We are designing a battery charger for LiFePO4 batteries but the charging temperature range for the particular battery model we'll use is wider: -30 to +60 °C. How to program the LTC4015 to handle that wider temperature range?
The LTC4015 has a JEITA function which allows the user to program a specific temperature-dependent charging profile. You can configure this function to operate in your desired temperature range. The JEITA function is described starting on page 32 of the datasheet.
The LTC4015 assumes a thermistor with a beta value of 3490k. However, there are digital limits to the JEITA_Tn registers and a thermistor with a beta value of 3490k can only work within a range of about -18°C to 74°C. So, we do need to make a tweak here in order to get this to work in your application.
I have attached a guide and calculator that should help to explain the problem and assist you in choosing an appropriate NTC thermistor. Let me know if you have any questions.
Don't forget to ensure that the JEITA function is enabled with the en_jeita bit in register 0x29. Also, you will likely want to tweak the charging voltage/current in each temperature region. Detailed instructions for doing so can be found on pages 62-63.