48% duty-cycle ceiling — the flyback designer's hard stop
The UC3844D: It drives a single transistor output and operates from a 12 V to 28 V supply rail, covering the bias range of common offline SMPS designs. The 48% ceiling guarantees the core resets every cycle in a single-ended forward converter.
Switching frequency and topology reach
Switching frequency goes up to 500 kHz. The part supports boost, buck, buck-boost, flyback, and forward topologies, but the 48% duty limit restricts the step-down ratio in a forward converter. No clock sync input and no synchronous rectifier drive — this is a straightforward, low-pin-count controller for cost-sensitive designs, not a multi-phase or high-efficiency synchronous supply.
The part is no longer in production, so new builds cannot rely on factory supply. Sourcing is through independent distribution and the open market. We can quote the UC3844D against your BOM quantity — confirm the date-code range and package condition (tube, 14-SOIC) at RFQ time.
Package and temperature grade — board-fit constraints
The UC3844D comes in a 14-SOIC (0.154", 3.90 mm width) package, surface-mount. If your application sees an engine bay, outdoor telecom cabinet, or motor-drive enclosure, the 0°C lower limit will cause start-up failures below freezing.