This part is a kind of semiconductor called 3843CM.

Function of this product has CURRENT MODE PWM CONTROLLER.

Manufacturers : BCD Semiconductor

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Some of the text within the PDF file :

Data Sheet CURRENT MODE PWM CONTROLLER General Description The AP3842C/3C/4C/5C are high performance fixed frequency current-mode PWM controller series. These integrated circuits are optimized for off-line and DC-DC converter applications with minimum external components. They feature under-voltage lockout (UVLO) circuit with low start-up current, trimmed oscillator for precise duty cycle control, current sense comparator providing maximum current limiting and a totem pole output stage for increasing output current. In addition, these ICs also feature accurate protection against over-temperature, over-current and maximal output power. The AP3842C and AP3844C have UVLO thresholds of 16V(on) and 10V(off); The corresponding thresholds for AP3843C and AP3845C are 8.4 V(on) [ ... ]

3843CM PDF Datasheet Download