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Application Note : LDO Regulators (Linear Regulators)

Inrush Current Preventing Circuit

At the start-up of a LDO regulator, an inrush current equivalent to a peak current flows until the output capacitor is charged up to the set output voltage if the LDO regulator does not have an inrush current protection circuit or soft-start circuit.
When the inrush current is large, the input voltage of a LDO regulator may be unstable. If there is a large input voltage drop, the operation of another connected circuit 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.

* RP115 has both of the inrush current protection circuit and the constant slope circuit built in, and it switches the circuits according to the capacitance of COUT. Please refer to a datasheet for details.

Inrush Current Limit Circuit

Inrush Current Limit Circuit has a function that limits the output current for a fixed period of time to reduce the inrush current at the start-up and prevents the fluctuation of the input voltage.

Limiting the inrush current(Imaginary graph)

The volume of inrush current is affected by the capacitance of the output capacitor.
The time of the inrush current becomes longer when the output capacitor is large.
A sufficient effect of inrush current limitation might not be achieved when the output capacitor is large.

Limiting not enough because of the limiting time is too short(Imaginary graph)

If it is necessary to limit the inrush current completely in case of using the large output capacitor, Ricoh recommends the product equipped with an Adjustable Inrush Current Limit Time Function.
The inrush current limit circuit of the R1190 Series and RP132 Series has a DELAY pin, and it is possible to adjust the time of the inrush current limitation by connecting a capacitor to the DELAY pin.

Extended the inrush current limiting time(Imaginary graph)

Inrush Current Limit Circuit Included

  • *1 Inrush Current Limit Time can be adjusted by external capacitor.
  • *2 The RP115 prevents the inrush current by the constant current limit circuit during the starting up operation.

Constant Slope Circuit

RP110x25xB/D Inrush Current Characteristics

Some products have a Constant Slope Circuit which prevents the overshoot of the output voltage.
The constant slope is a kind of soft-start circuit which allows the output voltage to start up gradually.

The capacitor to create the start-up slope is built in the IC that does not require any external components. The start-up time and start-up slope angle are fixed inside the IC.

If the capacitance of the external output capacitor exceeds a certain capacitance, the output current limit circuit minimizes the incoming current of the output capacitor at the start-up.
As a result, the start-up time becomes longer and the start-up slope angle becomes less steep.


See "Inrush Current Characteristics Example of RP110x25xB/D" on the right, if the output capacitor is less than 4.7 µF, the Constant Slope Circuit operates during start-up, likewise, if the output capacitor is over 10 μF, the output current limit circuit operates during the start-up.
The boundary point of using these two circuits is inversely proportional to the output voltage.
If the output voltage is higher, the output current limit circuit operates even if the output capacitor capacitance is small.
The boundary points vary with each products.

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