Battery charger ICs evolve to meet new demands

Battery charger ICs evolve to meet new demands

Some recent advances in charger ICs can help distribute power quickly and for changing needs

BY TOM DELURIO
Advanced Analogic Technologies (AnalogicTech)
Santa Clara, CA
http://www.analogictech.com

Today’s smart phones, navigational devices, and Mobile Internet Devices (MID) pose tremendous new challenges to battery charger IC designers. Their escalating capabilities, ranging from larger memories and higher resolution screens to a dizzying array of new features, now demand higher capacity batteries that, in turn, are redefining battery charge rate requirements. At the same time, the sheer complexity of these new portable systems requires new approaches to power management and battery charging to ensure the system continues to operate without interruption throughout the charging cycle.

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.

Linear vs. switching topologies

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 battery charger ICs take advantage of the higher efficiency of switching devices. These new switch-mode battery charger ICs offer designers an opportunity to supply much higher current to the battery than comparable linear chargers while using less power. Historically battery charger IC designers have stayed away from switch-mode topologies for two reasons: the switching function generates higher noise levels than typical linear chargers and they require a higher external component count. The noise generated by these devices is primarily generated during light load operation, particularly during preconditioning and taper charge modes. As the charge current decreases, some switching chargers will enter pulse skipping operation where the PWM frequency changes in an asynchronous manner. This characteristic causes system level EMI noise issues that are difficult to filter.

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 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.

Fig. 1. The switching/linear battery charger supports constant current charge rates up to 2 A.

Dynamic control

As handheld designers integrate a host of new functions into their systems, particularly capabilities such as GPS, Wi-Fi, codecs, and FM receivers/transmitters, today’s the battery charger ICs require more sophisticated control of the power source. Many of these new functions must be powered continuously either from the battery, or during the charge cycle, from the charging source. Ideally these complex systems need a dynamic battery charger capable of automatically switching between the charging source and the battery as load demands change.

Figure 2 depicts a linear battery charger IC with these new capabilities. This single-input battery charger and power control IC is compatible with either an ac power adapter or USB port power source. The IC’s power control circuit allows the system to charge a single-cell Li-ion battery and power a load simultaneously.

Fig. 2. The dynamic battery charger IC simplifies system power management by automatically routing power for the battery to the system when load requirements exceed adapter capabilities.

This capability allows the charger IC to continue to power many functions while providing maximum charge rate to the battery. Charge regulation pass devices control the charge current or voltage from the adapter input power to the battery. Two additional load switches control and route input power to supply the system load while also managing power from the battery to the system load when needed. This charge control and switch array permits dynamic charging of the battery cell and control of power to the system load simultaneously. When an input power source is applied, the adapter input will provide power to the system load and charge the battery. If an outside power source is not available, the battery automatically powers the system. The battery disconnects from the load if the cell voltage falls below 2.9 V to protect the battery cell from over-discharge which results in shorter battery life.

Today’s increasingly complex and feature-rich portable electronics devices need more sophisticated charging solutions than ever before. New features and functions are driving up battery capacities and placing new demands on battery charger efficiency and charge rate. At the same time the widespread adoption of “always-on” wireless features require battery charger ICs capable of managing system loads and automatically rerouting power from the battery to the system when necessary. The recent introduction of hybrid switching/linear chargers and dynamic dual-path devices offer designers an exciting new opportunity to address these emerging issues. ■