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BQ25672: Thermistor usage, VT value calculation ?

Ahto Moorast
Prodigy 10 points
Part Number: BQ25672

I'm evaluating BQ25672 on EVM.
I'm planning to test this charger chip with external NTC thermistor.

In the datasheet SLUSEB9 in section 8.3.8.5.1, I find formulas on calculating the resistances RT1 and RT2.

But I can't seem to understand how one would get the VT1-VT5 values used in RT1 and RT2 calculations.

Following the datasheet example I can't replicate the resistor RT1 and RT2 values shown in the example.

a) In the datasheet there is NTC_Control_1 register where I can set the VT2, VT3 to a specific % of VREGN.
b) VREGN value is vague. According to the spec it is 5V, but only at VIN=15V. Using 5V value won't give me correct RT values as in datasheet example.

For example datasheet claims, using:
RT2 fixed at 30.31k (to set T5=60deg C)
T1=0deg C setting
RTHcold as 10k 103AT NTC at 0deg is 27.28k 

RT1 should then be 5.24k according to the example.

I assume VT1 to be 72% of VREGN during charging (as in datasheet) -> VT1 = 5V*72% = 3.6V ?

This makes RT1 = -10.369k ??

The sentence on page 34 seems to be missing something:
" The [??] provides the comparators with fixed thresholds for VT1 and VT5, and the comparators with programmable
thresholds for VT2 and VT3. "


So how to find values for VT1-VT5 to calculate RT1 and RT2 ?

  • 0
    TI__Guru**** 166226 points

    Hi Ahto,

    The values on the BQ25672EVM for RT1 and RT2 are for a 103AT, 10kohm thermistor.  If you want to compute, you can use the spreadsheet at the link below.  It is for another device but it works the same.  The exact REGN voltage cancels out in the computations because the internal TS ladder comparators use REGN for the reference as well as to bias the TS pin. VT# is the voltage created by REGN x ((RT2||R(NTC))/(RT2||R(NTC)+R1).

    https://www.ti.com/lit/zip/slurax7

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hello,
    Thanks for the reply,

    It did confirm VT calculation, that it is indeed a percentage from VREGN which means, VREGN value at specific VIN is not irrelevant.
    Sure the VREGN possibly cancels out in RTx calculations but is still needed to derive VTx values.

    Checking on bq2589x for which the spreadsheet was for, the data matches. When porting this for BQ25672 I see some discrepancies.
    Please see the attached spreadsheet for example.
    BQ25672_NTC.xlsx

    I get different result using formula b) (from original spreadsheet) and d) from BQ25672 datasheet (b and  d references in the spreadsheet).
    For BQ25672 there is no VREF or VREGN value in the very first term of RT1. 

    ----- about spreadsheet:

    I wrote down all of the VT percentage values from the BQ25672 datasheet and used either Rise and Fall values to compare the results.

    I also double checked the used formulas in the original spreadsheet and noticed that it uses formula c) with slight modifications, where  VTCO is used as VT1. VLTF is used as VT5. Thus making it the same as b) and calculates the same.


    So how comes d) in BQ25672 to be ? is this a mistake or am I missing something ?


  • 0 in reply to Ahto Moorast
    TI__Guru** 105740 points

    Hello Ahto,

    The Excel shared is used for calculating RT1 and RT2 reliably and they are essentially the same for both chargers. If you change the VREGN and the threshold percentages in the original Excel you should get the VTx, RT1, and RT2 values you're looking for. I will need to check why the equations aren't giving you the correct answers when you input them in Excel.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 10 points

    Hey

    Any luck looking at the calculations with data from bq25672 ?

  • 0 in reply to Ahto Moorast
    TI__Guru** 105740 points

    Hello Ahto,

    The equation for RT2 in the bq25672 should include the VREF as in the Excel equation if you use the VTx voltages (VREGN*threshold_percantage), if you use the percentage thresholds only, it should give you the correct results. My original statement to use the Excel sheet and change VREF and threshold percentages to find the values you need will still calculate the correct results.

    Sincerely,

    Wyatt Keller