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BQ78350-R1A: Interfacing with AFE bq7694003 and MCU

Shubham Kumar1
Prodigy 75 points
Part Number: BQ78350-R1A
Other Parts Discussed in Thread: BQ76940, BQ78350

Hi

I need some help to understand how to interface gauge IC bq78350-R1A with AFE bq7694003 and MCU. Here MCU is working at 3.3V(typ) and AFE regulator output is at 3.3V while the gauge IC works at a max. voltage of 2.6V and a typical voltage of 2.5V. The problem here is the another line mentioned in the datasheet stating the input and output voltage range of SMBC and SMBD lines as 5.5V.

What I'm not able to get is, during communication when my MCU is working at 3.3V and gauge IC working at 2.5V, what will be the levels for high side and low side. Is there going to be any compatibility issues in detecting the levels or are they just going to be working fine? Please help me understand this.

Thanks

  • 0
    TI__Guru* 81034 points

    Hi Shubham,

    It is definitely possible to use the BQ76940 with the BQ78350-R1A gauge and an MCU using 3.3V SMBus. However, you need to use the BQ7694001 (instead of the BQ7694003). The BQ7694001 provides a 2.5V REGOUT to power the VCC pin of the BQ78350-R1A. This is important, otherwise the BQ78350-R1A may be damaged.

    The communication between the BQ76940 and BQ78350 is an I2C Bus. This bus will have pull-up resistors to REGOUT (2.5V). The communication between the BQ78350 and the MCU is the SMBus. This will have pull-up resistors to the MCU 3.3V rail. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 75 points

    Hi Matt,

    Thanks for the reply. What I have been meaning to ask with this question is to know the voltage levels for high and low determination for AFE, Gauge and MCU. For example, a signal voltage greater than 75% of Vcc is treated as HIGH in MCU and lesser than 25% of Vcc is treated as LOW in MCU. Similarly, in this situation if Gauge IC is working at 2.5V but MCU is working at 3.3V, if I talk about the voltage levels for HIGH/LOW determination say when SMBus is pulled up at 2.5V, what exactly will be the voltage levels for HIGH/LOW determination?

    And if I do the same with AFE Gauge connection keeping AFE regulator output at 3.3V and Gauge at 2.5V with I2C bus pulled-up at 2.5V. Why this won't work? Please help me understand the rationale behind your suggestion above.

    Thanks.

  • 0 in reply to Shubham Kumar1
    TI__Guru* 81034 points

    Hi Shubham,

    On the BQ78350-R1A, the Abs Max voltage for the VCC pin is 2.75V. Applying 3.3V to this pin can damage the device. Normally the REGOUT pin of the AFE is connected to the VCC pin of the gauge.

    Regards,

    Matt