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BQ25798: Battery Only Mode configuration for VSYS

Richard Earl
Prodigy 20 points
Part Number: BQ25798
Other Parts Discussed in Thread: USB2ANY

Tool/software:

Hi


I'm sorry but I'm being stupid. I have the following system and I need some help to set up the Battery Only Mode for the BQ25798.

* Single Cell battery - 3.8V max

* Desired VSYS - 5V 

* I am using the BW25798 Evaluation Module and so I can control it through the TI Charger GUI and my USB2ANY device.

The intention is to run external circuitry from the VSYS of the BQ25978 whilst we are in battery only mode. Therefore the voltage from the battery needs to be stepped up to the desired VSYS voltage but I can't work out how to do this.

Can you please advise on what configuration / register settings would be required to achieve this. 

Many thanks in advance for your help!

Regards
Richard

  • 0
    Prodigy 20 points

    I forgot to mention that currently I can only get VSYS to match the battery voltage (VBAT). I can't seem to get the BATFET to be enabled to boost the VBAT to the desired VSYS.

  • 0 in reply to Richard Earl
    TI__Guru**** 166626 points

    Hi Richard,

    The BATFET between BAT and SYS is only a switch.  In battery only mode, the BATFET turns on to connect BAT to SYS.  In charge mode with a voltage at VBUS, the converter can regulate SYS to a minimum system voltage (less than battery regulation voltage) but then SYS follows the battery voltage up to the battery regulation voltage.

    In reverse/OTG mode, the BATFET turns on, connecting BAT to SYS, and the converter runs in reverse, compared to charge mode, to provide a regulation voltage at VBUS.  If there is a load at VBUS, the converter can power it in reverse/OTG mode.

    Alternatively, if your load is connected at PMID not SYS, then VBUS, which is connected to PMID through another switch, powers the load.  When VBUS goes away, the charger can be configured for backup mode.  Backup mode is essentially fast turn on reverse/OTG to maintain a voltage PMID instead of VBUS.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points

    Hi

    Thank you so much for the detailed response. So it sounds like that without an external power supply (on vbus) it isn't possible to generate a VSYS that is greater than VBat.

    So there are two scenarios for me then I think

    1) connect my load to VBUS and engage OTG mode. I have managed to make this work.

    2) Backup mode where I need to connect my load to PMID. 

    i) I configured OTG mode and saw a 5V Voltage on PMID but when I connected the load, the voltage collapsed. Do you know why?

    ii) what are the appropriate registers to set in order to enable back up mode

    Your help is really appreciated!


    Regards


    Richard

  • 0 in reply to Richard Earl
    TI__Guru**** 166626 points

    HI Richard,

    OTG mode from VBUS has a current limit that is set in REG 0.0D[6:0].  If the applied load is higher than this limit, the converter changes from regulating voltage to regulating current at that current level which causes the output voltage to drop.  

    In backup mode or OTG mode, with load at PMID, there is no output current limit.  If the load collapsed, then it pulled too much current from the converter causing the converter to enter cycle by cycle current limit..  What was the applied load?

    For back up mode:

     When VBUS is high and supplying the PMID load, set EN_BACKUP = 1.  When VBUS drops lower than the threshold, the charger itself forces EN_OTG = 1 to enter the backup mode after set DIS_ACDRV=0 to disable both ACDRVx pins.  This isolates external adapter port (presumably without valid voltage) from VBUS.  

    If there is an adapter reconnected while the charger is in backup mode, the user may transition the source which powers the PMID load from the battery back to the adapter. 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.

    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 20 points

    Thank you so much. I will give this a go.
    FYI - the applied load was just a simple LED+resistor.