The LTC4054 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.
Suitable USB Charging Device
This device operates within a supply voltage range of 4.25 V to 6.5 V, making it ideal for connecting to a computer’s USB port. To ensure the safety of the cells, the charging process is divided into three stages. Initially, a brief phase delivers constant power to the cell. In the subsequent stage, a constant current is supplied, causing the cell voltage to increase linearly. Finally, the device switches to a constant voltage mode, leading to a sharp drop in current.
Smart Charging Management
The LTC4054 features a high-impedance state mechanism when the input voltage falls below a predetermined level, preventing battery discharge. The CHARGE pin (pin 1) indicates the charging status. This open drain output is pulled down to ground via a low impedance during charging, allowing for LED connection to visualize the charging process. In standby mode, where the Li-ion cell voltage is between 2.9 V and 4.05 V, the pin sinks approximately 20 μA to ground. If the cell voltage drops below 2.9 V, the LTC4054 resumes charging. CHARGE transitions into a high-impedance state if the input voltage is not at least 100 mV higher than the cell voltage. In this scenario, the under-voltage lockout circuit activates, drawing less than 2 μA from the cell.