Dual-voltage regulator meets USB-power needs
Figure 1 This circuit meets USB specs for controlled power-up sequence and slew rate.
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.
A 200-kΩ resistor that connects to the 10-nF bypass capacitor at the voltage reference controls the 1.8V power-up rise time. The resulting rise time is approximately tRISE=20 µA×1.235V/10 nF=2.5 msec. The 3.3V supply follows the 1.8V supply, according to the 10-msec time constant of its 100-kΩ, 100-nF input filter. A small Schottky diode connected between 1.8V and 3.3V guarantees the 3.3V to be within 0.5V of the 1.8V during start-up. Inrush current of approximately 38 mA is IINRUSH=CLOAD(dV/dt), where CLOAD is the total load capacitance, dV=1.8V, and dt=tRISE. The total operating quiescent current of this dual regulator measures just 56 µA, and the worst-case maximum spec for the circuit in Figure 1 is 64 µA. This figure leaves 194 µA available for the load during suspend mode. Figure 1's application requires regulator current of less than 150 mA from each output. You can easily modify the circuit to provide more than 200 mA per channel by substituting a 2N4401 for the 2N- 3904 and adding active current limit with a 2N3906 (Figure 2).
Figure 2 This configuration boosts the output current of Figure 1’s circuit to more than 200 mA.
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.
Figure 3 For USB applications requiring 5V, this circuit provides precise inrush limiting.
- Dual regulator handles two input voltages
- USB Power Delivery 2.0 Enables Power Distribution Flexibility