The LM311 is a single high-speed differential voltage comparator that compares two input voltages and changes its output state according to which voltage is higher. Unlike an operational amplifier, it is designed mainly for fast switching instead of linear signal amplification. Its separate collector and emitter output connections allow it to work with different logic levels, LEDs, relays, and other loads. It also supports offset adjustment and strobe control. These features make the LM311 useful in voltage monitoring, zero-crossing detection, waveform shaping, oscillators, and protection circuits.

Catalog

LM311 Pinout Details

How the LM311 Work Internally

The LM311 compares the voltage at the non-inverting input, IN+, with the voltage at the inverting input, IN−. Its input transistor stage detects even a small voltage difference between these two pins and sends the result through several internal gain stages. These stages amplify the difference until the circuit can clearly decide which input voltage is higher.

When the voltage at IN+ is higher than IN−, the output transistor normally turns off. Because the LM311 uses an open-collector output, an external pull-up resistor then raises the collector output voltage. When IN− is higher than IN+, the output transistor turns on and pulls the collector output toward the emitter voltage, which is usually ground or the negative supply.

The internal circuit also includes balance and strobe functions. The balance pins can reduce input offset error, while the strobe pin can control the output stage. The separate collector and emitter output pins make the LM311 flexible for different logic voltages, relay drivers, level shifting, and other switching circuits.

LM311 Features and Specifications

LM311 Key Features

LM311 Absolute Maximum Ratings

LM311 Circuit Design Requirements

The LM311 requires a few external components and careful voltage selection for reliable operation. Proper supply decoupling, valid input voltages, and a suitable pull-up resistor help prevent false switching, incorrect output states, and excessive output current.

Supply Decoupling

Place a 0.1 µF ceramic bypass capacitor close to the LM311 supply pins. For a single-supply circuit, connect it between VCC+ and ground. For a dual-supply circuit, place one capacitor from VCC+ to ground and another from VCC− to ground. Keep the connections short to reduce unwanted inductance.

These capacitors reduce supply noise and provide current during fast switching. Poor decoupling can introduce supply glitches that affect the input common-mode range and cause inaccurate comparisons or false output transitions. An additional 1 µF to 10 µF bulk capacitor may be useful when the power source is noisy or connected through long wires, but it does not replace the nearby 0.1 µF ceramic capacitor. TI recommends bypassing each supply rail used by the device.

Input Voltage Limits

Several voltage limits must be checked separately when designing an LM311 circuit:

• Supply-voltage limits define the permitted voltage between the positive and negative supply pins. The LM311 can operate from a single supply or split supplies, but the supply voltage must remain within the operating range specified by the selected manufacturer.

• Input common-mode limits define the input-voltage range in which the comparator is specified to make a valid comparison. With ±15 V supplies, TI specifies a common-mode range of approximately −14.5 V to +13 V over the full temperature range. These values apply to the stated test conditions and can change with the supply voltage, temperature, and device manufacturer.

• Maximum differential input voltage is the greatest permitted voltage difference between the non-inverting and inverting inputs. It is mainly a device-protection limit and does not define the normal input operating range.

• Absolute maximum ratings are stress limits intended to prevent immediate damage. They are not recommended operating conditions, and normal circuit operation is not guaranteed near these limits.

A circuit can remain below its absolute maximum ratings and still operate incorrectly. For example, when both inputs rise above the valid common-mode range, the output may become undefined even though the IC is not immediately damaged. For reliable operation, keep both inputs within the common-mode range specified in the relevant datasheet.

Pull-Up Resistor Selection

When pin 7 is used as the normal open-collector output, the LM311 requires an external pull-up resistor. The minimum resistance needed to limit the output sink current can be estimated with:

Where:

• R_PUis the pull-up resistance

• V_PU is the pull-up supply voltage

• V_OL is the comparator’s output-low voltage

• I_SINK is the selected output sink current

For example, with a 5 V pull-up supply, an estimated 0.4 V output-low voltage, and a desired sink current of 5 mA:

A standard 1 kΩ resistor would limit the current to approximately 4.6 mA under these assumed conditions:

The final resistor value must also meet the logic thresholds of the connected circuit. A lower resistance produces a faster rising edge and makes the output less sensitive to coupled noise, but it increases sink current and power consumption. A higher resistance reduces current consumption but creates a slower rising edge because it takes longer to charge the output, wiring, and input capacitance. Output leakage and load current must also be considered to ensure that the output reaches a valid high voltage.

Unused Balance and Strobe Pins

Pins 5 and 6 provide offset-balancing and strobe functions. When these functions are not required, follow the instructions in the datasheet for the exact LM311 version and manufacturer.

For TI’s LM311, the BALANCE pin and BAL/STRB pin may be left unconnected. TI also states that connecting these two pins together is acceptable. Do not connect the BAL/STRB pin directly to ground. Pulling current from this pin activates the strobe function and forces the output transistor into its off, or high-impedance, state regardless of the comparator inputs. These recommendations are confirmed in the TI LM311 datasheet.

LM311 Application Circuits

Zero-Crossing Detector

This circuit uses the LM311 to detect when an input signal crosses 0 V. One input receives the signal, while the other input is connected to ground as the reference level. When the input rises above 0 V, the LM311 output transistor turns off, and the 20 kΩ pull-up resistor raises the output toward V_(CC+). When the input falls below 0 V, the output transistor turns on and pulls the output toward V(CC-). The circuit therefore converts a signal that crosses zero, such as a sine wave, into a high-or-low output signal.

