7.9 inch display / Model A1432 / Available in black or white / Announced October 23rd, 2012 / 16, 32, or 64 GB capacity

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Power Issues on VCC_Main

Something pulling VCC_Main voltage down...

Okay, First time replacing PMIC and Tristar on an Ipad Mini.

Symptoms... Worked the night before, put it on charge and next morning dead.

I opened it, put in new charged battery...no change.

Ordered and replaced with new PMIC and no change.

Replaced Tristar, no change

Now... Probing around measuring voltages, PPBAT_VCC is right at 3.9 to 4 and VCC_Main is 3.5.

Took off PMIC and pads read same.

Any suggestions on where to go from here? Q8104?

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I would certainly check Q8104/L8112/D8100 to see of those components are working properly. VCC_MAIN is created by the buck regulator internal to the PMU. Seeing as how you have already changed it, it could be diode or coil related.

That said, with a low VCC_MAIN, I would also suspect that one of the output rails generated by the PMIC is drawing too much current and therefore possibly current limiting VCC_MAIN. Check the other outputs for shorts/voltage levels.

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@refectio , thank you for your suggestions. Would you mind providing example or suggestion on how to check Q8104? L8112 =000.4ohm, D8100 = OL/0.158v... It's quite hard to check voltages on the other PMU outputs as they were all fluxuating and I just realized I need to either get a stencil or another PMU to put it back on :(

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A MOSFET is like a switch. This is a p-type enhancement MOSFET so the channel (from Drain to Source) is normally open (hence the dotted line). If the Gate voltage is lower than the Source Voltage, then the channel conducts. If the Gate voltage is the same as the Source, then the channel opens and no longer conducts. The body diode is inherent in the design.

So you could test the FET with a multimeter in diode mode. Source (Red) to Drain (Black) should read 1.5-3V. If you switch the probes around, then you should read ~0.5V. Unless the FET is still conducting (it "remembers") in which case you will see 0V (channel is shorted). Measuring between the Gate and either Drain or Source should be infinite resistance, or 3V in diode mode (depending on your meter). Of course, all that depends on what else is connected to the FET in the circuit.

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