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UCC25600: accuracy of setting the minimum switching frequency

Lutz Naumann
Expert 6460 points
Part Number: UCC25600

Dear Team,

the "Features" describe a +/-4% accuracy for setting the minimum switching frequency. Under "Electrical Characteristics" one can find the Kico (switching frequency gain / I (RT) ) with a typical of 80Hz/uA but a min / max of 60Hz/uA / 100Hz/uA. this deviation is much larger than +/-4%.

Practical tests over several production lots confirm values in the range of 77Hz/uA and so the +/-4% spec.

My question is: Do we really have to expect the huge deviation which would result out of the Kico for setting the setting the minimum switching frequency? What is the explanation of the discrepancy between the +/-4% spec and the Kico rating? 

Thanks Lutz

  • 0
    TI__Guru 73905 points

    It depends on the temperature range. The Feature says  so please read the whole sentence to make understanding.

    Programmable Minimum Switching Frequency The UCC25600 high performance resonant mode

    With 4% Accuracy (3% accuracy at controller is designed for dc-to-dc applications

    temperature range: -20°C to 105°C)

     

    Kico (switching frequency gain / I (RT) ) with a typical of 80Hz/uA but a min / max of 60Hz/uA / 100Hz/uA. If you read the table carefully, that is with temperature -40C to 125C.

    So your expectation should be made in accordance to temperature in your application.

  • 0 in reply to Hong Huang
    TI__Expert 6460 points

    Hello,

    I agree with the statement of the features (in brackets) mentioning +/-3% accuracy for the temperature range -20°C to 105°C.

    I also agree with the statement that Kico with (min 60Hz/uA) < (typ 80Hz/uA) < (max 100Hz/uA) is speced over full temp -40°C to 125°C.

    So in what temperature range are the +/-4% valid then?

    many thanks

    Lutz

  • 0 in reply to Lutz Naumann
    TI__Guru 73905 points

    In range of -40 to 125C. If you look at the table on page 3, you will see Fsw(min) min to typ 40.04 to 41.7 kHz to show -3.98%, and max to typ 43.36 to 41.7kHz to show +3.98%. 

  • 0 in reply to Hong Huang
    TI__Expert 6460 points

    OK, I see.

    So once the frequency is set to the 41.7kHz it will stay at that frequency over full temp with +/-4%. My problem is, that setting the frequency to the typical 41.7Hz is not accurate then. For example. If I set the IRT = 600uA my typical switching frequency results in 48kHz. And the typical would remain over full temp within +/-4%. However, the set frequency may vary between 36kHz and 60kHz due to the variation of the Kico.

    Form my understanding the spec of the Kico tolerance is the initial switching frequency tolerance and the +/-4% the switching frequency temperature drift.

    So in total over full temp the part will have a +/-24% tolerance wrt. a calculated and set IRT of 600uA.

    Is this understanding correct?

    kind regards

    Lutz

  • 0 in reply to Lutz Naumann
    TI__Expert 6460 points

    Hi Team,

    do you have any feedback for me?

    thanks

    Lutz

  • 0 in reply to Lutz Naumann
    TI__Guru 73905 points

    Kico does not affect FSW(min) tolerance.

    When FSW(min) is reached, it is in the range of 40.04kHz to 43.36kHz, in -40C to 125C.

    How much current is needed from RT to reach FSW(min) can vary 20% (Kico) but FSW(min) variation is +/-4% after it is reached.

  • 0 in reply to Hong Huang
    TI__Expert 6460 points

    OK, understood.

    so those are two different things. One is the accuracy of Fsw min ( once reached) and the other is how accuret one can set the switching frequency.

    thanks

    Lutz