Electronic – Limiting supercapacitor charging inrush


This is somewhat a duplicate.

I'm looking for a way to limit the inrush current of a small supercapacitor circuit. Input is regulated 5V (USB), output feeds a 3.3V regulator. Goal is a 10s runtime at 100mA, so the supercap is 5V, around 500mF if my math are correct.

The simplest inrush limiter is well presented in the question above, but I'd like to find something that charges the supercap faster, without loosing the ability to charge it to (almost) the supply voltage. Something like constant current, around 300mA.

The other typical answer to this question is the LTC4425, or another supercap charger from Linear. From what I read, those are

  1. expensive and
  2. designed to handle and equalize two 2.5/2.7V supercap, which isn't warranted given the large availability of 5V supercaps.

What's the intermediate solution?
The system isn't battery powered, so efficiency is not a major consideration.

Best Answer

Answering my own question.

Using a P-channel FET as a current limiter seems to be a quite common technique. A good introduction can be found in this EETimes article.

The technique has been packaged in multiple $1-$2 ICs, including: https://www.fairchildsemi.com/datasheets/FP/FPF2123.pdf http://www.diodes.com/_files/datasheets/AP255x.pdf http://ww1.microchip.com/downloads/en/DeviceDoc/mic20xx.pdf

This page is an example of someone using the Fairchild chip in a supercap charging application.

Using LTSpice, I compared the "limit the current with a resistor" approach (presented in the first link of the question) with the PNP+FET approach (presented in the EETimes article above). The PNP+FET circuit is roughly 3x faster.