Battery life is arguably the most important feature in many of today's high-volume portable electronic products. While much of the portable electronics industry has migrated to the lithium-ion battery, drawn by its high capacity, small size, light weight and durability, no similar consensus has been reached about the battery charger IC.
As the power component that controls Li-ion charging conditions, battery charger ICs play a pivotal role in portable system design. Yet designers use a broad range of devices, from older, relatively primitive and lower-cost chargers to newer, more-complex ICs that integrate increasingly sophisticated levels of intelligence to extend battery life and protect the integrity of the system under charge.
The challenge for designers of portable systems is how to charge their systems quickly and efficiently without risking user safety or damage to the battery, while at the same time occupying minimal board space. Li-ion chargers typically employ three charging modes to maximize charging efficiency and ensure user safety. Deeply discharged batteries must be gradually precharged with a fraction of the full current to raise the voltage level slowly above a safe threshold.
For example, a charger for a single-cell Li-ion battery, which has an operating range of 2.8 V to 4.2 V, typically measures the voltage of the battery under charge and enters precharge, or “trickle charge,” mode whenever the battery charge falls below 3 V. Trickle charge mode prevents the battery from charging at a high rate when the battery resistance is lowest, and thereby minimizes heat dissipation. Generally, the charge rate during precharge runs about 10 percent of the full charge capacity.
Once the battery reaches that minimum voltage level, the charger switches to a constant-current or fast-charge mode, which charges the battery at full capacity until it reaches a second threshold, close to the full-charge level. At this point, the charger enters a constant-voltage mode, where the battery voltage is held constant, allowing the charge current to taper off. Once the charge current drops below a preset termination threshold, the constant-voltage charging mode ends.
Microchip Technology's MCP73837 dual-input lithium-ion/polymer charger features a power-source selection
capability that lets it charge automatically from either an ac or a USB source.
Thermal management is a major challenge for battery charger designers. Every charger IC experiences a voltage drop during the charging process that takes the form of heat dissipation. To avoid battery damage or system shutdown, most chargers integrate some form of control mechanism to manage heat buildup. Older charger ICs often employed an “all or nothing” approach to overtemperature or overcurrent conditions. They simply shut down the charge process when heat levels reached a prespecified threshold. Newer devices use more-sophisticated feedback techniques that constantly monitor die temperature and dynamically or algorithmically adjust charge current in proportion to changes in ambient temperature. This built-in intelligence allows today's charger ICs to reduce the charge current gradually until equilibrium is reached and the temperature stops rising. It shortens battery charge time by allowing the charger to continue charging the battery at the maximum possible current without incurring system shutdown.
Charger ICs such as the MAX8804, introduced by Maxim Integrated Products (Sunnyvale, Calif.) last July, use proprietary thermal regulation circuitry to limit die temperature during the fast-charge stage or when the system is exposed to high ambient temperatures. Capable of withstanding a 30-V dc input, the charger requires only 6 mm2 of board space. Others, such as the 1-A bq24060 charger from Texas Instruments (Dallas), offer a thermal foldback regulation feature that allows the device to run continuously in harsh environments where ambient temperature is high, such as in an automobile during the summer or when connected to the incorrect adapter with a higher input voltage. Typically, most of today's newer devices also add overvoltage protection schemes.
Multiple power sources
One increasingly common requirement is the need to charge the battery from a variety of power sources. The trick is to offer this capability without overloading the power source. It is achieved by adjusting the charge current dynamically as input conditions change.
Operating across a broad, 4.0- to 13.2-volt input range, AnalogicTech's AAT3663 can charge up to two lithium-ion cells in series. The charger is fabricated in a proprietary bipolar/CMOS/DMOS technology.
For example, mobile users often do not have the time to find an ac outlet to recharge their devices; instead, they may prefer to recharge their battery from one of the many USB ports found on most electronic equipment, including other battery-powered devices such as notebooks. As a result, many charger ICs on the market today support the ability to charge single-cell lithium batteries from both ac and USB inputs. The challenge here is to compensate for the constantly changing level of power available from the USB port. As load levels change, the system must manage the constant-charge-current level to ensure USB port operation.
Some charger ICs support USB operation with a simple two-tier approach that uses two preset charge levels–usually 500 mA and 100 mA–to support charging at either the maximum current level the USB port allows or, if conditions require, a minimal level to ensure port integrity. Typically, this requires interface logic embedded in the charger IC so the system microcontroller can constantly read the status of the USB port and instruct the charger to switch from one charge level to another.
Recently, some charger IC manufacturers have begun offering schemes that automatically sense the power available on the USB port and adjust the charge current to maximize charge efficiency. In addition, many automatically detect the type of power source and adjust their charging process without user intervention.
When the LTC4075 charger from Linear Technology Corp. (Milpitas, Calif.) detects power at the input, it automatically selects the appropriate power source for charging without requiring external MOSFETs, sense resistors or a blocking diode.
A similar automated power-source selection function is available on the MCP73837, offered by Microchip Technology Inc. (Chandler, Ariz.).
Move to dual cells
As portable system designs grow in complexity and expand the number and types of subsystems, displays and processors they use, single-cell lithium-ion battery sources are proving insufficient to meet system needs. As a result, some portable media players, high-performance SLR-type digital cameras and GPS-based navigation systems now operate off dual Li-ion cells connected in series. These portables generally require input between 8.4 V and 8.8 V to charge. Designers have traditionally used discretes to support such designs, but a new class of charger ICs has entered the market that supports the requirement.
A representative example is AT3663, the first in a new family of 1-A linear battery charger ICs from Advanced Analogic Technologies Inc. (AnalogicTech; Santa Clara, Calif.) for charging up to two Li-ion cells in series. The device is fabricated using the manufacturer's proprietary Modular BCD process technology, which integrates fully isolated CMOS with high-speed complementary bipolar transistors and 30-V DMOS power devices without using complex and expensive epitaxy or high-temperature diffusion.
The new process allows the AAT3663 to support input voltages from 4 V up to 13.2 V, which gives designers the freedom to use lower-cost, unregulated adapters.