I am making a portable charger and I need to convert 3.7V to 5V/2A and I was going to use the LTC3872 and its suggested use. However, it says the max input current is .4mA and I don't understand how that will produce 2A. Am I reading this wrong or should I use a different boost converter?

http://www.analog.com/media/en/technical-documentation/data-sheets/3872fc.pdf

 

According to the first graph in that pdf load current is up to 2A , that will need more than 3A input ....

But you're making this from scratch??? seems a lot of work , I'm sure if I looked on eBay I could find a ready made device for a $

 

0.4 mA is the Input DC Supply Current of the chip. The 2A will depends on the current capability of your 3.7V battery.

Check this:
https://forum.allaboutcircuits.com/threads/how-to-design-a-3-7v-to-5v-2a-stepup-circuit.141108/
https://forum.allaboutcircuits.com/...cuit-design-to-power-up-raspberry-pi3.133717/