I am currently having this issue... The iPhone would show apple logo then immediately go to dfu mode then power off and repeat. I have now plugged the device into the wall using my iPads transformer and with that in place the device does not power off now but it stays stuck in dfu mode now. I try to put the device in restore mode with iTunes logo but I am getting no luck. Here are some possible explanations: 1. The juice of charge doesn't surpass a certain % just stays at the level of being able to just power on (I tried this with battery disconnected from MB and was getting same result: power on-dfu-off 2. Completely corrupted software. Right now. I'm leaving the device in dfu mode with the iPad charger. Hopefully will be able to restore. Will let everyone know soon. (1) UPDATE: SUCCESFULLY OUT OF BOOTLOOP! USE AN IPAD CHARGER!
I am going through this as well. However, I damaged the front camera port on the MB and this is why my device probably doesn't boot up properly. Check the connectors on the MB. One of the replacement screens I bought didn't have the connectors properly aligned so I kept pushing & pushing hard and bent the pins.