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BQ76952: Simple isolated 12S balancer

Teemu Suikki
Prodigy 10 points
Part Number: BQ76952
Other Parts Discussed in Thread: ISO1641, ISO1640

Tool/software:

I'm designing a 12S balancing circuit. I have no need for most of the protection features. Circuit will only have access to the balancing wires, the "big power" wires don't enter the PCB.

I assume BQ76952 will work fine if I simply disable most of the protection features in the config?

The balancing circuit needs to be isolated from the main MCU as the potential is not the same. I'm thinking i2c through ISO1641, and then a few optocouplers for wakeup/shutdown signals. ISO1641 can be powered from REG1 or REG2, if the circuit can be waken up by opto signal.

Is it safe to connect TS2 to RST_SHUT and pull them down with a single optocoupler, and maybe a weak pullup to PACK+ ?

Opto not active -> RST_SHUT high and device enters shutdown. Opto active -> TS2 low and device wakes up.

Or maybe it's safer to use two optos, one for each? In that case, can I use LD instead of TS2 for wake up?

Any other pitfalls I might encounter? :) Most of the example circuits have the protection features enabled and MCU connected without isolation.

  • 0
    TI__Genius 17935 points

    Hello Teemu,

    Table 16-3 Terminating Unused Pins in the BQ76952 Datasheet recommends what you should do if you plan on not using certain pins. Based on Table 16-2 Startup Sequence and Timing in the datasheet, you can pull TS2 pin low to wake up the device, however, it is not a good idea to do that while the device is active when SHUTDOWN mode is available.

    Some additional notes:

    1. Some users end up getting stuck in the device’s Soft SHUTDOWN mode. Please refer to BQ769x2 Soft SHUTDOWN FAQ if that occurs.
    2. If you are planning to use ISO1641, there might be issues with it communicating to the MCU if our device decides to do clock stretching, as the ISO1641 only has a unidirectional clock. 

    Best Regards,

    Alexis

  • 0 in reply to Alexis_H
    Prodigy 10 points

    Thanks! I did not know BQ76952 uses clock stretching. I will use ISO1640 instead.

    I understand about the soft shutdown. But I'm still not sure what is the simplest way to make sure BQ76952 is shut down when MCU side loses power, and then reliably wake it up when MCU wakes? Also if ISO1640 is powered by REG1 there is no comms until regulators are working.

    When operating, MCU would hold GPIO1 up so RST_SHUT is low. When MCU loses power, pullup R212 pulls RST_SHUT high and after 1sec it will enter shutdown. Is it guaranteed that REG1 stays enabled long enough for device to reliably enter shutdown?

    When MCU wakes up, it uses GPIO2 to pull TS2 which wakes the device. Correct?

    I was initially thinking about using only one opto, like this:

    Same signal connected to both RST_SHUT and TS2. Diode D32 added so weak pullup of TS2 would not leak into REG1 when the regulator is powered off. I think this SHOULD work as well but leaves little room for manouvering if there are issues?

  • +1 in reply to Teemu Suikki
    TI__Genius 17935 points

    Hello Teemu,

    REG1 doesn’t turn off until the SHUTDOWN process is initialized so it will stay enabled long enough for the device to reliably enter shutdown.

    Either the TS2 pin is pulled low or the LD pin is pulled up to exit SHUTDOWN mode. More specifics on the SHUTDOWN mode can also be found in this BQ76952 SHUTDOWN mode FAQ.

    Best Regards,
    Alexis