Convergent products mixing computing, consumer and communication functionality are pushing state-of-the-art handsets beyond notebook computers in complexity, capability and features. It's no surprise that the power demands of such devices make power management critical to achieving acceptable battery life in small form factors.
The centerpiece of the power system in these new convergent products is the lithium-ion (Li-ion) battery, which uniquely combines high capacity, small size, light weight and durability. Equally essential is the battery-charger IC, which controls Li-ion cell-charging conditions to minimize charging time while maximizing battery life, the number of charging cycles and safety.
To minimize charging time, existing battery-charger ICs deliver a predetermined charging current matched to the battery's capacity. However, these devices are agnostic to the power source. Therefore, systems demanding more current than the ac wall adapter can deliver may cause the charger's input voltage to sag and a malfunction to occur. For this reason, manufacturers must be careful to pair handsets to specific adapters — an extremely difficult task for suppliers managing broad product portfolios.
Users' increasing preference to charge their portable devices from the USB port on a notebook computer further complicates charger IC requirements. Attempts to charge at more than 500 mA can cause the port voltage to sag and provoke the USB host to shut down the USB port for safety's sake and terminate charging.
Many existing chargers provide only rudimentary safety features — blindly attempting to charge at a maximum rate and then shutting off if a fault condition occurs. This all-or-nothing approach is particularly problematic in hot climates where overheating of the charger IC is likely to occur at the maximum charging rate. Once overtemperature shutdown has occurred, attempts to restart the charging sequence can damage the battery cell through incomplete electrochemical reactions caused by repeatedly initiating and terminating charging. Moreover, since most chargers only use blinking lights to indicate charging status, they are unable to communicate their condition to the baseband or applications processor, making intelligent systemwide fault recovery impossible.
Managing battery charging in today's feature-rich portable products requires a new generation of smart battery chargers. To address the limitations of current solutions, an improved charger IC should:
Charge at the maximum rate from a variety of power sources without disturbing or overloading the source of power by dynamically or algorithmically adjusting charging current as input conditions change.
Charge at the maximum rate safely without overheating the charger IC by dynamically or algorithmically adjusting charging current in accordance with ambient temperature and self-heating characteristics.
Digitally communicate the battery's charging condition to the host processor and allow the system to control charging conditions and manage fault recovery.
Charger ICs with these features are becoming available. For example, one smart-charging technique dynamically matches the charging rate to the pack's electrical and thermal environs. When the IC becomes too hot or the input voltage begins to sag, the charging current is reduced by some predetermined amount to avoid the impending fault. Thereafter, the current is adjusted up or down until an acceptable steady-state condition is achieved so that some degree of charging is maintained in all but the most extreme conditions.
Meanwhile, some charger ICs are using a single-wire interface to provide high-speed, bidirectional digital communication between the analog charger IC and host processor or baseband chip without the need for complex protocols, precision timing or multiple pins.
By providing the three benefits listed above, second-generation charger ICs promise to vastly outperform today's more-primitive chargers, and in the process improve battery life, reduce charging time and maximize the number of cell charging cycles. As handheld systems become even more complex in the future, smart battery charging will become an increasingly essential product component.
An acknowledged device physics expert in power management and IC technology, Richard Williams has invented and developed several milestone devices, including the first production Trench Power MOSFET. Williams holds a BSEE degree from the University of Illinois at Urbana-Champaign and an MSEE degree from Santa Clara University in California.