The relatively flexible battery charging profile of a Li-Ion battery, and the wide variety of available batteries and battery charger ICs on the market, makes the design even more difficult, especially if battery management is a new task for the engineer. Understanding the key parameters and features of battery charging solutions allows us to better examine pros and cons and create a more sophisticated decision process. The ultimate goal is to balance the ability to provide good consumer experience (charging time) with a robust and safe system design, while keeping the cost at acceptable levels. An analysis of the different charging topologies, the several charger IC features and some of the key charging parameters permits design engineers to make a more intelligent choice.
Battery selection
It is good design practice to always start with an analysis of the average system power, board space and cost requirements. Since in most cases these requirements do not go well together, trade-offs have to be made on a case-by-case basis. It is extremely important for design trade-offs to take place early in the development process; last-minute changes can impact the entire power management sub-system as well as other application specifications. The average power requirements of the application, in combination with an acceptable usable battery life will determine the necessary battery capacity. Choosing a specific battery capacity and form factor can result in significantly lower cost if the same battery type is widely adopted in high-volume portable applications. As we will discuss in the following paragraphs, higher battery capacity can also translate to a longer charging time, unless this is proactively addressed in the system power design. Last but not least, requirements for weight and specific form factor can also pose a challenge while trying to meet the system's power requirements and usable battery operating life.Once the battery type, make and model are selected, the recommended charging algorithm is available from the battery manufacturer (battery datasheet). Deviation from the datasheet specifications is not recommended, since it could lead to early battery degradation. Even though the charging profile of Li-Ion batteries is well defined, certain charging parameters can be adjusted based on system needs (see Figure 1). For example, charging current needs to be below a certain level for reliability (usually 0.5 to 1C), however the system might not be able to provide this current level because of power dissipation or other design constraints. The termination current threshold is another parameter that can differ from one design to the next, since delaying the charge termination from a certain point onwards does maximize battery capacity but results s in a significantly longer charge cycle. Hence, system requirements need to be carefully analyzed before finalizing the power conversion and battery management architecture.
Charging topology
The selected battery capacity and target charge cycle times also determine the charging IC architecture to be used for a specific design. For small battery capacities, a linear-mode charging solution is ideal because of low cost and complexity. As the required charge current levels increase, a switch-mode topology becomes inevitable. The significantly higher efficiency of a switch-mode battery charger IC allows for higher charge current levels (i.e. shorter charging times), and at the same time minimizes hot spots, a key issue in compact designs. A switch-mode charger IC is also more desirable in systems that use unregulated or simply higher-voltage wall-adapters, since their power dissipation is not a direct function of input (adapter) to output (battery) voltage differential. New linear charger IC offerings do incorporate current foldback. This allows the charge current to be reduced as the charger IC die temperature increases, thereby protecting the IC itself. The downside of this operational mode is longer charging times that are the result of the thermally-reduced charge current.
Charging IC features
System philosophy will define the necessary secondary protection features for a specific design. The primary protection features (over-voltage, under-voltage, over-current, etc.) are always integrated into the battery pack, and are the responsibility of the cell itself, as well as the battery protection circuitry in the pack. However, given the "sensitivity" of Li-Ion battery chemistry and consumer perception (as a result of the few but significant accident reports), a lot of applications incorporate additional protection in their designs. This becomes especially important when certain standards need to be met, like the recently introduced IEEE 1725 from the IEEE Power Engineering Society.One of the most popular safety features in hand-held system designs is the monitoring of cell temperature. High temperature levels are the main source of Li-Ion instability, therefore battery packs incorporate a thermistor element that can monitor temperature levels and provide this information to the "outside" world. In response to this, many of the charging ICs have the ability to "read" this thermistor output and suspend charging when the cell temperature is outside a battery manufacturer's specified range (usually 0 C to +45 C). Another very important protection feature is the monitoring of the battery voltage level. Protecting the battery from an over-voltage condition is one of the main functions of the protection IC located inside the battery pack. Having secondary protection allows for higher system reliability and meets the most stringent safety requirements in the industry. Input over-voltage protection is also a desirable feature in many new designs. It prevents the charging IC from charging when the input voltage is higher than a specific voltage threshold. This feature is extremely important when companies are concerned with their devices being connected to "non-compliant" wall-adapters. Additionally, safety (also called charge) timers are used frequently in babattery charging applications. These timers provide protection against defective battery cells by suspending charging when the duration of the charging process exceeds the time expected under normal charge (and operating) conditions. This is a parameter that is adjustable in most charging ICs, since it needs to allow some design flexibility associated with charge current levels, and therefore normal charging duration.
Charging IC parameters
The most critical parameters to look after when selecting a battery charging solution are input voltage, float voltage and charge current. Input voltage seems trivial, since everyone knows that it defines the voltage that the charging IC solution can accept from a wall adapter. However, in many cases a wider (i.e. higher) input voltage operating range can have a positive effect on total system cost, since is eliminates the need for a well-regulated wall adapter. Float voltage level and accuracy are two of the most important parameters when charging a battery. A higher than expected float voltage degrades the life of the battery and could allow the battery pack to enter an over-voltage condition, resulting in charge suspension. On the other hand, a lower than expected float voltage leaves the battery cell under-charged, thereby reducing the battery's usable operating life. In addition to float voltage accuracy, system designers need to start taking into account short-term transitions to new battery technologies with higher float voltage levels. The high majority of today's charging ICs provide a 4.2V float voltage, and only very few solutions exist that are able to address the new higher float-voltage requirements. As mentioned earlier, the battery pack datasheets provide recommendations on charge current levels that ensure safety and reliability. While the charger IC should ideally provide the maximum current provided by the battery manufacturer in order to achieve the shortest charging times, in reality there are many system parameters that affect the "real" charge current level. It is recommended that power dissipation under worst-case system conditions be analyzed to ensure that "advertised" charge current for a specific charger IC is feasible. Charge current accuracy is also very critical, since the more precise the current setting the less design headroom is required for the system. For example, in USB-powered charging applications, the host device needs s to draw less than 500mA to meet the USB2.0 specification, but at the same time draw a current level that is as close to 500mA as possible to accelerate the charging process.
Summary
The tremendous popularity and adoption of the Li-Ion and Li-Polymer battery chemistries in consumer applications has resulted in the development of a high number of battery charging ICs by various manufacturers. While this wide product offering allows for great design flexibility, it can also delay or complicate the selection process. At the same time, an increasing number of engineers is getting involved in the design of the battery charging circuitry. Understanding the key charging parameters and functions, and doing the appropriate prep-work on the system requirements, can significantly accelerate time-to-market, optimize system performance and ensure safety and reliability.
