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BQ25188: BQ25188 after the first switch-on

bob shnaider
Prodigy 10 points
Part Number: BQ25188
Other Parts Discussed in Thread: BQ25186

Tool/software:

Hello, I would like to use BQ25188 together with 1-cell LiFePo4 battery. But I do not understand the algorithm of the initial power-on.
From the datasheet - after reset (switching off, replacing the battery) the registers are reset to default values ​​-

VBAT_CTRL Register -- VBATREG -- 4.2 V

ICHG_CTRL Register -- CHG_DIS -- 1b0 = Battery Charging Enabled

In other words, after switching on, 4.2 V will be supplied to the battery (instead of the permissible 3.65) even before the MCU HOST has time to make the required changes to the registers ?? Wouldn't it be more logical to set the default "Battery Charging Disabled", make all the settings in the registers and only then allow battery charging?
Please help me understand the correct startup algorithm, how to prevent 4.2 V from being supplied to the battery after the first switch-on.

  • 0
    TI__Expert 7880 points

    Hello Bob,

    You're correct that the registers are reset to their default values during a power-on reset (POR). Therefore:

    • I recommend changing the VBATREG setting on startup so the charger doesn't charge above 3.65V before an input adapter is connected.

    In the startup case you described where the LiFEPO4 is at 3.65V and the VBATREG setting is the default 4.2V, the charger will output the default 10mA fast charge current as long as VBAT is below VBATREG. As the battery voltage approaches the VBATREG threshold, the constant voltage (CV) loop becomes more dominant and the charge current tapers off. To clarify, the charger will not immediately output 4.2V to the battery.

    The primary reason charging is enabled by default is so that the device can have the ability to charge a battery without a host controller. If you absolutely need to prevent default charging, there are a few options:

    • Implement external logic on the TS/MR pin to prevent charging until the host enables it.
    • Consider using the BQ25186, which has a dedicated charge enable pin.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 10 points

    Thank you very much for your answer, but the solutions you proposed require additional components. I think you should remove the mention of LiFePo4 from the datasheet.. it is not only misleading, but can also be dangerous when used.
    A very simple solution to this problem would be to make VBATREG=3.5 V by default, 3.5V is absolutely safe for Li-Ion, Li-Pol and LiFePo4.

    BR

    Bob

  • +1 in reply to bob shnaider
    TI__Expert 7880 points

    Hello Bob,

    I appreciate your feedback and concerns regarding safety. This is definitely something we are aware of and actively considering.

    Please refer to this thread where I discuss the concern of overcharging a LiFePO4 cell when the registers are set to their default settings:

    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1419307/bq25180-otp-programming/5449010?tisearch=e2e-sitesearch&keymatch=BQ25180%2520POR#5449010

    The situation where a LiFePO4 cell at 3.65V is connected to the BQ25188 with the default register settings is problematic if:

    1. An input adapter is always connected.
    2. The host controller/firmware cannot communicate with the device under any circumstances.
    3. A battery protector IC is not being used.

    Otherwise, the device registers, including the VBATREG setting, can be configured on startup in battery-only mode.

    Implementing external logic on the TS/MR pin does not necessarily require additional components if a FET is already being used in place of a push button for control. For instance, the host can pull TS/MR low until the registers are configured, as charging is disabled while the TS/MR pin is low.

    Best regards,

    Alec