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UC3842: SOFT-START CIRCUIT

prasanth Chigurupati
Prodigy 20 points
Part Number: UC3842

Tool/software:

Hi TI,

How to calculate Css and Rss for soft start in UC3842? 

  • 0
    TI__Guru 73395 points

    Hi,

    COMP pin effective range is from 1V to 4.5V. Then, soft start time can be considered from Css = 0.5V to Css = 3.8V.

    Considering to approximation of a linear charge, if soft start time tss = 1ms, and if you select Rss = 10k, then

    Css = tss x (Vref/Rss) / (3.8V-0.5V)

          = 0.001 s x (5V/10k) / 3.3V) = 1.51 x 10^-7 F = 0.151 uF, or 0.15uF

    As the real charge is in exponential, so the time would be slightly longer. But the above is good enough to have an initial set of parameters. Then you can do some tuning during bench test. 

    ,