Current Limit Sense⚠ unverified
Electrical / Power Electronics · Compute the peak current limit set by a sense resistor
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| Rsense | Rsense | — | 1.0 | Sense resistance |
| Vlimit | Vlimit | V | 1.0 | Comparator limit voltage |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | Ipeak | A | Peak current limit, in amperes (A). Returns 0.0 when ``Rsense`` is not positive |
The science & history
Understanding the Parameters
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$R_{sense}$ — resistance of the shunt in series with the path being limited (switch, inductor, or load return). Smaller $R$ means less voltage drop and less loss ($I^2 R$) but needs a lower noise floor or gain to resolve the same $V_{limit}$. The live calculator currently labels $R_{sense}$ as dimensionless; it is ohm.
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$V_{limit}$ — the voltage the control IC or comparator treats as "too much current" (fixed internal reference, external DAC, or resistor-programmed threshold).
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$I_{peak}$ — the trip current. Designers choose $R_{sense}$ so that normal peak (load + ripple + margin) stays below $I_{peak}$, while faults exceed it quickly.
Derivation (Approaching a Proof)
By definition of resistance (Ohm's law), the voltage across the sense element is $V_{sense} = I\,R_{sense}$. At the trip boundary $V_{sense} = V_{limit}$, so
$$I_{peak} = \frac{V_{limit}}{R_{sense}}.$$
No dynamics are modelled: this is the static threshold. Real circuits add blanking time, leading-edge blanking, filtering, and temperature coefficients of both $R_{sense}$ and the comparator reference.
Power loss in the shunt at trip is $P = V_{limit}\,I_{peak} = V_{limit}^2/R_{sense}$, another reason milliohm shunts (or lossless current mirrors / Hall sensors) are preferred at high current.
History
Resistive current sensing is as old as ammeters; in SMPS it became standard for cycle-by-cycle current limit and peak-current-mode PWM (Unitrode/TI-style controllers from the 1980s onward). Modern designs also use $R_{DS(on)}$ sensing or DCR sensing across the inductor to avoid a discrete shunt.
Related Concepts: Conduction Loss, Buck Inductor Ripple, Diode Forward Loss, Watt's Law
Notes: Registry calculator current-limit-sense (unverified). Static Ohm's-law model.
Registry unit bug: $R_{sense}$ labelled dimensionless — should be ohm. Flag for Track B.