Electronics > Projects, Designs, and Technical Stuff
Regulator dies and takes whole circuit down with him
evansg:
I have seen 3-4 cases so far in similar circuit designs that has the following issue:
For a (strange) reason 5V regulator (I have seen that on LDL1117 from STMicroelectronics) explodes (with a large hole on it) and takes down the whole 5V line as it passes through the 12V DC input. Of course all 5V tolerant components are fried out!
Texas instruments same regulator (LM1117) or similar 3.3V is not affected. Both ST and TI exists in the same circuit. ST dies and destroys whole 5V line. TI 3.3V works flawlessly.
Any idea what can cause that? ESD? I get reports that this has happened always when the user plugs the power inside the socket. (voltage spike?) and always using a 12-13.8V battery (lifepo or lead acid). Unfortunately I cannot reproduce that Tried many times but coudn't cause a damage to the regulator
Check attached photo of the damage from a board that was returned back.
Thanks
SiliconWizard:
Would be interesting to see your schematic, at least just the power supply section.
The LDL1117 and LM1117 have different characteristics on a number of points, but their rated input voltage range is the same (20V absolute max).
(The LDL1117 has a much lower dropout, higher max output current, higher PSRR...)
The thing that doesn't seem documented is the ESD rating, whereas it is for the LM1117. I don't know if it's just an omission, or if it was never tested...
ESD is something to consider here. Is there any kind of protection against that on your board?
As a side note, if a LiFePo4 battery is allowed, a typical such "12V" battery will be at 14.4V when fully charged. Still under the max input voltage for your regulator(s), but higher than your rated 12-13.8V... (Probably doesn't matter much here, but just thought it could be worth mentioning.)
One other thing that comes to mind is the possible reverse current when the power source (battery/power switch?) is removed. I haven't read the datasheets thoroughly, but I don't know if those regulators are fully protected against it. You may need to add a diode between IN and OUT for good measure.
DBecker:
Is this an LD1117 or a LDL1117 from ST?
The LDL part has a MOSFET pass transistor, while the LM1117/LD1117 has a NPN. The MOSFET will conduct in the reverse direction, dumping the capacitor charge through the part if the input jack is shorted.
How certain are you that the part isn't counterfeit? There are many pin compatible regulators that cost about a penny each in Asia, while a real ST part will cost far more. Some of the low-end *1117 parts have a datasheet max voltage of 12V or under. I've blown them out with a 9V supply. They are really intended for easy duty e.g. regulating 5V down to 3.3V at well under their max current rating.
free_electron:
Not visible in the picture but .. where are your local anti-oscillation caps ?
paragraph 6.2 : The LDL1117 requires external capacitors to assure the regulator control loop stability.
Any good quality ceramic capacitor can be used but, the X5R and the X7R are suggested
since they guarantee a very stable combination of capacitance and ESR over the
temperature range. The input/output capacitors should be placed as close as possible to
the relative pins. The LDL1117 requires an input capacitor with a minimum value of 1 μF.
This capacitor must be placed as close as possible to the input pin of the device and
returned to a clean analog ground. The control loop of the LDL1117 is designed to work
with an output ceramic capacitor. Other type of capacitors may be used, as long as they
meet the requirements of minimum capacitance and equivalent series resistance (ESR), as
shown in Figure 20: "Stability plan (VOUT = 5 V)" and Figure 21: "Stability plan (VOUT =
1.2 V)".
To assure stability, the output capacitor must maintain its ESR and capacitance in the
stable region, over the full operating temperature range.
The suggested combination of 1 μF input and 4.7 μF output capacitors offers a good
compromise among the stability of the regulator, optimum transient response and total PCB
area occupation.
This is a common mistake when designing with linear regulators. ALWAYS read the datasheet carefully. Some regulators do not need them , some do not want them , some must have them ! If you don't put them on a regulator that needs them , or do put them on a regulator that doesn't want them , or the ESR is too low ( yup, there is such a thing ! everybody is gunning for the lowest esr, but that too is completely wrong. ) the regulator will become unstable and can self destruct.
Simply assuming that all 1117 and 'compatibles' out there behave the same way is wrong. Check your datasheet carefully when selecting alternates.
evansg:
ST Regulators are always sourced from Mouser (all components were bought from Mouser and Farnell) and moreover the in/out capacitors are Murata X7R 10uF. There is a N mosfet /zener diode at the input which does efficiently the reverse polarity protection.
Regarding the "reverse current when the power source (battery/power switch?) is removed". I do not think so as this happens always (based on customer reports) when they plug the socket in the DC IN.
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