LM393N

The LM393N from Texas Instruments is a dual differential comparator—the comparator counterpart of the LM358 op-amp. Unlike an op-amp, the LM393 has an open-collector output that can only sink current (pull LOW) and requires an external pull-up resistor to produce a HIGH output. This open-collector architecture enables wired-OR connections: multiple LM393 outputs can share a single pull-up resistor, and any comparator pulling LOW dominates. The input common-mode range includes ground (0V), allowing single-supply operation for comparing signals near ground. The 1.3µs response time is suitable for most DC and low-frequency comparison tasks. The ±36V differential input voltage range allows the two inputs to differ by up to 36V without damage, useful when comparing signals from different voltage domains. The N suffix denotes the PDIP-8 through-hole package.

The LM393N consists of two independent voltage comparators. Each comparator has an inverting input (-), a non-inverting input (+), and an open-collector output. The comparator continuously compares the voltage at the + input with the voltage at the – input. When V+ > V-, the output transistor is OFF, and the external pull-up resistor pulls the output HIGH. When V+ < V-, the output transistor turns ON, sinking current through the pull-up resistor and pulling the output LOW. There is no negative feedback; the comparator operates open-loop with very high gain, so even a tiny difference between inputs drives the output to one rail or the other. The 2mV typical offset voltage means the switching point may be shifted by up to 5mV from the ideal threshold. The 1.3µs response time includes the internal propagation delay plus the time for the output transistor to switch. In a battery monitor application, the + input connects to a voltage divider from the battery, and the – input connects to a reference voltage (e.g., from a zener or TL431). When the battery voltage drops below the threshold, V+ < V-, and the output goes LOW, triggering an interrupt or LED indicator.

LM393N uses the PDIP-8 package with standard dual comparator pinout: Pin 1 = Output A (open-collector), Pin 2 = Inverting Input A (-), Pin 3 = Non-inverting Input A (+), Pin 4 = GND, Pin 5 = Non-inverting Input B (+), Pin 6 = Inverting Input B (-), Pin 7 = Output B (open-collector), Pin 8 = VCC. The open-collector outputs require external pull-up resistors to VCC or a separate logic supply. This allows output level shifting — the pull-up voltage can differ from VCC. Maximum output sink current is 16mA. Input common-mode range includes ground. Pinout is identical to LM2903, LM393A, and LM2903V comparators.

• Battery Monitor: IN+ from battery divider, IN- from TL431 (2.5V ref); output LOW when battery < threshold; LED indicator
• Window Comparator: Two comparators with different thresholds; wired-OR outputs; LOW when signal is outside window
• Zero Crossing: AC signal through resistor to IN+, IN- at GND; output transitions at each zero crossing
• Overcurrent Detect: Current sense amp output to IN+, threshold on IN-; output goes LOW on overcurrent
• RC Oscillator: Positive feedback via resistor divider; timing via RC on IN-; free-running square wave output

LM393 is the industry-standard dual voltage comparator with open-drain output and 2-36V supply range. The 1.3µs response time suits general-purpose threshold detection. LM393AN offers lower offset voltage. LM339 provides 4 comparators in DIP-14. For high-speed applications, TLV3202 offers 40ns response with push-pull output at 2.7-5.5V supply.