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Original question:

TPS2121: USB Inrush protection

TPS2121: Inrush and CSS

d_zero
Genius 4775 points
Part Number: TPS2121

Dear Arthur,

one question if we want that inrush be 500mA and the CL is 280uF (Vin = 5V9

We get the calculation SR_ON = 1785V/s.

in your Table 1 in DS you only have for 780V/s = 100nF

How can i calculate the CSS for SR=1785V/s ? 

Also there is figure 6 on page 9 in DS but there are different values as in table 1 ?

in table 1 100nF = 780V/s and in figure 6 100nF = 3.5V/ms = 3500V/s

Best Regards,

D. 

  • 0
    TI__Mastermind 32650 points

    Hi D,

    Unfortunately, I can only provide some back of the envelop calculations for this question.

    I cannot provide a direct equation for the Css capacitor but it is possible to use linear interpolation between the points.

    When Css capacitance is dropped by a factor of 10, Slew Rate increases by a factor of 10 (approximately).

    Hence linearly interpolating between 100nF and 10nF (with corresponding slew rates of 780V/s and 7800V/sec), I would say that you need a Css capacitance around 85nF to get a Slew Rate of 1800V/s.

    Do note that this is only an approximate calculation.