Dealing with LDO heat - Page 1
Forum section: electronics / beginners

author: bryce1
author_rank: Regular Contributor
timestamp: August 07, 2014, 09:31:33 am
content: Hi guys,I am using this one IC on my board that draws about 200mA at 3.3v. The rest of the circuit runs at 5V, and I have problems dealing with the heat of the LDO I am using (for 5V to 3.3V).The reasons mainly being the density of the board and my lack of experience. Since cost is not a big issue, should I just go for a switching regulator like the SC189ZSKTRT? Do you have any layout / calculation advice?Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2014, 10:09:24 am
content: Only 0.34W... what, is it SOT23 or something? If you can reserve some area for heat spreading copper and vias, that's plenty fine. Maybe risky for automotive or something. If you can't do that, a larger device like SOT89 or -223 would do better.Tim

author: mij59
author_rank: Frequent Contributor
timestamp: August 07, 2014, 10:43:12 am
content: Hi,What's the 3.3V IC, LDO?The inductor for the SC189ZSKTRT would also take up space.

author: bryce1
author_rank: Regular Contributor
timestamp: August 07, 2014, 10:51:36 am
content: Right now, I am using a NCP1117ST12T3G. It is a SOT-223 package.

author: mij59
author_rank: Frequent Contributor
timestamp: August 07, 2014, 11:01:14 am
content: Quote from: bryce1 on August 07, 2014, 10:51:36 amTake a look at page 8 of the data sheet fig. 21, minimum pad size is about 3mm.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2014, 11:09:12 am
content: How hot is it getting, or, how is it being a problem?

author: bryce1
author_rank: Regular Contributor
timestamp: August 07, 2014, 11:13:53 am
content: I think it might not be the cooling after all.It gets untouchable after milliseconds. I checked, it is corrected the right way, so I have no idea what is going wrong.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2014, 11:24:50 am
content: Milliseconds? Like hundreds of them, or..?Now I'm curious, as any kind of human response takes a very long time, electrically speaking. If you're accidentally measuring temperatures within (single digit) milliseconds, that would actually be pretty exciting..Tim

author: bryce1
author_rank: Regular Contributor
timestamp: August 07, 2014, 11:31:46 am
content: Maybe I am that good :-DSeriously, there must be something wrong with the part or the PCB. I'll try to figure it out tomorrow.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2014, 03:22:30 pm
content: If you want the higher efficiency needed to lower the power dissipation, then a switching regulator is the way to go. Using a linear regulator to drop 5.0 volts down to 3.3 volts yields 66% efficiency which is not bad but a typical switcher with an efficiency of 83% would drop the heat requirements down to half which may or may not be worth it. A good switcher could drop the losses by half again.This is probably about perfect if your layout skills are good:http://www.linear.com/product/LTC3542A larger SO-8 package and lower operating frequency might be easier to deal with:http://www.linear.com/product/LTC1627http://www.linear.com/product/LTC1707

author: tszaboo
author_rank: Super Contributor
timestamp: August 07, 2014, 03:35:33 pm
content: I think you should be able to handle that amount of heat without too much issue. Typically a SOT223 has <150K/W thermal resistance.You either underestimated the current consumption, you have a short on the PCB or a component is toast. Also 1117 doesnt like ceramic capacitors directly on the output, it needs something bigger and with more ESR. I would measue the resistance between 3.3V and GND (while off) to start with.

author: Neilm
author_rank: Super Contributor
timestamp: August 07, 2014, 06:25:43 pm
content: Personally, I'd take it off the board and put a PSU in. That will allow you to measure the current so you will be able to see what is going and what the current draw actually is.

author: bryce1
author_rank: Regular Contributor
timestamp: August 08, 2014, 12:58:12 pm
content: Quote from: NANDBlog on August 07, 2014, 03:35:33 pmWhy?datasheet:"The capacitor type can be ceramic, tantalum, or aluminum electrolytic as long as it meets the minimum capacitance value and ESR limits over the circuit’s entire operating temperature range".

