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BQ78350-R1: BMS-chip turns off discharge (and charge) mosfet when mcu resets

jan stappers
Prodigy 160 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ76940, , BQ78350, BQ76200

Tool/software:

Hello,

Our BMS chip (BQ78350-R1 and BQ76940) turn their discharge and charge mosfets off when a reset in the MCU occurs. However if we set FET_EN = 0 and manually control the fets, then a reset on the mcu does not affect the state of the mosfets.

Our first suspect was that the floating pins of the mcu during a reset cause the pres-pin to toggle, but we have a pull down on both keyin (disabled) and pres-pin. the MRST from the BQ78350-R1 has also a pull up (and pull down on the gate). So They seem to be safe for the events happening on the MCU. The boot-pin on the BQ76940 should also be stable i think, the mcu might give a brief high level, but I don't think this might cause the mosfets to toggle. So we are not sure what is causing this to happen and what difference it makes when we set FET_EN = 0 and control the fets ourselfs. 

The 2.5V power rail seems to be stable so it looks like we can rule that out.

The reset happening on our mcu is a soft-reset due to a crash (that we are yet figuring out) which sometimes happens. But it should not affect the working of the BQ78350/BQ76940. We would prefer to keep FET_EN = 1 so that the BQ78350 can handle the operations of the BMS.

Side note: we also use the high side driver (BQ76200), but from my understanding the charge pump is always enabled if CE = 1 or one of the inputs = 1. So The mcu shouldn't have any effect on this and FET_EN doesn't affect the working of the high-side driver.

Can someone point out what we might be doing wrong or are we missing something?

What's the difference when FET_EN = 0 or 1, which parameters might cause the reset/state change to happen when FET_EN = 1 and do not happen when FET_EN = 0?

Here is the schematic of our bms part:

Best regards,

Jan

  • 0
    TI__Guru 61536 points

    FET_EN = 1 means that the protection algorithm inside the gauge controls the FETs. So if they turn off, then one of the protection conditions was met. Please verify your system against all protection events.

    If the FETs would turn off regardless of FET_EN state then I would check if your MCU reset somehow causes a gauge reset (which also will at least temporarily turn off the FETs).

  • 0 in reply to Dominik Hartl11
    Prodigy 160 points

    No, there are no "real" protection-events happening. We have figured out that a pull down of 100k is not enough on the PRES-pin. When our mcu resets it still rises a bit and causes the BQ to turn off as well. A stronger 1k pull down seems to resolve this. It was however not very clear for us that the PRES-pin would not have any impact when FET_EN = 0. It only seems to have an impact when FET_EN = 1.