LM79

The LM79 is a family of fixed negative-voltage linear voltage regulators in three-terminal packages. They provide a regulated negative output voltage with respect to ground and are the complementary counterparts to the LM78xx positive regulators. The LM79xx devices were developed by National Semiconductor and later produced by other manufacturers, including Texas Instruments after acquisitions. They are available in common packages such as TO-220, TO-3, and various surface-mount forms.

Operation and features: LM79xx regulators hold the output at a fixed negative voltage, such as -5 V, -12 V, -15 V, and so on, relative to ground. The input must be more negative than the desired output by a dropout amount, typically around 2 V, depending on load current and temperature. They include protection features such as current limiting and thermal shutdown, and can deliver about 1 A of output current in standard packages with adequate heat sinking. Their performance is suitable for supplying negative rails in dual-supply power systems and other analog circuits requiring a stable negative reference.

Pinout and usage: In many TO-220 variants, viewed from the front with the leads down, the common pinout is pin 1 ground, pin 2 output, and pin 3 input, with the tab usually connected to ground. However, pin arrangements vary between package types and manufacturers, so consulting the specific datasheet is essential for correct wiring. Capacitors are recommended for stability: typically a decoupling capacitor on the input (around 0.33 µF) and a capacitor on the output (around 0.1 µF), with larger electrolytics used if required for load transients.

Variants: The suffix xx in LM79xx denotes the magnitude of the negative output (for example, LM7905 provides -5 V).