LTC1760 LOPWR trip

Hello,

I'm testing a board with three LTC1760s in parallel connected to 6 Inspired Energy NH2034HD43 packs, (4s, 2p). I set the LOPWR trip voltage to be at 11V, but I find the controllers let the batteries discharge all the way down to 9.6V where the internal battery protection kicks in and shuts off the battery output.

Any ideas what I might be missing? I had understood the LOPWR function to disable discharge when when no other power sources are present, but perhaps that is not the case. Are there any other ways to set the cutoff voltage without connecting to a dedicated SMBus host?

Thanks!

Paul

  • 0
    •  Analog Employees 
    on Oct 15, 2020 8:51 PM

    Hi Paul,

    That is not what the LOPWR trip does - its function is to attempt to keep the load powered, not to disable discharge from the batteries. The LTC1760 offers no such function inherently.

    It is perfectly fine to rely on the battery's own protection to do this instead.

    If you want to manually disable a battery's path to the load, you can do so following the instructions in the attached document.

    PDF

    Regards,

    Zack

  • Hi Zack,

    Thanks for the feedback! That explains the behavior I'm seeing. I actually added the ability to disconnect the batteries from the circuit for storage using a pfet in series with the battery + terminal where the gate is pulled up to the battery voltage when shut off and pulled to ground when enabled (schematic attached). In the next version I could likely modify this to add a low-voltage cutoff for the battery.

    I initially planned to use the battery protection to end discharge, but the issue is that the LTC1760 will not detect the battery after this happens and so will not charge it (or perhaps tries to trickle charge very slowly). Could this be a side effect of the pfet addition to the circuit? Otherwise it's a straight copy of the demo board for the LTC1760 x3.

    Paul

  • 0
    •  Analog Employees 
    on Oct 19, 2020 6:28 PM in reply to paul.l.d.roberts@gmail.com

    Hi Paul,

    The PFET seems like it will allow wake-up charging to occur since that does not require a voltage to be present. However, it will need to be turned on in order to start the normal charge cycle. Does that happen? Once it is turned on, everything should be normal.

    Make sure your thermistor value is in a valid range for wake-up charging. Certain ranges will have a wake-up charge timeout, though that may be what you want. There is a table in the smart battery spec (section 4.4.4) which describes this behavior.

    Regards,

    Zack

  • Hi Zack, that's a good question, I was relying on the fet conducting when BAT_EN is pulled to ground even when the smart battery has disabled discharge, but perhaps that's not correct. Do you have a suggestion on how to turn the fet on in that case?

  • +1
    •  Analog Employees 
    on Jan 4, 2021 10:09 PM in reply to paul.l.d.roberts@gmail.com

    Hi Paul,

    Do you have the weak pull-up path shown in the DC486B reference design?

    See R25 and D5:

    That will ensure the FET can always be biased and you should always be able to turn on the FETs by pulling the gate to GND.

    Regards,

    Zack