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UCC2897A: I want to know about FB & CS PIN in UCC2897A

YongSung Bae
Prodigy 185 points
Part Number: UCC2897A

I will use UCC2897A to design active clamp forward converter

, then I don't understand the principle of operation of the comparators in the blue line. In particular, I am curious about the output from the input at pins 12 and 11 to the operation of the two comparators. I want to understand the behavior of these pins for the open loop test, so I hope you can help

  • 0 in reply to YongSung Bae
    TI__Intellectual 2640 points

    Hi YongSung! 

    Thank you for reaching out and for your interest in the UCC2897A! 

    Essentially, SS/SD and FB take turns governing the switching of the controller. The pin with the lower voltage controls the switching. At turn-on the SS pin ramps up, eventually starting switching. As the FB pin begins getting voltage information back from the output, it will take over controlling the output pulses and widths. 

    The first comparator (with 0.75 V negative input) compares the incoming current sense signal (from CS pin 9) to an internal threshold. This comparator checks to see if the system is in need of over current protection and kicks the controller by restarting if it needs to. 

    The second comparator (above the first) checks the current sense signal against the FB (or SS) information in order to determine the duty cycle of the output. 

    More detailed information is available in the Feature Description section of the datasheet. 

    Thanks again for getting in touch, I hope this information helps! 

    Aidan Davidson

  • 0 in reply to Aidan Davidson1
    Prodigy 185 points

    Hello Aidan
    Thanks for your answer.

    When I look at the schematic, it seems that the SS pin already has a voltage of 5V. If so, isn't it the right way to control the low voltage rather than the higher voltage of the FB pin and the SS pin to control the switching?

    And if you have any advice on OPEN LOOP test using this IC, please.

    Have a happy day.
  • 0 in reply to YongSung Bae
    TI__Intellectual 2640 points

    Hi YongSung! 

    You are correct, the lower of the 2 pin voltages will control the switching. I have updated my original post to reflect this. The SS pin ramps up to 5V depending on the size of the external capacitor connected to it. 

    I don't have any open loop advice to offer at this time, I will add on to this thread if I find something. 

    Thanks again for reaching out, I hope this helped! 

    Aidan