100-kHz Free-Running Multivibrator

This circuit configures the LM311 as a free-running oscillator that produces a square wave of about 100 kHz. The 1200 pF capacitor repeatedly charges and discharges through the resistor network. When the capacitor voltage reaches one switching threshold, the LM311 changes its output state. Feedback then changes the threshold and makes the capacitor voltage move in the opposite direction. This cycle repeats automatically without an external input signal. The 1 kΩ resistor pulls up the open-collector output, while the other resistors set the switching thresholds and oscillation timing. The output can drive up to two standard 54-series logic-gate inputs or an equivalent load.

LM311 Voltage-Level Indicator Circuit

This circuit uses the LM311 to compare an adjustable input voltage with a fixed 6 V reference. The Zener diode and 1 kΩ resistor produce the reference voltage for pin 2, while the 10 kΩ potentiometer supplies the adjustable input voltage to pin 3.

When the input voltage rises above the 6 V reference, the LM311 output transistor turns on and pulls pin 7 low. Current then flows through the 560 Ω resistor and LED, causing the LED to light. When the input falls below 6 V, the output transistor turns off, and the 4.7 kΩ pull-up resistor raises the output voltage, turning the LED off. This circuit can serve as an adjustable overvoltage indicator or voltage-level alarm.

LM311 Application Curves

The curves show how quickly the LM311 output responds to different input overdrive voltages. Overdrive is the amount by which the differential input exceeds the switching threshold. The tests use ±15 V supplies, a 2 kΩ output load, a 100 mV input step, and a temperature of 25°C.

The left graph shows the output changing from approximately −15 V to +15 V, while the right graph shows the opposite transition. A larger overdrive, such as 20 mV, makes the output switch faster. A small 2 mV overdrive causes a longer delay and slower transition because the input is only slightly beyond the threshold. These curves show that a clear input-voltage difference improves the LM311 switching speed and produces a more reliable output transition.

LM311 vs Similar Comparator ICs

LM311 vs LM393

The LM311 is a faster single comparator with stronger output-current capability, separate collector and emitter connections, and balance and strobe functions. The LM393 contains two comparators, consumes less power, and accepts input voltages near ground. Choose the LM311 for fast switching and flexible output control. Choose the LM393 for low-power circuits that need two comparison channels.

LM311 vs LM339

The LM339 contains four low-power comparators, while the LM311 contains one faster and more flexible comparator. The LM339 is suitable for monitoring several voltage, temperature, or sensor signals in one circuit. The LM311 is better when one channel needs faster switching, stronger current sinking, or strobe control.

LM311 vs LM319

The LM311 is a single comparator, while the LM319 contains two faster comparators. The LM319 is suitable for high-speed pulse detection, timing, and waveform-shaping circuits, but it generally consumes more power and has a different pinout. The LM311 is better when one comparator with strobe, balance, and flexible output connections is required.

LM311 vs LM358

The LM311 is a comparator designed for threshold detection and fast switching. The LM358 contains two operational amplifiers intended mainly for amplification, filtering, and buffering. Although the LM358 may work as a slow comparator in some circuits, it can recover slowly from saturation and may not switch as predictably as the LM311.

LM311 vs Modern Rail-to-Rail Comparators

Modern comparators often operate at lower supply voltages, consume less current, and accept input voltages close to both supply rails. Many also provide push-pull outputs, built-in hysteresis, internal references, and smaller packages. The LM311 remains useful for higher-voltage circuits, strong current sinking, floating-output connections, and designs that need strobe or balance control.

LM311 Replacement and Equivalent ICs

LM311 Applications and Use Cases

• Battery-voltage monitoring – Compares battery voltage with a reference and activates a warning when the voltage becomes too high or too low.

• Zero-crossing detection – Detects when an AC signal crosses 0 V for timing, switching, and frequency-measurement circuits.

• Overvoltage protection – Detects when a supply voltage exceeds a safe limit and triggers a shutdown or protection circuit.

• Undervoltage lockout – Prevents equipment from operating when the supply voltage is too low for reliable operation.

• Temperature monitoring – Compares a temperature-sensor signal with a preset limit to control a fan, heater, or alarm.

• Light-level detection – Works with a photodiode or photoresistor to switch lights or alarms according to brightness.

• Motor-speed monitoring – Compares a speed-sensor signal with a reference to detect overspeed or low-speed conditions.

• Waveform shaping – Converts slow, noisy, or irregular analog signals into cleaner square-wave signals.

• Oscillator circuits – Works with resistors and capacitors to generate repeating square-wave signals for timing or clock applications.

• Pulse detection – Detects short voltage pulses from sensors, encoders, and communication circuits.

• Relay and LED control – Uses its current-sinking output to control indicator LEDs, relay drivers, or transistor stages.

• Logic-level interfacing – Uses the open-collector output to connect analog signals to TTL, CMOS, or other logic-voltage systems.

• Window detection – Uses two comparators to check whether a voltage remains between an upper and lower limit.

• Current monitoring – Compares a current-sense voltage with a reference to detect overload or short-circuit conditions.

• Industrial alarm systems – Monitors pressure, level, temperature, or voltage sensors and activates an alarm when a set limit is crossed.

Mechanical Dimensions

Manufacturer

Texas Instruments designs and manufactures analog ICs such as the LM311 through its global semiconductor production network. The manufacturing process includes wafer fabrication, circuit formation, wafer testing, assembly, packaging, and final electrical testing. These steps help ensure that the LM311 meets its specified input, output, switching-speed, and temperature requirements. TI also controls product quality through device qualification, material testing, process monitoring, and ongoing reliability checks. It provides manufacturing and quality information such as fabrication location, assembly location, material content, moisture-sensitivity rating, and reliability estimates.