Napkin Sketches

Above you see the building block for the USB charger thingy that I'm planning that I sketched out over drinks while waiting for dinner tonight.Up on top is the USB port with the +5 and GND. The D+ and D- pins are connected to a pair of voltage dividers to pull the voltage to +2.0V and +2.8V to indicate the charger is capable of high-current operation.To the right is the current sense resistor (figure around 0.02Ω) which over a 2.1A current would give a 0.042V voltage drop. That pipes into a 100x sense amp.Finally you have a MOSFET to turn the whole beast on and off.I'm figuring that each of these beasties would be on the back side of the board from the USB connector to get the highest packing density.Finally each bank of eight ports will need three bigger (relatively) chips to support. One for the current sense multiplexing. One for the FET control multiplexing and the final one to control the switch to short out D+ and D-. I think I can back this pretty densely so from the top it looks like two 16-port USB plug arrays.Alternately I can shorten the board to be two banks of eight ports.No matter what I think I'd need to run this on a 4-layer process to get the most copper between the voltage source and the USB ports. This has a 68A load going over it so even a small resistance will have an appreciable voltage drop that I'm trying to avoid.