Ideas on the Design Method of Single-Cell Lithium Polymer Battery Charger
Single-cell lithium polymer batteries are becoming the main choice for portable and ultra-portable applications. The AAT3681A developed for this end market is a high-performance battery charger designed to charge a single-cell Li-ion/polymer battery from an external power source with a current of up to 300mA. This charging current can be customized in a wide range from 15mA to 300mA using an external resistor. This allows the adapter/USB port to perform fast charging in different applications. It is an independent charging solution that only requires an external component to achieve all functions. This device can accurately adjust battery charging voltage and current for 4.2V lithium-ion/polymer battery packs.
The device's rated operating temperature range is -40℃ to +85℃. Under a given constant current charging level, when the working environment temperature exceeds the power dissipation capacity of the device package, the charging control will enter the thermal limit state.
The device provides a status monitoring output pin, which can directly drive an external LED to indicate the battery charging status.
The junction temperature and charge state of the device are fully monitored to prevent error conditions.
AAT3681A implements a complete protection system. When an over-voltage or over-temperature error condition occurs, the device will automatically shut down to protect the charging equipment, control system, and battery under charging.
AAT3681A has four basic charging modes for the entire battery charging cycle: pre-charge/trickle charge, constant current/fast charge, constant voltage charge and charge termination.
Before charging starts, in order to ensure a safe charging environment, AAT3681A will check several conditions. The input power supply must exceed the minimum operating voltage or under-voltage lockout threshold before the charging sequence can be initiated. When these conditions have been met and the battery is connected to the BAT pin, AAT3681A checks the status of the battery. If the battery voltage is lower than the pre-charging pinch value, the charging control circuit starts to pre-charge the battery. The trickle charge current of the battery pre-charge is one-tenth of the constant current of the fast charge.
For example, if the programmable fast charge current is 300mA, then the precharge current is 30mA. Battery cell pre-charging is a safety precaution for deeply discharged battery cells. It also helps limit the power consumption on the pass transistor, because the voltage difference on the device is the largest at this time. The precharge of the constant current rechargeable battery unit continues until the voltage on the BAT pin exceeds the precharge voltage threshold. After reaching this point, the device starts the constant current charging phase. The constant current charging amplitude is programmed by the user by setting the resistance. AAT3681A maintains constant current charging mode until the battery reaches the voltage regulation point.
Constant voltage charging
In the constant current fast charging stage, when the battery voltage reaches the output charging regulation value, the system enters the constant voltage charging mode.
The adjustment voltage level is set to 4.2V (?.5%) at the factory. The charging current in the constant voltage mode drops because the battery cell being charged is about to reach its maximum capacity. End of charging cycle and recharging sequence
In the constant voltage mode, when the charging current drops to 10% of the fast charging current, the device terminates charging and enters a sleep state. The charger will stay in sleep state until the battery voltage drops below the battery recharge voltage threshold.
Because it consumes very low current during sleep, AAT3681A can minimize battery leakage when not charging. This feature is particularly useful for such applications where the input power level may be lower than the rechargeable battery level or undervoltage lockout level. In these cases, when the AAT3681A input voltage drops, the device will enter a sleep state, and once the input power is restored from the error state, it will automatically resume charging.
Heat dissipation considerations
AAT3681A adopts sC70JW-8 package, it can dissipate up to 687mW of power, when it is correctly installed on the PCB board, and has a maximum of 160℃/W The thermal resistance. There are many factors that should be considered when designing the PCB and arranging the location of the charger IC with other heat sink components in a specific application design.
Protection circuit; Overvoltage protection
An overvoltage event is defined as: the voltage on the BAT pin exceeds the maximum battery charging voltage and is set at the overvoltage protection threshold. If an overvoltage event occurs, the AAT3681A charging control circuit will shut down the device until the voltage on the BAT pin drops below the OVP voltage level. AAT3681A will resume normal charging operation after the overvoltage condition is eliminated.
Protection circuit; thermal shutdown
AAT3681A has a thermal protection control circuit, once the internal die temperature exceeds the preset thermal limit threshold, it will shut down the charging function. Once the internal die temperature drops below the thermal limit, it will automatically return to the previous normal charging state.
Programmability of charging current
Users can program the constant current charging level with the help of a setting resistor placed between the ISET pin and ground. The fast charge accuracy and precharge trickle charge current are determined by the tolerance of the setting resistor used. Therefore, metal film resistors with a tolerance of 1% are recommended. The constant current range of fast charging is from 15mA to 300mA.
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