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BQ25798: Re-enabling backup / OTG mode under battery power only.

Chris Damper
Prodigy 10 points
Part Number: BQ25798

Tool/software:

I have previously used the BQ25798 in a float standby application with a LiFePo cell using backup mode. This is working fine as backup mode is enabled while under external power.

I now have an application for portable battery powered equipment with USB charging using the same battery and charger.

In both cases, we are using the Figure 10-2.configuration with the load on PMID. The second one does not have the ship FET fitted. It also has a diode from the battery itself to the PMID to allow the processor to remain powered - this may be the issue although it is putting less than 3V

As before, it works fine when powered externally and switches automatically to battery power when the USB is removed. However, if the chip is switched off via I2C as follws:

WriteRegister(0x0F, 0xAE); // clear backup mode

WriteRegister(0x12, 0x00); // and OTG mode

WriteRegister (0x11, 5); // ship mode

it powers down and draws very low current.

If I try to re-enable with:

WriteRegister (0x11, 0); // ship mode off.

WriteRegister (0x10, 0xF0); // set Vindpm as threshold to start backup

WriteRegister(0x0F, 0x83); // enable backup mode

WriteRegister(0x12, 0x40); // and OTG mode

It does not start boosting the PMID output.

From reading elsewhere it would appear that backup mode cannot be set under battery power, only with external power available.

Reading the registers afterwards confirms that REG0F EN_BACKUP is still zero however REG12 EN_OTG is set and REG1C VBUS_STAT is 7 i.e. OTG mode.

The Vsys voltage is below 3V, well below the enable OTG threshold set by REG10 and default Vindpm.

Is this chip designed to be able to go from a low power ship mode to providing a boosted PMID output under battery power alone ?

Are there any examples of this I can use as a base for settings ?

  • 0
    TI__Guru**** 166576 points

    Hi Chris,

    The EN_BACKUP_MODE bit can only be set to 1 if there is a voltage > UVLO at VBUS. When VBUS< UVLO, the host can write EN_OTG = 1 to enable reverse/OTG mode.

    Regards,

    Jeff

  • 0
    Prodigy 10 points

    I had confirmed that the EN_BACKUP bit would not set so this was expected.

    I was setting the EN_OTG bit, and then checking every few seconds. The EN_OTG was remaining set and the status indicated OTG_MODE. I was also checking the OTG voltage was still set correctly, however no switching waveforms on the inductor and no boosted voltage out.

    I've sidestepped the issue for the second iteration prototype by using a buck/noost regulator from the largest of battery or PMID and then using the BQ chip as just a charger.

    It would be useful to know for future use if this chip is actually capable of use in a portable rechargable battery application or whether it's only suitable as charger only or backup float standby. If so, examples so I can see what I'm doing wrong.

  • 0 in reply to Chris Damper
    TI__Guru**** 166576 points

    Hi Chris,

    Many other customers use OTG mode to provide a voltage back to VBUS. To enable the OTG operation, the following conditions have to be valid:

    • The battery voltage is higher than VBAT_OTG rising threshold, and not trigger the VBAT_OVP protection.

    • The VBUS is below VVBUS_UVLO.

    • The voltage at TS pin is within the range configured by BHOT and BCOLD register bits

    If the input MUX FETS are used, the host must also write either EN_ACDRV1 or EN_ACDRV2 for the OTG output to appear at VAC1 or VAC2.

    Regards,

    Jeff