The BQ76905 device integrates an Undertemperature in Charge (UTC) Protection that digitizes the voltage of an external negative temperature coefficient thermistor on the TS pin using the ADC, and triggers an alert or fault when the digital ADC measurement at the thermistor is greater than a programmable threshold VUTC. The thermistor is biased using an on-chip 20-kΩ pullup resistor (which is trimmed in TI factory), which is biased by the internal 1.8-V LDO rail only when the thermistor 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. The ADC uses this same internal 1.8-V LDO rail as its voltage reference for this measurement, thereby implementing a ratiometric conversion.

The VUTC threshold is programmable from 0 to 19635 16-bit ADC counts in steps of 77 ADC counts using the Protections:Temperature:Undertemperature in Charge Protection Threshold configuration register. The UTC protection is enabled using the Settings:Protection:Enabled Protections B:[UTC] configuration bit.

The UTC protection triggers an alert signal when an undertemperature in charge event is first detected, then triggers a fault if it persists after a programmable number of measurements have been taken, UTC_DLY, which can be set from 0 to 255. The delay is set by the Protections:Temperature:Undertemperature in Charge Protection Delay configuration register. The time interval for measurements is based on the settings for the voltage and current ADC measurement speeds. Note that during SLEEP mode this protection only operates during the duty-cycled measurement burst every Power:Sleep:Voltage Time, so there can be a longer delay before it triggers an alert. If an alert is triggered, the device returns to NORMAL mode until either the alert is cleared or a fault is triggered.

When a UTC fault is triggered, the device turns off the CHG FET if configured for autonomous FET control, based on the setting in Settings:Protection:CHG FET Protections A[UTC]. The device recovers when the thermistor measurement is less than or equal to the threshold set by Protections:Temperature:Undertemperature in Charge Protection Recovery (which has the same threshold range from 0 to 19635 16-bit ADC counts in steps of 77 ADC counts). If the Protections:Temperature:Undertemperature in Charge Protection Recovery is set to zero, autonomous recovery is disabled, and recovery must be initiated by the host sending the PROT_RECOVERY() subcommand with the [TEMPREC] bit set.