The current-limit value in a buck-converter data sheet is not always the maximum regulated output current. Some controllers detect the peak of inductor current; others detect its valley. Since output current approximately equals average inductor current in steady continuous conduction, the ripple waveform changes the relationship between the threshold and available load current.
Identify what is sensed
The TI TPS53318/TPS53319 data sheet, for example, explicitly identifies a valley-current threshold. Read the selected part’s control description, threshold tolerances, and protection behavior. A threshold derived from MOSFET on-resistance may include temperature compensation and additional uncertainty that differs from a dedicated shunt measurement.
For triangular ripple of 1 A peak-to-peak, an illustrative 4-A peak threshold corresponds to an average near 3.5 A at onset. A 4-A valley threshold corresponds to an average near 4.5 A. These are simplified geometric relationships, not permission to operate either hypothetical converter at those currents. Thermal ratings, separate peak protection, and timing limits still apply.
Use worst-case ripple and limits
Calculate ripple across input voltage, output voltage, switching frequency, and inductance tolerance. Inductance can fall with DC bias and temperature. Use the minimum limit to check whether the required load is supported, and the maximum limit to check component stress during faults. These two calculations deliberately use different corners.
| Question | Corner to examine |
|---|---|
| Will normal load trip protection? | Low threshold and relevant ripple extreme |
| Can the inductor withstand a fault? | High threshold and maximum overshoot |
| Will startup finish? | Load plus capacitor charging demand |
| Will the board overheat? | Sustained loss and retry behavior |
Correlate waveforms with protection state
Increase load in a controlled test while recording output voltage, inductor current, and any fault indication. Check probe offset before comparing a valley threshold with the waveform. Observe whether the controller skips pulses, extends off time, folds back, or enters hiccup after an additional timer; the first threshold crossing and complete shutdown may be different events.
Combine this budget with inductor saturation and thermal ratings and minimum on-time constraints. Document the actual sense method with the power budget so a pin-compatible replacement is not approved from a similar headline current rating alone.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist