LDO Regulator Application Note

Various protection circuits are built into Ricoh regulators.

The Inrush Current Preventing Circuit

At the start-up of a regulator, a large current flows until the output capacitor is charged up to the set output voltage if the regulator does not have an inrush current protection circuit or soft-start circuit. When the inrush current is large, the input voltage of a regulator may be unstable. If there is a large input voltage drop, the operation of another connected device may be affected.
Ricoh provides two kinds of Inrush Current Protection Circuits, and both have different features.

  Control Method Start up Time
Inrush Current Limit Circuit Limiting with constant current. When COUT is small, it is possible to start up quickly compared with a constant slope circuit.
Constant Slope Circuit Limiting with constant time, so-called Soft Start Function. Start-up time is fixed in the IC and start-up time does not change by the capacitance of COUT.

RP115x have build-in both the inrush current protection circuit and the constant slope circuit, and switches the circuits according to the capacitance of COUT. Please refer to a datasheet for details.

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