• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 47 subscribers
  • Views 924 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PROBLEMS WITH IC: LM339MX/NOPB

Mamadee Fofana
Prodigy 30 points
Other Parts Discussed in Thread: LM339, LM139, LM360, LM339-N

Problem:

*LM339 not switching when input voltages are close to each other(i.e. 2.62Vref on inverting ) and 2.8V on non-inverting input)

* Open collector output of the LM339 is pulled to +5V through a 10K resistor; When inverting input gets 187mV applied, with 2.8V present at non-inverting input , the LM399MX/NOPB operates as intended(i.e. open collector output gets pulled up-to +5V).

 However, when Vref/inverting input = 2.62V, with the 2.8V still at +Vin, The  LM399MX/NOPB  output stays LOW. The IC does not seem to be discerning 180 – 200mV input difference.

Is there some internal hysteresis that we don’t know about…?

Digikey’s specifies the LM339MX/NOPB  as a General purpose LOW power comparator type.

They also have the LM139AD which is of a Differential type.

I am confused as to what the difference (if any) is between a general purpose LM339 and a differential LM339.

I need the comparator’s output to float to +5V under both non-inverting input voltage (187mV & 2.62V) conditions.

Please advise on a viable alternative.

Thanks

Mamadee V. Fofana

Contract Embedded Systems Design/Development Engineer

  • 0
    TI__Guru 53036 points

    Hello Mamadee,

    First... There is NO difference between the LM339 and a "differential LM339". All comparators are "differential" input. The "differential" title was an unfortunate choice of some marketer years ago, as the LM339 is NOT what we would consider a "differential" comparator as it only has one output. A true "differential" comparator would have two complementary outputs (like the LM360). The LM139 is the military temp range version of the LM339.

    So there are NOT two different types of LM339's. The LM339MX is the National Semiconductor version and the LM339AD is the Texas Instruments version of the same basic device. Since the TI/NSC merger, the National device is now known as the LM339-N and the TI version is the LM339.

    As for the failure - what is the supply voltage for the LM339? The LM339 has an input common mode range that extends from GND to 1.5V below V+. So if your supply is less than 4V, you may be seeing the effects of a common mode range violation.

    With the LM339 series, as long as one of the inputs remains within the legal common mode range, the output will be valid. But if both inputs are in violation, the output will be invalid and indeterminate. It sounds like at the 2.8V & 2.6V condition, both inputs are violating the input range resulting in an invalid output.

    If the LM339 has a 5V supply, then we need to dig further.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    The LM339 is being powered by a +5V wall adaptor: http://www.digikey.com/product-search/en?WT.z_header=search_go&lang=en&site=us&keywords=271-2575-nd&x=0&y=0

     

    This rules out the common mode range violation theory…so we do need to dig a bit further.

    I just applied +5.0Vdc to the non-inverting input with the inverting input still set to Vref of 2.62V and the

    Output swings to +5V (via pull up) as it should; my next step is to determine exactly how much of a voltage difference there must be between (–) VIN and (+) VIN to cause the desire output swing.

     

    From your experience…would a zero crossing detector work in this situation?

     

    Thanks

  • 0 in reply to Mamadee Fofana
    TI__Guru 53036 points

    Hmmm..

    Can you please provide a schematic of the circuit - including what is providing the input signals and what the output is driving? It is easier than continuing to play 20 questions.

    We are missing something - this is too simple of a failure. Have you tried replacing the LM339?

    You mention zero cross...are you measuring AC? Are you using a scope or DMM?

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    I’ve figured out the issue Paul. The LM339 needs a minimum differential voltage of 200mV for the output to swing. Using a POT and fixed resistor in a divider network, I gradually adjust Vref from 2.62V to 2.54V to get the LM339 output to swing.

    How come this information isn’t specified in the datasheet…or is it specified and I just interpreted it wrong?

    Please advice.

    Thanks for all your help.

  • 0 in reply to Mamadee Fofana
    TI__Guru 53036 points

    Hello Mamadee,

    Something is still wrong. 200mV is way too high. It should not be more than 5mV (the maximum offset voltage) at the inputs.

    To see 200mV would mean the device is damaged. Do you see 200mV on the other channels? Have you tried another LM339? Are there large value resistors (>100K) in series with the inputs? Are there any AC signals, intentional or unintentional on the inputs?

    I think we are overlooking something. Please provide a full schematic of your application and your "pot" test circuit. If you do not wish to post your circuit publicly, then please send it to paul.grohe@ti.com .

    Regards,