LM393 dual comparator: pinout, the pull-up, hysteresis, the LM339 and the LM2903

The 393 is the dual and the 339 the quad. An A after the number (LM393A, LM339A) marks a part sorted for lower offset. A B (LM393B, LM339B) is TI's newer die, covered under substitutes. The last letter is the package: P or N is the through-hole DIP, D the SOIC. An LM393P is the plain dual in an 8-pin DIP.

The LM2903 and the LM293 sit in the same datasheet with the same pins; only the temperature range and a few limits change. An LM2903 is rated for −40 to 125 °C, so it turns up in cars and outdoor boards.

TI allows 36 V across the LM393 and recommends 2 to 30 V. Either input may go from 0.3 V below ground to 36 V, whatever the supply, and the differential limit is the full ±36 V. That is unusual: you can feed a 12 V signal into a comparator running from 5 V without damage.

The output can be pulled up to anything up to 36 V, which is how a 5 V comparator drives a 12 V relay driver or a 3.3 V microcontroller pin. It may be shorted to ground indefinitely; TI warns that a short to VCC can overheat and destroy it. An input pulled more than 0.3 V below ground turns on a parasitic transistor and can make the output wrong until the current stops.

The inputs work from 0 V up to 1.5 V below VCC at 25 °C, and up to 2 V below over 0 to 70 °C. On 5 V that is 0 V to 3.5 V, or 3 V when warm. TI also notes that only one input has to be inside that range. As long as the reference sits there, the other input may go all the way to VCC and the output still gives the right answer.

The output transistor sinks at least 6 mA with 1.5 V across it, and at 4 mA it sits at 130 mV typical, 400 mV at most (700 mV when hot). When it is off it leaks 50 nA at most. Response time is 1.3 µs for a 100 mV step that overdrives the input by 5 mV, and 0.3 µs for a full logic-level step. The whole package draws 0.45 mA typical, 1 mA at most, on 5 V.

• Turning an analog level into a logic signal: a light sensor, a thermistor divider, a battery voltage against a threshold.
• Level shifting. The comparator runs from 5 V or 12 V and its pull-up goes to the 3.3 V of the microcontroller that reads it.
• Zero-crossing and window detectors. Two outputs pulled up through one resistor form a wired-AND, so both comparators of one LM393 make a window.
• Overcurrent and undervoltage trips, where the output shuts something down.

It is the wrong part for edges that must arrive faster than its 1.3 µs response time, for a logic output with no room for a resistor, and for anything that needs a linear output. For an amplifier, use the LM358, which has the same pins and a push-pull output. The LM358 vs LM393 page covers what breaks when you swap them.

One half of an LM393 on 5 V watches a sensor voltage on pin 2. Its output goes to an input pin of a 3.3 V microcontroller and should go low when the sensor passes about 2.5 V, without chattering while the signal drifts across that point.

Fit 100 nF from pin 8 to ground, close to the package. Without hysteresis a slow input crossing the threshold can make the output switch many times as noise rides on it; the comparator lesson shows why.

The LM339 puts four of the same comparators in a 14-pin DIP, with the same input range, offset, bias current and response time in TI's tables. Watch the supply pins: VCC is pin 3 and GND is pin 12, and the four outputs sit on pins 1, 2, 13 and 14. TI also notes that some makers swap the names of channels 1 and 2; the pins themselves are the same.

The whole LM339 draws 0.8 mA typical and 2 mA at most on 5 V, and its output sinks 16 mA typical at 1.5 V. Tie the inputs of any unused comparator to ground so it sits in a known state.

The LM2903 uses the LM393's pins and the same data sheet. It is rated from −40 to 125 °C against 0 to 70 °C, and pays for it with a wider offset: 7 mV maximum at 25 °C and 15 mV over temperature, against 5 and 9 mV. In the example above the worst band of 135 mV is still nine times that 15 mV, so the same resistors work.

• Every output needs its own pull-up unless you want two outputs to share one as a wired-AND. With no resistor the output just floats.
• Pull up to the voltage of whatever reads the output, not the comparator's own supply. A 5 V pull-up into a 3.3 V microcontroller pin overdrives it.
• Keep the reference input below VCC − 2 V. The signal input can go higher, as long as the other one stays in range.
• Add hysteresis for any input that changes slowly or carries noise. A feedback resistor ten times the divider resistance gives about 150 mV here.
• Never let an input go more than 0.3 V below ground. A series resistor of a few kilohms protects an input that comes from off the board.

The LM393A, LM393B and LM2903 drop into the same socket. The LM393B is the one to buy for new work: lower offset, lower bias current, less supply current and better ESD. For four comparators use the LM339, and rewire the supply.

1. Power the chip from 5 V, fit 10 kΩ from pin 1 to 5 V and put 2.5 V on pin 3 from two equal resistors.
2. Put 2 V on pin 2. Pin 1 should read close to 5 V. Raise pin 2 to 3 V: pin 1 should drop to a few tens of millivolts.
3. Remove the pull-up and measure pin 1 with the input at 2 V. A multimeter reads nothing useful, which is normal: the output is off and floating.
4. Measure the supply current with no load: about 0.45 mA, 1 mA at most. Much more points to a damaged part or an input taken below ground.

Use the LM393 wherever an analog level has to become a logic signal, pull each output up to the supply of whatever reads it, and add a feedback resistor so the band is at least ten times the 9 mV offset. Buy the LM393B for new boards, and use the LM2903 when the board lives outdoors or in a car.

The LM393P in DIP-8 and the LM339N in DIP-14 are stocked by most Indian component sellers. Ask for TI or another named maker, and check one from each batch with the bench test above before it goes into a board. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.