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LM393LV: OV detection circuit not working as expected

Anand Meharwade
Expert 1090 points
Part Number: LM393LV

Hello All,

We have considered the LM393LV dual comparator package for the design of Over Voltage detection circuit. One of the comparator is used for under voltage and other is used for over voltage.

We have set the Under & Overvoltage threshold as shown below. The non inverting under voltage circuit is working correctly and the comparator output transitions are as expected at correct threshold values. But for Overvoltage circuit, the output goes low before the set threshold and is toggling. The input to these comparators are same and fed from a op amp circuit. Could you please help me to understand the problem and to resolve this at the earliest.

Below are the threshold values set for OV.

VL = 2.527V (seen toggling from 2.44mV and become low at ~2.53V)
VH = 2.419V (for recovery to high the toggling starts a ~2.5V and becomes high at ~2.419V)

OV hysteresis is 108mV

I tried increasing hysteresis but toggling of the output is still there and the transistion values are not correct. I have simulated these values in TINA and are looks to be fine)

Looking forward to hear from you soon,

Regards,

Anand M

  • 0
    Guru 260440 points

    C1080 prevents the hysteresis feedback from working quickly. Remove it.

  • 0 in reply to Clemens Ladisch
    Expert 1090 points

    Dear Clemens,

    Thanks for the response, after removal of the capacitor, the behavior is same. The observation is comparator output goes low and toggles continuously before the the reference voltage is reached. My set points for OV is 2.527V but the comparator output goes low and toggling before this voltage. The output toggles until 2.527V and then become LOW.

    The toggling behavior is the issue which I want to eliminate. Could you please let me know if there is anything to be looked at.

    Regards,

    Anand M

  • 0 in reply to Anand Meharwade
    Guru 260440 points

    This sounds as if the hysteresis is not working.

    The pullup R1161 is very weak; it and R1179 form a voltage divider. Make it 10 kΩ to ensure that the output voltage is higher.

    Please show an oscilloscope trace with pins 5, 6, and 7.

  • 0
    Guru 140200 points

    Hi Anand,

    are you sure you want R1161 to be that high ohmic? I would decrease it to 10k.

    Also, as Clemens already stated, C1080 is counterproductive and must be removed.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru 51606 points

    Agreed.

    Decrease the R1161 value to 10k to beef-up the output high voltage.

    Also, is there a bypass capacitor nearby on the comparator supply voltage? 0.1uF?

    You could try re-adding the C1080, but connecting to the negative input side (right side of 1k) to help filter the incoming signal.

    Is the "toggling" random, or periodical, like an oscillation?

  • 0 in reply to Paul Grohe
    TI__Genius 11701 points

    Hi Anand,

    We have not heard from you in a while so I will be closing this post. If you have any more questions, feel free to reply back to this thread.

    -Chi