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BQ77915: BQ77915: 8-series cell configuration with BQ77915

Part Number: BQ77915

Hi.

I am designing 8S  PCM with bq77915.

You said, "Generally keep the cell count balanced between devices. When the cell count does not match, section 9.3.13 of the data sheet recommends using the higher cell count on the upper device."

And, In the Section 9.3.13

It is highly recommended to connect higher cell count to the upper devices (for example, for a 7-series configuration, connect four cells on the upper device and three cells on the bottom device).

This is to provide stronger CTRx signal to the bottom device.

So, which is better? 5-3 configuration and 4-4 configuration.

Thank you.

Pi

  • Hi,

    In thiscase, the 4-4 configuration would be best.

    When considering the CHG/DSG signal strength from the upper device to the CTRC/CTRD pins of the lower device, we look at the electrical spec. When VDD for the upper device is under 12V, the spec for the CHG/DSG output is lower than if it is above 12V. This should be okay to meet the input voltage spec for the CTRC/CTRD pins. If there are only 3 cells connected to the upper device (for example using a 6S solution), it would give a little bit lower margin in the case of transients in the system. This will likely not be an issue, but this is why it is recommended to connect 4 cells to the upper device in the case of 7S.

    Best regards,
    Matt