LM311

The LM311 is a voltage comparator integrated circuit originally developed by National Semiconductor and now produced by Texas Instruments. It compares two analog input voltages and outputs a digital signal indicating which input is higher. It is widely used in threshold detection, level shifting, zero-crossing detection, and general comparator circuits in consumer electronics and industrial applications.

The device features an open-collector output, which means the output transistor can sink current but cannot source it. An external pull-up resistor to a positive supply is required. The open-collector design also allows wired-OR connections, enabling simple logic combining of multiple comparators.

The LM311 has differential input stages with a common-mode input range that can include ground when operated from a single supply, though inputs must remain within the specified range relative to Vcc. It includes a strobe (enable) input that can disable the output, and without strobe the device behaves as a standard comparator. Inputs are high impedance and protected against overrange conditions within specified limits.

The device operates over a wide supply voltage range, typically from about 5 to 30 volts, with various package options such as 8-pin DIP and small-outline packages. Its fast response and simple interface make it a common choice for reference thresholds, window detectors, battery monitoring, and power-supply protection circuits.

The LM311 remains a staple in analog and mixed-signal design and is often discussed alongside related comparators such as the LM393 and LM339 families.