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BQ24610 TS calculation is negative

Ignacio Linares
Expert 1655 points

Other Parts Discussed in Thread: BQ24610

I have a customer using the BQ24610.  He is getting a negative value for the RT2 resistor.  I have verified his numbers.  What does a negative resistor value mean?

------------------------

We are looking to use this thermistor in our battery pack http://www.digikey.com/product-search/en?keywords=490-7167-ND

 

I am trying to calculate the resistor values to go with the TS pin. The recommended temperature range for charging our battery pack is +10C to +45C. I followed the same calculations we have done and ended up with negative number for the resistors. Could you please check and see if you get similar results?

 

 

Hieu,

 

This is odd – if I use +10C as you indicated, I get the following (RT2 = -46.8 ohms).

 

Vvref

3.3

Vvref/Vtco -1

1.906977

RTHcold

1.79E+01

Vvref/Vltf - 1

0.360544

RTHhot

4.92E+00

1/vltf

0.412286

0.735

Vltf

2.4255

1/vtco

0.880902

0.344

Vtco

1.1352

1/RT2

-0.02137

1/Rthcold

5.58E-02

RT2

-4.68E+01

RT1

1.05E+01

 

 

If I use 0C, then I get RT2 = 474 ohms.

 

Vvref

3.3

Vvref/Vtco -1

1.906977

RTHcold

2.72E+01

Vvref/Vltf - 1

0.360544

RTHhot

4.92E+00

1/vltf

0.412286

0.735

Vltf

2.4255

1/vtco

0.880902

0.344

Vtco

1.1352

1/RT2

0.002112

1/Rthcold

3.67E-02

RT2

4.74E+02

RT1

9.28E+00

 

 

 

 

 

 

  • 0
    TI__Expert 8690 points

    Hi Ignacio,

    I get negative results as well: 

    R_COLD (10C) 17.93 k-ohm
    R_HOT (45C) 4.92 k-ohm
    VREF 3.30 V
    VLTF 2.43 73.50%
    VTCO 1.14 34.40%
    R9 10.48  k-ohm
    R10 -46.78  k-ohm

    This is with beta = 3380K and r25 = 10k.

    I have tried to calculate this with different NTC thermistors, but they all give negative, even with different Beta and R25 values. The issue is that 10C doesn't work, but as soon as I calculate for 0C, they all give real values. Seems that the equation does not work for anything higher than 0C.

    Regards,

    Steven

  • 0 in reply to Steven Bangdiwala
    Expert 1655 points

    Steven,

    Thanks for the reply.  The customer decided that for this particular battery he needs 10C to be the cold temp limit.  I assume because he picked this temperature, because it is a battery supplier recommendation and there are some implications such as damage or degradation of the battery if the temp goes lower than 10C.

    What then is the recommendation/resolution?  It seems silly to have a formula in the datasheet that doesn't work.

  • 0 in reply to Ignacio Linares
    TI__Expert 8690 points

    Ignacio,

    The only solution I find is that you find a NTC thermistor with high beta value. The equation assumes a bigger variance in NTC resistance in order to work.

    With more beta, the slope is more steep and could give you real values. The internal references are calculated using these equations. Another method would be to create your own comparators and then feed that data to the TS pin.

    Regards,

    Steven