Typically, 3.7V (final charge or float voltage 4.2V) lithium-ion batteries have been used as primary power sources due to their high energy density per unit weight (Wh/kg) and per unit volume (Wh/m³). In the past, many high-power devices used two 7.4V (8.4V float voltage) lithium-ion batteries to meet power requirements. However, with the availability of inexpensive 5V power management ICs, more and more handheld devices are adopting lower voltage architectures, allowing the use of a single lithium-ion battery. Typical portable medical or industrial devices have many functions and very large (for portable devices) displays. When powered by a 3.7V battery, its capacity must be in the thousands of milliwatt-hours. Charging such a large capacity battery over several hours requires several amperes of charging current.
However, even with such a large charging current, users still want to charge their high-power devices via the USB port when a high-current AC adapter is unavailable. To meet this requirement, the battery charger must be able to charge at a high current (>2A) when an AC adapter is available, while still efficiently utilizing the 2.5W to 4.5W power supplied by the USB port. Furthermore, the IC must protect sensitive downstream low-voltage components from overvoltage events that could result from damage, and efficiently direct high current from the USB input, AC adapter, or battery to the load to minimize power loss as heat. Simultaneously, the IC must safely manage the battery charging algorithm and monitor critical system parameters.
The low 3.6V float voltage of lithium iron phosphate (LiFePO4) batteries makes them unsuitable for standard lithium-ion battery chargers. Improper charging can cause irreparable damage to these batteries. Accurate float voltage charging will extend battery life. Compared to cobalt-based lithium-ion batteries, LiFePO4 batteries offer advantages including higher volumetric energy density (capacity per unit volume) and less susceptibility to premature failure (if new batteries are subjected to deep cycling too early).
Large-capacity batteries require high charging current and high efficiency.
Many portable applications, including industrial and medical equipment, rely on the convenience offered by USB-compatible charging.
Lithium iron phosphate batteries have specific charging requirements, namely a lower float voltage, which offers some encouraging advantages compared to lithium-ion batteries.
