LM393 Pinout Guide — Dual Differential Comparator, DIP-8/SOP-8, Specifications & Common Mistakes | ICMASS

LM393 Pinout and Pin Functions

Typical Application Circuit

Common Mistakes When Using the LM393

Mistake 1: Missing pull-up resistor (the #1 cause of "LM393 not working"). The output transistor can only sink current to ground. Without a pull-up resistor (typically 3.3kΩ to 10kΩ) from OUT to VCC, the output floats — it never reaches a valid logic HIGH. Your multimeter reads 0V at the output and you assume the comparator isn't switching. It is switching just fine; there's nothing pulling the output up.

Mistake 2: No hysteresis = oscillation at the threshold. When the input signal crosses the reference voltage slowly (any real-world sensor signal, a charging capacitor, a temperature change), noise around the threshold causes the output to chatter at hundreds of kHz. The fix: a feedback resistor (Rf) from OUT to IN1+. This creates two separate thresholds (Vth+ and Vth-) separated by a few tens of millivolts. The input must cross the full gap to trigger a state change. On StackExchange, users report that adding 10–50mV of hysteresis eliminated instability in every case.

Mistake 3: Both inputs outside the valid common-mode range. The LM393's inputs work from 0V to VCC−1.5V. If both inputs of a channel go above VCC−1.5V (or below ground), the output becomes unpredictable and usually goes low. This bites designers who divide a 5V rail with equal resistors expecting a 2.5V threshold on a 3.3V VCC — 2.5V is above 3.3−1.5=1.8V. The fix: use resistor ratios that keep at least one input below VCC−1.5V, or increase VCC.

Mistake 4: Pull-up voltage higher than the downstream logic can tolerate. The open-collector output lets you pull up to any voltage within the 36V rating — a feature, not a bug. But if you pull up to 12V and connect the output directly to a 3.3V microcontroller GPIO, you'll destroy the MCU. Pull up to the logic supply voltage that your downstream device expects. If you need level translation, the LM393 does it natively: connect the pull-up to the target voltage rail.

Mistake 5: Forgetting that each channel needs its own pull-up. OUT1 and OUT2 are independent open-collector outputs from separate comparators. They share a package and a VCC pin, but they do not share an output stage. If you add a pull-up on OUT1 and expect OUT2 to work, it won't. Pull each output separately.

Mistake 6: Resistor divider values too low. The LM393's inputs draw ~25nA of bias current. You don't need 1kΩ divider resistors to drive them. Use 100kΩ to 1MΩ resistors for reference dividers — the bias current error is negligible (25nA × 100kΩ = 2.5mV), and you save milliamps of quiescent current. On battery-powered designs, this alone can double run time.

Mistake 7: Connecting the output directly to a capacitive load. Long traces, MOSFET gates, and decoupling capacitors on the output pin slow down the rising edge (the pull-up resistor forms an RC filter). The comparator still works, but the output rise time stretches from nanoseconds to microseconds. For driving MOSFETs, add a gate driver or at minimum lower the pull-up to 1kΩ. For long PCB traces, keep the pull-up close to the LM393 pin, not the destination.

PCB Layout Tips

• Keep input traces short and away from the output. The output swings rail-to-rail in nanoseconds. Even 1pF of parasitic coupling from OUT1 to IN1+ can cause oscillation. Separate input and output traces by at least 2mm, and never route them parallel.
• Place the bypass capacitor within 5mm of the VCC pin. 0.1µF ceramic, directly from pin 8 to the ground plane. The LM393 switches in ~300ns; without a local bypass, the supply rail dips and both channels misbehave.
• Pull-up resistors close to the output pin. Minimize the trace length from OUT to Rpull. The high-impedance node between the open-collector transistor and the pull-up is sensitive to capacitive loading.
• Star ground for single-supply designs. Connect the LM393's GND (pin 4), the reference divider ground, the input signal ground, and the bypass cap ground at a single point. This prevents output switching currents from modulating the reference voltage through shared ground impedance.

