SLUSDQ4 April 2019 BQ79606A-Q1
When measuring an external temperature sensor, the GPIO connections require a resistor divider from TSREF to AVSS, with the GPIO input connected to the center tap. The NTC is connected from TSREF to GPIO or from GPIO to AVSS, depending on the application requirements. The connections are shown in Figure 45. The resistors linearize the NTC curve to provide the best accuracy over the temperature range of interest.
The resistors, R1 and R2, are calculated based on the desired temperature range of interest and the NTC used. For the following calculations, the linearization is highest between 10% and 90% of full scale. First, the temperature range of interest must be selected. This range sets the best resolution (and therefore accuracy) of the temperature sensor. The resistance of the NTC must be calculated for the extremes of this range. Use the following equation to calculate RHOT (the resistance at the hottest temperature) and RCOLD (the resistance at the coldest temperature):
Where RTS is the calculated NTC resistance, R0 is the room temperature value of the thermistor, β is the temperature coefficient of the thermistor and T is the temperature for the calculated resistance.
Once RHOT and RCOLD are calculated, use the following equations to calculate R1 and R2. For the case where the NTC is connected from GPIO to AVSS, R1 and R2 are calculated using:
For the case where the NTC is connected between TSREF and GPIO, R1 and R2 are calculated using:
Additionally, connect a 1- kΩ resistor from the center tap of the resistor divider to the GPIO input ( GPIO pin used as input to AUX ADC to measure the temperature) and bypass VGPIO to AVSS with a 1-μF capacitor.
When the NTC is connected from GPIO to AVSS the temperature of the sensor is calculated as:
When the NTC is connected from TSREF to GPIO the temperature of the sensor is calculated as:
Where RATIO_ADC is the result of Equation 3, R1 and R2 are the linearization resistor values, R0 is the NTC value at room temperature (25C), and β is the temperature coefficient of the NTC.
When measuring a voltage, these channels require a simple external low-pass filter to reduce high frequency noise for best operation. The RC values correspond to the customer's application requirements.