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BQ24070: Detailed Operation of Current Limiting, DPPM

Jerry Molnar
Prodigy 160 points
Part Number: BQ24070

Hi again Garrett,

I have a question about DPPM as it relates to current limiting and prevention of damage to the BQ24070.

In our design, the battery is a single 18650 Li-Ion cell and the chip is powered by a 5V output AC to DC wall adapter that has a 12W rating.

The nominal charging current to the battery is 1A and the nominal load that the BQ24070 has to supply is 200ma.

Assume that the unit is operating normally and then for some reason a high load current develops due to a fault, but it isn't a short circuit...let's say it is (in theory) 6A at 4.4V on OUT.

From what I understand from the data sheet, the device will source the maximum current from the AC adapter and then supplement that with battery current.

However, the device has a maximum output current of 4A (from the absolute maximums table) which I assume is based on transistor sizes or bonding wire ratings etc.

Here is my question:
An 18650 battery has lots of capability to supply current, so if the voltage on OUT doesn't drop, will the device limit the current in some other way to prevent damage due to supplying too much current?

I imagine if there is short, the voltage on OUT will be low enough to trigger the short circuit protection, but the example I wrote is a bit different.

Thanks in advance!
Jerry Molnar

  • 0
    TI__Mastermind 25805 points

    Hi Jerry, 

    The BQ24070 has thermal protection such that OUT pin will be disconnected from IN pin if power dissipation leads to the IC getting too hot. The same thermal protection does not exist for battery supplement. BAT will be allowed to continue to discharge to SYS. If output load is large enough that OUT voltage falls 200mV below battery voltage the battery FET (Q2) will also turn off.

    This combination of short circuit protection (BAT to OUT) and thermal protection should limit current through the BQ24070 for the example you provided. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 160 points

    Hi Garrett,


    Thanks for your answer. I'll give you a bit more description of our system which will lead to another question.

    Most of the time the unit is in a lower power consumption mode where it is drawing 100-200ma on the 4.4V output of the BQ24070. When it is performing it's main task, it will be running from battery and we've implemented a parallel high current path that supplies the load current (The BQ output is disabled to the load when the high current path is on). The high current load will have bursts of 6-7A of current that the battery supplies.

    I'm thinking through fault conditions and my question is what will happen to the BQ24070 if the CE pin is low (Battery enabled to OUT) and if due to a faulty circuit load current in excess of 4A passes from the battery to the BQ24070 to the load. Will it fry the bonding wires? Another way to ask this question is what is the reason for the absolute max load current rating of 4A?

    Thanks

    Jerry

  • 0 in reply to Jerry Molnar
    TI__Mastermind 25805 points

    Hi Jerry, 

    The absolute max current rating is based on the maximum current the BQ24070 can handle. There is risk of device damage if > 4A passes from BAT pin to OUT. 

    I can test this week and get back to you, but if OUT pin load current increases above 4A Q2 FET should turn off to prevent further current flow. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 160 points

    Hi Garrett,

    That would be great if you can test the scenario of current >4A passing from BAT to OUT to see if Q2 shuts off.

    Thanks!
    Jerry

  • 0 in reply to Jerry Molnar
    TI__Mastermind 25805 points

    Hi Jerry, 

    I have verified in lab that when load current is increased to 4A the internal FET (Q2) turns off to disconnect battery from OUT and prevent any further current flow through BQ24070.

    After testing this scenario multiple times I confirmed the BQ24070 can still charge as expected. 

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 160 points

    Hi Garrett,

    That is great news! Great feature!

    Thanks once again!

    Best Regards

    Jerry Molnar