Checking Quick Output Discharge on a Rail With Another Power Source

A load switch with quick output discharge can pull its output down after it is disabled. That helps establish a known rail state, but it can also draw current from another source connected to the same output. Review every source path before treating the discharge feature as harmless cleanup after shutdown.

Identify the discharge connection

Read how the selected switch enables and connects its discharge element. The TI TPS22918 data sheet describes a configurable QOD path using internal and optional external resistance, or leaving the function unused according to its specified connection method.

Draw that path explicitly in the off-state schematic. Include output capacitance, downstream loads, other regulators, connector power, and signal related backfeed paths. A disabled main switch does not necessarily mean the output has no energy source.

Calculate the intended discharge

For an isolated capacitor discharging through a fixed resistance, voltage follows an exponential response with time constant equal to resistance times capacitance. A hypothetical 100 ohm path and 10 microfarad capacitor have a 1 ms time constant. The time to a particular logic or reset threshold depends on the initial voltage and threshold.

Use the actual discharge resistance range and operating conditions. Internal resistance may vary with voltage and temperature, so a nominal RC estimate is only a starting point for the required timing.

Check externally sustained current

If another source holds the output at a voltage while QOD is active, the path can conduct continuously rather than only dissipating stored capacitor energy. A hypothetical 3.3 V across 100 ohms would produce 33 mA and about 109 mW in that resistance, before other circuit effects.

Compare the resulting current and dissipation with the selected device and external component limits. Also review reverse current through the main switch separately; quick discharge and reverse blocking are different functions.

Verify the complete power sequence

Measure output voltage, discharge timing, and current with every realistic combination of main supply, alternate supply, and control state. Include accessories or programming connections that can keep the downstream rail alive.

Save the off-state path diagram, resistance and capacitance assumptions, measured waveform, and permitted source combinations. If the design requires the rail to remain powered from another source, use a supported configuration that respects that requirement. A successful shutdown test with all alternate sources removed does not establish behavior when the finished product is connected to the rest of its system. Record the source combination associated with each observed current.