The circuit in the figure derives its power from a USB port and produces 5 and 3.3V supply rails for portable devices, such as digital cameras, MP3 players, and PDAs. The circuit allows the port to maintain communications while, for example, charging a lithium-ion battery. IC2 boosts the battery voltage, VBATT, to 5V, and IC3 buck-regulates that 5V output down to 3.3V. IC1, a lithium-ion battery charger, draws power from the USB port to charge the battery. Pulling its SELI terminal low sets the charging current to 100 mA for low-power USB ports, and pulling SELI high sets 500 mA for high-power ports. Similarly, pulling SELV high or low configures the chip for charging a 4.2 or 4.1V battery, respectively. To protect the battery, IC1’s final charging voltage has 0.5% accuracy. The CHG terminal allows the chip to illuminate an LED during charging. IC2 is a step-up dc/dc converter that boosts VBATT to 5V and delivers currents as high as 450 mA. Its low-battery detection circuitry and true shutdown capability protect the lithium-ion battery. By disconnecting the battery from the output, “true shutdown” limits battery current to less than 2 _A. An external resistive divider between VBATT and ground sets the low-battery trip point. Connecting the low-battery output, LBO, to shutdown, SHDN, causes IC2 to disconnect its load in response to a low battery voltage. The internal source impedance of a lithium-ion battery makes IC2 susceptible to oscillation when its low-battery-detection circuitry disconnects a low-voltage battery from its load. As the voltage drop across the battery’s internal resistance disappears, the battery voltage increases and turns IC2 back on. For example, a lithium-ion battery with 500-m_ internal resistance, sourcing 500 mA, has a 250-mV drop across its internal resistance. When IC2’s circuitry disconnects the load, forcing the battery current to
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