I would like to thanks everyone who has contributed above. Finally, I will see if I can find MAX4080 easily where I am.

There is one new problem, I don't know if I should start a new thread or carry on here, but I will give it a shot here first: So here I go:

I have one Doorbell, it runs on mains 230V currently, few zeners drop the voltage after the rectifier and through 78L05 the PIC & the RF receiver get their 5 V.

Now I would like to modify this circuit in such a way, that if there is a power cut, the system should switch onto two AA standby batteries for brief period of times (like appx 1 hour or so every few days) . Both PIC & RF receiver can work on appx 3V minimum (PIC can go down upto 2.5V). Now, these batteries are not chargeable, so no voltage should enter in them, I had a eureka moment and thought that it can be done with diodes, but the voltage drop across them makes them unsuitable as I only have around 0.6 V (new AA have appx 3.6V) appx to play with and I can't share it with the diode. Can someone think of a suitable circuit that will make this a possibility(with easily available components)?