Use of COMP Pin in LM5576 Regulator

[vishweshgm]

Hello,

I am trying to design a simple regulator circuit using LM5576
Datasheet Link : Link

Input : 12 to 60V
Output : 12V
Nominal Current : 0.5A (I am using 5576 because 5574 or 5575 not in stock)
Ripple current : 100~200mA


It has a COMP pin. connected to output of error amplifier. Can some one explain in simple terms, what is it's function and how do I decide C5 and R4 values in-general?
Datasheet explains this as loop compensation. But I am unable to understand this properly in google.

 

[FvM]

Datasheet paragraph 8.2.2.14 explains in detail.

There are many hits for "error amplifier compensation"

 

[cupoftea]

To start with put in R4 = 1K and C5 = 10n, C6=100pF
Those values should be conservative, and give stability.
You can tweak them later.
Then calculate the loop bode plots by multiplying the error amplifier transfer function by the equation 2A-5 in the attached page 227 (225 and 226 tell the meanings)
In 2A-5, Ri is your sense resistor value; R is your effective load resistor value = vout/ioutmax

You will ned to use complex numbers so do it in excel.

Then plot out Gain vs freq
and
phase vs freq

Use the excel IMABS function to get the gain......then do 20log10(gain) to get it in decibles...then you can add things together when in decibels.

2a-5 is from the Basso book, switch mode power supplies.

--- Updated Feb 22, 2023 ---

If you dont want to use 2A-5, then G(s) equation in fig6, pg 10 of the below is simpler...but really , you need 2A-5 because you will be using slope compensation aswell.

https://ww1.microchip.com/downloads/aemDocuments/documents/PMIC/ApplicationNotes/ApplicationNotes/DCDC_TN_206.pdf

--- Updated Feb 22, 2023 ---

I presume you know how to get transfer function for the error amplifier?
Its Zfb/Zin, but as discussed, you have to do complex numbers....so get excel to do it for you.
Remember your Zin is R5 in your case....unless you put other components in there aswell.....in fact, i like to add a R IPW an RC between R5 and the FB pin...gives you a bit more loop flexibility

 

[cupoftea]

Also i take it you cant log into ti.com WEBENCH either because you dont have a company email address?.....that , others will know, may well do the bode plots for you.

 

[vishweshgm]

@cupoftea : Thanks a lot for giving me right direction to start the study. Really appreciate!

 

[cupoftea]

Yes,,,
The drill is get the feedback loop equation...
you do that by multiplying together...
1... power stage transfer function
2....error amp txfer function
3...modulator txfer function

but take logs of 1,2,3 instead, and then add them instead of multiply, as its easier.

Then you plot the bode gain and phase vs freq....
..then see where your crossover freq is etc, and the gain and phase margin

to make it more confusing for you, 2 and 3 above are often combined into a single equation.


There is a buck reg exampel in this from page 12...

https://www.ti.com/lit/an/snva555/snva555.pdf?ts=1677071970497&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM5576

...i am actually trying to work thru it myself too.
The "Km" parameter is defined at top of page 3.......but i am seeing what is "VIN" in that equation....is it literally the "Input voltage" to the SMPS, i believe so?
...The "Fm" parameter is given at the bottom pf page 2....so you need to know the "VRAMP" parameter...but is that even given in the LM5576 datasheet?...i think the RAMP is 2.1V, looking at the block diagram on page 9....its an emulated ramp of course, not to worry about that though.

Ri = Gi.Rs (bottom of pg 3)...so for you that is (0.5 x Rs)

LM5576 datasheet

https://www.ti.com/lit/ds/symlink/lm5576.pdf?ts=1677076890537&ref_url=https%253A%252F%252Fwww.snapeda.com%252F

..but oh deepest joy !!!.......all the equations given in that doc are no good for you since they dont account for the slope compennsation...so its back to my 2A-5 for you.