LM337BD2TR4G: Key Specifications and Application Circuit Design for the onsemi Negative Voltage Regulator
The LM337BD2TR4G from onsemi is a venerable and highly reliable negative adjustable voltage regulator. As the complement to the ubiquitous LM317 positive regulator, this device is designed to deliver a user-adjustable output voltage from approximately -1.2V down to -37V, with a guaranteed output current of 1.5A. Its robust performance, internal current limiting, and thermal shutdown protection make it an ideal choice for a wide range of power supply applications requiring a negative rail.
Key Specifications and Features
Understanding the core electrical characteristics is crucial for effective implementation. The LM337BD2TR4G excels in several key areas:
Adjustable Output Voltage: The regulator maintains a -1.25V reference voltage (VREF) between the output and adjustment terminals. By using an external resistor divider network, the output can be set to any value within the range of -1.25V to -37V.
High Output Current: It can deliver a continuous output current in excess of 1.5A with proper heat sinking, making it suitable for powering larger loads.
Excellent Line and Load Regulation: The device exhibits tight regulation, typically 0.01%/V (line regulation) and 0.3% (load regulation), ensuring a stable output voltage despite fluctuations in the input voltage or changes in the load current.
Integrated Protection: Built-in thermal overload protection shuts down the regulator if the internal junction temperature exceeds safe limits. Internal short-circuit protection provides current limiting that is a function of the junction temperature, safeguarding the device and the load.
Package: The "D2PAK" (TO-263) surface-mount package offers a low thermal resistance for efficient heat dissipation, which is critical when operating at high input-output differentials and high currents.
Application Circuit Design
A basic adjustable negative voltage regulator circuit using the LM337BD2TR4G is straightforward to implement. The design centers on two external resistors and two capacitors for stability.
1. Basic Circuit Configuration:
The input voltage, which must be more negative than the intended output voltage, is applied to the Input (IN) pin. The Output (OUT) pin provides the regulated negative voltage. The Adjustment (ADJ) pin is the control input for setting the output voltage.
The core of the design is the resistor divider formed by R1 and R2. Resistor R1 (typically 120Ω to 240Ω) is connected between the OUTPUT and ADJ pins. Resistor R2 is connected between the ADJ pin and the ground (or the positive rail, which serves as the common reference point for this negative supply).
2. Output Voltage Calculation:
The output voltage (VOUT) is set using the formula:
VOUT = -VREF (1 + R2/R1) - IADJ R2
Where:
VREF is the reference voltage (-1.25V typical).
IADJ is the adjustment pin current (typically 50 µA), which is very small and often negligible for most practical calculations, simplifying the formula to:
VOUT ≈ -1.25V (1 + R2/R1)
3. Critical Component Selection:
Input Capacitor (CIN): A 0.1µF to 1µF ceramic or tantalum capacitor should be placed close to the input pin to bypass high-frequency noise and improve circuit stability. This is especially important if the regulator is located far from the primary power supply filter.
Output Capacitor (COUT): While not always strictly required for stability, a 10µF tantalum or aluminum electrolytic capacitor on the output improves transient response and stability. For the adjustable version, a small 1µF solid tantalum capacitor is recommended directly at the output to damp oscillations and enhance performance.
Adjustment Pin Bypass (CADJ): Adding a small capacitor (e.g., 10µF) from the ADJ pin to ground can significantly improve the regulator's ripple rejection ratio, filtering out noise that might otherwise be amplified by the resistor divider.
Heat Sinking: Due to its power-handling capability, thermal management is paramount. The power dissipated by the regulator is calculated as PD = (VIN - VOUT) IOUT. This power must be dissipated via a heatsink to keep the junction temperature below its maximum rating of 125°C.
Design Example:
To create a -5V output from a -9V input with a 500mA load:
1. Choose R1 = 240Ω.
2. Calculate R2: VOUT = -1.25V (1 + R2/240) = -5V. Solving for R2 gives R2 = 720Ω.
3. Calculate power dissipation: PD = ( | -9V - (-5V) | ) 0.5A = 4V 0.5A = 2W. A suitable heatsink must be attached to the D2PAK package to handle this dissipation.
ICGOODFIND: The onsemi LM337BD2TR4G remains a fundamental building block for generating negative voltage rails in power supply designs. Its simplicity, robust protection features, and ability to deliver up to 1.5A make it indispensable for applications ranging from op-amp circuits and audio amplifiers to industrial control systems. Successful implementation hinges on proper heat sinking and the correct selection of just a few external passive components to ensure stable, reliable, and long-term operation.
Keywords: Negative Voltage Regulator, Adjustable Output, 1.5A Output Current, Thermal Protection, Application Circuit Design.