Frequently Asked Questions

Q1: Why is my LM393 output always LOW?

A: Three likely causes, in order of probability: (1) Missing pull-up resistor on the output. The LM393 is open-collector — it cannot drive high by itself. Add 3.3kΩ to 10kΩ from OUT to VCC. (2) Both inputs are outside the valid common-mode range (0V to VCC−1.5V). Keep at least one input within range. (3) The inverting input really is higher than the non-inverting input, and the output is correctly LOW. Check your input voltages with a multimeter before blaming the IC.

Q2: How do I add hysteresis to the LM393?

A: Add a resistor (Rf) from the output to the non-inverting input. This creates positive feedback. The two thresholds become: V_th+ = V_REF + (VCC − V_REF) × R1/(R1+Rf) and V_th- = V_REF − V_REF × R1/(R1+Rf), where R1 is the resistor from VIN to IN+. For a 5V supply with V_REF=2.5V, R1=100kΩ, Rf=1MΩ: hysteresis window is about 25mV. Increase Rf for less hysteresis, decrease for more.

Q3: Can I power the LM393 from a single 3.3V rail?

A: Yes, but watch the input range. Single supply works from 2V to 36V. At 3.3V VCC, the valid input common-mode range is 0V to ~1.8V (VCC−1.5V). If your reference voltage is 2.5V from a bandgap reference, it will be out of range. Either increase VCC to 5V, or use resistive dividers to keep inputs below 1.8V.

Q4: What is the difference between LM393 and LM339?

A: LM393 = dual (2 comparators in 8-pin). LM339 = quad (4 comparators in 14-pin). Same silicon, same specs, same behavior. Use LM393 when you need 1–2 comparators, LM339 when you need 3–4. Pinout is different (14-pin vs 8-pin), but the building block inside each channel is identical.

Q5: Can I use an LM393 as an op-amp?

A: No — and don't try. The LM393 is a comparator, not an op-amp. It has no internal frequency compensation, its output stage is open-collector (not push-pull), and it's optimized for saturation, not linear operation. In a closed-loop op-amp configuration, it will oscillate violently. Use an actual op-amp like LM358 for amplification. Use the LM393 for comparing two voltages and giving a digital HIGH/LOW answer.

Q6: How fast can the LM393 switch?

A: Large-signal response: ~300ns. Small-signal: ~1.3µs. Large-signal means the input overdrives the threshold by 100mV or more — the output slews at its maximum rate. Small-signal means the input is within a few mV of the threshold — the output transistor comes out of saturation slowly. For clean switching with noisy sensor signals, drive the input at least 50–100mV past the threshold.

Q7: Can the LM393 outputs be connected together (wired-OR)?

A: Yes — this is a feature of the open-collector output. Tie both OUT1 and OUT2 together with a single pull-up resistor. If either comparator pulls low, the common output goes low. Both must be off for the output to go high. This works because open-collector outputs can share a pull-up without fighting each other. Push-pull outputs would short-circuit if you tried this.

Q8: What is the maximum voltage I can pull the output up to?

A: Up to 36V, regardless of VCC. You can run the LM393 at VCC=5V and pull the output up to 24V. The open-collector transistor is rated for 36V V_CEO. This is how the LM393 does level translation without extra components: power the IC from your logic rail, pull the output up to whatever voltage the next stage needs.

Related Products

• LM393DR — Dual Comparator, SOIC-8 (STMicroelectronics)
• LM339DR — Quad Comparator, SOIC-14 (STMicroelectronics)
• LM358DR — Dual Op-Amp, SOIC-8 (STMicroelectronics)
• LM393 vs LM339 vs LM358 — Comparator vs Op-Amp Comparison
• TLV1702 — Dual Comparator, Rail-to-Rail Inputs, 2.2V–36V (TI)