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BQ77915: Operation of external MOSFETs

Radel Design Team
Intellectual 460 points
Part Number: BQ77915

Hi,

We are intending to use external MOSFETs as we require higher balancing current than 50mA.

When will the external MOSFET turn on ?

Is there any specific part number for the external MOSFETs ?

How will the external MOSFET turn on?

Thank You,

With reagrds

Raahul Bharadwaj

  • 0
    TI__Guru 54715 points
    Hi Raahul,
    External balancing can be used with the BQ77915 as shown in figure 8 of the data sheet. The pins pull together and can be used to control either N or P channel FETs, N channel are recommended here. While it is not written for the BQ77915, www.ti.com/.../slua749a.pdf section 4 shows circuits for both FET types and current flow for turn on of the FETs. Gate protection may be needed depending on the Vgs limits of your FETs and transients in your system.
    The FETs should be selected to turn on with the voltage you will be balancing. If you are balancing at 3.9V you will likely want a FET with an identified RDSON at 1.9V. Of course be sure the Vds can take the cell transient voltage.
  • 0 in reply to WM5295
    Intellectual 460 points

    Hi,

    Thank you very much for the response.

    The other query is regarding orderable information.

    Table 1 in the datasheet of BQ77915 lists the different selectable Over Voltage(OV), Under Voltage(UV) and other parameters. But the steps to generate a unique part number with required settings is not mentioned.

    Also the website lists only 1 type(P/N: BQ7791500PWR).

    My preferred settings is as below:

    Over Voltage: 3.6V with 1second delay

    Under Voltage: 2.7V with 1 second delay

    Start Threshold for Cell Balancing: 3.5V

    Vhys t(Hysteresis b/w over voltage and full charge voltage range): 50mV

    Vstep(difference b/w cell balancing threshold voltages): 50mV

    Thanks in advance.

    Regards,

    Raahul

  • 0 in reply to Radel Design Team
    TI__Guru 54715 points
    Hi Raahul,
    The bq77915 configurations defined are shown in the data sheet device comparison table section 6. Orderable configurations are shown in the website.
    The bq77915 is capable of thresholds for LiFePO4 cells, but the 50 mV OV hysteresis is not available, see table 1 as a summary of capability.
    To define a new device configuration, please contact your local TI representative to discuss the business opportunity.
  • 0 in reply to WM5295
    Intellectual 460 points
    Thank you for your reply

    Regards,
    Raahul