This part is a kind of semiconductor called AP1539.

Function of this product has 18V 4A 300KHz BUCK CONVERTER.

Manufacturers : Diodes Incorporated

Image and pinout :

Some of the text within the PDF file :

AP1539 18V 4A 300KHz BUCK CONVERTER Features • • • • • • • • • • Input voltage: 3.6V to 18V Output voltage: 0.8V to VCC Output current: up to 4A Duty ratio: 0% to 99% PWM control Oscillation frequency: 300KHz typ. Soft-start like, Current limit and Enable function Thermal Shutdown function Built-in internal SW P-channel MOS SOP-8L-DEP: Available in “Green” Molding Compound (No Br, Sb) Lead Free Finish / RoHS Compliant (Note 1) General Description AP1539 consists of step-down switching regulator with PWM control. These devices include a reference voltage source, oscillation circuit, error amplifier, internal PMOS. AP1539 provides low-ripple power, high efficiency, and excellent transient characteristics. The PWM control circuit is able to vary t [ ... ]

AP1539 PDF Datasheet Download