BQ76940:Voltage monitoring and balancing operation of BQ76940

Hi Sato-san,

I am translating your post to English:

question 1:
Please let me know how the input impedance of the VC0-VC15 pins changes during voltage monitoring. It is assumed that the input signal from each pin is switched inside the IC and input to the 14-bit AD converter. What is the approximate input resistance of the VC0-VC15 terminals (electrodes)? Is the input resistance affected by internal signal switching or is it constant at each terminal?
The input resistance of the AD converter may be 5KΩ. *Example: AD1674
If you change it, please let me know how it changes and the pattern. What is the nature of the change in input resistance, switching time, etc. for each terminal?

Question 2:

What does the IC do to keep the cell voltages balanced? It is correct that the IC compares the voltages of each cell and if there is a cell with a higher voltage the VC pin lowers the input resistance to lower the charge Does the input resistance of each VC terminal decrease when there are multiple cells with higher voltages?

Thank you for your reply.

Best regards,

Matt

Hello Sato-san,

In regard to your questions:

1) Please note that in regard to the VCX input pins and ADC, each differential cell input is factory-trimmed for gain or offset, such that the resulting reading through I2C is always consistent from part-to-part and requires no additional calibration or correction factor application.

• The input impedance of the VC0-VC15 pins is negligible.  Resistance is provided by external cell input resistors.  These resistors are used with capacitors to filter cell balancing current.  Section 7.3 Recommended Operating Conditions provides recommended values for these resistors.
• Figure 7-2. in Section 7.7 Typical Characteristics shows the BQ76930 VCx Error Across Input Range at 25°C with VIN at 3.6 V.  This may help provide a visual for how the VCX pins affect voltage readings.
• When using internal cell balancing, the internal cell balancing driver resistance ranges from 1 - 10 Ohms.  These values can be found in Section 7.5 Electrical Characteristics.  Section 7.5 also provides electrical characteristics for the 14-bit ADC.
• I will link the data sheet that provides this information here: BQ769x0 3-Series to 15-Series Cell Battery Monitor Family for Li-Ion and Phosphate Applications datasheet (Rev. I).
• You may also want to watch this video for more schematic considerations: How to create schematic for BQ76920, BQ76930 and BQ76940 | TI.com Video.

2) Cell balancing for the BQ769x0 family works by controlling cell balancing FETs.  The device does not vary an internal resistance value.  It uses either internal or external FETs to direct current flow around cells to dissipate excess power through resistors instead of charging overcharged cells.

• Section 4 Cell Balance of the Top 10 Design Considerations application note discusses balancing FETs and how to size the input resistors to select the balance current within the capability of the part.  Section 4 also covers different cell balancing configurations and considerations. I will link the app note here: bq769x0 Family Top 10 Design Considerations (Rev. A).
• Additionally, Section 8.3.1.3.3 Cell Balancing in the datasheet discusses cell balancing.

Best,

Andria