onsemi MC79M05BDTG: Fixed -5V Linear Voltage Regulator Datasheet and Application Circuit Analysis
The onsemi MC79M05BDTG is a classic fixed-output negative linear voltage regulator belonging to the renowned 79MXX series. Designed to deliver a constant -5V output voltage at currents up to 500mA, this device is a fundamental component for powering a wide array of analog and digital circuits requiring a stable negative bias or supply rail. Its robustness, simplicity, and built-in protection features make it a perennial choice for engineers.
This analysis delves into the key specifications from its datasheet and breaks down its typical application circuit.
Key Datasheet Specifications and Characteristics
The MC79M05BDTG is packaged in a TO-252-3 (DPAK) package, offering a good balance between size and power dissipation capability. Understanding its absolute maximum ratings and electrical characteristics is crucial for reliable design.
Output Voltage: -5V ±4% (typically under specified conditions of line, load, and temperature). This tolerance ensures sufficient stability for most applications.
Output Current: A maximum of 500mA. The "M" suffix denotes the medium-current version of the 79XX family.
Input Voltage: The maximum recommended input voltage is -35V. For optimal performance and to minimize power dissipation, the input voltage should be maintained at least 2V more negative than the output (i.e., V_IN < -7V).
Line Regulation: 6mV (Typ.) / 100mA (Typ.). This measures the output voltage change for a variation in input voltage.
Load Regulation: 10mV (Typ.) / 500mA (Typ.). This measures the output voltage change for a variation in output current.
Thermal Protection: A critical feature that automatically shuts down the regulator if the internal junction temperature exceeds safe limits (~150°C), preventing catastrophic failure.
Short Circuit Protection: Limits the output current to a safe level if the output is shorted to ground, safeguarding both the regulator and the power source.
Internal Power Dissipation: The DPAK package has a maximum power dissipation of approximately 1.5W at 25°C, which is heavily dependent on the PCB's copper area acting as a heat sink.
Typical Application Circuit Analysis
The standard application circuit for the MC79M05BDTG is straightforward but requires specific external components for stability and protection.
```
[Simplified Circuit Diagram]
Unregulated Negative Input (V_IN) ----+---[C_i]---+----+---- V_OUT (-5V)
| | |
GND GND [C_o]
|
GND
```
Key Components:
1. Input Capacitor (C_i): A 0.33μF ceramic or tantalum capacitor placed close to the input pin is essential. Its primary role is to bypass high-frequency noise on the input supply line and improve the circuit's stability, especially when the regulator is located far from the primary power filter.
2. MC79M05BDTG Regulator: The core component. The input (pin 1) receives the unregulated negative DC voltage, the ground (pin 2) is connected to the system common, and the regulated -5V output is provided on pin 3.
3. Output Capacitor (C_o): A 0.1μF ceramic capacitor is recommended to be placed at the output. This capacitor significantly improves the transient response of the regulator, ensuring stability and reducing output impedance at high frequencies. While not always strictly required for stability, it is considered mandatory for robust performance.
Considerations for Reliable Operation
The major design challenge with linear regulators is thermal management. The power dissipated by the regulator is calculated as:
P_D = ( |V_IN| - |V_OUT| ) I_OUT
For example, with an input of -9V and a full 500mA load, the power dissipated would be (9V - 5V) 0.5A = 2.0 Watts. This exceeds the package's rating without an adequate external heat sink. Therefore, for high current or high input-output differentials, a proper heat sink attached to the DPAK tab is mandatory to keep the junction temperature within safe operating limits.
ICGOOODFIND Summary
The onsemi MC79M05BDTG is a highly reliable and easy-to-use solution for generating a stable -5V supply. Its value lies in its simplicity, integrated protection features (thermal and short-circuit), and predictable performance. While switching regulators offer higher efficiency, the MC79M05BDTG's low noise output and minimal external component count make it an excellent choice for low-to-medium power analog circuits, op-amp power rails, and as a secondary regulator in larger systems where efficiency is not the primary concern. Proper attention to input/output capacitance and thermal design is the key to maximizing its performance and longevity.
Keywords: Negative Voltage Regulator, Fixed -5V Output, 500mA, Linear Regulator, Thermal Protection
