The UCC39421 family of synchronous PWM controllers is optimized to operate from dual Alkaline/NiCd cells or a single Lithium-Ion (Li-Ion) cell, and convert to adjustable output voltages from 2.5V to 8V. For applications where the input voltage does not exceed the output, a standard boost configuration is utilized. For other applications where the input voltage can swing above and below the output, a 1:1 coupled-inductor (Flyback or SEPIC) is used in place of the single inductor. Fixed frequency operation can be programmed, or synchronized to an external clock source. In applications where at light loads variable frequency mode is acceptable, the IC can be programmed to automatically enter PFM (Pulse Frequency Modulation) mode for an additional efficiency benefit.