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BQ24072T: Using this part for higher discharge currents

Victor Claver
Prodigy 175 points
Part Number: BQ24072T
Other Parts Discussed in Thread: BQ24040

Hi, 

I started a design with the BQ24072T a while back and now the specs have changed so a higher discharge currents need to be used. I thought that a possible solution would be to connect the  electronics system directly to VBAT, adding some protection for the battery and not using Vout as my system input. 

I have done some tests and the discharging works fine, however, I have problems charging the battery when is fully discharged (protection kicks below 2.8V).

I have attached an image for a better understanding..

 

So the questions are, can this work? if so, why does not charge when fully discharched and a load connected? (if disconnect the load it starts charging)

  1.  When not charging:
    • OUT = 3.4V
    • BAT = 0.98V
    • CE = 0V
    • EN1 = 0
    • EN2 = 3.3V
    • 120 ohms resistor in paralel with battery
    • 10k load on OUT

 When charging okay:
  • OUT = 3.4V
  • BAT = 3.38V
  • CE = 0V
  • EN1 = 0
  • EN2 = 3.3V
  • open circuit
  • 10k load on OUT

Thanks in advance!

  • 0
    TI__Mastermind 21590 points
    Hello Victor,
    The BQ24072T is a power path device that can support up to 4.5A of discharge current from OUT. Since the path for charge current is different from the load, you'll have issues if you connect your load directly to the BAT, you'll have issues with proper termination when a load is present and since the charge current is set by the ISET pin, the charge current to the battery will be the ISET current setting minus the load you're pulling from BAT.
  • 0 in reply to Raheem
    TI__Mastermind 21590 points
    What is your load requirement that couldn't be met anymore?
  • 0 in reply to Raheem
    Prodigy 175 points

    Hi Raheem,

    Thank you for such a quick response. 

    We are looking at 6-7 amps discharge. When the product is charging, the high current sources are shut down, so when charging the current load is below 50 mA. Not sure the termination current is a problem as the protection circuit goes "open" if the voltage of the battery is below 2.8V. Please, correct me if I am wrong. 

    Thanks again. 

    Best Regards,

    Victor Claver 

  • 0 in reply to Victor Claver
    TI__Mastermind 21590 points
    Victor,
    Termination current is the current that is reached for the charger to determine that charging is done so the battery voltage will be around 4.2V not 2.8. If you connect the load directly to the battery, with a 50mA load present when charging, it'll be hard to terminate since the 50mA will be present. Is there any way to disable the load when charging?
  • 0 in reply to Raheem
    Prodigy 175 points

    Hi Raheem, 

    Of course it is, sorry about it. Long working hours...

    Regarding the termination potential issue, couldn't I disable it by TD pin? Or the timer will terminate the charging? I could also potentially reduce the current of the load when charging.

    Are these 2 the only issues I could have using the configuration showed (system load connected to BAT instead of OUT) ?

    1. Proper termination when load is connected (I probably can reduce it)
    2. Charge current to the battery will be the ISET current setting minus the load you're pulling from BAT (1.3A - 0.05A will still give me a 1.25A charging, which it is okay)

    Just to double check, is the Termination Current = 0.1*Ichg? That would mean that my termination current is around 130mA.

    Again, thanks for your help!

    Best Regards,

    Victor Claver

  • 0 in reply to Victor Claver
    TI__Mastermind 21590 points
    Victor,
    You can disable termination by pulling TD high and this will also disable the safety timers. This could work if 50mA is the load when charging. Even if termination is enabled, the device will terminate a little later with the load which should be fine. You would need to add additional circuits for short circuit protection on the BAT pin since the load will be connected to the battery. You can also take a look at the bQ24040 if the power path function isn't needed.
  • 0 in reply to Raheem
    Prodigy 80 points

    Hi Raheem,

    Thanks for your response. Yes, adding additional protection circuits was in the original plan.

    Best Regards,

    Victor Claver

    PS. I can't find the "this resolve my issue" button (maybe because TI thinks is resolved? dont know)