UM1542 datasheet(25/43 Pages) STMICROELECTRONICS

25 / 43 page

UM1542

Thermal sensor linearization

Doc ID 023172 Rev 1

25/43

Appendix A

Thermal sensor linearization

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

The thermal law is exponential, as shown in Equation 3:

Equation 3

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

constant resistor (R49 in schematic, Appendix B) added in series with the NTC. A voltage

divider is then implemented. The voltage across R49 follows the supply voltage (Vdd)

according to the relationship below:

Equation 4

To make the relationship of Equation 4 vary linearly, it's sufficient to ensure that the second

order derivative is zero. Equation 5 gives the R49 value to ensure this condition.

Equation 5

To linearize the voltage response of the M2020 5 k from EPCOS, between -20 and +5 °C, a

30 k

Ω resistor should be chosen for R49.

In this temperature range, VS varies according to Equation 6, for a 5 V supply:

Equation 6

Figure 10 gives the variation of VS and the linear value given by Equation 6, versus the

temperature sensed by the NTC.

)

(

)

(

NTC

−

NTC

⋅

)

(

)

(

)

(

)

(

)

(

NTC

−

⎟

⎠

⎞

⎜

⎝

⎛

⋅

l T

()

51.84

16.1943

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

≡

UM1542 Datasheet(PDF) 25 Page - STMicroelectronics

UM1542 Datasheet(HTML) 25 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 33 34 35 36 37 38 39 40 41 42 43

Datasheet Download

Link URL