LDO keeps blowing up but without any magic smoke... - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 07:55:42 am
content: Hi,I am trying out the Micrel MIC5205 LDO I bought from ebay (item #311513963103) and they keep blowing up.No magic smoke though.After the failure there is always a short between Vcc and Ground.The load is very small at around 14mA and this can handle up to 150mA.Enable pin is always pulled down with a 100k resistor and I enable it by connecting the EN pin to Vcc.A 1uF tantalum capacitor is connected to Vout.http://ww1.microchip.com/downloads/en/DeviceDoc/mic5205.pdfAt first power on +8Vcc, the output was ramping up at a random manner. After some tries switchingthe power supply on and off, it went to 3.3V and then it stayed there with a very stable output.I applied a small load and everything was fine but whenever I tried +10V Vcc the output dropped toalmost zero. At 9.8V it started to draw excessive current. According to the datasheet this will work up to 16V.The second IC had the same behavior but it was damaged immediately at 10V.At first I thought that I damaged it thermally when soldering, because I used 300 degrees C.The datasheet suggests a 250 degrees for 5 seconds as an absolute maximum.The second time I used 230 degrees and very quickly. Same results though...Am I doing something wrong, or did I receive a faulty batch ?Any help is appreciated. (I have another 3 left...)Thank you.p.s. by the way, this fine pitch is simply evil !

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2016, 07:57:54 am
content: No input bypass?

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 07:59:18 am
content: No, I did not use any. None mentioned in the datasheet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2016, 08:08:19 am
content: Quote from: hgg on May 12, 2016, 07:59:18 amQuoteI like that they give so many PSRR graphs, but... they're upside down, the "rejection ratio" ones. Rejection is positive if it's, well, rejecting. So they're apparently claiming >100dB ofgainat 10Hz, which is obviously false. The "ripple rejection" ones are positive dB, though... so it's worse than just being wrong, they're inconsistent too.But more to the point, there is no spec on GND pin current versus Vin (safe to assume it's notoriously bad unless shown), and no stability graph versus C + ESR for either input or output capacitors. I would not design-in this part.Tim

author: wraper
author_rank: Unknown
timestamp: May 12, 2016, 08:22:15 am
content: Quote from: hgg on May 12, 2016, 07:55:42 amThat's reflow temperature (when all device gets heated), not solder iron temperature.

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 08:48:43 am
content: QuoteIndeed the input wire was more than 10 inches, which I forgot, but is this a reason forthe LDO to get damaged ? Can it be damaged by oscillation ? (none observed though).QuoteAny suggestion of a quality part with an enable pin?QuoteSo what is the max safe temperature with a soldering iron?Thanks.

author: wraper
author_rank: Unknown
timestamp: May 12, 2016, 08:55:04 am
content: 300oC is certainly safe, 350oC should be safe too, if doing the job fast enough. All you should care about is that part itself don't heat up above that max temperature.

author: djacobow
author_rank: Super Contributor
timestamp: May 12, 2016, 08:56:09 am
content: I can't speak for blowing up, but I inherited a design with this very part last year and it oscillated like nobody's business the way they had it in there. That output cap, it really needs to be a tant, or at least put a small resistance in series with an MLCC.When I revved the board, I designed it out. I don't remember what I put in there, but I actually didn't need the enable.

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 09:03:59 am
content: I used a tantalum capacitor.So, it looks like that its not one of the best LDOs out there...Thats why I like ebay. You can find the worst parts at the greatest prices.I will give it one more try fully 'caped'....

author: EPTech
author_rank: Regular Contributor
timestamp: May 12, 2016, 09:37:41 am
content: Hi there,QuoteYes it can because each wire and trace has a parasitic inductance. When you power on, you actually make a step. The voltage on the input will overshoot very shortly but surely and could go way past the rated input voltage.

author: danadak
author_rank: Super Contributor
timestamp: May 12, 2016, 10:28:01 am
content: When it oscillates it is dissipating a lot of CxV**2xf power dissipationbecause of load cap, so die meltdown real possibility. Keep in mindpower is the rate of change of energy, eg. the flow of energy in andout of the cap constitutes power, the power to move energy in and outof the cap.Regards, Dana.

