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Original question:

LM139-SP: input negative voltage

LM139A: Negative input

Gary Berrios
Prodigy 10 points
Part Number: LM139A
Other Parts Discussed in Thread: LM339, LM139

With a negative input of -12V through 3K Ohms, we've never experienced a false state but are we "on the edge"?  Opposite input = +1.2V through 750 Ohms..  Operation at +15 to +30C. The datasheet states that "input current flows through parasitic diode to ground and will turn on parasitic transistors that may increase Icc and may cause output to be incorrect".  I would have added a clamp diode but the design has been in use for 20+ years so overall enthusiasm for adding a diode now is rather less than non-existent.

"May cause the output to be incorrect".  And, obviously, may not.  But under what circumstances specifically?  It appears that at least some of the current would flow through the forward biased input transistor collector-base junction to ground.  Is this path more predominant than the parasitic diode?

Thanks

  • 0
    TI__Guru 53026 points

    Hello Gary,

    Please see the LM339 family application note - it describes this, particularly sections 2.6 and 2.9:

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators

    It is hard to pin down an exact failure mode. The LM139 family is built on a junction-isolated process, so there is a reversed diode under every node to V-, and even between internal semiconductor device nodes. There is not a dedicated ESD structure to take the brunt of the reverse current, so all that reverse current is flowing through all these various internal paths - many of which are not designed for current flow.

    While we think of the internal schematic as discrete devices, the internal transistor/diodes/resistors are actually just separated by reversed PN junctions. So excessive currents can flow from one portion of the circuit to another. We do know that a large reverse voltage on one comparator input can affect another channel when stressed - even when that other channel is running in a valid state. So the internal devices can "talk" to each other during stress.

    Current is the biggest factor. Remember that the Abs Max ratings just states that damage will not occur, but does not guarantee proper operation. So the rather generous 50mA current can still cause malfunctions (but not damage). To get 50mA takes almost a volt below the negative supply. It is best to minimize the amount of reverse current to as little as possible.

    TI will always recommend using external clamping if negative voltages are part of normal operation. You may need dredge up some enthusiasm for an external clamp diode...:^)