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BQ76920: BQ76920

Jan Volkering
Expert 8190 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ78350, BQ76940EVM, BQ76930EVM, , BQ76930, BQ76940, TIDA-00449

rHi, a customer I support wants to use the BQ76920 with their own MCU.

Questions: Referring to the datasheet example schematic and EVM schematic:

1: Why are there on the South battery two capacitors in series with the mid point to ground and only a single capacitor on the other cells?

2: Are there guidelines for determining the resistor and capacitor values connected between the batteries and the BQ76920?

3: Can they use the BQ78350 EVM firmware (downloadable) as guidance for implementing the control functionality in their own micro?

  • 0
    TI__Guru 54715 points

    Hi Jan,

    1. If you are referring to the Cc capacitors for VC0 and VC1 of figure 8-1 of the data sheet, this configuration prevents a step load on the battery which will drop the voltage of the cells from pushing VC0 below its reference VSS.  A differential connection of the caps here would tend to push VC0 below VSS which can disrupt operation on some part designs.

    2.  There are no concise guidelines.  The input resistors Rc are selected for the balance current.  They should not be so small to exceed the maximum balance current.  Smaller resistors will provide higher balance current but will have less resistance to the IC for transients.  Larger resistors will give lower balance currents and better isolation from transients.  Resistances above the recommended range may begin to show measurement errors.  (For the bq76930 and bq76940 the largest recommended value is common due to the architecture and external balancing is typical).  The capacitor is sized in the recommended range to provide noise and transient filtering for the input pins. For smaller Rc a larger Cc would typically be used.  The Cc discharges into the IC when balancing turns on, so Cc should not be arbitrarily large.  Also the filter must settle after balancing current stops within 12.5 to avoid voltage measurement artifacts.   The 100 ohm and 1 uF of the bq76920EVM seems to perform well.  (The 1k ohm and 1uF on the bq76930EVM and bq76940EVM show voltage effects during balance, a smaller filter cap is usually preferred.)

    3. The algorithms and code for the bq78350 are proprietary, the general description is available in its documentation.  The SBS Smart Battery System sbs-forum.org may be a reference of interest.  Code references which may be useful are TIDA-00449 which has a control implementation for the family (bq76930 was used), and the application report and associated code Multi-Cell Li-Ion Battery Management System Using MSP430F5529 and bq76PL536  which uses a different monitor.