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 10:39:04 am
content: Interesting.I never saw oscillation on the scope though. The voltage was a flat line all the time.Just soldered another IC with hot air but this time with all the capacitors.Worked fine for a while and after some on/off cycles and a small load it cannot regulate more than 9V.At 10V the output voltage drops in half. Never had this kind of problem with any other LDO.Am still not sure what is going on. The first time I thought that I might have damaged it when I checkedfor shorts with the multimeter before powering it on. The UT61E outputs +3V in continuity mode and ifyou check with wrong polarity maybe you can damage it with -3V (?). The UT136C outputs a safer 0.44V.I am at a loss of what is the real problem, but I am definitely not going to use this LDO even if I find thereal cause. ...which is...

author: Gyro
author_rank: Super Contributor
timestamp: May 12, 2016, 11:17:13 am
content: Those LDOs are also very easy to damage by shorting the input... discharge current spike of the output capacitor through the internal parasitic reverse diode.

author: danadak
author_rank: Super Contributor
timestamp: May 12, 2016, 11:56:41 am
content: Protection of linear regs, see link, page 4......http://www.ti.com.cn/cn/lit/an/snoa402b/snoa402b.pdfRegards, Dana.

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 12:05:12 pm
content: Hi,Maybe I should have used protection diodes.Thanks for the link, I will read it.I have two more ICs. I'll give them a try with all the protection possible.Do you think that this has anything to do with the problem?https://youtu.be/9DwytzdpQOcThanks.

author: krivx
author_rank: Frequent Contributor
timestamp: May 12, 2016, 12:15:41 pm
content: Just because no one has mentioned it yet - eBay parts are always something of a gamble. These things could be seconds, fake or ESD damaged before they get to you.

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 12:20:38 pm
content: Indeed, and fake might be more possible because of the constant failure above 9.8V exactly! (Its rated for 16V)They arrived in a plain plastic bag... so ESD damage is a possibility as well.

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 01:44:57 pm
content: This looks like a properly designed LDO, what do you think? Any experience with this part?http://www.ti.com/lit/ds/symlink/lp2985-n.pdf

author: umagarz
author_rank: Unknown
timestamp: May 12, 2016, 02:03:59 pm
content: Quote from: krivx on May 12, 2016, 12:15:41 pm+1I recently bought some 7seg serial drivers on ebay, and none of them would communicate with SPI...

author: rstofer
author_rank: Super Contributor
timestamp: May 12, 2016, 02:49:55 pm
content: To be clear, there should probably be a reverse biased diode between Vout and Vin. Many regulators don't like reverse current at shutdown. This becomes a more serious problem when there is a lot of capacitance on the load. They have to discharge somewhere.You may see a flat line on the output but how about the input. You mentioned a long wire, a perfect tuned circuit for oscillation. You might try some capacitance at the input. Something like the one used at the output.I buy my parts from DigiKey or Mouser. Absolutely nothing from eBay! Those chips are floor sweepings at best.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2016, 04:28:54 pm
content: Most likely seems to me, either reverse conduction, or input overvoltage or reversal.Long wires exhibit inductance, which causes voltage spikes when connected. This is exacerbated with ceramic capacitors, but dampened with electrolytics. (And tantalums, but tants don't appreciate current surges.)Tim

author: hgg
author_rank: Frequent Contributor
timestamp: May 12, 2016, 04:33:21 pm
content: I will try once more with all the protection passives in place.Maybe a 12V zener as well !It usually works up to 9.8V but above that the LDO starts to draw more current and fail.

author: hgg
author_rank: Frequent Contributor
timestamp: May 13, 2016, 07:18:20 pm
content: Ok, I soldered another one, with a protection diode and input/output capacitors.Proper soldering temperature and dwell time. Again, it can regulate only up to 9.8 Volts.Put 9.9 Volts and the output drops rapidly to 1.36V also drawing a lot of current from thepower supply.Asdjacobowsaid, the original ones oscillate very easily. These ones oscillate only if Ileave the enable pin floating. They are stable even without any capacitors but they canregulate only up to 9.8VThey must be %100 fake.Cannot think of something else.