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BQ25620: BQ25620/BQ25622: Low battery cut-off

Nicholas Zambetti
Prodigy 230 points
Part Number: BQ25620

I'm performing some low/dead battery tests with the BQ25620, and I've found that it's possible to drain a battery completely.

My questions are:

1) Can the BQ25620 somehow disconnect the battery to prevent over-discharge? I don't see any mention of this in the datasheet.

2) One idea that comes to mind, is to use the BQ25620 watchdog timer (with INT output), to wake the MCU every 40/80/160sec. The MCU would wake and check VBAT using the ADC of the BQ25620: if VBAT is acceptable, the MCU would return to sleep, otherwise the MCU would instruct the BQ25620 to activate shutdown. Does this sound like a reasonable strategy? Is this one of the intentions of the periodic watchdog timer, or is it a hack?

3) It might be a nice feature if, when not powered by VBUS, VSYSMIN could act as a threshold to activate shutdown mode. Reading the datasheet, it seems VSYSMIN is only relevant when powered by VBUS, is this correct?

  • +1
    TI__Mastermind 31140 points

    Hi Nicholas, 

    Please see my comments regarding your questions below. 

    1)Yes the device has a battery undervoltage protection. The BQ25620 will turn off the BATFET to disconnect BAT pin from system when VBAT falls below the Vbat_uvlo threshold, which is defined on page 9 of the datasheet. Via a register setting you can set the over discharge threshold to either typ 2.2V or 1.8V. For some batteries this threshold is too low and in that case you will need to use some form of external protection circuitry. 

    2) I would not call your idea the intention of the watchdog timer, but I believe it is a reasonable way to signal to the MCU to check VBAT ADC value. The watchdog timer's main purpose is to ensure the IC maintains communication with the host MCU. One key point is the watchdog timer expiring will reset the majority of registers back to their default value. If this is not a concern for you then I see no issues with your proposed strategy. 

    3) You are correct that VSYSMIN threshold is only relevant when powered by VBUS. Thank you for the feedback regarding the potential feature to include in future charger ICs. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 230 points

    Hello Garrett, your reply is very helpful.

    Thank you for informing me about Vbat_uvlo, I did not take notice of that field when reading the datasheet. For the low/dead battery tests I've performed so far, I'm using a 3.7V Li-Ion battery with over-discharge-detect at 2.4+/-0.05V. Since Vbat_uvlo was at the default the default 2.2V (2.1-2.3V) during this test, it makes sense that the battery's protection triggered before the BQ25620. I verified this (after system power was lost) by disconnecting the battery and measuring 0V on the connector terminals.

    However, going into production, we will ship the product with a protected LFP battery pack. The manufacturer specifies that the protection of the LFP battery pack has over-discharge-detect at 2.1+/-0.05V, and over-discharge-release at 2.3+/-0.1V. So it seems the BQ25620 should work perfectly with Vbat_uvlo set to the default 2.2V (2.1-2.3V). We are awaiting samples, I can verify this after receiving them.

    As an aside, I made a quick proof-of-concept to test the watchdog periodic-wake idea:, and it does wake the MCU according to the watchdog period. After wake, the MCU is able to request and receive ADC readings from the BQ25620, allowing some milliseconds of delay between enabling the ADC and reading the registers. I will consider this a fall-back solution, but it seems useful for other low-power applications in the future as well.

    Thanks again,

    Nicholas