I have a board that is USB powered, literally just tapping into VBUS without any downstream devices that need actual USB support, so don't care about protocol compliance etc.
However, I am in a situation where the board draws up to 100mA on start-up from all caps I have on the PCB, which sometimes causes issues with the operation of the device on start-up.
I would like to add a soft-start circuit to limit the in-rush current to around 20mA which is the idle power draw of my device.
Possible solution #1:
Do I add my 100uF bulk capacitor before or after the mosfet? My intuition says before since I want the charge time of the capacitor to be slowed down.
Possible solution #2:
https://www.ti.com/lit/an/slva670a/slva670a.pdf1. For the discrete option, how do I calculate component values?
2. For the voltage regulator option, what options would you recommend? I found NCP330 mosfet. The data sheet shows a sample application in Figure 1., but in my case I can't control the Enable pin so, maybe just tie it to VBUS instead?
https://www.onsemi.com/pdf/datasheet/ncp330-d.pdfPossible solution #3:
Section
2.4.2 Bulk capacitance vs. inrush current – USB peripheral deviceshttps://www.ftdichip.com/Support/Documents/AppNotes/AN_146_USB_Hardware_Design_Guidelines_for_FTDI_ICs.pdfSame question as earlier, do I add my bulk capacitor before or after the mosfet?
My desire is to go with an integrated solution, but the discrete option works too as I'm not space constrained.