USB Soft Start Circuit options - Page 1
Forum section: electronics / beginners

author: newtekuser
author_rank: Frequent Contributor
timestamp: August 08, 2024, 10:57:13 pm
content: 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:Section2.4.2 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.

author: ledtester
author_rank: Super Contributor
timestamp: August 09, 2024, 05:11:33 am
content: QuoteThat idea relies on the gate capacitance of the MOSFET - so the value of R_SR depends entirely on the MOSFET you use. It just uses RC-charging of the gate capacitor to slow down the turn on of the MOSFET. Since the gate capacitance of a MOSFET is usually very small (typically in the 10s of pF) you need a R_SR in the megaohm range to get an appreciable RC time constant.You can be less dependent on the MOSFET's gate capacitance (and use a smaller R_SR) by adding your own capacitor as shown in this Falstad simulation:(Falstad simulation link)The scope display at the bottom shows the current through the 100R load resistor. Pressing the switch will show how the current ramps up (and down when the switch is opened.)

author: ledtester
author_rank: Super Contributor
timestamp: August 09, 2024, 05:34:43 am
content: QuoteWhat do you mean by "before" or "after"? As you say, you want the cap to be affected by the soft-start circuitry so in the schematic below it should on the VCC_SWITCHED bus.

author: ledtester
author_rank: Super Contributor
timestamp: August 09, 2024, 05:55:43 am
content: QuoteIt is possible you can leave it unconnected. From the datasheet:Note where it says "The high side MOSFET is automatically turned on if the Vin voltage is applied thanks to internal pull up connected between Vin and EN pin."

author: newtekuser
author_rank: Frequent Contributor
timestamp: August 09, 2024, 06:20:52 am
content: Thanksledtester. Would you recommend going with the solution/circuit using the IRLML6402TRPBF, or the solution/circuit using the NCP330?

author: newtekuser
author_rank: Frequent Contributor
timestamp: August 10, 2024, 12:11:16 am
content: Would this work if I used the NCP330 this way?

author: LoveLaika
author_rank: Frequent Contributor
timestamp: August 13, 2024, 03:58:08 pm
content: I'm not sure how much this will help, but when I was designing my own soft-starter circuit, I used the following links below. I'm not all that experienced or intuitive unlike the other users here, so it took me some time to understand how it works and how to play around with it in simulation. But using those guides as a rough guideline, I incorporated them into my design as a slow starter. It worked for me, but I modified it a bit by replacing the NPN with an NMOS, and I added a resistor across the PMOS like what you did in your circuit.https://circuitcellar.com/cc-blog/issue-304-eq-answers/https://www.onsemi.com/pub/Collateral/AND9093-D.PDF