Say I'm using an LDO to power an MCU such as the ATmega328p with 3.3V from 3 NiMH AA batteries.
What it isn't clear to me how to deal with brown-out situations.
I can think of 2 possible risks (i.e. when using an LDO such as the MCP1700):
- deep discharge of the batteries when their voltage drops to MCU brown-out levels (say 2.7 V) and the MCU continues to draw some current in reset mode
- damages to the LDO itself when V_in drops below V_out
Is it a good idea to look for LDO with on/off pin (such as the S-1132) and connect it to a voltage detector such as the MCP112 or TC54?
In that way one would work-around the brown-out detection of the MCU, and basically implement an external brown-out detection that just switches the MCU off (instead of keeping it in reset).
Or are there LDOs available that already include a voltage detector and just turn off V_out if V_in drops below a threshold?
Regarding LDO damages, I stumbled over a paragraph in the S-1132 datasheet:
In the S-1132 Series, a low on-resistance P-channel MOS FET is used as the output transistor. Be sure that VOUT does not exceed VIN + 0.3 V to prevent the voltage regulator from being damaged due to reverse current flowing from the VOUT pin through a parasitic diode to the VIN pin, when the potential of VOUT became higher than VIN.
(Section Operation, page 13)
How would V_out exceed V_in? I mean, I thought if V_in drops to V_out + V_dropout or less, then the LDO works in its dropout zone where it isn't regulating anymore, i.e. V_out has some linear relation to V_in. See for example the graph for the S-1132VB50 (V_out=5V) variant (page 16), where V_Out/V_in pretty much equals 1 for V_in=2.5..5:
So is the paragraph warning against (accidentally) connecting another power-source to V_out that exceeds V_In+0.3V?
Or is it possible to damage an LDO somehow merely by letting V_in drop under a minimum threshold for an extended time?
Another thing I don't understand in this datasheet are the output voltage/output current graphs on page 16, e.g. for S-1132B30:
I mean, e.g. for I_out=400 the curves have two V_out values?