Brain-dead iPhone 13 Won't Turn On, No voltage on VDD_MAIN

Context: This customer sent me their iPhone 13, which was working without issues until one day, when it turned off and stopped working while charging. Upon further inspection, the phone appeared pristine and unopened, with no signs or traces of water damage or previous repair.

I have never seen this happen, and this fault point is rare. My only explanation for this fault is that a voltage surge may have been caused by using a non-MFI charger, which potentially damaged an essential chip on the device's board. The customer confirmed that he was perhaps using a cheaper charger he had bought from a gas station, as he had lost his original one.

Symptoms: When the board is connected to a power supply, a healthy board will output around 4 volts on the main line (PP_VDD_MAIN). That voltage for this board was not present, which is very rare. The power supply is essential because it supplies power to every circuit in your phone and puts the majority of chips into standby mode.


(When connected to the battery, around 4V should be present on VDD_MAIN)

It must be present before the PMU (power management unit) can be put on standby. Once it does, it will send all the important power rails to your NAND and CPU chip and begin the boot process.

Luckily, using schematics, it's not hard to find where PP_VDD_MAIN is outputted. It first goes from the battery to the battery connector and then through a resistor that powers the U4200 chip (charging IC), which outputs VDD_MAIN.


From J10000 (Battery connector) U4200 (Charging IC) will be turned on

From that, we can immediately deduce that our fault is U4000, since it's dependent on 4.0 volts being shown on VDD_MAIN.

With that out of the way, we can desolder the chip, grab another from a donor, and replace it. Reball if needed. You can perfect-pull using flux and proper handling.