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<div class="moz-cite-prefix">On 12/08/2015 05:36 PM, Brian via
TriEmbed wrote:<br>
</div>
<blockquote cite="mid:56675B7C.2060005@cheetah.dynip.com"
type="cite">Hi,
<br>
<br>
If a linear voltage regulator sees its nominal output voltage
across its GND/OUT pins while its IN pin floats, is it going to be
damaged?
<br>
<br>
For example, a 7805 regulating a 9 volt battery, but the battery
is disconnected and an AC adaptor is supplying 5 volts.
<br>
<br>
This seems like a thing I should /just know/, but I can't figure
out a good way to google for it, or to divine it from a datasheet.
<br>
</blockquote>
Dear Brian (not to be confused with either Brian or Brian),<br>
The scenario you describe is classic and raises an interesting
point. A lot of semiconductor devices cannot tolerate the situation
you describe because internal clamping diodes end up conducting more
than the few milliamperes they're intended to handle. As Carl said,
the data sheet is the first place to look. <br>
Most of us have read a thousand times that the limit for a pin
such as a GPIO pin is (for example) "VCC+.3v". But guess what: if
VCC is zero <b>the limit is .3 volts!! </b>Applying more than this
to the GPIO pin causes current flow through internal circuits
(typically a diode first) that can only tolerate a few milliamperes
at most. Without a resistor in series with the pin to limit current
this is a recipe for escape of the magic smoke. If the internal
diode destroys itself in a shorted state, then the next time VCC is
applied some other parts of the chip will be fried. If the diode
ends up as an open circuit then the normal input voltage protection
provided by the diode is lost (i.e. a "VCC+x volts" applied to the
pin will be passed into the more sensitive parts of the chip instead
of being safely clamped to VCC or ground). Again, this varies with
different devices, but for MSP430 (from memory) the current limit is
just two milliamperes.<br>
But returning to linear regulators, on page 8 of an <a
href="https://www.sparkfun.com/datasheets/Components/LM7805.pdf">old
TI datasheet</a> I found is a description of the standard
precaution for simple regulators (a diode "backwards" across the
regulator from output to input).<br>
A more detailed treatment is <a
href="http://www.physics.utah.edu/%7Elebohec/P3610/LecNotes/Lecture09.pdf">here</a>
(see page 9-4).<br>
Carl's solution would be OK if you weren't picky about what
voltage actually shows up on the other side of the schottky diode
(the Vf changes a lot with current and temperature!).<br>
<br>
-Pete<br>
<br>
<br>
<br>
<blockquote cite="mid:56675B7C.2060005@cheetah.dynip.com"
type="cite">
<br>
<br>
<br>
Thanks!
<br>
-Brian (one of many)
<br>
<br>
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</blockquote>
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