Need 25mA switching converters for power rails but get 3A rated beasts. - Page 1
Forum section: electronics / projects

author: Houseman
author_rank: Regular Contributor
timestamp: October 24, 2024, 08:26:59 am
content: Hi Folks.In a new project I am working on I need to get from a 5V power supply (could be from USB port) two reference voltages of -12V and +24V to be fed into a DAC. the DAC consumes no more than 25mA for each supply rail and the input has to be very stable.The Idea is to generate the supply voltage and then cascade an LDO to filter out the noise.Consulting various TI IC I have everytime found huge Inverted Buck-Boost solutions to provide -12V rail and Boost for the 24V one. But they are rated for 3A with huge iductors and mostly oversized.Often in the 25mA range their specs lacks of efficiency going down to 65%. oftern the good efficiency starts at 2A.Now the question. Do I really need to pass to such beasts consuming a fraction of the rated current or are there some other solutions to get a clean output I am unaware of?Best regards and thanks.Steve

author: Andy Chee
author_rank: Super Contributor
timestamp: October 24, 2024, 08:42:10 am
content: For 25mA, consider capacitor charge-pump topology, instead of inductor buck/boost.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 24, 2024, 09:15:39 am
content: Try Digikey parametric search - plenty of other manufacturers other then TI if they don't do anything suitable.

author: tom66
author_rank: Super Contributor
timestamp: October 24, 2024, 09:40:37 am
content: I have used this LT part in the past:https://www.analog.com/media/en/technical-documentation/data-sheets/3463f.pdfIf you don't expect much current from the output you can get away with much smaller inductors (just choose ones that have Lsat of the maximum switching current).

author: PGPG
author_rank: Frequent Contributor
timestamp: October 24, 2024, 10:11:46 am
content: Among examples at the end of MIC2290 datasheet there is 5V -> 24V/40mA.

author: Houseman
author_rank: Regular Contributor
timestamp: October 24, 2024, 10:20:30 am
content: Thank You all.You have opened me a new world.Do you think I still need to use a LDO voltage regulator in cascade?

author: PGPG
author_rank: Frequent Contributor
timestamp: October 24, 2024, 11:49:42 am
content: LDO gives you noise filtering and may be better voltage stability than DCDC converter.Noise you can also filter using passive components.

author: tom66
author_rank: Super Contributor
timestamp: October 24, 2024, 12:27:47 pm
content: Compare the filtering bandwidth of an LDO to passive filter components. In many cases a combination of RC-Ferrite-LC filters can achieve similar performance. One thing an LDO can benefit is low-frequency stability, many DC/DC converters will vary their output voltage under load, which will have PSRR effects at low frequencies.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 24, 2024, 08:34:58 pm
content: Quote from: Houseman on October 24, 2024, 08:26:59 amThe trend is to higher capability, so why not focus on package and price?You can often find limiting searches to lower current parts gives more expensive or less integrated parts.Inductor size you can scale by current and ripple.At very light loads you may just run in pulsed burst mode and use a ldo if your dac needs a clean rails.Pulsed mode is more efficient and very tolerant of inductor choice but gives a bit more ripple.If the load is stable, more supply filtering may be enough.What is the DAC PSRR ?

author: Houseman
author_rank: Regular Contributor
timestamp: October 25, 2024, 10:49:57 am
content: Quote from: PCB.Wiz on October 24, 2024, 08:34:58 pmThank You PCB.WizThe DAC used is the TI7716 12-bits DAC.The datasheet expresses PSRR as +-1LSB.Here attached.I have used these schematics for the two power rails, however I get horrendous amount of noise in the DAC output.I attach the PCB (first one is just to understand more)The board is a 4 layer board: mainly routed on the TOP with 3 layers GND with vias to these ones.I know passing the DAC+24V signal near to the -12V inductor was not a good choice so I cut the traces and routed it manually on the BOTTOM layer by a cable.However situation seems not to get better...Any help really appreciated.Thanks

author: Houseman
author_rank: Regular Contributor
timestamp: October 25, 2024, 10:51:21 am
content: Quote from: tom66 on October 24, 2024, 12:27:47 pmSorry TOM66 .Do you mean by comparing it by instruments? measuring it?Regards

author: Terry Bites
author_rank: Super Contributor
timestamp: October 26, 2024, 08:29:21 am
content: Loads of low power solutions but take care with your layout and wiring as they can contribute to noise.Many noise issues can be traced back to layout.Houseman is right:An LDO will lower the over all efficiency so a simple LC filter may be sufficent to clean up HF noise.You might consider a ready rolled solution, eghttps://www.digikey.co.uk/en/products/detail/tdk-lambda/CCG3-24-12DR/15991448

author: Houseman
author_rank: Regular Contributor
timestamp: October 28, 2024, 08:31:19 am
content: Hi Terry thank you for your reply.Proposed solution has 200mV ripple and noise, that's way to high for me.. Did I got right if you proposed such a solution to get rid of layout problems?Best regards and thanks.

