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question on buck soft start

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totoro

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Hi
I have some questions on the soft start process:
when start up the external load is zero or any possible value?
in general method, the vref of error amp is ramp up slowly to implementate the soft start, my question is how slow should it be? 10us 100us or 1ms?
BTW should over current protection be added to the low side NMOS switch? In my simulation there is a negative current peak due to the inductor at the beginning of the start up?
Thanks!
 

How slow the Tc should be depends on the time constant of the inductor and capacitor.

It doesn't really matter if it is too long.
If you have a load during start up the Tc won't need to be as long, and leaving it at the minimum required to prevent overshoot will work just fine.

The current through the low side switch won't exceed the current stored in the inductor, which was limited by T(on) of the high side switch, and under startup conditions, will be dependent on the soft start Tc.
 

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    totoro

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