Figure 1).
Specifically, the core and I/O voltages track within 0.5V until the core voltage reaches 1.8V. The controlled slew rate limits inrush current to less than 100 mA. Two micropower linear regulators use a very-low-power bandgap voltage reference and a dual op amp. The dual op amp must draw low power, have inputs active to ground, provide rail-to-rail drive, and not reverse polarity as you apply power. Each op amp has an npn transistor buffering its output to provide greater than 100 mA. The regulator loops are stable with these components and values. Simple current limiting accrues from a resistor in series with each 2N3904 collector lead.
Figure 2).
Many USB-powered supplies also require a 5V output.
The circuit of Figure 3 provides precise inrush limiting for 5V and a signal to enable other supplies or loads. The portion of the circuit in broken lines limits inrush current to less than 100 mA at power-on. The 51.1Ω resistor charges the 5V load capacitance to approximately 4.5V, and the 2N3906 then releases the PFET's gate, allowing it to short-circuit the resistor. Finally, the 2N3904 turns off, enabling the linear regulators to start. This inrush circuit precisely limits peak inrush current independently of capacitive load. Use of a large load capacitance prevents load-current spikes from reaching the USB input line.