An idea for USB inrush current limiter
USB inrush current limiter using a NTC thermister and delay ON circuit with SIMetrix/SIMPLIS simulation.
I am currently working on a device which gets power supply via USB connector as one of my personal project. The device should be able to get powered by USB charger and power bank. I had to put a larger input capacitor than the maximum capacitance which is defined in USB specification. This is why I started considering a simple inrush current limiter.
USB specification
| Parameter | Requirement |
|---|---|
| Max. input capasitor | 10uF |
| Max. inrush | 50uC (Q=CV) |
Chapter 7.2.4.1 Inrush Current Limiting in Universal Serial Bus Specification Revision 2.0
Design idea
Output of the following circuit is fed into a step-down DC/DC regulator. The main functions of this circuit are
- Limit the inrush peak current to less than 100mA using NTC thermistor(100Ohm@25degC)
- When the input capacitor is almost fully charged, pass though the supply voltage
| Parts | Comment |
|---|---|
| V1 | USB VBUS |
| R8 | USB cable resistnce, USB connector contact resistance, PCB pattern registance |
| R2 | NTC thermistor |
| R1, R3 | Define threshold voltage to turn on P-MOSFET Q2 to 4.8V |
| C1 | Input capacitance |
| R5 | Gate current limiter for N-MOSFET Q1 |
| R4 | Load (step-down DC/DC regulator) |