Onsemi MC7924CTG: -24V Fixed Negative Linear Voltage Regulator Datasheet and Application Circuit Analysis

The Onsemi MC7924CTG is a member of the classic MC79XX fixed negative voltage regulator family, designed to deliver a stable -24V output at currents up to 1.0A. These components are renowned for their robustness, internal current limiting, thermal shutdown, and safe-area compensation, making them highly reliable for a wide range of industrial and consumer applications. This analysis delves into the key specifications from its datasheet and explores a standard application circuit.

Key Datasheet Specifications and Characteristics

The MC7924CTG is a three-terminal device in a TO-220 package, facilitating easy mounting to a heatsink for power dissipation. Its primary electrical characteristics include:

Output Voltage: -24V ±4%. This tolerance is guaranteed over the entire specified line and load condition range.

Input Voltage Range: -27V to -38V. For stable regulation, the input must be at least 3V more negative than the output voltage (-27V), while the absolute maximum input voltage is -40V, beyond which the device can be damaged.

Output Current: Up to 1.0A continuous.

Line Regulation: 120 mV (Typ.). This measures the output voltage change for a specified input voltage change.

Load Regulation: 240 mV (Typ.). This measures the output voltage change for a change in load current from 5mA to 1.0A.

Thermal Resistance, Junction-to-Case (RθJC): 5 °C/W. This is a critical parameter for heatsink calculation.

Operating Junction Temperature (Tj): 0°C to +150°C. The internal thermal shutdown activates to protect the IC if this limit is exceeded.

Standard Application Circuit Analysis

A typical application circuit for generating a fixed -24V supply is shown below. The circuit requires only two external components for basic operation, though additional components are highly recommended for stability and protection.

1. Input Capacitor (C_IN):

A 0.33μF ceramic or tantalum capacitor should be connected between the input pin and common ground (GND). This capacitor is essential for improving transient response and stability, especially when the regulator is located several inches from the power supply filter.

2. Output Capacitor (C_OUT):

A 0.1μF ceramic capacitor is connected from the output pin to GND. This component is crucial for enhancing the regulator's stability and transient response. It prevents the output from oscillating under certain load conditions.

3. Protection Diodes (Optional but Recommended):

While not always mandatory, adding diodes protects the regulator from potential fault conditions:

A diode (e.g., 1N4002) connected across the input and output terminals protects the regulator against reverse voltage if the input is shorted to ground. This is because the output capacitor can discharge backwards through the low-impedance path of the regulator, potentially causing damage.

A diode from the output to the input pin can protect against a similar reverse-bias condition.

Heatsinking Requirement

A fundamental aspect of using linear regulators like the MC7924CTG is managing power dissipation. The power dissipated by the regulator is calculated as:

P_D = ( |V_IN| - |V_OUT| ) × I_LOAD

For example, with an input of -30V, an output of -24V, and a 0.5A load, the power dissipated would be (30V - 24V) 0.5A = 3 Watts. This significant heat must be dissipated through an adequately sized heatsink attached to the TO-220 package to keep the junction temperature within safe limits.

ICGOODFIND Summary

The Onsemi MC7924CTG is a highly reliable and simple-to-use solution for generating a stable -24V power rail. Its key advantages are its minimal external component count and built-in protection features like current limiting and thermal shutdown. Designers must pay close attention to the input-output voltage differential, provide adequate input and output capacitance for stability, and most critically, implement a properly calculated heatsink to ensure reliable operation under full load conditions.

Keywords:

Negative Voltage Regulator

Linear Regulator

Heatsink

Application Circuit

Thermal Shutdown