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I'm currently trying to design a supply using a low dropout regulator capable of 1.5 A that has a high capacitance of up to 1 mF at its output. To limit inrush current I would like to ramp up the output voltage at a specified rate.

Most common LDOs do not have a soft start function and if they do, it is usually not configurable and too fast for such high capacitances (< 0.5 ms).

So my question is: how can one implement an external slow-start circuit for a simple LDO? Is that even reasonable? My thought was that maybe you could change the voltage on the ADJ-Pin in the feedback network in some way to slowly turn on the output? I tried to simulate something like that using an op-amp integrator but it didn't work. I have also found this appnote which discusses something similar but for a boost converter.

My back-up solution would be to use a p-channel MOSFET and an RC-Network (like here).

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  • \$\begingroup\$ What's wrong with putting a resistor in series with the capacitor? \$\endgroup\$
    – Puffafish
    Nov 26 '21 at 9:23
  • \$\begingroup\$ Having 1 mF at the output of an LDO is quite uncommon, does your LDO support that? What LDO are you using, include a link to a datasheet. Also explain WHY you need to limit the inrush current. In my view the inrush current will be limited by the LDO as most LDOs have current a limiting function and over-temperature protection. Also explain WHY such a large 1 mF cap is needed at the output of the LDO. \$\endgroup\$ Nov 26 '21 at 9:30
  • \$\begingroup\$ @Bimpelrekkie Sorry, I forgot to outline the situation a little better. I have not settled on any specific LDO yet. The supply will be powering an RF Module at 3.6 to 4 V. The datasheet recommends such a high capacitance as a bypass to lessen the load on the supply if it is unable to deliver currents up to 2.5 A. Is it safe to use these limiting functions in regular operation? \$\endgroup\$ Nov 26 '21 at 9:42
  • \$\begingroup\$ Then edit your question and add the information. Also include a link to the datasheet of the RF module. It is better to provide more information than you think is needed. Why not simply use an LDO that can deliver 2.5 A? That might be a cheaper solution than adding a 1 mF cap. Is it safe to use these limiting functions in regular operation? If "safe" means: will it not break the LDO? No, the LDO will protect itself (assuming it has that function). As long as maximum ratings are not exceeded, there is no issue. But take a step back, try to not use a 1 mF cap. \$\endgroup\$ Nov 26 '21 at 9:59
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    \$\begingroup\$ Be careful if you use the mosfet solution! Very easy to exceed to safe operating area but yet be far from anywhere near maximum current but yet the mosfet dies. \$\endgroup\$
    – Kartman
    Nov 26 '21 at 10:19

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