This post explains for the semiconductor 2YL6.
The Part Number is IN4054 ( Marking Information is 2YL6).
The function of this semiconductor is Voltage Charge IC.
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The IN4054 is a complete single-cell lithium-ion battery constant current and constant voltage linear charging IC. It uses a very small SOT-23-5 package and requires only a few external components, making it truly suitable for portable product applications.
Moreover, the IC is specially designed for USB power characteristics. At the same time, the device can also be used as a stand-alone linear Li-Ion battery charger.
Since it has a complete internal MOSFET structure, no external sense resistors and diodes are required. When working under high power loads or high temperature environments, thermal feedback will automatically control the charging current and thus control the temperature of the wafer. The charging voltage is fixed at 4.2V, and the charging current is set by connecting a resistor.
8. Soft start can effectively limit the inrush current.