Dual-voltage regulator meets Usb-power needs

This Design idea stems from the limited availability of ICS voltage regulators that can meet key usb-power specs, coupled W ith the need for turn-on sequencing and rise-time control on each output. As always, the for pc-related designs, the minimum cost is a primary motivation. USB specs require all loads to the limit inrush current to less than the plus 50µc of charge when powered on. If permission is granted to increase the load to the MA, inrush limiting could be required again to prevent excursions over The 500-ma limit. The other troublesome usb-power requirement is the "suspend"-current maximum of 500µa, of which, if you have only 250µa; A termination resistor requires the rest. Suspend requires the load to power down and keep alive just enough to listen for permission to power up again. So, the sum of the regulators ' operating currents plus load current must is less than 250µa. The Dual-regulator circuit meets the USB spec and powers an Asics that requires a core voltage of 1.8V and I/O VOLtage of 3.3V to rise with a controlled sequence and slew rate (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. The micropower linear regulators use a very-low-power bandgap voltage reference and a dual op amp. The dual op amp must draw low power, with inputs active to ground, provide Rail-to-rail drive, and not reverse polarity as You apply power. Each op amp have an NPN transistor buffering it output to provide greater than. The regulator loops is stable with these components and values. Simple current limiting accrues from a resistor in series with each 2N3904 collector leads.

A 200-kωresistor that connects to the 10-NF bypass capacitor at the voltage reference controls the 1.8V power-up rise Tim E. The resulting rise time is approximately t_{RISE=20ΜAX1.235V/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 approximatelyinrush=cLoad (DV/DT), where C load is the totalLOAD capacitance, dv=1.8v, and Dt=tRISE. The total operating quiescent current of this dual regulator measures just 56µa, and the worst-case maximum spec for the Circuit in Figure 1is 64µa. This is leaves 194µa available for the load during suspend mode. Figure 1 's application requires regulator current of less than for each output. You can easily modify the circuit to provide more than MA per channel by substituting a 2N4401 for the 2n-3904 and ad Ding active current limit with a 2n3906 (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 is at than. The 51.1ωresistor charges the 5V load capacitance to approximately 4.5V, and the 2n3906 then releases the PFET ' s gate, AL Lowing 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.

Dual-voltage regulator meets Usb-power needs