Calculator KB-04
Voltage Divider Calculator
Choose feedback divider resistors for a DC-DC converter or voltage regulator to get the desired voltage.
Result
| R1 | R2 | Output | Error |
|---|
In brief: Select feedback voltage divider resistors for DC-DC converters and regulators: E12–E192 series, actual output voltage, error, and divider current. Example: Best R1 / R2 pair: 6.2 kΩ / 1.2 kΩ.
Calculation example
| Required output voltage | 5 V |
|---|---|
| Feedback voltage | 0.81 V |
| Series of standard values | E24 |
| Minimum | 1k Ω |
| Maximum | 90k Ω |
| Best R1 / R2 pair | 6.2 kΩ / 1.2 kΩ |
|---|---|
| Output voltage | 4.995 V |
| Error | -0.1 % |
| Current through the divider | 675 µA |
How it is calculated
R1 is connected between the output and the feedback input, and R2 between the feedback input and ground. The controller maintains a voltage U at its inputfb, so the output voltage is determined only by the resistor ratio. The calculator checks all values in the selected series within the specified range and shows the pairs with the smallest error.
What to look for
- Resistors that are too large pick up interference, and the controller’s input current starts to affect the result. Resistors that are too small unnecessarily load the output. The divider current is usually kept between 0.1 and 1 mA.
- The actual voltage also depends on the resistor tolerances and the variation in the controller’s reference voltage, which is typically 1–3% itself.
- You can enter the value with a letter: 4k7, 10k, 1M. For fine adjustment, a trimmer resistor is often used in place of part of R1.
Thanks to user zapimir for the first version of this calculator for the blog.
Frequently asked questions
How do I calculate the feedback voltage divider?
Vout = Vref · (1 + R1 / R2). With Vref at 0.81 V and a 5 V output, 6.2 kΩ and 1.2 kΩ will do: the result is 4.995 V.
What current should flow through the divider?
Usually 0.1–1 mA. At lower currents, the divider picks up interference and the controller’s input current starts to affect it; at higher currents, it needlessly loads the output.
Which resistors should I use for an accurate voltage?
Use E96 or E192 series resistors with 1% tolerance or better. But the controller’s reference voltage itself varies by 1–3%, so the most accurate option is a trimmer resistor in place of part of R1.
