1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

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

    Joined:
    Jan 5, 2007
    Messages:
    2,009
    Likes:
    34
    Hello,

    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

    Joined:
    Mar 14, 2008
    Messages:
    10,623
    Likes:
    479
    Location:
    L.A., USA Zulu -8
    Just add a little RC delay at the input if you need to slow it down.
     
    • Like Like x 1

Share This Page