Regulating Negative Voltage: A Circuit Design Guide Using the onsemi MC7915CT
In many electronic systems, particularly those involving operational amplifiers, analog-to-digital converters, or audio circuits, the need for a stable and reliable negative voltage supply is paramount. While positive voltage regulation is commonplace, generating a clean negative rail presents unique challenges. This guide explores the design and implementation of a negative voltage regulator circuit using the robust onsemi MC7915CT, a classic -15V fixed linear regulator.
The onsemi MC7915CT belongs to the venerable MC79XX series of negative fixed voltage regulators. Its primary function is to take an unregulated, negative input voltage (more negative than -15V) and provide a precisely regulated -15V DC output with a maximum current of 1A. This device simplifies power supply design by integrating a voltage reference, an op-amp, current-limiting circuitry, and thermal shutdown protection into a single three-terminal package (Input, Common Ground, Output).
Key Circuit Design Considerations
A typical application circuit for the MC7915CT is elegantly simple but requires careful attention to a few critical details to ensure stable operation and long-term reliability.
1. Input Voltage (VIN): The input voltage must be more negative than the desired output voltage. For the MC7915CT, the input must typically be between -19V and -35V. A minimum voltage difference, or headroom (dropout voltage), of 2V is required, but a larger margin is advisable, especially under varying load conditions.
2. Capacitor Selection: The choice of capacitors is crucial for preventing oscillations and ensuring good transient response.
Input Capacitor (CIN): A 0.33 μF ceramic or tantalum capacitor should be placed as close as possible to the input pin. This capacitor bypasses high-frequency noise and helps suppress any tendency to oscillate.
Output Capacitor (COUT): A 1 μF tantalum or 10 μF aluminum electrolytic capacitor on the output is recommended to improve stability and transient response. Like the input capacitor, its placement near the regulator's output pin is critical.
3. Heat Sinking: Linear regulators dissipate excess power as heat. The power dissipated (PD) is calculated as:
PD = (VIN - VOUT) ILOAD
For example, with an input of -25V, an output of -15V, and a 500mA load, the power dissipated would be (10V 0.5A) = 5W. This significant power necessitates an adequate heat sink attached to the regulator's metal tab to keep its junction temperature within safe limits and prevent thermal shutdown.
4. Protection Diodes: In complex systems, it is good practice to add protection diodes. A diode from the output to the input can prevent the internal circuitry from being damaged if the output is held at a higher voltage than the input (e.g., during shutdown or if a backup supply is present on the output rail).
A Basic Application Circuit
The schematic for a core -15V power supply is straightforward:
An unregulated negative DC input is connected to the Input pin of the MC7915CT.
The Common Ground pin is connected to the system ground (0V).
The Output pin delivers the regulated -15V.
Capacitor CIN (0.33μF) bridges the Input and Ground.
Capacitor COUT (1μF) bridges the Output and Ground.
A heat sink is mounted on the component.
This circuit will reliably convert a noisy input like -20V into a clean, stable -15V output for powering sensitive analog components.
The onsemi MC7915CT remains a highly effective and straightforward solution for generating regulated negative voltage rails. Its integrated protection features and simple application requirements make it an excellent choice for designers seeking reliability and performance in analog and mixed-signal power systems. Proper attention to heat dissipation and capacitor placement is the key to a robust implementation.
Keywords:
Negative Voltage Regulation
Linear Regulator
MC7915CT
Circuit Design
Power Supply
