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Did we stress our LM6142 op-amp?

Edward Riess
Prodigy 120 points
Part Number: LM6142

Did we stress our LM6142 op-amp?
At ambient temperature, with about 20V applied between V+ and V-, the pin-1 output of an LM6142 amplifier, while at maximum output voltage, was connected through a 47-ohm resistor to a node that possibly was at -16VDC with respect to V-, for a time period of several minutes.  The LM6142 TI datasheet specifies a maximum output current of ±25mA with a note that states that this "Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in exceeding the maximum allowed junction temperature of 150°C". Can it be safely inferred that at ambient temperature this would cause no stress to the part, and that the maximum allowed junction temperature of 150°C would not be exceeded, especially since the TI datasheet also lists the “Output Short Circuit Current LM6142” in its “Sourcing” mode to be a maximum of 35mA?

We would greatly appreciate your determination on this question.

Thank you.

 

  • 0
    TI__Guru*** 140016 points

    Hello Edward,

    If this is something that only happened once to one unit then I suggest just replace the LM6142 and move on.

    If there are many devices that suffered this fate and maybe some of them have made it out into the real world, then that would warrant different advice.

    For output voltages within the supply rails, figure 17 would suggest under 20mA output current for a stress of (V+ - VOUT)*20mA. At ambient temperature, a short to V- is a power stress of (20V - 0V)*20mA = 400mW.  At a Tja of 193C/W would give a temperature rise of 77C, making die temperature approximately 25C + 77C = 102C (just one channel shorted). Several minutes should not age the device very much. However, a short to ground is not what was reported to have happened.

    In the reported case, the output could have been pulled towards -16V. Almost every op amp's output pin has a diode from output to V- (I think LM6142 is the same). Therefore the output would be a diode current that could up to 16V / 47 ohm = 340mA. That current may pull the output a couple volts negative creating high V- current. This current can also turn on parasitic transistors that can steal current from V+; this current has over 20V of voltage drop which can create very power dissipation and very high temperatures. 

    Tell me about the 47 ohm resistors; what power rating are they? Did they overheat? Has their resistance changed due to the stress.

  • 0 in reply to Ron Michallick
    Prodigy 120 points

    Ron,

    Your analysis and reply are much appreciated.

    On the first point, part replacements would be extraordinarily difficult, time-consuming, and expensive.

    Further analysis of our situation showed that I was mistaken to think that the 47-ohm resistor could have been connected to a node that was at a level below V-.  Your 400mW worst-case situation therefore applies, and we are able to use your determination in our stress analysis which also shows that the resistor was not stressed.

    As much as I regret making that mistake, the added insight you provided about turn-ons of parasitic transistors is good to know.

    Appreciatively,

    Ed Riess