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Unstable comparator LM393 with small Hysteresis

Randy Fung
Prodigy 60 points
Other Parts Discussed in Thread: LMV393, TINA-TI

Dear Support,

I have the following requirement to compare with on the schematic attached.

whenever PSA_PSOK fall below or  in the range of 2V to to 2.5V (3.3.V signal), I want to assert "PSA_PWRGD" high.

The circuit attached is not stable and some times it oscillated. I think because the Hysteresis is too small.

The vref is at 2.8V and the the power in is about around 3.3V.  

Based on the formula, Vin1 and Vin2 are very closed too each other. I think they need to be half between VIN1 and VIN2. Is that right?

VCC R1 R2 Vref Vin1 Vin2
3.28 10000 1000000 2.78 2.8078 2.775

What is your recommendation to  change the circuit to stabilize and  meet the specs requirement of < 2.5V and flag.

Please advise.

Thank you.

Randy 

  • 0
    TI__Guru 53846 points

    Hi Randy,

    32mV is a bit on the low side. You want to be at least 2x the maximum input offset voltage of the comparator (since the comparator offset will skew the thresholds) plus the window needed for any p-p noise on the input signal line.

    But you are exceeding the upper input common mode limit of the LMV393. The limit is 0.7V below V+, or 2.6V. You are operating in the "grey zone" where it still appears to work, but the offset voltage will be varying quite a bit, which could interfere with the hysteresis.

    I would shift the trip-point down at least 300mV (or more). You can do this by changing the reference divider and also adding a third resistor to the non-inverting input to ground to create a second voltage divider. See AN-74 for help with the equations.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 60 points

    HI Paul,

    Thank you for your reply and suggestions. I did modify the circuit as you suggested.

    I added the R982 with V+ = 2.59V  and Vref about 1.8V.  It stabilized. However, it did not behave as I wanted. See my scope capture pic. I noticed if Vin is shutdown slowly linear. The circuit worked. But Vin is sharp drop. The Op did not work. I think because it's not fast enough.

    Can you comment and suggest to improve it to response to faster switching on the VIN?

    Thank you.

    Randy

     

    Slow Vin response Opamp ok.

    Faster Vin drop. Opamp Failed to detect

  • 0 in reply to Randy Fung
    Intellectual 480 points

    I would experiment using Tina-TI (free spice simulator) and substitute a voltage generator set to a triangle at 2.5 +/- 0.5 Volts for PSA_PSOK and putting a meter on the output. Where the triangle crosses the square wave is an easy measure of the trip in both directions so both trip points and hysteresis can be seen and no soldering!