Onsemi MC79M12BDT: 12V Negative Voltage Regulator Datasheet and Application Circuit Design
The Onsemi MC79M12BDT is a robust, fixed-output, negative voltage regulator belonging to the popular 79MXX series. Designed to deliver a precise –12V output, this component is essential for applications requiring a stable and reliable negative voltage rail from an unregulated negative input. Housed in a thermally enhanced D²PAK surface-mount package, it is built to handle substantial power dissipation, making it suitable for a wide range of industrial and consumer electronics.
Key Datasheet Specifications and Features
A thorough review of the MC79M12BDT datasheet reveals its core electrical characteristics and operational limits. Key parameters define its performance and ensure safe operation within a system.
Output Voltage: –12V ±4%
Output Current: Up to 500mA
Input Voltage Range: –14.5V to –27V (for –12V output)
Line Regulation: 6mV (Typ.)
Load Regulation: 12mV (Typ.)
Ripple Rejection: 62dB (Typ.)
Operating Junction Temperature: 0°C to +125°C
The device incorporates internal current limiting, thermal shutdown, and safe-area compensation. These features are critical for protecting the regulator against overload conditions and potential burnout, significantly enhancing the reliability of the end application. The thermal shutdown feature deactivates the device if the junction temperature exceeds safe limits, reactivating only when the IC has cooled sufficiently.
Typical Application Circuit Design
Implementing the MC79M12BDT in a circuit is straightforward, but proper external component selection is vital for stability and performance. A standard application circuit is shown below.
```
Unregulated
Negative Input
──╱╲───┐
│
█ █ Ci
│
├───┤╲│ │
│ │ │ MC79M12BDT
│ │╱│ │
├───────┬─── -VOUT (–12V)
█ █ Co │
│ █
│ █ RL Load
│ █
│ │
──────────────┴─── GND
```
1. Input Capacitor (Ci):
A 0.33μF ceramic or tantalum capacitor should be connected between the input pin and ground. This capacitor is essential for improving circuit stability and input ripple rejection, especially when the regulator is located far from the power supply filter.
2. Output Capacitor (Co):
A 1μF tantalum or 10μF aluminum electrolytic capacitor is recommended on the output. This component is crucial for ensuring stability and improving the transient response of the regulator. It helps maintain a stable output voltage during sudden changes in load current.
Design Considerations:
Heat Sinking: The D²PAK package has excellent thermal performance but may still require a PCB heatsink (copper pour) for maximum power dissipation at full load current. The power dissipated (P_D) is calculated as P_D = (|V_IN| – |V_OUT|) I_OUT. Ensure the junction temperature does not exceed 125°C.
Protection Diodes: While often optional, adding reverse protection diodes (from input-to-output and output-to-ground) is considered good practice. They safeguard the regulator if the input is shorted to ground or if a large output capacitor holds charge after the input is removed.
The Onsemi MC79M12BDT stands out as a highly reliable and simple solution for generating a –12V power rail. Its integration of critical protection features like thermal shutdown and current limiting makes it exceptionally robust for demanding environments. While switching regulators offer higher efficiency, the MC79M12BDT remains the preferred choice for applications where simplicity, low noise, and cost-effectiveness are the primary design drivers.
Keywords: Negative Voltage Regulator, Thermal Shutdown, Application Circuit, Ripple Rejection, D²PAK Package
