How it works
A switching regulator such as a PMIC adjusts its output so that the feedback pin voltage equals an internal reference, Vfb. The most common way to set the output is a divider made of a top resistor Rtop and a bottom resistor Rbot.
Vout = Vfb × (1 + Rtop / Rbot)Rtop (ideal) = Rbot × (Vout / Vfb − 1)Vout,max = Vfb(1+a) × [1 + Rtop(1+t) / (Rbot(1−t))]Vout,min = Vfb(1−a) × [1 + Rtop(1−t) / (Rbot(1+t))]
t: resistor tolerance, a: Vfb accuracy
The calculator scans every standard value in the chosen Rbot range. For each Rbot it finds the standard Rtop closest to the ideal value. It then lists the five pairs with the smallest actual output error.
Worked example
With Vfb 0.6 V, target 3.3 V, Rbot 10 kΩ to 100 kΩ, the E96 series, 1 % resistors and 1 % Vfb accuracy, the best pair is Rtop = 115 kΩ and Rbot = 25.5 kΩ. The actual output is about 3.306 V (+0.18 %), and the divider current is about 23.5 µA.
Including both the resistor tolerance and the Vfb accuracy, the maximum output is about 3.39 V (+2.9 %) and the minimum is about 3.22 V (−2.4 %). Check the target voltage against this range.
Design tips
- Start from the Rbot range recommended in the datasheet and pick the pair inside it. The recommended range accounts for the feedback pin bias current and noise.
- A larger Rbot reduces the divider's standby current and improves light-load efficiency. It also makes the feedback node more sensitive to leakage and noise, so do not go too high.
- A feed-forward capacitor (tens of pF) in parallel with Rtop can improve transient response and phase margin. Follow the datasheet formula for its value.
- Vfb accuracy changes with temperature and input voltage. Use the full-temperature-range value from the datasheet to be safe.
Limitations
- The calculation includes only the resistor values. Feedback pin bias current, PCB leakage and resistor temperature coefficients are not included.
- Standard series are E24 and E96. E96 has finer spacing but more part numbers to stock.
- Results are for early design review. Confirm the final values by measuring the output voltage on the real board.
Frequently asked questions
How should I choose Rtop and Rbot?
First check the Rbot range the datasheet recommends. Without a recommended range, values from a few kΩ to a few tens of kΩ are common. A very small Rbot wastes standby current; a very large Rbot makes the feedback pin more sensitive to leakage and noise.
If the calculated error is 0.5 %, will the real output also be within 0.5 %?
No. The calculation uses only the nominal resistor values and nominal Vfb. The real output is also affected by resistor tolerance, Vfb accuracy, temperature and feedback pin bias current, so judge the design by the maximum and minimum values within the tolerance range.
Should I use E24 or E96?
E96 has finer spacing, so it can find a pair closer to the target voltage, but it means more part numbers to stock. Use E96 when you need precision and E24 when inventory is the priority, then recheck the error.