onsemi MC7818BTG 18V Positive Voltage Regulator: Datasheet, Pinout, and Application Circuits
The onsemi MC7818BTG is a classic, fixed-output, positive linear voltage regulator belonging to the renowned 78xx series. It is engineered to deliver a stable +18V DC output voltage from a higher, unregulated input voltage, making it a fundamental component in countless power supply designs. Its primary role is to provide simple and reliable voltage regulation for a wide array of electronic circuits, from industrial equipment to consumer electronics.
This device is packaged in a robust TO-220 package, which is favored for its excellent thermal characteristics and ease of mounting to a heatsink. The MC7818BTG can supply a maximum output current of 1A, provided that adequate heat dissipation is maintained to keep the internal junction temperature within safe operating limits.
Key Datasheet Specifications
Understanding the critical parameters from the datasheet is essential for proper implementation:
Output Voltage (VO): +18V (typically at 5 mA, 25°C)
Output Voltage Tolerance: ±4% (for the MC7818BTG)
Input Voltage (VI): Up to 35V (recommended to stay a few volts above the 18V output for proper regulation)
Dropout Voltage: Typically 2V at 1A output current. This means the input must be at least 20V to ensure regulation.
Line Regulation: 18 mV (Typ.)
Load Regulation: 18 mV (Typ.)
Ripple Rejection: 68 dB (Typ.) - This highlights its excellent ability to filter out AC ripple from the input.
Operating Junction Temperature Range: 0°C to +125°C
Pinout Configuration
The pinout for the TO-220 package is standard for three-terminal regulators:
1. Pin 1 (INPUT): This pin receives the unregulated DC input voltage.
2. Pin 2 (GROUND): This pin is connected to the common ground of both the input and output circuits.
3. Pin 3 (OUTPUT): This pin provides the regulated +18V DC output.
Application Circuits
The most common application for the MC7818BTG is as a basic voltage regulator circuit.
1. Standard Regulation Circuit:
This is the fundamental setup. The input capacitor (Cin, typically 0.33μF) helps neutralize inductive effects from long input leads and suppresses high-frequency noise. The output capacitor (Cout, typically 0.1μF) improves the regulator's transient response and stability.
2. Dual-Supply Power Source:
By pairing the positive MC7818BTG with its negative counterpart, the MC7918 (-18V regulator), you can create a symmetrical ±18V dual power supply. This is a crucial configuration for powering operational amplifiers, audio amplifiers, and other analog circuits that require both positive and negative supply rails.
Design Considerations:
Heatsinking: The TO-220 package must be attached to a suitable heatsink if the power dissipation (PD = (Vin - Vout) Iout) exceeds a few watts.
Input Voltage: Ensure the input voltage remains above the dropout voltage (Vout + 2V) but does not exceed the absolute maximum rating of 35V, especially under no-load conditions.
Protection Diodes: For added robustness, especially when large output capacitors are used, protection diodes can be added to prevent the regulator from being damaged if the input is shorted to ground.
ICGOODFIND: The onsemi MC7818BTG remains a cornerstone of linear power supply design. Its exceptional simplicity, robust performance, and built-in protections like internal current limiting and thermal shutdown make it an enduring and reliable choice for designers seeking a no-fuss solution for generating a stable +18V rail. While switching regulators offer higher efficiency, the MC7818BTG's low noise output and minimal external component count secure its place in applications where these attributes are paramount.
Keywords: Linear Voltage Regulator, 18V Fixed Output, TO-220 Package, Positive Regulator, Power Supply Circuit
