Not every rail in a schematic comes with its voltage written next to it. Sometimes the net name is cryptic, sometimes the note is missing, and sometimes you measure a rail that looks wrong and want to know what it should be. For most adjustable regulators the answer is in two resistors and one number from the datasheet. This guide shows how to find them, do the calculation and avoid the traps that make the result misleading.

How feedback sets a voltage

An adjustable regulator compares a fraction of its output voltage with an internal reference voltage and adjusts the output until the two match. The fraction comes from a resistor divider between the output and ground, with its middle point connected to the feedback (FB) pin. The idea of feedback is described under feedback loop, and the divider under voltage divider.

The relationship is:

Vout = Vref × (1 + Rtop ÷ Rbottom)

where Rtop goes from the output to FB, and Rbottom from FB to ground.

Step 1: find the feedback pin

On the regulator page, find the controller's FB pin (also named VFB, FB1, FB2 on dual controllers). Follow its net:

  • one resistor goes to the output rail — that is Rtop;
  • one resistor goes to ground — that is Rbottom;
  • there may be a small capacitor across Rtop — a feed-forward capacitor, which does not affect the DC voltage.

How to read the rest of the page is covered in Reading Voltage Regulator Pages in Laptop Schematics.

Step 2: find the reference voltage

Vref comes from the controller's datasheet — the electrical characteristics table, usually listed as the feedback or reference voltage. Typical values for laptop regulators are somewhere between about 0.5 V and 1.25 V, but it varies by part. Make sure you read the datasheet for the exact part number, including any suffix; variants of the same family sometimes have different references. Datasheets in general are described under datasheet.

Step 3: calculate

With Vref and the two resistors, the formula gives the target output. The arithmetic can be done in Regulator Feedback Resistor Calculator, which also shows the nearest standard resistor value if you need to replace one.

A quick example

A controller has Vref = 0.6 V. Rtop is 20 kΩ, Rbottom is 10 kΩ. Vout = 0.6 × (1 + 20 ÷ 10) = 1.8 V.

When the formula does not apply

Fixed-output regulators

Some regulators have the divider inside. Their FB or VOUT pin connects straight to the output, and the output voltage is fixed by the part number. The datasheet states it.

Selectable outputs

Some controllers choose between preset voltages with a pin tied high, low or to a resistor. Others use a resistor from a GPIO to the FB node to shift the voltage when a signal changes — for example to support two memory voltages on the same board. In those cases, the output depends on the state of that pin or signal, and the schematic usually has a note explaining the options.

Digitally controlled rails

Processor and GPU core regulators set their voltage from commands sent by the processor or GPU. There is no fixed divider to calculate; the voltage varies with load.

Remote sensing

Some regulators sense the output voltage at the load rather than at the inductor, using a separate sense line. The divider is still there, but the voltage it controls is the one at the sense point.

Dual controllers

Controllers that make two rails, such as the always-on 3 V and 5 V pair, have two feedback pins, each with its own divider — or, on fixed-output parts, internal ones. Make sure you pair each FB pin with the right output; the schematic usually draws them on either side of the controller.

Pitfalls

  • Wrong Vref — using the reference of a similar part gives a plausible but wrong answer.
  • Swapped resistors — swapping Rtop and Rbottom gives a very different result; check which one goes to the output.
  • Optional parts — the schematic may show two resistor positions, one of which is not fitted on your board. Check the board.
  • Additional resistors — an offset resistor from another net changes the result; include it.
  • Tolerances — 1 % resistors and a reference with its own tolerance mean the real output can differ slightly from the calculation.

Measuring the feedback node

On a running regulator, the FB pin should sit at Vref — that is the whole point of the loop. Measuring it is a quick health check:

  • FB at Vref and output correct — the loop works.
  • FB at Vref but output wrong — the divider is not what the schematic says: a wrong or drifted resistor, or an extra leakage path.
  • FB well below Vref, output at maximum — the loop is open: Rtop missing or cracked, so the regulator cannot see its output and drives it up.
  • FB above Vref, output low or zero — something is forcing FB high, or the regulator is in protection.

Measure at the resistors rather than the controller pin, and be careful: the FB node is a high-impedance point next to switching pins.

Feedback faults in practice

The most common feedback faults on laptop boards are corrosion around the FB node after liquid, a cracked resistor after a drop or rework, and a wrong part fitted by a previous repair. Corrosion is the trickiest, because it creates a leakage path that is not a component at all; cleaning the area thoroughly before measuring resistor values avoids chasing a part that is actually fine.

Using the result in diagnosis

Compare the calculated voltage with the measured one:

  • Within a few percent — normal.
  • Noticeably high or low by a fixed proportion — suspect a drifted feedback resistor, a corroded FB node or a leakage path from FB to ground or to another rail.
  • Zero — the regulator is not running; the feedback is not the problem.
  • Output at the input voltage — a shorted high-side MOSFET, or an open feedback path letting the regulator run away.

Example: a rail at 1.3 V instead of 1.05 V

A chipset rail measures 1.3 V where the net name suggests 1.05 V. The controller's datasheet gives Vref = 0.7 V, and the schematic shows Rtop = 5.1 kΩ and Rbottom = 10 kΩ, which gives 1.057 V — correct. Measured out of circuit, Rbottom reads 6.9 kΩ. With that value the formula gives about 1.22 V; the remaining difference comes from corrosion residue around the FB node acting as an extra leakage path. Cleaning the area and replacing Rbottom brings the rail to 1.05 V. Running the board at 1.3 V for long could have damaged the chipset.

FAQ

What is Vref?

The internal reference voltage the regulator compares its feedback against. It is in the controller's datasheet.

Why is there a capacitor across the top resistor?

It is a feed-forward capacitor that improves the regulator's response. It does not change the DC output voltage.

Can I change a rail's voltage by changing a resistor?

Technically yes, but every chip on that rail must tolerate the new voltage. In repair, restore the original value.

Why is my calculation slightly off from the measurement?

Resistor and reference tolerances. A few percent is normal.

What about processor core voltages?

They are set digitally by the processor and change with load. There is no divider to calculate.

Bottom line

For adjustable regulators, find the FB pin, identify the top and bottom resistors, take Vref from the exact datasheet and apply Vout = Vref × (1 + Rtop ÷ Rbottom). Watch for fixed-output parts, selectable outputs, offset resistors and not-fitted positions. Then compare with your measurement: a rail that is off by a fixed proportion points straight at the feedback network.