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2014, 01:35:12 pm
content: Yes, and what limits are those?

author: mariush
author_rank: Super Contributor
timestamp: August 08, 2014, 01:51:16 pm
content: lm1117http://www.ti.com/lit/ds/symlink/lm1117-n.pdfpage 9Output CapacitorThe output capacitor is critical in maintaining regulator stability, and must meet the required conditions for bothminimum amount of capacitance and ESR (Equivalent Series Resistance).The minimum output capacitancerequired by the LM1117-N is 10?F, if a tantalum capacitor is used. Any increase of the output capacitance willmerely improve the loop stability and transient response. The ESR of the output capacitor should range between0.3? - 22?.In the case of the adjustable regulator, when the CADJ is used, a larger output capacitance (22?ftantalum) is required.ncp1117-dhttp://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDFSee graphs on page 6Also see page 8.Frequency compensation for the regulator is provided bycapacitor Cout and its use is mandatory to ensure outputstability.A minimum capacitance value of 4.7 uF with anequivalent series resistance (ESR) that is within the limits of0.25 ohm to 2.2 ohm is required.The capacitor type can beceramic, tantalum, or aluminum electrolytic as long as itmeets the minimum capacitance value and ESR limits overthe circuit’s entire operating temperature range. Highervalues of output capacitance can be used to enhance loopstability and transient response with the additional benefit ofreducing output noise.I typically use 33-47uF 35-50v rated electrolytics , they usually have 0.45-0.7 ohm ESR and the can size is just as large as caps rated for lower voltages (so i don't have to stock this capacitance in several voltage ratings and can sizes). See Panasonic FC or Nichicon PW for example, good series for linear regulators.If you want something more modern, you could go with 68-100uF 16v or 47uF 25v Panasonic FR which are rated for 5000h@105c but the esr @ 100kHz is already down to about 0.3 ohm so in some extreme cases the regulator may not like it.

author: tszaboo
author_rank: Super Contributor
timestamp: August 08, 2014, 02:59:22 pm
content: Quote from: bryce1 on August 08, 2014, 12:58:12 pmThere are various sources on TI, AD or Linear website explaining the phenomenon. The loop stability is not good with low ESR caps. But, this is from TI's datasheet:"The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for bothminimum amount of capacitance and ESR (Equivalent Series Resistance). The minimum output capacitancerequired by the LM1117-N is 10?F, if a tantalum capacitor is used. Any increase of the output capacitance willmerely improve the loop stability and transient response. The ESR of the output capacitor should range between0.3? - 22?. In the case of the adjustable regulator, when the CADJ is used, a larger output capacitance (22?ftantalum) is required"Typical ceramic cap goes down to 0.01Ohm ESR easily. Source:www.avx.com/docs/techinfo/mlc-tant.pdfFig. 1.I know, that you dont necessarily use TI LDO, or that capacitor from AVX, but as a general rule of thumb, if they dont say in the LDO datasheet, that it is stable with ceramic, than don't use it with ceramic. It is a feature of the newer ones, and I'm sure there are several thousands of engineering work-hours making newer LDOs stable with them.mariush: did you just qoute the same thing as me like five seconds before me?

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2014, 03:36:10 pm
content: Low dropout regulators rely more on the output capacitor for frequency compensation because the common emitter or common source output stages have voltage gain. The negative regulator version of the LM317, the LM337, suffers from the same problem despite not having low dropout because it also uses a common emitter output stage which is why it needs an output capacitor for stability unlike the LM317.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: August 08, 2014, 03:56:30 pm
content: Before we go to self oscillation as an explanation, let's look at how you have connected the chip, in particular the tab. The tab is connected to pin 2. In fact, pin 2 and the tab are likely the same metal piece, which is used as a holder for the die, as shown in the attached image. Pin 2 on your particular device is the output, meaning that if you connected the tab to ground for heat dissipation, you are shorting the output to ground. What would happen then is that device would heat up rapidly, and likely shut off due to thermal overload protection.Another thing, NCP1117ST12T3G is a12V regulator, not a 3.3 V regulator. Maybe you used that as a generic name for the whole range of those regulators, but if you used an actual NCP1117ST12T3G, you've probably fried every 3.3 V chip on the output side, and maybe some of the capacitors as well if 12 V is out of range for them, and possibly created a short somewhere inside a chip in the process. The good news is that the regulator likely survived, so if you need a 12 V regulator in the future, you can probably salvage that one.

