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Original question:

TLC3704-Q1: Protecting comparator input for when Vdd is 0

TLC3704: TLC3704 safe input current-level when it is unpowered

Edward Riess
Prodigy 120 points
Part Number: TLC3704

Dear expert(s):

 The TI datasheet for TLC3704 specs an absolute maximum input current of +/- 5mA.  That, of course, applies to the part when it is powered.

During a Flying Probe test of a transient voltage suppressor connected to a TI TLC3704 IC by a resistor, inputs of this device may experience a pulse of 5mA, of either polarity, for 5ms, while the part is unpowered.  Since this test is on an avionics flight module, and practically everything gets a high-level review, it would be helpful to have an official statement from TI that states that not only would a 5ms pulse of either polarity be non-injurious to an unpowered TLC3704 IC of any-grade, but a 5mA pulse applied for some longer time (such as 10ms, 100ms, or 1s) would similarly be non-injurious. 

The best statement would be this: "No stress or damage would be caused to an unpowered TI TLC3704 IC of any grade by applying, to any one of its inputs, a continuous current of 5mA of either polarity with respect to its negative-supply terminal (pin 12 in the package used in this assembly)."

We'd be grateful for such a statement.  Please advise us on this matter.

Thank you.

Ed Riess

  • 0
    TI__Guru 53266 points

    Hello Ed,

    Officially, TI cannot warrant or give an official statement on conditions outside the datasheet specifications.

    Unless there is a statement saying otherwise, the datasheet specifications are assumed for a powered-on device (V+ highest potential and GND lowest).

    However...

    Pulling the input negative will activate the ESD diode to GND pin, clamping to a diode drop from ground (the more current, the more drop).

    Going positive is a little more complicated. Pulling any input above V+ will cause the top ESD diode to forward-bias and back-feed the positive supply.

    If it is a continuous DC voltage, you could end up back-powering the V+ bus if there is enough input current available. If it is a pulsed voltage, the supply rise time will be determined by the supply bypass capacitors and available current. The input pin may be almost 1V above V+ depending on the current.

    The ESD diodes and connecting metal are designed to dissipate an amp or two in <1us without damage to handle occasional ESD strikes.

    If you limit the current to 5mA as mentioned in the Applications section, and also limit the open circuit voltage and pulse duration, I would not even worry about it.

    To ease your fears, we do similar tests on the ESD diodes in production test to see if they are there and functioning. The currents and voltages are usually limited to <100uA and <2V.

  • 0 in reply to Paul Grohe
    Prodigy 120 points

    Paul,

    Thank you for your enlightening information.  It has provided the needed certainty of this test's safety.

    Respectfully,

    Ed Riess