Temperature sensing is a top priority in many battery charger applications. Oftentimes, accurate temperature monitoring is essential even when an adapter is not present and power is not being supplied to the charger IC. Particularly on the BQ21040, temperature sensing is handled by supplying 50uA current from the TS pin across an NTC thermistor. However, this typical mode of operation is not possible without power being supplied to the BQ21040 via an adapter at the IN pin. This application note serves to outline how TS functions at a high level and how one might use an external MCU to be able to monitor battery temperature even when an adapter is not present
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This document serves as a guide on how to monitor temperature on the BQ21040 when an adapter is not present at VIN and the device is in battery only mode.
Typically, when monitoring temperature on the BQ21040, a 50uA current is supplied at the TS pin. This current then travels through an NTC thermistor to ground. As the resistance of the NTC varies with temperature, so does the voltage drop across it. This voltage drop is then measured to derive the temperature.
When an adaptor is not present on the BQ21040, the IC is operating in battery only mode. During this mode of operation, there is no 50-uA current being supplied at TS and therefore essentially 0 voltage drop occurs across the NTC thermistor. Without this voltage drop, the method used to obtain temperature measurements with an adapter present as described above cannot be used. Instead, an external MCU will be used to bias 50uA of current across the thermistor. Voltage will then be measured by the MCU in the same manner that the BQ21040 does when an adapter is connected.
