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Question

What is an inrush current preventing circuit?

Answer

LDO Linear RegulatorsProtection

An inrush current is a large current temporarily flowing immediately after the start-up of an LDO linear regulator.
A large inrush current may cause a large voltage fluctuation of the device antecedent to an LDO regulator, which may have a negative impact even on other circuits connected to the device.

To reduce and prevent adverse effects caused by an inrush current, inrush current preventing circuit is incorporated in LDO regulators.

An inrush current limit circuit 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 larger the output capacitor, the longer the time of inrush current becomes.
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 while using a large output capacitor, we recommend products 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)

  Control Method Start up Time
Inrush Current Limit Circuit Limiting inrush current at a certain value When the COUT is small, it is possible to start up quickly compared with a constant slope circuit.
Constant Slope Circuit Limiting inrush current in a certain period of time, so called soft start function The start-up time is fixed inside and independent of the output capacitance.

* The RP115 has both the inrush current protection circuit and the constant slope circuit, and switches over the operating modes according to the capacitance of COUT. Please refer to the datasheet for details.

* To understand a soft-start function in DC/DC converters, please refer to "What is a soft-start function?".

For example, according to Inrush Current Characteristics of RP110x25xB/D below, with an output capacitor less than 4.7 µF, the inrush current at its start-up is regulated by the constant slope circuit.
Likewise, if the output capacitor is over 10 µF, the output current limit circuit operates during the start-up.
The boundary point of these two circuits is inversely proportional: the higher output voltage and the smaller capacitance, the more likely to be operated by the current limit circuit.
Each product has its own boundary point.

RP110x25xB/D Inrush Current Characteristics

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