4 Calculate Temperature With TMP6x

A silicon-based linear thermistor has a linear positive temperature coefficient (PTC), TMP61, is used for better temperature measurement accuracy. The TMP61 has good linear behavior across the whole temperature range. Unlike an NTC, which is a purely resistive device, the TMP61 resistance is affected by the current across the device and the resistance changes when the temperature changes. The TMP6-THERMISTOR-DESIGN only offers the R-T table for TMP61 with a fix 10kohm bias resistor so the user needs to use specific R-T table provided by TI if BQ769x2 is used to measure the TMP6x.

To convert V_sense to temperature, one of the common software methods is using a look-up table. This method results in a simple setup for the R-T table but puts high demand on the flash memory requirements for the MCU and a lengthy array-parsing program. This can also be inaccurate due to system errors such as tolerance variations and temperature coefficients which can lead to divergence from the R-T table.

With the use of the TMP6 linear thermistor, however, a much better algorithm is available for converting is the 5^th order polynomial regression model as Equation 1 shows:

where T = temperature (in Celsius); R = measured resistance value; A0-A5 are the calculated polynomial coefficients. R_T is back-calculated from V_sense, R_pu, R_pad and R_on as Equation 2 shows:

To obtain the coefficients value, a 5^th-order polynomial fit is performed to obtain the best fit to a series of data points in the R-T table. Based on the R-T table of TMP61DEC package, these are:

• A0 = - 3.513960E+02
• A1 = 9.021910E-02
• A2 = - 1.011904E-05
• A3 = 7.112242E-10
• A4 = - 2.612301E-14
• A5 = 3.863465E-19

The coefficients are only for a 1.8V bias voltage, a 18kΩ pullup resistor and TMP61DEC package.