voltage-regulator
I was looking for a low quiescent voltage regulator to use in a low power atmega328 device and came across some regulators such as LM2574, LM2575 or MC33761, that has an on/off pin. How does one use that pin?
I'm a bit confused. The only way I could think of controlling this pin would be using the uC, but turning the regulator off wouldn't turn the uC off as well?
Edit: What happens when the regulator is turned off?
I think your assessment that there are fewer N.C. type switches is somewhat short sighted. It is true that most TACT type switches are indeed N.O. type but many other switch types offer Form C type contact arrangements that offer both N.C. and N.O. on the same part.
If the input voltage on your regulator is always present you can leverage that to your advantage with some simple additional circuitry to provide the inversion logic that you require for use of a N.O. type switch. Here is how to connect it up.
Use an NPN transistor with its emitter connected to GND. Tie the collector to the inhibit pin on the regulator chip. Then connect a resistor from the base of the NPN to the input voltage supply rail. A 10K resistor may be a suitable value. Finally connect the N.O. tact switch between the base and GND. A small signal type transistor such as a 2N3904 is probably the best type to use.
Pressing the switch turns off the NPN which in turn frees the GND connection at the inhibit input.
Based on the comments in the question, if you want to use two AA Li-Ion cells, charged from 5V USB input, you will need to boost the 5V up to 8.4V because each of the cells will need 4.2V charge voltage. Furthermore, you will need a special charge management chip; you can't just run 8.4V across the two cells.
There are far fewer charge management chips for two cells that for a single cell, the MCP73213 is one. There is a wider range of boost regulators that will take 5V in and can be configured to output 8.4V out; I chose the LMR62014.
Note that the maximum input you can expect from a USB port is 1A; according to the datasheet you will only get about 600 mA out from the boost regulator. So charging 2 AA Li-Ion 2600 mA cells to full capacity at 0.2C will take some time.
For NiCd or NiMH batteries, you can use essentially the same circuit, substituting a suitable charge management IC for the MCP73213 for those chemistries such as the LT1511.
Best Answer
Enable/Shutdown pins on voltage regulators are used from externally controlled circuits. Unlike most hobbyist circuits where a single regulator powers everything, more complex circuits might need to control multiple regulators for different purposes.
Even in a single regulator circuit, an on-off pin could be used by a microcontroller on that regulator to reset itself.
And some regulators allow the On/Off pin for rapid toggling. Essentially allowing PWM control of the regulator, for example, for led dimming.