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bq24707: Circuit changes to support long output cable?

John M
Expert 2135 points

We're hoping to use a bq24707-based charger in our next design and I was wondering if there were any changes recommended for handling long output cables? Assuming that our circuit is similar to the one used for the EVM and that the cable to the battery can be up to 4ft long (8ft. "round trip"), is there anything we should be worried about? Typical charge current levels are 2A and 5A at 14.0V - 17.0V.

  • 0
    TI__Guru 56650 points

    When a battery is close to its full capacity during the charge, the charging current tapers down. The host can setup a termination current to stop charge when a battery is fully charged. For example, it can be 1/10 of the charge current level. So, the termination current could be 0.2A or 0.5A. If you know the cable resistance and the termination current and accuracy, the host controller can set a higher charge voltage to compensite the cable voltage drop.

  • 0 in reply to Wang5577
    Expert 2135 points

    Thanks for the reply!

    Unfortunately, I didn't explain my concern very well. My apologies.

    I was worried about the possible effect of the long cable's inductance and was wondering if there were any recommended circuit changes? The cable's DC resistance isn't a problem for the wire gauge we're using.

  • 0 in reply to John M
    TI__Guru 56650 points

    For the long cable's inductance, you have to do a battery insert and remove test. If the charger output or system bus has a high voltage spike during a battery insertion or the phase pin has a large negative voltage when remove a fast charging battery, please add TVS diode on the charger output to clamp the voltage spike and limit the negative voltage on PH pin.

  • 0 in reply to Wang5577
    Expert 2135 points

    Ahh...I hadn't considered the effect of connecting/disconnecting the battery under load.

    For the prototypes, I was adding a unidirectional TVS for ESD protection so it can now do double duty. I'll be sure to verify that any spikes are suppressed. Thanks again!