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BQ76920: Circuit Design

Demetrios Katsaduros
Prodigy 80 points
Part Number: BQ76920

Hello,

In the datasheet of the BQ76920, it states that a microcontroller is required to oversee and control the AFE. Does this mean that to activate the OV/UV, short circuit protections and cell balancing, the microcontroller should be connected to it?

Thank you,

Demetrios

  • 0
    TI__Guru 54715 points
    Hi Demetrios,
    The bq76920 protections will always be active, by default the FET outputs are off. The microcontroller (MCU) must come talk to the bq76920 to set the FET outputs on. Once on, if the MCU does not need readings it could go to sleep. If a OV/UV/short circuit protection occurs, the bq76920 will turn off the FET outputs as detailed in the data sheet without intervention by the MCU. The MCU must assess the fault condition, clear the status and enable the outputs if desired.
    For cell balancing the algorithm and control are completely up to the MCU. If the MCU goes to sleep with balancing running the balancing will continue.
    I see the title does not match the text of the question, I will edit so searches will show a consistent part number. If I misinterpreted please post again.
  • 0 in reply to WM5295
    Prodigy 80 points
    Hello and thank you for your reply! Temperature protection is strictly achieved by a thermistor and MCU support?

    Also, within the datasheet, it states "Multiple cells may be simultaneously balanced. It is left to the user’s discretion to determine the ideal
    number of cells to concurrently balance. Adjacent cells should not be balanced simultaneously." I would like to balance 4 cells only, which i will have connected from Vc0-Vc3, would the internal balance circuitry be able to simultaneously balance each cell?
  • 0 in reply to Demetrios Katsaduros
    TI__Guru 54715 points
    Hi Demetrios,
    If enabled the thermistor is biased and measured by the part. The MCU must provide temperature conversion, threshold comparison and any response as desired.
    The part has the inputs numbered from bottom up, VC0 being the bottom of the bottom cell, VC1 being the top of the bottom cell, etcetera. Each input has a single ended voltage limit and each pair has a differential voltage limit. When balancing is enabled for a cell, it will attempt to pull the pin pair together if the internal bias voltage is available. When multiple cells are enabled to balance, multiple pins will attempt to pull to the same voltage, this could violate abs max voltages, or could give ineffective balance current due to the bias voltage for the internal FETs being affected by the shifting inputs. One cell voltage condition which may occur is to have 1 cell low. If cell 5 is low and the other 4 cells are to be balanced down the recommendation to balance every other cell would suggest to balance cells 1 & 3 for some time, then switch to cells 2 & 4. Some designers do choose to balance only 1 cell at a time for simplicity and to limit power on the board although this will extend the balancing time.
  • 0 in reply to WM5295
    Prodigy 80 points
    hello and thank you for all of the insight to the chip. i am proceeding forward with this chip for my application. the resistors and capacitors connected to the cells labeled as "Rc" and "Cc", are these the built in resistance/capacitance of the cell? "Rf" is a 10k ohm resistor?
  • 0 in reply to Demetrios Katsaduros
    TI__Guru 54715 points
    Hi Demetrios,
    The Rc and Cc capacitors are filter components between the cell and the IC. See figure 8-1 of the bq76920 data sheet. See the recommended values in section 6.3 and an example in the bq76920EVM. Rf is the power filter resistor, 100 to 1k ohm would be more typical. Rf could be shared with the REGSRC supply, you may want to block discharge under heavy load, see the EVM and www.ti.com/lit/slua749 for discussion.