TI__Prodigy 370 points

    Hi Max,

    The expected input signal is within the standard RS-485 signal range.

    -Daniel

  • 0 in reply to Daniel Carlson
    TI__Guru 62910 points
    Hi Daniel,

    Sorry, I was a little mixed up initially - I thought you were talking about voltages on the RS-485 interface (Y and Z pins). For that, there would be no problems tolerating RS-485 voltages when the device is unpowered.

    If a high level voltage is applied to the A or G inputs while VCC is unpowered, you would forward bias a diode that is used for protection on those pins. This would mean that the pin would sink more current than usual, since you would be partially powering the device through this leakage path. It would be unlikely to cause any kind of permanent damage if the VCC rail were undriven. If the VCC pin were forced to some voltage (like 0 V), though, the amount of current flowing through the protection diode could exceed its capabilities and result in damage.

    Max