I expect that I'm making some sort of math error, but I've checked my calculations and can't find an error. I'm trying to calculate RLO and RHI for the TS divider per the bq24160 datasheet, and I'm getting odd results when I then calculate RCOOL and RWARM. Here are some details:
The NTC thermistor is a fairly standard type, 10 kohms @ 25 C and B(25-50) = 3380 K. Using the data from the NTC datasheet, RCOLD (0 C) = 27.4 kohms and RHOT (60 C) = 3.02 kohms. RLO and RHI calculate out to 12.3 and 15.1 kohms, respectively. Plugging those back into the equations for RCOOL and RWARM I get -34.4 and +37.4 kohms, respectively. Clearly, no value should be negative, and they should be close to the 10 C and 45 C values from the NTC datasheet. Those values are 18.0 and 4.91 kohms, respectively.
Any ideas on what I'm doing wrong? Is there some sort of limitation around the type of NTC thermstor I can use?
Thanks,
Scott