author: bryce1
author_rank: Regular Contributor
timestamp: August 09, 2014, 02:47:46 pm
content: it is the 3.3v version. so higher board density => smd tantal?

author: mariush
author_rank: Super Contributor
timestamp: August 09, 2014, 04:01:38 pm
content: Did you bother to read our comments?Want to use smd tantalum capacitors? => select your capacitor => read datasheet => is the esr above the minimum value recommended? => if yes, you can use it.

author: bryce1
author_rank: Regular Contributor
timestamp: August 11, 2014, 06:06:17 am
content: No need to get angry. I'll rephrase the question: If electrolytic is not an option due to the density and ceramics all have too low ESR, is my best bet a tantalum capacitor?

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2014, 06:20:53 am
content: Quote from: bryce1 on August 11, 2014, 06:06:17 amYes, tantalum capacitors work well for this. Alternatively a ceramic capacitor may be used with a resistor in series. Some low dropout regulators are better than others as far as tolerating output capacitor characteristics.

author: george graves
author_rank: Super Contributor
timestamp: August 11, 2014, 06:49:08 am
content: Quote from: T3sl4co1l on August 07, 2014, 11:24:50 amCome on - you know what he meant.

author: sleemanj
author_rank: Super Contributor
timestamp: August 11, 2014, 08:48:16 am
content: Quote from: bryce1 on August 11, 2014, 06:06:17 amYou can use a ceramic with a low value resistor in series instead of a tantalum, seehttp://www.ti.com/lit/an/slva214a/slva214a.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: August 11, 2014, 06:54:29 pm
content: Quote from: bryce1 on August 11, 2014, 06:06:17 amI wouldnt use tantalum in the 21 century (unless no other choice). There is a high chance (over 50) it was sold to buy weapons to make genocide in Congo.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: August 11, 2014, 11:38:29 pm
content: Stupid question, but I didn't see it mentioned:Is the output actually +3.3V as you expect? (Did you measure it?)

author: george graves
author_rank: Super Contributor
timestamp: August 12, 2014, 09:35:52 am
content: [quote author=sleemanj link=topic=34857.msg494585#msg494585 date=1407746896You can use a ceramic with a low value resistor in series instead of a tantalum, seehttp://www.ti.com/lit/an/slva214a/slva214a.pdfWow - that's a really low value resistor. What gives?

author: mariush
author_rank: Super Contributor
timestamp: August 12, 2014, 10:12:20 am
content: Like the application note, that regulator in the appnote needs capacitor with esr of 0.05 ohm or higher, so they used a 0.06 ohm + ceramic cap esr to make sure the esr even at various temperatures and voltages will remain above 0.05 ohm.

author: TMM
author_rank: Frequent Contributor
timestamp: August 12, 2014, 02:13:38 pm
content: Quote from: bryce1 on August 08, 2014, 12:58:12 pmIf the ESR is too low the reg could oscillate, which would cause an overheating problem as you describe. Try putting a 1-10ohm resistor in series with the existing output cap.edit:oops beaten >_<

author: bryce1
author_rank: Regular Contributor
timestamp: August 14, 2014, 04:15:52 am
content: The way I read the datasheet of my 10uF ceramic, I have at least 50 milliohm, which would be more than the 33 milliohm required by the NCP1117.http://www.samsungsem.com/servlet/FileDownload?type=data&file=CL21A106KOQNNNE.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: August 14, 2014, 09:05:58 pm
content: Quote from: bryce1 on August 14, 2014, 04:15:52 amThan you are reading the datasheet wrong. It goes to 5mOhm at 1 Mhz. Or your link is broken.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: August 14, 2014, 10:53:13 pm
content: Quote from: bryce1 on August 14, 2014, 04:15:52 amAre you actually measuring +3.30V on your output?