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BQ24250: Adding NTC Stops The Deice Charging.

Lee Wade
Prodigy 100 points
Part Number: BQ24250

Hi all,

I am on my 3rd and final prototype build using the BQ24250RGER. On the previous units I did not fit the NTC, instead a 10k resistor was used from TS to GND (*R3) with a 5k from LDO to TS (*R2)

I have had only good results from these previous build.

Now I have fitted a 10k NTC with the recommended beta (3500k?) from the datasheet, it now doesn't charge. If I add back in the 10k fixed resistor (*R3) in most cases it charges. I may have damaged a device on the way.

I also don't understand why a 4000k NTC requires a 9.82k resistor ( it is a small deviation? ) and a 3500k doesn't require a resistor at all.

Any help appreciated.

Lee.

* datasheet annotation

  • 0
    TI__Guru**** 165886 points

    Hi Lee,

    The datasheet is incorrect and a revision is pending.  I apologize for the confusion. You will need a pull up and pull down resistor in parallel with NTC to ground as calculated by the xls spreadsheet at the link below:

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi Jeff,

    thanks for the very prompt reply.

    I couldn't get the linked spreadsheet to work; it complained of a missing library I tried to resolve it but didn't get anywhere.

    So, for a 3500k beta 10k NTC the value of R3 the parellel resitor should be 10k?

    Many thanks.

    Lee.

  • 0 in reply to Lee Wade
    TI__Guru**** 165886 points

    Hi Lee,

    For an NTC thermistor with Ro=10kohm and Beta=3500, the solver reports resistor from LDO to TS = 5.38kohm and resistor from TS to GND (in parallel with NTC thermistor) = 11.1kohm.

    Regards,

    Jeff

     

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi Jeff,

    OK so the values are not so different to stop a device working when fitted, just not achieving the JEITA spec and if the paralled 11.1k is missing it is a problem?

    Must have been marginal with the fixed 10k fitted for the NTC. It also worked without the 10k fixed and no NTC.

    I think production may have also damaged some devices by fitting the NTC after the batteries were fitted and the BQ24250 was running?

    Back at work on Monday to do more testing.

    Many thanks,

    Lee.

  • 0 in reply to Lee Wade
    TI__Guru**** 165886 points

    Hi Lee,

    Correct about not achieving JEITA and being marginal.

    I would not expect attaching a thermistor to the TS pin to damage the charger because most thermistor are part of the battery pack so will be attached when a battery is reattached. 

    Regards,

    Jeff