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BQ76940: SMBUS and I2C ESD protection

rohsik_j
Prodigy 165 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350-R1A, EV2400, BQ78350, STRIKE, TIDA-00792

Hello,

The circuit is regarding BQ76940 as AFE and BQ78350-R1A as companion and STM32 as MCU.

the following screenshot shows the protection circuit for SMBUS line ( Companion to EV2400 Module interface)

Diode used for ESD protection is : DESD5V0S1BA-7

FROM TI reference Design for SMBUS

     

QUERIES:  1) The ICs are getting damaged frequently so was wondering what might be the issue, we use EV2400 to tap I2C and SMBUS to verify the Signal when companion is connected .do we need to disconnect companion with AFE  and MCU with companion when we want to interface through EV2400?

                   2) significance of 1mega resistor(R116 &R117) connected to GND and Zener(D53 and D54) connected to BAT-ve

                    3)why there is two 100 ohm resistor connected in series

                   4) Significance of spark gap connected  to BAT-ve

                   5)what protection/methods can be implemented to improve I2C and SMBUS for ESD.

Thanks in advance

Regards

  • 0
    TI__Guru 54715 points

    Hi Rohsik,

    1. When the BQ76940 is used with BQ78350-R1A the BQ76940 version should be 2.5V REGOUT to power the BQ78350.  3.3V from the EV2400 will exceed the voltage limit of the BQ78350 and may be a cause for the damage.  ESD from connecting the systems could be a possibility also.  When communicating with the BQ76940 from the EV2400 the gauge should be disconnected.  You might be able to use a level shifter between the EV2400 and the I2C bus with the BQ78350 attached, however remember that the BQ78350 does not expect to share the bus, if it sees registers changed it will change them back to what it intended, if it has a bus collision with the EV2400 it will go into an error mode and may spend most of its timeline setting registers or trying to establish reliable communication with the BQ76940.

    2. R116 and R117 help insure the SMB lines go to 0V when disconnected from a host.  D53 and D54 limit the voltage from an ESD strike or voltage on the connector pins.

    3. One resistor limits the current from the external source into the Zener.  The other resistor limits current from the Zener diode's voltage into the IC pin

    4. The spark gaps will limit voltage during an ESD strike with sufficient voltage.    

    5. The circuit shown has been used in several TI battery reference designs.  Check the ESD performance of the Zener diodes selected.  You might look at ESD devices specifically designed for communication signals.  TIDA-00792 has used a TPD1E10B06DPYT with a single 100 ohm series resistor.