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LM139A: Effect of driving input >0.3V below the negative rail on other comparator inputs with the same package.

Brian Koerner
Prodigy 10 points
Part Number: LM139A
Other Parts Discussed in Thread: LM139

The LM139A is a 5962-87739012A.  This is a dual comparator package.

With the rails at +5V/0V the -input was driven to -1.2V this caused the second comparator in the package to pull its +input to -0.32V while being driven by 2.5V with a source impedance of 7.5K.  Can you confirm that driving the input below the rail in this manner would result in this failure mode and shed some light on the mechanism?  

  • 0
    TI__Guru 60786 points
    Hello Brian,

    The LM139 family is built on a junction-isolated process, and does NOT contain specific ESD protection devices on the pins.

    All the internal circuit nodes are isolated from the substrate by reverse biased junctions (think of every node on the internal example schematic connected to a common die substrate through a reversed bias diode).

    If one pin is brought more than a diode drop below V-, it is possible to forward bias these diodes and "drag" the other nodes down through the substrate. Since there is no defined reverse-voltage path, reverse currents will find their ways though the various internal junctions.

    Unlike circuits built with discreet packaged components, devices on a die can be connected to each other through various paths and reversed junctions.

    This is why the datasheets warns many times about applying negative voltages to the inputs. You *really* do not want to do that...

    7.5k/2.82V=376uA flowing into the + input. To pull the -Input to -1.2V, this would require several millamps as you would be clamped by the substrate junctions - which would consist of many paths through the device. One of those "various paths" was probably other input device.

    So, yes, pulling one input 1.2V below ground can cause currents to flow through the other pins. That is not unexpected, and, obviously should be avoided! :^)