Regulator feedback resistor calculator

The formula

Vout = Vref · (1 + R1 / R2) + Ifb · 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 into the divider. Choose the regulator type first: a feedback-pin part has Vref across the bottom resistor, a three-terminal adjustable part across the top one. 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, and the bias current is not constant: it has a maximum and drifts over temperature. Keep the divider current at least 250× the worst-case bias (maximum plus drift), so 1.25 mA for a part that can reach 5 µA. More is better, at the cost of wasted current.
• 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. The Vout range column shows the worst case for the tolerance you pick.
• This tool covers the arithmetic and the resistor tolerance, not the whole error budget. The regulator’s own initial reference accuracy and its drift over temperature, and the bias current’s spread and drift, add to the result. When the output voltage has to be accurate, a fixed-output part with a specified accuracy over temperature is usually the better and cheaper choice.

Related note