What the 48% duty ceiling means for your power stage
The UC3845AD limits the maximum duty cycle to 48%, which caps the step-down ratio in buck and forward topologies and sets the reset winding requirement in single-ended flyback designs. For a given input voltage, the output cannot exceed 48% of the input minus losses — that is the hard boundary for the transformer turns ratio and the output capacitor stress. Switching frequency runs up to 500 kHz, so the magnetics can shrink compared to a 100 kHz controller, but the 48% duty window still forces a larger minimum off-time than a 50%+ part would allow.
The Vcc range is 7.6V to 30V, with the 7.6V undervoltage lockout threshold meaning the bias rail must stay above that during startup or the controller hiccups. That matters for offline flyback designs where the startup resistor charges the Vcc cap slowly. No industrial or automotive temperature range here; if the board sees an engine bay or outdoor enclosure, this part is not the fit.
That means the only path to fill a BOM line is through independent distribution or surplus stock — no factory orders, no last-time-buy window unless a distributor holds inventory. For a board already laid out for the 14-SOIC footprint, the mechanical fit is straightforward, but the supply chain risk is real: once existing stock dries up, the part becomes a spot-market item.