Onsemi LM393NG: Datasheet, Application Circuits, and Design Considerations
The LM393NG from Onsemi is a classic, industry-standard dual differential comparator integrated circuit. Renowned for its reliability, low power consumption, and cost-effectiveness, it serves as a fundamental building block in a vast array of electronic systems, from battery monitoring to motor control circuits. This article delves into the key specifications of the LM393NG, explores common application circuits, and outlines critical design considerations for optimal performance.
Datasheet Highlights and Key Specifications
Understanding the LM393NG begins with its core electrical characteristics as defined in its datasheet. The device contains two independent, high-gain voltage comparators designed for single-supply operation over a wide voltage range, from 2V to 36V, or dual supplies of ±1V to ±18V. This flexibility makes it suitable for both 3.3V/5V logic systems and higher voltage industrial applications.
A defining feature is its low supply current drain, typically 0.4 mA per comparator, which is virtually independent of the supply voltage. This is crucial for battery-powered applications. The inputs can swing to the negative rail, enabling ground-sensing capability. The outputs are open-collector, allowing for easy interfacing with other logic levels (e.g., 5V, 12V) or for wire-OR'ing multiple comparator outputs together. However, this output structure requires an external pull-up resistor to the desired logic high voltage.
Other critical specs include a low input bias current (25 nA typical), low input offset voltage (2 mV typical), and a response time of approximately 1.3 μs. Designers must note that the LM393NG is not a high-speed comparator; it is optimized for low power and general-purpose use.
Common Application Circuits
The open-collector output and wide supply range make the LM393NG incredibly versatile.
1. Basic Comparator: The most straightforward application is comparing a signal voltage against a fixed reference. The inverting (-) input is connected to a reference voltage (e.g., from a resistor divider or a Zener diode), while the signal is applied to the non-inverting (+) input. The output goes high only when the signal voltage exceeds the reference.
2. Zero-Crossing Detector: This circuit detects when an AC signal passes through zero volts. The non-inverting input is grounded, and the AC signal is fed to the inverting input through a current-limiting resistor. The comparator's output toggles each time the AC waveform crosses the zero point, useful for generating timing signals in dimmers or motor controllers.
3. Window Comparator: Two LM393NG comparators can be configured to determine if a signal is within a specific voltage "window." One comparator checks if the signal is above a lower limit, while the other checks if it's below an upper limit. The outputs are logically combined (often with pull-ups and diodes) to create a single output that is high only when the signal is within the designated range.
4. Darkness/Light Sensor: A classic use case pairs the comparator with a light-dependent resistor (LDR) and a potentiometer. The LDR and a fixed resistor form a voltage divider. The comparator compares this voltage against a threshold set by the potentiometer. The output state changes based on the light level, enabling automatic night lights or security systems.
Critical Design Considerations
To ensure stable and accurate operation, several factors must be addressed:
Hysteresis (Schmitt Trigger): Without hysteresis, a comparator operating near its threshold point can produce multiple output transitions due to noise on the input signal. Adding positive feedback (a resistor from the output back to the non-inverting input) creates hysteresis, which provides noise immunity and ensures a clean, single output transition. This is arguably the most important design step for any real-world application.
Pull-Up Resistor Selection: The value of the external pull-up resistor on the output is a trade-off. A lower value provides a faster rise time but increases power consumption. A value between 1kΩ and 10kΩ is typical for many applications.
Input Voltage Range: While the inputs can go below the negative rail (ground), they must not exceed the positive supply rail (VCC). If the input signals can surpass VCC, external clamping diodes must be used to protect the IC.
Decoupling and Layout: A 0.1μF to 1μF decoupling capacitor should be placed as close as possible to the VCC and GND pins to suppress noise on the power supply lines, which can couple into the inputs and cause erratic switching. Keep input traces short and away from noisy output or power traces.
Driving Loads: The open-collector output can sink up to 16 mA (sink current), but it cannot source significant current. It is ideal for driving LEDs, small relays (with a flyback diode), or logic-level inputs directly.
The Onsemi LM393NG stands as a testament to timeless design, offering an unbeatable combination of versatility, robustness, and low power consumption. Its open-collector output and wide operating voltage range make it an indispensable tool for engineers designing sensor interfaces, voltage monitoring systems, and basic logic functions. By carefully applying hysteresis and proper board layout, designers can leverage this ubiquitous comparator to create reliable and effective circuits for countless applications.
Keywords:
1. Voltage Comparator
2. Open-Collector Output
3. Low Power Consumption
4. Hysteresis
5. Single-Supply Operation
