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Original question:

UCC28780EVM-002: Unable to start up when connected to power

UCC28780EVM-002: Unable to start up when connected to power

Nisarg Mehta
Prodigy 160 points
Part Number: UCC28780EVM-002

I have a doubt regarding the feedback current (Ifb), it is not clear to me from the datasheet that Ifb should be between 0-75uA. At what value of Vout, what should be the value of Ifb, that I am not understanding. For eg., If nominal output is 20V, then at 20V should the feedback current be 50uA or 75uA or 0uA or what? at which exact value of Ifb, the control unit will consider the output is as per required? Please help me with this query.

Could you provide me in detail the range of the Rdm and Rtz? As in the following description from the datasheet, it says about too small and too large current, could you specify the limits ? And also what is the voltage level to check the pin open or short condition?

  • 0
    TI__Guru 66135 points
    Hello Nisarg,

    The title of your thread is “UCC28780EVM-002: Unable to start up when connected to power”, is that correct? You are asking questions about device operation you should consider changing that.

    Figure 23 that shows control law should have an IFB on the X Axis. At IO(OPP) the IFB current should be 0uA and 75uA at IO = 0A. I will have to ask the systems engineer about the other IFB points and get back to you.

    In regards to the RDM and RTZ resistor range, I will need to ask the systems engineer about this as well.

    Regards,

    Mike
  • 0 in reply to Mike O'
    TI__Guru 66135 points

    Hello Nisarg,

    I discuss the RDM and RTZ with the design engineer. He said that if you design for applications between 5 W and 150 W, you should not have an issue in regards to triggering open/short pin protection.

    In regards with how IFB current moves with control law I discussed this with the system engineer. Please refer to figure 23 of the data sheet.

    At Io max, IFB ≈ 0uA

    AAM, IFB 25 uA to 0uA

    ABM, IFB ≈ 25 uA to 35 uA

    LPM, IFB ≈ 48 uA to 35 uA

    SBP, IFB ≈ 75 uA to 48 uA

    Io = 0A, IFB ≈ 75 uA

    Regards,

    Mike

  • 0 in reply to Mike O'
    Prodigy 160 points
    Hello Mike,

    Thank you for your help..