Temperature Sensor Application Tips

What are Linear Active Thermistors?

Linear Active Thermistor™ Integrated Circuits (ICs) are Microchip's analog temperature sensors that convert temperature to an analog voltage output. MCP9700/01 and MCP9700A/01A are families of Linear Active Thermistor devices.

These sensors compete with a thermistor solution in price and performance. Unlike resistive sensors (such as thermistors), Linear Active Thermistor ICs do not require an additional signal conditioning circuit. Therefore, the biasing circuit development overhead for thermistor solutions can be eliminated by implementing these low-cost devices. The Voltage Output (VOUT) pin can be directly connected to the Analog-to-Digital (ADC) input of a microcontroller. The sensor output voltage is proportional to ambient temperature with temperature coefficient of 10 mV/°C and 19.5 mV/°C with output voltage at 0 °C scaled to 500 mV and 400 mV, respectively. These coefficients are ideal for 8-bit ADCs referenced at 5V and 2.5V. The operating current is 6 μA (typical) and they use a Printed Circuit Board (PCB) space-saving 5-pin SC-70/SOT-23 and 3-pin TO-92 packages.

MCP9700/01 and MCP9700A/01A Typical Applications:

  • Entertainment Systems
  • Home Appliance
  • Battery Packs and Power Supplies for Portable Equipment
  • General Purpose Temperature Monitoring

Sensor Application Tips

The MCP9700/01 and MCP9700A/01A ICs are designed to drive large capacitive loads. This capability makes the sensors immune to board parasitic capacitance which allows the sensors to be remotely located and to drive long PCB traces or shielded cables to the ADC. In addition, adding capacitive load at VOUT helps the sensor's transient response by reducing overshoot and undershoot. This provides a more stable temperature reading. IC temperature sensors use analog circuitry to measure temperature. Unlike digital circuits, analog circuits are more susceptible to power-supply noise. We recommend that a bypass capacitor CBYPASS of 0.1 μF to 1 μF be placed at close proximity to the VDD and VSS pins of the sensor (see figure below). The capacitor provides protection against power-supply glitches by shunting fast transient noise to ground. The effectiveness of the bypass capacitor depends upon the power-supply source resistance. A larger source resistance has the effect of providing the Resistor-Capacitor (RC) network with the CBYPASS and adds a corner frequency to filter out the power-supply noise. Adding a series resistor to the power-supply line is therefore adequate to increase the source resistance. CLOAD is optional.

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