voltage regulators

Hi
can someone tell me how works voltage regulator? and why you need MOSFET??

What is that from?

I believe the MOSFETs job is reverse battery protection.

Not sure what the regulation voltage is for the IRU1205 because the variant isn't specified in the schematic.

The variation is as follows:
IRU1205CL= Adjustable
IRU1205-18CL= 1.8V
IRU1205-25CL= 2.5V
IRU1205-28CL= 2.8V
IRU1205-30CL= 3.0V
IRU1205-33CL= 3.3V
IRU1205-36CL= 3.6V

The drop out voltage is between 10mV and 400mV depending on the amount of current you are drawing.

Pin 1 is the input voltage (max of 10V)
Pin 2 is the ground pin
Pin 3 is the enable pin
Pin 4 is the adj/Cbp - either the voltage adjust pin for variable regulator version, or the bypass capacitor for all other versions
Pin 5 is the output

Here is the link to the data sheet:
http://www.datasheetcatalog.org/datasheets2/41/41896_1.pdf

I'm going to agree with ronsimpson on the MOSFET, I'ts probably there to protect the battery although a regular diode could have done that job just as well. I thing the MOSFET is a redundancy and unless the designer intended to use it to shutdown the circuit, you can probably get away with not using it.

A diode has 0.7 (or 0.4) volts loss. When power is coming from a 6V (or 3V) coin cell this is major.
The MOSFET can have very little voltage loss.

Depending on the load, I was assuming an unknown load and a signifigant input voltage in which case a didode may be more appropriate.