PEX Rail on Pascal GPUs

This page talks about the PEX rail on Pascal series of GPUs, how it works, what it's used for, and common problems with it.

The Controller Circuit.
PEX is created by a uP1728Q by uPI semiconductor (datasheet) or similar IC by switching 3.3V to 1V Markings on the schematic and board could differ from GPU model to another but the circuit is almost always the same.

The uP1728Q uses 3.3V as Vin coming from the PCIe bus, 5V from 5V Rail at VDD, and gets enabled by the PGOOD signal from the Vcore controller.

The controller regulates the 3.3V on the input to 1V on the output. It regulates that voltage through a resistive divider on the FB pin (Figure 2)

Usage.
PEX rail goes directly to the core to power the pcie express logic on it.

No voltage out.
First thing to check is the EN signal which should be 2V+ followed by Vin which should be 3.3V (Figure 2) then VDD which should be 5V. If one of those is missing then check the schematic and follow the signal to find the culprit, it's usually a faulty resistor. Replace the controller if everything else is in order but still no voltage out.

Short on PEX rail.
Most likely a blown capacitor, feed it external 1V 2A from a LBP and apply some alcohol to spot the faulty capacitor.

If it's not a capacitor then it's the chip that's shorted and in that case you're out of luck..