STMicroelectronics L78L09ACD

Overview

L78L09ACD from STMicroelectronics is a three-terminal positive linear voltage regulator delivering a fixed 9 V output at up to 100 mA, featuring internal current limiting, thermal overload protection, and short-circuit protection. It requires no external components and is rated for ±4% output voltage tolerance (A-grade), with 1.7 V dropout voltage and 70 µV RMS output noise (10 Hz–100 kHz) - used in local on-card regulation for microcontroller power rails, sensor interfaces, and low-noise analog subsystems.

For engineers reviewing the L78L09ACD datasheet, L78L09ACD pinout, L78L09ACD application, or L78L09ACD equivalent, this page provides verified technical context, package-specific design meaning, real-world use scenarios, and validated alternative options for fixed-voltage local regulation where stability, low quiescent current (6 mA typ.), and thermal robustness are critical.

Technical Context

The L78L09ACD operates as a monolithic series pass regulator with built-in thermal shutdown and foldback current limiting, eliminating need for external protection circuitry. Its architecture delivers stable 9 V output across input voltages from 11.5 V to 23 V and load currents from 1 mA to 100 mA, with junction temperature range of 0 °C to 125 °C.

It achieves 37–44 dB supply voltage rejection at 120 Hz under 40 mA load, and maintains output regulation within ±5% over full operating conditions. The device's 1.7 V dropout enables operation from modest input margins, while its 70 µV output noise supports noise-sensitive analog circuits such as ADC references and op-amp signal chains.

Key Specifications

Pinout & Package

Supplied in TO-92 package (JEDEC TO-226AA), with molded plastic case, axial leads, and through-hole mounting. Thermal resistance junction-to-ambient is 200 °C/W (no PCB copper heatsink); junction-to-case is 15 °C/W.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar fixed-output linear regulator applications.

Compared with LM78L09ACZ and MC78L09ACP, the L78L09ACD offers superior noise performance (70 µV vs. ≥85 µV), tighter tolerance (±4% vs. ±5%), and lower dropout (1.7 V), making it optimal for analog-intensive, space-constrained, or thermally limited applications.

Availability

L78L09ACD is available at Aetrix Electronics and suitable for industrial sensor nodes, medical wearables, automotive body electronics, and point-of-sale peripherals requiring stable component supply, long-term lifecycle support, and RoHS-compliant packaging.

Supply support for L78L09ACD includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, designing and manufacturing microcontrollers, power management ICs, analog chips, and MEMS sensors for industrial, automotive, and consumer markets.

The L78L series belongs to ST's legacy linear regulator product line, engineered for cost-effective, robust, and easy-to-use local voltage regulation in space- and reliability-constrained embedded systems.

FAQ

What is the minimum input voltage required for stable 9 V output from the L78L09ACD?

The L78L09ACD requires a minimum input voltage of 11.5 V to maintain regulated 9 V output across all load and temperature conditions, based on its 1.7 V dropout specification. Below this, output voltage drops linearly with input; operation below 11.5 V is not guaranteed per datasheet electrical characteristics.

Can the L78L09ACD be used in parallel to increase output current?

No - the L78L09ACD is not designed for parallel operation. Its output voltage tolerance (±4%) and lack of current-sharing control make load imbalance likely, risking thermal runaway. For >100 mA, use a higher-current regulator (e.g., L7809CD) or external pass transistor configuration per ST application note AN1051.

Is a heat sink required for continuous 100 mA operation?

A heat sink is not required for 100 mA operation at ambient temperatures ≤50 °C with standard TO-92 mounting on FR-4 PCB (no copper pour). At 100 mA with 23 V input, power dissipation reaches ~1.4 W; junction temperature exceeds 125 °C above ~60 °C ambient unless 6 cm² copper pour is used per ST's R_thJA = 55 °C/W spec.

Does the L78L09ACD support reverse input polarity protection?

No - the L78L09ACD lacks reverse-polarity protection. Applying negative voltage to VIN relative to GND may cause irreversible damage. External protection (e.g., series diode or MOSFET-based reverse-voltage circuit) is required in environments prone to wiring errors or battery reversal.