This part is a kind of semiconductor called L6599AT.

Function of this product has Improved high-voltage resonant controller.

Manufacturers : STMicroelectronics

Image and pinout :

Some of the text within the PDF file :

L6599AT Improved high voltage resonant controller 61 Features  50% duty cycle, variable frequency control of resonant half bridge  High accuracy oscillator  Up to 500 kHz operating frequency  Two-level OCP: frequency-shift and latched shutdown  Interface with PFC controller  Latched disable input  Burst mode operation at light load  Input for power-ON/OFF sequencing or brownout protection  Non-linear soft-start for monotonic output voltage rise  600 V - rail compatible high-side gate driver with integrated bootstrap diode and high dv/dt immunity  -300/700 mA high-side and low-side gate drivers with UVLO pull-down  Guaranteed for extreme temperature ranges Datasheet - production data Applications  LCD and PDP TV  Desktop PC, entry-level serve [ ... ]

L6599AT PDF Datasheet Download