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Adjustable LDO Output Voltage Calculator
This calculator determines the output voltage of an adjustable LDO regulator from feedback resistor values. It can also calculate the required bottom feedback resistor for a target output voltage. It is useful for adjustable LDOs, low-noise power rails, sensor supplies, analog circuits, and embedded power design.
Input Parameters
Results
In most adjustable LDO circuits, R1 connects from output to feedback pin, and R2 connects from feedback pin to ground. Always confirm the resistor naming convention in the datasheet.
Equations Used
Output Voltage:
Vout = Vref × (1 + R2 / R1)
Required R2:
R2 = R1 × (Vout / Vref - 1)
Feedback Divider Current:
Idiv = Vref / R1
Divider Power Loss:
Pdiv = Vout × Idiv
Where:
Vout = adjustable LDO output voltage
Vref = internal feedback reference voltage
R1 = resistor from output to feedback pin
R2 = resistor from feedback pin to ground
Frequently Asked Questions (FAQ)
Q1: What does this adjustable LDO calculator do?
It calculates the output voltage of an adjustable LDO from R1, R2, and Vref, or calculates the required R2 value for a target output voltage.
Q2: What is Vref in an adjustable LDO?
Vref is the internal feedback reference voltage used by the regulator. Common values include 0.6V, 0.8V, 1.2V, and 1.25V.
Q3: Which resistor is R1 and which is R2?
In this calculator, R1 is the top resistor from output to feedback pin, and R2 is the bottom resistor from feedback pin to ground.
Q4: Does Target Output Voltage affect Calculate Vout?
No. Calculate Vout uses Vref, R1, and R2 only. Target Output Voltage is used only when you press Calculate R2.
Q5: Does resistor tolerance affect output voltage?
Yes. Resistor tolerance directly affects the feedback ratio and output voltage accuracy. Precision rails often need 1% or better resistors.
Q6: What feedback current should I use?
Many designs use tens of microamps to hundreds of microamps through the divider, but the exact recommendation should come from the LDO datasheet.
