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BQ24031: Extending the charge safety timer

dnilsson
Prodigy 170 points
Part Number: BQ24031

Hi,

We have a sensor device under development and test using the BQ24031, together with a rather large 6.7Ah Li-Ion battery. Input power is AC or USB, and I have set the AC input current limit to 480mA and USB input limit to 500mA. This causes a rather slow battery charging, which is OK for this product, but seems to prevent us from using the charge termination feature (being able to detect when the battery is fully charged via STAT1/STAT2). Maximum R(TMR) according to datasheet is 100k, causing a typical charge safety timer of 10hours which isn't going to be enough for our battery if fully discharged. Disabling the timer (tying TMR high) does disable the timer but also causes the charger to indicate "fast charge" forever which is according to spec since it disable charge termination.

I've been looking into increasing the AC charge current to lets say 1000mA, but keeping the USB charge current to 500mA. The datasheet isn't very clear what happens in this case with the timer when charging through USB, is current limiting via USB considered one case of DPPM? Chapter 10.3.8 of the datasheet states: "In DPPM mode the charge termination is diabled" so I guess any kind of extension to the timer based on DPPM will also prevent us from getting a "charge complete" status back from BQ24031?

In other words, is it possible to charge a fully depleted 6.7Ah battery from USB 500mA using BQ24031 while also expecting the BQ24031 to signal "charge complete" via STAT1 / STAT2 pins? I suppose putting this under software control where an external MCU resets BQ24031 via CE input at some time interval is some kind of workaround, but I would prefer to avoid that logic.

Thanks
Daniel

PS. Chapter 10.3.10 states that the recommended capacitor on DPPM is 1 nF to 10 nF, chapter 11.2.2.1 recommends 1 nF to 100 nF on the same capacitor. Maybe both places in the datasheet should recommend the same range DS.

  • 0
    TI__Mastermind 22140 points

    Hi Daniel,

    The safety timer should be the same whether switching between AC or USB mode (it is simply a fast charge timer).

    dnilsson said:
    is current limiting via USB considered one case of DPPM?

    DPPM can occur due to current limiting via USB but this will depend on your OUT load. If the input current cannot supply sufficient current to both the system load and the charging current, and the voltage continues to drop below the set VDPPM point, the device will be in DPPM. 

    dnilsson said:
    "In DPPM mode the charge termination is diabled" so I guess any kind of extension to the timer based on DPPM will also prevent us from getting a "charge complete" status back from BQ24031?

    Yes.

    dnilsson said:
    In other words, is it possible to charge a fully depleted 6.7Ah battery from USB 500mA using BQ24031 while also expecting the BQ24031 to signal "charge complete" via STAT1 / STAT2 pins? I suppose putting this under software control where an external MCU resets BQ24031 via CE input at some time interval is some kind of workaround, but I would prefer to avoid that logic.

    Yes, you would need to extend the safety timer through this method. Judging from the capacity of the battery, it may not be possible to charge with USB 500mA charging. 

    dnilsson said:
    PS. Chapter 10.3.10 states that the recommended capacitor on DPPM is 1 nF to 10 nF, chapter 11.2.2.1 recommends 1 nF to 100 nF on the same capacitor. Maybe both places in the datasheet should recommend the same range DS.

    Noted! Appreciated!