The Original 78L33ACUTR Positive Fixed Linear Voltage Regulator is a precision three-terminal monolithic regulator integrated circuit manufactured by STMicroelectronics. Encapsulated in a compact, thermally enhanced SOT-89-3 surface-mount package, it is designed to deliver a stable, regulated +3.3V DC output with load currents up to 100mA from input voltages up to 30V. Featuring an internal thermal overload shutdown, internal short-circuit current limiting, and tight ±5% output voltage tolerance, the 78L33A serves as an ultra-reliable point-of-load voltage regulator. It is widely employed in 3.3V microcontroller power rails, sensor signal conditioning circuits, battery-powered instrumentation, and noise-sensitive RF/analog subsystems.

Specifications

• Part Number / Model: 78L33ACUTR / L78L33ACUTR
• Manufacturer: STMicroelectronics
• Device Function: 3-Terminal Fixed Positive Linear Voltage Regulator
• Package Type: SOT-89-3 
• Nominal Output Voltage (Vout): +3.3V DC (Output Tolerance: ±5% / 3.135V to 3.465V over temperature)
• Maximum Output Current (Iout): 100 mA (Continuous)
• Input Voltage Range (Vin): 5.3V to 30V DC (Absolute Maximum: 30V DC)
• Dropout Voltage (Vd): Typ. 1.7V to 2.0V (at Iout = 40mA to 100mA, Ta = 25°C)
• Quiescent / Bias Current (Iq): Typ. 3.0 mA / Max. 5.5 mA (at Vin = 8.3V, Iout = 40mA)
• Line Regulation: Typ. 20 mV / Max. 150 mV (at Vin = 5.3V to 20V, Iout = 40mA)
• Load Regulation: Typ. 15 mV / Max. 60 mV (at Iout = 1mA to 100mA, Vin = 8.3V)
• Ripple Rejection Ratio (SVR / PSRR): Typ. 49 dB to 54 dB (at f = 120Hz, Vin = 6.3V to 16.3V)
• Output Noise Voltage (eN): Typ. 40 µV (f = 10Hz to 100kHz)
• Short-Circuit Current Limit (Isc): Typ. 140 mA (at Vin = 8.3V)
• Power Dissipation (Pd): Up to 800 mW to 1000 mW (Internally limited, mounted on standard PCB copper pad)
• Operating Junction Temperature (Tj): 0°C to +125°C (Commercial / Industrial Grade "A" Suffix)
• Storage Temperature Range (Tstg): -65°C to +150°C
• Thermal Resistance, Junction to Ambient (Rth(j-a)): ~100°C/W to 180°C/W (depending on PCB copper area)
• Pinout Configuration (SOT-89-3 - Top View / Pin Side Down):
  • Pin 1: Output Voltage (VOUT - +3.3V)
  • Pin 2 / Center Tab: Ground (GND)
  • Pin 3: Input Voltage (VIN)

Features

• Stable +3.3V Point-of-Load Regulation: Provides clean, low-noise +3.3V DC power directly at the load, eliminating ground loop noise and trace resistance drops from central supplies.
• High Input Voltage Range up to 30V: Handles input voltage spikes, high-voltage battery rails, and 12V/24V industrial bus supplies without requiring pre-regulation stages.
• Internal Thermal Overload Protection: Automatically shuts down or folds back output current if the silicon junction temperature exceeds safe operational limits (typ. 150°C).
• Short-Circuit Current Limiting: Built-in internal current-limiting circuitry prevents catastrophic device destruction during accidental output short circuits to ground.
• Thermally Enhanced SOT-89-3 Footprint: Wide center ground tab dissipates heat directly into the host PCB ground plane, allowing significantly higher continuous power dissipation than standard SOT-23 packages.
• Low Output Noise & High Ripple Rejection: Delivers clean analog power with ~49dB power supply rejection, ideal for precision ADC references, audio preamps, and wireless modules.

Common Applications

• 3.3V Microcontroller & DSP Power Rails: Supplying low-power 3.3V MCUs (STM32, ESP8266/ESP32 low-power sub-circuits, CH32V, PIC) from 5V or 12V supplies.
• Precision Sensor Nodes: Ultra-clean 3.3V power source for analog sensors, I2C/SPI environmental sensors, and MEMS accelerometers.
• Industrial 12V/24V Auxiliary Rails: Stepping down industrial auxiliary control voltages to local 3.3V logic levels.
• Battery-Powered Portable Instruments: Linear regulation from 9V alkaline or multi-cell Lithium batteries for measurement meters.
• RF & Wireless Front-End Biasing: Low-noise linear power regulation for Bluetooth, LoRa, and Sub-GHz RF transceiver modules.

Usage Tips

• Input & Output Capacitors: Connect a **0.33µF (or 1µF) ceramic/tantalum capacitor** from Pin 3 (VIN) to GND to prevent input oscillations, and a **0.1µF to 2.2µF low-ESR capacitor** from Pin 1 (VOUT) to GND to ensure stability and improve transient response.
• Minimum Input Voltage Headroom: The 78L33 requires a minimum dropout headroom of ~1.7V to 2.0V; ensure the input voltage (VIN) is at least **5.3V DC** under full load to maintain solid +3.3V regulation.
• PCB Heat Sinking via Center Tab: Solder Pin 2 and the wide center thermal tab directly to a generous copper pour on the PCB (connected to the ground plane) to maximize power dissipation and prevent premature thermal shutdown.
• Thermal Dissipation Calculation: Calculate power dissipation using Pd = (Vin - Vout) × Iout + (Vin × Iq). For example, with Vin = 12V and Iout = 50mA: Pd = (12V - 3.3V) × 0.05A ≈ 0.435W. Ensure adequate copper area so junction temperature remains below 125°C.
• Reverse Bias Protection: If driving high-capacitance loads on Vout or circuits where Vout might exceed Vin during power-down, place a 1N4148 diode in reverse across Pin 1 (Anode) and Pin 3 (Cathode) to prevent reverse current damage.

Datasheet