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Question about UCC28950 Slope compensation formula

Go
Genius 4190 points

Hi,

I'd like ask about the UCC28950 Slope compensation formula in app note SLUA560B page24 fomula(146).

Compered with the Excel Calculator (SLUC222C) formula in C222 cell,

The position of (1-Dtyp) term is different between these formulas

which one is correct? 

Kindly advice.

Regards,

Go

  • 0
    TI__Mastermind 25570 points
    Hello Go

    Interesting.

    Actually the RSUM is calculated using VSLOPE which is taken to be the greater of VSLOPE1 or VSLOPE2. Usually VSLOPE1 will be greater and the discrepancy you note in the position of the (1-D) term in the VSLOPE2 formula will have no effect - you can see this in the typical calculation results below.

    Calculate Magnetizing Current during ILOUT down slope dILMAG 0.234 A
    Calculate VSLOPE1 VSLOPE1 4.00E-02 V/us
    Calculate VSLOPE2 VSLOPE2 1.19E-04 V/us
    Calculate VSLOPE VSLOPE 4.00E-02 V/us
    Calculate RSUM RSUM 125.0 kΩ
    Closest Standard Resistor Value (E48) RSUM 127.0 kΩ
    Select Standard Resistor Value RSUM 127 kΩ


    Now, Having said that, the discrepancy you noted is real. I think that the Excel Calculator is wrong and that the App note is correct -

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Genius 4190 points

    Hi, Colin

    Thank you for your advice.

    As for my understanding, if the VSLOPE2 calculation in the app note is correct.

    Supposed CS pin voltage ramp down time (toff) will be as below,

    toff=(1-Dtyp)/fsw

    and supposed CS pin ramp down voltage will be as below,

    dVcs=(dIL_OUT/a1+dIL_MAG)＊Rs/a2

    if we set the target Vslope2 value to half of the Vcs change,

    Vslope2 will be as follows, (same as the formula in app note)

    Vslope2 =  dVcs/(2＊toff) = (dIL_OUT/a1+dIL_MAG)＊Rs/a2 / (2＊(1-Dtyp)/fsw)

                = ((dIL_OUT/(2＊a1)+dIL_MAG)＊Rs＊fsw) / (a2 ＊ (1-Dtyp)) 

    Is it correct?

    Regards,

    Go

  • 0 in reply to Go
    TI__Mastermind 25570 points

    Hello Go

    The formula is almost correct  -  the +dIL_MAG term should be -dIL_MAG - corrected version is below.

    = ((dIL_OUT/(2＊a1)-dIL_MAG)＊Rs＊fsw) / (a2 ＊ (1-Dtyp))

    You also need to calculate the VSLOPE1 value and then use the larger of VSLOPE1 and VSLOPE2 to calculate RSUM - Relevant extract from the App note below

    There is a lot of useful information at

    ]http://www.ti.com/ww/en/power-training/login.shtml?DCMP=psdslibrary&HQS=tlead-power-psdslibrary-apec2015-pwrhouse-20150312-lp-en 

    in particular the article at https://www.ti.com/seclit/ml/slup075/slup075.pdf

    deals with slope compensation (written in 1985 but physics hasn't changed and Lloyd Dixon knows his stuff)

    Regards Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points

    Hello Go

    Sorry, for some reason the App Note extract didn't 'stick' when I pasted it earlier - here it is again.

    Regards
    Colin