Power Supply-Switching vs Linear - Page 1
Forum section: electronics / projects

author: gilbenl
author_rank: Regular Contributor
timestamp: May 03, 2016, 08:37:55 pm
content: I've been working on a DAQ for precision voltage (~0.5uV) and resistance measurements. I need a power supply that efficiently provides low-noise +5v and +12v @~200mA. The plan is to power from a 14.8v Li-ion. I'm assuming the most efficient way to do this is with a switching supply.Given that the ADC and sensor power bus are isolated using Murata MEV1S0505DC and MEV1S1212DC DC-DC isolators, do I need to worry about noise from the switching PS?Does anyone know of an IC that would provide both +5 and +10 (or 12) volts or am I stuck with having 2 separate regulators?

author: lincoln
author_rank: Regular Contributor
timestamp: May 06, 2016, 12:04:13 am
content: > I'm assuming the most efficient way to do this is with a switching supply.Are you speaking strictly of power efficiency? Dose BOM or PCB area matter? Low noise don't really mix well with switching regulators. You could do the trick of Switching regulator > LC Filter > Linear. What kind of sampling rate are you targeting?>Does anyone know of an IC that would provide both +5 and +10 (or 12) volts or am I stuck with having 2 separate regulators?Linear and TI both have 2 channel switching regulators, I don't know if they will run up to 10 V as the ones I have been looking at are geared towards digital logic / FPGAs / ASICs

author: danadak
author_rank: Super Contributor
timestamp: May 06, 2016, 11:27:12 am
content: Micrel has a series of compound switchers followed by LDOs to getthe best of both worlds, noise.Regards, Dana.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 06, 2016, 03:11:58 pm
content: AFAIK, LDO behind a switch mode converter has little to do with reducing noise. The noise reduction comes from extra LC filtrationg stage, which further degrades load response of the converter, so the LDO is there to improve load regulation. Of course, linear regulator offers some noise reduction, but line regulation / PSRR is poor at higher frequencies which is often the problematic noise people are talking about, and it needs to be dealt with using different methods.

author: danadak
author_rank: Super Contributor
timestamp: May 06, 2016, 03:35:25 pm
content: LDO has vastly superior PSRR, as such helps in noise reduction (to a point)from supply rails. Also control loop for LDO can enhance transient response.See Micrel, On Semi discussions about this.Regards, Dana.

author: ez24
author_rank: Super Contributor
timestamp: May 06, 2016, 03:42:18 pm
content: QuoteIs it possible to get to the individual cells in the battery so you can use only LDOs. Otherwise you will have noise no matter what you do.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: May 06, 2016, 04:04:35 pm
content: Quote from: Siwastaja on May 06, 2016, 03:11:58 pmYou just need to pick the right linear reg. Eg:http://cds.linear.com/docs/en/datasheet/3042fa.pdfoffers 78+ dB PSRR from 10 Hz to ~3 MHz.

author: dmg
author_rank: Regular Contributor
timestamp: May 06, 2016, 04:36:27 pm
content: I'd go the switchmode + LDO way for efficiency. I've powered quite sensitive stuff with switch + LDO without problems. The thing is, you need to evaluate inside which frequency band are you noise sensitive and take into account the PSRR of your involved devices and the extra PSRR you get (or not) from decoupling capacitors.Then do the math and check if you're safe.Sometimes you are designing something apparently very sensitive to noise (for example high speed data converters) and look through reference designs that go the fancy way, but then you do the math and it turns out that you can get away with something an order of magnitude less fancy and still have the supply noise contribution 10 dB below some other unavoidable or inherent noise contribution.

author: Marco
author_rank: Super Contributor
timestamp: May 06, 2016, 06:10:36 pm
content: Quote from: Siwastaja on May 06, 2016, 03:11:58 pmSimple ripple can be too much for the voltage reference to handle alone (and amplifiers, etc).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 06, 2016, 07:04:58 pm
content: The logical way to reduce ripple from a switched mode regulator is an LC filter. Typical LDOs don't offer very much reduction on the high frequency ripple anyway. However a downside of the LC filter is that load regulation, especially to fast load changes will suffer and it is not that great with a switched mode supply anyway. So the LDO is more about giving a better regulation and thus allowing for a filter.Switched mode regulators are also different, some have much lower noise than others. So using a lower noise version can help. The small DC/DC converter modules tend to be rather noisy as they are more build to a price and size than noise. Noise visible outside also very much depends on the layout, not just the chips used.For the 12 V you might get away with just an LDO - not much to gain for the 14 V to 12 V step.

author: ez24
author_rank: Super Contributor
timestamp: May 06, 2016, 07:06:35 pm
content: Quote from: suicidaleggroll on May 06, 2016, 04:04:35 pmThis looks like a good LDO for a switcherOP see if you can find the datasheet for your switcher and see how it matches this datasheet

author: PartialDischarge
author_rank: Super Contributor
timestamp: May 08, 2016, 07:51:34 am
content: Linear Technology's solutions are great, specially the power products, I only wish they sold their products through Mouserrecently I ended up choosing a TexasI converter instead of LT just because of that....

author: gilbenl
author_rank: Regular Contributor
timestamp: May 11, 2016, 05:16:41 pm
content: Didn't see any responses and gave up after a few days. Seems I missed a lot--Sorry folks!Here's the current working schematic for the PS. Went with an LT1613 SEPIC w/soft start and a MAX16054 latching soft ON/OFF running off a 4.7v Li-Ion. Originally (and currently), the idea is to have this be a modular device w/a master with uProcessor and multiple sensor modules w/microcontrollers. As this is effective one module and not the master, there's no immediate need to have the big battery and PS w/multiple rails. It would've been nice to have that checked off, however, there is an immediate need for this little unit.The LT uPower units would be fantastic, however, non-handsolder-able packages are currently out of the question. I haven't done a SEPIC w/soft start before so maybe someone with experience can throw me a few tips. Here's what I got:This unit is centered around the LT2983, which is a multi-sensor IC. It has it's own internal voltage reference and will be isolated from the rest of the board with a Murata NTE0505 DC/DC Isolator w/local RC filter (47uH, 4.7uF) and ADuM3150 SPI Isolator. So (and correct me if I'm wrong), I think my biggest noise concern is keep noise away from the front end rather than PS noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2016, 08:10:47 pm
content: The DCDC converter may very well be the larger noise source. I don't know that special model, but some of the small ones are rather bad. Especially common mode "noise" can be rather bad and filtering might be difficult. Even a classical comon mode choke might be an option here - though being larger than the DCDC converter.It can also be tricky combining two switched mode converters, especially if the frequencies get close to each other.A 4.7 V LiIon is odd. Usually they are 3.6 V nominal and something like 3-4.2 V depending on charge. So in this case a simple boost converter would be OK, not sepic needed.

author: akis
author_rank: Frequent Contributor
timestamp: May 11, 2016, 08:43:00 pm
content: Quote from: gilbenl on May 03, 2016, 08:37:55 pmBe warned the MEVs will blow instantly on output shorts. Try the MMV range instead.