Budget Load-Switch Inrush with the Downstream Circuit Running

A controlled load-switch rise time can reduce capacitor charging current, but the downstream circuit may start drawing current before the rail reaches its final voltage. The upstream supply sees both demands. A calculation based only on capacitance can therefore miss the startup condition that causes a reset or supply collapse.

Estimate the capacitor contribution

Use current equals capacitance times voltage slew rate for the charging component. As an illustrative linear ramp, 220 microfarads rising from zero to 3.3 V in 5 milliseconds requires about 145 milliamps. This calculation assumes that effective capacitance and ramp shape; it excludes operating current drawn by the load.

Texas Instruments’ inrush management report discusses controlling charging current through voltage rise time. Confirm how the selected switch specifies and implements that rise time, including tolerances and whether its behavior changes with load or operating conditions.

Map the load’s startup events

Identify when downstream regulators, processors, displays or other circuits begin operating. Include their input capacitors and any internal startup behavior. A downstream converter can present a changing load while the switch is still ramping.

Where possible, sequence high-demand functions after the rail is established. Check the definition and timing of a power-good output rather than assuming it means every downstream device is ready. A voltage threshold at the switch and successful application startup are different events.

Check the upstream source and switch

Compare the combined current waveform with the upstream supply’s transient capability and current-limit behavior. Include cable and connector resistance. A supply with an adequate steady current rating can still dip or enter protection during startup.

Evaluate switch dissipation during the ramp using its applicable operating guidance. A slow rise reduces charging current but extends time with voltage across the switch. Review repetitive cycling, ambient temperature and the actual capacitance rather than assuming any arbitrarily slow ramp is acceptable.

Test the complete sequence

Capture switch input voltage, output voltage, current and important reset or enable signals. Begin from a defined discharged state and test supported rapid off-on cycles. Residual charge can create a different waveform from the first cold start.

Repeat with the maximum intended downstream population and representative source impedance. Compare the measured charging component and load activation with the budget. If the rail collapses, identify the first current surge or threshold crossing before simply increasing capacitance elsewhere.

Keep the rise-time setting, capacitor assumptions and startup traces with the power sequence. A new module or firmware change that activates a load earlier can invalidate a previously successful inrush design even when the switch and nominal capacitor values remain unchanged.