How to generate negative supply voltage? - Page 1
Forum section: electronics / beginners

author: wfsuper
author_rank: Newbie
timestamp: March 19, 2014, 02:23:24 am
content: A quick question:Some op amps need positive and negative supply, and this seems to be a smart solution:https://www.kickstarter.com/projects/pnmini/pnmini-positive-negative-power-supply-module-for-mI am just wondering how does it work. How come both negative and positive can be integrated into one? Any comments about the trick they use?

author: Nerull
author_rank: Frequent Contributor
timestamp: March 19, 2014, 02:28:32 am
content: They're just reversing the terminals on an floating supply. It works just like a battery - connect it one way and you get a positive voltage, connect it the other way and you get a negative voltage. If you have a bench supply with multiple isolated outputs, you can do the same thing.

author: Rerouter
author_rank: Super Contributor
timestamp: March 19, 2014, 09:42:08 am
content: Nerull, thank you very much, that just occurred to me after you explained itI've been hunting down a cheap 5V to -24V chip and failed to realize i could just use an isolated 5V to 24V one....

author: wfsuper
author_rank: Newbie
timestamp: March 21, 2014, 12:34:37 am
content: The one on kickstarter looks like an non-isolated one...it's an inverting buck-boost. but how come it supports both positive and negative?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2014, 12:57:39 am
content: This is a standard operating principle for most switching regulators. You'll find examples in the datasheets.Operation is, it bootstraps its ground terminal to 5V (or whatever) below Vout, but if Vout is 0V, ground is -5V. The switcher has to handle input + output current, which isn't a big deal for small ratios or low power.Thing is, those RECOM 78xx three terminal switchers (and I forget which other companies are making them too) are already on the market; what does this Kickstarter hope to do?Tim

author: FrankBuss
author_rank: Unknown
timestamp: March 21, 2014, 01:48:24 am
content: There was a posting with some schematics for creating a negative supply voltage, if you don't need much power (depends on your op amp) :https://www.eevblog.com/forum/projects/design-charge-pump-for-lcd-backlight/15/If you have already a microcontroller, which can produce some PWM signal, or any other clock, all you need is some diodes and capacitors. Much cheaper than the Kickstarter solution.

author: mpi
author_rank: Newbie
timestamp: September 19, 2017, 10:50:48 pm
content: Sorry for grave-robbing a necro-thread.In which datasheets does it describe getting a negative voltage from a positive output buck converter?The only thing I've been able to find isthis paperfrom TI, but it's written as if this is a strange quirk of the TPS series converters.Can other converters sharing the same architecture (such as theROHM BD9G401EFJ) generate a negative voltage the same way?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2017, 11:25:39 pm
content: Of course!The main limitations are:- Supply voltage range: the regulator sees (Vin - Vout) across it- Output current range: the regulator must deliver (I(Vin) + I(Vout)) amps through its (normal) output (because it's bootstrapping itself against ground). If the positive-side load comes on first, then the regulator must be able to deliver this current into a zero voltage load (no foldback).- Fault handling: ensure that, if a positive supply gets shorted to the negative supply, the chip can handle it (usually, the result is that it should have active current limiting).- Logic signals (enable, power good, etc.), if applicable: these are usually ground referenced, but "ground" is the negative output. So you will probably need level shifting to interface these. (If you don't need them, they can be strapped high/low as usual for the chip, without worry.)Tim