Inverting Buck boost Analog Devices AN-1083 app note - error in diagrams? - Page 1
Forum section: electronics / general

author: bobof
author_rank: Contributor
timestamp: March 16, 2021, 04:14:08 pm
content: Just sanity checking my understanding. I was looking up some of the current loop stuff for IBB power supplies for optimal layouts and came across this app note from AD:https://www.analog.com/media/en/technical-documentation/application-notes/AN-1083.pdfIf these are IBB, aren't the COUT caps shown backwards? (ie +ve on cap should be connected to 0V, -ve should be connected to VOUT). Or is there some subtlety why it makes sense to show them in this orientation on this current loop diagram?

author: aheid
author_rank: Regular Contributor
timestamp: March 20, 2021, 02:57:18 pm
content: The text specifically says Vout is negative with respect to ground, so Cout is wrong way around. It also seems to me the current loop on input side is wrong orientation, surely Vin is positive for the circuit to make sense as an inverting one. In that case the Cout error is surely just a copy paste error.

author: bobof
author_rank: Contributor
timestamp: March 20, 2021, 03:29:43 pm
content: Quote from: aheid on March 20, 2021, 02:57:18 pmYes, I did think the current loop looked odd as well...!With so much inconsistency (disappointing really for major manufacturer) I just decided to disregard it totally.This info from TI seemed much more useful:https://e2e.ti.com/blogs_/b/powerhouse/posts/laying-out-an-inverting-buck-boost-converter-for-successThough disappointingly it doesn't discuss the "missing" cap which it seems should go between VIN and VOUT (it only have caps between VIN and GND and VOUT and GND.The 3rd cap is discussed here:https://www.ti.com/lit/pdf/snva856

author: Siwastaja
author_rank: Super Contributor
timestamp: March 21, 2021, 08:33:55 am
content: Typical "appnote written by an intern in hurry" stuff.Never base your designs around appnotes. Sometimes they are very good, but be careful.

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 09:58:44 am
content: Some appnotes mention reference designs which are actual builds tested by manufacturer. AN-1083 has 4 reference designs as well, thou no PCB layouts provided. Newer documents usually do. Note that AD has "circuit notes" as well. In short - you may consider copying appnote design if it has clear signs of actual build - has BOM and PCB layout or even measurement results provided. Could be in separate document. When you are (professional) in a hurry - consider buying evaluation module for initial tests & prototyping.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 21, 2021, 10:19:08 am
content: Yes, do note though that manufacturer's actual reference designs, even if they exist as physical products, are not guaranteed to be any better. Of course, they have tokindawork, they have at least powered them up with no instant smoke, but that's about it.Those evaluation modules you can purchase from Digikey have gone through a bit more testing, but even that doesn't necessarily guarantee they are well designed, may not be worth copying. They are still worth looking at and often just fine, of course.

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 12:03:55 pm
content: I would love to see some pre-built reference design board which is as bad as "no smoke but that's about it". Any examples?Apart from some minor mistakes I do not recall any problems with ref.designs of Analog Devices RF, A/D, D/A chips and TI/LT power solutions. Support engineers often asks for demonstration of the problem (if any) on manufacturer's reference design so they can quickly re-create it in their lab. Reference design is exactly that - reference, usually overengineered using quality components.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 21, 2021, 12:30:39 pm
content: I don't have any in mind particular, but I do remember seeing some quite poor intern reference designs on some of the TI's BQ series power management chips, calling them "references", appnotes describing the complete design and photographs showing manufactured and tested PCB. Those boards were not for sale, though.

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 02:04:23 pm
content: Seems like it depends on division/lab/engineer. Generic msp430 designs could have some flaw as wellAnyway we shall not generalize because TI have way more good than badreference designs.