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BQ78350-R1: BQ78350-R1 with 3.3 V?

FelixG
Prodigy 50 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ76920EVM, , BQ78350, BQ76920

Hi

We designed a BMS similar to BQ76920EVM and have no REGOUT after booting the chip. I just noticed, that we used a 3.3 V version (BQ7692006) to power the gauge IC (BQ78350-R1) which tolerates max. 2.5V. Since I can't measure a shortcut when powered off, there must be another issue within the REGOUT path.

How does the gauge controller behave on overvoltage? Could this be a reason for short-cutting REGOUT after booting the gauge?

Thank you for you help and regards

Felix

  • 0
    TI__Guru* 76230 points
    hi Felix,
    The bq78350 would be damaged if you use it with a bq769xx device that is 3.3V

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 50 points

    Hi Onyx

    thank you for your fast response. Without BQ78350 I still get 0 V on REGOUT after booting. Could a broken BQ78350 be the reason for destroying the AFE (bq76920)? Changing both ICs got the same result.

    Thanks Felix

  • 0 in reply to FelixG
    TI__Guru 54715 points
    Hi Felix,
    It has been a couple weeks since your post, hope you have resolved your issue.
    Generally I would not expect a short circuit to damage the REGOUT pin, but ICs can fail in unpredictable ways when overstressed. If REGOUT is missing on the bq76920 without the gauge device, be sure REGSRC is connected. Also check for unexpected shorts on the board or across filter capacitors. If everything looks correct, be sure to check that the boot signal is effective, TS1 should transition from some low voltage to > 1V for boot. At the boot time CAP1 should rise from 0-V to 3.3V as REGOUT rises from 0-V to its regulated value, checking CAP1 may be an indication that the boot signal was received.
  • 0 in reply to WM5295
    Prodigy 50 points

    Hi

    thank you for your help. I checked the signals you mentioned. With an oscilloscope I can see 2.5 V on both pins (REGOUT and CAP1) for some miliseconds and then breaking down with an capacitive discharge curve. My schematics are similar to the evaluation module (BQ76920EVM) and I can´t find a difference resulting in capacitive supply break down. Maybe this is a known behavior to you?!

    Thanks and regards

    Felix

  • 0 in reply to FelixG
    TI__Guru 54715 points
    Hi Felix,
    In a system with the bq78350 if REGOUT & CAP1 go low in about 12s I would expect the gauge runs, finds a problem and sends a shutdown command. The voltages will drop off exponentially as the capacitors discharge.
    Since yours is shutting down in some ms, I would expect the part is getting a low voltage shutdown. Check the BAT pin voltage during this cycle to see if it is falling into VSHUT (see the data sheet). Also if both REGOUT and CAP1 are only rising to 2.5V this indicates some problem, CAP1 is always 3.3V and if you still have the bq7692006 REGOUT is also 3.3V. If one of these outputs is overloaded perhaps that is pulling BAT down by the drop on the power filter resistor. CAP1's current comes from BAT pin input, REGOUT's current comes from the REGSRC pin input. CAP1 should normally have no current external to the part.
    As in the earlier discussion, if the bq78350 received 3.3V it could be damaged and may then draw excessive current. Damage can be unpredictable, some may fail short, some may fail open.
  • 0 in reply to WM5295
    Prodigy 50 points
    Thank you a lot. This solved my issue. In my design, the supply voltage is tapped after the current limit resistor on VC5. This one has a value of 1k. With 10 V battery voltage, the limit is at 10 mA. Seemingly the booting process needs a bit more current. After fixing REGOUT and CAP1 are at the right level.