5V-to-3V3 LDO compatible with 0 ESR output cap? - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: March 15, 2019, 12:34:09 am
content: Hi,I realise these requirements are somewhat conflicting, but I didn't expect them to be conflicting to the extent that I've had difficulty finding a singling matching candidate. So I'm looking for:A linear voltage regulator, taking:Input voltage: 5V to 12VOutput voltage: 3V3 (so, a max dropout of 1.7V then I guess, which is where the trouble starts)Stable with ceramic output capacitors (i.e., basically 0ESR, if only I could use a non-LDO regulator)Output current: > 500 mA (not a hard requirement)Through-hole package (not a hard requirement)Could anyone please suggest matching parts? I've looked through a dozen parts on digikey, and setting the parametric search of requirements #1-#3 seems to only yield parts failing to meet #4.And as something of a meta-query, what's the best place to do parametric searches where you can specify the 0 ESR output capacitor condition?To preempt the often reasonable response, "It's much easier to find a 1ESR cap than an LDO matching these requirements"; the application here is for a parts bin in a shared space for random fooling around. IMHO, that usecase calls for an idiotproof part, and in 2019 idiotproof means compatible with perfect capacitors. Also, I've never understood why it could be OK to have a tantalum/electrolytic cap at the output of the regulator, but then having a dozen 1uF ceramic MLCCs on the same net as input caps to various ICs downstream. Am I supposed to be relying on a certain level of ESR/loop inductance in my power distribution traces to prevent those other caps from bringing the LDO into instability?? Anyway, I'm drifting off topic...

author: kony
author_rank: Regular Contributor
timestamp: March 15, 2019, 01:00:07 am
content: Polymer cap and series resistor is a viable path as well sometimes, even the most cost effective and long device life choice as well.

author: Deepak
author_rank: Regular Contributor
timestamp: March 15, 2019, 01:11:28 am
content: So an LDO relies on the ESR of the cap for its feedback system therefore you won't find any such chip through a parametric search. If you must have a 0 ESR cap then you could get away with adding a resistor at the output of the LDO, but it would need to be small value and within tolerance over a wide temp range.And there's no such thing as idiot-proof...

author: rs20
author_rank: Super Contributor
timestamp: March 15, 2019, 02:44:13 am
content: Thanks for the suggestions of using a separate ESR resistor (although I'm puzzled by the "within tolerance over a wide temp range" bit; absolutely any half decent 1 ohm resistor can stay within 0.1 to 2 ohms over temperature??), however such external solutions don't meet my "idiotproof" requirement.Quote from: Deepak on March 15, 2019, 01:11:28 amCounterexample:MIC5203-3.3. It is an LDO (0.3V dropout) and specifies no minimum ESR. So although it's SMD and only supports 80mA output; I think it's not reasonable to conclude that the design couldn't be stretched to a through-hole 500mA part. The question remains whether such a part is available from distributors though.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 15, 2019, 03:17:23 am
content: There are quite a number of LDOs that specify ceramics as their output capacitor, not sure about a "0 ESR" specification, but at least expecting very low. It could be that most of them are designed for lower operating currents, though, meant for boards with a slim profile or local regulation that doesn't require all that much current.

author: Deepak
author_rank: Regular Contributor
timestamp: March 15, 2019, 05:56:48 am
content: Quote from: rs20 on March 15, 2019, 02:44:13 amI stand corrected, though for all we know they just did what I suggested inside of the package - or it may be something more exotic. In either case, but there's no free lunch and this thing still has specific requirements for capacitor selection. At least an LCR meter makes ESR easy to test.

author: exe
author_rank: Unknown
timestamp: March 15, 2019, 10:56:22 am
content: I normally go to a vendor's website and play with parametric search. E.g. I set here "output capacitor type: ceramic":http://www.ti.com/power-management/linear-regulators-ldo/products.html#p238max=12;125&p238min=-40;5&p1154=3.3;3.3&p984=Ceramic(feel free to reset the search if it doesn't display what you want).I also have some parts, I believe, from micrel and microchip. Linear/Analog also had mlcc-friendly LDOs.

