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BQ76PL455A-Q1: Cell balancing circuit check & standby current measurement

Kay Chen53
Expert 2225 points
Part Number: BQ76PL455A-Q1

Hi there:

greeting, We're under checking customer schematic for using the BQ76PL455A-Q1 with 15 or 16 cell application, (could refer attached file for customer schematic)

We'd like to double confirm the value of the cell balancing circuit whether is correct or not.

And also we'd like to know how to measure the stand-by mode (or sleep mode) current with this one?  (from data sheet is saying 50uA max) 

BQ76PL455Q1.pdf

  • 0
    TI__Genius 11480 points
    Hi Kay,

    I would recommend changing the LED and balance resistors to the drain of the FET rather than the source. This circuit looks like its set up to balance with a PMOS, while the pl455 uses an NMOS. If they have this connection, then the EQ pin may have trouble driving the FET, especially at lower cell voltages.

    Also note that 11 ohms will give a significant balance current at higher cell voltages. Please be aware of the thermal implications here, and have proper layout. I would even consider using four resisters to spread out the power.

    You can measure the current into the TOP pin of the pl455. This is the main supply. If you have a resistor in series with the pin you can just measure the voltage across the resistor when it is in sleep mode...
  • 0 in reply to David Wiest Jr
    Expert 2225 points
    Hi David:

    Thanks again! out customer also consider this design whether will cause the battery cell power consumption itself?

    If their currently design:

    change LED and balance resistors to the drain side.

    change Rsense resistors from 100 ohm into 1K

    would it help to improve the issue symptom?

    or any other consideration need to take to prevent the battery cell consume?
  • 0 in reply to Kay Chen53
    TI__Genius 11480 points
    Hi Kay,

    My concern with the balance resistor configuration was that balancing would not work properly, because the EQ pin would have to overcome Ibalance*RSNS + VGS. This would mean that there could be some regions where the FET is not driven correctly, since the EQ pin comes from the Cell.

    Changing the sense resistor will help filter, but won't help leakage current much. You will also see more I*R drop.

    Remember, almost all semiconductors have some leakage... FETs, Diodes, IC's, etc. They will all have some Iq that drains the battery. Look for passives with low leakage.