Cost-Effective Solution for Regulated -12V from +15V DC Input - Page 1
Forum section: electronics / projects

author: Halban
author_rank: Contributor
timestamp: January 22, 2025, 10:17:52 am
content: Hello everyone,I'm working on a project that requires generating a regulated -12V output from a +15V DC input (up to 1A).Here are my key specifications for the negative output voltage:- Regulated DC voltage: -12V (±10%)- Output current: 100mA max- Cost constraint: Total solution must be under $3I was previously using the integrated DC-DC converterP78E12-1000(configured for a negative output), but unfortunately, it's no longer available. It was affordable and worked well for my application.As a replacement, I've considered the DC-DC converter173951236, but I'm exploring whether there might be a cheaper alternative that doesn't rely on integrated DC-DC converters, as this seems like a common circuit requirement.I've looked into inverting buck-boost converters like the LM2662, LM2577, LT1054, and MAX766, but these options seem too expensive for my budget (total design cost exceeds $3). Additionally, charge pump converters don't seem viable, as they can't handle the 100mA output current I need.Does anyone have suggestions for a cheap and efficient solution that meets my specifications?Thank you very much for your time and help!Have a great day!

author: JohanH
author_rank: Frequent Contributor
timestamp: January 22, 2025, 12:08:07 pm
content: You should manage the cost if you can use surface mount components and select other relatively common parts, e.g. something like LM25005, LM5005, LM5575, LM5576. Something like this was suggested by webench.ti.com:

author: tszaboo
author_rank: Super Contributor
timestamp: January 22, 2025, 12:14:16 pm
content: You can use some buck converters- typically older, simpler ICs, with high voltage input- as an inverting converter.https://www.analog.com/en/resources/design-notes/making-a-voltage-inverter-from-a-buck-stepdown-dcdc-converter.htmlThis will be typically cheaper than an equivalent inverting circuit, with some drawbacks.

author: JohanH
author_rank: Frequent Contributor
timestamp: January 22, 2025, 12:25:25 pm
content: Yes, all of those cheap examples listed by webench, where older step-down buck converters.

author: Halban
author_rank: Contributor
timestamp: January 22, 2025, 02:56:50 pm
content: Thanks a lot for your precious help!By any chance, do you have some IC reference you can recommend?I read some datasheet of these buck DC converters, but unfortunately, none of them are including examples of schematics for negative configuration (and of course, they are not including the maximum Iout they can provide)

author: JohanH
author_rank: Frequent Contributor
timestamp: January 22, 2025, 09:23:09 pm
content: Some data sheets I've read of buck converters do include negative configuration examples, at least I've seen some. Basically you use the same circuit, the only difference is make the GND side of the IC your -12V and tie SW/output to the circuit ground. Just as in the two schematics above that I posted from the webench.ti.com site. I don't like to promote a certain manufacturer, but I like how easy it is to use the webench.ti.com site. Just enter your desired input and output voltages and it will suggest circuits and ICs for you. You can sort by cost, footprint size or other factors. You can see the schematic directly with suggested component values and even manufacturer codes in the BOM. If you want to edit the circuit further you have to register on the site. You can even export the circuit to a PCB program (but I wasn't able to in all examples that I tried, and they didn't offer a KiCAD output that I use). In contrast, another competitor have their own similar program, but if I remember correctly I had to download some software and I didn't get far before it became way too complicated.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 23, 2025, 03:39:32 am
content: Quote from: Halban on January 22, 2025, 10:17:52 amLook athttps://www.ti.com/lit/an/snva856b/snva856b.pdfShows how to use a conventional positive buck converter as a negative output.You need enough voltage rating to span your -12 to +15V ( ie some margin > 27V)

author: Halban
author_rank: Contributor
timestamp: January 23, 2025, 01:43:24 pm
content: Thanks a lot, that's everything I needed!