Quick question on interpreting formula. - Page 1
Forum section: electronics / projects

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 17, 2014, 04:31:36 pm
content: Just a quick question in regards to the formulas for selecting an inductor in the Datasheet for theLT3791-1on page 14;I'm getting numbers I know are plain crazy, probably an order of magnitude or two out.There is only one thing that pops into mind that could be the cause, and that is my interpretation of the format of the values I need to substitute, specifically for the inductor ripple current percentage.Usually the formulas want it like as a decimal/fraction eg, 0.2 or 20/100 for 20%, 0.35 or 35/100 for 35%, but I get the feeling it wants it as the discret percent value, eg, 20 for 20%, 35 for 35%.Cheers in advance.I'm going to take a nap, been up late trying to figure out where I stuffed up...

author: Rudane
author_rank: Regular Contributor
timestamp: February 17, 2014, 04:53:33 pm
content: At a first glance, that 100 on the top of the equation could be a percentage, so in order for the percentage to cancel I think the % ripple may be in straight percent.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 18, 2014, 01:43:13 am
content: Yeah, that seems the case. With a fresh set of eyes that now seems obvious. I guess that's what you getfor math at 3 in the morning.Thanks.

author: georges80
author_rank: Frequent Contributor
timestamp: February 18, 2014, 03:30:55 am
content: I've designed a LED driver with the lt3791. The percent is as you've figured out the actual number, so 10% put in 10 in the equation.I'll give you a BIG hint... you MOST likely will need a 4 layer board to tame the chip and the switching energy in the various power paths from coupling into the sense paths.Linear has a demo board and ALL the design files are on their website for it - LOOK carefully at the layout they used and learn from it. I fortunately scored one of their demo boards for free via a 'friendly' distributor. I also emailed back and forth with the demo board's app engineer - 4 layer board is my not so subtle hintcheers,george.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 18, 2014, 04:03:10 am
content: I built up a couple of the app note designs to get a good layout, as well as a dev board design, while I worked out some other things on the project, layout shouldn't be an issue - time spent optimizing layout for 2L vs 4L is well worth it in the long run. That said, 700khz high-ish power SMPS is serious business compared to lower freq designs.

author: georges80
author_rank: Frequent Contributor
timestamp: February 18, 2014, 04:11:10 am
content: Good luck on your project. I could get a 2 layer board (with various cuts in the ground plane) working up to around the 40W output range, above that things got tricky. The design I did was a buck/boost and that adds a twist to things as well.With 4 layers the board runs happily at 70W and higher with excellent efficiency which is a benefit of the synchronous rectification that the lt3791 provides.cheers,george.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 18, 2014, 04:33:30 am
content: Quote from: georges80 on February 18, 2014, 04:11:10 amThanks, i'll need itThe load mine powers is a pretty horrible load, big transients, HF noise, wide power consumption range. Even worse is what the input power is, 10-50v for 36v out...