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Regulator feedback resistor calculator
Every adjustable regulator and DC-DC converter sets its output with two resistors against an internal reference. Pick the reference, say what you want out, and get the standard pairs that land closest.
VOUT — — ≥ 100× FB bias · three-terminal: 5–10 mA R1 — · R2 —
Best standard pairs
The formula
Vout = Vref · (1 + R1 / R2) + Iadj · R1
When to use it
Use it for any part whose datasheet gives the output as a function of a feedback divider: LDOs, buck and boost converters, and the classic three-terminal adjustable regulators, where the Iadj term matters because the adjust pin sources a small current through the top resistor. R1 sits between the output and the feedback pin, R2 between the feedback pin and ground.
Pitfalls
- Read the reference voltage from the datasheet, not from memory: 0.6, 0.8, 1.0, 1.21 and 1.25 V are all common and a wrong Vref gives a wrong board.
- The divider current must swamp the feedback pin’s bias current — at least 100× it — or the output drifts with temperature. Aim for tens of microamps to a few milliamps.
- Three-terminal adjustable regulators also need a minimum load current to regulate; the divider itself is usually that load, so keep it around 5 to 10 mA for those parts.
- Use 1 % resistors. A 5 % pair can put the output outside the tolerance of what it powers before you have even added the reference’s own error.
Need the whole board designed, not just the feedback divider?