Onsemi LM317LBDR2G: Technical Specifications and Application Circuit Design
The LM317LBDR2G from Onsemi is a quintessential three-terminal adjustable positive voltage regulator, renowned for its reliability, ease of use, and robust performance. Housed in a surface-mount D²PAK package, this component is engineered to deliver over 1.5 A of output current with a voltage range from 1.25 V to 37 V. Its internal architecture incorporates short-circuit current limiting, thermal overload protection, and safe-area compensation, making it exceptionally resilient in demanding applications.
A primary advantage of the LM317 is its ability to regulate voltage using only two external resistors, which set the output voltage. The device maintains a typical line regulation of 0.01%/V and a typical load regulation of 0.1%. A critical specification is the 1.25 V reference voltage (Vref) that appears between the output and adjustment terminals. This stable reference is the cornerstone of its adjustable operation.
A Basic Application Circuit Design
The fundamental adjustable regulator circuit requires only the LM317 and two resistors. The output voltage is calculated using the formula:
Vout = Vref (1 + R2/R1) + Iadj R2
Where:
`Vref` is typically 1.25 V.
`Iadj` is the adjustment pin current (typically 50 µA), which is often negligible compared to the current flowing through R1 and R2.
R1 is typically chosen to be around 240 Ω to ensure a minimum load current of approximately 5 mA, which is crucial for maintaining regulation when no other load is present.
For a stable output, the use of capacitors is highly recommended. An input capacitor (Cin) of 0.1 µF is advised, especially if the regulator is located far from the primary power supply filter. An output capacitor (Cout) of 1 µF provides improved transient response. While the LM317 is stable without these, they are essential for mitigating instability caused by parasitic impedances in real-world layouts.
For enhanced performance, adding a diode (D1, e.g., 1N4002) across the input and output prevents reverse voltages from damaging the IC during shutdown. Placing a 10 µF capacitor (Cadj) across the adjustment resistor R2 can dramatically improve the ripple rejection ratio, reducing output ripple by up to 80 dB.
Key Design Considerations:
1. Heat Dissipation: The D²PAK package offers a low thermal resistance, but a heatsink is mandatory for high input-output differentials or high load currents. Power dissipation is calculated as `Pdiss = (Vin - Vout) Iload`.
2. Stability: Ensuring a minimum load current is critical. If the design might operate with very light loads, the value of R1 must be selected to draw at least 3.5 mA to 10 mA.
3. Protection: The internal current limit protects the device, but external diodes provide an extra layer of security against unexpected voltage reversals.
ICGOOODFIND: The Onsemi LM317LBDR2G stands as a versatile and robust workhorse for linear power supply design. Its simple two-resistor configuration, coupled with comprehensive internal protection features, makes it an ideal choice for a vast array of applications, from industrial controls and automotive systems to consumer electronics and DIY projects, where a stable and adjustable voltage rail is required.
Keywords: Adjustable Voltage Regulator, Thermal Overload Protection, Ripple Rejection, Heatsink, Linear Regulator.
