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LM393: LM393 hysteresis circuit questions

Part Number: LM393
Other Parts Discussed in Thread: LM2903

Hi Support team 

my customer adopted LM393 in hysteresis circuity for power up control as following 

they suffered hysteresis circuity invalidation with unknow reason, but after change the source to ST, the waveform is normal 

may i have your help to check the different between TI and ST

do you have any countermeasure to overcome it   

thanks a lot 

abnormal waveform 

Normal waveform 

  • Hello Red,

    Please confirm the supply voltage for the comparator. Is it 5V or 12V? Is the comparator supply also rising? Or steady when these photos were taken?

    What is the value of C846 (10nF?). What is the B+ voltage?

    Are they seeing this issue with all TI devices, or just a few?

    I do see some issues. When using positive feedback (hysteresis), it is not recommended to place capacitors on the positive input node (C843, C846), or on the output (C871). These cause a delay in the feedback and can affect the hysteresis action (causing bursting).

    It looks like there is a large transient that occurs at 360ms and 570ms, just before the transition in the "normal" waveform. Do they know where this is coming from? It looks like it goes negative by at least -1V. The LM2903 family does NOT like negative voltages and spikes like that can cause abnormal operation. The transient occurs just before the output switches in the "abnormal" plot. Have they also looked at the supply line for transients?

  • Hi Support team 

    C846 is NC (no component)、C843 is 1nf, VCC is 14~15V, B+ is 325~330V

    there is no.C871, I assumed you are talking about the C844?

    BTW, my customer would like to know the delay time while VCC power up ready and differential feed into voltage, when the output will generate the voltage?

    thanks a lot 

  • Hi Red,

    what is the purpose of D803 and C844?


  • Red

    we will continue to review and get back to you.  In the meantime, please comment on Kai's questions as he attempts to assist as well.

    Thanks Kai for your review as well.


  • Hello Red,

    It looks like in the "abnormal" photo, the positive input (blue) starts drooping about 15ms after the rise, then crosses back down under the reference (green), where the comparator output then starts to go low, then the 12V shuts down. Then the positive input comes back up above the reference, and the comparator output starts rising. Seems like the comparator is functioning.

    So the question is why the positive input node "droops" between 100ms and 130ms, whereas the "good" does not seem to droop.

    There also seems to be a burst on the reference (neg input) at that time. There is no bypassing on the reference. Try placing a 10nF cap in parallel with R870 to provide bypassing for the reference.

    Have you monitored the comparators supply? To watch for transients or glitches?

    Is there a bypass capacitor directly across the comparators supply pins (0.1uF directly across pins 4 and 8). I see a lot of transients and noise on the traces,

    What is the other half of the comparator (U803A) doing? Have you looked at those pins to make sure signals are within ranges? Check for transients.

    Also try removing C843 - it may be delaying the hysteresis.

    The LM393 should "power up" within a few microseconds - faster than the time scale of the plots. We cannot see the sequence of the comparator supply, but the comparator seems to be reacting correctly in the beginning.