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BQ77915: Can we increase cell balancing LED indicator current?

John M
Expert 2135 points
Part Number: BQ77915

Hello,

On page 16 of the bq77915 EVM User Guide, SLUUBU2B, the Cell Balance Indicator Schematic with five LEDs is shown. We would like to use a higher LED current level of 10mA to allow for much more visible balancing status LEDs.

For our design can we increase the value of the Rin input filter resistors to compensate for the increased LED current, limiting the cell balancing current to 50mA or less, and have the LEDs drawing 10mA?  If yes, do we need to reduce the value of the input filter caps to keep the same RC time constant?

If possible would like to avoid the expense and space needed for external FETs just to have brighter LEDs. Thank you.

Regards, John M.

  • +1
    TI__Guru 54715 points

    Hi John,

    The VCn to VCn-1 pins pull together when cell n balances using the internal N-ch FET shown in the Functional Block Diagram of the BQ77915 data sheet.  You can adjust the current in the LED by adjusting the resistors in series with the LEDs (shown on page 16 of the user guide as you note).  Normally the pins pull together symmetrically.  As the current in the LED is increased, the LED in the VCn-1 path blocks current, so the pins no longer pull together symmetrically.  Since the LEDs have a required Vf there may be a limit, but I'm sure you have some range of adjustment beyond the EVM circuit. 

    An external FET to sink most of the balance current and the LED current would make the circuit more versatile, but as you note it is a lot more components.

  • 0 in reply to WM5295
    Expert 2135 points

    Hello,

    Thank you for the quick response! I did neglect to consider the effect of the LEDs on the VCn-1 current flow. Hmm…looks like we’ll have to grab a couple of EVMs, swap some resistors, and see what happens. Please consider my issue resolved. Thanks again!

    Regards, John M.