low dropout regulator problem

[jmelson]

Interesting issue.  I make some gadgets that use either a Beagle Board (old) or Beagle Bone (newer) for a net interface.  These run in a rack that supplies +6 V power, so I use an L4941 low dropout regulator to provide the +5 for the Beagles.  These have run fine for several years.  Now, a couple of them started crashing the Beagles every few minutes.

Well, on some of these units I did not heat sink the regulators.  The Beagles, when idling, draw 250 - 300 mA, with a 1 V drop from 6 V down to 5 V, that should be 250 - 300 mW.  The L4941 is rated at 50 C/W junction to ambient in frree air, if I read the data sheet correctly.  (Maybe they are playing with numbers in the datasheet and it really doesn't mean "free air".)  Anyway, some checks I did when first setting these up seemed to indicate this was reasonable, as the regulators didn't get over about 40 C after a long run.

Anyway, bolting the existing regulator to the aluminum equipment case fixed the issue.  So, either the regulators got more sensitive to temperature, or the thermal conductivity changed, or something.  I've never see this before.

Jon

[mariush]

Could it be weak/cracked solder joints due to heat/cooling cycles over long time?
maybe adding heatsink just put more pressure on joints or you resoldered and that actually fixed it.

[Psi]

The ones that started crashing, are they newly build units of exactly the same design, or old units that have been ruining fine for years?

If newly build units i might suspect that your voltage regulators are fake. ie stamped with L4941 but actually some other reg.

[jmelson]

The ones that started crashing, are they newly build units of exactly the same design, or old units that have been ruining fine for years?

If newly build units i might suspect that your voltage regulators are fake. ie stamped with L4941 but actually some other reg.

The units had old Beagle BOARD computers from 2013.  These are no longer made, they are replaced by the Beagle BONE.  Yes, these units have been running in the field, on and off, for several years.  They are not run 100% of real time, but may have to run for several months each time they are used.

I buy all these parts from major franchised distributors like Digi-Key, Mouser, Newark, etc., so there should be just about zero chance of them being fakes. 

There's no way these got hot enough to deteriorate the solder on the regulators.  For one thing, that leaves obvious darkening of the PC board.

Jon