Buck Converter stability

[Sumeet]

Hi,
My doubt is related to the solution using low ESR ceramic caps for
buck converter stability proposed in this Maxim app note.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1121/ln/en

I cant figure out how they have arrived at the relations for R1 and
Cff. Cff I can probably solve, considering a constant current charging
it through R1 during the duty ON cycle and assuming some value of
ripple reqd at the feedback node.

but it appears as though they divide the vripple(p-p) reqd at the
feedback point by 2 before they apply this charging equation. I dont
understand this. also I dont understand the statements made about load
transient and its relation to the size of capacitor (Cff) as also why
we need to match the inductor time constant with R1 Cff.

Can anyone throw more light on this please ?

thanks
Sumeet

 

[Genome]

Hi,
My doubt is related to the solution using low ESR ceramic caps for
buck converter stability proposed in this Maxim app note.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1121/ln/en

I cant figure out how they have arrived at the relations for R1 and
Cff. Cff I can probably solve, considering a constant current charging
it through R1 during the duty ON cycle and assuming some value of
ripple reqd at the feedback node.

but it appears as though they divide the vripple(p-p) reqd at the
feedback point by 2 before they apply this charging equation. I dont
understand this. also I dont understand the statements made about load
transient and its relation to the size of capacitor (Cff) as also why
we need to match the inductor time constant with R1 Cff.

Can anyone throw more light on this please ?

thanks
Sumeet

You are not alone.

If this is all Maxim are prepared to say about the thing, your above link
and the data sheet, then I would recommend that you tell them to go get
stuffed and DO NOT use it.

On the other hand, being forced to read between the lines and think a bit
deeper (something that should not be necessary!!!) This looks like a
hysteretic voltage mode controller with a bit of added arse end.

For fun you might play with

http://www.transim.com/champion/Tutorial.html

and click on item 4) under Control Strategies. Use my 'not' e-mail of

[email protected] to log in. Otherwise register using a bogus load of
gibberish. You should get to play with a simulator model of a similar
controller. Won't tell you what you want to know but it might begin to make
sense as to what's happening.

At which point I'm going to stop thinking about it. Ideally you want to find
out some information for stability analysis of such things..... As they say,
Google is your friend.


Then again Maxim should be doing this.....

DNA

 

[Active8]

hi:

do you and Watt Sun travel together or something. visit the group
according to the phase of the moon, maybe? i haven't seen any posts
from either of you in a while and here you both are all of a sudden.

brs,
mike