author: Houseman
author_rank: Regular Contributor
timestamp: October 28, 2024, 11:42:48 am
content: Ok.So first things first I wanted to make some measurements. A friend of mine (Thanks Fausto!) borrowed me his Rigol DHO 914 so I could make some measurements.And so I started from the beginning of the chain. The Power Supply.By the way I am deeply studying this threadhttps://www.eevblog.com/forum/projects/my-psu-design-ripple-and-noise-with-picture-measurements/?all#pnavin order not to overfill you with dumb questions, or least I will do my best..I am using a TRACO TMM24112C power supply. Here the datasheet:https://www.tracopower.com/sites/default/files/products/datasheets/tmm24c_datasheet.pdf?t=1729814401I have used this setup with a 1A variable load connected directly to the output and the oscope with short connection. no ground loops although the PSU itself is inner isolated and has no PE connection.First photo is the OFF condition NOISE FLOORAs the photo2 shows the digital load itself brigs some noise.Although I cannot see and can't understand why, the switching main frequency of approx. 132kHz here are the NO LOAD condition and LOAD condition.Now the most important question. Am I doing something horribly wrong I cannot foresee or is it a good beginning point?Why can't I see a straight line in the FFT that I indeed see after my self designed +24V PSU? Last photo (PEAK OUT) is attached.Best regards

author: coromonadalix
author_rank: Super Contributor
timestamp: October 28, 2024, 02:41:06 pm
content: even good filtered smps psus will still show some switching remnants, unless you begin to play with inductors capacitors filters to kill the noise perfectly ....some psu's with no minimal loads wont love that, and could heat moresay feeding a 25 ma circuit with a 2-3 amp psu is not good, and as you saw the efficiency, load etc .. play many roles ... you need to check all the spec of a psu, not only the voltage and current outputeven the switching frequencies play a role, 100 200 300 khz can be common, even saw 500khz, in some cases you have to manage harmonics if they happensit depends of the needed applications ....a linear model could be better in some cases too, use a toroid x-former, and as you know a battery is a perfect psu ... once again all depends of your needs

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 29, 2024, 01:30:54 am
content: Quote from: Houseman on October 28, 2024, 11:42:48 amIf you are doing noise measurements, it is usually better to use simple power resistors, with toggle switches to series/parallelIf building a test fixture, you could also add LC filters to get a simple feel for how much inductance will filter out the worst noise.Some test benches have deliberate filters to check known noise sources - eg mains ripple artifacts.

author: Houseman
author_rank: Regular Contributor
timestamp: October 29, 2024, 09:45:59 am
content: Thank You for the hints.Any good place to start from (LC filters to filter out main noise?)Regards

author: electron_plumber
author_rank: Regular Contributor
timestamp: October 29, 2024, 12:49:10 pm
content: Your noise measurements aren't easy to decipher -- can you get a clean high-bandwidth scope shot of a single switching interval?Looking at your layout briefly, there are some basic things you can improve. I recommend looking at some sample layouts. Looking at the 24V converter:your input/output caps should be closer to the switch cellyour switch node copper should be smalleryou should have some high-frequency (smaller package) caps on the input/output filter for better switch-noise rejectionyour feedback path is referenced to ground copper that appears to be adjacent to (and connected to) ground copper right under the inductor. Those eddy currents are probably adding noise to your (small-signal) analog feedback. Move the inductor away from the feedback.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 29, 2024, 05:27:06 pm
content: Quote from: Houseman on October 24, 2024, 10:20:30 amDepends on how low noise / low ripple you actually need and how far you get with other methods of filtration and their cost effectiveness compared to the linear regulator. Usually the answer is no. In the rare case where the answer is "yes", the linear regulator has to be chosen carefully. There are special parts with PSRR up to very high frequencies up to where it matters. If you slap in any random linear regulator IC, it's not much different from a piece of wire.

author: Houseman
author_rank: Regular Contributor
timestamp: October 29, 2024, 05:31:09 pm
content: Got it!Thanks.Any good book on specific Filtering techniques for PSU to begin with?Does a like filter kit exist where to try some configs or a bunch of inductor kit and capacitor kit put together could be a good choice? I am in a town where no electronic stores / radio shacks exists anymore and so must rely on Major E-comm like farnell/mouser that charges You with shipping fees.Best regards

author: Houseman
author_rank: Regular Contributor
timestamp: October 30, 2024, 09:38:34 pm
content: Made new measurements on the +30V and -30V power rail on two symmetric 300mA loads with no filteringAttached first image.Apart from the horrendous 20mA noise, what concerns me most are the peaks of the harmonics on the FFT of both rails.Starting from the switching frequency of the converter, there are even and odd harmonics all over the spectrum.How can I read them? What is the reason behind that?Where to start in such a case with the filter design?Do I need multiple stages filtering?Best regards