How do the common low-dropout LP2950 and buck LM2596 regulators behave when their input voltage drops below their output voltage?
I'm driving a micro-controller board rated for 3.7-5.5 V with a 4.8V battery pack built from four 1.2V NiMH cells. 1.2V NiMH cells range from 1.45V freshly charged to slightly below 1V, or whenever you wish to start worrying about polarity reversal.
As I understand it, one should usually use a buck like the LM2596 for efficiency reasons. LM2596 boards all claim the input voltage must exceed the output voltage by at least 1.5V or even specify a minimum input voltage of 4V. I'll want the device running while the pack still exceeds 4V though, but 2.5V = 4V - 1.5V sounds too low, although not necessarily.
What actually happens when an LM2596's input voltage drops below output plus 1.5V? Does it stop working or become unpredictable? Or does it supply the input voltage minus 1.5V?
Are there LM2596 that bypass themselves or switch to an internal LDO when the voltage drop gets too low?
Also, I imagine an LDO like the LP2950 would continue dropping the voltage by it's minimum drop. Is that correct?