- Ask a related questionWhat is a related question?A related question is a question created from another question. When the related question is created, it will be automatically linked to the original question.
Hello,
I am planning to design a BMS using the BQ76952 and BQ25756. (See design overview in picture below) The battery charger should also be integrated on the BMS board and communicate with the same MCU (host) as the monitor. In the data sheets it is recommended to connect the ground of the monitor via a net tie to the battery.
The question is if the bq25756 charger is on the same board and communicates with the same microcontroller, which ground should it be connected to? VSS or PGND? If it is connected to PGND, the shunt can cause small voltage differences between the MCU and BQ25756 (I2C bus and other signal lines between host and charger)
The same question arises for the MCU. In the datasheet of the BQ76952 the MCU is connected to PGND (see picture below). In the TIDA-010208 reference design, the MCU is connected to VSS. (See picture below)
The communication to the outside world will be isolated
What would you recommend for the GND connections?
Thank you in advance
Lukas
Datasheet:
Hello Lukas,
Yes, Vss is recommended to be connected through a net-tie to the BAT-. This is mostly to easily separate the high-current path of the battery current, from the low-current path of the IC ground, to keep ESD and high currents away from sensitive IC components.
The answer about the ground is... it depends. Since the MCU is in the same board as the AFE, it is fine to connect to the same ground reference. This would mean that there is no voltage modulation due to the difference between Vss and PGND. It is also completely find to put MCU ground at PGND, although as you said, the I2C signal may be modulated by the voltage drop of the sense resistor (Which is typically a very small voltage drop).
The other biggest difference is that MCU current could be measured by the AFE only if placed on the PGND side, otherwise MCU current would not be measured.
So for the MCU either would be okay in my opinion, although it is more commonly placed on the Vss/BAT- side when on the same board as the AFE.
On the charger side however, I would recommend this to be on the PGND side, the current of the charger needs to be measured by the AFE in order to make any protection decisions, if the ground of the charger is placed on BAT-/Vss, then the charger current bypasses the current sensing element of the AFE.
Hope this helped! Let me know if you have any further questions 
.
Best Regards,
Luis Hernandez Salomon
