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100-200ns comparator needed for DIY Constant off time controller

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Flyback, Sep 28, 2017.

  1. Flyback

    Flyback Well-Known Member

    Jan 5, 2007

    We are doing a 230VAC offline, 150W, Power Factor corrected, single stage LED driver.

    We need constant off time control. However, there are no OffTheShelf control chips available for this purpose. Therefore , we have to make our own as in the attached schematic.

    As you can see, the DIY constant off time controller unfortunately consists of three separate IC’s and a signal FET….As you can see, there are also a number of resistors involved there. There are fast, high rise time signals within the bounds of the controller, and shielding this lot of components from noise will be a challenge.

    As such, we need to use hysteresis with the PWM comparator. The LT1713 comparator gives a very convenient Q! signal which is ideal for giving hysteresis. However, the LT1713 comparator is way too fast for our purpose, (it’s a 7ns comparator). We need a 100-200ns comparator instead, to make noise free operation easier to achieve. However, there are no OffTheShelf comparators which have the Q and Q! outputs that the LT1713 does. Can you confirm this? Do you know of a 100-200ns comparator with Q and Q! outputs?

    Schematic and LTspice sim attached.

    Attached Files:

  2. crutschow

    crutschow Well-Known Member Most Helpful Member

    Mar 14, 2008
    L.A., USA Zulu -8
    Just add a little RC delay at the input if you need to slow it down.
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