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BQ25798: TS pin and NTC

James Panko
Prodigy 160 points
Part Number: BQ25798
Other Parts Discussed in Thread: TIDA-050047,

Hi Ti Team, 

Referencing BQ25798 SLUSDV2B Revision B:

2 Questions

1. Page 6 recommends 103AT-2 thermistor but does not provide the B specification which specifies the thermistor slope. What is the B specification for the recommended 103AT-2?

2. The Resistors specified when using 103AT-2 thermistor, RT1 = 5.24k and RT2 = 31.31k, not common resistors. The values for RT1 and RT2 specified for the TIDA-050047 and 

BQ25798BKUPEVM are RT1 = 5.23k and RT2 = 30.1k. What are the recommended values for RT1 and RT2?

The Datasheet has several references to the RT1 and RT2 Values:

  Page 46: Figure 9-12 and the paragraph defining the calculations for RT1 and RT2 with 103AT-2 thermistor.

  Page 83: REG18_NTC_Control_1 register: TS_COOL_1:0 and TS_WARM_1:0 descriptions

  Pages 130, 131 and 132: Reference schematic shows the Resistor values

Regards,

Jim

  • +1
    TI__Guru**** 165886 points

    HI Jim,

    Regarding 1, NTC thermistor datasheets usually have a Beta value so you can use the equation below to estimate resistance vs temp instead of a table:

    The value for a 103AT-2 thermistor is shown below:

    Regarding 2, the R1 and RT2 values are the closest to the values computed by the datasheet equations.  The recommend values for a 103AT-2 are those on EVM RT1 = 5.23k and RT2 = 30.1k.  The TS_COOL and TS_WARM temps are by products of RT1 and RT2.  The 798 has registers than allow some flexibility in charger operation at TS_COOL and TS_WARM .

    Attached is a spreadsheet that should help you size RT1 and RT2.2084.BQ2579X_TS_Calc.xlsx

    Regards,

    Jeff