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BQ78350-R2-DEVICE-FW: PreDischarge and shutdown.

Devis Fracassi
Prodigy 185 points

Part Number: BQ78350-R2-DEVICE-FW

Hi,

we designed a 36V BMS with N-mosfet on the positive line, using the BQ78350-r2, BQ76930 and BQ76200.

We also added a pre-discharge circuit, as the one of the BQ78350-r2 Addendum (SLUUBM6).

The power supply of the BQ78350 (VCC) is the RegOut voltage, from the BQ76930.

We are facing the following issue: when the BQ78350 goes to shutdown, the pre-discharge mosfet is activated for 10-20 ms.

At shutdown, the RegOut switches off and, on the falling edge, the BQ78350 resets and leave free the GPIO_B output. On the GPIO_B there is Rpullup1 resistor (470K) that activate the mosfet.

Probe 1: GPIO_B;      Probe 2: RegOut

Due to power on the Pack+ for 10-20 ms, a pulse on the TS1 boots on the BQ76930 again. So, the battery never shutdown.

What is the simpler way to avoid this pre-discharge activation during the shutdown?

Can we add a 1uF capacitor between the GPIO_B and ground?

Is there any BQ78350 parameters setting to avoid this issue?

Many thanks for any your help. Regards.

Devis

  • 0
    TI__Guru 54715 points

    Hi Devis,

    A supervisory function would avoid the issue you describe.  That may be a lot of circuitry.

    The BQ78350-R1 data sheet does not caution against a capacitor directly on the GPIO pin, it should work.  You may want some resistance between the pin and capacitor.  

    The only BQ78350-R2 adjustment which seems related would be the polarity, selecting active low when PDSG is on.  The switch circuit would then need to detect a difference between VCC and the GPIO rather than a simple voltage on the pin.

  • 0 in reply to WM5295
    Prodigy 185 points

    Thanks a lot for the reply.

    Instead to use a capacitor, probably it is better set GPIO_B active low and add a transistor as on the following image.

    What do you think?

    Thanks a lot for any your advice.

    Regards.


    Devis

  • 0 in reply to Devis Fracassi
    TI__Guru 54715 points

    Hi Devis,

    Yes, I think the proposal would solve the issue.  Things to check would be the current required through RPULLUP2 and transient coupling through the transistor to the GPIO.