Voltage regulator with alternate voltage source...

Mouse mouse at Rodents-Montreal.ORG
Fri Apr 8 13:27:05 CDT 2016

> I archive all data sheets I run across and I found this information
> on both TI and Fairchild data sheets for the 78H05, LM340 and LM78XX
> devices - it is usually under a heading called Application Hints and
> for some reason Fairchild have removed these Hints from the sheets
> you quote.

> In ALL cases the manufacturers caution that putting voltage to the
> output when the input is not powered CAN DAMAGE THE CHIP.

I have the datasheet PDF for the On Semiconductor version, and I have
been unable to find anything of the sort in it.

Whether On was sloppy when writing their documetnation or whether some
manufacturer's devices are safe or whether I just missed something, I
don't know.  But, when dealing with a device of unknown provenance, it
does seem clear you're better off not powering the output side if the
input side has a low-impedance path to ground.  (Powering the output
with the input disconnected appears to be a completely different thing;
I have trouble imagining a design that would be damaged by that.)

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