Simplest way to get +/-12V or +/-15V from +3.3V but at low power - Page 1
Forum section: electronics / projects

author: peter-h
author_rank: Super Contributor
timestamp: November 06, 2023, 02:32:20 pm
content: I have a requirement to add a -10V to +10V DAC output to my +3.3V powered product.I am not yet sure if I can use the 32F417's internal 12 bit DAC (0 to +2.5V output) or if I will need to SPI-interface some 16 bit DAC. Probably the latter. But the biggest challenge is getting the two supply rails generated at low power. DC-DC converters all draw way too much. I want to get it below about 5mA drawn from the +3.3V. For smaller rails like -10V and +10V (nominal) one can use a MAX232 (and ignore all the I/O pinsbut that needs a +5V supply, and the MAX3232 generates only about -6V and +6V).Probably the Q is whether anybody makes a capacitor pump based solution.I could build one of course (2 x diode + cap Cockcroft–Walton circuit) but it is a lot of components.For a -3.3V rail my usual choice is an ICL7660 which I use in 10k+ volumes in something else.One can build a circuit around a high permeability toroid core which draws a few mA and I make products already which use these. This one is a 5V to 5V isolatorbut these transformers are a PITA unless you buy 10k, and very expensive to make in the West. Plus a lot of parts.

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 07, 2023, 12:05:30 am
content: First you must do the math.You seem to expect more power out than power in.Assuming 100% efficiency(not at all possible), and with a 3.3V supply only supplying lest than 5mA means you have only <17mW power drawn from the 3.3V supply.If you figure out some magical way to convert 3.3V to +/- 10V (without DC to DC converters?) regulated supplies(again with 100% efficiency) then their total power output (including operating current) must be <17mW.You can then expect around a maximum of few hundred uA output per +/- 10V supply the current needed by your 16-bit DAC and SPI interface pull-up resistors.You can't create more power out than you supply power in.Of course, if you don't like DC to DC converters because they use too much power, then must find some yet to be discovered voodoo device that can accomplish this task.That being said, if you have a 12-bit DAC available in a 32F417, you must certainly find a way to use it.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 07, 2023, 03:35:42 am
content: Quote from: peter-h on November 06, 2023, 02:32:20 pmNot that I've seen : 3v3 to +/- 10V is x3 both ways.Little SOT26 style flyback switchers would be cheapest. eg lcsc have MT9284BS6 for 3.5c/100, that will do the +10V easily, claims 600uA switching.Or, you can get split rail DC-DC parts like ROHM BD8316GWL or TOREX XC9519A can do +/- 9V in a single package.

author: MathWizard
author_rank: Super Contributor
timestamp: November 07, 2023, 05:19:26 am
content: What can charge pumps do? I've heard of them, but never tried to analyze a circuit of one. Aren't they good for low current or low power situations , and these days made as all in 1 chips+external caps, and no doubt internal cap versions for very low power rails.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 07, 2023, 05:58:53 am
content: Quote from: MathWizard on November 07, 2023, 05:19:26 amThey use a flying capacitor, that alternates between charge from a main supply and dump into a reservoir, using mosfet switches.They are quite efficient, and create raw outputs 2x or 3x the supply.The classic 7660 can convert up to +3~10V to -3~10V, with some droop at a few 10's of mAQuote from: MathWizard on November 07, 2023, 05:19:26 amSome are, but there are also 10A rated charge pumps, so not all are wimps

author: peter-h
author_rank: Super Contributor
timestamp: November 07, 2023, 07:43:57 am
content: The MT9284BS6 looks great, and indeed a single inductor doesn't need a custom mfg. However I don't see that this can do a negative rail. Price is great tooI guess I could use some sort of diode pump to get a negative rail; just need a square wave to switch a mosfet...The BD8316GWL doesn't appear to go more negative than -9V.I really need +/-12V or preferably 15V so I can get a +/-10V output from the DAC.Thank you allWhat is the most commonly used 16 bit SPI DAC? Preferably internal ref because an ADR441 is almost £10 (I am using it with an MCP3550).

author: tooki
author_rank: Super Contributor
timestamp: November 07, 2023, 08:02:33 am
content: Quote from: peter-h on November 06, 2023, 02:32:20 pmThat’s an awfully broad assumption, with emphasis on the “assumption”.For my final project in my electronics apprenticeship, I was given a task which included as a requirement a DC-DC converter (any type) from 3.3V to 15V with a quiescent current under 500 μA. I used an off the shelf part that got me far below that.There are many parts out there today for such things because it’s not an uncommon requirement, especially now that local regulation has become very popular (as opposed to generating all voltages centrally and distributing them all over the board).

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 07, 2023, 08:03:29 am
content: Quote from: peter-h on November 07, 2023, 07:43:57 amThe Microchip charge pumps TC7660SE / TC1044SE are rated at 13V max for 12V in/out and the TC7662B are rated higher. May be ok for modest currents.If that is not enough current, parts like R1283 from Ricoh Elec Devices (now Nisshinbo Micro Devices) are DFN12 for dual supplies, up to +/-20V

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2023, 08:06:39 am
content: Charge pump ain't gonna be great unless you can get one highly integrated, but then current is likely to be way too little.I'm fond of a combined SEPIC/Cuk for that kind of thing:Oh, as long as everything else is showing on there, it's probably worth adding the transformers were Coilcraft LPD4012-333MRC.Part values can be adjusted for the lower current, of course it's up to you to find a controller/regulator suitable for very low current consumption. You may find it's better to use oversized components and just run in burst mode (with a regulator that idles at low Iq).You could do worse (though, to be fair, not much worse) than one of those dumbass gated-clock style hysteretic regulators; the one advantage they do have is very low idle current, since they're truly little more than a comparator.Incidentally, this is excerpted from another analog board, for which the extended rails did mux + diffamp service ahead of a 24-bit converter (for extended CM range, cross-wiring protection, EMI, 4-20mA current loop, etc. purposes). Hence also the TVSs, for clamping diodes elsewhere. Didn't do extensive noise testing, but the sample variance was consistent with datasheet expectations (i.e. ~60 counts p-p, or >18 bits useful range), so it seems it was at least not grossly out of whack.Tim

author: Psi
author_rank: Super Contributor
timestamp: November 07, 2023, 08:12:58 am
content: Don't fall into the assumption a DCDC will need some big-ish inductor on the board.For low currents a 0603/0805 SMT chip inductor can be fine, and the reg IC usually have a built in mosfet and sometimes a diode too.So your final circuit can be pretty minimal and physically small, like a SOT32-6 DCDC regulator IC, two caps, and an inductor.And you might need two resistors for feedback if it's not a fixed output version. But often they sell the regs in a fixed 9/12/15V output versions.The parts all tend to be pretty cheap, passives are all cheap and basic low-current SOT32-6 dcdc regs aren't expensive either.

author: peter-h
author_rank: Super Contributor
timestamp: November 07, 2023, 11:02:13 am
content: QuoteYes, and I posted similar. I was referring to ones you can buyThe R1283 looks just the right thing!