1.8V Rail on Navi GPUs Explained

1.8V Rail on Navi GPUs Explained
Type	Circuit
Device(s)	RX 5700, RX 5700XT
Difficulty	◉◉◌◌ Medium

Here you'll find information about 1.8V circuit used on Turing cards.

The Controller Circuit

In Navi cards, you'll most likely find either a GS7260 (Full datasheet not available, used on non reference cards) or MPQ8633A buck converter (on reference cards) that steps down either 12V or 3.3V (Depending on the model) to 1.8V used to power low level logic inside the core and in some models the bios chip(s)

1.8V rail location on the PCB (RX 5700XT Ref) (Figure 1)

Schematic view of the 1.8V rail (Figure 2)

Enable signal for 1.8V rail controller (Figure 3)

The controller steps down 12V/3.3V on the input, from the PCI-E slot.

Enabling this controller is a detection circuit for 12V_BUS and 3.3V shown in figure 3.

V_CC comes from 12V_BUS through a 1 Ohm resistor.

Usage

1.8V is mainly used on Navi to power low level logic inside the core.

Its P_GOOD is used to enable the 0.75V rail

Common Problems

Here are common problems for the 1.8V rail and possible fixes.

No voltage out on the 1.8V rail

First thing to check is the EN signal which should be 3+ V followed by V_CC /V_IN (Figure 2). For the MPQ, V_CC should be 3.3V and V_IN should be 12V/3.3V. For the GS, V_CC should be 12V. if one of them is missing then check the schematic and follow the signal to find the culprit, it's usually a faulty resistor.

After making sure both of the above are there, check the FB circuit, it is possible that one or more resistors are faulty.

Short on 1.8V rail

To find out what is shoring 1.8V, you need to inject 1.8V through the rail and with isopropyl alcohol or thermal camera find out what is heating up. If it is not a capacitor, then the core is a high possibility(!).

No PowerGOOD signal

P_GOOD signal is pulled high through an external resistor to 3.3V. If the output voltage is normal but there is no P_GOOD then you need to check that resistor (Figure 2). If the resistor is fine but the signal is held low by the controller then the controller could be faulty.

Low voltage on 1.8V rail

This usually only happens with a faulty controller or faulty resistors in the FB/COMP circuit. First check if the FB resistors are fine. If yes, then the controller is faulty.