Review an LDO When Its Output Can Remain Above Its Input

An LDO’s output can remain energized after its input collapses. A large output capacitor, an external interface or another supply on the rail may create that condition. Whether current then flows backward depends on the regulator and surrounding circuit, so inspect shutdown and alternate-power states explicitly.

Identify every output energy source

List output capacitance, batteries, external supplies and connected I/O that can energize the rail. Compare discharge paths at input and output. A lightly loaded output may decay more slowly than an input connected to a switched supply or another active load.

Texas Instruments’ LDO reverse-current discussion describes the issue and several protection approaches. The chosen regulator’s documentation determines its actual behavior. Reverse-current protection, reverse-polarity protection and an enable input are different features and should not be treated as interchangeable.

Check the exact guaranteed states

Read the conditions under which reverse blocking is specified. Determine whether protection applies with enable high, enable low, input absent or only under a particular voltage relationship. Include the permitted voltage at each pin when the input supply is off.

Trace where reverse current would go if it occurs. Charging an upstream rail can unintentionally power other circuits even when the regulator itself is not damaged. A protection measure should address the required system behavior as well as component limits.

Evaluate proposed external circuitry

A series diode, blocking switch or different regulator can change voltage headroom, loss and startup. A diode placed across input and output can provide a current path in some designs rather than block reverse current. Evaluate the intended function and current destination instead of adding a familiar protection sketch without analysis.

For an illustrative stored-charge calculation, a 100 microfarad capacitor at 3.3 V stores about 0.545 millijoules using one half times capacitance times voltage squared. That energy calculation alone does not predict peak reverse current; circuit resistance, inductance and device behavior determine the waveform.

Capture shutdown and recovery

Measure input voltage, output voltage and relevant current during the supported power-down sequences. Include loss of input while an alternate source remains connected. Use a controlled setup within component limits and record both the initial transient and slower rail decay.

Verify the next startup as well. Residual voltage can affect reset, sequencing or the regulator’s behavior even after the immediate reverse-current event ends. Keep the power-state matrix and measured traces with the regulator selection. A board that regulates correctly while powered still needs evidence for the off and partially powered states it can encounter.