5 V Rail on Pascal GPUs

This page explains how the 5 V rail is created and what it is used for in Nvidia Pascal GPUs and the most common problems that could occur.

The Controller Circuit

Pascal cards typically use either an RT7296F or MP1475 buck converter to create 5 V out of 12V_Bus rail

The RT7296F is a synchronous step-down converter made by RICHTEK. You can find the datasheet Here

MP1475 is manufactured by Monolithic Power Systems and its datasheet can be found here

5V rail location on the board (Figure 1)

5V rail schematic (Figure 2)

Many GPU boards don't have component marking on them and if they do sometimes marking can be different from the schematic above but the circuit itself is the same. U2 on figure 1 is U27 on the schematic and so on.

5 V is a fairly simple rail that is usually the first to turn on. The controller gets enabled by 3.3 V rail through a 1 kΩ resistor, capacitors are used to smooth out the voltage.

Most controllers need 5 V on the VCC pin to operate but the RT7296F generates its own 5 V to its VCC pin. It uses 12V_F (12V_Bus fused) to feed the Vin pin with some capacitors.

The controller regulates the 12 V on the input to 5 V on the output. It regulates that voltage through a resistive divider on the FB pin (Figure 2)

Lastly, if the power output is within 90% of the normal voltage the PG (Power Good) pin will go high, used in power sequencing. In most Pascal GPUs PG pin on the 5 V controller goes through 0 Ω resistors to the EN pin on the 1.8 V controller.

Usage

The 5 V rail is most commonly used to power the VCC pins for other ICs on the board. Below are examples for 5 V powering on the PEX and Vmem ICs.

5 V powering on Vmem controller on a GTX 1060 (Figure 3)

5 V powering on PEX rail on a GTX 1080 (Figure 4)

Common Problems

Here are common problems for the 5 V rail and how to fix them.

No voltage out on the 5 V rail

First thing to check is the EN signal which should be 2+ V followed by Vin which should be 12 V (Figure 2) 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 VCC pin. If there is no 5 V on it then most likely the controller is faulty and needs changing. If there is 5 V on VCC, check out the resistor divider and make sure their values are the same as in the schematic (Figure 2).

Short on 5 V rail

5V supplies the VCC voltage for almost all the ICs on the board, thus you should look there first. If not, a capacitor might be shorted. To find the source of the short, Follow the steps in this guide: Base Voltage Rail Short on Pascal GPUs.

Here is everything that is connected to 5 V and might be causing a short:

Front 5 V shorts on pascal GPUs (Figure 5)

Back 5 V shorts on pascal GPUs (Figure 6)

Everything marked with red is potential short location.

No PowerGOOD signal

PG signal from the controller is the same signal that enables the 1.8 V controller, if the 5 V buck converter doesn't form that signal the rest of the card won't work. The most likely cause is the controller itself. If 5 V is being outputted, however, if even after replacing the controller you still don't have PG then check the FB circuit (Figure 2) and make sure every resistor is the right resistance.