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BQ78350-R1: Return path of the SMBUS/ SYS PRES and potential issues

JPAlleyne
Expert 1565 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350, BQ76200, BQ76930

Recently, I did a PCB design consisting of the bq78350, bq76930, and the bq76200. The header that is in the battery is shown in Figure 1 and the header it connects to in the system is shown in Figure 2.

Pin Explanations:

Pin 8 and 7: The SMBUS (SMBD and SMBC): These pins go from the bq78350 to a microprocessor in the system.
Pin 6: BATTV: It is a current limited pin on the Battery side for us to check the direct battery voltage. It is unconnected on the system side.
Pin 5: The System present pin which is connected to the bq78350 and is used to turn on the battery only when the battery is in the system by pulling to low. It is pulled to Pack- on the system side.
Pin 4 and 3: PACK - : This is the high current return path that, on the battery side, is connected to BATT - through the current sense resistors and is the High Power Return path for the system.
Pin 2 and 1: PACK + : This is the high current  supply that, on the battery side, is connected to BATT + through the high-side FETs and it supplies power to the system.

The Questions: 

1) Once mated, the only return path for the SMBUS is PACK - . Are there any issues that can come up by doing this? Is there anyway that noise on this line can cause system failure?
2) Once mated, the System Present pin is connected to PACK - . Are there any issues that can arise from this? Can this cause system failure?

 

I am asking this question because we have had a couple system failures where it seems as if the system just stopped getting power. We want to investigate the battery system as a potential issue.

I have recently changed the pin out of the connectors where pin 6, instead of being the battery test voltage is now a digital ground return that connects to ground to which the bq78350 is connected and the ground  to which the microprocessor is connected on the system side. See the last picture: figure 3

Thanks,

JP

Figure 1: Battery Side Connector

Figure 2: System Side connector

Figure 3: New Battery Side Connector

  • 0
    TI__Guru* 81034 points
    Hi JP,

    I do not see any issues with this configuration. You mentioned that you made a change recently to connect the GND reference as well? Did the issues observed occur before and after this change? I suppose it is possible for noise on PACK- to cause the issues you have observed, but I have not seen any similar instances.

    I will also review your schematic and connections with my colleague in case he has observed something like this in his years supporting this product. However, he will be out of office until next Thursday so it will be a few days before I can provide his feedback.

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Expert 1565 points

    Hi Matt, 

    When you say that you don't see any issue with the configuration are you referring to the first two pictures? The case when Pack - is being used as the return path for the SMBus and SYS PRES? The change I did, which is the 3rd picture where I added a GND reference, has not been tested yet. I'm looking forward to hearing from you and your colleague.

    Thank you,

    JP

  • 0 in reply to JPAlleyne
    TI__Guru* 81034 points
    Hi JP,

    Here is the feedback from my colleague:

    Current in PACK- will modulate the apparent voltage on PRES, SMBC and SMBD. Typically this is not expected to be large (100A x 1 mOhm = .1V) although designs vary and transients can occur. SMBus has PEC which could/should help, I don’t know how to quantify that.
    A ground reference would be quieter, but raises the risk of a short circuit from PACK+ to GND. If PRES opens the FETs should eventually open since the gauge has some filtering. The placement does not look at risk of this casually, and as a “header” it may not be user accessible.

    Loss of battery power sounds like some protection event, I presume it was unexpected. Would lifetime or blackbox have any data to help?

    Regards,
    Matt
  • 0 in reply to Matt Sunna
    Expert 1565 points
    During this suspected shutdown all protections were off, but we did see a lot of inconsistent data from the battery that happened before the suspected shutdown. This is why I asked if any issues on the SMBUS to cause a shutdown.