author: rs20
author_rank: Super Contributor
timestamp: March 15, 2019, 09:23:33 pm
content: Thanks so much, exe!! Taking the parametric search that you had kindly already mostly pre-populated and adding in the current requirement/throughhole restriction yielded exactly one part, theLM2937, with these properties:Input voltage: 4.75 V to 26 VOutput voltage: 3V3Stable with ceramic output capacitorsOutput current: up to 500 mATO-220 Through-hole packageI.e., meeting every requirement I outlined in the OP (which I thought was probably a bit too much of a stretch!)Thanks!

author: sdouble
author_rank: Frequent Contributor
timestamp: March 15, 2019, 09:37:47 pm
content: lt3045 by analog

author: exe
author_rank: Unknown
timestamp: March 15, 2019, 10:40:57 pm
content: You are welcome! I have good results with lt3080, it's 1.1A device, I've been using in my diy bench power supply, works quite well. It's not a fixed voltage regulator, but voltage is set with just one resistor. There is also lt3081 that allows current limit. There are many interesting devices, too bad not easy to find. I had to read many tens of datasheets to find ceramic-friendly ldos.There are some "more obscure" parts from Asia that work with mlcc, but all I have in my list at hand are for lower current or voltage than you need.

author: AE7OO
author_rank: Regular Contributor
timestamp: April 24, 2019, 08:31:18 am
content: You may want to take a look the Analog DevicesADP124ARHZ-3.3. It is SMD, but very easy to work with, even without hot air.The only requirements for both input and output caps is that they are at least 0.7uF capacitance and have less than 1ohm ESR. I use 1uF X7R and it just works.

author: mariush
author_rank: Super Contributor
timestamp: April 24, 2019, 10:02:28 am
content: The generic 1117 regulators can do 0.8A, 13v/15v/20v max (varies on manufacturer) and typical 1.3v drop at 0.8/1a (bit less at lower currents)Unless datasheets say otherwise, most do require a capacitor on output with ESR between 0.1 ohm and 1 ohm ... but you could simply use ceramic capacitor(s) in series with a 0.15/0.22/0.47 ohm resistor for example to artificially increase ESRSome datasheets will say that particular 1117 is safe to use with ceramic caps but even with those it would be better to be safe andat least add footprint for a resistor in series with cap. it would allow you to use regulators from other brands that require minimum esr,without having to redesign pcb.if resistor is not needed you can use jumper link or 0 ohm resistor on that resistor footprint.Here's example of such 1117 regulator:https://www.digikey.com/product-detail/en/diodes-incorporated/AZ1117CH-3.3TRG1/AZ1117CH-3.3TRG1DICT-ND/4505206or to252 version for higher surface, better heat transfer, just sorted by highest quantity available :https://www.digikey.com/product-detail/en/diodes-incorporated/ZLDO1117K33TC/ZLDO1117K33DIDKR-ND/2095639(but there's a catch... note datasheet says ceramic caps compatible but still has to be 0.05 ohm ... 0.5 ohm , so adding a 0.15ohm or 0.22 ohm in series with ceramic cap(s) wouldn't hurt. )Seehttps://www.digikey.com/products/en/integrated-circuits-ics/pmic-voltage-regulators-linear/699?FV=ffe002bb&quantity=10&ColumnSort=1000011&page=1&k=1117&pageSize=25&pkeyword=1117for all 1117 regulators on Digikey

author: David Hess
author_rank: Super Contributor
timestamp: April 24, 2019, 04:19:11 pm
content: 1 microfarad with a ceramic output capacitor is awfully low. Most (all?) devices which will work with a ceramic output capacitor will require more than 1 microfarad like the LT1963, LT1965, LT3045, and LM2937 which specify 10 microfarads or more. The LT3080 requires 2 microfarads or more.Beware that 8.7 volts of voltage drop at 0.5 amps results in 4.35 watts of power dissipation which is doing to require a larger package and heat sinking either externally or through the board. For reference, a free standing TO-220 package is good for about 2 watts.