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BQ78350-R1: What is the specific circuit for the SMBC and SMBD for?

Paul K52
Intellectual 640 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350

Hello,

refering to the BQ78350 datasheet on page 14, I would like to know what the circuit is about, which is connected to the SMBD and SMBC pin?

I see two spark gaps, two Z-diodes, and to resistors, connected to ground and resistors, which are in series connected between the pin header and the pins of the chip

I could imagine that I need the Z-diodes to stabilize the voltage across SMBD and SMBC.
The sense of the spark gaps is to reduce the ESD to the remaining circuit.

And what are the remaining resistors for? ( 4x 100 Ohms, 4x 1 MOhms)

I could imagine that the voltage at the SMBD and SMBC are measured across this pins and ground. The bigger the resistors R73,R76 the better is the voltage, measured across this pins.

But I have no clue why I need 100 Ohm resistors in this configuration.

Thanks for figuring this out.

  • 0
    TI__Guru 54715 points
    Hi Paul,
    The circuit is a common SMBus input circuit for battery circuits exposed to the connector. It is to limit the voltage and reduce transients.
    There are only 2 1M resistors, these keep the inputs low when the connector is not mated to the system. These are not always used.
    The zeners prevent excessive negative or positive voltage. In some cases ESD prevention components are provided. In some cases the zeners and 100 ohm resistors can limit DC signals applied to the pack. The resistor between the connector and zener will limit current into the zener. The resistor between the zener and pin will reduce current into the IC. Obviously when the continuous voltage of the pack is high enough power dissipation will be high in the outside resistor, in some cases you may want fusable resistors.
  • 0 in reply to WM5295
    Intellectual 640 points
    "There are only 2 1M resistors, these keep the inputs low when the connector is not mated to the system."

    Refering to this sentence, these resistors are just pull-down-resistors, correct?
  • 0 in reply to Paul K52
    TI__Guru 54715 points
    Hi Paul,
    Yes.