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BQ24075 control charging temp with thermister

Tom Smith
Expert 2930 points
Other Parts Discussed in Thread: BQ24075, BQ27500

We would like to use the BQ24075 DPPM charger ic which is in our design
To control charging temperature.  we had been using the BQ27500.
All we need to do is connect a thermister to the TS line of the BQ24075
And this should work.  However, it is not working right it seems.
The thermister kicks in and the charging is stopped while hot, but the charging
Will not resume once the temperature returns to in range.
We are on a very tight schedule with implementing this change and could really use
Some help.  It may be a very simple issue that we just are not aware of yet.

-Tom-

  • 0
    Expert 2930 points

    How about this for a possible solution;

    I think I figured out the BQ24075… it may be related to the Termination current.
    It may be that the charger wants to be plugged back in if it is halted for any reason,
    Including a temperature event on TS.

    Comments...................

    -Tom-

     

  • 0 in reply to Tom Smith
    TI__Genius 17340 points

    Tom,

    Thought I'd throw in my two cents. First of all, what type of thermistor is being used? If its not a 103AT-2 type thermistor, than the thermal limits will not be set accurately.

    Second, make sure the device has not stopped the charge cycle. Accorinding to page 28 of the datasheet, the CHG pin will remain low during a temperature fault which indicates that the device is still charging. If for some reason the device has stopped charging during the temperature fault, then the device will only restart charging when Vbat has dropped below the recharge voltage threshold. It seems to me that pulling the battery and plugging it back should also restart the charge cycle. Either way, during a temperature fault, the device should continue to indicate that its charging and it should continue the charge cycle after the temperature has dropped. Figure 41 on page 27 of the datasheet shows the modified charge cycle  with the thermal loop.

    Hope this leads you in the right direction, otherwise one of the battery management guys may be of more help.

    Regards,
    Matt Guibord

  • 0
    TI__Genius 14985 points

    What connection do you have between the bq24075 and the bq27500? 

    You mentioned "To control charging temperature.  we had been using the BQ27500."

    I'm trying to see if that might be related to your problem.

  • 0 in reply to dMax
    TI__Mastermind 31120 points

    The circuit is very simple.  when the TS votlage is in range charging is possible and when out of range the charging PENDs, meaning that the charging stops, safety timer stops, termination disabled until the TS voltage returns in range.  Check the voltage on the TS pin.  Is it in rage or out of range?

    Is there anything tied to the TS pin other than the thermistor? Put a scope on the pin and see if it is a DC voltage.

    If you can't find the issue post the schematic and maybe I can see something.

     

  • 0 in reply to Charles Mauney
    Expert 2930 points

    Initial info to for your review,

    I noticed that it would work as advertised when it required a lot of current.  When the battery was almost charged, it would stop and require the plug in.
    We are using the same thermister that goes with the BQ27500.  I think it is a 103AT-2 type thermistor.

    More info to follow regarding your above responses.

    -Tom-

  • 0 in reply to Tom Smith
    Expert 2930 points

    I talked with Ming Yu at TI today, and he encouraged us to resolve our temperature issue with the BQ27500.  The thermister on the BQ24075 is not working out.  So far, the BQ27500 changes have been working, but we have some more work to do.

    -Tom-