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Question of LMV7271 for Hysteresis Circuit

Jacky Chen29
Genius 4735 points
Other Parts Discussed in Thread: LMV7271, TINA-TI

Hola,

We used LMV7271 as our new design of hysteresis circuit as below：

Vin = 36V --> Vout : low

Vin = 39V --> Vout : high

And the calculated result as below：

But the actual result after we testing become as below：

Vin = 31V --> Vout : low

Vin = 40V --> Vout : high

Are there something wrong which about setting or missing？ 

Another question is if I want to achieve this hysteresis function, does TI have another suitable solution or LMV7271 is the best solution？

Brs

  • 0
    TI__Mastermind 22825 points
    Hi Jacky,
    I agree that the computed values (36V and 39V) should be the transition points you should get. The LMV7271 and other similar RRIO comparators should behave similarly, as your circuit is operating based on the external resistor values you are setting.

    Be careful about the accuracy of your 2.5V reference. A 5% shift upward moves the threshold significantly by 2V (38V and 41V). A 5% shift downwards results in the same 2V shift (34 and 37V). However, these are still lower than what you are measuring (31V to 40V). Also, make sure you decouple your Vref signal at the LMV7271 inverting input (cap to ground).

    The other source of possible error is the accuracy / tolerance of your resistor values. Make sure you account for that in case you are not using 1% values.
    If your Vin (36V) source has 60Hz hum (or other power supply fluctuations), it'd be a good idea to shunt the VA node to ground with a capacitor to get rid of these and get a more "averaged" waveform.


    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    Genius 4735 points

    Hola Hooman,

    Thank you for your prompt reply and below is my calculated result for your kind reference, is it correct？

    But I'm confusing about the description of the equations as below：

    The description is wrote VIN1 but the equation shown DELTA VIN1！

    If VA need equals VREF, and my calculated result might be wrong, right？because my VA is 0.27(see above)！It should be 2.5V, right？

    Besides, you can see my original design, our power source is 36Vin and I'm use resistor divider to let VA equals 2.5V as below, is it right？

  • 0 in reply to Jacky Chen29
    TI__Mastermind 22825 points

    Hi Jacky,

    I agree that LMV7271 datasheet equation 1 should be "Vin1" (not "delta_Vin1"). I'll inform the engineering team responsible for the LMV7271 to correct this typo.

    I'm not exactly following the computations you have shown. However, I've attached a spreadsheet which hopefully helps:

    LMV7271 Hysterisis 5_26_16.xlsx

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    TI__Mastermind 22825 points

    Hi Jacky,

    My apologies!

    I had an error in my spreadsheet (I had ignored the current flow through R1 when I computed the Vin trip point). I've corrected it below:

    LMV7271 Hysterisis Corrected 5_27_16.xlsx

    I've confirmed the trip points with TINA-TI as well:

    Here is the TINA-TI simulation file:

    LMV7271 Hysterisis 5_27_16.TSC

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Genius 4735 points

    Hola Hooman,

    Clear and helpful. Thank you very so much.