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LM393: LM393 output voltage

Ganapathi rao
Intellectual 320 points
Part Number: LM393

Hello,

I am using LM393 IC in one of our designs. Vref  (3.43V) is connected to +Ve terminal of comparator. Attached reference schematics and voltage wave forms at -Ve terminal (A) and comparator output (B). Can you please explain why comparator output start raising towards 5V and then starts falling.

  • 0
    Guru 140200 points
    Hi Ganapathi,

    what is the pink curve showing? What do we see in the scope plots? Input signal "A" is changing between 3V and 1V? Output signal "B" is changing between 5V and -0.8V?

    What is connected to the output of LM393A?

    Also, your input signal is a bit high. Common mode input voltage range is 0V...3V for full temperature range.

    Kai
  • 0 in reply to kai klaas69
    Intellectual 320 points

    Hi Kai,

    Thanks for quick reply. We have a current transducer. Output of current transducer is given at point A in attached schematics. Pink color graph shows input signal to current transducer. Ideally, when pink color graph reaches above 1V ( it shows short circuit current in board is 50A), comparator output shall go to GND. but it is  not going like that. It starts raising towards 5V from 3.3V then falling towards 0. Output of comparator is given to FPGA in our design. Kindly let me know if you need more information.

    As comparator output is pulled up to 3.3V, so output voltage is at 3.3V until V- voltage raises above Vref. But,  What could be the reason for comparator output to change from 3.3V to 5V then falling ?.

    As comparator has open collector inside, not sure how output reaches to 5V.

    Output at open collector shall be either open circuit or GND,No relation with VCC.

    Thanks & Regards

    Ganapathi

  • 0 in reply to Ganapathi rao
    Guru 140200 points

    Hi Ganapathi,

    hhm, 50A short circuit current is quite a bit. It could shift the ground potentials somewhere. This shift could be superimposed to your measurement. Have you connected the ground tip of scope probe directly to pin 4 of LM393A?

    Kai

  • 0 in reply to kai klaas69
    Intellectual 320 points

    Hi Kai,

    I have measured voltages just near the comparator. Everything seems OK. May be during my last measurements, ground might have shifted and its super imposed on comparator outputs. Pink wave form is t +Ve terminal, Blue waveform is at -Ve terminal and Yellow waveform is comparator output. Thanks a lot for your quick responses.

    Thanks 

    Ganapathi

  • 0 in reply to Ganapathi rao
    Guru 140200 points
    Hi Ganapathi,

    yes, looks much better now!

    But your input voltages are a bit too high. I would modify the voltage dividers so that no more than 3.0V is reaching the inputs of LM393A.

    Kai
  • 0 in reply to kai klaas69
    Intellectual 320 points

    Hi Kai,

    VCC is 5V and Common mode voltage range is 3V in my design. So does it mean that if V- reaches to VCC (5V in my design)  and V+ can be at 3V max ?

    Thanks 

    Ganapathi

  • 0 in reply to Ganapathi rao
    Guru 140200 points
    Hi Ganapathi,

    both input voltages should stay below 3V. Exceeding the common mode input voltage range can not only cause phase reversal but can also ruin the timing.

    Kai
  • 0 in reply to kai klaas69
    Intellectual 320 points
    Hi Kai,

    As per data sheet:

    (2) The voltage at either input should not be allowed to go negative by more than 0.3 V otherwise output may be incorrect and excessive
    input current can flow. The upper end of the common-mode voltage range is limited by VCC – 2V. However only one input needs to be in
    the valid common mode range, the other input can go up the maximum VCC level and the comparator provides a proper output state.
    Either or both inputs can go to maximum VCC level without damage.


    VCC is 5V and Common mode voltage range is 3V in my design. So does it mean that if V- reaches to VCC (5V in my design) and V+ can be at 3V max ?

    Thanks
    Ganapathi
  • 0 in reply to Ganapathi rao
    Guru 140200 points
    Hi Ganapathi,

    your last scope plot shows that the voltage at the +input of LM393 is 3.44V, even 3.6V including switching noise. The input voltage at the -input goes up to 4.91V. So, both input voltages exceed the common mode input voltage range!

    The passage you have copied from the datasheet explains that there is no damage and no phase reversal under the conditions decribed there. But this has nothing to do with the timing and switching characteristics. If the timing is critical I wouldn't let the input voltages exceed the common mode input voltage range. But, of course, you are a free man and can do what you like...

    Kai
  • 0 in reply to kai klaas69
    Intellectual 320 points
    Hi Kai,

    Thanks a lot for your continuous support. Sure, i will consider all your inputs and proceed further for final design.


    Thanks
    Ganapathi