Feedback Resistor Calculator | Delta Method

Instructions

  1. Enter the Feedback Pin Voltage (min, typ, max) usually found in the main IC datasheet.
  2. Set the Target Output Voltage (min & max bounds).
  3. Choose the Resistor Series. Eg: E12, E24 and so on. The resistor tolerance is set separate.
  4. Adjust additional parameters or keep the defaults(recommended).
  5. In the result table, adjust the slider to sort and highlight the pair with the best balance of accuracy and quiescent current.

Note: Below lookup provides ppm rejects:

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About Feedback Dividers
Schematic Diagram
Vout = VFB (1 + Rup / Rdown) + Rup IFB

Feedback resistor dividers are commonly used to set the output voltage in DC-DC converters, such as buck, flyback, LDO, etc.

The regulator IC maintains a constant voltage VFB at the feedback pin to match its internal reference, while the resistors Rup, Rdown scale VFB to a higher target output voltage Vout. In practice, the internal error amplifier sinks or sources a tiny bit of bias current, known as feedback leakage current IFB, which creates an undesired offset (Rup IFB) in the output voltage.

When accounting all these parameters and their manufacturer tolerances, a traditional worst-case analysis often leads to over-engineering. This tool evaluates component tolerances through gaussian distribution model and calculates the Vout min/max using Second Order Delta Method, providing much more realistic and practical design data, matching the precision of Monte Carlo simulations

Besides this, there is a current IRdown that flows through Rdown which contributes to overall quiescent current of the converter. Minimizing IRdown is critical for battery connected converters. This tool will help designers choose resistors with best balance of statistical Vout accuracy and IRdown current.

Step 1: IC Specification
Min
Typ
Max
Sigma
Step 2: Design Target
V
V
ppm
Targeting 4.9σ
Step 3: Resistor Options
Series
Tol (%)
Sigma
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