Detailed introduction to lithium battery charger design requirements

The factors that affect the safe and effective charging of lithium-ion batteries are voltage, current and temperature. Therefore, these three factors must be taken care of in the design of the charger.

According to the above requirements, the charger IC should have active power path management. This power path management requires real-time adjustment of the lithium battery charging current in the case of loading/unloading of the load current, effectively monitoring and managing the current in accordance with the USB-IF specification. Restrictions and soft activation features. The temperature monitoring function should be integrated in the IC. If the IC temperature exceeds the limit value, the charging current will be automatically reduced. The charger's power adapter ensures that the 5V DC power supply can be step-down and steady current charging. If the adapter has an overcurrent, the external resistor can be used to set the maximum charging current control to avoid USB overload.

The charger should also have an adaptive power path management function. When the power consumption of the system terminal exceeds the supply limit of the input power source, the lithium battery can actively activate the discharge function and supply the required power to the system end. The charger consumes a small electrode. When the battery is lower than 3V, the IC should preset the current at 10% of the set charging current.

The suitable temperature for charging the lithium battery is 0~45°C. If it is charged at 0.5C, the temperature rise of 1C can be set to 90°C. At this time, the charger is operated at an ambient temperature of 35°C, and the lithium battery is best charged. The current is best charged at a 2 hour rate.

The fundamental problem when charging is that the temperature of the chip is too high during charging. Therefore, a balance must be found between the charging current and the heat dissipation mechanism during design, because the portable charger is applied to a thin and portable product. The use of a plastic housing with poor thermal conductivity, regardless of the metal heat sink, and finally to reduce the charging current is to extend the charging time to achieve a lower operating temperature.

Therefore, the ordinary lithium battery charger is suitable for a lithium battery of 1500 mAh or less, and for a high voltage and high capacity lithium battery a synchronous switching charger is used for charging.

The lithium battery charger charging process described above is divided into three phases:

1. The charging IC first detects whether there is a protection mode of output short circuit or overload. If the system is normal, then the battery voltage is 3V. If it is higher than 3V, it can directly enter the constant current charging. Below 3V, the current is 10% of the constant current. Precharged;

2. Constant current fast charge phase: increased to 4.2V;

3. Constant voltage stage: After the battery rises to 4.2V, the voltage no longer rises, and the charging current shows a decreasing trend until it reaches 10% of the constant current to stop charging.

Future lithium battery chargers will move toward faster, safer and more comprehensive charging management systems.

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