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bq20z75-v180 Smbus max pull-down current

Bruce Butts
Prodigy 30 points
Other Parts Discussed in Thread: BQ20Z75-V180

I'm having noise issues talking to a smart battery pack that internally uses a Benchmark bq20Z75-v180.  I've found that if I violate the Smart Battery v1.1 spec, which in turn, references the System Management v1.1 spec, for SMBC and SMBD max pull-down current of 350 uA, that I can communicate with the bq20Z75 without any trouble.

All I can find on the bq20Z180-v180 data sheet (bq20z75-V180 slusa22) with regards to the specified SMBC and SMBD pull-down current is under the Absolute Maximum Ratings section, is the parameter Iss, 'Maximum combined sink current for input pins' /PRES, /PFIN, SMBD, SMBC, with a value of 50 mA.

I'm getting reliable communications at a SMBC, SMBD pull-down currents of 1.0 and 3.0 mA, with a logic low level of about 0.4V, so the chip is clearly able to pull-down more than 350 uA, at least in this circumstance.

Can you provide a more precise characterization of SMBC, SMBD pull-down current?

-- Thanks.

  • 0
    TI__Intellectual 2650 points

    Hi  Bruce,

    externally the SmBus should be pulled up by around 10K. If you are having to exceed that I am wondering if maybe you have too much capacitance on the line somewhere?

    I have attached a note on SMBus which covers some common gotchas.

    Mike

    7167.SMBus made simple_v6.pdf 

     

  • 0 in reply to Mike Ewer
    Prodigy 30 points

    Hi Mike,

    We are operating in a noisy environment, and ~330 uA (3.3V / 10k-ohms) pull-down current isn't sufficient.

    The 350 uA pull-down current is specified to allow a 1.1k ohm series resistance in the SMBC and SMBD lines for ESD protection.  The battery protection board used by our custom battery implements the ESD protection scheme shown in the bq20Z75-v180 (slusa22) p16 -- a T-section with 200 ohms series resistance.  Without exceeding the same voltage drop across 200 ohms, the pull-down current can be increased to ~1.9 mA.

    At this point, if we can't increase the SMBus pull-down current, we are going to have to look for a different battery solution.  I'd sure appreciate it if you could give us a max SMBus clock and data pull-down current value, even if it is 350 uA, so we can make a clear-cut decision on this matter, and get on with our board release.

    /b

    p.s. Thank you for the SMBus .pdf.  You sent that to me several weeks ago, as part of an email exchange involving Larry Norman of PTI, and I did study it at that time, and again today.  I've also read the relevant parts of the Smart Battery and System Management Bus specs.  I also have worked with I2C successfully in the past.  //b