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LM393: LM393

Rohini E
Intellectual 610 points
Part Number: LM393
Other Parts Discussed in Thread: TLC372, TLV3702, TLV3402, TLV7032, TLV7011, TLV4041

Hi Team,

We are using LM393D to indicate battery low in our design. Please find our circuit and observations below

During design phase we used 5.1K at R98. So  Vref=2.2V.

Now after battery tuning it has to be 2.8V. so, we have replaced R98 with 1.78K.

Note: We gave Hysteresis loop for output stability.

Observations:

Comparator's output is low even when V+=5V (PIN 3) and V-=2.8V (PIN 2)

We tried without Hysteresis loop also(without R117), but the result is same - Output is low.

I request you to share your input on the design and the above issue.

We are awaiting for your response

Thanks & Regards,

E.Rohini

  • 0
    TI__Guru 53296 points

    Hello Rohini,

    Your schematic did not attach. Click the paper clip icon to attach a file, or the "picture" icon (to the left of the icon) to attach an image/jpeg.

  • 0 in reply to Paul Grohe
    Intellectual 610 points

    Hi Paul,

    Please find the attached schematics. R98 is tuned to 1.78K for 2.8V Vref

    Thanks & Regards,

    E.Rohini

  • 0 in reply to Rohini E
    TI__Guru 53296 points

    Hello Rohini,

    You are violating the input voltage range. The LM393 input range is 0V to 2V below Vcc.

    At 3.3V supply, the limit is 1.3V. You are applying 2.2V and 5V, so both are out of range and the output is indeterminate.

    It may "seem" to work at room temp, but it will fail at cold temps.

    At 5V supply, the limit is 3V, so it should work, but you are still violating the input range with 5V. The output will still be correct because you are relying on the "feature" that as long as one input is in the valid range, the output will be correct. But operation is not warranted since you are still violating the input range.

    Please see the following appnote for more info:

     Application Design Guidelines for LM339/LM393/TL331 Family Comparators

  • 0 in reply to Paul Grohe
    Intellectual 610 points

    Hi Paul,

    Yes, we have gone through the design guide.

    Since it was a very old design we didn't relay on the input range.

    Thanks for your input. We will search for new footprint compatible comparator

    Regards,

    E.Rohini

  • 0 in reply to Paul Grohe
    Intellectual 610 points

    Hi Paul,

    Can we use TLC372 for our design. I request you to share your thoughts on the same

    Thanks & Regards,

    E.Rohini

  • 0 in reply to Rohini E
    TI__Guru 53296 points

    Hi Rohini,

    Are you using a 5V supply or 3.3V? This makes a big difference if you are applying 5V on pin 3.

    If you need a drop-in replacement, that complicates things since few comparators can handle inputs above the supply voltage.

  • 0 in reply to Paul Grohe
    Intellectual 610 points

    Hi Paul,

    We are using 2 supplies. Either 5V from adapter or 3V from battery

    Thanks & Regards,

    E.Rohini

  • 0 in reply to Rohini E
    TI__Guru 53296 points

    Hello Rohini,

    Because your input is potentially above the supply voltage, then you would need a device such as the TLV3402, which allows for the input to exceed the supply by up to 5V (20V max). It is a standard dual pinout, so it should drop-in replace the LM393D. It is a nanopower device, so it is slow (30us), but batteries don't move that fast.. If you wish to eliminate the pullup resistor, the TLV3702 is the push-pull output version.

    Also, please remove the large capacitors on the input and output. C85 negates the hysteresis feedback. C88 should not be needed if the hysteresis is functioning correctly (which it is not). If input filtering is needed because of noise, then split the 3k in into two 3k resistors with the cap in the middle

    If you are contemplating a redesign, have a look at the TLV7032 (TLV7011 family), which will tolerate up to 6V on the input pins, regardless of supply voltage. Also conciser the TLV4041 family with built-in reference.


  • 0 in reply to Paul Grohe
    Intellectual 610 points

    Hi Paul,

    Thank you for your suggestions.

    Regards,

    E.Rohini

  • 0 in reply to Rohini E
    TI__Mastermind 24330 points

    Thank you Rohini

    I am going to close this post.  if you need any further assistance, you can create a new thread.

    Chuck