VDD_Main and VDD_Boost fluctuations.

Hi,

We have an odd case at the moment, the user was using the phone when a few apps started to slow down and now the phone won’t boot. The logic board is pristine, not a single short detected.

The phone doesn’t get hot but draws 0.60mA. It won’t enter DFU or Recovery Mode, even with 2UUL ResDog. The Apple logo appears for 2 seconds then reboots. The Resistance of the C12 pad on the Audio IC is good.

Voltages are good in a lot of places too.

1.8V, 1.8V SD RAM, 3V0_NAND, 0V9_NAND.

One peculiarity though is VDD_Boost and VDD_Main. I’ve been looking with ZXW and Blackfish this gives 3.9V while at 0.60mA then consumption drops to 0.44mA and this goes up to 4.2V / 4.4V for only a second, the device then reverts to the original power draw of 0.60mA and 3.9V for both of these lines.

Could anyone tell me a bit more about the VDD_Boost and its functionality, I don’t think I understand it fully. Thank you in advance!