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THS4304-SP: Driving inputs while unpowered

Brian Giesinger22
Prodigy 50 points
Part Number: THS4304-SP
Other Parts Discussed in Thread: THS4304

I am using the THS4304-SP in an application where it is buffering/amplifying a ~10MHz sine wave reference clock.  The ~10MHz oscillator may get powered prior to the supply for the THS4304-SP.  The oscillator is transformer isolated, then AC-coupled to the THS4304-SP, where we have a DC-bias setup at the input.  The nominal output level of the oscillator is +6dBm.

Is it OK to drive the unpowered THS4304.SP in this situation?

Thanks,

Brian

  • 0
    TI__Genius 14540 points
    Typically, we recommend avoid driving devices that are not powered as it does have the possibility to damage to device depending on voltage levels and time period where high currents could result from forward biased ESD structures. You mention +6dBm out of oscillator through a balun. Do you know that voltage levels this puts on the THS4304-SP input? Is the DC bias setup also powered down so that common mode input is 0V? And for how long would you expect this unpowered state in which input is fed a signal?

    Thanks
    Christian
  • 0 in reply to Christian Yots
    Prodigy 50 points

    Hi Christian -

    Yes, understood on the preference for not driving while un-powered.  I am trying to adding sequencing to my system.

    The voltage level is approximately 1.3Vpp, centered on 0V (so yes, common mode input is 0V), since the bias is not yet powered (bias if formed by voltage divider from the THS4304-SP supply rail).

    This condition would happen for a few hundred milliseconds every power cycle.  Figure a few hundred to a few thousand power cycles over a lifetime.

    Thank you,

    Brian

  • 0 in reply to Brian Giesinger22
    TI__Genius 14540 points
    Hi Brian,

    So the input will swing up to +0.65V and down to -0.65V, correct? This is about the level at which the ESD diodes will start to conduct current with the rails at 0V. This level does not forward bias the diodes significantly and for a short period of time. Also, the signal to the diodes is an AC signal the currents are alternating from one diode to positive rail to the other diode to negative rail. This reduces the likelihood of electromigration damage.

    Thanks,
    Christian
  • 0 in reply to Christian Yots
    Prodigy 50 points
    Hi Christian,

    Thank you. Is there an input current limit that we should be aware of as we drive these unpowered inputs? (often times manufacturers state something like 10-20mA in datasheets as an acceptable current limit to drive to unpowered pins). Do you have an input circuit diagram for the device (I presume it's just standard ESD diodes from ground and to supply)? We are currently not planning to mitigate driving this device while unpowered, so I just want to make sure we really are safe.

    Thank you,
    Brian
  • 0 in reply to Brian Giesinger22
    Guru 140200 points
    Hi Brian,

    an ESD protection scheme can usually withstand 10mA for a brief period. But if the ESD protection scheme is used to repeatedly shunt input overvoltages to the supply rails, the current through the ESD protection scheme should be limited to 1...2mA. But again, as the datasheet is not specifying a maximum input current during input overvoltage conditions, it's at your own risk.

    Kai