Last week, Leiditech's EMC support team received an urgent request from a customer — an industrial sensor passed all production line tests without issue, but after being deployed at the customer site, it began to experience frequent "ghost failures": the device would occasionally reboot for no apparent reason, or the collected voltage signals would exhibit erratic jumps, leading to system-wide misjudgments.

The customer's engineers spent an entire week troubleshooting — from software logic to power supply ripple, they left no stone unturned. In the end, they traced the culprit to an unassuming low-voltage comparator on the PCB.

After a thorough analysis by Leiditech's EMC support team, the root cause emerged: in the dry field environment, static electricity accumulated on operators or the equipment enclosure caused "soft damage" to the comparator through signal lines or housing gaps. This damage does not make the chip fail immediately, but quietly shifts its input offset voltage or causes it to falsely toggle at critical thresholds, ultimately manifesting as intermittent system failures. This is the most insidious aspect of electrostatic discharge (ESD) — it acts like an invisible assassin, striking at the very moment you least expect it.

I.Why Are Comparators So Vulnerable to ESD?

To understand why comparators require special ESD protection, let's briefly review how they work. The core function of a comparator is to determine the relative magnitude of two input voltages. A voltage comparator typically consists of two inputs and one output: a non-inverting input (+) and an inverting input (-), and an output (Vout) for high/low level signals. When the voltage at V+ is higher than at V-, the output toggles to a high level; otherwise, it outputs a low level. It functions like a precision "voltage referee" and is a core element in circuits such as analog-to-digital conversion, overvoltage/undervoltage detection, and signal conditioning.

Classic and common voltage comparator models include: single-channel LM311, dual-channel LM393, and quad-channel LM339.

Low-power options include: single-channel TLV3011 and TS881, suitable for battery-powered devices.

High-speed models include: single-channel TLV3501 and MAX9010.

Precision model: single-channel AD790.

However, it is precisely this "precision" characteristic that makes comparators particularly sensitive to static electricity. Modern low-voltage comparators are often manufactured using deep submicron CMOS processes, in which the internal gate oxide layer is extremely thin and can have a breakdown voltage as low as tens of volts. Yet, an ordinary human body electrostatic discharge can easily reach thousands or even tens of thousands of volts. More critically, comparators typically operate in an open-loop configuration with very high gain. If an ESD pulse couples into the input — even a very narrow spike — it can be amplified and cause an instantaneous inversion of the output state, leading to system misoperation. Even worse, if the ESD energy directly punctures the gate oxide of the input stage, it can cause permanent damage and render the comparator completely nonfunctional.

II.ESD Protection Solution: Leiditech's EMC Support Team Puts "Bulletproof Vests" on Comparators

So, how do we build a robust defense for vulnerable low-voltage comparators? An effective solution is to use TVS (transient voltage suppression) diodes.

TVS diodes are the "frontline soldiers" of ESD protection. They act like intelligent voltage switches, presenting very high impedance under normal operating voltages and having no effect on the circuit. Once an ESD high-voltage pulse is detected, they can turn on within picoseconds to nanoseconds, diverting the large transient current to ground and clamping the voltage at a safe low level. For comparator input/output pins, we recommend using low junction capacitance (Cj < 10pF) TVS devices, such as the ESDA05CPLV (Cj = 3.5pF). The low capacitance characteristic ensures that high-speed signals (such as rapidly changing signals from sensors) are not distorted during transmission, thereby preserving the comparator's judgment accuracy. Of course, this cannot be applied universally — each case requires individual analysis.

III.Recommendations for Selecting ESD Junction Capacitance for Comparators

The operating speed of the comparator (signal frequency) determines the maximum junction capacitance you can tolerate.

Comparators do not operate at rates as high as USB 3.0 or HDMI, which often reach Gbps levels. However, their application scenarios span a wide range — from power frequency signal detection (50Hz/60Hz) to high-frequency pulse capture (at the MHz level). Therefore, a one-size-fits-all approach is not appropriate.

