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UCC28951: Design Equation Correction

AhmadBashar Taji
Prodigy 40 points
Part Number: UCC28951

Tool/software:

Hello,

In the datasheet for UCC28951, the equation (2) to calculate the soft-start capacitor during follower mode seems wrong.

The numerator should not have 25uA.

Regards

  • 0
    TI__Guru 73825 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 73825 points

    Hello,

    I am not sure equation 2 is correct.  

    Equations 2 is trying to correct for the 825 k ohm resistor hanging off the SS pin when the controller is being used in slave mode.

    There should still be current in the numerator, so I don't think just removing the 25 uA will correct equation 2.

    I looked at this have the following thoughts. I believe that I have a calculation that can be used as a better estimate.

    We should be able to use charge control to derive an equation.

    I = Css*dv/dt,

    Css = I*dt*dV

    The PWM ramp CS signal when level shifted should operate between 0.8 V and 2.8V  Based on charge control I cam up with the following equation to select Css based on Tss.

    Css = (25uA - (2.8 V-0.8V)/(2*(825k ohm))*(Tss/(2.8V-0.8V)

    Css = (25uA - (2V)/(2*(825k ohm))*(Tss/(2V)

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards,