SLUUCJ0 November 2023 BQ76907
The TS pin on the BQ76907 device can be connected to an external thermistor for temperature measurement or can be used as a general purpose ADC input (ADCIN mode). When used for thermistor measurement, the thermistor is connected between the TS pin to VSS, and the pin is internally connected to a 20-kΩ trimmed pullup resistor, which is biased by the internal 1.8V LDO (REG18) voltage. The voltage at the TS pin is digitized by the ADC using the same 1.8 V LDO voltage as the ADC reference, thereby implementing a ratiometric measurement. When the pin is configured for ADCIN mode, the 20-kΩ pullup resistor is disconnected, and the ADC uses VREF1 for its reference.
The selection between the two modes is made using the Settings:Configuration:DA Config[TSMODE] bit. When the bit is set, the TS pin operates in thermistor measurement mode, versus when the bit is cleared it operates in ADCIN mode.
The output of the TS pin ADC measurement is provided by the 0x2A TS Measurement() command. When the pin is configured for thermistor mode, the LSB size of the resulting data is given by
1 LSBThermistor Mode≅ 5 / 3 x VREG18 / 2N-1 ≈ 5 / 3 x 1.8 V / 215 = 92 μV
When the pin is configured for ADCIN mode, the LSB size of the resulting data is given by
1 LSBADCIN Mode≅ 5 / 3 x VREF1 / 2N-1 ≈ 5 / 3 x 1.1962 V / 215 = 61 μV
When the TS pin is configured for thermistor measurement the device enables the internal pullup resistor only while the pin is being measured (which is the default recommended setting). Alternatively, the pullup resistor can be biased continuously by sending the 0x69 REGOUT_CONTROL() command with the [TS_ON] bit set.