Flyback controller with 300 kHz fixed switching
The LTC3803ES6-3#TRMPBF: The LTC3803 is a current-mode flyback PWM controller that drives an external MOSFET. Its 300 kHz fixed switching frequency is a practical midpoint — high enough to keep the transformer small, low enough that switching losses in the FET and core don't dominate the thermal budget. The output configuration is positive and isolation-capable, meaning the controller can regulate a secondary-side output across a transformer with an optocoupler feedback path. Maximum duty cycle is 80%, which sets a floor on the achievable step-down ratio in continuous-conduction mode — a design consideration when the input-to-output voltage difference is large.
The Vcc supply range is 5.7 V to 8 V — a narrow window that typically requires a regulated bias rail rather than a loosely-tapped line. An auxiliary winding on the flyback transformer is the usual source once the converter is running. The single-output transistor-driver stage drives an external N-channel MOSFET; there is no synchronous rectifier on chip, so the secondary-side rectifier is a Schottky diode in the standard flyback arrangement.
TSOT-23-6 — footprint and board fit
Housed in a 6-lead TSOT-23-6 (thin SOT-23) package, the part occupies roughly 9 mm² of board area — suitable for space-constrained isolated supplies in IGBT gate-drive bias, auxiliary housekeeping rails, or telecom line cards. The control features are limited to current limit and enable — no soft-start ramp, no frequency synchronization, no external compensation ramp. The enable pin provides a simple on/off shutdown that pulls the supply current to near zero.