Onsemi MC7818CTG: 18V Positive Voltage Regulator Datasheet and Application Circuit Analysis
The Onsemi MC7818CTG is a classic example of a robust, fixed-output linear voltage regulator, providing a stable +18V DC output from a higher, unregulated input voltage. As part of the ubiquitous 78xx series, this component is a workhorse for power supply design, offering designers a simple and reliable solution for a wide range of applications. This article delves into the key specifications from its datasheet and analyzes a standard application circuit.
Key Datasheet Specifications and Features
The MC7818CTG is a three-terminal device (Input, Output, Ground) housed in a TO-220 package, which is renowned for its excellent power dissipation capabilities. Its primary function is to maintain a constant +18V output.
Output Voltage: +18V ±4% (typically at TJ = 25°C, IO = 1 mA, VI = 23V)
Output Current: Capable of delivering up to 1A of continuous output current with adequate heatsinking.
Input Voltage Range: For stable operation and to ensure the input-output differential (dropout voltage) is maintained, the recommended input voltage is typically between 20V and 33V. Exceeding the absolute maximum input voltage (35V) must be avoided.
Line Regulation: 6 mV (Typ.) - This indicates the output voltage's stability against changes in the input voltage.
Load Regulation: 12 mV (Typ.) - This specifies the output voltage's stability against changes in the load current.
Dropout Voltage: Approximately 2V at 1A output. This is the minimum voltage difference required between the input (VI) and output (VO) for the regulator to function properly.
Internal Protection: The IC includes vital internal thermal shutdown and short-circuit protection, safeguarding the device from damage due to excessive heat or current overloads.
Standard Application Circuit Analysis
The basic application circuit for the MC7818CTG is straightforward, requiring only a few external components to ensure stable and noise-free operation.
1. Input Capacitor (CIN - 0.33μF): A ceramic capacitor placed close to the regulator's input pin is crucial. It helps to bypass high-frequency noise on the input line and prevents any potential oscillations. While not always strictly necessary if the unregulated supply is close to the regulator, it is considered a critical best practice for reliability.
2. Output Capacitor (COUT - 0.1μF): A ceramic capacitor on the output is essential for improving the transient response—the regulator's ability to quickly respond to sudden changes in load current. It further reduces output noise and ensures stability.
3. Voltage Regulator (MC7818CTG): The core component that provides regulation and protection features.
4. Optional Larger Capacitors: In practice, larger electrolytic or tantalum capacitors (e.g., 10μF to 100μF) are often added at both the input and output. The input capacitor smooths out ripples from a rectified AC source, while the larger output capacitor provides a charge reservoir for handling higher load transients.
Design Considerations
Heatsinking: The TO-220 package requires a heatsink when operating at high input-output differentials and high load currents. Power dissipation (PD) is calculated as PD = (VIN - VOUT) IOUT. Without a proper heatsink, the internal thermal protection will activate, shutting down the regulator.
Minimum Input Voltage: Always ensure the minimum input voltage remains above VOUT + Dropout Voltage (VDO), e.g., 18V + 2V = 20V, to prevent loss of regulation.
The Onsemi MC7818CTG stands as a testament to the enduring value of simple, linear regulator design. Its robust internal protection, ease of use, and ability to deliver a stable 18V at 1A make it an excellent choice for powering analog circuits, op-amp systems, and as a reliable secondary regulator in larger power systems where efficiency is not the primary concern.
Keywords:
1. Linear Voltage Regulator
2. Fixed Output
3. Thermal Protection
4. Application Circuit
5. Power Dissipation
