Add hysteresis to buck Enable?

Discussion in 'The Projects Forum' started by apqo1, Jun 1, 2013.

  1. apqo1

    Thread Starter Well-Known Member

    Oct 5, 2008
    50
    3
    I'm working with a buck converter (RT8298ZSP -- see page 11) that features an enable pin allowing programmed undervoltage lockout (UVLO). I have no problem programming the UVLO threshold...
    Ren1=100k
    Ren2=18.2k
    UVLO≈13V
    ...but I'd like to add about ±200mV of hysteresis (13.2V rising, 12.8V falling).

    Is there a relatively simple way to accomplish this?

    Thanks.


    NOTE: The data sheet is somewhat contradictory as to the actual enable comparator threshold voltage. In some places it's 1.7V; in others it's 2V. I emailed the manufacturer and was told that 2V is the correct value, despite the example given on page 11.
     
    Last edited: Nov 25, 2013
  2. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    For hysteresis you need a small amount of positive feedback and I can't think of any way to do that except to add an external comparator with the desired hysteresis.
     
  3. apqo1

    Thread Starter Well-Known Member

    Oct 5, 2008
    50
    3
    That's what I figured. To use a comparator, I'd have to provide a Vref exactly equal to my desired threshold center point (13V), right?
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    That's correct. You could use a zener or a voltage reference chip for that.
     
  5. apqo1

    Thread Starter Well-Known Member

    Oct 5, 2008
    50
    3
    OK, thanks. I'm familiar with a basic comparator schematic and how to use a zener.

    Could you suggest an appropriate comparator to get me started, or at least what type? The parametric search on Digi-Key is a blizzard of terms I don't understand well enough to choose...
     
    Last edited: Nov 25, 2013
  6. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    You could likely use the old LM339 or LM393. Not real fast or super accurate but should suffice for your needs. Note that the output is an open collector so you need a pull-up resistor to V+ to get an output voltage change.
     
    apqo1 likes this.
  7. apqo1

    Thread Starter Well-Known Member

    Oct 5, 2008
    50
    3
    Quick update on recent progress: I did some more studying on comparators and hysteresis and came up with the attached circuit. Enable is calculated at 13.13V rising, shutdown at 12.73V falling.

    Parts are here and I should have a board soon. I'll post again once I have it built and tested.
     
    Last edited: Nov 25, 2013
  8. apqo1

    Thread Starter Well-Known Member

    Oct 5, 2008
    50
    3
    Prototype board built and tested. It works as expected unless it's connected to the buck regulator's enable pin, in which case the falling threshold voltage increases to 13V. I finally figured that the regulator's internal 5k pull-up resistor was effecting the comparator's feedback. Some experimentation resulted in an external pull-up of 10k instead of 100k. Current through the comparator's FET is still limited to 1.8mA with enable low, and the falling threshold problem is solved. It now provides rising threshold of 13.19V and falling threshold of 12.82V.

    Test board and final circuit attached below. Thanks again, crutschow, for all your help!
     
    Last edited: Nov 25, 2013
  9. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    Glad you found a solution to your anomaly and it's all working well. Congrats. :D
     
Loading...