Onsemi LM317LBZG: Technical Specifications and Application Circuit Design

The Onsemi LM317LBZG is a robust, industry-standard positive voltage regulator renowned for its versatility and reliability. This device is capable of supplying over 1.5 A of output current across an adjustable output voltage range of 1.25 V to 37 V. Its internal architecture is designed for exceptional performance, featuring internal current limiting, thermal shutdown, and safe-area compensation, making it remarkably resilient to overload conditions and simplifying circuit design.

A primary advantage of the LM317 is its need for only two external resistors to set the output voltage, solidifying its status as a quintessential adjustable regulator. This design ensures that the output voltage is maintained with high precision, typically exhibiting a line regulation of 0.01%/V and load regulation of 0.1%.

Key Technical Specifications:

Output Voltage Range: 1.25 V to 37 V

Output Current: > 1.5 A

Line Regulation: 0.01% / V (Typ.)

Load Regulation: 0.1% (Typ.)

Operating Junction Temperature: 0°C to +125°C

Package: TO-263 (D2PAK) for high-power applications

Basic Application Circuit Design

The fundamental adjustable regulator circuit is both elegant and effective. The output voltage (Vout) is determined by the ratio of two external resistors, R1 and R2, according to the formula:

Vout = Vref (1 + R2/R1) + (Iadj R2)

Where:

Vref is the internal reference voltage (typically 1.25 V) between the output and adjust terminals.

Iadj is the adjust pin current (typically 50 µA), which is negligible in most designs compared to the current flowing through R1 and R2.

For stable operation, a minimum load current of 10 mA is required. This is typically ensured by selecting R1 to be around 240 Ω, which sets a current of approximately 5.2 mA through itself, with the remainder supplied by the load.

Input and Output Capacitor Selection:

Input Capacitor (C1): A 0.1 μF to 1 μF ceramic disc or tantalum capacitor is recommended, especially if the regulator is located more than a few inches from the primary power supply filter capacitor. This helps nullify the effects of line inductance.

Output Capacitor (C2): While not always strictly necessary for stability, a 1 μF tantalum or 25 μF aluminum electrolytic capacitor on the output significantly improves transient response. For the adjustable version, adding this capacitor prevents the impedance of the voltage divider from affecting the loop response.

Advanced Configurations:

Beyond the basic regulator, the LM317LBZG can be configured for a wide array of functions:

Precision Current Limiter: By using the device to regulate current instead of voltage, it can serve as a simple, efficient constant-current source for charging batteries or driving LEDs. The formula Iout = Vref / R (where R is a single set resistor) provides a stable current.

High-Current Voltage Regulator: The internal current limit can be boosted by adding an external pass transistor to handle significantly higher currents while the LM317 controls the base drive.

ICGOOODFIND: The Onsemi LM317LBZG remains a cornerstone of linear power supply design, offering an unmatched blend of simplicity, flexibility, and built-in protection. Its ability to be configured for both voltage and current regulation with minimal external components makes it an indispensable component for engineers and hobbyists designing reliable, adjustable power sources.

Keywords:

1. Voltage Regulator

2. Adjustable Output

3. Application Circuit

4. Current Limiting

5. Thermal Shutdown