BQ76940: Question about Thermistor

Hi M,

The architecture of the device uses 3 cell groups, each has a single TS support.  Since these groups operate on different cells, they have a different reference potential, and the 3 thermistors must use different reference voltages as shown in the data sheet and you note above.

The part will not work properly if the TS2 and TS3 thermistors are referenced to VSS.  If this is done current is pulled from the TS2 and TS3 pins and will discharge the cells. The part also will not boot properly and will give an XREADY fault after boot which will can not be cleared. 

Two other topics come up with TS2 and TS3 which you may want to know:

1. The EVM shows a capacitor on the thermistors.  This is not required, but can avoid noise pick up differentially on the TSn pin from waking the upper groups, this can again show an XREADY fault on boot.  Operation of the upper groups only can also cause an imbalance in the cell groups over time.  If used the capacitor should be close to the IC.  If remote thermistors are used either the thermistors should be attached or a substitute resistance attached  for board test to avoid residual voltage on the pins which could prevent proper device boot and XREADY after boot.  A pull down to the proper reference is required for operation.

2. TS2 and its reference, and TS3 and its reference can form antennas when routed to remote locations in the battery.  The BQ76940 application circuit expects the power filter caps Cf can maintain a constant voltage on the power pins to avoid effects of noise pick up.  If needed, attenuate any noise pickup on these lines in your system appropriately for your environment.