STMicroelectronics L78L33ACUTR

The L78L33ACUTR provides a nominal 3.3 V regulated output for local or on-card power rails. Internal current limiting and thermal shutdown simplify overload protection, while the fixed-output topology requires no feedback divider. The AC selection provides the A-grade output-voltage limits over a 0 degrees C to +125 degrees C operating junction-temperature range.

ST identifies the U package as SOT-89. The catalog records the original package as TO-243AA, a corresponding three-lead surface-mount outline, and the existing registry image matches this body style.

Layout checks

• Use ST's current SOT-89 land pattern and verify pin-1 orientation.
• Connect pin 2 and the package tab to ground.
• Place input and output bypass capacitors close to their active pins and ground.
• Provide adequate PCB copper for heat spreading.

ST specifies 55 degrees C/W maximum junction-to-ambient thermal resistance for SOT-89 when evaluated with 6 square centimeters of PCB copper; actual performance depends on layout and airflow.

The SOT-89 assignment below follows ST's top-view drawing.

Do not assume that another SOT-89 regulator uses the same pin assignment; verify every proposed substitute.

ST's L78L33AC electrical table uses an 8.3 V input, 0.33 microfarad input capacitor, 0.1 microfarad output capacitor, and 40 mA load unless otherwise stated.

A replacement must preserve the fixed 3.3 V output, required current, SOT-89 footprint, VOUT/GND/VIN pin assignment, input limits, dropout behavior, bypass guidance, temperature range, and thermal performance. The similarly named AB variant has a different operating-temperature qualification, so the exact suffix matters.

No internal replacement link is asserted without an exact API-confirmed catalog match and engineering review.

STMicroelectronics lists L78L33ACUTR as an active product in volume production. In the order code, 33 identifies the nominal 3.3 V output, AC identifies the A-grade 0 degrees C to +125 degrees C device, U identifies SOT-89, and TR identifies tape-and-reel packing. ST documents the top marking as 8A.

Confirm the exact L78L33ACUTR order code, 8A marking, SOT-89 dimensions, reel label, lead finish, and lot traceability during incoming inspection. Package appearance alone does not establish authenticity. Electrical sampling should use the latest ST limits for the AC order code.

Validate input headroom, startup and load transients, capacitor placement, ground routing, and junction temperature in the final circuit. Regulator dissipation is approximately the input-to-output voltage difference multiplied by load current, with an additional bias-current term, so thermal limits can dominate before the absolute input rating is reached.