500 kHz switching ceiling — what it buys the BOM
The UC3843AD8TR is a current-mode PWM controller from TI's UCx84x family, rated for switching frequencies up to 500 kHz. That ceiling lets you shrink the transformer core in a flyback or forward converter compared to a 47 kHz part like the UCC2806DWTR, but there is no clock-sync pin — if your design shares a switching rail across multiple converters, you either free-run or add external sync logic. The 96% maximum duty cycle is the headline number for low-dropout step-down or forward-converter operation. A 50% duty-cycle controller forces a two-switch topology at the same input/output ratio; this part keeps the bill of materials simpler for a single-ended design.
Vcc range is 7.6 V to 30 V — the undervoltage lockout thresholds are fixed inside the UC3843 variant (8.4 V on, 7.6 V off). That UVLO window suits a primary-side bias winding that needs a clean start-up before the auxiliary winding takes over. Output is a single transistor-drive stage, positive and isolation-capable. The driver sources and sinks the gate charge of an external MOSFET directly; no buffer stage needed for most 1 A–5 A FETs at 500 kHz. Control features include frequency control — an external RC sets the oscillator timing. No soft-start or enable pin on this base variant; those live on the UC3844/45 siblings if the BOM needs them.
ROHS3 compliant. For an under-hood or industrial extended-temp build, the LM5150QURUMRQ1 covers -40°C to 150°C but at a higher per-unit cost and with a different pinout.
Package and footprint — 8-SOIC
8-SOIC, 0.154-inch body width, 3.90 mm. Surface-mount only. No thermal pad underneath — the die dissipates through the leadframe and the copper traces on the PCB. A 2-layer board with solid ground pour under the controller handles the thermal budget for most 10 W–50 W converters.