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LM119 input voltage below -Vcc

Brent Reschak
Prodigy 110 points
Other Parts Discussed in Thread: LM119

In a application, the LM119 inputs were accidentally pulled as much as 30V below -Vcc pin.  What is the maximum voltage that the inputs can be below -Vcc before the input transistor fuse short?  I have reviewed the datasheet and understand that the Absolute max that the part is stressed when the input is below -Vcc.  The question is how far can the input go before the input transistors are certain to fuse short? 

I would like to discuss our situation with a subject matter expert to go over these details.  Is that possible?

  • 0
    TI__Guru 56800 points
    Hi Brent ,

    I will be moving this into precision amplifiers for support .
  • 0
    TI__Guru 51396 points

    Hello Brent,

    Unless the datasheet specifically states a pin can be taken below V-, as in some current shunt monitors, it is ALWAYS bad to pull pins below V-.

    The LM119 is on a classic junction-isolated process, which means there is basically a reverse biased diode to V- under every internal circuit node (commonly called the "Substrate Diode"). It is also old enough that it does NOT have ESD protection diodes on all the pins like the newer devices.

    If you take a pin below V-, these distributed substrate diodes forward bias, and currents start to flow where it is not supposed to..and this is when damage can occur.

    These didoes start to conduct at about 400-500mV below V-. High currents (100's mA) start flowing through these random junctions when the voltage approaches 600-800mV. Now high currents are flowing through paths that were never designed to handle high currents.

    Since there is no controlled current path for a reverse input voltage (as with a ESD diode), the current spread is random and flows through the paths of least resistance. This can damage devices and/or damage traces if the current is high, or cause latch-up. Something else may be damaged before the input transistors would "fuse".

    So if the pins were really pulled down to -30V, even for just nanoseconds, damage will be done.

    Is there any resistance in series with the inputs? If there was at least 30V/1mA = 30kOhms in series with the input, then the current may have been limited to a tolerable level and it may have "survived". I say "survived" (in quotes) because there may have been shifts in offset or bias current, or other hidden damage, that may not be immediately apparent. Lifetime will be reduced.

    If you expect this event will occur during normal operation, then external Schottky clamping diodes and series current limiting resistance MUST be used to keep the inputs from going below V-.

    The part subjected to -30V should be replaced.

    Regards,