![]() ![]() Safety features include the gentle precharging of over-discharged batteries at a reduced current and the ability to detect faulty batteries. The charger includes a state machine that turns off the charger after charging is complete and automatically restarts charging when some of the charge has been drained from the battery. Because the charger limits the input current, a smaller, and generally cheaper, AC wall adapter can be used to power the circuit. When this current rises to the limit, the controller automatically reduces the current charging the battery, putting a cap on the current that can flow into the circuit's input. For example, the charger circuit's controller permits a limit to be placed on the current that flows into the circuit. The charger circuit shown in Figure 1 includes many other features that improve both battery life and system operation. The controller's fixed switching frequency, however, allows electrical noise to be easily filtered, but care must be used in circuit layout and component selection to prevent interference problems. Other disadvantages include the EMI and electrical noise caused by the switching action of the charger and the radiation caused by the output filter inductor. The controller, along with its external switches and LC filter, consumes more board space than other types of chargers. The main disadvantages of this type of charger are its size and complexity. Switch-mode chargers also hold an advantage over pulse chargers: they perform well over a wide range of input voltage, allowing the use of a smaller and cheaper AC wall adapter than when using a pulse charger. Switch-mode chargers have consistently low power dissipation over wide variations in input and battery voltage, a definite advantage over linear chargers. This circuit can be easily scaled up to allow as many as four series cells to be charged at currents of up to 4A.įigure 1. The power dissipation of this circuit remains below about 1W over the entire battery voltage range and over a wide range of AC adapter voltages. It uses the MAX1737 Li+ battery-charger controller with dual n-channel MOSFETs to step the AC adapter voltage down to the battery voltage. Switch-Mode Chargers Figure 1 shows the schematic of a typical switch-mode Li+ charger circuit. However, they do require a current-limited AC adapter, which is usually more costly. Pulse chargers don't present this problem. Although a linear charger may not require much board space to accommodate the IC and its external components, it may need additional board area to dissipate the heat generated by the charger's pass transistor. Linear and pulse chargers take up little board space and require a minimum of external components. Switch-mode chargers tend to be larger and more complex and require a large passive output LC filter the extra board space buys added efficiency. The major difference between these topologies is the size and cost vs. Li+ battery chargers come in three types: switch-mode, linear, and pulse. Circuit designers decide which features to include and how to implement them based on the specific application and the level of cost or complexity that is tolerable. These features can be implemented in the charger itself, in an ASIC or a discrete circuit, or possibly within the software found within a microcontroller. These include reduced charging current for over-discharged cells, detection of faulty cells, battery voltage monitoring and/or fuel gauging, input-current limiting, turning off the charger after charge completion, automatically restarting charging after partial discharge, charge state indication, and external charger enable/disable control. Other features are often added to improve the life of the batteries or the operation of the charger. ![]() Designers should consult the battery manufacturer to determine what's required to safely charge a particular battery. Knowing the types of chargers that are available and the tradeoffs between them allows a designer to choose the right charger for a specific application.Ī Li+ battery charger must limit the charging current and the battery's maximum voltage. ![]() On the other, a charger needs to be small, to fit inside ever-shrinking mobile-phone and communicating-PDA form factors. ![]() On one hand, high current is needed to quickly replace the energy drained from the battery while transmitting voice or data. Charging Li+ batteries in mobile phones and PDAs is a balancing act. ![]()
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