design opamp regulator

Discussion in 'General Electronics Chat' started by hansy05, Apr 27, 2012.

  1. hansy05

    Thread Starter New Member

    Apr 27, 2012
    Can anyone tell me how to design a hysteretic op-amp regulator (max. window of +/- 0.5V)?

    I don't exactly understand the meaning of "a hysteretic op-amp regulator (max. window of +/- 0.5V)". Window? hysteretic?
  2. WBahn


    Mar 31, 2012
    Is this homework? If so, it would probably get better service if it were over in the Homework Help forum. But at this point, let a moderator make that decision and move it if they wish; don't repost it over there.

    Where is the term coming from? Your textbook? An assignment presented by your instructor? Those are the best places to look for these kinds of answers.

    The term "hysteresis" basically means, "is sensitive to the history of". For instance, adding hysteresis to a comparator (such as a Schmitt trigger) means that the threshold is dependent on whether the recent history of the system has placed the comparator in a HI or a LO state.

    What is being regulated? The voltage on some load?

    I can think of a number of applications where the use of these terms would be reasonable and the resulting circuits would be very different. You really need to describe the entire assignment, and perhaps give us some information on the kinds of circuits you have been working with in the course up to this point.
    hansy05 likes this.
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    In most cases, a "hysteretic regulator" is a switcher that operates in burst mode, which is to say turns the PWM driver on when the output voltage decays to a lower threshold level and the switcher turns off when it reaches the upper voltage threshold.

    Needless to say, if you don't even understand what it is you are supposed to be designing, you probably can't. Go back to your teacher and explain that you can't read his mind and he needs to explain this better.
    hansy05 likes this.
  4. WBahn


    Mar 31, 2012
    I was trying to think in more direct terms, but this makes perfect since -- and it means that the opamp is probably configured as nothing more than a Schmitt trigger intended to go HI (or whichever direction turns on the driver) when the output voltage drops 0.5V below the target output and then keeps it on until the output voltage rises to 0.5V above the target output. So the "window" is simply the range over which the output is allowed to vary and still be acceptable.

    To make the term make a bit more intuitive sense, imagine there is a toy duck on a toy locomotive that is running back and forth along a track. You have a switch that either tells the train to move left or to move right (no "stop" position, it is always one or the other). Now imagine that someone builds a brick wall between you and the track but they put a window in the wall. They then tell you that your objective is to keep the duck "in the window", but they also tell you not to wear out the switch. So instead of waiting until the duck is in the center of the window and then rapidly flipping the switch back and forth to get the duck to stay right there, you let the duck move to the right edge of the window before switching it to go to the left and, similarly, you let it move to the left edge before switching it back. Consequently, your decision of where the duck is in order to make you switch is not fixed, but rather is dependent on the history of the duck, namely is it moving to the left or to the right.
    hansy05 likes this.
  5. crutschow


    Mar 14, 2008
    I believe, in many cases, the "hysteretic regulator" generates the PWM directly, at least for a buck type regulator. That is, when the voltage drops below the lower voltage threshold, it turns the switch on, and when the voltage rises above the upper voltage threshold, it turns the switch back off. The frequency of this PWM depends upon the load, the voltage delta, and the inductance and capacitance of the output filter.