The LTC4054 from Linear Technology (www.linear.com) is a very simple device for charging 4.2 V Li-ion batteries. This SMD IC comes in a five-pin SOT-23 package and needs just two external components (the LED is not absolutely essential): a decoupling capacitor of at least 1 μF and a resistor connected to pin 5 (PROG) to set the charging current. The value of 1.62 kΩ shown here gives a charging current ICELL of 600 mA when the device is in constant current mode. The formula
ICELL = 1000 (VPROG/RPROG)
where VPROG = 1 V, gives the charging current in terms of RPROG.
The device works from a supply voltage of between 4.25 V and 6.5 V and is therefore suitable for connection to a USB port on a computer. To avoid risk of damage to the cells, the charging process is divided into three stages. In the first stage, which is brief, a constant power is delivered to the cell. In the next stage a constant current is delivered, and the cell voltage rises linearly. Finally the devices switches to a constant voltage mode, andthe current drops off sharply.
The LTC4054 goes into a high-impedance state when the input voltage falls below a set value to ensure that the battery does not get discharged. The CHARGE pin (pin 1) indicates the charging state. It is an open drain output which is pulled down to ground via a low impedance during charging, allowing an LED to be connected. The pin sinks approximately 20 μA to ground when the Li-ion cell voltage is between 2.9 V and 4.05 V: this is the standby state. If the cell voltage falls below 2.9 V, the LTC4054 begins charging again. CHARGE goes into a highimpedance state if the input voltage is not at least 100 mV higher than the cell voltage. The under-voltage lockout circuit is then activated, with less than 2 μA being drawn from the cell.