UM0277 datasheet(21/32 Pages) STMICROELECTRONICS

21 / 32 page

UM0277

Thermal sensor linearization

21/32

Appendix A

Thermal sensor linearization

An NTC thermistor is a thermal resistor whose value decreases when its temperature

increases. The thermal law is exponential, as presented in Equation 1.

Equation 1

To achieve a simple voltage sensor, it is better to linearize the temperature response using a

constant resistor (R12) added in series with the NTC. A voltage divider is then achieved.

The voltage across R12 will follow the supply voltage (Vdd) according to the relationship

below:

Equation 2

To make the relationship of Equation 2 to vary linearly, it's enough to ensure that the second

order derivative will be zero. Equation 3 gives the R12 value to ensure this condition.

Equation 3

To linearize the voltage response of a M2020 5 k NTC thermistor from EPCOS, between -20

and +5 °C, a 30 kOhm resistor should be chosen for R12.

In this temperature range, Vs varies according to Equation 4, for a 5 V supply:

Equation 4

Figure 14 gives the variation of Vs and the linear value given by Equation 4 (Vl), versus the

temperature sensed by the NTC thermistor.

Rc T

() Ro

---

-------

–

⎝⎠

⎛⎞

•

Rc T

()

Rc T

() R12

----------------------------------

Vdd

•

R12

-------Rc T

()

⎝⎠

⎛⎞

•

----------Rc T

()

-----------------------------------------

Rc T

()

–

Vl T

()

51.84

16.1943

-------------------------

UM0277 Datasheet(PDF) 21 Page - STMicroelectronics

UM0277 Datasheet(HTML) 21 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL