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BQ24296M: Thermistor sense thresholds during charge termination

Vasyl Vorobei
Intellectual 355 points
Part Number: BQ24296M

Hi.

The datasheet for this PMIC declares several TS limits for the charging mode:

As one can assume, these thresholds must also apply in a charged state (REG08[4:5)=11). And I see from the experiment that they do.

However, in REG08[4:5)=11 states, the hot temperature hysteresis (difference in HFT and TCO limits) seems to disappear. I've done two experiments to check that, in both, there is a multi-turn potentiometer instead of the NTC:

1) REG08[4:5)=11, REG05[7] = 1. PMIC shows <10 mV window between HOT-NORMAL temp states.

2) REG08[4:5)=11, REG05[7] = 0 (disabled termination). PMIC shows a ~120 mV window, as expected from the datasheet values.

Could you confirm that in REG08[4:5)=11 states, there is no HFT-TCO hysteresis window? If so, which one is applied?

Best regards, Vasyl.

  • 0
    TI__Mastermind 27710 points

    Hi Vasyl,

    We will perform some research into your inquiry and get back to you.

    So you are aware tomorrow 3/29 is a TI holiday, therefore please expect an update from us early next week. 

    Best Regards,

    Garrett 

  • +1 in reply to Garrett Kreger
    TI__Mastermind 27710 points

    Hi Vasyl, 

    Please see my comments below. 

    I observed the same behavior as you report when testing the BQ24296M in lab and it is expected. 

    The transition point from HOT -> NORMAL is constant no matter if termination is disabled or enabled. The transition point from NORMAL -> HOT will change depending on if REG05[7] = 1 or 0 and if the device is in the charge terminated state. 

    This difference exists because with charge termination disabled the device is considers still in charging state. Therefore device will indicate HOT when TS voltage falls below TCO threshold. When charge is terminated (termination enabled) device is not actively charging. Device will indicate HOT when TS voltage falls below HTF threshold. 

    As figure 17 shows VHTF and VTCO are not the same. Therefore the 10mV verse 120mV difference is expected. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Intellectual 355 points

    Hi Garrett, thanks for your comment. I have one more question regarding thermal sensing.

    I'm observing that the IC's limits are made so that for the majority of 10k 0603 and 0805 NTCs, I'm getting their maximum permissive current at around +25 °C temperature. That leads to a minor (~1 °C) self-heating effect at elevated temperatures (resistance drops, current increases).

    Could you advise which NTC vendor or series I might look for for the MPN with proper permissive current, allowing me to use it without self-heating at +45 °C?

    Best regards, Vasyl.

  • 0 in reply to Vasyl Vorobei
    TI__Mastermind 27710 points

    Hi Vasyl, 

    We recommend a 103AT-2 thermistor be used. 

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Intellectual 355 points

    Hi Garrett.

    Semitec 103AT-2 is a PTH type. Could you advise SMD type as well?

    BR.

  • 0 in reply to Vasyl Vorobei
    TI__Mastermind 27710 points

    Hi Vasyl, 

    Apologies I am not a thermistor expert and therefore I do not have a specific SMD type thermistor to recommend for your design. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    TI__Mastermind 27710 points

    Hi Vasyl, 

    The best option may be to minorly adjust to resistor network to account for the self heating. The idea would be to set the HOT threshold to 44°C to offset the self heating. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Intellectual 355 points

    Without changing the NTC - yes, for sure. I've asked in case you might advise an alternative.