Help reading Data Sheet

Thread Starter

doronraz

Joined Jun 7, 2022
4
Hi friends,
I try to build my circuit and I am not understanding the data sheet.
I attached the whole data sheet, and add the specific sections.

I need to calculate those 4 components:
1655243808756.png
The data sheet tell me to calculate "Rc" and other components through trial and error test.
1655243674890.png
1655243927716.png

I download analog design software "LTspice" and the circuit from analog website, but my graphs are completely different.
1655244103087.png
(The blue trace is I(L))

My questions are:
1. Did I understand correctly the datasheet? (by calculate the components by trial & error)
2. How can I do that? what I make wrong about their software?


Thank you very much!
 

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Papabravo

Joined Feb 24, 2006
18,793
First things first. In order to know what compensation is required you need to know what the loop response of your system is. Once you know what your loop response is you can locate the pole and the zero of the compensator to add phase margin at the unity gain crossover and add additional rolloff at higher frequencies. The reason they tell you to do it by trial and error is that it depends on your inductor L1 and the capacitor Cout and its Equivalent Series Resistance. The chip designers have less than zero knowledge of what components you might select to use with their part.

Your graph is completely different because it includes the startup sequence. What you want is a step function on the load after the device has reached steady state, which is what is being show in Figure 3., of the datasheet.
 

crutschow

Joined Mar 14, 2008
30,103
The startup transient is a large-signal transient that is quite different from a small-signal load-change transient.

The value of the Css capacitor connected to the SS (Soft-Start) pin controls the start-up transient.
A larger capacitor should reduce the observed overshoot upon startup
 

MisterBill2

Joined Jan 23, 2018
12,402
What are the requirements of your load?? The load determines what response is needed to deliver adequate performance. So the very first thing you need to know is what your load requires.
So let us know how critical the regulation for the load is. Then there can be some help that is not a guess.
 

Thread Starter

doronraz

Joined Jun 7, 2022
4
First things first. In order to know what compensation is required you need to know what the loop response of your system is. Once you know what your loop response is you can locate the pole and the zero of the compensator to add phase margin at the unity gain crossover and add additional rolloff at higher frequencies. The reason they tell you to do it by trial and error is that it depends on your inductor L1 and the capacitor Cout and its Equivalent Series Resistance. The chip designers have less than zero knowledge of what components you might select to use with their part.

Your graph is completely different because it includes the startup sequence. What you want is a step function on the load after the device has reached steady state, which is what is being show in Figure 3., of the datasheet.
Wow, Thank you very much!
I will look more information about the loop response & poles and zeros, I need to see example how its done.
I didn't succeeded to edit the Analog's circuit, I will try again later.

Thank you again :).

The startup transient is a large-signal transient that is quite different from a small-signal load-change transient.

The value of the Css capacitor connected to the SS (Soft-Start) pin controls the start-up transient.
A larger capacitor should reduce the observed overshoot upon startup
Thank you, I have mistake about Css, I know is size.
The other components need to be found.
I started to find them :).

Thank you!
I started to read it, it seem very helpful.
 
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