I've learned something while creating a test procedure for a product I've
been developing - and written it up (with pictures and schematics) here:
http://www.gooligum.com.au/blog-section/blogart13

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Short version:

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I assumed that, if an ADC chip (MCP3008 in this case) passes its tests
(samples an input and returns a result in a defined acceptance range, a few
times in succession), it's safe to conclude that, not only in that chip ok,
but it's also being powered and that therefore the 3.3V rail that it's
sitting on and therefore the LDO regulator and upstream 5V power are also
ok.

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"Ok" in this case meaning "there is one".  Sure, it's not an exhaustive
power supply test by any means.  Sure, maybe the supply is marginal.  Maybe
it would fall over if pushed.  Maybe it's supplying the wrong voltage.  But
this was supposed to be a final acceptance test on a known (more fully
tested) design - a "go / no go" before each board leaves the factory.  If
there's a power supply there - enough to power the ADC - then presumably th=
e
regulator wasn't let off the board, it's not DOA, the micro-USB connector
(from which 5V comes) wasn't incorrectly fitted, etc.  The idea is to check
for things that might have gone wrong in PCB assembly, or major component
failures. =20

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In other words, I assumed, if the ADC works, there must be nothing majorly
wrong with the power supply.

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But in fact - I found, to my initial amazement, that the ADC worked just
fine, with no power supply at all!  I could remove the 5V supply, with no
apparent effect.  No power coming in at all, and it still worked.

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To prove this, I tried making up a basic circuit with a MCP3008 connected t=
o
the SPI bus, a couple of resistors to provide an analog voltage to measure,
and no connection to Vdd at all.  Didn't work (as it shouldn't), but adding
a 1uF cap across Vdd/GND, and still no power supply in sight, made it work
just fine.

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Eventually I figured it out - for more detail read the blog, but anyway -
enough power was supplied via a digital input (normally held high, but goes
low for each conversion) to power the rest of the chip via (I assume) the
internal protection diode on that input.  3.3V into an input led to 2.5V
being supplied out of (yes, out of!) the Vdd pin, charging the cap, which
was then able to hold the chip up for the short period that that input
dropped.

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And this seems to be common behaviour - a FTDI FT232RL on the same board di=
d
the same thing, powered quite happily from its RXD input, and supplying
power out of (!) it's Vcc pin.

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Obvious now that I see it, and maybe obvious to most folk here, so sorry if
that's the case.  But I'd never observed this before - presumably because
I've always applied power to any ICs I've used.  Going back to university
days, one of our mantras when starting EE pracs was "power to the chips!"
Ah, but it seems that they don't always need it...

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Cheers,

David

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