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BQ24296M: BQ24296M occasional VSYS drop after VIN disappear

Vasyl Vorobei
Intellectual 460 points
Part Number: BQ24296M

Tool/software:

Hi.

We observe an occasional VSYS drop in our design.

It happens under the VIN switching stress test. In this test, we simulate short bursts of unstable VIN by switching it on and off every 4 seconds for 10 attempts, with a relaxation period of a few minutes to allow battery charge.

The observation is:

1. After switching off the external supply, VIN drops down to the VINDPM limit

2. After 60 ms, there is a short (~500 us) dip in VSYS voltage.

Diagram of this event:

Unfortunately, this diagram doesn't capture the VBAT voltage, but the battery is ok (ESR <150 mOhm).

The reproduction happens at different VBAT voltage levels, slightly correlating with the VSYSMIN limit.

The reproduction correlates with a system load.

PMIC register settings:

REG00 = 0x17
REG01 = 0x1F
REG02 = 0x08
REG03 = 0x11
REG04 = 0xB2
REG05 = 0x9C
REG06 = 0x42
REG07 = 0x4B

Could you suggest possible reasons for such a VSYS drop? The issue is tough to reproduce, and understanding which events may lead to such behaviour may help us localize the issue.

Best regards, Vasyl.

  • 0
    TI__Guru 57321 points

    Hi Vasyl,

    What were the device operating conditions such as VBUS (voltage at VBUS pin), IVBUS, VBAT (voltage at BAT pin), IBAT, VSYS, ISYS right before removing the VBUS power? 

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Intellectual 460 points

    Hi Ning.

    It's difficult for me to tell the IVBUS, ISYS, and IBAT values because doing so requires heavy modification of the DUT, which almost certainly will disrupt the issue reproduction (which is already very challenging).

    Could you suggest a better oscilloscope signal combination for your answer? If the four voltage signals aren't enough, I may try expanding it up to eight with two oscilloscopes with a daisy-chain trigger.

    In case the current waveform is mandatory, I'm limited to the two current probes. But as I said, it will possibly destroy the repro conditions.

  • 0 in reply to Vasyl Vorobei
    TI__Guru 57321 points

    Hi,

    Let us check and get back to you.

    Thanks,

    Ning

  • 0 in reply to Ning T
    TI__Guru 57321 points

    Hi Vasyl,

    Do you have any rough idea what are ISYS and IBAT values when removing the VBUS power? 

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Intellectual 460 points

    Hi Ning.

    Regarding the ISYS, the average value must be between 200 and 400 mA. Peaks caused by LTE transmission may reach 2.0 A. The worst-case scenario for the whole system may reach 3.0 A (assuming the system reached VSYSMIN = 3.7 V).

    Regarding the IBAT, it may have proportionally higher values due to the lower minimum voltage (3.55 V)

  • 0 in reply to Vasyl Vorobei
    TI__Genius 17485 points

    Hi Vasyl,

    If possible, can you share waveforms including VIN, VSYS, VBAT, and IBAT or ISYS?

    This could be caused due to a load pulse applied to VSYS while VIN is VINDPM limited. The current limitation on VIN indicates that a load pulse on SYS would require additional current to be pulled from VBAT. I would confirm that VBAT voltage close to the IC pin remains stable. In some applications we can see that the trace and equivalent resistance of the battery will cause the VBAT pin voltage to drop due to the IR drop when a load pulse is applied. Including the VBAT waveform might help to identify if this is a possible cause.

    Best Regards,

    Juan Opsina

  • 0 in reply to Juan Ospina
    Intellectual 460 points

    Hi Juan, thanks for the suggestion.

    I'll change the setup to monitor the +VBAT, and (possibly) after reproduction, will try to add the current monitor for IBAT. It will take some time to reproduce.

    Regarding the IR drop. While waiting for the reproduction, I can measure the IR drop on a bare PCB. At what level must the VBAT voltage drop for the complete shutdown? VBAT_DPL?

  • 0 in reply to Vasyl Vorobei
    TI__Genius 17485 points

    Hi Vasyl,

    Yes the VBAT_DPL is a threshold to keep an eye out for. In this input current limited state your VSYS will closely track your VBAT voltage since there isn't enough current to sustain VSYSMIN regulation. In this case it is very possible that VBAT voltage is dropping without crossing the VBAT_DPL threshold and SYS is simply tracking that voltage down. In either case a capture of VBAT voltage close to the BQ's pin should provide useful context.

    Best Regards,

    Juan Ospina