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.
This thread has been locked.
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.
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.
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