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LM339: Non inverting input voltage

Anusree P V
Prodigy 20 points
Part Number: LM339

Hi,

As per data sheet of LM339, the input voltage range is from -0.3V to 36V. We are using LM339 as a Comparator in our circuit , in certain cases, we have a voltage of -0.7V at Vin ( inverting). which is above the specification. Will it cause damage to Opam? What will be the behavior of Opam in this case? VCC is +24V and GND.

  • 0
    TI__Mastermind 49956 points

    Hello Anusree,


    The LM339 series does not have input ESD protection diodes, so any negative input voltages go through various substrate junctions, which are not designed to handle current. Obviously, this is bad. This is why the datasheet warns several times about not applying negative voltages. Applying negative voltages can cause malfunctions, parametric shifts (offset, bias current) and even permanent damage.

    If negative voltages are expected as part of normal, external Schottky clamping diodes and series current limiting resistors should be used. Current should be limited to about 1mA or less. The series resistance may be part of and voltage dividers.

  • 0
    TI__Expert 3350 points

    Hello Anusree,

    Under overvoltage condition for this particular device, it is recommended that the current into (out of) the pin be limited to less than 1mA by adding an external series resistance (>1k in your case). The current limiting resistance can be part of any input dividers.

    Best regards,

  • 0 in reply to Bahram Mirshab
    Prodigy 20 points

    Above is our current circuit. The -0.7V is coming from a relay switching ( freewheeling).

  • 0 in reply to Anusree P V
    TI__Mastermind 49956 points

    Hello Anusree,

    Place a Schottky diode directly between pin 8 and 12. The series 10k is enough to safely limit the current.

    We specify Schottky diodes as they will conduct earlier (~400mV) and take the brunt of the current.

    Don't rely on the relay fly-back diode to clamp as it will have large peak currents and may have a peak voltage of up to -1V across it. The second diode will safely clamp any negative voltages since they are outside the high current path.

  • 0 in reply to Paul Grohe
    Prodigy 70 points

    Hi,

    This is a question related to the above query by Anusree.

    The datasheet of LM339DE4 mentions:  

    a) Absolute maximum Voltage Input min as -0.3V

    b) in operating Condition section Voltage Input range is not mentioned but  it is mentioned in the footnote that "  The voltage at either input or common-mode must not be allowed to go negative by more than 0.3 V. " 

    --What is not clear is the operating range of Voltage Input . Could you please let me know ?

    --If the Voltage input min is 0V. What would happen between 0V to -0.3V  ( if we limit the back emf to less than -0.3V , say we limit using a schottky as suggested above)

    Thanks

  • 0 in reply to Ajit Nandakumar Mannargudi
    TI__Mastermind 49956 points
    Hello Anusree,

    The maximum operating input voltage is listed in the 'Common mode input range" specification in the Electrical Characteristics table. It is specified as Vcc - 2V, or 2V below the Vcc voltage.

    So if your supply voltage is +24V, the legal input range is -0.3 to +22V.

    Going below -0.3V could cause improper operation (false output), and even damage depending on the available current. The 10k resistor will limit the current.