5 V Rail on Pascal GPUs

This page explains how the 5V 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 uses RT7296F or similar step down converters to create 5V out of 12V_Bus rail

The RT7296F is a Synchronous Step-Down Converter made by RICHTEK. You can find the datasheet Here 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.

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

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

The controller regulates the 12V on the input to 5V 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 5V controller goes through 0 ohm resistors to the EN pin on the 1.8V controller.

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

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

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

Short on 5V 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 5V and might be causing a short: Everything marked with red is potential short location.

No PowerGOOD signal.
PG signal from the controller is the same signal that enables the 1.8V controller, if the 5V buck converter doesn't form that signal the rest of the card won't work. The most likely cause is the controller itself. If 5V 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.