is it necessary to use internal regulator for NFC DC/DC or we can use normal internal LDO in design.
also if we need to use internal DC/DC regulator, what is the inductor requirement other than 10uH and 15nH? can we use two 4.7uH inductors in series with 15nH?
thanks in advance
It is not necessary to use the DC/DC, it is just the wisest choice. Assuming you are running with nRF VDD greater than 1.8v than you are going to waste a lot of power using the internal LDO. At VDD= 3.3V you will waste about 45% of your battery power in the LDO.
Thus everyone uses the internal DC/DC unless they just really do not have the space for the 3 components required to operate it.
On the inductors, sure you can put two 4.7uH in series as long as the combined solution gives you an SRF, saturation current, and DC resistance number similar to the specified 10uH. The net 9.4uH is probably close enough for the DC/DC. Nordic have commented on this before in the devzone. You may want to choose two 5.1uH's for 10.2uH.
There isn't any good reason to do this, it is just that you can if you wish. There are plenty of cheap 0603 parts out there that satisfy the requirements for the 10uH so I can't see a reason to use two 0402 or even two 0201's.
If you are trying to use an 0402 for it there are probably parts that will work too. I haven't looked lately. All of our designs use an 0603 for that part.