How is my Arduino Pro Mini running off 4.5v?

I have a 5v Pro mini. I connected my 4.5v supply to VCC and GND to try powering it. I expected that this would work as it is within specs. It did.

Then just for kicks, I connected the 4.5v supply to the RAW pin and GND. I expected that this wouldn't work because despite the onboard regulator being a low dropout regulator, it still needs something like 5.35v, and perhaps more if it has that diode in front of it Grumpy Mike's power regulation tutorial mentions which would drop the input voltage by 0.7v.

So how is it the Arduino is working? I thought below the dropout voltage the regulator stops working? Or does the dropout voltage simply mean that below 5.35v it can't supply 5v, but it will continue to supply something less than that, and cease functioning as a stable source of power?

If it's the latter, well, I guess that's great actually. I mean the Arduino can run at less than 5v, so this potentially means more battery life for my device...

But still, I wonder what happens to the voltage from the regulator when you get into this condition? If I'm supplying 4.5v, do I get 4.5v out, or something less than that, like 4.5v = minus the dropout voltage, minus whatever the diode takes away?

Or does the dropout voltage simply mean that below 5.35v it can’t supply 5v, but it will continue to supply something less than that, and cease functioning as a stable source of power?

That is the case, below the drop out voltage spec it stops being able to regulate but still passes through what voltage is avalible (minus some voltage drop through the regulator. At some point the voltage will drop below the brown out detector on the AVR chip and enter a reset condition and stay there unless Vcc returns higher.

Lefty