Most adjustable regulators on a laptop board — buck converters, LDOs, backlight drivers — set their output voltage with two resistors. The regulator adjusts its output until the voltage at its feedback (FB) pin equals an internal reference. Knowing the reference and the two resistors tells you exactly what voltage the rail should have; knowing the target tells you what a missing resistor must be.
How to use it
- Find the regulator's reference voltage in its datasheet (the FB voltage).
- Enter R1 (from the output to the FB pin) and R2 (from the FB pin to ground) as shown in the schematic.
- Read the output voltage.
To find a missing resistor, leave it empty and enter the target output voltage instead.
The formula
Vout = Vref × (1 + R1 ÷ R2)
It is a voltage divider viewed from the other side: the regulator makes Vout whatever it needs to be for the divided voltage to equal Vref. Because the result depends on the ratio R1 ÷ R2, both resistors must be accurate — feedback networks use 1 % parts from the E96 series.
Worked example: a rail that comes up high
A 1.2 V memory rail measures 1.38 V. The regulator's datasheet gives Vref = 0.6 V, and the schematic shows R1 = 10 kΩ and R2 = 10 kΩ, which gives exactly 1.2 V. Measured out of circuit, R2 has drifted to 7.7 kΩ — with that value the calculator gives about 1.38 V, matching the board. The drifted resistor is the fault, not the regulator. The general method is in How to Test a Resistor with a Multimeter (In and Out of Circuit), and reading regulator pages in Reading Voltage Regulator Pages in Laptop Schematics.
Where it matters on a laptop board
- Confirming the voltage a rail should have when the schematic gives only the resistors.
- Diagnosing rails that are high or low by a fixed proportion.
- Choosing a replacement when a feedback resistor is missing or burnt.
Some processor and graphics regulators set their voltage digitally rather than with resistors; this calculator does not apply to them. More on the principle in Feedback Resistors: Working Out a Rail Voltage from the Schematic.
FAQ
What if the datasheet gives a different formula?
Some regulators add a third resistor or use an offset. Follow the datasheet; the basic formula covers most parts.
Why use 1 % resistors in feedback?
The output voltage is proportional to their ratio. A 5 % resistor can move the rail by several percent.
Can I change a rail voltage by changing a resistor?
Technically yes, but only if every load on the rail tolerates the new voltage. In repair, restore the original values.