It has come to our attention that some users of the TLV1701 comparator have observed unexpected output leakage current, leading to concerns about potential device damage. This article aims to clarify that this behavior is actually expected and is not indicative of a faulty component. Understanding the nuances of Comparator Sourcing, especially regarding internal protection mechanisms, is crucial for effective circuit design. Let’s delve into the specifics of why this leakage occurs and how to manage it in your applications.
The Role of ESD Clamps in Comparator Design
To protect sensitive integrated circuits like the TLV1701 from electrostatic discharge (ESD) damage, internal ESD clamps are implemented. In the TLV1701, an ESD clamp is present between the output pin and the V+ supply pin. This clamp is designed to withstand significant voltage differences, and it’s not a simple diode, but a more complex structure to ensure robust ESD protection.
Leakage Current and Voltage Differentials
Depending on the voltage difference between the output “high” voltage and the V+ supply voltage, a leakage current can flow through this ESD clamp. In certain conditions, this leakage can be as high as 1uA. This phenomenon is particularly noticeable when the output is intended to be “high” but is weakly pulled down, creating a substantial voltage differential between the output and V+.
Normal vs. Uncommon Comparator Usage
In typical applications, a comparator output is either actively pulled high using a relatively low-value pull-up resistor (less than 100kΩ) to a voltage above 3V, or it is driven low by the output transistor or another device on the output bus. These are considered normal operating conditions where the leakage is either minimized or effectively managed by the circuit design.
However, an uncommon scenario arises when the output, intended to be in a “high” state (non-saturated), is pulled down towards zero volts by a high resistance. This situation creates a large voltage difference between the output and V+. This significant voltage differential is the primary driver for the observed leakage current through the ESD clamp. Under normal pull-up or sinking conditions, this leakage is typically negligible or masked by the intended output behavior.
Datasheet Specifications and Practical Implications
The TLV1701 datasheet specifies a typical leakage current of 70nA. It’s important to note that this specification is usually measured under conditions where the pull-up voltage is close to the supply voltage (Vsupply). In such cases, the voltage difference across the ESD clamp is minimal, hence the low leakage current.
In the reported scenario, a 20V supply voltage and a 3.3V output “high” level were used, resulting in a 16.7V difference between V+ and the output. This large voltage difference naturally leads to a significantly higher leakage current than the datasheet typical value. The observed 700nA leakage in such conditions is indeed within expected parameters and not an anomaly.
Solutions for Managing Output Leakage
To mitigate the effect of this leakage current in designs where it becomes problematic, two primary solutions are available:
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Reduce the Pull-up Resistor Value: Decreasing the pull-up resistor value will strengthen the “high” state and reduce the effect of the leakage current attempting to pull the output down.
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Add a Pull-Down Resistor: Incorporating a second high-value pull-down resistor can provide a path to bleed away the leakage current, effectively pulling the VOL voltage down to the desired specification, despite the leakage.
Future Datasheet Enhancements and Alternative Options
We are planning to further characterize this output leakage behavior and include a detailed graph in a future update to the TLV1701 datasheet to provide clearer guidance for users. While the described behavior is inherent to the TLV1701’s ESD protection design, for applications where input voltages do not exceed 18V and output leakage is a critical concern, alternative comparator devices without this specific output leakage characteristic, albeit with different pinouts, might be considered.
Disclaimer: This explanation is for informational purposes and based on the provided technical context. For specific design concerns and component selection, always refer to the official datasheet and consult with qualified engineers.
