When the output transistor turns off, it releases the node, and the pull-up establishes the high level. The resistor also limits current when the output is low. Its resistance must suit the receiver's logic thresholds, loading, and timing.
When the LM393P output transistor is on, current flows from the pull-up supply through the resistor and into the output pin, then to ground. When the transistor turns off, the output becomes high impedance. The pull-up supply, resistor, and connected load then determine the high-level voltage.
If the output does not rise, check for an effective pull-up path, a working pull-up supply, and a load that may be holding the node low. A missing high level alone does not establish that the comparator is faulty. Connecting the output directly to a supply does not provide the current limiting of a pull-up resistor.
There is no single best LM393 pull-up resistor value. Lower resistance increases the current the output must sink when low, so the low-level voltage and power dissipation need checking. Higher resistance can slow the rising edge and increase the high-level voltage drop caused by leakage current.
The resistor current is the pull-up voltage minus the output-low voltage, divided by resistance. For the table below, assume a 3.3 V pull-up supply and neglect the low-level output voltage and current from other branches. The 3.3 V is the pull-up voltage, not a requirement for the comparator's own supply.
| Pull-up supply | Pull-up resistance | Estimated resistor current with output low |
|---|---|---|
| 3.3 V | 10 kΩ | 0.33 mA |
| 3.3 V | 1 kΩ | 3.3 mA |
These estimates show the change in load; they do not establish that either resistance suits a particular circuit. Check the actual total sink current against the comparator's low-level output specifications at the operating temperature, together with the receiver's low-level input threshold.
For a separate timing example, assume a total output-node capacitance of 100 pF and a high-impedance receiver. With a 10 kΩ pull-up, a first-order RC model gives a 10%–90% rise time of approximately 2.2RC, or 2.2 µs. The 100 pF is an assumed total capacitance, not a recommendation to add a filter capacitor. This calculation describes the rising edge after the output releases; it does not include comparator propagation delay.
Confirm that the resistor connects the correct output pin to a suitable logic supply and that the comparator and receiver share a ground. Keep the comparator inputs within their permitted operating conditions and exercise both output states. With the actual load connected, use an oscilloscope to check the low and high levels and the rising edge against the receiver's input thresholds and timing requirements.
If no resistor value meets the current, logic-level, and timing requirements together, review the load or interface arrangement. Continuing to reduce resistance may simply increase the sink-current requirement.
Technical basis: TI data sheet SLCS005AH (April 2025), Sections 6.1, 6.4.1, and 7.2.2; application note SNOAA35F (December 2024), Sections 6.10, 7.1, 7.2, and 7.4. The numerical examples are calculations under the stated assumptions, not guaranteed device specifications.
For a component review, gather the full comparator part number, pull-up supply, receiver thresholds, and load conditions. Include the package, operating temperature, required quantity, and target delivery date to keep the inquiry specific to your project. For LM393P or alternative-part inquiries, send these details through the SUNTEC inquiry form.
An LM393 powered from 5 V can use a resistor pull-up to 3.3 V when the comparator and receiver share a ground and the receiver's thresholds, loading, and power-off limits are met. Check for any additional 5 V pull-up. The comparator's input-voltage limits remain unchanged.
A microcontroller's internal pull-up may serve an LM393 output. Verify its resistance range, whether it stays enabled during reset, and the circuit's leakage, capacitance, and timing requirements. Software enablement alone is insufficient.
On an LM393 module, a resistor already connecting the output to a suitable supply provides a pull-up path. Adding another resistor to the same supply lowers the effective resistance and increases current when the output is low. Check the schematic before adding it.
Two LM393 open-collector outputs can share a pull-up to a suitable supply when they share a ground. Either output can pull the node low; both must release it for a high level. Check the combined load, total leakage, and intended logic function.
An LM393 startup glitch may occur while supply or input conditions are not yet valid. Check the supply sequence and waveforms before attributing it to the pull-up resistor; changing resistance alone is not a guaranteed fix.
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