What will happen if the input voltage becomes lower than the output voltage?

The output capacitor sometimes holds electric charge after the whole system is turned off. In that occasion, the input voltage may be lower than the output voltage.

In this case, reverse current may flow from the output to the input. If the resistance in the input circuit is high and the reverse current is limited to 30 mA or around, the IC will not be damaged. However, if large reverse current (several hundred mA or more) flows for a few seconds, a schottky barrier diode (SBD) must be inserted between the input and output (Fig. 1).

Moreover, if the voltages applied from another source to the Vout exceeds the input voltage by 0.3 V or more, reverse current flows inside the IC via the parasitic diode, which may activate parasitic elements as electrical paths for large current, and the LDO regulator may be damaged. In order to protect regulators from reverse currents, insert an SBD between the input and output (Fig. 2).

Our LDO regulators including reverse current protection do not need external components such as an SBD.

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