1、Low-speed / DC signal comparators (e.g., overvoltage/undervoltage detection, temperature comparison, battery level monitoring)

u Signal characteristics: The signal changes slowly, has a very low frequency (< 1kHz), or is even just a DC level.

u Junction capacitance recommendation: < 100pF is sufficient.

u Reason: Since the signal changes slowly, the charging/discharging time constant caused by junction capacitance has almost no effect on the signal. In this case, priority can be given to selecting ordinary TVS diodes with lower clamping voltage and higher surge current capability, which actually provide better protection. There is no need to sacrifice clamping performance or increase cost in pursuit of extremely low capacitance.

2. Medium-speed signal comparators (e.g., PWM signal conditioning, intermediate-frequency square wave detection, sensor pulses within 1MHz)

u Signal characteristics: Signal frequency ranges from tens of kHz to several MHz, with rise/fall times in the hundreds of nanoseconds.

u Junction capacitance recommendation: <10pF.

u Reason: At this speed range, the junction capacitance of ordinary TVS diodes (tens of pF) begins to cause visible delay and distortion on the signal edges, potentially shifting the comparator's trip point. It is recommended to use dedicated low-capacitance ESD protection diodes, such as Shanghai Leiditech's ESDA05CPLV (Cj ≈ 3.5pF) or ULC0311NH (Cj ≈ 1.2pF). Components in this class offer excellent cost-effectiveness and represent the "sweet spot" for most comparator applications.

3. High-speed comparators (e.g., high-speed ADC driving, RF detection, high-speed pulse counting > 10MHz)

u Signal characteristics: Signal frequency reaches tens of MHz or even higher, with rise times within a few nanoseconds.

u Junction capacitance recommendation: < 1pF, preferably < 0.5pF

u Reason: At this stage, signal integrity is the top priority. Any capacitance exceeding 1pF will significantly increase insertion loss, cause signal reflections, and may even prevent the comparator from correctly responding to high-speed pulses. Ultra-low capacitance ESD devices must be selected, such as the ULC0542CLV (Cj ≈ 0.3pF) and the ULC3311CDN (Cj ≈ 0.45pF).

To summarize:

Final recommendation: For most general-purpose low-voltage comparator applications (such as the industrial sensor scenario mentioned in the article), selecting a low-capacitance ESD diode with a junction capacitance between 1pF and 5pF is the most reliable choice. It provides adequate ESD protection, does not significantly affect most medium- and low-speed signals, and keeps costs under control.

For comparators operating at 3.3V and 5V, the following are recommended common ESD protection models:

IV.What are the advantages of this solution?

The advantages of this TVS-based protection solution are very clear:

1、Extremely fast response, precise protection: TVS diodes offer picosecond-level response speed, effectively clamping the main ESD pulse with its extremely steep rising edge and firmly limiting the voltage within a safe range. This fundamentally prevents gate oxide breakdown or output false toggling caused by high-voltage spikes.

2、Balancing protection and performance: By selecting TVS diodes with low junction capacitance, a high protection level can be achieved without compromising signal integrity. This is critical for comparator applications that need to process weak or high-speed signals, such as high-precision sensor interfaces.

3、Simple design, controllable cost: The components required for this solution are common, mature models that are low in cost and readily available. Engineers can flexibly select the clamping voltage of the TVS diode based on the specific comparator model and operating voltage to achieve optimal cost-performance.

In summary, ESD protection is by no means an optional "nice-to-have" add-on, but rather a "cornerstone" for ensuring the long-term reliable operation of electronic products. For sensitive devices such as low-voltage comparators, implementing precise protection with low-capacitance TVS diodes is a wise choice to eliminate "ghost failures" and confidently transition products from the lab to demanding real-world environments.

V.Which industries is it applicable to?

Low-voltage comparators are most commonly used in portable, battery-powered, and space-constrained devices, primarily covering the following types of products:

1、Portable devices such as mobile phones, smart wristbands, and Bluetooth earbuds;

2、Battery level monitoring and power management;

3、Sensor signal processing (e.g., temperature control, light control);

4、Automotive electronics and industrial control;

5、Medical instruments (e.g., blood glucose meters).