onsemi MC79M08BDTRKG: A Comprehensive -8V, 500mA Negative LDO Voltage Regulator Guide
Voltage regulation is a cornerstone of stable and reliable electronic design, particularly for systems employing negative supply rails for analog circuitry, operational amplifiers, or sensors. The onsemi MC79M08BDTRKG stands as a robust and dependable solution in this domain. This component is a fixed -8V, 500mA negative voltage regulator, offering designers a simple and effective way to derive a well-regulated negative voltage from an unregulated or noisy input source.
As a member of the classic 79MXX series, this regulator is characterized by its internal current limiting, thermal shutdown, and safe-area compensation, which make it remarkably resilient to overload conditions and virtually indestructible when properly heatsunk. The `D2PAK` (TO-263) package provides an excellent balance between compact board space and superior thermal performance, allowing it to dissipate heat efficiently and deliver its full 500mA output current.
Key Features and Electrical Characteristics
The MC79M08BDTRKG is engineered for performance and durability. Its primary specifications include:
Output Voltage: -8V ±4% (under specified conditions)
Output Current: Up to 500mA
Dropout Voltage: Typically 2V at 500mA (ensuring the input must be at least ~-10V)
Line Regulation: 60mV (Typ.)
Load Regulation: 120mV (Typ.)
Ripple Rejection: 62dB (Typ.), which is highly effective at attenuating input noise
Application Circuit Design Guide
The basic application circuit for the MC79M08BDTRKG is straightforward, requiring only a handful of external components for stable operation. The standard configuration is shown below:
```
Unregulated
Negative Input
(e.g., -12V to -20V)
|
[C_i]
|
+----[MC79M08BDTRKG]----+-----> Regulated -8V Output
| IN GND OUT |
| |
[GND] [C_o]
|
[GND]
```
Input Capacitor (C_i): A 0.33μF ceramic or tantalum capacitor placed close to the input pin is recommended to improve circuit stability and transient response. It helps neutralize inductive effects from long input traces.
Output Capacitor (C_o): A 0.1μF ceramic capacitor is essential for ensuring stability and preventing oscillations. For improved transient response, a larger aluminum electrolytic capacitor (e.g., 1μF to 10μF) can be added in parallel, especially for dynamic loads.
Heatsinking: For continuous operation near the maximum current rating or with a high input-to-output voltage differential, proper heatsinking is critical. The tab of the D2PAK package is electrically connected to the ground pin and must be soldered to a sufficient copper pour on the PCB to act as a heatsink. The power dissipation is calculated as Pd = ( |V_in| - |V_out| ) I_out. If Pd is significant, a dedicated heatsink attached to the package tab is mandatory to prevent the internal thermal shutdown from activating.
Design Considerations and Best Practices
1. Protection Diodes: In applications where the input is switched or the output is connected to a large capacitive load, the regulator can be susceptible to reverse voltage stress. Adding protection diodes—a reverse-biased diode from input to output and another from ground to output—can safeguard the IC against these reverse bias conditions.
2. Noise Reduction: While the built-in ripple rejection is excellent, for extremely noise-sensitive applications, adding a small bypass capacitor (e.g., 10nF) across the upper feedback resistors (in adjustable configurations) or using an LC filter on the output can further reduce noise.
3. PCB Layout: For optimal performance, keep the input and output capacitor loops as small as possible by placing the components immediately adjacent to the regulator's pins. Use a solid ground plane to provide a low-impedance return path and aid in heat dissipation from the package.
The onsemi MC79M08BDTRKG is a proven workhorse for generating local -8V power rails. Its combination of robust protection features, a thermally-adept package, and simple implementation makes it an ideal choice for designers seeking a no-fuss, reliable solution for analog and mixed-signal power requirements. Its ability to deliver a stable -8V output from a widely available, higher negative voltage source simplifies power architecture design in countless applications.
Keywords:
Negative Voltage Regulator
LDO (Low-Dropout Regulator)
500mA
Application Circuit
Thermal Protection
