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BQ24780S: BQ24780S charger IC ACDET Issue

Kavitha Ajay
Prodigy 60 points
Part Number: BQ24780S
Other Parts Discussed in Thread: BQ24773, BQ24780

Hi,

We are using BQ24780S charging circuit in our unit, it has to work with both 12V and 24V Battery.

we have 10mtr cable between the Battery and the unit, so the ACDET Divider circuit i used was 470K and 178K in order to work from 9.8V to 24V as ACDET absolute max is 6.5-7V.

Now the problem is the charger has to work at 9.8V, but it will work properly only at 10.7V with the load of 1-1.5A for 3S during TX, else around 0.6A regular,. and with cable of 10mtrs, it is working only after 12.7V, else charger is resetting means the charger goes off when tx happens with high current hence failing.

Charger parameters like Charger i/p current, Discharge current windows are in max,. 

one experiment i did by changing ACDET to 470K and 324K, R1/R2 then it works fine from 11V itself with 10mtr cable, but problem is it is nt safer range at 24V input for the charger, as ACDET goes beyond 7V.

what if i increase ACDET above 7V ? will it effect the charger? any other means to verify that the charger with load and cable length nt effecting charger performance at lower vtg inputs.

help me out to solve the issue.

  • 0
    TI__Mastermind 33305 points

    HI, Ajay,

    I suspect other issues in the system.  The long 10 meter cable caught my attention. The cable is associated with resistance. This may raise the SRN voltage if not carefully designed. BQ24780S detects the battery voltage at SRN. Remote sense is critical for the accurate sensing of the battery voltage. 

    Regarding the ACDET voltage, I wonder if you can put a small Zener on the  ACDET node, for example 5V zener. Do not exceed the 7V abs max rating. Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under recommended operating conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

    Note that BQ24780S is a buck charger, which means the input voltage needs to be higher than the battery voltage for charging. The max input voltage is 24V. 

    Regards,

  • 0 in reply to Tiger Zhou
    Prodigy 60 points

    Hi Zhou,

    Thanks for the reply. i will add a Zener of 5.2V at ACDET net and check. 

    This Issue on ACDET will be same with BQ24773 too right?, as we are changing from BQ24780 to BQ24773 due to unavailability of BQ24780S. 

    So with BQ24773 also this Zener solution will work the same way as BQ24780?

    Battery Voltage is 8.4V fully charged condition 2S2P. requirement is to work from 12V and 24V input, so this wide input range is effected with the Cable length.

    Thank you

  • 0 in reply to Kavitha Ajay
    TI__Mastermind 33305 points

    This Zener is a general solution to clamp the pin voltage. It applies to other parts as well.