Back up battery for GSM module

[m.waqas]

Hi
i want to design a circuit for Backup battery for my GSM module SIM300DZ whose max voltage and current values are 0-3.6v,250mA.
power supply Vin is 12v and For main power supply of module i am using the refrence circuit of the source power supply input given in SIM3XXDZ hardware design manual by simcom.
i want to use a Nicd battery pack (3 cells 3.6v,750mAh) for back up battery.
I don't know how to design the circuit so that when main power supply is absent back up battery should work.

Thanks

 

[m.waqas]

i have found a power supply circuit

GSM man please help me can i use this circuit for my power supply after making the changes,suggested by you in the thread.

my board will have a SIM300DZ nodule max 3.4-4.5v,3A
PIC18F65J15 max 0.3-3.6V,250mA
GPS receiver +3.5~5.5Vdc
can i use the power supply circuit attatched along for my board

Plz give your suggestions which circuit i should use