bq76940: Low-side x High-side switching - Ground/BATT-/PACK-

Hi Mauro,

Let me clarify. In the first paragraph you cite above what is being discussed with communications lost is talking to strictly the outside world not the gauge inside the battery pack. That paragraph is explaining that if you want your BMS system to be able to talk to the outside world (your actual system) then you would want to have some kind of high side protection as the ground plane inside your BMS system will no longer be the same potential as your actual systems's ground.

Your second and third cites on the 10S TIDA and EVM, are correct inside the BMS system you will always reference to BAT- as you don't want to lose you ground IF you have low side FETs and the FETs are open.

Hope this clears up why high side is importance. To sum up, high side helps your BMS system have the same ground level as your overall system (i.e. a drone for instance) to keep communications alive and have your MCU (in your overall system) still be able to talk to the BMS MCU or BMS Gauge to recover after a fault.

Thanks

Miguel

Hi Mauro,
For low cost batteries users usually like the simplest interface possible such as SMBus. SMBus referenced to the BATT- implies that BATT- comes out of the pack unprotected which may be unsafe in some situations. The EVM does this but it is a subsystem to demonstrate operation, not a finished product. The next common approach is to reference SMBus to the PACK- which works when the battery is normal but can stress the interface signal lines or attached components and lead to leakages during protection. High side protection switching is a common method to avoid switching the ground and to maintain a communication path during protection. The signals are still referenced to PACK- so that any current in the battery can be measured by the protector/gauge even though current does modulate the reference voltage some.
Isolated communication is an excellent solution to the issue, but may be undesired in simple low cost batteries due to the power for the isolator, the component cost and the circuit size.
Since you have an isolated RS485 communication path specified it seems you have the issue solved and could use either high or low side protection switching, and since you already have spent power on the isolator you would not want the additional power of the bq76200 unless it provides some other benefit in your system. Practical guidelines for the isolation would be to be sure the isolation will accomodate the maximum battery voltage + any known system transients and margin for the unexpected. Be sure to also check for any industry standards applicable to your product which may indicate specific isolation levels which must be met.

Hello, Miguel.

It's all clear now.

Thank you very much for the fast response.

Hello, WM5295.

Thank you also the fast and complete response.

That's why we chose TI, you have the best support!