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UC1843: Soft start Calc.

Part Number: UC1843

How do I calculate the soft start timing? I followed the soft-start circuitry on figure 22 and is seeing some delay but I want know how to correctly calculate it. 

www.ti.com/.../detailed-description

  • Hey John,

    The soft start circuitry will slowly bring the base voltage of the transistor up in a fashion similar to how a capacitor charges.

    A good estimate of when the voltage will turn the transistor off is after 4 time constants. These time constants are similar to the time constants for a capacitor charging.

    Thus:

    tss = 4Rss*Css

    Where:

    tss is your soft start time

    Rss is the soft start resistance

    Css is the soft start capacitance

    Thanks,

    Daniel

  • So I am testing my simulation with different Rss and Css.  I keep Rss constant at 1Mega and change Css.  The soft starts work but I cant make out the timing.  

     The above image is the current across the buck Inductor (IL) with Rss=1M, Css=1uF, starting at 0.20 to 0.40.    

    If I increase my Css to 3uF, the starting time would be aroung 0.60s to 1.20s. Can you explain what is going on? or how can I make tss starts earlier?    

  • Hey John,

    To decrease the tss of the design you could always look into decreasing the resistance of the Rss as the timing is dependent on both resistance and capacitance.

    1 MOhm seems quite large.

    Thanks,

    Daniel