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BQ25798: Entering Backup Mode

Wolfang Philipps
Prodigy 10 points
Part Number: BQ25798

Tool/software:

Can someone please explain to me the following sentence of the manual at page 37:

“Set EN_OTG = 0, in order to exit OTG mode and enter the forward charging mode without PMID voltage crash. Setting BKUP_ACFET1_ON = 1, also clears BKUP_ACFET1_ON to 0 and sets EN_BACKUP to 1.”

Will setting BKUP_ACFET1_ON = 1 trigger it to go back to 0 again?

  • 0
    TI__Guru**** 166466 points

    Hi Wolfgang,

    Yes.  From the datasheet, that statement should be moved to item 1:

    The following sequence is used to switch from the battery power back to ACIN1 while simultaneously re-arming the backup mode:

    1. Write BKUP_ACFET1_ON (REG0x16[0]) register bit to 1. Setting BKUP_ACFET1 _ON = 1 will cause the device to set DIS_ACDRV = 0 and EN_ACDRV1 = 1. After that, backup mode is disabled, however, the charger remains in the normal OTG mode. The ACFET1-RBFET1 is turned on to connect the adapter to VBUS. The user must ensure the adapter voltage is equal to or higher than the charger VOTG voltage setting, otherwise, the charger OTG output might back drive the adapter connected to VBUS. Setting BKUP_ACFET1_ON = 1, also clears BKUP_ACFET1_ON to 0 and sets EN_BACKUP to 1.

    2. Determine the source at ACIN1 is valid (is not in overvoltage and did not fail poor source detection) by reading back EN_ACDRV1 as 1.

    3. Set EN_OTG = 0, in order to exit OTG mode and enter the forward charging mode without PMID voltage crash. Setting BKUP_ACFET1_ON = 1, also clears BKUP_ACFET1_ON to 0 and sets EN_BACKUP to 1.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hi Jeff,

    Thank you for your quick response.

    Your explanation helped clarify things.

    I however still having problems enabling the backup mode after a reset:

    I’m setting bit 6 (REG_RST) of the REG09_TERMINATION_CONTROL register to reset the BQ25798 registers and timers. After issuing the reset, I wait approximately 100ms. Then I attempt to enable backup mode by setting the EN_BACKUP bit in the REG0F_CHARGER_CONTROL_0 register, while 5V is present at AC1.

    However, when I read back the EN_BACKUP bit, it is not set.

    Could you clarify the correct procedure for enabling backup mode after a reset?

    Best regards,

    Wolfgang

  • 0 in reply to Wolfang Philipps
    TI__Guru**** 166466 points

    HI Wolfgang,

    Backup mode auto enables OTG/reverse mode if the VAC1 voltage drops below the VINDPM threshold x % setting in REG0x10[7:6]. There is a100%*VINDPM option REG0x10 so you only need VINDPM REG0x05 to control the auto turn on.  Backup mode is not intended to be on demand.  On demand OTG/reverse is implemented using EN_OTG bit.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Hi Jeff,

    I'm working with Wolfgang on this project. This issue is not with entering backup mode based on the VINDPM threshold, we realize that Backup mode is only entered when VAC1 drops below the threshold. However, if we're reading the datasheet correctly, the EN_BACKUP bit needs to be set while VBUS is high so that Backup mode is triggered when VAC1 drops below the threshold.

    The issue we are seeing in our testing is that after the REG_RST bit is set (when external VAC1/VBUS is present) and registers are cleared, it will not allow us to set EN_BACKUP to 1 but we expect it to allow us to do so.

    (There are inconsistencies about EN_BACKUP: Figure 9-37 says it is Read-only but Table 9-25 says it is R/W.)

    It is our understanding from the datasheet that we need to set EN_BACKUP to 1 (or have it automatically set to 1 via BKUP_ACFET1_ON = 1) every time VBUS is being powered from VAC1 to enable the automatic engaging of OTG when VAC1 drops below the threshold.

    Are we misunderstanding the operation here?

    Thanks,

    Craig

  • 0 in reply to Craig Morin
    TI__Guru**** 166466 points

    HI Craig,

    I apologize for the datasheet typo.  Backup mode must be re-armed while a valid voltage is at VAC1. I pulled the original spec and see no reason why REG_RST prevents writing to EN_BACKUP bit.  Does REG_REST reset EN_ACDRV1 bit to 0?

    Regards,

    Jeff