A Guide to Using the Microchip MCP9700 Linear Active Thermistor IC for Temperature Sensing
Temperature measurement is a fundamental requirement in countless electronic applications, from environmental monitoring to system health management. While many solutions exist, the MCP9700 linear active thermistor IC from Microchip Technology offers a compelling combination of simplicity, accuracy, and low cost. This guide explores its operation, key features, and practical implementation.
Understanding the Linear Active Thermistor Concept
Unlike a traditional negative temperature coefficient (NTC) thermistor, whose resistance change is non-linear and requires complex signal conditioning, the MCP9700 is an integrated analog sensor. It internally linearizes the response of its sensing element, providing a voltage output that is directly proportional to the ambient temperature. This eliminates the need for additional linearization circuits or lookup tables, dramatically simplifying the design process.
Key Features and Specifications
The MCP9700 is designed for ease of use and broad applicability. Its standout characteristics include:
Linear Analog Voltage Output: The device outputs a voltage scale factor of 10 mV/°C, making calculations straightforward.
Low Operating Voltage: It functions with a supply voltage from 2.3 V to 5.5 V, making it ideal for both 3.3V and 5V systems and battery-powered applications.
High Accuracy: The typical accuracy is ±2°C from 0°C to +70°C and ±4°C over the full extended range of -40°C to +125°C.
Low Power Consumption: Drawing a mere 6 µA (typical) of operating current, it is perfect for power-sensitive designs.
Small Form Factor: Available in SOT-23-3 and TO-92 packages, it requires minimal board space.
Basic Application Circuit
Implementing the MCP9700 is remarkably simple. The basic circuit requires only three connections:
1. Vdd: Connect to the positive power supply (2.3V to 5.5V).
2. GND: Connect to the system ground.
3. Vout: This is the analog output pin. The voltage on this pin is given by the formula:
Vout = V0 + (Tc × Ta)
Where:
Vout = Output Voltage
V0 = Output Voltage at 0°C (typically 500 mV)
Tc = Temperature Coefficient (typically 10 mV/°C)
Ta = Ambient Temperature (°C)
For example, at 25°C, the typical output voltage would be 500 mV + (10 mV/°C × 25°C) = 750 mV.
This Vout pin is typically connected directly to an analog-to-digital converter (ADC) input on a microcontroller (MCU). Since the output impedance is low, no buffer amplifier is required.
Design Considerations for Improved Accuracy
To achieve the best possible performance from the MCP9700, consider the following tips:
1. Power Supply Decoupling: Place a 0.1 µF ceramic decoupling capacitor close to the Vdd pin to filter out noise on the power supply rail.
2. PCB Layout: Keep the device away from heat-generating components like voltage regulators or power resistors. Even a small amount of self-heating from nearby parts can affect the reading.
3. ADC Reference: The accuracy of your measurement is only as good as your ADC's reference voltage. For precise readings, use a stable voltage reference for the MCU's ADC instead of the power supply rail, which may be noisy.
4. Averaging: To reduce the effect of noise, take multiple ADC samples in software and average them to get a stable temperature reading.
Typical Applications
The MCP9700's characteristics make it suitable for a wide array of applications, including:
HVAC systems
Battery temperature monitoring
Consumer appliances
Industrial control systems
Automotive cabin temperature sensing
The Microchip MCP9700 stands out as an exceptionally user-friendly and cost-effective solution for adding temperature sensing to any electronic project. Its linear voltage output eliminates complex math, its low power consumption preserves battery life, and its simple 3-wire interface allows for rapid prototyping and integration. For designers seeking a reliable, no-fuss analog temperature sensor, the MCP9700 is an outstanding choice.
Keywords: Temperature Sensor, Analog Output, Low Power, Linear Active Thermistor IC, Microcontroller Interface
