Questions About the DC2132A Schematic - Page 1
Forum section: electronics / beginners

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 20, 2019, 04:21:40 am
content: Hey everyone,Before I get started, I know that there have been posts about this Demo board and schematic in the past and I don't want to beat a dead horse but at this point they have been dead for nearly 2 years, so I thought that I would be better to just make a new post about it.Anyway, I have been trying to wrap my head around the schematic for the DC2132A demo board but there are a few things that really confuse me about it.First, I understand that the LT8612 is being used as a tracking pre-regulator to improve the efficiency, but I'm not sure how they were able to have it's output be 1.7v higher than the set output voltage as stated in the demo manual. I see that they are using some sort of offset circuit connected to the feedback pin on the LT8612 but I don't know how they are able to create the voltage offset. Couldn't they just use a resistor voltage divider connected to the output of the LT3081's to create the offset?Something that also confused me was the -5v DC-DC converter for the 0v output. Why does the output pass through Q1 and not directly into the output? Is that transistor used to keep the -5v from going into the output when there is a positive voltage? This part of the Demo board isn't mentioned in the manual so I'm not sure about why Q1 is there.I would also like to know if there are any cheaper alternatives to the LT8612? I did some digging on Digikey but all of the LT chips that had similar specs and pinouts to this one were in the same price range ± $3. I'm still an amateur and only 16 years old, and that means that I make a lot of mistakes unknowingly. I have tried to build this circuit before and so far I have been unsuccessful which means at roughly $12 a pop plus all of the other expensive LT chips, the cost quickly gets out of hand. It's also a pain in the but to solder this chip even with a hot air station and if there was a chip that came in a TSSOP package that would be much easier to solder.Lastly, is there any way that I could get a 0-30v output rather than a 0-24v output? I would like to make a bench supply using this circuit and if it's possible I would like a larger v-out range.Thanks!,John---Demo Board Schematic and Design Files ---https://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/dc2132a.html#eb-documentation

author: mvs
author_rank: Frequent Contributor
timestamp: February 21, 2019, 02:01:53 pm
content: Quote from: FotatoPotato on February 20, 2019, 04:21:40 amFB pin has absolute max rating of 4V. You can not offset it with just resistor voltage divider.QuoteIt acts together with R2 as basic constant current source. (5V-0.7V)/549 = 8mAQuoteYou can use any buck converter with sufficient voltage and current capabilities. But you will need to compensate feedback loop correctly. If you need only 1 PSU do not bother with it, just use LT design.QuoteYou can try to tweak R24 a bit

author: Ultron81
author_rank: Contributor
timestamp: February 21, 2019, 10:42:40 pm
content: Quote from: FotatoPotato on February 20, 2019, 04:21:40 amYou can use a constant current source returned to a negative voltage supply instead of this set up. I haven't built the supply yet, but I did some design. I was planning on using a LM334 set to a 5mA constant current, and use a 7660 charge pump to supply a -5V supply. Much cheaper.As far as the max voltage for the supply, you just need to mess around with the R24, 27, 28.