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LM358: LM358 output question

Redick Lee
Prodigy 570 points
Part Number: LM358

Hi Team,

 

Sorry to bother you.

We use LM358 to do comparator application (V+ 5VSB and V- GND). Currently, the measured result on pin7 is 3.72V.

As we can see from datasheet, it define the minimum voltage range of Voh is Vcc-1.5V, but we can’t find the data of maximum voltage range.

We have two questions as below, may I have your comment? Thanks for your help and time.

 

  1. Per my understanding, the output voltage range on pin7 is 3.5V~5V, am I correct?
  2. May I know how to get a stable output voltage on pin7?

 

Yours Faithfully

Redick Lee

  • 0
    TI__Genius 13445 points
    Hi Redick,

    Thanks for inquiring about our part.

    1. Yes you're correct in your assessment of the Vout range.
    2. Directly connecting a capacitor at the inverting pin of an amplifier is always recipe for oscillation. What are you hoping to achieve with C664 on your schematic? It's not a good idea to filter signal at inverting input.. I would suggest removing the cap

    Regards,

    -Mamadou
  • 0
    TI__Guru*** 149776 points

    Redick,

    The output voltage moves 1:1 with Vcc voltage.  The voltage loss between VCC and output is two PN junctions (diodes)  and some resistance * load current.  The internal resistance varies a little (up to +/-15%) device to device. The resistance increases with higher temperature. The PN diode voltage decreases with temperature at a rate near -2mV/C or -4mV/C for both diodes.  

    Your load resistance is high so the variance for internal resistance is not a significant factor.  Temperature and VCC will be the most significant factors.

    This chart should help.

    In conclusion; for a stable output voltage, keep VCC and temperature constant.

    Otherwise use a voltage regulator or use an analog switch to 'switch in' a known stable voltage. 

  • 0 in reply to Ron Michallick
    TI__Prodigy 570 points

    Hi Team,

    Thanks for your reply, one more question.

    According to OPA compare circuit, the V1 will impact on Vout value (If open loop gain is fixed at VCC). Do you think that is really suitable for this BATT_PRESENT function? We are not sure the detection is still workable if BATT_SIOC have different voltage level. Please advise your comment or suggestion.

     

    Yours Faithfully

    Redick Lee

  • 0 in reply to Redick Lee
    TI__Guru*** 149776 points
    Redick,

    Most of the time, (V1 -V2) will be large and Aol is large so the limitation of VOL and VOH will apply most of the time.
    When inputs are the 'same' value, then expect [noise * Aol] to cause output to jump around as the input voltages match each other. Hysteresis can fix that issue.

    As for the drawing and formula in your post, I don't really see how this applies to a comparator application.
  • 0 in reply to Ron Michallick
    TI__Prodigy 570 points

    Hi Team,

     

    Currently, the measure result of the U23 pin7 is around 3.72V.

    According to the VOH-VCC picture as below, the VOH max is about 3.8V(5-1.2), am I right?


    Yours Faithfully

    Redick Lee

     

  • 0 in reply to Redick Lee
    TI__Guru*** 149776 points

    Redick,

    I'll make it easy for you. I scaled the VOUT based on VCC being 5.000V and I added a load line matching your load of 54.5 kohms

    -40C is 3.5V, 25C is 3.78, 125C is 4.2V.

    LM358 is 0C to 70C so interpolate the results and add some margin to cover variance or just use 3.5V to 4.2V as a range that includes already includes margin.