How does the designer ensure these new higher-capacity batteries are charged in a reasonable amount of time? And what role can the battery charger IC play in the intelligent distribution of power when system load requirements suddenly change? This paper will review some recent advances in charger IC development and how they offer designers promising solutions to these problems.
Most portable systems currently available use simple linear battery chargers. These devices are highly attractive to designers because they are relatively easy to implement and require a minimum number of external components. Moreover, a linear charger generates minimal noise because it uses no switching components. Its primary liability is its relatively high power dissipation through the linear regulator pass FET. Therefore, as long as battery-charging current requirements stays low, the linear charging topology offers an attractive option.
Unfortunately as portable devices add new functions, they require higher levels of battery capacity. In this environment the higher power dissipation associated with a linear charger presents a significant liability. If the portable-device user wants to both use the device and charge the battery at the same time, a linear charger will generate significant heat and potentially damage the system or the battery. Thermal charge reduction techniques can be used to manage heat dissipation, but they extend the charge cycle.
To supply higher levels of current to the battery some new Camera battery charger ICs take advantage of the higher efficiency of switching devices.
To address this problem and take advantage of the unique characteristics of both charging topologies, some power semiconductor manufacturers have introduced new battery charger ICs that can supply high charge current with minimal thermal impact to the system using a switching charger and then switch into a linear charger during low current charging modes to minimize noise (see Fig. 1). This new type of PWM switch mode charger with a linear mode provides high efficiency at the full constant current (fast charge) rate. The switching charger for Battery controls the constant current charge mode up to 2.0 A with a PWM switching regulator and automatically switches to linear mode during the battery preconditioning mode and near the end of constant voltage taper charge mode. This capability is especially useful when charging a portable device during the longer taper charge mode since the switch mode can be used to accelerate the charge cycle. Once the charge current level dips below 300 mA, the linear mode takes over and noise generated by the switching converter is eliminated.