Oh, you mean Q2-R5 in [1]? That was a late addition, and yes, it's a linear current regulator using a depletion-mode MOSFET. I did that because I didn't want to change the board layout much and it only took two components. Others have built this device, and having good protection circuitry means it works for them, not just me.
Q2-R5 is not the inrush current limiter. The inrush current limiter is U2. That's an AP2553W6.[2]
That's a part designed to solve a specific problem - plugging into a USB port. USB port ICs have overcurrent detection which will quickly turn off power from the port if you try to pull too much power. This keeps external devices from pulling down the +5 rail in a laptop or pad with limited power available. The port turns off until reset. (On Windows this used to take a reboot; Linux usually resets if you close and open the device.)
So if you plug in something with a large filter capacitor, the inrush current as the capacitor charges can momentarily cause an overcurrent condition and shut the port down. The AP2553W6 has both a linear regulator and a switch. When everything is good, the switch is closed and power flows through. If there's a momentary overload, it current limits. If there's a steady overload, it cuts power.
Devices which don't do this power startup properly are often the cause of problems mentioned in searches for "USB port stops working". Not doing this properly saves about $0.25 in parts cost in volume. Such devices will work fine plugged into a USB charger or a desktop machine, but may shut down a USB port on a laptop or tablet.
> When everything is good, the switch is closed and power flows through. If there's a momentary overload, it current limits. If there's a steady overload, it cuts power.
Yes I'm looking for a device family that can do exactly that, but for a large range of currents and voltages (48V, 5A max). Preferably with just two terminals.
That would probably be a self-resetting positive-temperature-coefficient fuse. Those are available for a wide range of currents. They're simple, cheap two-terminal devices.[1]
Those are safety devices. If you're worried about a sudden load introducing a glitch on the 48VDC rail, you may need faster response. It wouldn't have worked in my application, because I need something that will current limit before the overload detector at the power source end trips.
Here's a bigger current limit switch.[2] Versions are available that can handle 48VDC 5A. Texas Instruments calls it an "e-fuse". Not two-terminal, though; it needs some external resistors and caps, plus a connection to ground. Turns off in 280 nanoseconds on a short circuit. This is functionally similar to the part I used, but for higher voltages and currents in industrial applications. TI has many useful tech notes available.
Q2-R5 is not the inrush current limiter. The inrush current limiter is U2. That's an AP2553W6.[2] That's a part designed to solve a specific problem - plugging into a USB port. USB port ICs have overcurrent detection which will quickly turn off power from the port if you try to pull too much power. This keeps external devices from pulling down the +5 rail in a laptop or pad with limited power available. The port turns off until reset. (On Windows this used to take a reboot; Linux usually resets if you close and open the device.)
So if you plug in something with a large filter capacitor, the inrush current as the capacitor charges can momentarily cause an overcurrent condition and shut the port down. The AP2553W6 has both a linear regulator and a switch. When everything is good, the switch is closed and power flows through. If there's a momentary overload, it current limits. If there's a steady overload, it cuts power.
Devices which don't do this power startup properly are often the cause of problems mentioned in searches for "USB port stops working". Not doing this properly saves about $0.25 in parts cost in volume. Such devices will work fine plugged into a USB charger or a desktop machine, but may shut down a USB port on a laptop or tablet.
(This is for USB-A. USB-C is more complicated.)
[1] https://github.com/John-Nagle/ttyloopdriver/blob/master/boar...
[2] https://www.diodes.com/assets/Datasheets/products_inactive_d...