The popular USB interface can charge a portable device while transferring data. But for high-capacity batteries, the 500-mA output current of USB hosts and powered hubs greatly extends the charging time. (Unpowered USB hubs supply no more than 100 mA.) Thus, a system that accepts charging power from an ac adapter as well as the USB port is more convenient. Such a system can charge from a notebook USB port when you're traveling, yet can charge faster via the adapter when you're at home or in the office. An external transistor current source adds dual-input capability to a single-chip lithium-cell charger (Figure 1). The chip, IC₁, operates alone when you connect to USB power and allows you to pin-program it for a maximum charging current of either 500 or 100 mA. When you plug in an ac adapter, which the 600-mA components set, the external-transistor current source, Q₂ and Q₃, turns on and sets IC₁'s charging current to 500 mA. Because IC₁ and Q₂ both charge the battery under that condition, the total charging current is 1100 mA.

Q₂ and Q₃ form a current limiter for the ac adapter. The limiter allows Q₂ and R₁ to pass the additional 600 mA. When the voltage drop across R₁ exceeds that across R₂, which R₂ and R₅ set, Q₂ begins to turn off. Q₂ cancels V_BE, enabling R₁ to more accurately set the maximum current. Voltage across R₃ sets the reference voltage, and the output current limits when the voltage drop on R₁ matches the voltage on R₃. Q₃ should have a beta higher than 200 at 1A, so that IC₁'s  pin can sink enough current to turn on Q₃. High beta also minimizes error in the transistor current source. When IC₁ changes from current mode to voltage mode at approximately 4.15V, IC₁'s  output turns off the transistor current source. IC₁ remains on and finishes off the taper to full charge. It also remains on and continues to function when USB power is gone and only ac power remains.

IC₁ also controls the prequalification current, which is the current level necessary to safely recover deeply discharged cells at low battery voltage. The output assumes a high-impedance state during cell prequalification to ensure that the external current source remains off, and that the prequalification current of approximately 50 mA comes only from IC₁. When you plug in the ac power, Q₁ turns off to prevent back-feeding the USB input. You install Q₁ "backward" with the drain connected to USB input side, so that USB power remains connected to the IN pin (IC₁ pin 4) via Q₁'s body diode, even when Q₁ is off.

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