Onsemi MC7815CT: 15V Positive Voltage Regulator Datasheet and Application Circuit Analysis

The Onsemi MC7815CT is a classic example of a robust, fixed-output, positive linear voltage regulator. As a member of the ubiquitous 78xx series, it provides a simple and effective solution for deriving a stable +15V DC output from a higher, unregulated input voltage. This article delves into the key specifications from its datasheet and analyzes a standard application circuit.

Key Datasheet Specifications and Characteristics

The MC7815CT is a three-terminal device (Input, Output, and Ground) packaged in a robust TO-220 case, which facilitates easy mounting to a heatsink for efficient power dissipation.

Output Voltage: +15V ±4%. This tolerance is guaranteed over the entire operating conditions, ensuring a reliable voltage rail for downstream components.

Output Current: Capable of delivering a maximum output current of 1A, making it suitable for a wide range of low-to-medium power applications.

Input Voltage Range: For proper regulation, the input voltage must typically be maintained 2.5V to 3V above the output voltage. For a 15V output, this translates to a recommended input voltage between 17.5V and 35V. Exceeding the absolute maximum input voltage (typically 35V) can damage the device.

Line Regulation: Typically 6mV. This specifies the output voltage change for a given variation in input voltage, demonstrating its ability to reject input ripple.

Load Regulation: Typically 12mV. This specifies the output voltage change when the output current varies from 5mA to its full 1A load.

Thermal Protection: A critical built-in feature is internal thermal shutdown, which disables the regulator if the junction temperature exceeds ~150°C, protecting it from damage due to overheating.

Short Circuit Protection: The IC includes internal current limiting that safeguards both the regulator and the power source in the event of an output short circuit.

Standard Application Circuit Analysis

The basic application circuit for the MC7815CT is straightforward but requires careful consideration of a few external components for stable and reliable operation.

1. Input Capacitor (C_IN - 0.33µF): A ceramic or polyester capacitor placed close to the regulator's input pin. Its primary role is to bypass high-frequency noise on the input line and to improve the circuit's stability, especially when the regulator is located at a distance from the primary power supply filter.

2. Output Capacitor (C_OUT - 0.1µF): This capacitor, also placed very close to the regulator, is crucial for improving the transient response—its ability to react quickly to sudden changes in load current. It also further reduces output noise and high-frequency oscillations.

3. Heatsinking (TO-220 Package): Power dissipation is a fundamental consideration for linear regulators. The power dissipated (P_D) is calculated as (V_IN - V_OUT) I_LOAD. For example, with an input of 20V and a full 1A load, the regulator must dissipate (20V - 15V) 1A = 5 Watts of heat. Without an adequate heatsink, the internal junction temperature will rapidly exceed its limit, triggering thermal shutdown. The TO-220 package is designed to be attached to an external heatsink to efficiently radiate this heat.

4. Optional Diodes: For scenarios where the input supply might be switched off faster than the output (e.g., when a large output capacitor is used), a protection diode from the output to the input pin is sometimes added. This prevents the output capacitor from discharging back through the regulator, which could potentially cause internal damage.

ICGOODFIND Summary

The Onsemi MC7815CT is a quintessential, easy-to-use linear voltage regulator prized for its robustness, built-in protections (thermal and short-circuit), and stable 15V output. Its primary drawback is inefficiency at higher input-to-output differentials, where switching regulators become more favorable. However, for applications requiring a simple, low-noise, and reliable +15V power rail with currents up to 1A, the MC7815CT remains an excellent and cost-effective choice.

Keywords:

Linear Voltage Regulator

Fixed Output

Thermal Protection

Application Circuit

TO-220 Package