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BQ78350-R1: Disable both FETs on BMS

MaxIT
Prodigy 30 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350

Hi,

I have a question about BQ78350-R1 with FW 2.02 installed.

I want to disable both CHG and DSG FETs usign an external logic circuit. I can use both SMBUS or discrete signal to do it and I want to do it when BMS is in sealed mode.

I see that PRES pin can disable both FETs. Can I use this pin to disable or enable FETs even if the battery pack remains connected to BMS? Can I power-on BMS from shutdown or sleep with the PRES pin high (FET disabled)? Can I shutdown BMS with PRES pin high (FET disabled)?

Thanks in advance

  • 0
    TI__Guru* 81034 points
    Hi Max,

    The PRES pin can disable both FETs and could be used for this purpose, so you could have control of the disable. Keep in mind that PRES is polled every 250ms and a high value needs to be detected for 4 consecutive samples to initiate the turn off.

    However, in Shutdown the BQ78350-R1 is powered off (VCC is 0V coming from the BQ769x0 REGOUT pin), so a high logic level on this pin would violate the Abs Max rating for this pin. If PRES is detected low, the FETs will be controlled based on the BQ78350 algorithms. Also, the BQ78350-R1 has a body diode protection feature (see section 3.1 of the TRM) that will enable a disabled FET as needed to prevent FET damage. This is all to say, you will not have complete control over the FETs since the BQ78350-R1 will make decisions when to enable the FETs based on multiple conditions and features.

    Instead of using the PRES pin, I recommend an independent external circuit to drive the FETs that also monitors the outputs of the CHG and DSG pins of the BQ769x0.

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Prodigy 30 points
    Hi Matt,

    thanks for your answer!

    I know that for the power on of the FETs the BQ takes decisions based on various parameters, and this is ok, but for the project it is important that the shutdown can be controlled from the outside circuit. There are some decision on shutdown maked by BQ78350?

    When the BQ78350 is powered off, also the FET logic is powered off. So I think that I cannot violate the Abs Max for this pin.

    I noticed that if I keep the PRES pin high and give the shutdown command the BQ769x0 does not turn off. There is some reason?

    Thanks
    Best regards
    Max.
  • 0 in reply to MaxIT
    TI__Guru* 81034 points
    Hi Max,

    The BQ78350-R1 does have two alternative methods for Shutdown (voltage-based or time-based) in addition to the Shutdown command. These methods can both be disabled in the data flash so that only the command can issue the Shutdown.

    When the PRES pin is high, the Shutdown command should still work. I just tried this on my EVM and it works. Are you doing this on an EVM?

    Regards,
    Matt