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BQ25708: Charging in 4s

Andre Cilia
Prodigy 205 points
Part Number: BQ25708

Hi,

I have a design based on the BQ25708 chip which is meant to be able to charge 2s and 4s batteries. Additional circuitry has been implemented to determine whether 2s or 4s charging is required, and a microcontroller applies the settings through SMBus accordingly. The charger will be working from an input supply ranging from 9V to 20V.

The MCU also configures the CELL_BATPRESZ voltage by shorting out a resistor in a resistive network such that the CELL_BATPRESZ voltage is as per specifications for 2s and 4s charging.

When the MCU determines that 4s charging is required, the settings in the below image (4s charging.png) are applied to the BQ25708 charger:

However the charger immediately raises the SYSOVP flag - cutting off its output. Clearing the SYSOVP flag by the MCU causes the output voltage to momentarily rise to the desired voltage - but SYSOVP is set again - causing the output voltage to fall again.

Charging works perfectly however for 2s charging.

Am I missing some configuration change that is causing this behaviour?

Thanks and regards,

Andre Cilia

  • 0
    Prodigy 205 points

    An additional note with regards to the configurations: the charge current register (0x14) is actually set to 0x0640, not 0x000

  • 0 in reply to Andre Cilia
    TI__Genius 11385 points

    Hi Andre,

    My guess is that your configuration timing is a bit off. 

    BQ25708 reads the CELL_BATPRESZ to set SYSOVP at every converter startup (converter plug in). If you startup as 2S, then try to operate the charger as 4S (higher voltage) later, the settings may trigger SYSOVP.

    The better way is to change the CELL_BATPESZ configuration after the input power supply is unplugged and before re-plug (no VBUS should be available when you change CELL_BATPRESZ configuration). By doing so, the charger starts up at the right SYSOVP configuration.

    Thanks and I hope this helps,

    Peng

    *Please press "This Resolved My Issue" on the thread if my answer is satisfactory

  • 0 in reply to Pengzhao Zhu
    Prodigy 205 points

    Hi,

    Thanks for your help. Forcing the voltage divider which sets CELL_BATPRESZ to always select the 4s voltage level seems to work.

    However would this be advisable for charging 2ss batteries?

    Note that this is for a user configurable charger whereby a mechanical plate determines which type of battery is to be charged. 4s batteries cannot be fitted into a 2s plate and vice versa. 

    Thanks and regards,

    Andre

  • 0 in reply to Andre Cilia
    TI__Genius 11385 points

    Hi Andre,

    I would recommend starting up at the proper cell setting to reduce potential production issues. Typically, most applications use either a single type of battery or would not want to replace a battery when VBUS is present. 

    Ultimately this is the decision of the end equipment designer, but I have a few suggestions that you can look at:

    1. Have a VBUS switch. When you want to change the battery -> turn off the VBUS switch -> change CELL_BATPRESZ configuration -> turn the VBUS switch back on.

    2. Power down VBUS before changing battery. During this interval, the system can change CELL_BATPRESZ configuration.

    Thanks and I hope this helps,

    Peng

    *Please press "This Resolved My Issue" on the thread if my answer is satisfactory